Ilatronal Bureau of Standards

Library, E-01 Admin. BIdg. APR 2 8 1971 111Q3 DflSTlb

NBS SPECIAL PUBLICATION 260-27

Standard Reference Materials:

URANIUM ISOTOPIC STAKDARD

REFERENCE MATERIALS
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards^ was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and

technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Center for Computer Sciences and

Technology, and the Office for Information Programs. THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consistent system of physical measurement; coordinates

that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics—Electricity—Heat—Mechanics—Optical Physics—Linac

Radiation^—Nuclear Radiation-—Applied Radiation-—Quantum Electronics^

Electromagnetics^—Time and Frequency^—Laboratory Astrophysics'—Cryogenics'. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research lead- ing to improved methods of measurement, standards, and data on the properties of

well-characterized materials needed by industry, commerce, educational institutions, and

Government; provides advisory and research services to other Government agencies; and develops, produces, and distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials and the following divisions: Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Reactor

Radiation—Physical Chemistry. THE INSTITUTE FOR APPLIED TECHNOLOGY provides technical services to pro- mote the use of available technology and to facilitate technological innovation in industry and Government; cooperates with public and private organizations leading to the development of technological standards (including mandatory safety standards), codes and methods of test; and provides technical advice and services to Government agencies upon request. The Institute also monitors NBS engineering standards activities and

provides liaison between NBS and national and international engineering standards

bodies. The Institute consists of the following technical divisions and offices: Engineering Standards Services—Weights and Measures—Flammable Fabrics

Invention and Innovation—Vehicle Systems Research—Product Evaluation Technology—Building Research—Electronic Technology—^Technical Analysis

Measurement Engineering. THE CENTER FOR COMPUTER SCIENCF.S AND TECHNOLOGY conducts re- search and provides technical services designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selection, acquisition, and effective utilization of automatic data processing equipment; and serves as the principal focus within the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Center

consists of the following offices and divisions: Information Processing Standards—Computer Information—Computer Services —Systems Development—Information Processing Technology. THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific information generated within NBS and other agencies of

the Federal Government; promotes the development of the National Standard Reference Data System and a system of information analysis centers dealing with the broader

aspects of the National Measurement System; provides appropriate services to ensure

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Publications—Library—Office of Public Information—Office of International

Relations.

' Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

- Part of the Center for Radiation Research.

» Located at Boulder, Colorado 80302.
UNITED STATES DEPARTMENT OF COMMERCE • Maurice H. Stans, Secretary NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

Standard Reference Materials:

Uranium Isotopic Standard

Reference Materials

(Certification of Uranium Isotopic Standard Reference Materials)

E. L. Garner, L. A. Machlan, and W. R. Shields

Analytical Chemistry Division

Institute for Materials Research

National Bureau of Standards Washington, D.C. 20234

""lAU Of

I

. National Bureau of Standards Special Publication 260-27

» » * Nat. Bur. Stand. (U.S.), Spec. Publ. 260-27, 162 pages (Apr. 1971) CODEN: XNBSA

Issued April 1971

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.10:260-27). Price $1.25.
Library of Congress Catalog Card Number: 70-610921

il
PREFACE

Standard Reference Materials (SRMs) as defined by the

National Bureau of Standards are "well-characterized materials, produced in quantity, that calibrate a measurement

system to assure compatability of measurement in the nation."

SRMs are widely used as primary standards in many diverse

fields in science, industry, and technology, both within the

United States and throughout the world. In many industries

traceability of their quality control process to the national

measurement system is carried out through the mechanism and

use of SRMs. For many of the nation's scientists and technologists it is therefore of more than passing interest to know the details of the measurements made at NBS in arriving

at the certified values of the SRMs produced. An NBS series

of papers, of which this publication is a member, called the

NBS Special Publication - 260 Series is reserved for this

purpose

This 260 Series is dedicated to the dissemination of

information on all phases of the preparation, measurement,

and certification of NBS-SRMs . In general, much more detail

will be found in these papers than is generally allowed, or desirable, in scientific journal articles. This enables the user to assess the validity and accuracy of the measurement

processes employed, to judge the statistical analysis, and

to learn details of techniques and methods utilized for work

entailing the greatest care and accuracy. It is also hoped

that these papers will provide sufficient additional information not found on the certificate so that new applications

in diverse fields not foreseen at the time the SRM was originally issued will be sought and found.

Inquiries concerning the technical content of this

paper should be directed to the author(s). Other questions

concerned with the availability, delivery, price, and so forth will receive prompt attention from:

Office of Standard Reference Materials

National Bureau of Standards

Washington, D. C. 20234

J. Paul Call, Chief

Office of Standard Reference Materials

iii
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iv
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V
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Remittance from foreign countries should include an additional one-fourth of the purchase price for postage.

vi
TABLE OP CONTENTS

PAGE

1. INTRODUCTION 2 2. CHEMISTRY 3 A. Introduction 3 B. Ignition of U3O8 3 C. U3O8 Stoichiometry 8 D. Aliquoting of Uranium Solutions l4

E. Preparation of SRM Samples by Union Carbide

Nuclear Company 15

F. Preparation of Separated Isotope Samples by

Union Carbide Nuclear Company I8

G. Solution Preparation for Mass Spectrometric

Analysis I8

3. MASS SPECTROMETRY - INSTRUMENTATION 20

A. Introduction 20

B. General Description of Mass Spectrometer. ... 20

C. Source. 20

D. Collector 22

E. Ion Multipliers 23

F. Measuring Circuit 24

1. Slave VRE 2 4 2. Master VRE 25

3. Expanded Scale-recorder 25

4. MASS SPECTROMETRY - MASS DISCRIMINATION EFFECTS

AND AN EVALUATION OF PARAMETERS 27

A. Introduction 27

B. Mass Discrimination Effects 27

C. Filament Temperature 29

1. Sample Filaments 29

2. Ionizing Filaments 31

D. Sample Size 32

E. Time 35

F. Sample Mounting 35

G. Total Sample Composition 36

H. Acidity 37

I. Filament Material . 38

J. Pressure 39

K. Non-ohmic Response 40

L. R-C Response 40

M. Memory. 4l

5. MASS SPECTROMETRIC ANALYTICAL METHOD 43

A. Introduction 43

B. Sample Mounting 43 vii
PAGE

C. Degassing 44

D. Analytical Procedures (Heating Pattern) .... 45

1. Low Temperature Procedure 46

2. High Temperature Procedure 48

3. Outline of General Analytical Procedure . . 51

E. Role of the Operator in the Analytical Method . 53

F. Precision of the Analytical Procedures 54

6. ABSOLUTE 2 3 5^/2 3 ABUNDANCE RATIOS OP URANIUM

SRMs U-970 THROUGH U-005 57

A. Introduction 57

B. Experimental Procedure. 58

1. Mass Spectrometry 58

2. Isotopic Composition of "U235" and "U238"

Separated Isotopes 60

3. UaOe Ignition Procedure 6l

4. Preparation of Calibration Mixes Containing

2 3 5^/2 3 By Ratios of 9 , 1 and 0.1 65

5. Preparation by Solution Aliquoting of

Calibration Mixes Containing 23^^/2

Ratios of 0.005 to 0.05 69

6. Preparation of Calibration Mixes for

SRM U-970 72

7. Analysis of Calibration Mixes and SRMs

U-900, U-500 and U-100 74

8. Absolute 235u/238u Ratios by Interpolation. 78

9. Analysis of Calibration Mixes and SRMs

U-050 through U-005 79

10. Isotopic Analysis of Calibration Mixes and

SRM U-970 88

7. ABSOLUTE ISOTOPIC ABUNDANCE BY ^^^\J ISOTOPE

DILUTION 90

A. Introduction 90

B. Experimental Procedure 91

1. Mass Spectrometry 91

2. Isotopic Analysis of "U233" Separated

Isotope, Lot No. 3 92

3. Preparation of Spiked SRMs U-005 through

U-930 93

4. Preparation of Spiked SRMs U-0002 and

U-970 97

5. Analysis of SRMs U-005 through U-I50 to Determine Minor Isotope Composition with

Ion Multiplier Detection 100

6. Analysis of SRMs U-100 through U-930 to Determine Minor Isotope Composition . . . 101

vili
PAGE

7. Re-evaluation of Minor Isotope Composition

of SRMs U-030 through U-500 Using the

"Z" Source 103

8. Analysis of SRM U-970 to Determine Minor

Isotope Composition 103

9. Analysis of SRM U-0002 to Determine ^

by Isotope Dilution 106

10. Analysis of SRM U-0002 to Determine ^^'^U by Pulse Counting Ill

C. Results and Discussion 113

8. REFERENCES 121

APPENDIX 1 125

APPENDIX II (Certificates of Analysis) 131

LIST OP TABLES

TABLE NO.

1. Weight loss on ignition of 1 gram samples of

SRM 950a 4 2. Weight loss on ignition of 1 gram samples of

SRM 950a in an oxygen atmosphere 5 3. Weight loss on repeated ignition of 0.5 gram

samples of SRM 950a 6 4. Coulometric assay of ignited UaOs samples. ... 7 5. Ignition loss on 1 gram samples of SRM 950a

under different cooling rates 12

6. Spectrographic analysis of SRM U-050 17

7. Comparison of the calculated 2 3 5^/2 3 8^ p^tio to the observed 2 3 5yy 2 3 8y ^'atlo for low and high

temperature analyses of synthetic uranium

samples 59

8. Composition of "U235" and "U238" Separated

Isotopes 60

9. Calculated 2 35^/2 3 sy ratios of calibration mixes

for SRM U-900, SRM U-500 and SRM U-100 .... 68

10. Calculated 2 3 5^/2 3 ratios of calibration mixes

for SRM U-005 through SRM U-O5O 73

11. Calculated 235^^233^ r^^tios of calibration mixes

for SRM U-97O 74

ix
I PAGE

12. Correction factor for calibration of 2 3 5^/2 3 ratio of SRM U-500 75 1 13. Absolute 2 3 5^/2 3 By patio of SRM U-500 76 ! 14. Correction factors for calibration of 235^^238^

ratios of SRM U-900 and SRM U-100 77 i 15. Absolute 2 3 5^/2 3 patios of SRM U-900 and

SRM U-100 78

16. Correction factor for calibration of 2 3 5^/2 3

ratios by the Interpolation method 80

j 17. Absolute 235^/238^ patios for SRMs U-930, U-850, I U-800, U-750, U-350, U-200, U-150 and U-O5O. . 8I

18. Absolute 2 3 5^/2 3 By patio of SRM U-O50 82 ! 19. Absolute 2 3 5^/2 3 8u patio of SRM U-O30 83 i 20. Absolute 2 3 5^/2 3 patio of SRM U-020 84

| 21. Absolute 2 3 5^/2 3 8^ patio of SRM U-OI5 85 22. Absolute 2 3 5^/2 3 8^ patio of SRM U-010 86

' 23. Absolute 2 3 5u/ 2 3 8u patio of SRM U-OO5 87

24. Absolute 235^/238^ patio of SRM U-970 89 25. Isotoplc composition of "U233" separated

Isotope, Lot No. 3 92

26. Composition of "U233" spiked solutions of SRM | U-930 through SRM U-005 , series A 95 i 27. Composition of "U233" spiked solutions of SRM

U-930 through SRM U-005, series B 96

28. Spike calibration of "U233" separated Isotope

solutions 98

29. Composition of "U233" spiked solutions of SRM

U-970 99

30. Composition of "U233" spiked solutions of SRM

U-0002 100

31. Minor Isotope composition of SRM U-005 through

SRM U-200 using Ion multiplier detection . . . 102

32. Minor Isotope composition of SRM U-100 through

SRM U-930 using the NBS collector. ...... 104

33. Redetermination of minor Isotope composition of

SRM U-030 through SRM U-500 using the NBS

collector and the "Z" lens source 105

34. Minor Isotope composition of SRM U-970 107

X
PAGE

35. ^^^U in SRM U-970 by Isotope dilution 108

36. ^^^U in SRM U-0002 by isotope dilution 110

37. ^^'*U and ^^^U in SRM U-0002 by pulse counting. . Ill

38. Summary of NBS determined ^^^U/^^^U ratios and

the isotope dilution values of ^^'*U and ^^^U

for SRMs U-0002 through U-970 113

39. Comparison of ^^^U and ^^^U measurement by NBS,

UCNC and GAT Il4

40. Certified atom percent composition of SRMs

U-0002 through U-150 II8

41. Certified atom percent composition of SRMs

U-200 through U-970 119

LIST OP FIGURES

FIGURE NO.

1. Thermogravimetric analysis curves of blanks and

100 mg U3O8 samples under various pressures. . 9 2. Thermogravimetric analysis curves of 100 mg

UsOe samples in various percentages of oxygen. 10

3. Thermogravimetric analysis curves of 100 mg

U3O8 samples with different cooling rates. . . 11

4. Deposition of solution on a sample filament. . . 34

5. The yellow-orange oxide of uranium on a sample

filament 34

6. Platinum boat and glass desiccator used in

U3O8 ignition 62

7. Furnace with quartz ignition tube for ignition

of uranium oxides 63

8. Apparatus used for weight aliquoting 70

9. 12-inch radius of curvature 68° analyzer tube mass spectrometer from 196O to I963 127

10. 12-inch radius of curvature 68° analyzer tube mass spectrometer from I963 to 1966 128

11. 12-inch radius of curvature 68° analyzer tube mass spectrometer from I966 to 1970 129

12. 12-inch radius of curvature 90° analyzer tube mass spectrometer I30

xi

URANIUM ISOTOPIC STANDARD REFERENCE MATERIALS

E. L. Garner, L. A. Machlan and W. R. Shields

Institute for Materials Research

National Bureau of Standards

Washington, D. C. 2023^

An ignition procedure has been developed that will yield

reproducible st oichiometry for UsOe. The effects of temperature, length of ignition, rate of cooling, pressure and type

of atmosphere were investigated. This ignition procedure

has been used for the blending of high purity ^^^U and ^^^U

separated isotopes to prepare calibration standards for the

determxination of bias effects in the thermal emission mass spectrometry of uranium. Weight aliquoting was used to prepare

calibration mixes with 235^^238^ ]og^-^j_Qg Qf more than 10 and

less than 0.1 and to add a ^^^U spike for the determination of

minor isotope abundances in the uranium isotopic standards by

the isotope dilution technique.

A description of the unique features of the mass spectrometer instrumentation including the source, NBS collector and

expanded scale recorder are given. Two specific analytical

procedures were used for the isotopic analysis of uranium and

are adaptable, within a general framework, to fit the particular

ion current intensity requirements of a wide range of isotopic

distributions. Mass discrimination due to evaporation and

ionization on the filaments, and other parameters such as temperature, time, sample size, sample mounting, total sample

composition, acidity, filament material, pressure, non-ohmic

response, R-C response and source memory were studied as part

of the development effort to establish sound analytical

procedures

The absolute isotopic abundances of I8 uranium SRMs were determined by thermal emission mass spectrometry. The general

approach was to determine absolute 2 3 5yy2 3 8y patios by using

calibration mixes to correct for filament bias. Then the

absolute ^^'*U and ^^^U were determined by ^^^U isotope dilution. For SRM U-0002, isotope dilution was the only practical

means of determining the low abundance of ^^^U as well as the

^^"^U. The limits given for the isotopic composition of the

uranium SRMs are at least as large as the 95 percent confidence limits for a single determination and include terms for

inhomogeneities of the material as well as analytical error.

Key words: Absolute isotopic abundance- ignition procedureisotopic standards, mass spectrometry;

st oichiometry ; uranium.

1
1. INTRODUCTION

This publication describes the work by the Analytical

Mass Spectrometry Section of the National Bureau of Standards

on the development and characterization of standard reference

materials for the isotopic composition of uranium.

The initial measurement of the isotopic composition of

uranium Standard Reference Materials (SRMs) was completed in

1959 with 95 percent confidence limits of 0.5 percent for the

least abundant of the major isotopes (^^^U and ^^^U) and above

one percent for the minor isotopes (^^'*U and ^^^U). A 5 year

period of intensive evaluation and development of the chemical

procedure, the mass spectrometric instrumentation and the mass spectrometric analytical procedure followed this initial effort.

The SRMs played a unique and dual role of providing a group of

high purity working standards to develope methods to analyze

uranium, and also to provide a uniform set of standards with

a wide range of isotopic abundances to evaluate, develop and

improve the mass spectrometric instrumentation. Informational

fallout from the development of a uranium analytical method

was beneficial and applicable to the analysis of other elements. It was demonstrated that many of the general rules and

control procedures necessary to make a meaningful comparison

of ion currents were basically the same and were independent

of the elemental ions measured.

The complete analytical method for any isotopic measurement is divisible into three broad areas; chemistry, instrumentation and mass spectrometric procedure. A prerequisite

for an accurate isotopic measurement is an evaluation of the error contributions from each of these sources. Since any

of these can cause a serious loss of precision and accuracy,

equal attention must be given to all phases of the measurement.

2
2 . CHEMISTRY

A . Introduction

The chemistry requirements in the characterization of

these standards were the accurate blending of separated isotopes

to provide standards for mass spectrometer calibration and the

accurate additions of "U233" separated isotope for the isotope

dilution determinations of the minor isotopes. In addition it was necessary to provide solutions that were uniform in uranium

and acid concentrations, and that were very low in impurities.

B. Ignition of UgOe

The goal of the early investigation was to find an ignition

procedure that would give a reproducible stoichiometry for U3O8

Irrespective of the previous history of the starting materials.

If a reproducible oxide could be produced, weighed portions of

the high purity "U235" and "U238" separated isotopes could be

blended directly or aliquots taken to prepare accurately known

mixes for the mass spectrometer calibration. Even if the

ignited samples of U3O8 were not stoichiometric, as long as the oxides were both nonstolchiometric to the same degree, the

blending would produce the same isotopic composition as stoichiometric oxides. The same criteria also applied to the

^^^U isotope dilution for the minor Isotope determinations.

Previous work on the certification of uranium oxide (UsOe)

assay standards, SRMs 950 and 950a, indicated a difference of

about 0.06 percent between the assay and the value calculated

from impurity determination. This difference was considered

to represent deviations from the nominal stoichiometric com- position. The assay, 99.9^ percent, was based on material

ignited at 900 °C for one hour in an open crucible and cooled

in a desiccator.

The Ignitions in this investigation were conducted in a tube furnace. In a comparison with the previous work samples

of SRM 950a, ignited in the tube furnace for one hour at 900 °C, had a weight loss that agreed to within 0.005 percent

3
of samples from the same bottle of 950a Ignited in an open

crucible for one hour at 900 °C. Samples of high-purity,

99.98 percent, uranium metal were ignited at 900 °C for one hour after slow oxidation at a lower temperature. The increase

in weight, taking into consideration the metallic impurities,

was from O.O7 to 0.12 percent more than the calculated weight

increase to produce U3O8. This meant that the same stoichiometry was not obtained for these two starting materials and

further investigation was necessary. Ignitions at temperatures

from 900 to 1050 °C were tried on SRM 950a and the metal. It was found that the higher temperature (Table 1) gave greater

Table 1. Wei

of

ght loss on ignition of 1 SRM 950a.

gram samples

Loss in weight

Temperature

(°C)

Ignition time^ Sample

(%)

1 Sample 2 {%)

900 1 0 .096 0.098

Tt 17 0.108 O.lll

tt 18 0 .109 0.107

tt 34 0.112 0 .115

1000 1 0 .127 0.129

It 2 0.132 0.136

^ Total ignition time in hours at each temperature

ignition losses on 950a, all less than stoichiometric, and

increased length of ignition times also gave larger ignition

losses (Table 1). Experiments with ignition in an oxygen

atmosphere (>99%) showed slowly increasing losses with increasing temperatures to 1000 °C (Table 2). Ignition in oxygen

after ignition in air at 1050 °C appeared to give reproducible

ignition losses. Further investigation showed that several

hours, usually overnight, ignition at IO5O °C in air and then

4
Table 2. Weight loss on Ignition of 1 gram samples of

SRM 950a in an oxygen atmosphere.

Loss in weight

Temperature^

(°C)

Sample 1 {%)Sample 2 {%)

450 0.050 0 . 0 49 550 0.061 0.058

0 . U M U . U / U

7 c: n nU 0 7 7 . U / f n n 7 ll 800 0 .079 0 . 076

850 0 .082 0 .080

0.085 0.082

950 0.087 0.085

1000 0.088 0.087

Ignited for one hour at each temperature.

an ignition in oxygen from 700 °C to 1000 °C gave an oxide

that was reproducible. Repeated ignition cycles in air and

then in oxygen showed nearly constant weight (Table 3). Since

calculation showed both SRM 950a and the metal to have the same stoichiometry after ignition in air at IO5O °C for I6 hours followed by one hour in oxygen at 85O °C, several

uranium samples with different histories were ignited by this

procedure and titrated coulometrically by George Marinenko of

the Microchemical Analysis Section [1,2,3,4]. The results

are given in Table 4. The five samples represent starting

materials of metal; nitrate, roasted at 850 °C; and a peroxide

precipitate roasted at 8OO °C. The first four samples have

less than 100 ppm impurities and the fifth sample, the metal,

has been corrected for an assay of 99.977 percent which had

been determined by the same coulometric titration. Impurities

detected in the metal are about 200 ppm which agrees very well

5
Table 3. Weight loss on repeated ignition of 0.5 gram samples

of SRM 950a. Loss in weight

(°c)

-L^li-L L/-L\JLi U-Lllitr (hours

)

Q mr^ "1 0 (3 ~l d-iiip ± e ± {%)

Sample 2 {%)

QDD y u U ai r n n 7 c; u . u 0 0 .075

-L U

n u . ± u ± 0 .110

850 ]_ u 2 nu . nvQ U f y 0 .079

1! M It u . u 0 u 0 .080

1050 16 ai r 0.102 0 .114

]_ Do 0.082 0 .082

tt II II 0.082 0 .081

1050 16 ai r 0.105 0 .117

850 ]_ u 2 0 .085

II It n U . U 0 4 0 .084

1050 16 air 0.105 0 .115

850 1 O2 0.086 0 .085

Tf II It 0.086 0 .085

11 16 ti 0.086 0 .085

with the assay. When this correction is made, the assay value

of. the oxide, 99.929 percent, is in good agreement with the

other four samples. These results indicate that the ignition

procedure will give samples of the same st oichiometry regardless of the starting materials

This ignition procedure was used for all samples in this

paper

6
Table 4. Coulometric assay of ignited UsOe samples.

Sample Sample No. Assay

{% UsOe)

SRM 950a la 99.904

lb 99.946

2 99.918

Average = 99.923 s.d. = 0.023

SRM U-0002 la 99 .917

lb 99.911

2 99.960

Average = 99.929 s.d. = 0.027

"U238" la 99.913

lb 99.932

2 99.919

Average = 99-921 s.d. = 0.010

"U235" la 99.928

lb 99.918

2 99.921

Average = 99.922 s.d. = O.OO5

Dlngot la 99.929

lb 99.923

2 99.934

Average = 99-929 s.d. = 0.005

7
C . U3Q8 Stoichiometry

The stoichiometry of U3O8 was Investigated further in

hopes of finding an easily reproducible ignition procedure that

would give stoichiometric U3O8. Some of the previous work

Indicated the rate of cooling could affect the ignition loss

and there have been personal communications indicating slight

changes in ignition loss at different geographic locations

having different altitudes or atmospheric pressures. A series

of thermogravimetric analysis (TGA) ignitions were conducted

varying the air pressure and oxygen content. Accurate cooling

experiments could not be carried out conveniently with the

equipment available. Figure 1 gives blanks at two pressures

of air showing that the instrument was not sensitive to different pressures as long as they were constant. The temperature

was determined by using a calibrated platinum-platinum rhodium

thermocouple to correct the chrome 1-alumel thermocouple used

on the equipment. The correct temperatures are marked on the

figures . Figure 1 also gives TGA curves on 100 mg samples of

SRM 950a at decreasing atmospheric pressures. To obtain the

greatest sensitivity, 100 mg was suppressed by the instrument

and the sensitivity was increased to the maximum of 100 yg per

division. It can be seen from the curves that the loss in

weight occuring near 900 °C shifts to lower temperatures with

decreasing pressures. Figure 2 shows a similar shift when the

oxygen content is reduced. The first curve in Figure 3 is a cooling curve after ignition of SRM 950a to 1000 °C in air and

indicates some gain in weight below 850 °C. The same sample

being reignlted is shown in the second curve and shows approximately the same loss at 900 °C as a sample being ignited for

the first time. The third curve is the same sample reignlted

after it had been cooled by quickly removing it from the

furnace. The instability of the TGA instrument does not allow

a determination of whether all the weight gain occurs during

the cooling or if part of the gain is during the heating period

of the next ignition.

8



One gram samples of SRM 950a were ignited in a tube furnace

to obtain ignition losses under different cooling procedures

as well as under various temperatures and pressures. These

ignitions were carried out using the same equipment and under

the same conditions, except as noted, that were used for all

samples in the isotopic program, as described later in Section

6,B,3. Three rates of cooling were used in addition to the one normally used. For convenience the rates of cooling are identified as follows: "fast", meaning that the boat containing

the sample was quickly pulled into an iced portion of the

quartz tube; "normal", meaning that the furnace was quickly

moved back from the sample area of the tube; "slow", meaning

that the furnace was slowly moved back from the sample area of

the tube over a period of approximately 15 minutes; "very slow",

meaning that the furnace was turned off and allowed to cool over a period of several hours while still covering the sample

area. Table 5 shows the average loss on ignition of at least

two samples ignited for one hour at the temperature indicated.

Table 5- Ignition loss on 1 different cooling

gram samples of SRM

rates

950a under

Cooling rate

Temperature

(°C)

Very slow

{% loss)

Slow

{% loss)

Normal

loss) Fastloss)

850 0.076

900 0.084 0.087 0.096

950 0.086 0.102

1000 0.085 0.090 0.114 0.120

1050 0.082 0.094 0.126

The results show not only a significant increase in loss on ignition at increasing temperatures but increasing loss with

faster cooling rates. The results indicate that a very slow

cooling rate might give a reproducible ignition loss but that

12
it would be nonst olchlometrlc to approximately the same extent

as the Ignition described in Section 6,B,3. The TGA curves,

the assay, and the ignition data indicate that the U^Oq is very

close to stoichiometric after- the loss at approximately 925 °C in air is complete as shown on the TGA curves but that weight

is picked up, apparently oxygen by the U3O8, in amounts that

vary depending on the cooling rate, temperature of the ignition

and length of ignition.

In an air atmosphere at a reduced pressure of 550 mm of

Hg the loss on ignition for one hour at 900 °C was 0.104 percent

compared to O.O87 percent at a pressure of 750 mm with a normal

cooling rate for both. When a slow cooling rate was used the

losses were O.O83 percent at the reduced pressure and 0.084

percent at atmospheric pressure. These results indicate that

samples ignited at higher elevations could have an assay as much as 0.02 percent higher than the same sample ignited at sea level if normal cooling rates are used.

Ignitions in 5 and 10 percent oxygen atmospheres resulted

in greater losses. The loss on two samples at 1050 °C for one hour in 10 percent oxygen with normal cooling was O.I66 and

0.172 percent. The loss at 1000 °C for one hour in 5 percent

oxygen with normal cooling was 0.175 and O.I87 percent, with

slow cooling was 0.154 and 0.l4l percent, and with very slow

cooling after one hour at IO5O °C was O.O9I and 0.093 percent.

An ignition loss of approximately 0.15 percent would be required

on this sample to obtain stoichiometric U3O8. These ignitions

in 5 and 10 percent oxygen with normal cooling exceed the 0.15

percent loss but again with slower cooling the loss decreases

until it approaches, within 0.01 percent, the loss found in an

air ignition with a very slow cooling. It might be possible to obtain stoichiometric U3O8 by an ignition in an atmosphere of

less than 20 percent oxygen to an exact ignition temperature

and by a very carefully controlled cooling rate but, it would

not be practical because of the difficulty in reproducing

13
these Ignition conditions. For this reason the procedure

developed in the previous section {2 ,B) was used for all

ignitions in the SRM characterization.

D . Aliquoting of Uranium Solutions

The preparation of "U235" and "U238" separated isotope

calibration mixes for the range of 10 percent ^^^U to 90 percent ^^^U could be done by blending weighed portions of the

separated isotopes; but, outside this range and for all the

"U233" additions for isotope dilution determinations of the

minor isotopes, aliquoting of solutions would be required.

The aliquoting needed to be accurate to within a few parts in ten thousand, and preferably to one part in ten thousand, so that the aliquoting uncertainty would not contribute significantly to the overall uncertainty in these determinations.

An automatic buret was investigated to determine if it

could be used for aliquoting. Repeated aliquots of 5 ml were found to differ by as much as 0.01 ml, which was twenty parts

in ten thousand and would not be satisfactory. Since in this

laboratory it has been found difficult to do volume aliquoting

to an accuracy of much better than one part per thousand, it was decided to try weight aliquoting.

A teflon bottle used as a "squirt" bottle was investigated

for use in aliquoting by weight . The regular cap on the teflon

bottle was replaced with a similar cap that had inserted in it

a piece of polyethylene tubing shaped for use as a "squirt"

bottle and the outside end of the tubing was covered with a polyethylene cap to prevent evaporation. When the weight of

aliquot obtained by weighing the "squirt" bottle before and

after delivery of the aliquot, was compared with the weight of

solution delivered to a glass weighing bottle, the discrepancy

was 0.5 to 1 mg less in the glass weighing bottle. Since the

weighing bottle with solution lost about 0.5 mg per minute

when covered with a filter paper having a small hole, as had

been used for the aliquot checking, and since the aliquoting

14
required about one minute, the weight of aliquot using a "squirt" bottle should be accurate to within 1.0 mg. It was decided to use allquots of at least 3 g of solution from the

"squirt" type teflon bottle which should give allquots accurate

to at least three parts In ten thousand. This procedure was used on the "U233" Isotope dilution of SRMs U-005 through U-930.

A better weight allquotlng technique using plastic

syringes was developed before the remaining allquotlng was

done. In this technique the weight of solution was determined

and the cap of the teflon bottle was quickly replaced with a rubber serum septum. A platinum needle was Inserted thru the

septum and a short second needle served as a vent. A 2.5, 5 or 10 ml plastic syringe, with the rubber plunger covered by

a thin teflon sheet to prevent possible contamination, was attached to the Kel-P hub of the platinum needle and the

desired amount of solution was drawn Into the syringe. After

a small amount of air was drawn Into the tip of the syringe,

the syringe was disconnected and quickly capped with a Kel-F

cap. The syringe was wiped with a damp paper towel to remove

any static charge and weighed. The aliquot was delivered from

the syringe and the syringe was capped, wiped and rewelghed.

Repeated weighings of the filled or empty syringe gave weights

that duplicated to within 0.1 mg. Allquotlng by this procedure

[5,6] has been shown to be accurate as well as precise to at least one part In ten thousand.

E . Preparation of SRM Samples by Union Carbide Nuclear Company

The 18 SRM samples were supplied through the AEC from

Union Carbide Nuclear Company, Oak Ridge, Tennessee.

All but SRM U-0002 were prepared at Oak Ridge by the

following procedure

Approximately 1900 grams of uranium hexafluorlde of the

specified Isotoplc concentration was withdrawn from the cascade. The hexafluorlde was liquified, thoroughly mixed, vacuum transferred to a large nickel cylinder, cooled with

15
liquid nitrogen, "knocked out" into a 15 liter always-safe

polyethylene container, and hydrolyzed with 8 liters of cold

distilled water. After mixing, the solution was evaporated

to dryness in platinum dishes under infrared lamps and the

uranyl fluoride was converted to urano-uranic oxide by pyrohydrolysis at 800 °C. This oxide was dissolved in 2 N nitric

acid and the solution was filtered into a clean always-safe

container and thoroughly mixed by inverting and rolling the

container

Portions of the solution containing 100 to 125 grams of

uranium were transferred to Vycor beakers and the uranium was precipitated by adjusting the pH to 1.0 with ammonium hydroxide,

adding 1.3 ml of 30 percent hydrogen peroxide per gram of

uranium, and allowing the U0it*2H20 to settle at least 16 hours.

The supernatant liquid was decanted, the precipitate was washed,

and after allowing the precipitate to settle, the supernatant

was again decanted. The precipitate was filtered, washed,

transferred to a platinum dish, and ignited to urano-uranic

oxide at 85O °C for I6 hours. This oxide was ground in a boron carbide mortar, sieved through a 60 mesh screen, and

blended. All of the oxide was then placed in a polyethylene

bottle and again blended for 10 minutes.

The U-0002 sample was purified by a cupferron precipitation

and an additional peroxide precipitation in addition to the

procedure used on the other 17 SRMs

These samples were checked by spec trographic analysis for

64 possible Impurities. The total impurities detected on each

of these samples were less than 50 ppm. A typical spectrographic analysis is given in Table 6. If all the other elements

not detected were present at their limits of detection the

total could be approximately 200 ppm, this, however is highly

improbable

For each SRM sample, except the U-0002, one kilogram of

uranium in the form of oxide was shipped to the National Bureau

of Standards. Twenty kilograms of SRM U-0002 were shipped.

16
Table 6. Spectrographic analysis of SRM U-O5O.

Limit of ^ Limit of

Element detection Detected Element detection Detected^

( DDm

( ppm

(ppm

"•& 0 .

Pt 1 1 Al 1 Rb 10 As 3 Re 5 Au 1 Rh 1 B 0 .

1 .

Ru 1 Ba 10 Sb 1 Be 1 Sc 1 Bi 1 Si 1 3 Ca 1 Sn 1 Cd 1 Sr 1 Co 1 Ta 1 Cr 1 Th 1 Cs 10 Ti 0 .

Cu 0 .0 .

Tl 2 Fe 1 2 V 1 Ga 1 W 1 Ge 1 Zn 20 Hf 1 _ Zr 1 _ Hg 1 Lanthanide

In 3 Elements

Ir 1 La 1 K 5 Ce 1 Li 1 Pr 1

Me: 0 .Nd 1 Mn 1 Sm 1 Mo 1 Eu 1 Na 1 2 Gd 1 - Nb 1 Tb 1 Ni 1 5 Dy 1 - Os 20 Ho 1 P 3 Er 1 Pb 3 Yb 1 Pd 1 Lu 1

^ The symbol "-" means that the element was looked for but

not found.

17
These samples were packaged at NBS for distribution to AEC approved facilities. One gram of uranium in the form of

oxide, weighed to within 5 mg, was transferred to glass micro

bottles. A thorough cleaning of the hood was conducted between

SRM samples to prevent isotopic contamination by a previously

weighed sample.

P . Preparation of Separated Isotope Samples by Union Carbide

Nuclear Company

The "U233" (Lot No. 3), "U235" and "U238" separated isotope samples were supplied by the AEC through Union Carbide

Nuclear Company. The original "U235" sample, pile waste, was enriched by Calutron treatment. The sample was then purified

by two ether extractions, a hydrogen peroxide precipitation

and volatilization of the vanadium as vanadyl chloride. The

"U238" sample was prepared from natural uranium in a similar

manner. Spectrographic analysis indicated approximately 50 ppm detectable impurities in these samples.

G . Solution Preparation for Mass Spectrometric Analysis

Solutions were prepared with a uniform matrix so they

would give a uniform response on the mass spectrometer (Section

4,G and 4,H). All solutions were prepared to contain 5 mg U/ml

in nitric acid (1+19). Impurities, especially alkali metals, were kept to a minimum. The reagents used were selected for

their high overall chemical purity as well as being checked

for any uranium contamination. The uranium isotopic SRM

samples and the separated isotope samples were of high chemical

purity and did not require further purification.

Although the solutions described in this publication did

not require further chemical purification, a chemical purification step is included when the isotopic composition is being

determined on an unknown sample so the unknown and standard

are as near the same purity and concentration as possible.

The purification step used is an anion exchange separation.

For samples containing up to a few milligrams of uranium, a 18
column (0.6x7 cm) of strongly basic anion resin which has

been cleaned by alternately adding 0.3 N nitric acid and

nitric acid (1+1) is used. The sample is dissolved in a minimum of nitric acid (1+1) and added to the column. Most

elements are not held by the resin in nitrate form [7] and

are quickly eluted with nitric acid (1+1), a maximum of 3-^

ml of nitric acid (1+1) is used for both the sample and the

elution of the impurities. The uranium is then eluted with

0.3 N nitric acid. The uranium eluate is evaporated to dryness

and then dissolved in sufficient nitric acid (I+I9) to give a solution containing 5 mg U/ml of solution. On samples con- taining low concentrations of uranium, this anion exchange

separation should be repeated. Two anion exchange separations

have proven adequate for uranium at the ppm level in glass

samples [8], Other separation techniques may be required for

some samples, but the solutions prepared for mass spectrometric

analysis should be as nearly as possible identical to the

calibration solution.

19
3. MASS SPECTROMETRY - INSTRUMENTATION

A . Introduction

The objective of this section is to briefly discuss some

of the unique instrumental features and to indicate- their

effect in the measurement method. Instrumental parameters

restrict the selection of conditions for certain analytical

procedures and are frequently the limiting or decisive factor

in selecting optimum conditions to determine isotopic composition. All probable sources of systematic error attributable

to the mass spectrometers must be identified, eliminated,

reduced or controlled before bias components associated with

the ionization process can be studied effectively. Thus, the mass spectrometer is the second area to be mastered before

the maximum amount of information can be extracted from each

analysis. Whenever applicable, some historic information is included because of the long and continuous nature of the

instrumentation development period.

B . General Description of Mass Spectrometer

The mass spectrometers were single focussing, solid sample

instruments with a 12-inch radius of curvature 68° analyzer

tube and 60° sector magnet. During the past decade there were

no newly discovered basic or fundamental innovations in the

spectrometers utilized for uranium analysis, but all of the

major component systems were repackaged, rearranged and, in

many instances, extensively modified (see Appendix I). The

basic 12-inch 68° analyzer tube mass spectrometer and some of

the developmental changes are described in other publications

[9,10,11].

C . Source

The source is a linear thin lens with each succeeding

lens being approximately twice as far from the ionizing filament as the previous lens [9]. The source was constructed

without baffles or skirts to facilitate the maximum pumping

speed possible in the vicinity of the filaments. Initially,

20
the source defining slit was movable and was adjusted by an external micrometer screw mounted in the source housing. The

major shortcoming of this type assembly was the inability to precisely determine or reproduce a given slit width once the

micrometer was moved or the source was disassembled for cleaning or maintenance purposes. The results of this unavoidable

variation in the slit width was a change in transmission.

Since sample filament temperatures cannot be measured directly,

the analytical procedure is based on maintaining a constant

signal intensity within a limited temperature range with all

other focal parameters constant. Thus, the effect of slit

variations was a sample filament temperature compensation to maintain the intensity as a fixed parameter. If this temperature compensation was large enough and remained undetected,

it would be the cause of apparent differences in isotopic

composition between different sources, shifts in data between

source cleanings, or sudden shifts in data on a source when

the micrometer screw was inadvertently repositioned.

The problem was solved by redesigning the source [10] so that the micrometer adjustment was replaced by a slit assembly

formed by two movable knife edges firmly fixed in position by screws. The slit adjustment is external to the source housing

and is made on the work bench where the width can be accurately

determined with gauges. The newly designed source can be disassembled and cleaned without a compulsory change in the slit

width or, if necessary, the slit knife edges are removable

for cleaning purposes and the desired slit width reproduced

accurately

The redesigned source also included the addition of a "Z" focussing lens which increased the transmission of the 12-inch

spectrometer by a factor of approximately 3. The increase in

transmission permitted the option of obtaining the standard

signal intensity at slightly lower filament temperatures and

with a correspondingly lower rate of fractionation. Since

signal intensities between 5^10~^°A and lxl0~^A are obtainable

21
without large changes in the isotoplc composition with time,

the lower isotoplc abundance limit at which satisfactorymeasurements could be made without using ion multiplier detection was extended to the 100 ppm level with a 95 percent

confidence limit of approximately 1 percent.

D . Collector

The NBS collector is a conventional Faraday cage collector

and has a transmission grid and a series of suppression grids

[9]- Although the transmission grid monitors a fraction of

the total ion current (50% or less), the prime function is to mask the suppression grid system from the ion beam. The grid

is made of 0.030 inch tungsten ribbon separated by an opening

of equal width. The width of the tungsten ribbon is large

enough to completely shield the suppression grids from the

ion current. The purpose of the suppression grid system is to provide cubic suppression of secondary particles between the

defining slit of the collector and the transmission grid. By

applying appropriate potentials to the grids, a nearly constant

grid response is obtained as ion currents of the isotopes are switched on and off the Faraday cage. A collector that is

dirty or exposed to large quantities of the alkali elements

will produce an excessive number of secondary electrons. Thus,

an adequately designed collector, "aged" with time, will

gradually acquire a thin film deposit of extraneous material

on various surfaces. As the level of contaminant increases

the number of scattered particles increases and some of these

particles are collected on the transmission grid. When this

stage is reached, the grid response changes from a constant

for ion currents switched on and off the Faraday cage to a variable that increases with time. The change in the constant

nature of the grid response is the prime indicator in determining the need for a collector cleaning. The suppression

grids extend the operational life of a collector beyond the

normal expectancy for a non-grid system, and eliminate a 22
random bias component which can be several tenths of a percent. This bias component Is dependent upon the energy of

the Ions, magnitude of the Ion current, and the degree of

cleanliness of the collector.

E . Ion Multipliers

The Ion multipliers were either 8 or 12 stage units con- structed of copper-beryllium dynodes . Although these units were capable of operation at high gain, the standard procedure

was to operate at abnormally low gains of 30-100 for uranium

isotoplc measurements. Thus, the analytical objective of the

multiplier was to provide sufficient gain to maintain a minimum

amplified output signal from the vibrating reed electrometer

(VRE) of approximately 10 mV (100 chart divisions) for operation of the expanded scale recorder in measuring minor isotope

ion currents. Additional objectives were (1) to obtain nearly

uniform gain (flat top peaks) as the ion beams moved across

the first dynode, (2) to obtain long term stability and (3) to operate at approximately the same background noise level as the standard collector.

Ion multipliers were used sparingly in analyzing uranium

SRMs and were Installed as detector only when the precision

of ^^'*U and ^^^U measurements was deemed to be unsatisfactory

with conventional collectors. Prior to the development of

the "Z" lens source, the general guideline was to use ion

multiplier detection when the abundance of a uranium isotope

was equal to or less than 0.1 atom percent. At a 0.1 atom

percent composition it was possible to use both types of

detectors and obtain direct evaluation of the accuracy of the

total bias correction (filament and ion multiplier) for multiplier measurements. The standard procedure was to determine

experimentally a single correction factor which Includes a component for the ion multiplier and to use this single cor- rection factor to correct for all bias effects.

23
P . Measuring Circuit

The measuring curcuit consisted of (1) two vibrating reed

electrometers (VRE) operated as a master-slave combination

and (2) an expanded scale recorder.

1. Slave VRE

The slave unit was used to monitor that portion of the

total ion current intercepted or collected by the transmission

grid. The input resistor for the slave VRE was always 10^°

ohm and the output of the unit was displayed by the Xi pen

of a dual pen strip chart recorder. The grid signal provided

a quick and reliable means of establishing uniform quality

control for all analyses and facilitated the detection of collector failures and/or abnormal collector response. Abnormal

or sudden changes in grid response were indicative of malfunctions such as loss of suppression potential, unstable

suppression potentials and broken or malaligned grid wires.

The stability and behavior (growth or decay) of the grid

signal are used as STOP or GO flags to reject or accept an analysis without ever measuring the ion currents of the isotopes on the Faraday cage. Concentrations of the alkali

elements, either from the filament assembly or the uranium

sample, produce sharp erratic spiking of the grid signal, even when these isotopes were not focussed on the grid or Paraday

cage. This particular response was a clue to their presence

which was quickly verified by scanning to the appropriate mass positions. Even though the concentration of these elements

was not known, any concentration that produced the random

spiking response was considered to be undesirable. If this

response occurred within an analysis and did not end before

the ratio measurement was started, the analysis was terminated

and no data were taken. If this response was reproducible for

different filament loadings, the contaminant had to be identified and the necessary measures were taken to reduce the

background below the critical level.

24
2. Master VRE

The master VRE is used to measure the ion current collected on the Faraday cage. Input resistors of 10^^ and 10^°

ohms were available on all units and were switched in or out

of the measuring circuit at the option of the analyst.

Although 10^ ohm resistors were available on some units, they were not normally necessary for uranium analysis. Input

resistors of 10^^ ohms were not used because they were found

to be non-linear.

3 . Expanded Scale-recorder

The 10 mV and 100 mV outputs of the master VRE are utilized

in the expanded scale recorder circuit which is discussed in

detail in another report [9]. These two outputs permitted the

design and operation of the unit so that either standard

(unexpanded) or expanded modes of operation were selectable at the option of the analyst. In the standard mode of operation,

a 10 mV output of the master VRE produced full scale deflection

of 100 recorder chart divisions . When operated in the expanded

mode, a 100 mV output from the VRE produced 10 full chart

widths of deflection (1000 recorder chart divisions), and by

selecting the proper potential for the bottom of the slide

wire, the recorder is balanced "on scale". Independent of the

mode of operation, there is a recorder error component due to dead zone and linearity of the slidewire which is approximately

1/4 of a chart division (0.25^ of full scale deflection).

There is also an error component in peak height determinations

of about 1/4 chart division. The total error of approximately

1/2 chart division from these two sources is always present,

is constant in magnitude and is independent of peak height.

This error contribution, as a percentage of the peak height,

is a variable that reduces to approximately 0.5 percent for

a maximum deflection of 100 chart divisions for the standard

recorder, or to 0.05 percent for a peak height approaching

1000 chart divisions for the expanded scale recorder. Thus,

the limit of error of bias for the standard mode of operation

25
is 0.5 percent, and drops to 0.05 percent for the expanded

mode of operation because the uncertainty due to recorder and

peak height determination is a smaller percentage of the

larger deflection. The net result is a factor of 10 reduction

in the systematic error contribution from the recorder when

operated in the expanded mode. A secondary benefit of expanded

operation is more uniform wear of the slidewire over its entire

length and, consequently, a more linear response, than when

operated in the standard mode which produces excessive wear

at one end of the slidewire.

Since the benefits of an expanded scale recorder are not inherently obtained by moving the functional switch to the

expanded position, the analyst must, when possible, avoid

operating the measuring circuit with less than 100 chart divisions of deflection. The limit of error for this operational

condition is 0.5 percent and is independent of the mode of

operation. To obtain maximum precision, the measuring circuit

must be operated so that recorder deflection is a maximum for

the ion current collected on the Faraday cage. Unless it was impossible or impractical to achieve the necessary ion current

intensity, the expanded scale circuit was always operated so that the minimum deflection was 200 chart divisions. Because

of the recorder dead zone, slidewire non-linearity and the

inability to determine the true resistance of the VRE decade

resistors, the minimum error statement of a direct isotopic

ratio determination was 0.00002.

26
4. MASS SPECTROMETRY - MASS DISCRIMINATION EFFECTS AND AN

EVALUATION OF PARAMETERS

A . Introduction

Good analytical procedures are based upon (1) a knowledge

of the sources of bias, (2) establishing control of the various

parameters that contributed to this bias and (3) selecting an optimum combination of parameters that is uniform and reproducible for each analysis. Even when the error contributions

from chemistry and the instrument are negligible, large bias

components are possible because of irregularities in the sample

mounting or the pattern of heating the filaments to obtain ion

emission. The difference between the observed and true isotopic

ratio is not necessarily small [6,13,14,15] and no effort is

expended to obtain a minimum deviation from the true value or

to accurately ascertain the magnitude of each component of the

bias. The effort is to determine the degree of constancy of

the isotopic ratio under various conditions, choose the best

set of conditions, and verify the constancy of the total bias

under identical experimental conditions for chemically prepared

calibration mixes of known isotopic composition and the samples

to be calibrated. Thus, a single, experimentally determined

correction factor is calculated and used to correct for all

bias effects for all ratios.

B. Mass Discrimination Effects

For thermally produced ions the light isotope is preferrentially evaporated and ionized with respect to the heavier

isotopes of an element. This effect is temperature dependent

and is a function of the mass. The preferrential depletion

of the light isotope was observed for uranium and all other

absolute isotopic abundance studies by the Analytical Mass

Spectrometry Section [ 5 , 6 , 13 , l4 , 15 , l6 , 17 , l8 , 19 ] . The bias

due to preferrential depletion of isotopes is referred to in

this report as filament bias.

27
For uranium this effect was studied by measuring the

2 3 5y/2 3 8u ratio for many variations and permutations of the

analytical parameters and over a wide range of Isotoplc ratios.

The dependency of the bias on temperature. Its Irreversible

nature, and Its consistency under controlled analytical con- ditions, over the range of ratios represented by SRMs U-050

through U-93-0., formed the basis for the conclusion that the

observed bias was due to the effects of Ionization and evaporation In the source and was Independent of Isotoplc composition.

By selecting different combinations of parameters, usually

temperature, time and sample size, the filament bias for uranium

2 3 5^/2 3 8y measurements can be made additive, zero or subtractlve . For low temperature analysis of a 200 yg uranium sample

the filament bias of the observed 235^^238^ ^^tlo Is approximately 0.5 percent. A high-temperature analysis of the same amount of material gives a 2 35^/2 3 sy ]j±q_q of approximately

0.2 percent. The only difference In parameters for these

techniques Is a significant Increase In sample filament temperature for the high temperature procedure . Further Increases

In sample filament temperature can make the filament bias zero

or even subtractlve. Thus, one should be aware that preferrentlal depletion occurs during thermal Ionization even If the

observed bias Is small or nearly constant for long periods of

time or when the measured Ion current ratio Is Identical to

Its composition In the sample.

By developing fixed analytical procedures In which all

parameters are constant or maintained within narrow limits.

It was demonstrated that bias effects due to fractionation are

a characteristic of the combination of analytical parameters

used for the measurement and are reproducible to better than

0.05 percent for a given procedure. The degree of accuracy

achieved in evaluating bias effects due to fractionation is

directly dependent upon the ability to control and reproduce

all parameters between sample loadings. Consequently, it was necessary to investigate some of the parameters that controlled

28
the rate of depletion of the sample reservoir and to develop

the means of starting each analysis with the same amount of

material on the sample filaments. Even after the chemistry

fulfilled one of its functions and supplied solutions of uniform concentration, it was necessary to look for conditions

during sample mounting and the mass spectrometric analysis

that could produce an apparent loss in sample size by either

a failure to convert to the proper oxide or by material simply

flaking off the filament surface.

C . Filament Temperature

Filament temperature is one of the more critical parameters to be controlled because evaporation and the rate of

depletion of uranium from the filament are primarily a function

of temperature. Ideally, it is desirable to measure the

temperature of each filament and maintain it within the same limits for each analysis. The methods used to control sample

filament temperatures were constant current control and a combination of filament current and signal intensity. The

ionizing filament temperature was determined by adjusting to

a constant rhenium signal intensity and by direct measurement

with an optical pyrometer.

1 . Sample Filaments

Strict reliance on a constant filament current to produce

a constant temperature for a large lot of rhenium ribbons was unacceptable for precise isotopic measurements. Dimensional

variations within a lot of rhenium ribbon, 0.001x0.030 inch, were large enough to cause a 1 A difference for filaments at 2l60 °C. Sample filaments were normally operated at less than

1650 °C and the current variation attributed to dimensional

variations was at least 0.3 A. Dimensional variations in the

thickness of ribbon between lots supplied by commercial manufacturers were found to be large enough to cause a definite

shift in filament currents at a constant temperature.

29
Although the preferred method of sample filament temperature control was Indirect and imperfect, it accomplished the

goal of limiting the sample filaments to a constant temperature

range for each analytical procedure used. It was based on

the assumption, that, if all parameters were maintained nearly

constant and approximately the same amount of uranium was loaded on the filaments for each analysis, the signal intensity

was a coarse measure of the filament temperature. Therefore,

the signal intensity for each analytical procedure was restricted to very narrow limits and was reproduced for each analysis

of a given procedure. Using the long term performance of many

sample loadings, ion current intensity - filament current relationships were established. This type of relationship was used

as a cominon reference for each uranium procedure. Failure to maintain the signal intensity within the prescribed limits for

a fixed current range was sufficient reason to terminate an analysis without any further consideration. This type of con- trol was in some respects tenuous because it was subject to distortion by failure to properly focus for maximum intensity,

malfunction in the focus control circuit, changes in the

ionization efficiency of the filaments or the transmission

efficiency of the source. In spite of these handicaps, the

reproducible signal intensity method has worked very well for

uranium and many other elements, in lieu of a direct method

of measuring sample filament temperatures.

By measuring the ionizing filament temperature over the same current range as the sample filaments were operated,

estimates of the sample filament temperature limits were made

for analysis of 200 yg samples. The minimum sample filament

temperature for an analysis was 1200 °C (^2.2 A) and the

maximum temperature used for any of the analytical procedures

was approximately l600 °C (^3.0 A). Analysis of SRM U-500 at random temperatures over the entire range with all other

parameters maintained as constants gave a scatter in the

observed 235^^238^ ^ata of 0.3 to 0.5 percent. By operating

30
at either extreme or even in the middle of this temperature

range the scatter in observed 2 3 5^/2 3 8y ]-atios was reduced by

a factor of three. It was concluded that sample filament

temperature limits of less than 200 °C were essential for ratio

measurements with a 95^ confidence level (C.L.) of better than

0.1 percent.

2 . Ionizing Filaments

The optimum temperature of the ionizing filament was selected to provide efficient ionization, long filament life,

and to deplete the interstitial oxygen which was a source for

formation of the UO"*". The Re"^ signal from the hot ionizing

filament was useable as a reference point to precisely reproduce the optimum filament temperature for each analysis. This

technique was based on the assumption that a nearly constant

temperature would yield approximately the same Re"*" signal

intensity for all filaments. Verification of this assumption

was initially made by installing a clear plastic source flange

and using an optical pyrometer to determine filament temperatures. Rhenium filaments at 2150 °C yielded a ^^^Re ion

current of approximately 6xlO~^^A. This method was believed

to be reproducible to within ±20 °C for most rhenium filaments.

Modification of the source flange to include a small

diameter window made temperature determinations with an optical

pyrometer the preferred method. Because the scale of the

pyrometer was calibrated in increments of 20 °C, the optimum

temperature was changed to 2l60 °C for convenience. Pyrometer

measurements quickly revealed that the temperature drifted

downward approximately 20 °C during the first few minutes of

heating. After readjustment to 2l60 °Cj the temperature

remained nearly constant for long periods of time. As the

filament developed cracks or "hot spots" the temperature

increased, and continued to increase until burnout occurred.

The response of the rhenium signal closely paralleled the

trends of the temperature, and as the filament developed "hot

31
spots" the signal Intensity started a continuous growth cycle

that ended In filament burnout.

There are shortcomings In the exclusive use of either of

these methods but the optical pyrometer Is superior and more precise. The constant rhenium signal Intensity method Is dependent upon any parameters which change the transmission

of the source or the Ionization efficiency of the filament.

Pyrometer measurements are free of these restraints and only

require a stain free window and a minimum of shadow on the

filament. Monitoring the rhenium signal was not discontinued

but was used as a secondary reference to mutually support the

pyrometer measurements or serve as an Indicator of abnormal

Instrumental or analytical conditions.

D . Sample Size

The optimum amount of uranium necessary for an analysis

of the SRMs was determined experimentally and is primarily a product of performance and analytical experience. The prime

requisite was the ability to complete an analysis using only

a small fraction of the total sample on the filament and to avoid making ratio measurements under the gross fractionation

conditions of complete sample depletion. A large uranium

sample size also made it possible to measure a wide range of

ion current ratios under the same thermal fractionation con- ditions. Some of the prime indicators used to make a decision

about the optimum sample size were the rate of change of the

2 3 5y^2 3 8y ratio, the maximum signal Intensity obtainable and

the length of time it was maintained.

Initially, approximately 400 yg of uranium was considered

to be optimum for an analysis and was used for several years.

Prior to use of a "Z" lens source the sample size was reduced

to approximately 200 yg because of improvements in the transmission characteristics of the spectrometer. Improvements in

the analytical procedure, and identification of some of the

bias components necessary for precise and accurate Isotoplc

32
measurements. The development of a "Z" lens source made a further reduction of sample size by at least a factor of three

feasible but was rejected to keep this parameter constant for

all uranium SRM measurements.

Calibrated micro-pipettes were not used to measure the

volume of solution placed on each sample filament because of

the greater probability of sample cross contamination and the

increased cost and time involved in sample mounting. Uncalibrated, throw-away pipettes are prepared from 5 mm pyrex glass

tubing, are thoroughly cleaned, and are considered to be quick,

inexpensive and safe with respect to cross contamination. The

chief disadvantage of the uncalibrated pipette is visual determination of drop size, and the inability to precisely reproduce

the required volume of solution for each filament, or between

different sample loadings. Variation in the volume of solution

is believed to be large enough to cause a 25 percent variation

in the total sample size. With the relatively large amount

of uranium used for an analysis (^200 yg), this variation is not critical and the effect was less than 0.05 percent of the

ratio at the 95% C.L. [9]. Figure 4 shows the tip of a throwaway pipette after depositing a drop of solution on a sample

filament. Figure 5 is a close-up of the sample after conversion to the yellow oxide of uranium.

For years the volume of solution deposited on a filament

was estimated to be 1/50 of an ml or approximately 20 micrliters . Using this guideline each drop of solution from a

5 mg U/ml solution contained 100 yg of uranium. Recent calibration of the drop size with a calibrated micro-syringe

indicated the volume of solution was approximately 15 micro

liters and the estimated quantity of uranium per drop was

75 yg. This evidence strongly indicated that all previous

estimates of the amount of uranium per analysis was more of

an upper limit and a better average estimate was 150 yg.

33
Figure 5. The yellow-orange oxide of uranium on a sample

filament

34
j E. Time

Because of preferrential evaporation and ionization, the

isotopic composition of the uranium on the sample filaments

is continuously changing. The change in composition is also

reflected in the ratio of ^^^u/^^^u ion currents detected at

the collector. A plot of the observed 235^^238^ patio versus

time over several hours of heating and under controlled analytical conditions gave a fractionation curve with a measurable

slope. When sample filament temperature and sample size were

variables, a series of curves with different slopes was

obtained. When these two parameters were constants for all

analyses, a series of curves was obtained with approximately

the same slope. In general the shape of the fractionation

curve is a characteristic of the temperature, time and sample

size parameters used. The ratio of the light (^^^U) to heavy

(•2 3 8 y-) isotope decreased with time and was not reversible once heating of the filament was commenced.

P . Sample Mounting

The method of depositing a drop of solution on a filament

and converting it to the desired form is discussed as a parameter because of the potential of unwittingly effecting a net reduction in sample size. The objective of the mounting procedure is to efficiently convert uranyl nitrate to the oxide

and to bind the sample to the filament so that it does not

fall or flake off. Uranium oxide (UO3) is a preferred sample

form since it is more refractory than uranyl nitrate and is not depleted from the filaments as readily.

Conversion to UO3 was performed in a fume hood. The

magnitude of the electrical current used to heat the sample

and the final coloration of the sample were monitored to determine the effectiveness of the process. The sample was subjected to the cooling effect of airflow which was dependent

upon the degree of cleanliness of the absolute filter and the

position of the hood door. The electrical current necessary

35
to convert the sample fluctuated in direct relationship with

the volume of airflow and made current readings nothing more than a coarse indicator. The fine adjustment was to heat the

sample until the color of the oxide was observed to change to

a yellow-orange.

Failure to efficiently convert to uranium oxide meant

that a significant portion of the sample was the more volatile

uranyl nitrate and was evaporated from the filament in significant amounts at temperatures less than 1200 °C. The remaining

uranium oxide sample could be abnormally small and the net result would be a probable increase in the rate of change of

the isotopic ratios with time. Drying the sample too quickly

or heating it too hot usually produced poor contact between

filament and sample which invariably flaked off when subjected

to either mechanical or thermal shock. At the conclusion of

the mounting procedure filaments were inspected to ascertain

if there was any migration of solution to the ends or reverse

side of the filament away from the ionizing filament.

G . Total Sample Composition

Solutions of uniform and high chemical purity are essential

for accurate determination of the absolute isotopic abundance

of any element. High concentrations of impurities require an increased sample filament temperature to obtain the standard

signal intensity for an analytical procedure. The increased

sample filament temperature can be a source of bias when com- paring or pooling data from the analysis of samples of variable

chemical purity. Frequently, this type of temperature compensation was small and not always detected by the analyst but

the net effect was usually a decrease in analytical precision

of the measurement

.

Once contaminant ions are identified, chemical processing

will normally reduce the concentration below the critical level.

For uranium analysis sodium and potassium were the major interferring elements and, although normally present in only trace

36
quantities, required special precautions to maintain below the

critical level. Sodium and potassium were also constituents

of the rhenium ribbon and the filament post. Degassing prior

to analysis in the mass spectrometer was used to reduce the

concentration of these elements below the critical level that

produced interference

Potassium is also the source of a polymer (Ke) which produced background peaks at mass 234 and 236. When present the

background peaks produced broad bases and abnormally sloped

peak tops for ^^'*U and ^^^U. With adequate precautions to maintain a minimum potassium concentration in the uranium

solutions, and degassing of filaments, the potassium polymer

was not detected at a lxio~^^A ion current sensitivity.

Suspected organic peaks were occasionally detected during

the sample degassing phase of an analysis at an ion current

intensity of approximately 2xlO~-^^A. After a brief cooling

and pumpdown period, organic peaks were not detectable with

an ion current sensitivity of lxio~^^A. Organic peaks were observed when high gain ion multipliers were used. This background was persistent and was not degraded below the detection

level of the multiplier over extended periods of heating when

ionizing filaments were operated at 2l60 °C. Reducing the

ionizing filament temperature below 2000 °C reduced the background below the detection limit of the multiplier.

H. Acidity

Isotopic analysis of uranyl nitrate dissolved in water

revealed that a significant number of these samples were

"hard" to analyze and required higher sample filament temperatures to obtain the standard signal intensity. Operation at the higher tempei'atures produced an increased rate of fractionation which resulted in a systematic difference in the

data for "hard" solutions. This error term was estimated to

be as large as 0.25 percent and was eliminated by adjusting

all solutions to a constant acidity. With a constant acidity

37
all samples were "hard" to analyze and responded uniformly

without any detectable systematic temperature differential

between samples.

After specific analytical procedures were developed, the

effects of variable acidity and sample size were evaluated

under low temperature and high temperature signal conditions.

SRM U-500 was used as a working standard and was prepared in

a nearly neutral solution (nitric acid solution evaporated to dryness on a steam bath and dissolved in water) and in acid

solutions of 1, 5 and 10 percent [9]. Statistical analysis

of the data revealed no systematic differences for the acid

solutions as large as 0.05 percent at the 95% C.L. There was

a small (<0 .1%) systematic difference between the nearly

neutral solutions and acid solutions analyzed under low

temperature signal conditions.

The work on uranyl nitrate dissolved in water established

the need for the acidity to be constant for all solutions and was reinforced by the experiment under controlled analytical

conditions. For the certification analysis of the uranium

SRMs , the U3O8 was dissolved in nitric acid (1+1) and diluted

to a final concentration of nitric acid (1+19).

I . Filament Material * Rhenium was the only filament material used for uranium

analysis. Rhenium was preferred to tungsten and tantalum

because of a higher ionization efficiency and a lack of uranium

background which was not detected at a 10~^^A ion current

sensitivity. The material was prepared to be especially "free"

of organics and the alkali elements but there were always

detectable ion currents of sodium and potassium. The background of the filament assembly was further reduced before use

in the spectrometer by degassing for a minimum of one hour in

a vacuum and under a potential field. Failure to apply a potential allowed the contaminants to remain in the immediate

volume of the filament and the cleaning process was inefficient.

38
' The surface tension of a cleaned filament was reduced and

solutions migrated to the edges, ends, and reverse side of the

filament before drying. Thus, only limited degassing of the

sample filaments was beneficial. After the sample was inserted

Into the spectrometer, the filament and sample were heated for

a total of 45 minutes as part of the analytical procedure

before data were taken. This final heating step was sufficient

to Insure Insignificant background contribution from the sample

or filament.

J . Pressure

Early In the development of the uranium analytical method

samples subjected to overnight pumpdown frequently required

abnormally high sample filament temperatures or a brief period

of overheating to produce the same standard signal Intensity

as samples with only several hours of pumping. This temperature differential between samples was the source of a small

but significant bias component which was reduced by Installing

a cryogenic pump (liquid nitrogen coldflnger) In the source

housing. The cryogenic pump made It possible to achieve a more uniform vacuum between samples and, after a short pumpdown (<1 hour), to obtain vacuums comparable to overnight

pumpdown

Subsequent to the Introduction of the source cryogenic

pump, the flight tube was modified to Include a beam valve [11].

The beam valve made It possible to Isolate the flight tube and

eliminated the regular venting to atmosphere In order to change

the sample In the source housing. Except for sample degassing

and the Isotoplc analysis, the flight tube was Isolated and

was maintained at a pressure of 10~^ torr. Near the end of

the uranium program and during the analysis of SRM U-0002 It was observed that under normal operating conditions, the

collector vacuum was Improved to the extent that the mean free

path of scattered electrons was long enough to produce baseline

Interference of a few parts per hundred thousand of the major

39
peak. This Interference was a significant part of the ^^^U

peak height but was not detected during degassing and was not

a problem during previous uranium analyses. The interference

was eliminated by devising a procedure to complete the ratio

measurement before the collector vacuum could become too high.

K . Non-ohmic Response

Vibrating reed input resistors of 10^-^ ohms exhibited a non-ohmic response for ion currents significantly greater than

5x10 ^^A and became the source of an intensity dependent bias

component. The standard operating procedure to eliminate the

problem was the use of a 10^^ ohm input resistor for ion cur- rents less than 5xio~^^A and switch to a 10^° ohm input resistor

for ion currents significantly greater than 5^10"^ ^A. The 10^°

ohm resistors eliminated non-ohmic response as a major bias

component but reduced the amplified output of the VRE by a factor of 10. For some isotopic distributions, unless a compensating increase in the total signal intensity was made,

the reading error in determining peak heights significantly

limited the precision of the measurement.

L . R-C Response

R-C response of the measuring circuit is a source of bias

when comparing large ion current ratios (>20). It was not identifiable for an ion current ratio of 20, but was detectable

for a ratio of 30 and became more pronounced as a ratio of 200 was approached. When switching from a 10~-^°A to a 10~-^^A ion

current and allowing several seconds for the circuit to stabilize, the R-C response component was at least 0.3 percent of

the small peak height. Failure to fully recover would result

in an apparent enrichment of the smaller isotope and without

adequate calibration standards such data would be biased

significantly. Sufficient data was accumulated to prove that

the measuring circuit did stabilize within 30 seconds and the

recovery was more than 90 percent complete. Even with calibration standards it was necessary to change from the normal

40
cycle of equal 30 second observations per peak for 235^^238^

measurements to a cycle of 30 seconds for the major peak and

60 seconds for the minor peak. The first half of the 60

seconds observation was for the measuring circuit to stabilize

and the remaining half was used for data. Although the accuracy improved with sufficient time for the measuring circuit

to stabilize, the precision did not reflect it. Because of

the longer time intervals there was some difficulty in aligning

peak tops to determine average peak heights unless the ion

current emission was nearly linear.

When mass scanning to compare the minor isotopes to either

^^^U or ^^^U a minimum separation of 45 seconds between peak

tops was maintained to minimize the effects of R-C response.

The ^^^U peak was quickscanned (^^^2 seconds) during isotope

dilution analyses to avoid the full effect of a large ^^^U

ion current on the Faraday cage. For this type of analysis

the ^^^U intensity was maintained below the level that would

saturate the measuring circuit or require an abnormally longer

recovery period even when it was only momentarily collected

on the Faraday cage.

M. Memory

Memory or cross contamination between samples was not a significant bias component during thermal ionization analysis

of uranium. Only one documented case of source memory was observed. This occurred early in the uranium program and was easily detected because the contaminant was ^^^U and was not

a constituent of the sample analyzed. No other significant

indication of memory was ever detected. After the original

source shield was modified to remove a "skirt" that extended

over the drawing-out and discriminator plates, the chance of source memory was significantly reduced.

The standard procedure for minimizing possible memory

contributions was to limit the analysis of SRMs on a source

to a 20 percent ^^^U abundance range and to clean the source
after approximately 40 analyses. Further tests were made by

analyzing separated isotopes on sources with a large number

of SRM analyses and comparing the results with separated

isotope data obtained on a cleaned source. Another technique

was to analyze both separated isotopes on the same source

and compare the data with results from a cleaned source. The

final conclusion was that under normal operating conditions

source memory was not significant

.
5. MASS SPECTROMETRIC ANALYTICAL METHOD

A . Introduction

Because the light Isotope Is preferentially depleted from

a heated filament and since the Isotoplc composition Is con- tinuously changing. It Is necessary, for precise and accurate

Isotoplc composition measurements, to approximate the same fractionation curve and the same point In time on the curve

for each analysis. Under these restraints all ratio measurements are made over the same time span for each analysis and

the bias effects of fractionation are evaluated and appropriate

corrections made. A fixed analytical procedure Is the means

of reproducing the same fractionation curve for each analysis,

and It gives the parameters and procedure for obtaining a specified Ion current Intensity. All parameters must be accu- rately reproduced and the rules of the procedure strictly

adhered to In order to maintain the same experimental conditions for all analyses. Failure to be consistent will have a direct effect on the observed data and Is detrimental to the

precision and accuracy of the ratio measurement.

The analytical method for uranium Isotoplc analysis con- sisted of sample mounting, sample degassing, and the procedure

or heating pattern for obtaining Ion emission for the ratio

measurements

.

B . Sample Mounting

The same mounting procedure was used for all SRMs . One

drop of solution (75-100 yg of U) was placed on each sample

filament and evaporated to dryness with a heat lamp and an electrical current. The electrical current was adjusted In

the following manner: 1 A for 5 minutes; 1.3 A for 3 minutes;

slowly Increased until the yellow-orange oxide forms (between

1.8-2.3 A)

.

Rapid evaporation of the solution usually produced poor

contact between filament and sample. Under these conditions

significant amounts of material flaked off when subjected to

43
mechanical shock. Overheating of the sample produced a black

uranium oxide which was judged to be inferior in performance

to the yellow-orange oxide. Overheating also produced weak

contact between the filament and the sample which resulted in

material flaking off when the sample filaments were operated

at high temperature (l400-l600 °C).

The sample filaments were placed into the filament block

with an ionizing filament and alignment was made visually.

The ionizing filament was centered in the block and the sample

filaments were moved away (0.005-0.010 inch) from the edges of

the ionizing filament. The completed assembly was inserted

into the mass spectrometer and pumpdown commenced.

C . Degassing

Each sample was degassed for 15 minutes and then allowed

to cool for a minimum of 30 minutes before the analysis was begun. When the source pressure was less than 3^10"^ torr,

liquid nitrogen was added to the source coldfinger and degassing was accomplished in the following manner: adjust the ion

accelerating voltage to 8 kV; adjust the ionizing filament

temperature to 2l60 °C and set the sample filament currents

at 1.5 A. After 3 minutes of heating readjust the ionizing

filament to 2l60 °C and set the sample filament currents at 1.9 A. All filaments are turned off after a total of 15 minutes

of heating. The ^^''Re signal at the start of degassing was approximately 1.4xio~-^^A and decaying. The initial surge of

volatile material from the sample and filaments made an ion

accelerating voltage of 10 kV prohibitive because of the

greater frequency of high-voltage breakdowns.

Degassing dehydrated the sample and depleted the filaments

of interstitial oxygen which were sources for formation of

significant oxide ions (UO"*") . Once the oxygen background was depleted, a significant UO"*" ion beam was not observed unless

there was an air leak in the system. Initially, the u''"/UO''" ratio was small, but the UO"*" intensity decayed much faster

44
than the U ion current and was normally less than 10~-^^A within 10 minutes of heating. If U"*" ion currents were not

degradeable to the 10~^^A range during degassing, it was taken to be an indication of incomplete or inefficient con- version of uranyl nitrate to the oxide. Apparent failure to deplete the filaments of unconverted uranyl nitrate was a critical indicator and was cause to terminate the analysis.

Degassing was also helpful in the degradation of potassium, sodium, and organic materials below the critical level

that could cause interference. Organic background peaks were

not measurable and were difficult to detect because their

intensity was approximately the same as the lxl0~-^^A background

noise of the measuring circuit. Degassing followed by a brief

period of source cooling and pumping, reduced low level irregularities in the baseline below the detection limit of the

spectrometer. The critical check for organics was made during

the first 30 minutes of an analysis and, if suspected organic

peaks were observed, the analysis was terminated.

D . Analytical Procedures (Heating Pattern) . Two specific heating patterns were developed to obtain

standard ion current intensities for uranium ratio measurements. The low temperature and high temperature procedures

have been described previously [9] for instruments without

"Z" optics. Since publication of this report, both procedures

were modified to include use of an optical pyrometer to determine ionizing filament temperatures, and the high temperature

procedure was changed to take into account the increased

transmission of the "Z" source. Because of the wide isotopic

distribution of uranium SRMs , it was impossible to operate at two standard ion current intensities and obtain maximum precision for each SRM. A general procedure was developed and

is flexible enough to be used over a wide range of isotopic

distributions and sample sizes (1-400 yg).

45
1 . Low Temperature Procedure

The low temperature analytical procedure was developed

and used primarily for 235^^238^ ratio determinations of SRMs

U-930 to U-050. A total ion current of approximately 3xlO~^^A

was sufficient to make expanded peak height determinations

with the desired precision. When the "Z" lens source became

standard on all mass spectrometers in the laboratory, the

options were continued use of the old procedure or development

of an equivalent procedure for the "Z" source. Both types of

low temperature procedures were successfully used, and it was finally decided that the original procedure offered the best

chance of maintaining the sample filament temperatures well

within the desired limits (from ^1200 °C to <l400 °C). Thus,

the procedure described in this section is basically the same

as was previously reported.

Details of the low temperature procedure are: Time from start Procedure

(minutes

0-1 The ionizing filament temperature is adjusted

to 2160 °C and the sample filament currents

are set at 1.5 A. The ^^^Re peak is located

and focussed (course) for maximum intensity.

The ^^^Re is normally 1 . 5-1 . 8xl0"^ ^ A and

decaying. The stability of the rhenium

signal is considered critical and, if unstable

or erratic, the analysis is terminated. The

most probable causes of an unstable signal

are a defective filament, large alkali background, or electronic instability, etc.

5 The ionizing filament temperature will

normally drift downward approximately 20 °C

and is readjusted to 2l60 °C.

46
Time from start Procedure

(minutes)

6 The sample filament currents are increased

to yield a lxio"^^A (100 mV) u"*" grid signal.

The focus controls are adjusted for maximum

intensity

.

12 The sample filament currents are increased

to yield a 2xlO~^^A u"*" grid signal.

17 The ionizing filament temperature is checked

to verify stabilization at 2l60 °C and, if

necessary, adjustments are made.

18 The sample filament currents are increased

to yield a 3x10"^ U u"*" grid signal.

19 The U"*" signal is focussed for maximum

intensity

.

24 If the U''' grid signal intensity has changed

significantly (±5xl0~^^A), the sample filament

currents are readjusted to yield 3^10"^ ^A. The uranium mass range is scanned to determine

baselines

.

30 The ratio measurement is started. The u"^ grid

signal is expected to be approximately 3^10"^ ^A and growing slowly. Ten peak sets of data are taken by magnet switching between peak tops.

Each peak top is monitored for 30 seconds

except for measurement of ion current ratios

greater than 20. Then the observation cycle is

changed to 60 seconds for the small peak and

30 seconds for the large peak. One half of

the longer observation is allowed for recovery

from R-C response effects and the latter half

is used for data. The measurement is concluded

with a mass scan to determine baselines.

47
2 . High Temperature Procedure

The high temperature procedure was developed to provide

large U"*" Ion currents to make precise determinations of minor

Isotope composition. The procedure Is specifically designed

for an Internal normalization using the 235^^238^ ratio of

uranium as a known to evaluate bias effects for each Individual

analysis. The chief prerequisite Is an accurate value for the

2 3 5yy2 3 8y ]og^-tio and realization that this ratio and the ratios

to be corrected must be of the same magnitude or In the same response region of the measuring circuit. Also at Ion current

Intensities of 10"^ °A a constant awareness of non linear

responses of the measuring circuit must be maintained, and

adequate precautions taken to reduce their effect on the data.

Thus, the first step In determining the Isotoplc composition of a uranium sample is to use the low-temperature procedure, where applicable, in a separate experiment to obtain

an accurate value for the 2

a 5^/2 3 p^tio. Then the high

temperature procedure is used in another experiment to determine

^^'*U and ^^^U abundances. The best method of evaluating the

filament bias for. the minor Isotope measurement is an internal

normalization using the 235^^233^ j-atio as a known. This

technique is a very precise and sensitive method of correcting

for bias effects since a correction is made for each analysis

according to its position on the fractionation curve. For an internal normalization some of the analytical control procedures

and parameters, such as time, can be relaxed and the loss in

precision in the average bias correction between analyses does

not effect the corrected data.

The alternative technique for evaluating filament bias is

to analyze a standard of known 2 3 5^/2 3 Sy p^tio and the sample

to be calibrated under identical experimental conditions. The

observed 23 5^/2 3 (;jata of the standard is used to calculate

correction factors which are applied to the observed data for

the unknown sample to obtain corrected values for the isotoplc

composition. In contrast to internal normalization, there must

48
be strict adherence to the rules of the analytical procedure

or there will be a serious loss in precision in determining

the average filament bias correction which has a direct effect

on the corrected isotopic composition.

Details of the high-temperature procedure are: Time from start Procedure

(minutes

)

0-1 The ionizing filament temperature is adjusted

to 2160 °C and the sample filament currents

are set at 1.5 A. The -^^^Re is normally

1 .5-1. 8x10"^ and decaying. The stability

of the rhenium signal is considered critical

and, if unstable or erratic, the analysis is

terminated. The most probable causes of

unstable signals are a defective filament, a large alkali background, or electronic

instability, etc.

5 The ionizing filament temperature will normal

drift downward approximately 20 °C and is

readjusted to 2l60 °C.

6 The sample filament currents are increased to yield a lxlo"^°A u"*" grid signal. The focus

controls are adjusted for maximum intensity.

12 The sample filament currents are increased

to yield a 2xio"^°A u"^ grid signal.

17 The ionizing filament temperature is checked

to verify stabilization at 2l60 °C and, if

necessary, adjustments are made.

18 The sample filament currents are increased

to yield a 3xlO"^°A u"^ grid signal.

19 The u"^ grid signal is focused for maximum

intensity

.

49
Time from start (minutes

)

24

30

Seven ipeak seta of ^ss^yi^s^ taken

firsts by magnet switching between -peak tops.

Each peak is monitoved fov 30 seconds except

for measuvement of ion current ratios greater

than 20. Then the observation cycle is

changed to 60 seconds for the small peak and

30 seconds for the large peak. One half of

the longer observation is allowed for recovery

from R-C response effects and the latter

half is used for data.

Then ^^'*U and ^^^U are compared with a reference peak of ^^^U or a known ^^^U spike

by continuous mass scanning which is necessary

for accurate measurement of baselines under

the small peaks. The scan rate is adjusted

to yield a minimum of 45 seconds between peak

tops to minimize the effects of R-C response

.

Ten peak sets of minor isotope data are taken.

Finally the ratio measurement is con- cluded with another 7 peak sets of ^^^u/^^^U

data. The ^^^U and ^^^U baselines are determined by mass scanning

.

50

Procedure

If the U"*" grid signal intensity has changed

significantly (±3x10"^ ^A), the sample filament

currents are readjusted to yield 3xio~^°A.

The uranium mass range is scanned to determine

baselines

.

The ratio measurement is started. The U"*" grid signal is expected to be approximately

3x10"^ °A and slowly decaying or growing.
3 . Outline of General Analytical Procedure

Because of the different Isotoplc distributions of the

SRMs and the desire to operate with a maximum precision,

I analytical procedures that would yield intensities other than

I 3xlO~-^^A or 3xlO~^°A were necessary. Both the low and high

I temperature procedures follow a general analytical pattern that

: is flexible and adaptable to a wide range of signal Intensities

! The general outline for most uranium isotoplc measurements

I is as follows: adjust the ionizing filament temperature to

' 2160 °C; after 6, 12, and I8 minutes of heating adjust the

sample filament currents to yield 1/3, 2/3 and full intensity,

respectively; focus for maximum intensity after 7 and 19 minutes of heating; verify the stability of the ionization

filament temperature at 2l60 °C before the final sample filament adjustment is made; after 24 minutes of heating and when

necessary, reset the sample filament currents to yield full

intensity. The ratio measurement is started after 30 minutes

of heating. Baselines are determined by m.ass scanning at the

beginning and end of each ratio measurement.

The general guideline in selecting a procedure for the

isotoplc analysis of uranium samples is to fit the procedure

to the isotoplc abundance ratios so that the ion current

intensity is sufficient to provide the maximum precision in

determining peak heights, without exceeding the limit for

non-ohmlc response of the measuring circuit. Hence, linearity

of the input resistor is a limiting factor in determining how

much signal is to be used for a ratio measurement. The other

limit on the signal intensity is the ability of the collector

to adequately suppress secondary electrons when a large ion

beam impinges on collector surfaces. Ion current intensities

of 10~^A are available from a 200 yg uranium sample size but

meaningful measurements at this intensity are difficult for

most of the SRMs because of non-ohmlc response and the bias

effects of scattered electrons. Since obtaining sufficient

signal intensity is not a major problem, the rate of frac51
tlonation is the next factor to be considered. Once the

precision requirements for peak height determination are satisfied and adequate attention is devoted to possible non- linear effects, the best procedure is the one that yields the

smaller rate of fractionation.

Since the characteristic bias of both basic procedures

were well known, these procedures were used as known points

of departure for obtaining ion current intensities other than

those associated with the basic procedures. Therefore, select'

ing a procedure to fit a particular isotopic distribution was essentially modifying one of the existing procedures by

obtaining either more or less than normal intensity. The

only parameter that was changed to obtain a given ion current

intensity was the sample filament temperature. Once the

magnitude of the ion current intensity was fixed, temperature

adjustments were always made according to the restraints of

the general procedure described above. The advantage of this

technique was the ability to predict the direction of change

of the filament bias and also its approximate magnitude.

Thus, a modification of the low temperature procedure to provide approximately 6-7^10"^ of ion current would require an increase in sample filament temperature which would produce

an increased rate of fractionation. At the higher rate of

fractionation and with all other parameters constant, the

filament bias was less than the normal low temperature bias

of 0.45 percent. Under these conditions, and knowing the bias

for high temperature conditions, it was possible to accurately

predict a bias of between 0.4 and 0.3 percent for the modified

procedure .

' The basic procedures have been discussed along strict

functional lines of low temperature for 2 35^/2 3 sy patios and

high temperature for minor isotopes but these procedures are not restrictive in application. When the ^^^U abundance is

greater than 95% y minor isotope determinations are usually

made with a modified low temperature procedure to limit the

52
magnitude of the ^^^U ion current and to avoid non-ohmlc

response of the input resistor or saturation of the amplifier

circuits to the extent of not recovering in time for minor

isotope detection. Also, the high temperature procedure can

be used for 235^^238^ ratio measurements with approxlmiately

the same precision as the low-temperature procedure.

E . Role of the Operator in the Analytical Method

A direct comparison of isotopic data for multiple analysis

of a single sample or for analyses of many different samples

is meaningful and valid, only if all analytical parameters

are identical or maintained within experimentally established

limits, where the effect of a variation is known or is of

little influence on the measurement. The task of the operator

is to provide the necessary quality control to insure that

each analysis falls within the prescribed analytical specifications. To accomplish this task it is necessary to verify

the correctness of instrumental parameters, make a Judgement

as to the normalcy of the ion emission and stability, recognize

and detect instrument malfunctions, and avoid procedural errors

that could effect the precision and accuracy of the ratio

measurement. Although the operator may give the utmost in care and attention to remain within the constraints of the

procedure, a significant but small percentage of all uranium

analyses will deviate from the normal pattern and must be

terminated. Termination of an analysis is made by the operator

as soon as it is determined that the analysis is abnormal or that an unspecified or improper adjustment has been made.

Termination or rejection is always made before isotopic ratios

are calculated. If the ratio is calculated, it must be

accepted along with all other data until a sufficient number

of analyses are made to define the limits for the experiment.

If the limits are inadequate the entire series of measurements

is repeated.

53
When the operator performs Ideally and accomplishes his

task, he is merely an appendage of the instrument and does not significantly increase the confidence limits of the measurement. When the operator performs poorly or unsatisfactorily,

not only are the confidence limits expanded but the operator

is likely to become the limiting factor in the measurement.

Thus, a large number of man-hours invested in chemistry and

instrumentation are negated by a failure to be consistent or reproduce the same experimental conditions for all analyses.

P . Precision of the Analytical Procedures

The extensive development of the low and high temperature

procedures resulted in a degree of refinement that allows

these procedures to be used as reference techniques to evaluate

mass spectrometric instrumentation and also as a performance

evaluation in training an operator. The bias correction for

both procedures is reproducible to within 0.03 percent under

controlled analytical conditions.

After a new mass spectrometer is assembled^ the first

analytical measurement series is a determination of the

2 3 5^/2 3 By ratios of SRMs U-050 through U-930» This series of

measurements has been designated as a systems calibration and

is performed for each new or rebuilt 12-inch radius of curvature instrument before it is used for any other isotopic

measurements. The systems calibration tests the linearity of

the measuring circuit over a wide range of isotopic ratios by

evaluating the filament bias with a fixed analytical procedure.

The low temperature procedure was the preferred and most used

for the systems calibration because of the lower rate of

fractionation. Since the only bias measureable, over the

ratio range of 20 to 0.05, is a known constant for the low

temperature procedure and is due to fractionation effects in

the evaporation and ionization process, analysis of these

SRMs will quickly reveal any systematic error contributions

from the measuring circuit

.

54
' Once a systems calibration has been performed on a mass spectrometer, the Instrument is then available for general

Isotopic analyses and is also useful to evaluate the performance

I of a trainee in using a procedure for isotopic analysis. All

assigned personnel of the Analytical Mass Spectrometry Section

[i who regularly use an Instrument for isotopic analysis and all

' trainees or guest workers are required to successfully complete

j the systems calibration before attempting extensive analytical

I programs for other elements. Thus, the characteristic filament

I bias of the analytical procedure becomes a reference point and

; the operator must learn how to accurately reproduce the exactI ing conditions of the procedure for every analysis. When an operator makes the initial attempt to follow the instructions

of a fixed procedure, there may be some minor and unavoidable

differences in what he is doing and what the specifications

of the procedure are requesting. These differences are frequently systematic i.e. consistent use of a slightly different

filament temperature, sample size or time span for completing

the ratio measurement. Procedural variations of this nature

may yield a systematic difference between the data of the

operator and the known bias of the analytical procedure. These

differences are usually small (<0.05^) for uranium analysis

and are gradually reduced as the operator gains more experience

with the mechanics of the method. Although the low temperature

procedure was preferred for making the systems calibration,

other procedures are valid and were used. The high temperature

technique and an "intermediate" technique with biases of 0.2

and 0.3 percent, respectively, were used. Also, a systems

calibration was made using a 40 yg sample size instead of the

normal 200 yg sample size. Regardless of the type procedure

selected for the systems calibration, nearly constant use of

this procedure is recommended because it equates all instruments

and operators to the same reference point at a precision of a few parts in ten thousand.

55
When the objective of an experiment Is to determine the

absolute abundance ratios of a sample or a group of samples, more freedom is allowable for systematic differences in procedure between operators or between experiments, as long as these differences are reproduced for each operator or for each

experiment. If these differences are allowed, a correction

factor must be experimentally determined by each operator and

for each experiment. Different sets of parameters are allowable since the bias correction is due to fractionation effects

and, if each set of parameters is precisely reproduced as a set of constants, the abundance ratios will be averaged at fixed points on different fractionation curves. But as long

as known calibration standards are available and are analyzed

under the same experimental conditions as all the samples in

the same experiment, accurate corrections for differences in

fractionation between two different experiments can be made

and the corrected data should fall within the experimental

limits referenced to absolute. Thus, it is practical and

possible to make isotopic measurements without insisting upon

reproducing the exact procedure for each experiment or that

each operator obtain the same correction factor.

56
6. ABSOLUTE 2 3 5u/2 3 8u ABUNDANCE RATIOS OF URANIUM SRMs

U-9 70 THROUGH U-00 5 A . Introduction

Absolute Isotopic abundance studies by the Analytical Mass Spectrometry Section of the National Bureau of Standards Include

silver [16], chlorine [17], copper [13], bromine [I8], chromium

[15], magnesium [l4], lead [19], boron [5] and rubidium [6].

Exclusive of boron and lead, where sets of 2 and 3 SRMs were

certified, the end product was a single standard with an accurately known Isotopic composition and atomic weight. In all of these studies it was possible to make a complete set

of isotopic measurements in a relatively short time span.

The goal of the uranium isotopic program was to determine

the absolute isotopic composition of I8 samples that were to

be certified as SRMs. These SRMs were established by the

Atomic Energy Commission (AEC) to serve as quality control

standards within the AEC complex. Uranium SRMs are available

to AEC contractors, AEC or state licensees, and foreign governments which have entered an Agreement for Cooperation with

the U.S. Government concerning the Civil Uses of Atomic Energy.

Because of significant variation in nature [20,21,22,23,24,25,

26,27,28] and the wide range of isotope distributions from man

made processes, a well characterized set of uranium standards

that are available to meet the needs of commercial suppliers

of uranium are essential and invaluable. Since this is the most extensive set of isotopic standards with known absolute

Isotopic compositions for any element, they are finding

increased use in evaluating and calibrating mass spectrometric

measuring circuits.

The isotopic measurements were completed over an extended

period as a series of independent self contained experiments,

until the data was acquired for all I8 SRMs. The general

experimental approach was to determine the absolute 235^^233^

ratio using synthetic calibration mixes of known isotopic

57
composition and then in a separate experiment determine minor

isotopic composition by ^^^U isotope dilution. With these

bits of information, the smaller of the major isotopes (^^^U

and ^^^U) was calculated directly and the composition of the

remaining isotope was determined by difference.

The only measurable bias was the temperature and mass dependent fractionation factor related to the ionization process

in the source. Thus, for the instruments used in this study,

it was necessary to demonstrate that a single correction factor

was valid over a wide range of isotopic ratios. Calibration

mixes prepared from nearly pure separated isotopes of ^^^U and

^^^U were used to evaluate the bias factor at 235^^238^ p^-tios

of 10, 1, and 0.1. The bias was a constant, within experimental

error, when the samples were analyzed under identical experimental conditions. By significantly changing the heating

pattern or increasing the sample temperatures, the magnitude

of the bias was changed, but it was a constant for each set of

identical experimental conditions. The data for the low tem- perature and high temperature procedures are given in Table 7. B . Experimental Procedure

1 . Mass Spectrometry

Isotopic measurements of the calibration mixes and the

SRM samples were made on three different single-stage solidsample mass spectrometers. Each instrument had a 12-inch

radius of curvature 68° analyzer tube and 60° sector magnet

and was equipped with the expanded scale-measuring circuit,

the NBS collector, source coldfinger, and triple filament

rhenium ribbons (0.001x0.030 inch) in the ion source.

The "sample filaments were prepared for mass spectrometric

analysis according to the sample mounting procedure of Section

5,B. An ion accelerating voltage of 10 kV was used and ratios were determined by measuring u"*" ions. The low temperature

analytical procedure or a slight modification of this technique

was used for most measurements. In spite of the extended time

58
Table 7. Comparison of the calculated ^s^u/^^^U ratio to

the observed 235^^239^ ratio for low and high

temperature analyses of synthetic uranium samples.

Low temperature

Mix Ratio of 0.1 Ratio of 1 Ratio of 10

1 0.99564 0.99584 0.99597

2 0.99523 0.99476 0.99590

3 0.99574 0.99521 0.99619

4 0.99488 0.99567 0.99585

5 0.99520 0.99563 0.99608

Average = 0.99534 0.99542 0.99600

High temperature

1 0.99857 0.99742 0.99780

2 0.99852 0.99795 0.99786

3 0.99786 0.99720 0.99793

4 0.99830 0.99851 0.99800

5 0.99783 0.99779 0.99718

Average = 0.99822 0.99777 0.99775

period and the many instrumentation changes the fundamental

parameters for measuring ion currents were nearly the same

for each experiment in the series . Because of the continuous

development and upgrading of the instrumentation, a brief

description of the mass spectrometer is included for each

series of measurements in this Section and in Section 7. The standard procedure was to use the most advanced instrumentation available for the uranium isotopic abundance

measurements

59
2 . Isotoplc Composition of "U235" and "U238" Separated

Isotopes

.The isotoplc compositions of the separated Isotope samples were initially determined by a direct comparison with the major

isotope. Then the measured ^^'*U of the "U235" sample and the

^^^U of the "U238" sample were used as target values for a determination by isotope dilution. The isotope dilution

measurements were made on a mass spectrometer equipped with

an ion multiplier, and corrections for the effects of bias

were made by analyzing SRM U-500 under the same experimental

conditions

The difference between the isotope dilution data and the

data from comparisons with the major peak was less than 1 percent for the "U235" sample, and less than 1 ppm for the ^^^U

of the "U238" sample. The Isotoplc compositions as determined

by the dilution technique were used as best values and the

results are given in Table 8. The ^^''U and ^^^U isotopes were

not detected in "U238" and the limit of detection was less

than 1 ppm.

Table 8. Composition of "U235" and "U238" separated isotopes.

Isotoplc composition

(atom percent)

Separated isotope ^^'^U ^^^U ^^^U ^^^U

"U235" 0.0^42 99.8195 0.0574 0.0789

±.0005^ ±.0020 ±.0005 ±.0006

"U238" <.oooi 0.0004 <.oooi 99.9996

±.0001 ±.0002

a The uncertainties are 95^ C.L.

60
3 . UsOs Ignition Procedure

a. Apparatus - Platinum Boats (Figure 6 ): The

boats were formed from 20x33 mm pieces of 2 mil platinum foil.

The sides were 6.5 mm high and the folds were left slightly

open to prevent material from being trapped. The boats were

cleaned by heating In hydrochloric acid (1+1), Igniting to 1200 °C, heating In hydrochloric acid (1+1), rinsing with

water, heating In nitric acid (1+1), and rinsing with water.

Glass Desiccators (Figure 6 ) : The desiccators were made

from ground glass joints to contain the platinum boats during

movement and weighing. The weighings were done on a 20 g capacity mlcrobalance so the weight of the desiccator was limited to a maximum of 17 g. A minimum of silicone grease

was used as lubricant and sealer. The desiccators were cleaned In hot nitric acid (1+1) before they were used on samples with different Isotoplc composition.

Ignition Tube (Figure 7 ): A quartz Ignition tube, 3 ft long, 13 mm l.d., and 15 mm o.d. was used. End pieces of glass were butt joined with plastic tubing. The front end piece was

13 mm l.d., had a side arm for air or oxygen entry, and was closed with a sealed tube attached by a ball and socket joint.

The back end piece was a reducing adapter from 15 mm o.d. to

6 mm o.d. A separate quartz tube was reserved for each Isotoplc

level and was cleaned with nitric acid (1+1) before use. The

end pieces were cleaned by heating In nitric acid (1+1) before

use on the next tube

Air and Oxygen Supply : The air and oxygen supply lines were copper and glass tubing, butt joined with plastic tubing.

The gasses were passed through sulfuric acid and then ascarlte

and magnesium perchlorate before entering the sldearm of the

front end piece on the Ignition tube.

Furnace (Figure 7 ): A tube furnace capable of continuous

operation at I050 °C was used. It was mounted on wheels to enable It to be moved back and forth over the sample area of

the Ignition tube.

61

Figure 7. Furnace with quartz ignition tube for ignition of

uranium oxides

.

b. Procedure - The empty sample desiccator and tare desiccators, after sitting at least one hour to reach temperature equilibrium, were opened momentarily to equalize pressure

in the desiccators and then placed on a grounded steel plate.

After waiting 15 minutes, the sample desiccator and tare desiccators (these were weighed each time a sample desiccator was weighed) were weighed on a 20 g capacity microbalance . An empty platinum boat, ignited in the ignition tube at 1050 °C

with an air flow of about 8 to 10 ml per minute, was drawn

out of the tube directly into the sample desiccator. The

desiccator was quickly closed and allowed to stand approximately 1 hour. The desiccator was opened momentarily and

weighed as before.

The boat was removed from the desiccator and the sample

of uranium oxide was transferred to the boat. (All work

63
i involving open samples of radioactive material was conducted

in a glove box or hood.) After transferring the boat and

sample back to the desiccator; the desiccator, boat, and

sample were weighed as before.

The boat with sample was transferred to the ignition

tube, the furnace was moved to cover the boat, and the sample

was ignited at 900 °C for 1 hour with air flowing at about 8 to 10 ml per minute. The furnace was moved back and the

sample was allowed to cool 10-15 minutes. The boat was drawn

into the desiccator and weighed as before. The ignition was repeated at 1050 °C for l6 hours in air. After cooling the

boat was transferred to the desiccator and weighed as before.

The ignition was repeated at 85O °C for 1 hour in an oxygen

atmosphere {>99%) at a flow rate of about 8 to 10 ml per

minute. When the furnace had been rolled back and the sample

had cooled, oxygen was turned off and air was turned on at about 10 to 12 ml per minute. After 30 minutes of air flow

the sample was removed and weighed as before. After the boat

and sample were removed the empty desiccator was reweighed to check for any desiccator weight change.

c. Discussion of Procedure - Glass desiccators were used to contain the samples to prevent exposure of the sample

to moist air and to prevent contamination of the balance and

surrounding area. The desiccators were constructed to weigh

not more than 17 g, so that the combined weight of desiccator,

platinum boat and sample would remain within the 20 g capacity

of the microbalance used. To achieve accurate weighing to within a few micrograms several precautions were taken. The

desiccators were placed on a grounded steel plate and were handled with grounded tongs, the balance was grounded and was shielded from drafts, at least 30 percent relative humidity

was maintained in the room, and the balance was calibrated

with NBS calibrated weights. Pour desiccators were weighed

with the sample desiccators so that corrections due to changes

in temperature and pressure could be applied. This correction

64
has been as large as 200 yg although it was usually less than

100 yg. The platinum boats were checked for loss on ignition

at 1050 °C by heating at IO5O °C for 5-4 hours. They showed

an average loss of less than 2 yg. The ignition at 900 °C was done so that the sample could

be compared to earlier work. On samples that had consistent

ignition loses, the ignition at 900 °C and the weighing after

the 1050 °C ignition were eliminated.

To establish a limit to any change in the desiccator

weight it was weighed after the boat and sample were removed.

A loss in weight of the desiccator could result from such

things as a chip of glass broken off from the desiccator, a loss of lubricant or a loss of foreign particle which had been

on the desiccator before the first weighing. A gain in weight

of the empty desiccator could be due to U3O8 spilling from the

boat into the desiccator or to foreign particles landing on

the desiccator after the original empty desiccator weight had

been taken. These changes in weight could result in an error

in the apparent weight of U3O8 depending on when the weight

change occured. If the change happened after the empty boat

weighing and before the final sample weighing the apparent

U3O8 weight would be in error but if it happened before the

empty boat weighing or after the final sample weighing the

apparent U3O8 weight would not be in error. Since it might

or might not affect the U3O8 weight, one-half of the empty

desiccator weight change was applied to the apparent weight

of U3O8. This correction was very small in most cases, with

the largest correction being equal to 0.004 percent.

The weight of U3O8 was obtained by subtracting the empty

boat weight from the ignited at 85O °C in oxygen weight and

making the corrections for empty desiccator weight changes.

4 . Preparation of Calibration Mixes Containing 235^^233^

Ratios of 9, 1 and 0.1

a. Procedure - Each sample was ignited (Section

6,B,3) using the weight of sample calculated to give the desired

65
2 3 5y^2 3 8y patio. The platinum boat and sample were carefully

transferred from the desiccator to a 250 ml teflon bottle

wrapped in a damp paper towel to reduce any static charge on

the bottle. After both the "U235" and "U238" separated isotopes

were ignited and transferred to the bottle, sufficient nitric

acid (1+1) was added to the bottle to give a nitric acid (1+19)

solution containing 5 mg U/ml when diluted with water. The

bottle was sealed with a teflon- lined cap and heated on a steam bath for an additional 1/2 hour after all the sample had

appeared to dissolve. Sufficient water was added to give a nitric acid (1+19) solution and the solution was thoroughly

mixed

b. Discussion of Procedure - These samplet> were prepared for point and systems calibration of the mass spectrometer. The first mixes of "U235" and "U238" separated isotopes

were to bracket the SRM U-500. Five samples of approximately

0.25 g of the "U235" separated isotope were ignited and then

the calculated amounts of "U238" separated isotope to give a range of 235^/239^ ratios from 0.99 to 1.01 were ignited.

Five mixes were prepared to allow an evaluation of the mixing

procedure as well as the mass spectrometry measurement.

The solutions with 235^^238^ ratios of 9 were prepared

using 0.9 to 1.0 g samples of "U235" separated isotope and

0.10 to 0.11 g samples of SRM U-0002. SRM U-0002 was of

adequate isotopic purity to use for these mixes. The solutions

with 2 3 5yy238y ratios of 0.11 were prepared by the same procedure except for a reversal of the amounts of the "U235"

separated isotope and SRM U-0002.

Teflon picks up a static charge very easily and if the

charge is not discharged some of the uranium oxide may jump

out of the boat onto the bottle. It has been found that a damp towel will discharge the static charge on the teflon and

then the boat and sample can be safely transferred into the

bottle. Approximately 200 transfers using this technique have

been accomplished without any indication of a problem. All

66
nitric acid used for the uranium samples was checked for ura- nium by either "U233" isotope dilution mass spectrometry or

by fluorescence. The fluorescence procedure Indicated less

than 5 ppb U (limit of detection) in one lot of nitric acid

and the isotope dilution procedure indicated less than 0.01

ppb in another lot. The samples were dissolved in nitric acid

(1+1) by heating over a small steam bath in a radioactivity

hood. This required from one to four hours for all visible

oxide to dissolve. The samples were heated for about 30 minutes after all the visible material had dissolved to Insure

complete solution. The solutions were then diluted with water

to give a final solution of 5 mg U/ml in nitric acid (1+19).

The 2 3 5^/2 3 By patio (Table 9) was calculated for each

mix using the following equations:

Moles ^^^U "U235" + moles ^ ^ "U238"

Calculated 235^/238^ ^ Moles ^^^U "U235" + moles ^^^U "U238"

Moles ^^^U "U235" = moles of ^^^U from the

"U235" separated isotope

Moles ^^^U "U238" = moles of "^^U from the

"U238" separated isotope

Moles ^^^U "U235" = moles of ^^^U from the

"U235" separated isotope

Moles ^3^U "U238" = moles of ^^^U from the

"U238" separated isotope

The moles of ^^^U in the "U235" separated isotope were calculated by equation {2}.

67
A = 5C

^ D {2}

A = moles of Isotope ^^^U from the

"U235" separated isotope

B = g of "U235" separated isotope

C = atom percent of ^^^U in the

"U235" separated isotope

D = 1/3 of the molecular weight of U3O8

in the "U235" separated isotope

The values of the remaining isotopes were calculated in a similar manner.

Table 9. Calculated 235^^238^ patios of calibration mixes

for SRM U-900, SRM U-500 and SRM U-100

.

Separated isotope

Mix No. "U238"^ "U235" Calculated

(g UaOe) (g U3O8) 235u/23eu

13-186 0.102246 0.923314 9 .0481

13-185 0.103162 0 .926991 9.0038

13-35 0.26243 0.25911 0.9954

13-36 0.27698 0.27159 0.9886

13-37 0. 25841 0.25757 1.0049

13-38 0.25146 0.25206 1.0106

13-39 0.24989 0.24836 1.0020

13-177 0.911160 0.100724 0.11173

13-179 0.921865 0.101374 0.11114

"U238" !separated isotope was used for mixes No. 13-35

through 13-39 and SRM U-0002 was used for the remainder

68
The teflon, platinum and glass equipment such as bottles,

beakers, droppers and needles used to contain uranium, water

or nitric acid for all the work described In this publication,

were cleaned In detergent, rinsed with distilled water, heated

to boiling for at least one hour In nitric acid (1+1), rinsed

with distilled water and allowed to dry.

5 . Preparation by Solution Allquoting of Calibration

Mixes Containing 235^^238^ Ratios of 0.005 to 0.05

a. Procedure - The "U238" separated isotope samples

were Ignited (Section 6,B,3), transferred to teflon bottles

and dissolved as before (Section 6,B,^). The "U235" separated

Isotope samples were also ignited as above. A teflon bottle

was wiped with a damp towel to remove any static charge, allowed to stand 1-2 minutes and weighed. The "U235" separated

isotope sample was transferred to the weighed teflon bottle

and dissolved in enough nitric acid (1+1) to give nitric acid

(1+19) solutions containing 3 mg U/ml when diluted. The "U235"

solutions were then diluted to 3 mg U/ml with water, mixed

thoroughly and allowed to stand overnight before the final

weight was taken. The cap of the "U235" bottle was quickly

replaced with a rubber serum septum (Figure 8). A four inch

platinum needle was inserted through the septum. A short

second needle which just punctured the septum served as a vent.

A 10 ml plastic syringe with the rubber plunger covered by a thin teflon sheet was attached to the Kel-F hub of the platinum

needle and the desired amount of solution was drawn into the

syringe. The needle with the syringe still attached was raised

above the level of the solution and a small amount of air was drawn into the tip of the syringe. The syringe was disconnected from the hub and the tip was wiped with a dry paper

towel and quickly capped with a Kel-P cap. Any static charge

that might be on the syringe was dissipated by wiping it with

a damp paper towel, and the syringe and contents were weighed

on a seml-microbalance to the nearest 0.01 mg. The solution

was delivered from the syringe to the weighed bottle containing

69
Figure 8. Apparatus used for weight allquotlng.

70
the "U238" sample solution. The syringe was quickly capped,

then wiped and weighed again. The weight of the "U235" aliquot

was determined from the weights of the syringe before and after

delivery of the sample. The calibration solution was mixed,

diluted with water to give 5 mg U/ml and mixed again.

b. Discussion of Procedure - These mixes were prepared

for mass spectrometer calibration at the 235^^239^ ratios of

the six SRMs (U-005 through U-O5O) having less than 10 percent

^^^U. Unless very large amounts of "U238" separated Isotope

were used, the amount of "U235" separated Isotope needed could

not be weighed directly with the required accuracy. Since the

quantity of separated Isotopes Is limited, the "U235" Isotope

was added as a weighed aliquot. The "U238" separated Isotope

samples of 0.5 to 1.2 g were Ignited and transferred to teflon

bottles as before (Section 6,B,4). Two "U235" separated Isotope samples of O.3O and 0.35 g were Ignited and transferred

to separate weighed teflon bottles. Two samples of "U235"

separated Isotope were taken for greater reliability In the

results. With reasonable care the possibility of an error Is low, but an error In weighing, an error In recording the

weight, an unnoticed loss of sample or solution, or contamination can occur. Two samples of "U238", one spiked with one "U235" solution and the other "U238" sample spiked with the

second "U235" solution, were prepared to closely bracket the

2 3 5^/2 3 By patio of each of the SRMs.

Teflon bottles pick up static charges very easily and

can cause significant apparent weight changes. On 125 ml and

250 ml teflon bottles, static charges have resulted In apparent weight changes of as much as 30 mg . It was found that by

wiping the teflon bottle with a damp paper towel and allowing

it to stand for 1-2 minutes the reproducibility was ±1 mg. On

weighing 100 g of solution, the amount used on these samples,

the error would be no more than 2 parts in 100,000 from this source. A discussion of aliquotlng is given in Section 2,D.

71
The 2 3 5y/2 3 8u ratio was calculated using equation {1}

(Section 6,B,4). The moles of ^^^U from the "U235" separated

isotope were calculated from equation {3}.

A = {3}

A = moles of isotope ^^^U in the

"U235" separated isotope aliquot

B = g of "U235" separated isotope

in the solution

C = atom percent of ^^^U in the

"U235" separated isotope

D = 1/3 of the molecular weight of U3O8

in the "U235" separated isotope

E = g of aliquot taken

F = g of "U235" separated isotope solution

The moles of ^^^U were calculated in a similar manner. The

calculated 235^^238^ patios are given in Table 10.

6 . Preparation of Calibration Mixes for SRM U-970

a. Procedure - These solutions were prepared ac- cording to the procedure used for the solutions containing

235^/238^ ratios of 0.005 to 0.05 (Section 6,B,5) with the

"U238" separated isotope being thfe solution aliquoted by

weight instead of the "U235" separated isotope. The "U238"

separated isotope samples were diluted to approximately 1.3

mg U/ml and 5 ml plastic syringes were used to add weighed

aliquots of the "U238" solution to the "U235" samples.

72
Table 10. Calculated 23 5^/23 ratios of calibration mixes

for SRM U-005 through SRM U-O5O.

Solution No. I 'U238" "U235" "U235" Calculated

C fi, u 3 u 8 ;

( CT c; n 1 n ^

\ & 0 U J-Ii ;

c; n 1 n 1\T n

2 5U/238U

l6-ll|4 1 009929 7.01233 1^ u

n 9 n Q ii ^ Q 16-145 1 182429 5. 40564

h2^ U 16-146 1 188715 3.99410 1 U UlUl J /

16-147 1 204562 3.63029 2 U UlOi j J!:) 16-148 1 225201 1.94921 1 U 16-149 1 255977 1.83482 2 U 16-152 0 595380 9.27539 2 U

n c: Q Q n v 16-153 0 985517 5.97268 2 0 0207755

16-154 1 139857 5.86156 1 0 0155123

16-159 0 592706 10. 45382 1 0 0531937

16-160 0 665352 6. 09923 2 0 0314218

16-161 0 624362 6.62499 1 0 0320039

^ "U235" solution No. 1 contained 2.52977 mg U/g of solution.

^ "U235" solution No. 2 contained 2.87501 mg U/g of solution.

b. Discussion of Procedure - Pour mixes were prepared using 0.9 to 1.0 g samples of "U235" separated isotope

and two solutions of "U238" separated isotope for aliquoting.

The two "U238" solutions of 103 and IO6 g were prepared using

150 and 160 mg of U3O8 . Aliquots of from 2.5 to 3.0 g of the

"U238" solution were taken using plastic syringes. The

23 5y^238y patios Were calculated using equations {1} and {3}

except the calculation was for the "U238" separated isotope

in equation {3}. The values for the calibration solutions

are given in Table 11.

73
Table 11. Calculated 2

3 5^/2 3 patios of calibration mixes

for SRM U-970.

Separated isotopes

Sample No. "U235" "U238" Calculated

(g U3O8) (g SOln) 235u/238u

17-86 0.995261 2.88470^ 193.785

17-92 0.906216 2.67256^ 190.956

17-93 0.922357 2.67714^ 197.106

17-94 0.988828 2.97297^ 191.316

This "U238" solution contained 1.29004 mg U/g of solution.

^ This "U238" solution contained I.2626O mg U/g of solution.

7 . Analysis of Calibration Mixes and SRMs U-900, U-500

and U-100

The point calibration technique was used for each SRM in

this group and consisted of using synthetic mixes blended to have ^asyy'assu ratios approximately the same as the SRM to be

calibrated. This procedure provides an accurate means of

correcting for bias of a given ratio (point) but does not yield any information about the dependency of the bias on the

magnitude of the ratio.

The 2 3 5^/2 3 By patio of SRM U-500 was the first to be determined because all random and systematic bias components were nearly identical for each isotope. Two different operators and

two different 12-inch radius of curvature 68° analyzer tube mass spectrometers were used. Five calibration mixes and the SRM were analyzed to obtain a single analysis for all samples on Instrument No. land to obtain duplicate analyses for all samples on Instrument No . 2 . A modified low temperature procedure

was used and the u"*" ion current was approximately 5^10"^ ^A and

decaying slowly. Correction factors were determined for each

instrument by calculating the ratio of the "calculated"

2

3 5^/2 3 By observed ^3 5^/2 3 8^^ ,p^g correction factors and

the absolute ratio determination are given in Tables 12 and 13

.

74
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!^ coin hO Cd O O O rH rH rH Cd Cm ?H(U Cd > 0) O =3! H P hO O

Cd •H

Cm CQ C (u :=) Cd O VD OA VD o

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lue CD •H • cd rH -P o LPi VD CO C3A > ^ cd S OO OO oo OO OO

cd 1 1 1 1 1 EH OO OO oo OO OO o

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Table 13. Absolute 23 5^/23 ^atlo of SRM U-500.

Instrument No. 1 Instrument No. 2 Ob s e rve d. Corrected Observed C orre ct ed

1. 0029 0.99989 1,.0031 0.99999

1.0031 1.00009 1

,

.0030 0.99989

1.0026 0.99959 1,.0023 0.99919

1.0032 1.00019 1,.0022 0.99909

1.0035 1.00049 1,.0026 0.99949

1,.0037 1.00059

1,.0023 0.99919

1

,

.0027 0.99959

1,.0025 0.99939

1

,

.0020 0 .99889

Average = 1.0000 0.9995

The absolute 2 35^/2 a ratios of SRMs U-900 and U-100 were determined approximately one year after completion of the

measurement for SRM U-500. The 12-inch radius of curvature

68° analyzer tube mass spectrometer was used. Two calibration

mixes were prepared for each SRM and were used to determine

correction factors and calibrate for the effects of bias. Two

reference solutions were used for each SRM and were selected

to evaluate possible differences between samples that had been

dissolved for different lengths of time. Solutions labeled

"A" were dissolved approximately 1 year before the measuremient

and solutions labelled "B" were dissolved several weeks prior

to the analysis. The calibration mixes and SRM solutions were analyzed in an alternate pattern until 4 analyses were obtained

for each sample. Although memory was not detected, all measurements were completed on SRM U-100, and the source was disassembled and thoroughly cleaned to further reduce any chance of memory contributions before proceeding with ratio

76
measurement for SRM U-900. The correction factors and

absolute ratios are summarized In Tables 1^ and 15.

Table 14. Correction factors for calibration of 2 3 5^/2 3 8^ ratios of SRM U-900 and SRM U-100.

SRM U-900

Calibration

mix No

.

13-185

Calculated Observed

235u^238u 23 5^^23

9.0038 9.04750

9.04628

9.04301

9.04459

Correction

factor

0.99517

0.99530

0.99566

0.99549

13-186 9.0481 9.09248

9 .08780

9 .08157

9 .08903

Average =

0.99512

0.99563

0.99632

0.99550

0.99552

SRM U-100

13-177 0.11173 0.112149

0.112129

0.112176

0.112198

0.99626

0.99644

0.99602

0.99583

13-179 0.11114 0.111579

0 .111625

0.111674

0.111694

Average =

0.99607

0.99566

0.99522

0.99504

0.99582

77
Table 15.

Solution

A

Absolute ^ SRM U-100.'u/- U ratios of SRM U-900 and

SRM U-900

Observed Corrected

10.4249

10.4259

10 . 4111

10 . 4219

10 .3782

10.3792

10 . 3644

10.3752

SRM U-100

Observed Corrected

0 . 114083

0 .114085

0 .114081

0.114023

0 .113606

0 .113608

0.113604

0.113546

B 10.4203

10 , 4272

10 . 4213

10.4194

Average =

10.3736

10.3805

10.3746

10.3727

10.375

0.114046

0.114058

0.114173

0.114064

0 .113569

0 .113581

0 . 113696

0.113587

0 .11360

8 . Absolute 2 3 5u/2 3 8^ Ratios by Interpolation

Once it was demonstrated that the bias was independent of

isotopic composition for 235^^233^ current ratios between

10 and 0.1, it was decided to determine the remaining 235^^233^

ratios within this range by interpolation. The interpolation

method consisted of using samples of known 235^^233^ ratios to determine correction factors near the end points and at the

midpoint of the ratio range to be calibrated. If both the

calibration samples and the "unknown" samples are analyzed

under identical experimental conditions, then a single correc- tion factor is valid to correct all ratios within the limits

of the calibration.

SRMs U-900, U-500 and U-100 were used as standards of

known 2 35^/2 3 Sy ratios and were analyzed under the same experimental conditions as the SRM samples to be calibrated (SRMs

U-930, U-850, U-800, U-750, U-350, U-200, U-150 and U-O5O).

SRMs U-930 and U-O5O were included in the group although they were beyond the end points of the calibration standards.

78
A low temperature analytical procedure was used. Two

reference solutions were used for each SRM and were labelled

"A" for dissolution 1 year prior to the measurement and "B" for dissolution immediately before the measurement. Because

of the wide range of 235^^238^ ratios to be determined, alternate analyses of "calibration" samples with the SRMs was not practical. All samples were divided into three groups, with

each group covering a maximum ^^^U composition range of approximately 20 percent, and were analyzed in order of decreasing

^^^U composition. The sample groupings were U-930 to U-750,

U-500 and U-350, and U-200 to U-O5O. The source was disassembled and thoroughly cleaned for the analysis of each group.

The calculation of correction factors is summarized in Table

16 and the absolute ratios in Table 17 . Since there was no experimental difference between "A" and "B" solutions, this

designation was not included in the Table.

9 . Analysis of Calibration Mixes and SRMs U-050 through

U-005

The ratio of ^^^U and ^^^U ion currents of this group of

SRMs varied from approximately 0.05 to 0.005 • Non-ohmlc

response and the R-C response of the measuring circuit were

sources of bias that made point calibration of each SRM mandatory. A single focussing, 12~lnch radius of curvature 68°

analyzer tube mass spectrometer with a "Z" lens source was

used. The flight tube was equipped with a beam valve and was used to isolate the tube from the source housing except during

sample degassing and the analysis.

SRM U-050 was an end-point for the 2 3 5^/2 3 p^tio determination by the interpolation method and the ratio was redetermined by the point calibration method as a tie-point between

the two sets of measurements. SRM U-O5O was analyzed using an equivalent low temperature procedure for the "Z" lens source.

The signal intensity was approximately 5^10~'^^A and growing.

An optimum signal intensity was selected for each succeeding

SRM to yield maximum precision on the expanded scale peak height

79
Table l6 . Correction factor for calibration of 2 3 5^/2

3

ratios by the interpolation method.

SRM Absolute Observed

U-900 10.375 10.4278

10.4193

10 . 4299

10 .4180

U-500 0.9997 1.00460

1.00425

1.00422

1.00442

U-100 0.11360 0.114118

0.114110

0.114100

0.114144

Average =

Correction factor

0 .99494

0.99575

0.99474

0.99587

0.99532

0.99512

0.99547

0.99550

0.99530

0.99535

0.99546

0.99553

0 .99562

0.99523

0.99546

0.99538

80
Absolute 235u/238u patios for SRMs U-930, U-85O,

U-800, U-750, U-350, U-200, U-150 and U-O5O.

SRM Observed Corrected

U-930 17.4395 17.3589

17.4331 17.3526

17.356

U-850 6.17729 6.1488

6.17697 6.1484

6.149

U-800 4.28742 4.2676

4.28803 4.2682

4.268

U-750 3.17976 3.1633

3.18009 3.1654

3.164

U-350 0.548962 0.54643

0.548858 0.54632

0 .5464

U-200 0.252293 0.251127

0.252422 0.251256

0.25119

U-150 0.181931 0.181090

0.181910 0.181070

0.18108

U-050 0.052994 0.052749

0.053012 0.052767

0.05276

81
determination. Each SRM and its corresponding calibration

mixes were analyzed in an alternate pattern until there were

a total of 6 analyses for the SRM sample and 6 analyses for

the calibration mixes. A correction factor was calculated

using the calibration mixes and was used exclusively to correct

the SEN. with approximately the same 2 3 5^/2 3 ^^tio. The

calculation of correction factors and of absolute ratios are summarized in Tables I8 through 23.

Table I8. Absolute 2 3 5^/2 3 8^ ^^tio of SRM U-O5O.

Calculation of correction factor

Calibration Calculated Observed Correction factor

mix No. 16-152 0.053398 0.053624 0.995785

0.053640 0.995488

0.053622 0.995823

16-159 0.053194 0.053385 0.996422

0.053422 0.995732

0.053402 0.996105

Average = 0.99589

Absolute ratio

Observed

0 .053000

0.053010

0.053010

0.052997

0.052987

0.052984

Average

Corrected

0.052782

0.052792

0.052792

0 .052779

0.052769

0.052766

0.05278

82
Table 19. Absolute 23 5^/23 8^ patio of SRM U-O3O.

16-160

Calculation of correction factor

Calculated Observed Correction factor

0.031422 0.031531

0.031525

0.031526

0.996543

0.996733

0.996701

16-161 0.032004 0 .032094

0.032105

0.032102

0.997196

0 .996854

0 .996947

Average = 0 .99682

Absolute

Observed

0.031533

0.031535

0.031508

0.031515

0.031539

0.031531

Average =

ratio

Corrected

0.031433

0.031435

0.031408

0.031415

0.031439

0.031431

0.03143

83
Table 20 Absolute 2 3 5tt /2 3 'U ratio of SRM U-020.

Calibration

mix No. 16-144

Calculation of correction factor

Calculated Observed Correction factor

0.020944 0.021003

0 .021015

0.021015

0.997191

0 .996621

0.996621

16-153 0 .020776 0.020847

0.020843

0 .020843

0 .996594

0.996785

0 .996785

Average = 0.99677

Absolute ratio

Observed

0.020870

0.020882

0.020865

0,020877

0.020874

0.020881

Corrected

0.020803

0.020815

0 .020798

0 .020810

0.020807

0 .020814

Average = 0 .02081

84
Table 21. Absolute 235^^238 U ratio of SRM U-015.

Calculation of correction factor

Calculated Observed Correction factor

16-1^5 0.015673 0.015721

0.015720

0.015709

0.9969^7

0 .997010

0.997708

16-154 0.015512 0.015545

0.015564

0.015551

0.997877

0.996659

0.997492

Average = 0 .99728

Absolute ratio

Observed Corrected

0.015610

0.015609

0.015612

0.015598

0.015609

0.015612

0 .015568

0 .015567

0.015570

0 .015556

0 .015567

0 .015570

Average = 0.015566

85
Table 22. Absolute 2 3 5tt /2 3 'U ratio of SRM U-010

Calculation of correction factor

Calibration

mix No

.

16-146

Calculated Observed Correction factor

0.010137 0.010175 0.996265

0.010170 0.996755

0.010170 0.996755

16-147 0.010334 0.010365

0.010364

0.010364

0.997009

0.997105

0.997105

Average 0.99683

Absolute ratio

Observed

0.0101698

0.0101701

0.0101777

0.0101727

0.0101690

0.0101728

Average =

Corrected

0.010138

0.010138

0.010145

0.010140

0.010137

0.010141

0 .010140

86
Table 23. Absolute 2 3 5y^2 3 8 U ratio of SRM U-005

Calculation of correction factor

16-148

Calculated

0 .0048020

Observed

0.0048110

0.0048130

0.0048073

0.0048075

Correction factor'

0 .998129

0.997715

0.998898

0.998856

16-149 0.0050110 0.0050219

0.0050185

0.0050181

0.0050165

0.997830

0.998506

0.998585

0 .998904

Average = 0.99843

Absolute ratio

Observed Corrected

0 .0049273 0.,0049195

0 .0049245 0..0049167

0 .0049313 0.,0049235

0 .0049225 0..0049147

0 .0049275 0..0049197

0 .0049270 0,.0049192

0 .0049233 0,.0049155

0 .0049235 0,.0049157

Average = 0 ,. 004918

The standard high temperature procedure was used for this

SRM.

87
10 . Isotoplc Analysis of Calibration Mixes and SRM U-970

SRM U-970 was the most enriched of the uranium SRMs and

was the largest 235^^238^ ^atio to be determined by the

point calibration technique. The 12-inch radius of curvature

68° analyzer tube mass spectrometer with a "Z" lens source

and a tube beam valve was used. A modified low temperature

procedure was used and the total ion current intensity was approximately 4-5><10~^^A and growing. The four calibration

mixes were analyzed in an alternate pattern with two different

solutions of SRM U-970 until duplicate analyses were obtained

for each mix and a total of 4 analyses for each SRM solution.

Table 24 summarizes the calculation of correction factors and

the absolute 235^/238^ ^atio.

88
Table 24. Absolute -^^^u/^^^U ratio of SRM U-970.

Calculation of correction factors

17-86

Calculated Observed Correction factor'

193.785 194.791 0.994835

194.826 0.994656

17-92 190 .956 191.972

191.878

0.994707

0.995194

17-93 197.106 198.327

198.116

0.993843

0 .994901

17-94 191.316 192 . 400

192.283

0.994365

0 .994970

Average 0.99468

Absolute ratio

Observed Corrected

187.679 186,.680

187.738 186,.739 187.716 186,.717

187.687 186,.688

187.874 186 .874

187 .942 186,.942

187 .763 186,.764

187. 847 186,.848

Average = 186,.78

^ The standard low temperature procedure was used for this

SRM.

89
7. ABSOLUTE ISOTOPIC ABUNDANCE BY ^^^y ISOTOPE DILUTION

A . Introduction

The initial method of measuring uranium Isotoplc composition was to mass scan (magnet) all Isotopes at a constant rate^

select adequate decade sensitivity to give a maximum deflection

for each peak; then calculate the isotoplc composition over a small number of peak sets ("^10). The chief disadvantage of

this method was the relatively large amount of time spent on baselines that were constant between the major isotopes and

the relatively small amount of time on peak tops that were continuously changing in magnitude. An alternate method was developed to take advantage of the constant baselines of the

major isotopes and consisted of the following steps: measure

the 2 3 5y^2 3 8y pa^-^iQ ^T^Q beginning and end of the data period

of each analysis by the "peak hopping" technique; compare the

minor isotopes with respect to ^^^U as a reference peak;

calculate isotoplc abundance ratios and then the isotoplc com- position. This method provided superior 2 35^^-2 3 8y measurements

without compromising the precision of baseline determinations.

A constant rate of scanning was still essential for minor

isotope measurements in order to accurately determine baselines

of the smaller peaks on the tail of ^^^U.

The improved measurement technique and exclusive use of

the expanded scale recorder reduced the limit of error below

0.5 percent for most minor isotope measurements. Further

modifications were made to give better vacuums which produced

a significant reduction in the scatter from the ^^^U peak, but

the precision of the minor isotope measurements was larger than

could be accounted for under the analytical conditions of the

experiment. Since the ratio of the ^^^U with respect to the

minor isotopes was always between 50 and 66O, it was concluded

that much of the imprecision was simply inability to measure

such large ion current ratios without non-linear effects

increasing the scatter of the data.

90
Thus, the ^^^U Isotope dilution technique was utilized

to circumvent the requirement of measuring large ion current

ratios to determine minor isotope composition. The ^^^U spike

was added to each SRM to approximate the ^^"^U abundance and,

exclusive of SRM U-0002, the entire series of measurements

was reduced to comparing ion current ratios that were less

than twelve.

For SRM U-0002, the ^^^U isotope comprised 99-98 atom

percent of the sample and only two other isotopes were detected above the 1 ppm level. A precise and accurate comparison

of such a large ion current ratio was not practical, so isotope dilution was used to evaluate the ^^"^U and ^^^U

abundances of this SRM.

B . Experimental Procedure

1 . Mass Spectrometry

Isotopic measurement of the spiked SRMs and the calibration standards were made on the single-stage solid sample mass spectrometer briefly described in Section 6,B,1. The only

significant change in instrumentation was the use of an ion

multiplier for SRMs with isotopic abundance less than 0.1 atom

percent. After the "Z" lens source became a part of the standard instrumentation, minor isotope measurements for all but

four of these SRMs were reevaluated.

Exclusive of SRM U-0002 and SRM U-970, an unspiked solution and at least one spiked solution were analyzed for each

SRM. A single analysis of the unspiked solution was made

first and was then followed by three analyses of the spiked

solution. The ratio measurement consisted of measuring ^^"^U and ^^^U with respect to ^^^U for the unspiked solutions and

also ^3'*U and ^^^U with respect to ^^^U for the spiked solutions. Magnet scanning was at a constant rate with a quick

scan step ('^^2 seconds) used for the ^^^U peak of the spiked

samples. Each analysis was started and concluded with a

2 3 5y/2 3 8u patio measurement.

91
A high temperature analytical procedure or a modification

of this technique was used for all SRMs except SRM U-970.

Corrections for the effects of bias were determined by analyzing SRM U-500 under the same experimental conditions as the

spiked samples or by using the 235^^238^ ^atio of the spiked

SRM as a known to make an internal normalization for filament

bias effects.

2 . Isotopic Analysis of "U233" Separated Isotope 3 The 12-inch radius of curvature 68° analyzer tube mass spectrometer was equipped with an ion multiplier. When this

series of measurements was made, the most accurate isotopic

standard available with ion current ratios similar to "U233"

was a Belgian Congo natural uranium sample with the 235^^288^

ratio determined by uranium hexafluoride mass spectrometry

[20]. The Belgian Congo sample was used as a calibration

standard and was analyzed under the same conditions to determine correction factors. The isotopic composition calculated

from corrected ratios is given in Table 25. The ^^^U isotope

was not detected and was believed to be less than 1 ppm.

Table 25. Isotopic composition of "U233" separated isotope.

Lot No. 3

Lot No. 3. Isotope Atom percent

2 3 3 U 99.924 ±0.002 a U 0.0180 ±0.0004

2 3 5 U 0.0012 ±0.0004

2 3 8 0.0566 ±0.0006

a The uncertainties are 95^ C.L.

92
3 . Preparation of Spiked SRMs U-005 through U-930

a. Procedure - Each sample was Ignited (Section

6,B,3) and then the platinum boat and sample were carefully

transferred to a tef^^on bottle (0.5 g samples to 125 ml bottles and 1 g samples .o 25^ ml bottles) which had been wrapped

In a damp paper towel. Sufficient nitric acid (1+1) was added

to the bottle to give a solution of nitric acid (1+19) con- taining 5 mg U per ml when the "U233" spike and water had been

added. The bottle was sealed with a teflon lined cap and

heated on a steam bath for an additional 1/2 hour after all

the sample had appeared to dissolve. The "U233" separated

isotope samples were dissolved after the 16 isotopic standards

had been dissolved. After dissolution, the "U233" sample was diluted to an equivalent of approximately 1 mg of UaOs/ml of

solution

.

The cap on the "U233" bottle was replaced with a similar

cap that had inserted in it a piece of polyethylene tubing

shaped for use as a "squirt" bottle. The outside end of the

tubing was sealed with a polyethylene cap to prevent evaporation. The next more dilute solution of the "U233" was prepared

before the samples were spiked. The polyethylene tube sealer

was removed and a small quantity of solution was squirted into

a waste bottle. The sealer was quickly replaced, the bottle

was allowed to stand 2-3 mminutes and then wiped with a damp

.

paper towel. After standing 1/2 minute the bottle was weighed

to the nearest 0.1 mg on a 200 g capacity magnetically damped

balance. An empty 125 ml teflon bottle was wiped with a damp

towel, allowed to stand 1/2 minute and weighed. The sealer

was removed from the "U233" spike bottle and the calculated

amount of solution was squirted into the uncapped empty bottle.

Both bottles were reweighed after quickly resealing and wiping

as before. Sufficient nitric acid (1+19) was added to the

bottle containing the next dilution, to dilute it to the re- quired concentration of ^^^U. The amount of spike added was

93
i calculated from the weight change of the "U233" spike bottle ' and the aliquot receiving bottle, being open and subject to evaporation loss, was used as an approximate check. j The SRM samples were spiked as above except the final

dilution was with water to give a nitric acid (1+19) solution

containing 5 mg U/ml

b. Discussion of Procedure - These were the first i uranium SRM samples to have the "U233" spike added by weight ' aliquoting. The amount of "U233" separated isotope needed ! to nearly equal the ^^'*U isotopic content of each sample was calculated. The ^^'*U content of the samples varied over a large range, 0.002 percent to over 1 percent. This meant that

for practical size samples several concentrations of "U233"

separated isotope solution would be required. It was decided

to use 0.5 and 1.0 g samples of the SRMs with additional SRM

samples "spiked" at each change in dilution of the "U233"

solution to provide a check on the dilution accuracy. Two

"U233" separated isotope samples were used for duplication to ensure reliability of the results.

Approximately 0.1 g samples of "U233" separated isotope

were used and diluted to approximately 100 g of solution after

dissolving in nitric acid (1+1).

The sample bottles receiving the "U233" solutions were weighed before and after receiving their aliquot to give a check on the amount of aliquot added. These bottles were open

while receiving the aliquot and lost weight caused by evaporation but they did insure that weight recording errors or possible loss by splattering did not exceed a few milligrams.

The difference in weight of aliquot calculated from the spike

bottle and the receiving bottle was 2 to 7 mg

The moles of "U233" added were calculated using equation

{3} and the moles of SRM calculated using equation {2}. The

aliquoting data is given in Tables 26 and 27.

94
Table 26. Composition of "U233" spiked solutions of

SRM U-930 through SRM U-005, series A. Isotoplc standard

"U233"

tlon No. Aliquot Weight SRM

(g soln

)

(g U3O8)

lA^ 10 ,.9640 1.005215 U-930

It 7..8411 0.998426 U-900

ft 6,.3595 1.006923 U-850

tt 6,.6203 1.003400 U-800

tt 6,.0516 1.037086 U-750

tt 5..2925 1.017446 U-500

12

.9181 0 .506272 U-500

tt 12,.5021 0.999547 U-350

tt 6,.0825 0.997625 U-200

tt 4,.9750 1.004893 U-150

tt 3..3367 1.022924 U-100

12

.1196 0.453468 U-100

tt 11

.1938 0.996259 U-050

tt 7..4311 0.997973 U-030

It 4,.9790 1.009391 U-020

ha^ 12 ,.5783 0.515267 U-020

It 8,.8460 0.517468 U-015

tt 9..5006 0. 880029 U-010

tt 5^.2036 1.177292 U-005

3.6134 micromoles U/g of solution.

^ 0.72232 micromoles U/g of solution.

^ 0.089989 micromoles U/g of solution.

^ 0.017973 micromoles U/g of solution.

95
I Table 27. Composition of "U233" spiked solutions of ' SRM U-930 through SRM U-005, series B. Isotoplc standard

U233"

solution No. Aliquot Weight SRM

V 0 s oln

(g U3O8)

IB^ 10 8180 1 006699 U-930

7 7439 1 002125 U-900

6 6193 1 019177 U-850

II 6 5364 1 004216 U-800

II 6 0349 1 010581 U-750

II 5 2043 1 018733 U-500

2B^ 13 5146 0 508439 U-500

12 7793 1 037252 U-350

6 3829 1 013953 U-200

4 9651 1 015415 U-150

3 5400 1 054223 U-100

3B^ 12 2564 0 463396 U-100

II 11 1232 0 997781 U-050

II 7 6342 0 995414 U-030

II 4 9636 1 009075 U-020

4B^ 12 6011 0 502228 U-020

II 8 4375 0 499685 U-015

II 9 9743 0 897315 U-010

II 5 1542 1 177409 U-005

^ 3-5906 micromoles U/g of solution.

^ 0.71587 micromoles U/g of solution.

^ 0.089050 micromoles U/g of solution.

^ 0.017800 micromoles U/g of solution.

96
4 . Preparation of Spiked SRMs U-0002 and U-970

a. Procedure - The samples were Ignited (Section

6,B,3) transferred to teflon bottles and dissolved as before

(Section 6,B,4). The "U233" separated Isotope samples were diluted to approximately 1 mg of U/ml of solution for the SfM

U-970 samples and aliquots of these "U233" solutions were diluted to approximately 40 yg of U/ml of solution for the

SfM U-0002 samples. The "U233" solution aliquots were weighed

using a 5 ml plastic syringe as before (Section 6,B,5). The

spiked solutions were mixed, diluted with water to give

5 mg U/ml and mixed again.

b. Discussion of Procedure - The analysis of these

two SRM samples was started after the first l6 uranium isotopic

standards were completed. Although these two samples of widely

different isotopic composition were determined at the same

time, all operations in the handling of them were done separately, with care taken to insure that the chance of any cross contamination was minimized.

Six samples of SRM U-970 were spiked with two different

"U233" solutions. On three of the samples ^^^U was added to approximately equal the ^^'*U content of the SRM U-970 and on

the other three, ^^^U was added to approximately equal the

^^^U content.

Six samples of SRM U-0002 were spiked with the more dilute

"U233" solutions, approximately 40 yg U/ml of solution. Four

samples were spiked to equal the ^^^U content. The other two

samples of SRM U-0002 were spiked with enough ^^^U to equal

approximately ten times the ^^'*U content.

The first results on SRM U-970 samples indicated approximately O.k percent difference between samples spiked with

"U233" solution number 1 and those spiked with "U233" solution

2. Direct mass spectrometer determinations on SRM U-970 solutions agreed to within experimental error with samples spiked

with "U233" solution 1. This indicated that solution 2 probably

97
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did not contain as much ^^^U as calculated from the weight of

oxide. The ^^^U content of both solutions was determined by

Isotope dilution using two "U235" separated Isotope solutions

(Table 28). The concentration of ^^^U In "U233" solution 1 as determined by Isotope dilution mass spectrometry agreed to within 0.05 percent of the concentration calculated by using

the weight of oxide. The concentration of ^^^U In "U233"

solution 2 was 0.51 percent less than that calculated from

the weight of oxide. The reason for the low value was not ascertained. Three possibilities suggest themselves: an Impure or contaminated "U233" sample, loss of U3O8 sample, or

an Incorrect weight recorded. Any of these possibilities

could account for the difference but there Is no way of

determining which one. The corrected "U233" spike addition to SRMs U-970 and

U-0002 are given In Tables 29 and 30.

Table 29. Composition of "U233" spiked solutions of

SRM U-970.

ti U233 U-970

Sample No. Soln No. g of soln g of U3O8

17-76

17-77

17-78

17-83

17-84

17-85

3.18399

3.00664

2.95284

4 .67254

4.76840

4.81086

0.675190

0.657376

1 0 .651655

2 0.323509

1 0.323215

2 0 .328024

^ 4.0961 mlcromoles

^ 4.0582 mlcromoles

2 3 3 2 3 3 U/g solution.

U/g solution.

99
Table 30. Composition of "U233" spiked solutions of

SRM U-0002.

"U233" U-0002

Sample No. Soln No. g of soln g of U3O

17-68 1-2^ 0.36219 1.250375

17-69 1-2 4.79^68 1.206183

17-70 2-2^ 0 .62141 1.513389

17-71 2-2 4 . 43024 1.135712

17-72 2-2 4.72277 1.201320

17-73 1-2 4 .63259 1 .211331

^ 0.163531 micromoles ^^^U/g solution.

0.161327 micromoles ^^^U/g solution.

5 . Analysis of SRMs U-005 through U-I50 to Determine

Minor Isotope Composition with Ion Multiplier

Detection

Minor isotope compositions were determined in two steps

by using ion multiplier detection for abundances less than 0.1

atom percent and then by using the standard NBS collector for

abundances greater than 0.1 atom percent. An abundance of 0.1

atom percent was the crossover point at which both methods

were to be used. The source was the standard linear thin lens

without the "Z" focussing plates. A modified high temperature

procedure with a total intensity of approximately 8xio~-^^A was used and the intensity was further red^^jed when the ^^^U

abundance was large enough to saturate the VRE

.

The corrections for bias were made by analyzing SRM U-500

under the same experimental conditions as all other samples.

Because of the gross difference in isotopic composition, SRM

U-500 was usually analyzed on a clean source, but a small

number of samples were analyzed on the same source as the

spiked SRMs just prior to removal of the source for cleaning.

100
There was no significant experimental difference between the

two groups of data and it was added support for the conclusion

that uranium memory was not normally detectable over a large

isotopic composition range. SRM U-200 was analyzed to provide

a minimum of 3 points for comparison of multiplier and standard collector data and is not the preferred method for this

minor isotope abundance. The correction factors were determined to be 1.00^0 and 1.0120 for the ^^"JJ/^^^U and 235^/233^

ratios, respectively. Composition of the minor isotopes was calculated using equation {4} and is summarized in Table 31. [m m. — 1 ^ - — 100 {4}

^SRM "^SRmJ

R^ = Ratio of minor isotope with respect

to ^^^U corrected for bias

micromoles of spike

micromoles of SRM sample

micromoles of minor isotope in

"U233" spike

6 . Analysis of SRMs U-100 through U-930 to Determine

Minor Isotope Composition

The source was the standard thin lens without "Z" focussing plates. A high temperature analytical procedure was used

and the total u"*" ion current was approximately lxlO~-^°A. The

standard practice of analyzing uranium in groups of samples

covering approximately 20 percent in ^^^U Isotopic composition,

and cleaning the source prior to analyzing each group was used

to further minimize the chance of significant memory. Since

the 23 5^/23 sy patio of each SRM was known from previous measurement, corrections for bias were determined internally for

each analysis by calculating the ratio of the theoretical

101

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Table 32 for the average of 3 analyses per solution.

7 . Re-evaluation of Minor Isotope Composition of SRMs

U-030 through U-500 Using the "Z" Source

Subsequent to completion of the intial minor isotope determinations by ^^^U dilution, the "Z" lens source was developed

and became part of the standard Instrumentation. Since it was possible to achieve greater ion current intensities and maintain

the high degree of control necessary to accurately correct for

fractionation effects, a third series of measurements was undertaken to demonstrate an extension of the lower limit for precise

minor isotope measurements and to increase the number of SRMs

with isotope measurements using conventional detection. A high

temperature analytical procedure was used and the total u'*' ion

current ranged from approximately 3^10~^°k for SRM U-500 to 5xio~^°A for SRM U-030. Other than increased signal intensity

there were no major changes in the analytical method. Data

is given in Table 33 for the average of three analyses per

solution

8 . Analysis of SRM U-970 to Determine Minor Isotope

Composition

Two groups of three spiked SRM samples were prepared with

the ^^^U approximating either the ^^"^U or the ^ ^ ^U composition.

The three-fold purpose of the equal atom ^^^U group was to cross check ^^^U by isotope dilution j to provide a spike within

a factor of 3 of the ^^^U composition- and to obtain measurements on a ^^'*U/^^^U ratio other than 1. A low temperature procedure for the "Z" source was used

and the total u"*" ion current was 5-7^10"^ ^A. Larger ion

currents were noi. used because it could not be satisfactorily

demonstrated that the measuring circuit recovered completely

from the effects of R-C response in switching from a 10"^ °A to a 10 ^^A ion current. For this type of measurement, failure to recover was not correctable on the basis of a point

calibration and would result in a systematic enhancement of

103
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the ^^'^U and ^^^U peaks. Each set of equal atom samples were analyzed as a group. Within the equal atom ^^"^U group triplicate analyses were made for all spiked samples and, for the

equal atom ^^^U group, duplicate analyses were made for each

sample

.

The minor isotopes were compared by the normal magnet

scanning procedure and at all times the 235^/238^^ 238^^233^

ratios were measured by alternating between peak tops. The

2 3 5y^2 3 8y patio was used strictly as an indicator of the position on the fractionation curve for each analysis, and the

correction for bias effects was determined by analyzing SRM

U-500 on a cleaned source and under the same experimental

conditions as the spiked samples. The correction factors for

23>tu/ 2 3 3 u^ 2 3 6^/ 2 3 3^ 2 3 8^/ 2 3 3 ^ ^^^^ foUnd tO bO 1.00l4,

1.0042 and 1.0070 respectively. The calculation of isotopic

composition is summarized in Tables 34 and 35.

9 . Analysis of SRM U-0002 to Determine ^^^U by Isotope

Dilution

A high temperature procedure for the "Z" lens source was used and the total u"'' ion current was 4-5xlO~'^°A. The tube

beam valve was part of the standard instrumentation and it was possible to operate with higher collector vacuums than were previously obtainable. For this particular isotopic distribution and signal intensity it was demonstrated that the

collector vacuum was normally too high and scattered particles

rendered baselines at the ^^^U and ^^^U mass positions indeterminate. With respect to the ^^^U ion current of 4-5x10"^ °A the interference was less than one part in 100,000 but was approximately 2-5 percent of the ^^^U and ^^^U peak heights.

The continuous baseline curve was obtained with an estimated collector pressure of l-4xl0~^ torr. At an estimated

pressure of 4x10"^ torr, the scatter tail from the large ^^^U

peak was significant. As the pressure was reduced below IxlO

torr and approached mid-range 10~^ torr, interference from

scattered secondary particles was significant. An optimum

106
Table 34. Minor isotope composition of SRM U-970.

Observed ratios Atom percent

Sample 2 3 3u 2 236U/233U 23^U 2 3 6u No. added

17-76 0 .53642 3..10020 0.276844 1 .66524 0 .14912

3..10089 0.277498 1 .66561 0 .14948

17-77 0 .51545 3..22932 0.288310 1 .66679 0 .14923

3..22489 0.287253 1 .66450 0 .14868

17-78 0 .51545 3

.

0 0 Q 0 r\

. 22o2U r» 0 Q T n n ^0 . 2o

(

92d 1 .66621 0 .14903

3..22743 0.287446 1 .66581 0 .14878

17-83 1 .62776 1 . none l\ 1 .66322 0 14939

1 ,.02133 0.09131^ 1 .66451 0 .14926

1..02147 0.091141 1 .66474 0 .14897

TV ail 1 1 —OH 1 .67819 U .. 9 9 lcf9<d n n ft R Q 7 1 .66560 0 . 14930

U ,. 99IUDD n n ftft Ji ft^i 1 .66520 0 .14911

0..991412 0.088496 1 .66580 0 .14913

17-85 1 .65287 1

,

.00586 0

.

Oo99dO 1 .66458 0 .14931

1,.00621 0.089773 1 .66516 0 .14900

1,.00614 0.089848 1 .66504 0 .14913

Average = 1 .6652 0 .1491

^ Atom percent

.

107
Table 35. ^"^U in SRM U-970 by isotope dilution.

Sample

No. 17-76

17-77

17-78

17-83

17-84

17-85

added

0.53642

0.51545

0.51545

1.62776

1.67819

1.65287

Observed

2 3 Syy 2 3 3

0.969450

0.969326

1.00884

1.00782

1.00725

1.00715

0.319206

0.319655

0.319588

0.309926

0. 310564

0.310254

0.314909

0.314910

0.314968

Average =

Atom percent

2 3 8u

0.52336

0.52330

0.52335

0 .52282

0.52252

0.52249

0.52230

0 .52304

0.52293

0 .52280

0.52383

0.52335

0.52321

0.52321

0.52330

0 .5230

Atom percent.

pressure of approximately 2x10""^ torr gave a spectrum with

no baseline interference and a relatively small scatter con- tribution from the ^^^U peak but this pressure could not be

maintained long enough to complete the ratio measurement.

Thus, the objective of the modified procedure used for this

measurement was to raise the base pressure of the collector

108
from 10"^ torr and maintain it between limits ( <3xlO~^ >8xlO~^

)

that would avoid significant scatter contributions from the

tall of the ^^^U peak or baseline Interference by secondary

particles. To achelve this goal it was necessary to condense

the pumpdown time and to abandon the normal procedure of sample

degassing, and to reduce the long waiting period proceeding

the analysis. The normal pumping time for a uranium analysis

was modified as follows: allow 5 minutes of source pumping

by the mercury diffusion pump; open the tube beam valve for

an additional ten minutes of pumping; add liquid nitrogen to

the source coldfinger and start the degassing procedure.

Immediately after completing the normal 15 minute sample

degassing, the ion accelerating voltage was increased to 10 kV

and the sample was degassed at higher temperatures for another

15 minutes. At the beginning of the high temperature degassing

the u"^ signal intensity was adjusted to 1.3^10~'^°A and was Increased by this increment every 5 minutes until the total

intensity was approximately 4xl0~^°A. After a combined total

of 30 minutes of degassing, the filaments were turned OFF, the

beam valve was closed and the source coldfinger warmed to room

temperature. After a five minute waiting period the analysis

was restarted and a 4xl0~-^°A ion current was obtained in the

prescribed manner for a high temperature analysis during the

next 20 minutes.

Approximately one hour after the start of pumping the

ratio measurement was started with an estimated collector

pressure of l-2xio~^ torr. Extensive pumping either before

or after degassing, even with the beam valve open, was detrimental because the collector vacuum became too high before the

analysis could be completed. Completing the analysis within

2 hours after the start of pumping on the sample was mandatory

in order to avoid detectable interference at the baselines.

The correction for bias was determined by analyzing SRM

U-500 on a cleaned source and under the same experimental

conditions as the spiked samples. The correction factor for

109
the 2

3 5^/2 3 3u patio was found to be 1.0014. The ^^^U In SRM

U-000,2 is given in Table 36. Because of the small probability

of some organic contribution at an ionizing filament temperature of 2l60 °C, a single analysis of most spiked samples was made on the two-stage mass spectrometer using pulse counting

techniques. The measurements were made at a lower ionizing

filament temperature and under conditions of zero detectable

organic background. These results are given in Table 37 for

comparison with the conventional detector data.

Table 36. 'U in SRM U-0002 by isotope dilution.

Observed Atom percent

Sample

No. 17-69

2 3 3 added

0.018248

235U/233U

0.959330

0.960011

0 .961432

0.017530

0.017542

0.017568

17-71 0.017666 0.984336

0.983385

0.985569

0.017549

0.017532

0.017571

17-73 0.017556 0.999208

0.996700

0 .997662

0.017567

0.017522

0.017539

Average 0 .01755

Atom percent

.

110
Table 37. •U and ^^^U in SRM U-0002 by pulse counting

Sample No 17-68

17-70

17-69

17-71

17-72

17-73

^'U added'

0.001330

0.001859

0.018248

0.017666

0.017804

0.017556

Atom percent

U

0 .000164

0 .000162

0 .000157

0 .000158

0.000158

0.01759

0.01762

0.01749

0 .01758

Average = 0 .00016 0 .01757

Atom percent

10 . Analysis of SRM U-0002 to Determine ^^"^U by Pulse

Counting

Because of low Isotoplc composition, pulse counting was

the method of determining the ^^"^U content of SRM U-0002.

These measurements were made by L. J. Moore of the Analytical

Mass Spectrometry Section on the two stage mass spectrometer.

A description of the procedure is given below.

The same set of SRM U-0002 samples, which were spiked with

the "U233" solution and used for determination of the ^^^U

content, also presented an opportunity to determine the ^^'^U content, which was known to be <2 ppm. To provide a statistically useable ^^"^U signal of "^100 counts per second, an equivalent ^^^U signal of approximately 50 million counts was required. The single V filament technique used for the 0.02

ppm trace elements in glass series [8] proved to be inadequate

in terms of total signal intensity available. Therefore, a fourfold modification of the conventional triple filament

technique was used:

111
(1) Ionizing and sample filaments were degassed at

^^2050 °C for one hour.

(2) One-half the usual amount of uranium was used;

i.e. J approximately 50 yg per sample filament.

(3) A 30 minute degassing period was used prior to analysis, with the ionizing filament set at 2150 °C and the sample filament currents set

at 1.75 A.

(4) An ionizing filament temperature of 1900 °C was used during analysis to eliminate the contribution

of organic ions to the uranium spectrum.

A final ^^^U signal of ^1.6 V on a 10^^ ohm input resistor to

the VRE gave the requisite 100 cps ^^"^U signal.

A scan of the mass region from mass 237 through 233 during

an analysis of SRM U-0002 spiked with "U233" solution showed

there was no discernible organic or other contribution to mass position 236, nor in a scan of unspiked SRM U-0002 was there

any evidence of a contribution at the 236 or 233 mass positions.

Therefore, the only correction required for any contribution

to the various mass positions above the normal background was for the scatter due to the tail of the large ^^^U beam. To determine the magnitude of the correction, a count was taken,

immediately prior to analysis, for 100 seconds at the half-mass

positions on either side of each integral mass position of

interest. An average of the two half-mass counting results

then represented an approximate scatter correction for the

corresponding integral mass position. Since the scatter cor- rection for the ^^'*U amounted to percent of the total count,

the accuracy of the analysis was limited largely by the

inability to accurately correct for scatter.

A mass discrimination correction was applied by also

measuring the 2 3 3 yy2 3 5y i^q^2.o and comparing the corresponding

ratio as determined by the conventional triple filament technique. The ^^''U data for the pulse counting measurements is

given in Table 37. 112
C . Results and Discussion

The NBS determined values for 2 3 5^/2 3 ;patlos and the

minor Isotoplc composition by ^^^U dilution are given in Table

38. This data was used to calculate the isotoplc composition

of the least abundant of the ^^^U and ^^®U Isotopes, and com- position of the remaining isotope was determined by difference

Table 38. Summary of NBS determined 2 3 5^/2 3 ratios and

the isotope dilution values of ^^"^U and ^•^^U for

SRMs U-0002 through U-970.

SRM 235U/238U Ratio 2 3 tyS- 2 3 6ua

U-0002 0 . 00016 <0 . 00001

U-OO5 0 . 00 4910 0 . 00210 0 . Q04dd

U-010 0.0101^0 0.005^1 0 . OObol

TT n 1 tr n ri 1 c: vz U . Ul^^DD 0 . 00 050 0 . Ulo 39

TT n 0 r> U-U^U U . U d U 0 J. U . U 1^: 4 0 0 . 01051

TT n 0 n U-U jU U . (J jl4 j (J . u 10;:* / U-050 0 .05278 0 . 02787 0.04804

U-100 0.11360 0.06761 0.03790

U-150 0,18108 0.09934 0.06601

U-200 0.25119 0.12459 0.21168

U-350 0.5^64 0.24970 0.16730

U-500 0.9997 0.51808 0.07547

U-750 3.164 0.59243 0.25014

U-800 4.268 0.65614 0.24448

U-850 6.148 0.64352 0.37015

U-900 10.375 0.77751 0.33246

U-930 17.356 1.08183 0.20265

U-970 186.78 1.66520 0.14912

Atom percent.

113
i The ^^^U values of SRMs U-005 through U-350 and the ^^^U values ; of SRMs U-750 through U-930 were converted to weight percent I composition and are given in Table 39 with measurements by ! Union Carbide Nuclear Company (UCNC), Oak Ridge, Tennessee, I and by Goodyear Atomic Corporation (GAT), Portsmouth, Ohio. |

I Table 39. Comparison of ^^^U and ^^®U measurement by NBS,

UCNC and GAT.

np-pppTn-f- 2 3 5tt SRM NBS UCNC^ GAT^

U-005 0 . 4832 0 .4834 0 .4834

U-010 0.9911 0.9911 0.9915

U-015 1a. • 513 ^ -1. _) 1 513 1 513

U-020 2.013 2.013 2.013

U-030 3.009 3.009 3.008

U-050 4.949 4.948 4 .950

U-100 10.075 10.078 10.077

U-150 15.143 15.139 15 .146 U-200 19.807 19. 812 19.811

U-350 34.899 34.906 34 .904

Weight percent ^^^U

U-750 24.045 24.032 24.030

U-800 19.007 19.018 19.014

U-850 14.006 13.997 14.002

U-900 8.795 8.795 8.796

U-930 5.443 5.445 5.445

The only isotope determined in the SRM was the least

abundant of the two major isotopes.

114
The UCNC and GAT measurements were made by uranium hexafluorlde

anlaysis in which each SRM was compared to a set of bracketing

synthetic calibration mixes prepared from oxide blends of

enriched and depleted uranium. Separation of NBS data into a point calibration group and an interpolation group for comparison with values by UCNC and GAT does not reveal any significant

systematic differences. The magnitude of the deviations for

the interpolation data is, as might be expected, slightly

larger than that for the point calibration group. The only

SRM with a ratio determination by both techniques at NBS is

U-O5O and the agreement is well within experimental error.

The consistency of the NBS interpolation data with respect

to the independent measurements by GAT and UCNC provided the

foundation for evolution of the method into the systems calibration technique. The systems calibration is designed to test the linearity of the measuring circuit over a wide range

of isotopic ratios by evaluating the consistency of the filament bias under identical experimental conditions for all

samples. If the bias is constant, within experimental error,

for each ratio over the range tested, a single correction

factor is valid for all isotopic ratios within that range.

SRMs U-930 through U-O5O is the group of standards used for

this measurement and all of the 235^^238^ ratios are within

the limits 20 to 0.05 . The measurement of ion current ratios

outside of this range is subject to errors due to non-linearity

of the measuring circuit and all such ratios must be corrected

using the point calibration technique. The systems calibration

is a very sensitive and rigorous test for defining the linear

response range of the measuring circuit and is recommended for

each new or rebuilt mass spectrometer. Finally, the systems

calibration is useable as an exacting performance test for the

operator who is learning how to apply the rules of a fixed

analytical procedure and is a recommended means of evaluating

operator performance. The body of data accumulated from

systems calibration measurements gives strong support to the

115
assertion that, for the mass spectrometers of this laboratory,

the observed bias Is Independent of Isotoplc composition and

Is primarily a function of evaporation and Ionization In the source j a single correction factor Is valid within the linear

response region of the measuring circuit.

For ^^^U isotope dilution no significant systematic differences were observed for extra SRM samples spiked at each

change in dilution of the stock "U233" solution. A comparison

of minor Isotoplc composition by ion multiplier detection and

by conventional detection with a "Z" source for the SRMs U-200,

U-I50 and U-100, indicated no systematic differences, and thus,

an accurate correction for the additional bias effects of ion

multiplier measurements was made. Multiplier measurements were

also more imprecise than conventional detection with the "Z" lens source. The "Z" lens source made it possible to use the

conventional NBS collector in determining the minor isotoplc

composition of all SRMs except U-020 through U-005 and the

2^'*U composition of SRM U-0002.

Analysis of unsplked SRMs, as control samples, along with

the spiked SRMs revealed that minor isotoplc composition for

the unsplked samples was systematically greater {'\'0 .2%) than

the Isotope dilution values. This apparent enrichment was attributed to a non-linear effect of the measuring circuit

when comparing such large ion current ratios. When the minor

Isotope composition of SRMs U-500 through U-O3O were redetermined using the "Z" lens source, unsplked solutions were again

analyzed as control samples but with special precautions to minimize the effects of non-linear response. The values of

the VRE decade resistors were carefully checked, the ^^^U

signal intensity was maintained below non-ohmlc response levels

and the magnet scan rate was reset to give approximately one minute between peak tops. The Isotoplc composition of the

unsplked samples was again greater than the isotope dilution

values but the difference was less than 0.1 percent. On the

basis of these and a limited number of subsequent measurements

116
I it is believed that nearly all of the original difference between the spiked and unspiked data is caused by a R-C response

contribution for an ion current ratio of 100.

SRM U-970 was the only sample for which an attempt was

made to determine ^^^U composition by isotope dilution, and

was also the only effort to measure ^^"^U with a spike that was

not approximately the same magnitude. This limited test gave

no indication of dependence of ^^'*U composition on the magnitude of the spike. The ^^^U isotope dilution of the SRMs was inferior to the point calibration technique of determining

^^^U composition because of the detrimental effect of propagating small systematic errors ('^^0.02^ per mass unit) over a 5 mass unit range. For highly enriched ^^^U samples in which

the ^^^U composition is at the parts per million level, isotope

dilution may offer some advantages in spite of the error propagation

The certified composition (atom percent) of the uranium

SRMs is given in Tables 40 and 4l. All of the ^^''U and ^^^U

values are the results of NBS determinations by ^^^U isotope

dilution. The ^^^U and ^^^U values were calculated, where applicable, from point calibration measurements of the 235^^233^

ratio at NBS. For SRMs with 2 3 5^/2 3 p^tio determinations at NBS using the systems calibration technique, the values of NBS,

GAT and UCNC were given equal weight and averaged to yield a certified value. Some small and statistically insignificant

differences between minor isotope values in the certified data

tables and the NBS data of Table 38 exist, because once a certificate is issued, it is not reissued to reflect shifts of

a few parts in ten thousand as additional data is accumulated.

The uncertainty statement for the isotoplc SRMs is the 95%

confidence limits for a single determintion because this is

the limiting error statement that can be made for an isotoplc

measurement and is always approximately equal to the overall

limit of error. The overall limit of error is the sum of the

95% confidence limits for the ratio determination and other

117
Table 40. Certified atom percent composition of SRMs U-0002

through U-150

.

SRM

U-0002

U-005

U-010

U-015

U-020

U-030

U-050

U-100

U-150

0.00016

± .00001

0.00218

±.00004

0.00541

±.00005

0.00850

±.00009

0.0125

±.0001

0.0190

±.0001

0 .0279

±.0001

0 .0676

±.0002

0.0993

±.0002

2 3 5u 0.01755

±.00005

0.4985

±.0005

1.0037

±.0010

1.5323

±.0015

2.038

±.002

3.046

±.003

5 .010

±.005

10.190

± .010 15.307

±.015

2 3 6u

<0. 00001

0.00466

±.00005

0.00681

±.00007

0.0164

±.0001

0.0165

±.0001

0.0204

±.0001

0.0480

±.0002

0.0379

±.0001

0.0660

±.0002

2 3 8u 99.9823

±.0001

99 .504

±.001

98.984

±.001

98.443

±.002

97.933

±.002

96.915

±.003

94.915

±.005

89.704

±.010

84.528

±.015

118
Certified

through U' 23^U

0.1246

±.0003

0.2498

±.0006

0.5181

±.0008

0.5923

±.0009

0.6563

±.0013

0.6437

±.0014

0.7777

±.0015

1.0812

±.0020

1.6653

±.0017

atom percent

970.

composition of SRMs U-200

2 3 5 20.013

±.020

35.190

±.035

49 .696

±.050

75.357

±.025

80.279

±.021

85.137

±.017

90.196

±.011

93.336

±.010

97.663

±.003

2 3 6U 0.2116

±.0006

0.1673

±.0005

0.0755

±.0003

0.2499

±.0008

0.2445

±.0007

0.3704

±.0011

0.3327

±.0010

0.2027

±.0006

0.1491

±.0005

2 3 8u 79.651

±.021

64.393

±.036

49.711

±.050

23.801

±.024

18.820

±.019

13.848

±.014

8.693

±.008

5.380

±.005

0.5229

±.0006

119
known sources of possible systematic error and is determined

by performing the necessary experimentation to evaluate each

bias component. The overall limit of error has also been

determined for boron [5], rubidium [6], copper [13], magnesium

[1^], chromium [15], silver [16], chlorine [17], bromine [l8]

and lead [19] as part of the NBS absolute isotopic abundance

and atomic weight program. For each of these elements the

magnitude of the overall limit of error is approximately the same as the 95^ confidence limits for a single ratio determination. Thus, usage of the confidence limits for a single

determination to establish a minimum error statement which

approximates the summation of all uncertainty components

commonly associated with NBS isotopic abundance measurements

is based on an extensive volume of experimental data.

ACKNOWLEDGMENTS

We are indebted to the following: John L. Hague,

Inorganic Standards Coordinator, for his role in establishing

the uranium chemistry laboratory and for his counsel in

developing the uranium chemical procedures; Mrs. Martha S. Richmond for assistance with the preparation of most of the

^^^U Isotope dilution samples; Mrs. Joy Shoemaker for skill,

dedication and infinite patience in typing and assembling

the manuscript.

120
8 . REFERENCES

[1] Menls, 0., Editor, NBS Technical Note 275, 73 pages,

U.S. Government Printing Office, Washington, D.C. 20402

(1965).

[2] Taylor, J. K. , Editor, NBS Technical Note 273, 7^ pages,

U.S. Government Printing Office, Washington, D.C. 20402

(1965) . [3] Menls, 0., Editor, NBS Technical Note 402, 80 pages,

U.S. Government Printing Office, Washington, D.C. 20402

(1966) . [4] Taylor, J. K. , Editor, NBS Technical Note 403, 79 pages,

U.S. Government Printing Office, Washington, D.C. 20402

(1966).

[5] Catanzaro, E. J., Champion, C. E., Garner, E. L.,

Marlnenko, G., Sappenfleld, K. M. and Shields, W. R.,

NBS Special Publication 260-17, 70 prages, U.S. Government

Printing Office, Washington, D. C. 20402 (1970).

[6] Catanzato, E. J., Murphy, T. J., Garner, E. L. and

Shields, W. R., J. Res. NBS, 73A (Phys. and Chem.) 5,

511-516 (1969).

[7] Paris, J. P., Buchanan, R. P., Anal. Chem., 36_, 1157-

1158 (1964).

[8] Shields, W. R., Editor, NBS Technical Note 546, 110

pages, U.S. Government Printing Office, Washington, D.C.

20402 (1970).

[9] Shields , W. R.

,

U.S. Government

(1966) . [10] Shields , W. R.

,

U.S. Government

(1967) . [11] Shields, W. R.

,

U.S. Government

(1968) . [12] Shields, W. R. , U.S. Government

(1970).

Editor, NBS Technical Note 277, 99 pages.

Printing Office, Washington, D.C. 20402

Editor, NBS Technical Note 426, 53 pages.

Printing Office, Washington, D.C. 20402

Editor, NBS Technical Note 456, 47 pages.

Printing Office, Washington, D.C. 20402

Editor, NBS Technical Note 506, 48 pages.

Printing Office, Washington, D.C. 20402

121
[13] Shields, W. R., Murphy, T. J. and Garner, E. L., J. Res. ] NBS, 6QA (Phys. and Chem. ) 6, 589-592 (1964). | [14] Catanzaro, E. J., Murphy, T. J., Garner, E. L. and j] Shields, W. R. , J. Res. NBS, 70A (Phys. and Chem.) 6, i 453-458 (1966). ! [15] Shields, W. R., Murphy, T. J., Catanzaro, E. J., and

Garner, E. L., J. Res. NBS, 70A (Phys. and Chem.) 2,

193-197 (1966).

[16] Shields, W. R. , Garner, E. L. and Dibeler, V. H., J. Res., 66a (Phys. and Chem.) 1, 1-3 (1962).

[17] Shields, W. R., Murphy, T. J., Garner, E. L. and

Dibeler, V. H., J. Am. Chem. Soc, 84_, 1519-1522 (I962).

[18] Catanzaro, E. J., Murphy, T. J., Garner, E. L. and 1 Shields, W. R., J. Res. NBS, 68A (Phys. and Chem.) 6,

' 593-599 (1964). |

i [19] Catanzaro, E. J., Murphy, T. J., Shields, W. R. and

Garner, E. L., J. Res. NBS, 72A (Phys. and Chem.) 3,

261-267 (1968).

[20] Shields, W. R., Introductory remarks of Symposium on Precise Measurement of Isotopic Abundance Using Uranium

Hexafluoride, Eighth Annual Meeting of ASTM Committee

E-14, Atlantic City, New Jersey, June 27, I960.

[21] Smith, L. A., Variations in the Uranium-235 Content of

Fifteen Ores, Union Carbide Nuclear Company, Oak Ridge

Gaseous Diffusion Plant, 5 pages, January 19, 196I

(K-1462)

[22] Smith, R. P., Eby, R. E. and Turok, C. W. , Variations

in Isotopic Content of Natural Uranium, Union Carbide

Nuclear Company, Paducah Gaseous Diffusion Plant, 9 pages, June 26, 196I (KY-373).

[23] Isabaev, E. N. , Uastov, E. P. and Cherdyntsev, V. V.,

Radiokhimiya, 2, 94 (I96I).

[24] Cherdyntsev, V. V., Orlov, E. P., Isabaev, E. A. and

Ivanov, V. I., Geokhimiya, 840 (I96I).

[25] Thurber, D. L., J. Geophys . Res., 11, 4518-4520

(1962).

[26] Rosholt, J. N., Garner, E. L. and Shields, W. R.,

Science, 139, 224-226 (I963).

122
[27] Rosholt, J. N., Harshman, E. N. , Shields, W. R. and

Garner, E. L., Economic Geology, 59.5 570-585 (1964).

[28] Smith, R. F. and Jackson, J. M. , Variations in ^^"^U Concentration of Natural Uranium, Union Carbide

Corporation, Nuclear Division, Paducah Gaseous Diffusion

Plant, 14 pages, June 25, 1969 (KY-58I).

123

APPENDIX I

The "NBS type" 12-inch radius of curvature 68° analyzer

tube mass spectrometer with the combinations of components

that were standard instrumentation from 196O through 1970 is

shown in Figures 9, 10 and 11. Initially, the 12-inch

instrument (Figure 9) had differential pumping of the source housing and analyzer tube by mercury diffusion pumps. Gate

valves isolated the pumping system so that venting of the source and analyzer tube to atmospheric pressure was accom- plished without loss of vacuum in the diffusion pumps. Both

the source and collector [9] had slit assemblies that were externally adjustable by means of a micrometer screw.

A major modification in the instrumentation of the 12-inch

mass spectrometer during the I963-I966 period was replacement

of the analyzer tube diffusion pump with an electronic pump

(Figure 10). Other significant changes adopted as standard

instrumentation were a cryogenic pump (coldfinger) in the source housing and a small diameter window in the source housing flange.

Figure 11 shows the standard instrument during the I966-

1970 period. The major modifications include the following:

replacement of the gate valves by air-operated valves [11];

installation of a high vacuum beam valve [10,11]; the use of

a slot type mount for both the source and collector [10];

elimination of the externally adjustable slit assembly of the

collector and source; and the use of a "Z" lens source [10].

The "NBS type" 90°, 12-inch radius extended flight path

mass spectrometer is shown in Figure 12. The magnet and

analyzer tube are identical to the first stage of the multistage mass spectrometer [10] developed several years ago. The analyzer tube is mounted in a horizontal plane and is

pumped by a double-ended electronic pump. An air-operated

beam valve is located immediately behind the source housing

125
and is closed except when an analysis is made. The source housing is pumped by a mercury diffusion pump and is isolated

from the pump by another air operated valve. The source [10],

collector [10], expanded scale measuring circuit [9] and the

other electronic components are interchangeable with the

corresponding components of the 68° instrument.

The analytical performance of the 90° mass spectrometer

matches or exceeds that of the "work horse" 68° instrument in

all practical aspects. The 90° instrument has greater transmission efficiency and nearly the same dispersion as the 68°

spectrometer. In addition, all components of the 90° instrument are serviceable with the operator standing on the floor.

The most recent innovation, locating the filament control

panel in the console so that the operator can simultaneously

view the ionizing filament with the pyrometer and adjust the

filament current to obtain a given temperature, is shown in

Figure 12. The 1971 version of the 12-inch 68° analyzer tube mass spectrometer has been modified to include this feature

but will not be completed in time for a photograph to be

prepared for this publication. This modification is a direct

result of the increased use of the optical pyrometer in

measuring the temperature of the ionizing filament. Pyrometer

measurement is the exclusive method of reproducing an optimum

temperature for the ionizing filament for all triple filament

analyses. All of the current single filament procedures - boron, chromium, lead and nickel - are based exclusively on adjusting to temperatures measured with the optical pyrometer

rather than ion current intensity or current through the

filament. Since the introduction of the window in the source housing flange, the use of the pyrometer has increased until

it is now an Integral part of all mass spectrometric analytical

procedures

.

126
127

Figure 11. 12-lnch radius of curvature 68° analyzer tube mass spectrometer from I966 to 1970.

129
130
APPENDIX II

CERTIFICATES OF ANALYSIS

Uranium Isotopic Standard:

PAGE

U-0002 133

U-005 134

u-010 135

U-OI5 136

U-020 137

U-030 138

U-050 139

u-100 mo

U-150 I4l

U-200 142

U-350 143

U-500 144

U-750 145

U-800 146

U-850 147

U-900 148

U-930 149

U-970 150

131

S. Departraenf' of Commerce

MauriceiH. Stans

Certificate of (Analgia

STANDARD REFERENCE MATERIAL U-0002

Uranium Isotopic Standard

Atom Percent Weight Percent

2 3 4U

0.00016

± .00001 .00016

23SU

0.01755

± .00005

.01733

2 3 6U

<0.00001 < .00001

2 3 8 U

99.9823

± .0001 99.9825

1 The material consists of highly purified oxide, UsOg. The atomic weight of the material is

I calculated to be 238.0503 using the nuclidic masses 234.0409; 235.0439; and 238.0508. The value for ^^^U is calculated from measurements made on sampl(\s spfked with high purity ^^^U to approximate the ^^^U concentration, the ratio ^^-^IJ to ^^^U was measured on a triple-filament equipped thermal ionization mass spectrometer with d-c amplifier circuits. Ratio

I determinations were corrected for mass discrimination by measurements made under similar conditions on SRM U-500. The value for ^^'*U is calculated from'meastfl^r^^nts n measQj«rft€nts made on samples spiked with high purity ^^^U, the ratio ^^^U to ^^^IJ was measured on a two stage mass spectrometer using a pulse counting technique. The limits indicated for the isotopic compositions are at least as large as the 95 percent

confidence level for a single determination, and include terms for the inhomogeneities of the

material as well as analytical error. Mass spectrometry measurements at NBS were made by E. L. Garner and L. J. Moore using

solutions prepared by L. A. Machlan, The overall direction and coordination of the technical measurements leading to certification were performed under the chairmanship of W. R. Shields. The technical and support aspects in the preparation, certification and issuance of this standard

reference material were coordinated through the Office of Standard Reference Materials by J. L. Hague.

Washington, D. C. 20234

July 30, 1970

J. Paul Cali, Acting Chief

Office of Standard Reference Materials 133
U. S. DepartAiBnt' of Commerce

Maurice H. Stans Seweisfy

National Burea;U of Standards

A. V. As^i^r^irector Standard Reference Material U-005

Uranium Isotopic Standard

Atom percent

Weight percent

234U

0.00218

±.00004 0.00214

2 35 U

0.4895 ±.0005

0.4833

2 36U

0.00466

±.00005 0.00462

2 38U

99.504

±.001 99.510

The material consists of highly purified oxide. UaOg. The atomic weight of the material

is calculated to be 238.036 using the nuclidic masses 234.0409; 235.0439; 236.0457 and

238.0508. The values for ^^^U and ^^^U were calculated from measurements at the National Bureau of Standards. The samples were spiked with high-purity ^^^U to approximate the ^^^U concentration, the ratios ^^^U to ^^^U and ^^^U to were measured on a triple-filament equipped surface ionization mass spectrometer witlWipn-multiplier amplifier

circuits. The values for ^ ^ ^ U and ^ ^ ^ U were calculateji-f^o'l^^S^lJ^ments of the ^^^Uto^^^U

ratio made at the National Bureau of Stand^d^'onX^r^e-fi^ surface ionization mass

spectrometer equipped with dc amplifi^ ckciats. The observed ratios were corrected for mass discrimination effects by intereo^^iaris^hi sytithetic mixtures prepared at the 0.5 U.

topic concentrations are at least as large as the 95-percent

determination, and include terms for inhomogeneities in the

error. The ^ 3

s u to ^ 3 « U ratio for this standard. 0.004919, is

percent ^

s u level from high-pu The limits indicalet confidence limits^ material as known to al Mass spectpometry measurements at NBS were made by E. L. Garner on solutions prepared by L. A. Machlan. The overall direction and coordination of the technical measurements leading to

certification were performed under the chairmanship of W. R. Shields. The technical and support aspects in the preparation, certification, and issuance of this Standard Reference Material were coordinated through the Office of Standard Reference

Materials by J. L. Hague.

Washington, U. C. 20234

April 21, 1969 W. Wayne Meinke, Chief

Office of Standard Reference Materials

(This certificate supersedes certificate of March 2, 1959) 134
U. S. Departnient' of Commerce

Maurice. H. Stans

National Biuret's of Standards

A. V. Afitibr^irector Standard Reference Material U-010

Uranium Isotopic Standard

Atom percent

Weight percent

2 34U

0.00541 ±.00005 0.00532

235U

1.0037 ±.0010

0.9911

2 3 6U

0.00681 ±.00007

0.00675

2 3 8U

98.984

±.001 98.997

The material consists of highly purified oxide, U3O8. The atomic weight of the material

is calculated to be 238.020 using the nuchdic masses 234.0409; 235.0439; 236.0457 and

238.0508. The values for ^^"^U and ^^^U were calculated from measiirements at the National Bureau of Standards. The samples were spiked with high-purity U to approximate the ^•^^U concentration, the ratios ^^^U to ^^"^U and feK^^^^*|^ere measured on

triple-filament equipped surface ionization mass spectror?teter?w*l^r(5n-multiplier amplifier

circuits. The values for ^ ^ ^ U and ^ ^ ^ U were easurements of the U to U

ratio made at the National Bureau of ^andJ»M>^^fi^^ triple-filament, surface ionization mass

spectrometer equipped with dc<^ns(tolilte^iim^ The observed ratios were corrected for mass discrimination effecta;ijy iJitervprJt^rison with synthetic mixtures prepared at the 1 percent ^ ^ U level fro The limits confidenc

material as known to at h

Uand U.

te(a>t^p^fne isotopic concentrations are at least as large as the 95-percent

t(51<^ingle determination, and include terms for inhomogeneities in the

as>£malytical error. The ^ ^ ^ U to ^ U ratio for tliis standard, 0.010140. is

5t 0.1 percent. Mass spectrometry measurements at NBS were made by E. L. Garner on solutions prepared by L. A. Machlan. The overall direction and coordination of the technical measurements leading to

certification were performed under the chairmanship of W. R. Shields. The technical and support aspects in the preparation, certification, and issuance of this Standard Reference Material were coordinated through the Office of Standard Reference

Materials by J. L. Hague. Washington, D. C. 20234 W. Wayne Meinke, Chief

April 21, 1969 Office of Standard Reference Materials

(This certificate supersedes certificate of March 2, 1959) 135
U. S. Depart^tenf of Commerce Maurice-JI. Stans

of Standards

)irector Certificate

Standard Reference Material U-015

Uranium Isotopic Standard

234^ 23Sjj 236^ 238^ Atom percent 0.00850 1.5323 0.0164 98.443 ±.00009 ±.0015 ±.0001 ±.002 Weight percent 0.00836 1.5132 0.0163 98.462

The material consists of highly purified oxide, UjOg. The atomic weight of the material

is calculated to be 238.004 using the nuclidic masses 234.0409; 235.0439; 236.0457 and

238.0508. The values for ^^'*U and ^^^U were calculated from measurements at the National Bureau of Standards. The samples were spiked with high-purity ^ U to approximate the

^^"^U concentration, the ratios ^^^U to ^^"^U and ^-^^U to ^"^AU were measured on a

triple-filament equipped surface ionization mass spectromete|w^thvQn;Smultiplier amplifier

circuits. Ifv^

^''^

The values for ^ ^ ^ U and ^ ^ ^ U were calculat^^^^n^asurements of the ^ ^ ^ U to ^ ^ ^ U

ratio made at the National Bureau of Stanl3^r(lk on ajtrrple-filament, surface ionization mass

spectrometer equipped with dc aniplm^ c^uKS^^he observed ratios were corrected for mass discrimination effects by iarteXkpmp^soh with synthetic mixtures prepared at the 1.5 percent ^ ^ ^ U level from hi^^t^unity ^^"^^^nd ^ ^ ^ U. The limits indicates for ^he isbtopic concentrations are at least as large as the 95-percent confidence lirnrlATjqr a^^B^ determination, and include terms for inhomogeneities in the

material as^^ett) a^^^mtlytical error. The ^^^U to ^^^U ratio for this standard, 0.015565, is known to at lapt Ok)l percent. Mass spectrometry measurements at NBS were made by E. L. Garner on solutions prepared by L. A. Machlan. The overall direction and coordination of the technical measurements leading to

certification were performed under the chairmanship of W . R. Shields. The technical and support aspects in the preparation, certification, and issuance of this Standard Reference Material were coordinated through the Office of Standard Reference

Materials by J. L. Hague. Washington, D. C. 20234 W. Wayne Meinke, Chief

April 21, 1969 Office of Standard Reference Materials

(This certificate supersedes certificate of October 1, 1958) 136
U. S. Depart^teni^ of Commerce

MauriceiB. Stans

National B^rem of Standards Ol^tTltftC^t^ A. V. A«^^r-^)irector Standard Reference Material U-020

Uranium Isotopic Standard

234u 235^ 236^ 2 38^ Atom percent 0.0125 2.038 0.0165 97.933

±.0001 ±.002 ±.0001 ±.002 Weight percent 0.0123 2.013 0.0164 97.959

The material consists of highly purified oxide, UjOg. The atomic weight of the material

is calculated to be 237.989 using the nuclidic masses 234.0409; 235.0439; 236.0457 and

238.0508. The values for ^^'*U and ^^^U were calculated from measurements at the National Bureau of Standards. The samples were spiked with high-purity i^^^U to approximate the ^^'^U concentration, the ratios ^^^U to ^•^'^U and ^^^U to ^ ^ ^JJ were measured on a triple-filament equipped surface ionization mass spectrom£tenwith Mittj^hultiplier ampHfier

circuits.

-<r**«.^ \\ if^^^ The values for ^ ^ ^ U and ^ ^ * U were calculatpTfiYo^^easurements of the ^ ^ ^ U to ^ ^ ^ U

ratio made at the National Bureau of Stap^WcdsAon ajiripTe-filament, surface ionization mass

spectrometer equipped with dc amph^^cil^mtsr^he observed ratios were corrected for mass discrimination effects by r^teSkpmpMisc^ with synthetic mixtures prepared at the 2

percent ^ ^ ^ U level from higl<^puri|y ^^OytF^nd ^ ^ ^ U. The limits indjealeilfor me isotopic concentrations are at least as large as the 95-percent

confidence lipw^^o^t,^a^^i^ determination, and include terms for inhomogeneities in the

material as we^asorniJ^ical error. The ^^^U to ^^^U ratio for this standard. 0.02081. is known to at least O.T percent. Mass spectrometry measurements at NBS were made by E. L. Garner on solutions prepared by L. A. Machlan. The overall direction and coordination of the technical measurements leading to

certification were performed under the chairmanship of W. R. Shields. The technical and support aspects in the preparation, certification, and issuance of this Standard Reference Material were coordinated through the Office of Standard Reference

Materials by J. L. Hague. Washington, D. C. 20234 W. Wayne Meinko. Chief

April 21, 1969 Office of Standard Reference Materials

(This certificate supersedes certificate of October 1, 1958) 137
U. S. Departitoent' of Commerce

Maurice, H- Stans

National Burekti of standards ^^^^^

A. V. A«^r^irector Standard Reference Material U-030

Uranium Isotopic Standard

234^ 23Su 236^ 2 3St^J Atom percent 0.0190 3.046 0.0204 96.915

±.0001 ±.003 ±.0001 ±.003 Weight percent 0.0187 3.009 0.0202 96.953

The material consists of highly purified oxide, UaOg. The atomic weight of the

material is calculated to be 237.958 using the nucHdic masses 234.0409; 235.0439; 236.0457 and 238.0508. ^

The values for ^^'*U and ^^^U were calculated from>^e£^My^ewt^^t the National Bureau of Standards. The samples were spiked with hidii^mirety n^^v-to approximate the

^ ^ ^ U concentration, the ratios ^^^U to ^''*U ani;;^,^^'^!|6^ U were measured on a

triple-filament equipped surface ionization ma^ aoectrtomker with ion-multiplier amplifier

circuits. Y\ v>v_-^y ! The values for ^ ^ ^ U and ^ ^ ^'uJ vi^reT^cufated from measurements of the ^ ^ ^ U to i ^

^ ' U ratio made at the Natijaj^^miau c^Smidards on a triple-filament, surface ionization mass spectrometer equippjra~\^hh^^ circuits. The observed ratios were corrected

j for mass discrimingtighveHectsMy intercomparison with synthetic mixtures prepared at the

3 percent ^ ^ ^ lUg^^^^li^^urity ^ ^

s u and ^ 3 ^U. The limits ^dicated for the isotopic concentrations are at least as large as the 95-percent confidence limits for a single determination, and include terms for inhomogeneities in the material as well as analytical error. The ^^s^ to ^^Sjj j-^^Hq f^j. ^j^jg standard, 0.03143, is known to at least 0,1 percent. Mass spectrometry measurements at NBS were made by E. L. Garner on solutions prepared by L. A. Machlan. The overall direction and coordination of the technical measurements leading to

certification were performed under the chairmanship of W. R. Shields. The technical and support aspects in the preparation, certification, and issuance of this Standard Reference Material were coordinated through the Office of Standard Reference

Materials by J. L. Hague. Washington, D. C. 20234 W. Wayne Meinke, Chief

April 21, 1969 Office of Standard Reference Materials

(This certificate supersedes certificate of October 1, 1958) 138
S. Departita^ of Commerce

Maurice-JH. Stans

National B|QreSi& of Standards

A. V. Certificate

Standard Reference Material U-050

Uranium Isotopic Standard

234U 235U 236U 238^ Atom percent 0.0279 5.010 0.0480 94.915

±.0001 ±.005 ±.0002 ±.005

Weight percent 0.0275 4.949 0.0476 94.975 The material consists of highly purified oxide, U3O8. The atomic weight of the

material is calculated to be 237.898 using the nuclidic masses 234.0409; 235.0439; 236.0457 and 238.0508. The values for ^^*U and ^ae^j y^^j.^ calculated from Jtiez^^^meMAit the National Bureau of Standards. The samples were spiked with higli^puKkty nr*H^fo approximate the ^^^U concentration, the ratios ^^^U to ^^'^U and-.^^'^^^^i^ ^U were measured on a triple-filament equipped surface ionization mass s|rectr©mwer with ion-multiplier amplifier

circuits. ^ >\ \\. 1/ The values for ^^^U and ^ ^'i|^m-ec%;iHated from measurements of the ^^^U to ^^'U ratio made at the NationalJ&ureau crt^^E^ on a triple-filament, surface ionization mass spectrometer equippe^^^tl^B^^amp^lfier circuits. The observed ratios were corrected

for mass discriminatjpii eifccts intercomparison with synthetic mixtures prepared at the 5 percent 235xne^;^tg^

The limits ^mdicafed for the isotopic concentrations are at least as large as the 95-percent confidofice limits for a single determination, and include terms for inhomogeneities in the material as well as analytical error. The ^^^U to ^^'U ratio for this standard, 0.05278, is known to at least 0.1 percent. Mass spectrometry measurements at NBS were made by E. L. Gamer on solutions prepared by L. A. Machlan, The overall direction and coordination of the technical measurements leading to

certification were performed under the chairmanship of W. R. Shields. The technical and support aspects in the preparation, certification, and issuance of this Standard Reference Material were coordinated through the Office of Standard Reference

Materials by J. L. Hague.

Washington, D. C. 20234 W. Wayne Meinke, Chief

April 21, 1969 Office of Standard Reference Materials

(This certificate supersedes certificate of October 1, 1958)
U. S. Departing of Commerce

John T. €DTfrror, -Secretary

National Bureau of Standards

A. V. iAsJij^, Director Certificate of ^mlv^ii

Standard Reference Material U-100

Uranium Isotopic Standard

Atom percent

Weight percent

0.0676 ±.0002

.0666

.3nu 10.190 ±0.010

10.075

236U 0.0379 ±.0001

.0376

238U 89.704 ±0.010

89.821

The material consists of highly purified oxide, UaOg. The atomic weight of the material is calculated to be 237.741 using the nuclidic masses 234.0409; 235.0439; 236.0457; and 238.0508.

The values for - '^U and ""'U are calculated from measure^^^its "^t^e National Bureau

of Standards. The samples were spiked with high-purity^^Lnto^proximate the ^^*U concentration, the ratios -"U to -'^U and -"U to Jif:^p^et]^^jksured on a triple-filament equipped surface ionization mass spectrometeiyvv'T^o^ amplifier circuits.

The values for and

tional Bureau of Standards^

for mass discriminatioivrg^cT: 10-percent ^'^U level pfppar(

The limifetndi>^t€

icumted from measurements made at the Na-

"•^U ratio. The observed ratios were corrected infercomparison with five synthetic mixtures at the

. high-purity --^^U and ="U.

for the isotopic concentrations are at least as large as the 95- percent confidefi'lse level for a single determination. The -^"^U to -^''U ratio for this standard, 0.11360, is known to at least 0.1 percent.

Mass spectrometry measurements at NBS were made by Ernest L. Garner and William R. Shields on solutions prepared by Lawrence A. Machlan and Martha S. Richmond.

Washington, D.C. 20234 June 23, 1966 W. Wayne Meinke, Chief

Office of Standard Reference Materials

(This certificate supercedes certificate of 7-1-59) 140
S. Departn]^pt of Commerce

John T. COT^j^tr^ecretary

National ^

A. V, standard, £tVtitmtt Ot ^ttalpSlg ^D^rector Standard Reference Material U-150

Uranium Isotopic Standard

234U 23,SU -^aoU =3SU Atom percent 0.0993 15.307 0.0660 84.528 ±.0002 ±0.015 ±.0002 ±0.015 Weight percent .0978 15.143 .0656 84.693 The material consists of highly purified oxide, U.Os. The atomic weight of the material is calculated to be 237.585 using the nuclidic masses 234.0409; 235.0439; 236.0457; and 238.0508. The values for -^^U and - *''Uare calculated from measurements at the National Bureau,

of Standards. The samples were spiked with high-purity -''U to approximate the -"U concentration, the ratios -^^U to "^U and -^'U to -'''U were measured on a triple-filament equipped surface ionization mass spectrometer with d-c amplifier* circuits. The values for and ^^*U are derived from measuremdb^ m^^^^t the National Bureau of Standards, at Union Carbide Nuclear Co., Oak^id|^-^|enk:; and at Goodyear Atomic Corp., Portsmouth, Ohio, each laboratory'9«C^Li^^b«ng given equal weight. Values obtained at NBS are the result of direct/^iS§asa;remOTt of the -'"'U to - '^U ratio using triple filament thermal ionization. The^bsHrvedNra^s were corrected for mass dis- crimination effects by determining the/^stwp^iap/from measurements on standards U-500 and U-100. Experience at NB^hi^isbo'wy), through intercomparison of the standards, and synthetic mixtures at theUO-^0-,^j^d 90-percent -"' U level prepared from highpurity '"^U and -^"^U isotopes_,_m^t^|cons^nt bias for a given procedure can be maintained

over the range of 5- to^^^^B%erc&qt =^ U. Values from Union Carbide and Goodyear

Atomic are based on^d^reAt determinations of the - *'U concentration by oxide dilution and UFg analysis, aji4^'!)^»^]Wth^ calculated using the NBS values for -"U and -'^'U, and

the -^*U value\Q]^ir^ "i^ difference. The limits indicated for the isotopic concentrations are at least as large as the 95- percent confidence level for a single determination. The -'"U to -'''U ratio for this standard, 0.18109, is known to at least 0.1 percent; at the same time the pooled variance for the calibration system is significantly smaller. Mass spectrometry measurements at NBS were made by Ernest L. Garner and Wil- liam R. Shields on solutions prepared by Lawrence A. Machlan and Martha S. Richmond.

Washington, D.C. 20234 W. Wayne Meinke, Chief August 5, 1966 Office of Standard Reference Materials

(This certificate supersedes certificate of 7-1-59) 1^1
U. S. DepartmiS of Commerce

John T. eDTniOTr?ecretary

National E^^^i Standards

A. V. Ap*i;pSrector Certificate of ^nalpsig

Standard Reference Material U-200

Uranium Isotopic Standard

234U 235U 236U 238U Atom percent 0.1246 ±.0003

.1229

20.013 ±0.020

19.811

0.2116 ±.0006

.2103

79.651 ±0.021 Weight percent 79.856 The material consists of highly purified oxide, UsO.. The atomic weight of the material is calculated to be 237.440 using the nuclidic masses 234.0409; 235.0439; 236.0457; and 238.0508. The values for =^*U and ""U are calculated from measurements at the National Bureau

of Standards. The samples were spiked with high-purity -"U to approximate the -^*U concentration, the ratios -^^U to "^U and ="U to -^"U were measured on a triple-filament equipped surface ionization mass spectrometer with d-c ampl^er circuits. The values for -^^U and -^*U are derived from measur^mentsN^n^e at the National Bureau of Standards, at Union Carbide Nuclear Co., G&k R^^K^Tfenn., and at Goodyear Atomic Corp., Portsmouth, Ohio, each laboratopfl^^-^^^ueU^bemg given equal weight. Values obtained at NBS are the result of dir^^^^^^^ur&ment of the =^^U to ^^''U ratio using triple filament thermal ionization. T^e ((ibsery^av*'atios were corrected for mass disThe limit\>indicated for the isotopic concentrations are at least as large as the 95- percent confidence level for a single determination. The -^'U to ^''''U ratio for this standard, 0.25126, is known to at least 0.1 percent; at the same time the pooled variance for the calibration system is significantly smaller. Mass spectrometry measurements at NBS were made by Ernest L. Garner and William R. Shields on solutions prepared by Lawrence A. Machlan and Martha S. Richmond.

Washington, D.C. 20234 W. Wayne Meinke, Chief June 1, 1966 Office of Standard Reference Materials
S. Departn^^ of Commerce

John T. Comar, Secretary

Sfational B^jg^^ Standards

A. V. LSjpif^fii rector Certificate of ^nalpgis!

Standard Reference Material U-350

Uranium Isotopic Standard

234U ^:<^U -^«U -^^U Atom percent 0.2498 35.190 0.1673 64.393 ±.0006 ±0.035 ±.0005 ±0.036 Weight percent .2467 34.903 .1667 64.684 The material consists of highly purified oxide, U^Og. The atomic weight of the material is calculated to be 236.979, using the nuclidic masses 234.0409; 235.0439; 236.0457; and 238.0508. The values for --^^U and -^"U are calculated from measurements at the National Bureau

of Standards. The samples were spiked with high-purity -^HI to approximate the concentration, the ratios -"U to -'^^U and -•^*U to -"TJ were measu^i^ on a triple-filament equipped surface ionization mass spectrometer with d-c ampler ch^QU^. The values for ^^'U and -^*U are derived from mea^rem^rw-made at the National Bureau of Standards, at Union Carbide NuclearJ^^o^^^Sfc^^c^ge, Tenn., and at Goodyear Atomic Corp., Portsmouth, Ohio, each laboD^forVe^alue being given equal weight. Values obtained at NBS are the resultof^irect iTMasurement of the -'^''U to -^*U ratio using triple filament thermal ionizatu)nOt^eW)se^v^d ratios were corrected for mass dis- crimination effects by determiniflg'^vthe ~§^tm bias from measurements on standards U-500 and U-100. Experien£»^t ||jBK;'<|idS^hown, through intercomparison of the standards, and synthetic mixt^jyc^^s^^^t^ 10-,^'50-, and 90-percent -''U level prepared from highpurity ''^'U and -'"'U iso1(dpes,^at a constant bias for a given procedure can be maintained

over the range ^'^j t<>v5!54i)ercent U. Values from Union Carbide and Goodyear Atomic are baseid'di? direct determinations of the -

' U concentration by oxide dilution and

UFr, analysis, a;^'d tfbki the ratio calculated using the NBS values for -'"U and '^'U, and

the -^®U value obtained by difference. The limits indicated for the isotopic concentrations are at least as large as the 95- percent confidence level for a single determination. The -*''U to -'^U ratio for this standard, 0.5465 is known to at least 0.1 percent; at the same time the pooled variance for the

calibration system is significantly smaller. Mass spectrometry measurements at NBS were made by Ernest L. Garner and William R. Shields on solutions prepared by Lawrence A. Machlan and Martha S. Richmond.

Washington, D.C. 20234 W. Wayne Meinke. Chief May 23, 1966 Office of Standard Reference Materials

(This certificate supersedes certificate of 3-2-59) 143
Certificate of Snalpsiis!

Standard Reference Material U-SOO

Uranium Isotopic Standard

234U ...U 23r.U .,38U Atom percent 0.5181 49.696 0.0755 49.711 ±.0008 ±0.050 ±.0003 ±0.050 Weight percent .5126 49.383 .0754 50.029

The material consists of highly purified oxide, UaOg. The atomic weight of the material is calculated to be 236.534, using the nuclidic masses 234.0409 ; 235.0439 ; 236.0457

and 238.0508. The values for ^^^U and "'^U are calculated from measuretnents at tlM^fational Bureau

of Standards. The samples were spiked with high-purity-:'='* 'U to approximate the =^-'U concentration, the ratios "^U to ^^'^U and -^^U to -^.(^lif Vere measured on a triple-filament equipped surface ionization mass spectrometef\W^h d-c amplifier circuits.

The values for "^U and ='*U wfere\^alcii|Ated from measurements made at the National Bureau of Standards of the - '^U t^^^^JJ ratio. The observed ratios were corrected for mass discrimination effects by iiitercomparison with five synthetic mixtures at the 50-percent "'^U level prepared from high-purity "^U and "^V. The limits irniicate'd for the isotopic concentrations are at least as large as the 95- percent confidence level for a single determination. The -^""U to ^^^U ratio for this standard, 0.9997, is known to at least 0.1 percent. Mass spectrometry measurements at NBS were made by Ernest L. Garner and William R. Shields on solutions prepared by Lawrence A. Machlan and Martha S. Richmond.

Washington, D.C. 20234 W. Wayne Meinke, Chief May 24, 1966 Oflice of Standard Reference Materials

(This certificate supersedes certificate of 12-16-63) 144
S. Departmept of Commerce "":-rr?rri certificate of ^nalpgis

A. V. ;A^^ Director V j'j y ^ Standard Reference Material U-7S0

Uranium Isotopic Standard

Atom percent 0.5923 ±.0009

Weight percent .5880

.35U 75.357 ±0.025

75.129

23GU 0.2499 b.0008

.2502

23.801 ±0.024

24.033 The material consists of highly purified oxide, U^O.. The atomic weight of the material is calculated to be 235.756, using the nuclidic masses 234.0409; 235.0439; 236.0457; and 238.0508. The values for -^'U and -*"U are calculated from measurements at the National Bureau

of Standards. The samples were spiked with high-purity -"U to approximate the -^'U concentration, the ratios -^^U to -^*U and =^^11 to -^^'U were measured on a triple-filament equipped surface ionization mass spectrometer with d-c amplifier circuits. The values for =^''U and -''*U are derived from measureme^vts rMade.at the National Bureau of Standards, at Union Carbide Nuclear Co., Oak^^flidf^i^ny^^^nd at Goodyear

Atomic Corp., Portsmouth, Ohio, each laboratory'Sj^4;:aJu». bra^ equal weight. Values obtained at NBS are the result of direct m€^sf'Sfen?^>^f the -''U to -'"'^U ratio using triple filament thermal ionization. Thej)b^ve<f\ra|jlos were corrected for mass dis- m^ia^rom measurements on standards

tttfough intercomparison of the standa)^d ^0-percent - '"U level prepared from highbias for a given procedure can be maintained

Values from Union Carbide and Goodyear

Atomic are based on^ir^^t deMrmihations of the '''U concentration by oxide dilution and UF, analysis, anjd^tj^e)^^th^t^St^^ calculated using the NBS values for -"'U and ="'U, and

the -"-'U valuei^J]^i^(M"t^difference. The limits i^x|.icated for the isotopic concentrations are at least as large as the 95- percent confidence level for a single determination. The ^''^^U to -"'^U ratio for this standard, 3.166, is known to at least 0.1 percent; at the same time the pooled variance for the

calibration system is significantly smaller. Mass spectrometry measurements at NBS were made by Ernest L. Garner and William R. Shields on .solutions prepared by Lawrence A. Machlan and Martha S. Richmond.

crimination effects by determining the^ U-500 and U-900. Experience at NB^h/

ards, and synthetic mixtures at the^0-\50-

purity -^'U and -'*U isotopes, 45at 41 com

over the range of 5- to/S^^ri

Washington, D.C. 20234 February 11, 1966 W. Wayne Meinke, Chief

Office of Standard Reference Materials

(This certificate supersede rtificate of ;i-2-, 145
U. S. Departn]^ of Commerce

John T. Gor^l^t, Secretary

National I^^^fe Standards

A. V. AS!tt^D<rector Certificate of ilnalpgis;

Standard Reference Material U-800

Uranium Isotopic Standard

234U 235U 236U 23SU Atom percent 0.6563 ±.0013

.6519

80.279 ±0.021

80.088

0.2445 ±.0007

.2450

18.820 ±0.019

Weight percent 19.015 The material consists of highly purified oxide, UiOs. The atomic weight of the material is calculated to be 235.606, using the nuclidic masses 234.0409; 235.0439; 236.0457; and 238.0508. The values for -^^U and -'"'U are calculated from measurements at the National Bureau

of Standards. The samples were spiked with high-pui'ity --^^U to approximate the -^'U concentration, the ratios -^'U to -"U and -"U to -^''U were measured on a triple-filament equipped surface ionization mass spectrometer with d-c amplifier circuits. The values for ='^U and -'^"U are derived from measurements made at the National Bureau of Standards, at Union Carbide Nuclear Co., Oak Ridge, Tenn., and at Goodyear Atomic Corp., Portsmouth, Ohio, each laboratory's value being given equal weight. Values obtained at NBS are the result of direct measurement of the -'^^U to -''"U ratio I using triple filament thermal ionization. The observed ratios were corrected for mass dis- crimination effects by determining the system bias from measurements on standards i U-500 and U-900. Experience at NBS has shown, through intercomparison of the stand- ! ards, and synthetic mixtures at the 10-, 50-, and 90-percent -"''U level prepared from high- purity "'U and -^'^U isotopes, that a constant bias for a given procedure can be maintained

over the range of 5- to 95-percent -'*U. Values from Union Carbide and Goodyear I Atomic are based on direct determinations of the -'""U concentration by oxide dilution and 1 UFg analysis, and then the ratio calculated using the NBS values for -^^U and -^''U, and i the '^'^U value obtained by difference. The limits indicated for the isotopic concentrations are at least as large as the 95- percent confidence level for a single determination. The -''"U to -^''U ratio for this standard, 4.266, is known to at least 0.1 percent; at the same time the pooled variance for the

calibration system is significantly smaller. Mass spectrometry measurements at NBS were made by Ernest L. Garner and Wil- liam R. Shields on solutions prepared by Lawrence A. Machlan and Martha S. Richmond.

Washington, D.C. 20234 W. Wayne Meinke, Chief February 11, 1966 Office of Standard Reference Materials

(This certificate supersedes !rtificate of 3-6-59 146
. DepartnJl^ of Commerce

>hn T. eon^^ Secretary

itional l u^a^^ Standards

A. V. A^fciF^Di rector Certificate of Snalpgis!

Standard Reference Material U-850

Uranium Isotopic Standard

234U .35U 236U 238U Atom percent 0.6437 ±.0014

.6399

85.137 ±0.017

84.988

0.3704 .±.0011

.3713

13.848 ±0.014

Weight percent 14.001 The material consists of highly purified oxide, UsOk. The atomic weight of the material is calculated to be 235.458, using the nuclidic masses 234.0409; 235.0439; 236.0457; and 238.0508. The values for -'^^U and -^''U are calculated from measurements at the National Bureau

of Standards. The samples were spiked with high-purity -"U to approximate the -^^U concentration, the ratios -^^U to -^*U and "^'U to -''''U were measured on a triple-filament equipped surface ionization mass spectrometer with d-c amplifier circuits. The values for -^'^U and """U are derived from measurements made at the National Bureau of Standards, at Union Carbide Nuclear Co., Oak Ridge, Tenn., and at Goodyear Atomic Corp., Portsmouth, Ohio, each laboratory's value being given equal weight. Values obtained at NBS are the result of direct measurement of the U to -^•'*U ratio using triple filament thermal ionization. The observed ratios were corrected for mass dis- crimination effects by determining the system bias from measurements on standards U-500 and U-900. Experience at NBS has shown, through intercomparison of the standards, and synthetic mixtures at the 10-, 50-, and 90-percent -

' U level prepared from highpurity -

' U and -'"U isotopes, that a constant bias for a given procedure can be maintained

over the range of 5- to 95-percent -''"U. Values from Union Carbide and Goodyear

Atomic are based on direct determinations of the - ''^U concentration by oxide dilution and

UF,; analysis, and then the ratio calculated using the NBS values for -''U and -^^^U, and

the -"U value obtained by difference. The limits indicated for the isotopic concentrations are at least as large as the 95- percent confidence, level for a single determination. The -'"U to -''^U ratio for this standard, 6.148, is known to at least 0.1 percent; at the same time the pooled variance for the

calibration system is significantly smaller. Mass spectrometry measurements at NBS were made by Ernest L. Garner and William R. Shields on solutions prepared by Lawrence A. Machlan and Martha S. Richmond.

Washington, D.C. 20234 February 11, 1966 W. Wayne Meinke, Chief

Office of Standard Reference Materials

(This certificate supersedes certificate of 10-1-58) 147
S. Departn^l^ of Commerce

John T. Goimqr. Secretary

Standards ^rector Cettificate of ^nalpsiisi

Standard Reference Material U-900

Uranium Isotopic Standard

234U 235-Q 236^ 238-[J Atom percent 0.7777 90.196 0.3327 8.693 ±.0015 ±0.011 ±.0010 ±0.008 Weight percent .7735 90.098 .3337 8.795

The material consists of highly purified oxide, U.^Os. The atomic weight of the material is calculated to be 235.301, using the nuclidic masses 234.0409; 235.0439; 236.0457; and 238.0508.

The values for -"U and -^"^U are calculated from measurements at the National Bureau

of Standards. The samples were spiked with high-purity -^^U to approximate the ^''^U con- centration, the ratios "''U to -^^U and -"U to - "'U were me^^iired \)ii^':a triple-filament; equipped surface ionization mass spectrometer with d-c '^pif|fe^^ifcuits. The values for -^'U and -=""11 were calculated from measurements made at the National Bureau of Standards of the =''"U t^^^*^^"!! ratio. The observed ratios were- corrected for mass discrimination effects by inter^compaEison with five synthetic mixtures at the 90-percent "^u level prepare(|;|roija high-p«rity ='"U and -^^U. The limits indicated for the isotopic concentrations are at least as large as the 95- percent confidence level for a single determination. The U to -"'U ratio for this stand- ard, 10.375, is known to at least 0.1 percent.

Mass spectrometry measurements at NBS were made by Ernest L. Garner and Wil- liam R. Shields on solutions prepared by Lawrence A. Machlan and Martha S. Richmond

Washington, D.C. 20234 W. Wayne Meinke, Chief February 11, 1966 Oflice of Standard Reference Materials

(This certificate supersedes certificate of 10-1-58) 148
Departnjyg^ of Commerce

n T. Gori|W|L Secretary

ional l i^^^^^ Standards

A. V. A^fcl^Di rector Certificate of ^nalpsig

Standard Reference Material U-930

Uranium Isotopic Standard

'U 236U 238U

Weight percent

Atom percent 1.0812 ±0.0020

1.0759

93.336 ±0.010

93.276

0.2027 ±.0006

.2034

5.380 ±0.005

5.445 The material consists of highly purified oxide, UmOs. The atomic weight of the material is calculated to be 235.197, using the nuclidic masses 234.0409; 235.0439; 236.0457; and 238.0508. The values for -'"U and -^"U are calculated from measurements at the National Bureau

of Standards. The samples were spiked with high-purity - '^U to approximate the -^'U concentration, the ratios -'''U to -''^U and -'"U to -"'U were measured on a triple-filament equipped surface ionization mass spectrometer with d-c amplifier circuits. The values for -'^'U and -^'^U are derived from measurements jnade at the National Bureau of Standards, at Union Carbide Nuclear Co., Oak Ridge, Tenn., and at Goodyear Atomic Corp., Portsmouth, Ohio, each laboratory's value being given equal weight. Values obtained at NBS are the result of direct measurement of the -'"U to -''''U ratio using triple filament thermal ionization. The observed ratios were corrected for mass dis- crimination effects by determining the system bias from measurements on standards U-500 and U-900. Experience at NBS has shown, through intercomparison of the standards, and synthetic mixtures at the 10-, 50-, and 90-percent -'"'U level prepared from highpurity -''U and -'^U isotopes, that a constant bias for a given procedure can be maintained

over the range of 5- to 95-percent ''^ U. Values from Union Carbide and Goodyear

Atomic are based on direct determinations of the -' ''~U concentration by oxide dilution and

UF,; analysis, and then the ratio calculated using the NBS values for -"U and --^''U. and

the -'"U value obtained by difference. The limits indicated for the isotopic concentrations are at least as large as the 95- percent confidence level for a single determination. The -"'U to -'"U ratio for this standard, 17.349, is known to at least 0.1 percent; at the same time the pooled variance for the

calibration system is significantly smaller. Mass spectrcMiietry measurements at NBS were made by Ernest L. Garner and William R. Shields on solutions prepared by Lawrence A. Machlan and Martha S. Richmond.

Washington, D.C. 20234 W. Wayne Meinke, Chief February 11, 1966 Ofl^ce of Standard Reference Materials

(This certificate si (Ics certificate of IJ-l-r.S) 149
S. Departii^nj' of Commerce

MauriceiSLStans

National Bireat ofi Standards

L. M. Brartse^Btf, Director STANDARD REFERENCE MATERIAL U-970

Uranium Isotopic Standard

234U 235U 236U 238u Atom Percent 1.6653 97.663 0.1491 0.5229 ±0.0017 ±0.003 ±0.0005 ±0.0006 Weight Percent 1.6582 97.663 0.1497 0.5296 The material consists of highly purified oxide, UaOg. The atomic weight of the material is calculated to be 235.045, using the nucUdic masses 234.0409; 235.0439; 236.0457; and 238.0508. The values for ^^'^U and ^^^U are calculated from measurements made on samples spiked with high purity ^

^ to approximate the ^ and ^ ^^U concentrations, the ratios ^•'^Uto^^'*U and ^^^U to ^36u Pleasured on a triple-filament equipped surface iojMzation mass spectrometer with d-c amplifier circuits. Ratio determinations were corrected for mass akcrimination by measure- ments made under similar conditions on SRM U-500. The value for ^^'U is calculated from measureme^4^Mff?«4|o^ ^ ^ U to ^^*U, and calibrated by measurements of the same ratio on synthetic mixtiwfeswrepared from high-purity separated isotopes of ^ ^ ^ U and ^ ^ ' U to approximate thd^ cMnp^sitian of the sample. Because of the response time of the measuring circuit when switc|M«g^^m we^'^'^ peak to the ^

^ ' U peak, the ^ ^ ^ U peak was monitored for 1 minute and ordy^at!a.from)th€last 30 seconds, after the signal had stabilized, was used in the calculations. The ^/^ue for *v^Sns calculated by difference. The limits indicate^csforl the jisotopic compositions are at least as large as the 95 percent confidence level feisM^ftg^idererminat and include terms for the inhomogeneities of the material as well aaVHaly^M^rror. The ^^^U to ^ss^j

j-^^^Jq ^^j. ^y^^^ standard, 186.78, is known to at least 0.15 percerru^ ^

Mass spectrometry measurements at NBS were made by E. L. Garner using solutions prepared by L. A. Machlan. The overall direction and coordination of the technical measurements leading to certification were performed under the chairmanship of W. R. Shields. The technical and support aspects in the preparation, certification and issuance of this Standard Reference Material were coordinated through the Office of Standard Reference Materials by J. L. Hague.

Washington, D. C. 20234

July 9, 1970 150

J. Paul Cali, Acting Chief

Office of Standard Reference Materials
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