A11102 S7EDS3

NBS

PUBLICATIONS

NBS SPECIAL PUBLICATION 260-96

U.S. DEPARTMENT OF COMMERCE/National Bureau of Standards

Standard Reference Materials:

Uranium-235 Isotope Abundance

Standard Reference Materials for

Gamma Spectrometry Measurements

rno

5760- 96

986

.2 B.S. Carpenter, J.W. Gramlich, R.R. Greenberg,

L.A. Machlan, P. DeBievre, H.L. Eschbach, H. Meyer,

J. Van Audenhove, V.E. Connolly, N.M. Trahey, A.C. Zook
Tm he National Bureau of Standards was established by an act of Congress on March 3, 1901. The m 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 measurement 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's technical work is performed by the National Measurement Laboratory, the National Engineering Laboratory, the Institute for Computer Sciences and Technology, and the Institute for Materials Science and Engineering

.

The National Measurement Laboratory

Provides the national system of physical and chemical measurement;

coordinates the system with measurement systems of other nations and

furnishes essential services leading to accurate and uniform physical and

chemical measurement throughout the Nation's scientific community, in- dustry, and commerce; provides advisory and research services to other Government agencies; conducts physical and chemical research; develops, produces, and distributes Standard Reference Materials; and provides

calibration services. The Laboratory consists of the following centers:

Basic Standards2 Radiation Research Chemical Physics

Analytical Chemistry

The National Engineering Laboratory

Provides technology and technical services to the public and private sectors to address national needs and to solve national problems; conducts research in engineering and applied science in support of these efforts; builds and maintains competence in the necessary disciplines required to carry out this research and technical service; develops engineering data and measurement

capabilities; provides engineering measurement traceability services; develops

test methods and proposes engineering standards and code changes; develops and proposes new engineering practices; and develops and improves mechanisms to transfer results of its research to the ultimate user. The

Laboratory consists of the following centers:

Applied Mathematics

Electronics and Electrical Engineering2 Manufacturing Engineering

Building Technology

Fire Research Chemical Engineering2

The Institute for Computer Sciences and Technology

Conducts research and provides scientific and technical services to aid Federal agencies in the selection, acquisition, application, and use of computer technology to improve effectiveness and economy in Government

operations in accordance with Public Law 89-306 (40 U.S.C. 759), relevant Executive Orders, and other directives; carries out this mission by managing

the Federal Information Processing Standards Program, developing Federal ADP standards guidelines, and managing Federal participation in ADP

voluntary standardization activities; provides scientific and technological advisory services and assistance to Federal agencies; and provides the technical foundation for computer-related policies of the Federal Government. The Institute consists of the following centers:

Programming Science and

Technology Computer Systems

Engineering

The Institute for Materials Science and Engineering

Conducts research and provides measurements, data, standards, reference

materials, quantitative understanding and other technical information funda- mental to the processing, structure, properties and performance of materials; addresses the scientific basis for new advanced materials technologies; plans

research around cross-country scientific themes such as nondestructive

evaluation and phase diagram development; oversees Bureau-wide technical programs in nuclear reactor radiation research and nondestructive evaluation; and broadly disseminates generic technical information resulting from

its programs. The Institute consists of the following Divisions:

Ceramics

Fracture and Deformation 3 Polymers

Metallurgy

Reactor Radiation

'Headquarters and Laboratories at Gaithersburg, MD, unless otherwise noted; mailing address Gaithersburg, MD 20899.

2Some divisions within the center are located at Boulder, CO 80303. 'Located at Boulder, CO, with some elements at Gaithersburg, MD.
NBS

RESEARCH

INFORMATION Standard Reference Materials: center Uranium-235 Isotope Abundance Standard

Reference Materials for Gamma

Spectrometry Measurements

B.S. Carpenter, J.W. Gramlich, R.R. Greenberg, and L.A. Machlan

Center for Analytical Chemistry

National Measurement Laboratory

National Bureau of Standards

Gaithersburg, MD 20899

and

P. DeBievre, H.L. Eschbach, H. Meyer, and J. Van Audenhove

Central Bureau for Nuclear Measurements

Commission of the European Communities

B-2440 Geel, Belgium

with V.E. Connolly, N.M. Trahey, and A.C. Zook

Standards and Measurement Branch

Department of Energy

New Brunswick Laboratory

Agronne, IL 60439

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director Issued September 1986
Library of Congress Catalog Card Number: 86-600585

National Bureau of Standards Special Publication 260-96

Natl. Bur. Stand. (U.S.), Spec. Publ. 260-96, 96 pages (Sept. 1986)

CODEN: XNBSAV

U.S. GOVERNMENT PRINTING OFFICE WASHINGTON:

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402
PREFACE Standard Reference Materials (SRM's) as defined by the National Bureau of Standards are "well-characterized materials, produced in quantity, that cali- brate a measurement system to assure compatibility of measurement in the Nation." SRM's 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 SRM's. 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 in arriving at the certified values and of procedures used in producing

SRM's. An NBS series of papers, of which this publication is a member, called

the NBS Special Publication - 260 Series is reserved for the purpose. This 260 Series is dedicated to the dissemination of information on all phases of the preparation of the NBS uranium isotopic SRM for nondestructive

assay. In general, much more detail will be found in this 260 than is gene- rally allowed, or desirable, in scientific journal articles. This enables the user to assess the validity of processes employed, and to learn details of methods utilized for work entailing the greatest care. It is hoped that this

260 will provide sufficient additional information not found on the certificate

so that new applications of these SRM's may be found. Inquiries concerning the technical content of this paper should be directed to the authors. 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

Gaitherburg, MD 20899

Stanley D. Rasberry

Chief

Office of Standard Reference Materials

iii
OTHER NBS PUBLICATIONS IN THIS SERIES

Catalog of NBS Standard Reference Materials

(1984-85 edition), Catherine H. Hudson, ed., NBS

Spec. Publ. 260 (February 1984). $5.50* SN003-

003-02558-5.

Michaelis, R. E., and Wyman, L. L. Standard

Reference Materials: Preparation of White Cast

Iron Spectrochemical Standards. NBS Misc. Publ. 260-1 (June 1964). COM74-1 1061**

Michaelis, R. E., Wyman, L. L., and Flitsch, R., Standard Reference Materials: Preparation of NBS Copper-Base Spectrochemical Standards. NBS Misc. Publ. 260-2 (October 1964). COM74-

11063**

Michaelis, R. E., Yakowitz, H., and Moore, G. A., Standard Reference Materials: Metallographic

Characterization of an NBS Spectrometric LowAlloy Steel Standard. NBS Misc. Publ. 260-3 (October 1964). COM74-1 1060** Hague, J. L. Mears, T. W., and Michaelis, R. E., Standard Reference Materials: Sources of

Information, NBS Misc. Publ. 260-4 (February

1965). COM74- 11059

Alvarez, R., and Flitsch R., Standard Reference

Materials: Accuracy of Solution X-Ray Spectrometric Analysis of Copper-Base Alloys. NBS

Misc. Publ. 260-5 (March 1965). PB168068**

Shultz, J. I., Standard Reference Materials: Methods for the Chemical Analysis of White

Cast Iron Standards, NBS Misc. Publ. 260-6

(July 1965). COM74-I1068**

Bell, R. K., Standard Reference Materials: Methods for the Chemical Analysis of NBS

Copper-Base Spectrochemical Standards. NBS

Misc. Publ. 260-7 (October 1965). COM74-

1 1067** Richmond, M.S., Standard Reference Materials: Analysis of Uranium Concentrates at the

National Bureau of Standards. NBS Misc. Publ. 260-8 (December 1965). COM74-1 1066** Anspach, S. C, Cavallo, L. M. Garfinkel, S. B. Hutchinson, J. M. R., and Smith, C. N., Standard Reference Materials: Half Lives of Materials Used in the Preparation of Standard Reference Materials of Nineteen Radioactive Nuclides

Issued by the National Bureau of Standards NBS Misc. Publ. 260-9 (November 1965). COM74-11065**

Yakowitz, H., Vieth, D. L., Heinrich, K. F. J., and

Michaelis, R. E., Standard Reference Materials: Homogeneity Characterization of NBS Spectrometric Standards II: Cartridge Brass and LowAlloy Steel, NBS Misc. Publ. 260-10 (December

(1965). COM74-11064**

Napolitano, A., and Hawkins, E. G., Standard Reference Materials: Viscosity of Standard

Lead-Silica Glass, NBS Misc. Publ. 260-11 (November 1966). NBS Misc. Publ. 260-11** Yakowitz, H., Vieth, D. L., and Michaelis, R. E., Standard Reference Materials: Homogeneity

Characterization of NBS Spectrometric Standards III: White Cast Iron and Stainless Steel Powder Compact, NBS Misc. Publ. 260-12

(September 1966). NBS Misc. Publ. 260-12**

Spijkerman, J. L, Snediker, D. K., Ruegg, F. C,

and DeVoe, J. R., Standard Reference Materials: Mossbauer Spectroscopy Standard for the Chemical Shift of Iron Compounds, NBS Misc. Publ. 260-13 (July 1967). NBS Misc. Publ.

260-13**

Menis, O., and Sterling, J. T., Standard Reference

Materials: Determination of Oxygen in Ferrous

Materials - SRM 1090, 1091, and 1092, NBS

Misc. Publ. 260-14 (September 1966). NBS Misc.

Publ. 260-14**

Passaglia, E., and Shouse, P. J., Standard Refer- ence Materials: Recommended Method of Use

of Standard Light-Sensitive Paper for Calibrat- ing Carbon Arcs Used in Testing Textiles for Colorfastness to Light, NBS Misc. Publ. 260-15 (June 1967). (Replaced by NBS Spec. Publ.

260-41.) Yakowitz, H., Michaelis, R. E., and Vieth, D. L., Standard Reference Materials: Homogeneity

Characterization of NBS Spectrometric Standards IV: Preparation and Microprobe Characterization of W-20% MO Alloy Fabricated by Powder Metallurgical Methods, NBS Spec.

Publ. 260-16 (January 1969). COM74-1 1062** Catanzaro, E. J., Champion, C. E., Garner, E. L,

Marinenko, G., Sappenfield, K. M., and Shields, W. R., Standard Reference Materials: Boric

Acid; Isotopic and Assay Standard Reference

Materials, NBS Spec. Publ. 260-17 (February

1970). Out of Print. Geller, S. B., Mantek, P. A., and Cleveland, N.

G., Standard Reference Materials: Calibration of NBS Secondary Standard Magnetic Tape (Computer Amplitude Reference) Using the Reference Tape Amplitude Measurement

"Process A," NBS Spec. Publ. 260-18 (November

1969). (See NBS Spec Publ. 260-29.) Paule, R. C, and Mandel, J., Standard Reference

Materials: Analysis of Interlaboratory Measure- ments on the Vapor Pressure of Gold (Certifica- tion of Standard Reference Material 745). NBS

Spec. Publ. 260-19 (January 1970). PB190071**

Paule, R. C, and Mandel, J., Standard Reference

Materials: Analysis of Interlaboratory Measurements on the Vapor Pressures of Cadmium and

Silver, NBS Spec. Publ. 260-21 (January 1971). COM74-11359** Yakowitz, H., Fiori, C. E., and Michaelis, R. E., Standard Reference Materials: Homogeneity

Characterization of Fe-3 Si Alloy, NBS Spec. Publ. 260-22 (February 1971). COM74-1 1357**

Napolitano, A., and Hawkins, E. G., Standard

Reference Materials: Viscosity of a Standard

Borosilicate Glass, NBS Spec. Publ. 260-23 (December 1970). COM71-00157**

Sappenfield, K. M., Marineko, G., and Hague, J. L., Standard Reference Materials: Comparison

of Redox Standards, NBS Spec. Publ. 260-24 (January 1972). COM72-50058**

iv
Hicho, G. E., Yakowitz, H., Rasberry, S. D., and

Michaelis, R. E., Standard Reference Materials: A Standard Reference Material Containing

Nominally Four Percent Austenite, NBS Spec. Publ. 260-25 (February 1971). COM74-1 1356** Martin, J. F., Standard Reference Materials:

National Bureau of Standards-US Steel Corporation Joint Program for Determining Oxygen and Nitrogen in Steel, NBS Spec. Publ. 260-26

(February 1971). 85 cents* PB 81176620

Garner, E. L., Machlan, L. A., and Shields, W. R., Standard Reference Materials: Uranium

Isotopic Standard Reference Materials, NBS

Spec. Publ. 260-27 (April 1971). COM74-

11358**

Heinrich, K. F. J., Myklebust, R. L., Rasberry, S. D., and Michaelis, R. E., Standard Reference Materials: Preparation and Evaluation of SRM's 481 and 482 Gold-Silver and GoldCopper Alloys for Microanalysis, NBS Spec.

Publ. 260-28 (August 1971). COM71-50365**

Geller, S. B., Standard Reference Materials: Cali- bration of NBS Secondary Standard Magnetic Tape (Computer Amplitude Reference) Using

the Reference Tape Amplitude Measurement

"Process A-Model 2," NBS Spec. Publ. 260-29 (June 1971). COM71-50282

Gorozhanina, R. S., Freedman, A. Y., and

Shaievitch, A. B. (translated by M. C. Selby), Standard Reference Materials: Standard

Samples Issued in the USSR (A Translation from the Russian). NBS Spec. Publ. 260-30 (June

1971). COM7 1-50283**

Hust, J. G., and Sparks, L. L., Standard Reference

Materials: Thermal Conductivity of Electrolytic Iron SRM 734 from 4 to 300 K, NBS Spec. Publ. 260-31 (November 1971). COM71-50563**

Mavrodineanu, R., and Lazar, J. W., Standard

Reference Materials: Standard Quartz Cuvettes, for High Accuracy Spectrophotometry, NBS

Spec. Publ. 260-32 (December 1973). 55 cents* SN003-003-0 1213-1 Wagner, H. L., Standard Reference Materials: Comparison of Original and Supplemental SRM 705, Narrow Molecular Weight Distri- bution Polystyrene, NBS Spec. Publ. 260-33 (May 1972). COM72-50526**

Sparks, L. L., and Hust, J. G., Standard Reference

Materials: Thermoelectric Voltage, NBS Spec.

Publ. 260-34, (April 1972). COM72-50371**

Sparks, L. L., and Hust, J. G., Standard Refer- ence Materials: Thermal Conductivity of

Austenitic Stainless Steel, SRM 735 from 5 to 280 K, NBS Spec. Publ. 260-35 (April 1972.) 35 cents* COM72-50368**

Cali, J. P., Mandel, J., Moore, L. J., and Young, D.

S., Standard Reference Materials: A Referee Method for the Determination of Calcium in Serum, NBS SRM 915, NBS Spec. Publ. 260-36 (May 1972). COM72-50527**

Shultz, J. I. Bell., R. K. Rains, T. C, and Menis,

O., Standard Reference Materials: Methods of Analysis of NBS Clay Standards, NBS Spec.

Publ. 260-37 (June 1972). COM72-50692**

Richmond, J. C, and Hsia, J. J., Standard Refer- ence Materials: Preparation and Calibration of Standards of Spectral Specular Reflectance, NBS Spec. Publ. 260-38 (May 1972). COM72-

50528**

Clark, A. F., Denson, V.A., Hust, J. G., and

Powell, R. L., Standard Reference Materials: The Eddy Current Decay Method for Resistivity Characterization of High-Purity Metals, NBS

Spec. Publ. 260-39 (May 1972). COM72-50529**

McAdie, H. G., Garn, P.D., and Menis, O., Standard Reference Materials: Selection of Thermal

Analysis Temperature Standards Through a Cooperative Study (SRM 758, 759, 760), NBS

Spec. Publ. 260-40 (August 1972.) COM72-

50776** Wood, L. A., and Shouse, P. J., Standard

Reference Materials: Use of Standard LightSensitive Paper for Calibrating Carbon Arcs Used in Testing Textiles for Colorfastness to Light, NBS Spec. Publ. 260-41 (August 1972) COM72-50775**

Wagner, H. L. and Verdier, P. H., eds.. Standard

Reference Materials: The Characterization of Linear Polyethylene, SRM 1475, NBS Spec. Publ. 260-42 (September 1972). COM72-

50944**

Yakowitz, H., Ruff, A. W., and Michaelis, R. E., Standard Reference Materials: Preparation and Homogeneity Characterization of an Austenitic Iron-Chromium-Nickel Alloy, NBS Spec. Publ. 260-43 (November 1972). COM73-50760**

Schooley, J. F., Soulen, R. J., Jr., and Evans, G. A.,

Jr., Standard Reference Materials: Preparation and Use of Superconductive Fixed Point

Devices, SRM 767, NBS Spec. Publ. 260-44 (December 1972). COM73-50037**

Greifer, B., Maienthal, E. J., Rains, T. C, and

Rasberry, S. D., Standard Reference Materials: Powdered Lead-Based Paint, SRM 1579, NBS

Spec. Publ. 260-45 (March 1973). COM73-

50226**

Hust, J. G., and Giarratano, P. J., Standard Refer- ence Materials: Thermal Conductivity and

Electrical Resistivity Standard Reference Materials: Austenitic Stainless Steel, SRM's 735 and

798, from 4 to 1200 K, NBS Spec. Publ. 260-46 (March 1975). SN003-003-0 1278-5*

Hust, J. G., Standard Reference Materials: Electrical Resistivity of Electrolytic Iron, SRM 797, and Austenitic Stainless Steel, SRM 798, from 5 to 280 K, NBS Spec. Publ. 260-47 (February

1974). COM74-50176** Mangum, B. W., and Wise, J. A., Standard Refer- ence Materials: Description and Use of Precision Thermometers for the Clinical Laboratory, SRM 933 and SRM 934, NBS Spec. Publ. 260-48 (May 1974). 60 cents* SN003-003-0 1278-5 Carpenter, B. S., and Reimer, G. M., Standard

Reference Materials: Calibrated Glass Standards for Fission Track Use, NBS Spec. Publ. 260-49 (November 1974). COM74-51185

v
Hust, J. G., and Giarratano, P. J., Standard Refer- ence Materials: Thermal Conductivity and Electrical Resistivity Standard Reference Materials:

Electrolytic Iron, SRM's 734 and 797 from 4 to 1000 K, NBS Spec. Publ. 260-50 (June 1975). $1.00* SNO03-OO3-0 1425-7 Mavrodineanu, R., and Baldwin, J. R., Standard

Reference Materials: Glass Filters As a Standard Reference Material for Spectrophotometry;

Selection; Preparation; Certification; Use-SRM

930, NBS Spec. Publ. 260-51 (November 1975). $1.90* SN003-003-01481-8

Hust, J. G., and Giarratano, P. J., Standard Refer- ence Materials: Thermal Conductivity and Electrical Resistivity Standard Reference Materials 730 and 799, from 4 to 3000 K, NBS Spec. Publ. 260-52 (September 1975). $1.05* SN003-003-

01464-8 Durst, R. A., Standard Reference Materials: Standardization of pH Measurements, NBS

Spec. Publ. 260-53 (December 1975, Revised). $1.05 SN003-003-01551-2

Burke, R. W., and Mavrodineanu, R., Standard

Reference Materials: Certification and Use of

Acidic Potassium Dichromate Solutions as an

Ultraviolet Absorbance Standard, NBS Spec.

Publ. 260-54 (August 1977). $3.00* SN003-003-

01828-7

Ditmars, D. A., Cezairliyan, A., Ishihara, S., and

Douglas, T. B., Standard Reference Materials: Enthalpy and Heat Capacity; Molybdenum SRM 781, from 273 to 2800 K, NBS Spec. Publ. 260-55 (September 1977). $2.20* SN003-003-

01836-8

Powell, R. L., Sparks, L. L., and Hust, J. G., Standard Reference Materials: Standard

Thermocouple Materials, Pt.67: SRM 1967, NBS

Spec. Publ. 260-56 (February 1978). $2.20* SN003-003-0 18864

Cali, J. P. and Plebanski, T., Guide to United

States Reference Materials, NBS Spec. Publ. 260-57 (February 1978). $2.20* PB 277173

Barnes, J. D., and Martin, G. M., Standard Refer- ence Materials: Polyester Film for Oxygen Gas

Transmission Measurements SRM 1470, NBS

Spec. Publ. 260-58 (June 1979) $2.00* SN003-

003-02077 Chang, T., and Kahn, A. H. Standard Reference

Materials: Electron Paramagnetic Resonance

Intensity Standard; SRM 2601, NBS Spec.

Publ. 260-59 (August 1978) $2.30* SN003-

003-01975-5 Velapoldi, R. A., Paule, R. C, Schaffer, R., Mandel, J., and Moody, J. R., Standard Refer- ence Materials: A Reference Method for the Determination of Sodium in Serum, NBS Spec.

Publ. 260-60 (August 1978). $3.00* SN003-003

01978-0

Verdier, P. H., and Wagner. H. L., Standard Refer- ence Materials: The Characterization of Linear Polyethylene (SRM 1482, 1483, 1484), NBS

Spec. Publ. 260-61 (December 1978). $1.70* SN003-003-02006-1

Soulen, R. J., and Dove, R. B., Standard Reference

Materials: Temperature Reference Standard

for Use Below 0.5 K (SRM 768). NBS Spec.

Publ. 260-62 (April 1979). $2.30* SN003-003-

02047-8 Velapoldi, R. A., Paule, R. C, Schaffer, R. Mandel, J., Machlan, L. A., and Gramlich, J. W., Standard Reference Materials: A Reference Method for the Determination of Potassium in Serum. NBS Spec. Publ. 260-63 (May 1979). $3.75* SN003-003-02068

Velapoldi, R. A., and Mielenz, K. D., Standard

Reference Materials: A Fluorescence Standard

Reference Material Quinine Sulfate Dihydrate (SRM 936), NBS Spec. Publ. 260-64 (January

1980). $4.25* SN003-003-02 148-2 Marinenko, R. B., Heinrich, K. F. J., and Ruegg,

F. C, Standard Reference Materials: Micro- Homogeneity Studies of NBS Standard Reference

Materials, NBS Research Materials, and Other Related Samples. NBS Spec. Publ. 260-65

(September 1979). $3.50* SN003-003-021 14-1 Venable, W. H., Jr., and Eckerle, K. L., Standard

Reference Materials: Didymium Glass Filters for Calibrating the Wavelength Scale of Spectro- photometers (SRM 2009, 2010, 2013). NBS Spec.

Publ. 260-66 (October 1979). $3.50* SN003-003-

02127-0

Velapoldi, R. A., Paule, R. C, Schaffer, R., Mandel,

J., Murphy, T. J., and Gramlich, J. W., Standard

Reference Materials: A Reference Method for the Determination of Chloride in Serum, NBS Spec. Publ. 260-67 (November 1979). $3.75* SN003-

003-02136-9 Mavrodineanu, R. and Baldwin, J. R., Standard

Reference Materials: Metal-On-Quartz Filters as a Standard Reference Material for Spectrophoto- metry-SRM 2031, NBS Spec. Publ. 260-68

(April 1980). $4.25* SN003-003-02 167-9 Velapoldi, R. A., Paule, R. C, Schaffer, R., Mandel, J., Machlan, L. A., Garner, E. L., and

Rains, T. C, Standard Reference Materials: A

Reference Method for the Determination of Lithium in Serum, NBS Spec. Publ. 260-69 (July)

1980). $4.25* SN003-003-02214-4 Marinenko, R. B., Biancaniello, F., Boyer, P. A., Ruff, A. W., DeRobertis, L., Standard Reference

Materials: Preparation and Characterization of an Iron-Chromium-Nickel Alloy for Microanalysis, NBS Spec. Publ. 260-70 (May 1981). $2.50* SN003-003-02328-1 Seward, R. W., and Mavrodineanu, R., Standard

Reference Materials: Summary of the Clinical Laboratory Standards Issued by the National Bureau of Standards, NBS Spec. Publ. 260-71 (November 1981). $6.50* SN003-003-02381-7

Reeder, D.J., Coxon, B., Enagonio, D.,Christensen,

R. G., Schaffer, R., Howell, B. F., Paule, R. C,

Mandel, J., Standard Reference Materials: SRM

900, Antiepilepsy Drug Level Assay Standard, NBS Spec. Publ. 260-72 (June 1981). $4.25* SN003-003-02329-9

vi
Interrante, C. G.,and Hicho, G. E., Standard Refer- ence Materials: A Standard Reference Material Containing Nominally Fifteen Percent Austenite (SRM 486), NBS Spec. Publ. 260-73 (January

1982). $2.75* SN003-003-02386-8 Marinenko, R. B., Standard Reference Materials: Preparation and Characterization of K-411 and

K-414 Mineral Glasses for Microanalysis: SRM

470. NBS Spec. Publ. 260-74 (April 1982). $3.50 SN003-003-023-95-7

Weidner, V. R., and Hsia, J. J., Standard Reference

Materials: Preparation and Calibration of First Surface Aluminum Mirror Specular Reflectance Standards (SRM 2003a), NBS Spec. Publ. 260-75 (May 1982). $3.75 SN003-003-023-99-0

Hicho, G. E. and Eaton, E. E., Standard Reference Materials: A Standard Reference Material

Containing Nominally Five Percent Austenite (SRM 485a), NBS Spec. Publ. 260-76 (August

1982). $3.50 SN003-003-024-33-3 Furukawa, G. T., Riddle, J. L., Bigge, W. G., and

Pfieffer, E. R., Standard Reference Materials: Application of Some Metal SRM's as Thermometric Fixed Points, NBS Spec. Publ. 260-77 (August 1982). $6.00 SN003-003-024-34-1

Hicho, G. E. and Eaton, E. E., Standard Reference

Materials: Standard Reference Material Containing Nominally Thirty Percent Austenite (SRM

487), NBS Spec. Publ. 260-78 (September 1982). $3.75* SN003-003-02435-0 Richmond, J. C, Hsia, J. J. Weidner, V. R., and

Wilmering, D. B., Standard Reference Materials: Second Surface Mirror Standards of Specular

Spectral Reflectance (SRM's 2023, 2024, 2025), NBS Spec. Publ. 260-79 (October 1982). $4.50* SN003-003-02447-3

Schaffer, R., Mandel, J., Sun, T., Cohen, A., and

Hertz, H. S., Standard Reference Materials: Evaluation by an ID/ MS Method of the AACC

Reference Method for Serum Glucose, NBS

Spec. Publ. 260-80 (October 1982). $4.75* SN003-

003-02443-1 Burke, R. W.,and Mavrodineanu, R. (NBS retired), Standard Reference Materials: Accuracy in Analytical Spectrophotometry, NBS Spec. Publ. 260-81 (April 1983). $6.00* SN003-003-024-8

Weidner, V. R., Standard Reference Materials: White Opal Glass Diffuse Spectral Reflectance Standards for the Visible Spectrum (SRM's 2015 and 2016). NBS Spec. Publ. 260-82 (April 1983). $3.75* SN003-003-02489-9**

Bowers, G. N, Jr., Alvarez, R., Cali, J. P. (NBS

retired), Eberhardt, K. R., Reeder, D. J., Schaffer, R., Uriano, G. A., Standard Reference

Materials: The Measurement of the Catalytic

(Activity) Concentration of Seven Enzymes in NBS Human Serum SRM 909, NBS Spec.

Publ. 260-83 (June 1983). $4.50* SN003-003-024

99-6

Gills, T. E., Seward, R. W., Collins, R. J., and

Webster, W. C, Standard Reference Materials: Sampling, Materials Handling, Processing, and

Packaging of NBS Sulfur in Coal Standard

Reference Materials, 2682, 2683, 2684, and 2685, NBS Spec Publ. 260-84 (August 1983). $4.50* SNOO3-OO3-O2520-8

Swyt, D. A., Standard Reference Materials: A Look

at Techniques for the Dimensional Calibration of Standard Microscopic Particles, NBS Spec. Publ. 260-85 (September 1983). $5.50* SN003-003-

02521-6

Hicho, G. E. and Eaton, E. E., Standard Reference Materials: A Standard Reference Material

Containing Two and One-Half Percent Austenite, SRM 488, NBS Spec. Publ. 260-86 (December

1983). $1.75* SN003-003-02541-1 Mangum, B. W., Standard Reference Materials: SRM 1969: Rubidium Triple-Point - A Temperature Reference Standard Near 39.30 °C, NBS

Spec. Publ. 260-87 (December 1983). $2.25* SN003-003-02544-5 Gladney, E. S., Burns, C. E., Perrin, D. R., Roelandts, I., and Gills, T. E., Standard Reference Materials: 1982 Compilation of Elemental

Concentration Data for NBS Biological, Geological, and Environmental Standard Reference

Materials, Spec. Publ. 260-88 (March 1984). $7.00* SN003-003-02565-8

Hust, J. G., Standard Reference Materials: A Fine- Grained, Isotropic Graphite for Use as NBS

Thermophysical Property RM's from 5 to 2500 K, NBS Spec. Publ. 260-89 (September 1984). $4.50* SN003-003-02608-5

Hust, J. G., and Langford, A. B., Standard Reference Materials: Update of Thermal

Conductivity and Electrical Resistivity of

Electrolytic Iron, Tungsten, and Stainless Steel, NBS Spec. Publ. 260-90 (September 1984). $3.00* SN003-003-02609-3

Goodrich, L. F, Vecchia, D. F., Pittman, E. S., Ekin, J. W., and Clark, A. F., Standard Reference

Materials: Critical Current Measurements on an NbTi Superconducting Wire Standard Reference

Material, NBS Spec. Publ. 260-91 (September

1984). $2.75* SN003-003-02614-0

Carpenter, B. S., Standard Reference Materials: Calibrated Glass Standards for Fission Track Use (Supplement to NBS Spec. Publ. 260-49). NBS

Spec. Publ. 260-92 (September 1984). $1.50* SN0O3-O03-02610-7

Ehrstein, J., Preparation and Certification of Standard Reference Materials for Calibration of Spreading Resistance Probes, NBS Spec. Publ. 260-93 (January 1985). $2.25* SN003-003-02633-6

Gills, T. E., Koch, W. F, Stolz, J. W., Kelly, W. R., Paulsen, P. J., Colbert, J. C, Kirklin, D. R., Pei, P.T.S., Weeks, S., Lindstrom, R. M., Fleming, R. F., Greenberg, R. R., and

Paule, R. C, Methods and Procedures Used at the National Bureau of Standards to Certify Sulfur

in Coal SRM's for Sulfur Content, Calorific Value, Ash Content, NBS Spec. Publ. 260-94 (December 1984). $2.25* SN003-003-02629-8

vii
Mulholland, G. W., Hartman, A. W., Hembree,

G. G., Marx, E., and Lettieri, T. R., Standard

Reference Materials: Development of a 1 m

Diameter Particle Size Standard, SRM 1690, NBS Spec. Publ. 260-95 (May 1985). SN003-003-

02665-4

Carpenter, B. S., Gramlich, J. W., Greenberg, R. R., and Machlan, L. A., Standard Reference

Materials: Uranium-235 Isotopic Abundance Standard Reference Materials for Gamma

Spectrometry Measurements, NBS Spec. Publ. 260-96

' Send order with remittance to Superintendent

of Documents, US Government Printing Office Washington, DC 20402. Remittance from

foreign countries should include an additional one-fourth of the purchase price for postage. May be ordered from: National Technical Information Services (NTIS). Springfield

Virginia 22161.
ABSTRACTS

The accurate determination of isotope abundances by any method requires

that the measuring systems be calibrated using well characterized isotope reference materials. The National Bureau of Standards (NBS) and the Central Bureau for Nuclear Measurements (CBNM) have jointly produced and certified U3O3 nondestructive assay (NDA) reference samples to be used for calibrating gamma measurements. Five different uranium abundances have been certified (0.31,

0.71, 1.95, 2.95, and 4.46 nominal mass percent, 235U/U) . In this 260 series publication, the material fabrication and certification are described and a discussion of the measurement results affecting the accuracy of gamma spectrometry is given. These certified standards represent the first example of an international effort that establishes traceability to NBS, CBNM, and the basic

SI units. Key words: gamma energy; gamma spectrometry; homogeneity; mass spectrometry;

nondestructive assay; reference material; specifications aluminum; uncertainty; ^-> \J/V i so tope abundances.

ix
DISCLAIMER Certain chemicals, commercial equipment, or instruments are identified in this document in order to adequately specify the experimental procedure. In no cases does such materials identification imply recommendation or endorsement by

the National Bureau of Standards, nor does it imply that the equipment is necessarily the best available for the purpose.

x
TABLE OF CONTENTS

Pages Preface iii Abstract ix Disclaimer x

1. Introduction 1

2. Description and Intended Use 2 3. Preparation of Reference Samples 6

3 . 1 Fabrication of Cans and Plugs 6 3.2 Filling the Cans with 11303 Powder 9 3.3 Sampling for Verification During the Homogeneity Filling

Procedure

.

10 3.4 Ultrasonic Identification System 10

4. Determination of Properties Relevant for Gamma Measurement 12 4.1 Chemical Composition and Impurities 12 4.2 Uranium Content Measurements Performed at CBNM 12 4.3 Uranium Content Measurements Performed at NBL 12 4.4 Chemical Impurities 14 4.5 Determination of the Water Content 14 4.6 Estimation of the Light Element Content 15 4.7 Estimation of the Heavy Element Content 15 4.8 Radioactive Impurities 15 4.9 U30g Surface Sample Density 17 4.10 Can Window Thickness 19 4.11 Evaluation of the Uncertainty Contributions of a ^35^ Abundance Determination by Gamma Counting at CBNM 21

5. 235U/U Characterization 24 5.1 Measurements Made at CBNM by Mass Spectrometry 24 5.2 Measurements Made at NBS by Mass Spectrometry 45 5.4 CBNM Gamma Spectrometry Measurements 55 5.5 NBS Gamma Spectrometry Measurements 64

6. Conclusion and Discussion of Results 72

7. Acknowledgments 77

8. References 78

9 . Appendix 80

xi

1. Introduction

The traditional technique for uranium isotope abundance measurements is a destructive assay using mass spectrometry. In recent years, a number of studies have explored use of gamma spectrometry, a nondestructive method of assay. This method offers a number of advantanges . The cost for the equipment and the analysis are substantially less than the mass spectrometry cost. The equipment can be made portable for in- field measurements, and analysis time is short thereby allowing quick turn-around of results for possible safeguard

applications. In addition, substantial improvements in the accuracy of this method have been made with the use of Certified Reference Materials (CRM)

,

referred to by the National Bureau of Standards as Standard Reference Materials

(SRMs) . This report describes the development of these (CRMs) and the improved accuracy observed with the use of the CRMs in a measurement system.

To assure high quality measurements by any methods, it is necessary to have available accurately certified reference materials similar in composition

to the materials that are commonly measured. This is particularly true of gamma spectrometry SRMs. In this case, the material must be in the form of a chemically and isotopically homogeneous uranium layer that is carefully

adjusted for density and thickness as required when using the 185.7 keV

gamma-line of ^35^ for measurements (1-3). When measured under well defined

conditions as described in this report, the counting rate of this gamma line is proportional to the 235jj isotopic abundance.

In a joint research inititative between the National Bureau of Standards

(NBS) and the Central Bureau for Nuclear Measurements (CBNM) Geel Belgium, an SRM for nondestructive assay of uranium materials using gamma spectrometry has been developed. This SRM consists of a set of five U^Og powders separately

encased in special designed aluminum cans. The nominal ^^^U/U mass abundances

are 0.0031, 0.0071, 0.0194, 0.0295, and 0.0446, respectively. These materials were characterized and certified by CBNM and NBS , with supporting measurements

from the Department of Energy New Brunswick Laboratory. NBS will issue these materials as SRM 969 and CBNM will issue them as EC NRM 171, with a certification report (COM 4153) which is the equivalent to this NBS 260 publication.

This publication describes the material fabrication, a discussion of parameters that affect the measurement accuracy by gamma spectrometry, and the procedures for proper use of the SRM (4,5).

1
2. Description and Intended Use Standard Reference Material (SRM) 969 consists of a set of five uranium

oxide (u"30g) samples in aluminum cans, with isotope mass ratios of uranium- 235

to total uranium ( 235U/U) equal to 0.0031, 0.0071, 0.0194, 0.0295, and 0.0446 (referred to as samples or materials 031, 071, 194, 295, and 446 respectively). Each can (see Figure 1) contains 200.1 g of U3O3 as a pressed powder layer with

a diameter of 70 mm. A mean U^Og specific mass over the sample surface of 5.2 g/cm2 was achieved with sample heights of approximately 2.0 cm for all samples

with the exception of 446 which has a height of 1.5 cm. A surface density of 5 . 2 g/cm2 ensures that the characteristic 185.7 keV

gamma- radiation of 23 -*U, which is emitted through the can "window", is at least 99.9% of the emission rate that can be obtained from an infinitely thick

source. The radiation is normally detected using a high-resolution gamma detector for which a defined viewing angle is chosen with the use of a suitable lead (Pb) collimator (see Figure 2)

The intended use of this SRM is to provide scientists using gamma spectrometry a basis for accurate and worldwide accepted calibrations of isotope abundance measurements on uranium materials. These samples can be used

to calibrate working standards or plant- specific reference materials. Care must be taken to ensure that the measurement of a material with an unknown

abundance, be performed with the same detector-collimator configuration as the reference material. In addition, the use of sufficiently thick as well as 00c chemically pure uranium layers, homogeneous with respect to their "-'U abun- dance and chemical composition, are required (see details in the User's Manual)

(5). The unknown abundance (Ay) of a uranium material is obtained from gamma counting and from the abundance (Ar ) of a reference material by using the following equation:

Ay = Ar [(K

g

C) u/(K

g

C) r ] Kjjj Ks Kc

(1) Cu

,C r are the photo -peak counting rates for the 185.7 keV line determined

from the gamma spectra of both the unknown and the reference samples

(see typical spectra in Figure 3)

KgU,Kgr are small corrections of photo-peak counting rates for most of the unresolvable gamma lines near 185.7 keV, such as the protactinium

daughter products of 238U (6,7).

(Kg

) u

= l-0/Au , (Kg

) r = l-0/Ar

(2) where 0,^ which is <1.15 x 10" 5

, is the bias associated with the age of the uranium sample from last chemical separation; for older samples, 0 approaches

zero. The K values allow correction for differences between the unknown and

the reference materials with respect to the chemical composition of the uranium

materials (Km)

, is related to the surface density of the uranium samples com- pared to "infinite thickness" (Ks )

, and is related to the material type and the thickness of the can "window" (Kc )

2
3
CHANNEL NUMBER

256 512 768

^— 185.7 keV

235 U/U = 0.003 —« ^ „ \

1U.7 172.9 231.2 289. k ENERGY (keV]

1H.7 ENERGY [keV]172.9 231.2 289.6

256 512 CHANNEL NUMBER768

Figure 3. Gamma Spectra of U3O3 Reference Samples Measured with a High

Resolution Germanium Detector

1J
An empty aluminum can, identical to the ones used for the reference

samples, is supplied with each set of reference materials to facilitate the measurement of unknown materials. When this can is used Kc = 1

.

The reference materials are identified by numbers engraved in the wall of

the cans as shown in Figure 1. The numbers indicate the issuing organization,

a three digit number for the 235y abundances (in weight percent X 100) , and a three digit sequential number common to each set. In addition, all individual sample cans are equipped with a unique and tamper-proof seal inserted into the top part of the can. For identification, a special ultrasonic transceiver is used which can read the ultrasonic fingerprint in the seal via the connector on the cover of the cans (see Reference 5 and 8). Using this fingerprint, a verification of the signature of each sample can is possible for safeguards purposes.

5
3. Preparation of the Reference Samples

The main steps in the fabrication procedure were: Fabrication of cans and plugs to the required dimensional tolerances Acceptance control of cans and plugs

Filling of cans with U3O3 powder and accurate mass definition

Sampling for production control Pressing of the U3O3 powder

Automatic Tungsten Inert Gas (TIG) welding of the plug onto the can Final dimensional check and adjustment of the window flatness of the

can. Emphasis was placed on extensive control and inspection throughout the fabrication process to guarantee the required specifications.

3.1 Fabrication of Cans and Plugs The cans and plugs were machined from solid rod base materials at NBS using an automatic lathe. Since the windows could not be made to conform to required tolerances on the automatic lathe, they were machined with an over- thickness of 0.07 mm, then adjusted to the required tolerance at CBNM by

further machining.

Dimensions and tolerances of the can and plug are given in Figure 4. The material used for the preparation of cans and plugs is ASTM 6061 -T6 aluminum alloy with the following impurity specifications (values in units of mass %)

:

Si : 0.4 - 0.8 Mg : 0.8-1.2 Mn : < 0.15

Cu : 0.15 - 0.4 Fe : < 0.7 Ti : < 0.15

Cr : 0.04 - 0.35 Zn : < 0.25 U : < 0.00025

Other elements : each < 0.05

: total < 0.15

It should be noted that the 2.5 ng/g of uranium in the aluminum can material

will affect gamma measurements by less than 3 parts in 10^ for depleted uranium

with 235U/U = 0.0031. The control measurements as well as the selection of the cans and plugs

are described in this text. All metrological data pertinent to a particular can is provided with each reference material container. For an example see Figure 5.

6
7
Container N°

:

at. Mass U3 08

: A:

B:

C:

CBNM 194-024

(200.1 ±0.2)g

54.20 nun 11.98 30.00

69.88

1.989 88.98

1.01 70.03

66.0 79.95

Bottom thickness

D: 52.77

E: 20.78

12

34

5

67

89 10

11

1213 *13

s 13

2.002 mm

2.005 2.000

2.003 2.000

1.996 2.001

2.000

1.985

1.987

1.986

1.979

2.006 1.996 0.009

(All areas)

f: < 0.1 (Bottom Flatness)

x 5 1.989 0.010

(Five inner areas)

8
3.2 Filling the Cans with U3 08 Powder

The required amount of u^Og (200.1 ± 0.2g) was weighed into the can and pressed to 3 mm above the required final thickness by means of a steel plug

using a hydraulic press (diameter of steel plug = 69 mm, inner diameter of can

= 70 mm so that air could escape during pressing). A surface density of 5.2 g/cm^ was achieved. The U^Og in the gap between the plug and the wall of the can was carefully

brushed back into the can and the U^Og mass adjusted. This pressing was performed twice. The permanent Al plug was then placed into its final position

such that the lips of the plug and the can were pressed together. To avoid

deformation, the can was tightly fitted into a recess in a heavy steel block

during the entire hydraulic pressing procedure.

The U^Og masses were determined using a Mettler balance type PC 2000 with

a capacity of 2100 g, a reproducibility of ±0.01 g and a linearity of ±0.015 g. The balance was calibrated daily with a test weight calibrated against the CBNM standard kilogram. The indicated uncertainty of ±0.2 g on the u^Og mass includes all sources of error. The four U3 0g materials (031; 071; 194; and 295) had pour densities

between 0.7 to 0.9 g/cm^ and were pressed into the cans to an approximate

density of 2.5 g/cm . The 446 material had a much higher pour density of 1.77 g/cm^ . Shock and vibration tests monitored by gamma transmission measurements

showed that this material moved within the can when pressed to an approximate

density of 2.5 g/cm^ and caused an unacceptable reduction of the thickness of

the layer at distinct positions within the can. Re-pressing this material to a higher density (3.4 instead of 2.5 g/cm^) by inserting a 5 mm disk under the plug ensured stability of the material, as gamma transmission measurements

later confirmed.

The' lips of plug and can were TIG-welded together on a semi-automatic

machine that enabled repetitive production of precision welds. Pre-flow of inert gas, initial and final slope of the welding current, pulse operation of

the current and turntable speed were programmed for optimal reproducibility. Numbers were engraved into the top part of the cylindrical wall of the cans as shown in Figure 1

.

The numbering system was chosen as follows: The reference samples distributed by CBNM and NBS respectively for

the 031 material indicate the abundance as "031", and a sequential

set number of 001 to 070 for CBNM and 071 to 140 for NBS cans: CBNM 031 - 001 to 070 NBS 031 - 071 to 140 Reference samples with the same sequential number (013 or 120 in this example) with each of the different abundances, form a set. The empty can, provided in addition, has the corresponding sequential number as shown in the following example:

9
Example CBNM Set 013 NBS Set 120 CBNM 031 - 013 NBS 031 - 120 CBNM 071 - 013 NBS 071 - 120 CBNM 194 - 013 NBS 194 • 120 CBNM 295 013 NBS 295 • 120 CBNM 446 - 013 NBS 446 120 CBNM 013 NBS 120

3.3 Sampling for Verification During the Homogeneity Filling Procedure About 15 samples (2 to 5 g each) were taken during the filling procedure

of each of the U3O3 materials at a rate of one per day, i.e., about every tenth

can. These samples were used for chemical impurity and moisture determinations

as well as for checking isotopic homogeneity (see Section V)

.

3.4 Ultrasonic Identification System

Special attention was given to the individual ultrasonic identification

system for each of the reference samples, as described in (8). Integrated into the plug were identification sensors which work along the general principle of the ultrasonic signature:

one ultrasonic transducer uniquely identifies the can and is embodied

in the structure thus making tampering difficult (identifier), another ultrasonic transducer is glued onto the external cover to verify the integrity of the cover (integrity verifier),

both ultrasonic sensors are connected to a Lemo plug (see Figure 6a) for easy identity and integrity check by simple electronic connection

of an ultrasonic reader. These ultrasonic readers are commercially

available

.

Recording of the identity can be performed using a Polaroid camera to print the pattern that appears on the screen of the portable ultrasonic reader

(see Figure 6b) . This pattern of the ultrasonic spectrum is unique to each

sample can. The identifier and integrity verifier will indicate any changes in the pattern of the ultrasonic spectrum generated by any tampering with the can. The Safeguards Inspectorates of the U.S. Nuclear Regulatory Commission,

the International Atomic Energy Agency, and the EURATOM Safeguards Directorate can register the ultrasonic pattern of each seal within a set of reference materials for the purpose of safeguards and materials accountability. In order not to destroy the seals, storage temperatures between -10 °C and +40 °C should

be maintained.

10
Figure 6a. Ultrasonic Sensors as Integrated Part of Reference Samples
4. Determination of Properties Relevant for Gamma Measurement

4.1 Chemical Composition and Impurities

Sampling for chemical analyses was done both upon receipt of the material

and at the time of canning (see Section III on fabrication)

The uranium oxide was found to contain water and certain other impurities.

In addition, the oxygen- to -uranium ratio measurements performed by the supplier

(British Nuclear Fuels, Springfields , United Kingdom) indicated a slight excess

of oxygen when compared to stoichiometric U^Og

The following measurements were carried out either at CBNM and/or at NBL: determination of uranium content determination of the impurity content determination of the water content.

4.2 Uranium Content Measurements Performed at CBNM

The uranium content of the different batches (Table 1) was calculated on

the basis of a calcination of the material at 900 °C. The estimated sum of the total impurities in the form of oxides (see section on Chemical impurities) was subtracted from the mass determined after calcination. Table 1. Uranium Content at the Time of Packaging

Material g U/g material

0.8456

0.8453 0.8456

0.8452

0.8466

The calcination was on 2 to 3 g of material. The U3O3 was initially

heated in air for 2 hours at 110 °C and then heated in air for 15 hours at 900

°C. From each batch, three samples of the material were taken upon receipt, and three random samples were taken during the filling of the cans. No significant differences between the two sets of analyses were observed, so an average of the six measurements was made. Taking into account the uncertainty

of the mass determinations and the evaluation of the total impurities, the maximum uncertainty (95% confidence level) of the elemental content is estimated to be ± 1500 //g/g of material as received.

4.3 Uranium Content Measurements Performed at NBL

Four platinum dishes (75 mL) were cleaned, hot-acid washed, ignited in a furnace at 800 °C for one hour, cooled in a desiccator and weighed. The material in each container (21 g) was then poured into a platinum dish and the dish was reweighed. A special weighing procedure was instituted due to the instability of each material mass. Since the material proceeded to decrease in

031

071 194

295 446

12
weight during successive weighings, each set of weighings was plotted and the mass for time "zero" (material as received) was obtained by extrapolation.

To determine moisture content, the samples were dried at 110 °C for two hours, cooled and weighed. Next, the samples were placed in a furnace at 800

°C for two hours, cooled and weighed. The samples were reignited at 800 °C for an additional hour, cooled and weighed again. A weighing procedure similar to the above was necessary in each case. To ensure constant weight during aliquanting, samples were allowed to equilibrate with room air until the masses were constant. Aliquants were then weighed into beakers and the masses corrected back to ignited masses. Four aliquants from each sample were placed in beakers for isotopic analysis and chemical assay. The chemical assay aliquants were dissolved in phosphoric acid and titrated using the NBL High Precision Method (9). Aliquants of NBS 950b SRM were ignited at 800 °C for one hour and assayed as control standards. The uranium content and material weight loss determined by NBL is given in Tables 2 and 3

.

Table 2. Uranium Content Determinations by NBL- (After Drying and Ignition)

Material Uranium Content (g U/g) n

031 0.847685 ± 0.000027 25 071 0.847688 ± 0.000054 5 194 0.847800 ± 0.000083 5 295 0.847758 ± 0.000112 5 446 0.847488 ± 0.000020 5 SRM 950b 0.847783 ± 0.000072 3 15 0.847737 ± 0.000038 3 8

Dried, ignited at 800 °C, corrected for isotopic composition

Uncertainities are Is for n determinations

'Certified values = 99.968 ± 0.020% U3 08 (0.847731 g U/g)

Table 3. Total Weight Loss Determinations of NBL

Material Amount Lost (ug/g) 031 1800

071 2300

194 1800

295 2200 446 600 Table 4 gives the values obtained from NBL on the uranium content per gram

of material as received. These values are directly comparable to the data presented by CBNM (Table 1)

.

13
Table 4. Uranium Content Per Gram of Material (as Received)

Material Uranium content (g U/g) 031 0.8462

071 0.8458

194 0.8463

295 0.8459

446 0.8470

4.4 Chemical Impurities

The impurity contents of the different batches were calculated using data from spark source mass spectrometry (SSMS) and emission spectrometry analyses

performed by the Studiecentrum voor Kernenergie/Centre d' Etude de l'Energie

Nucleaire (SCK/CEN) , Mol , Belgium. A panoramic analysis, covering 72 elements of each base material was done by SSMS . Two samples taken during the canning of each material were analyzed

by emission spectrography . Thirty two elements were rechecked. The results of

the SSMS analyses indicated that no contamination occurred during the preparation step. The maximum impurity content has been estimated by summing the measured

concentrations and the detection limits for a number of elements (see Table 5). Table 5. Impurity Content Material Sum of Impurities Sum of Impurities

Z < 30 (/ig/g) Z > 30 (/ig/g) 031 < 500 < 6 071 < 300 < 6 194 < 400 < 6 295 < 200 < 6 446 < 1000 < 10 following elements were detected.

Z < 30 Li Be B F Na Mg Al Si P S CI K

Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn

Z > 30 Ga Ge As Se Br Rb Sr Y Zr Nb Mo Ru

Rh Pd Ag Cd In Sn Sb Te I Cs Ba La

Ce Pt Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Hf Ta W Re Os Ir Au Hg Tl Pb Bi Th

4.5 Determination of the Water Content

The moisture content of the material sampled at the time of canning was determined by heating the material to 300 °C under dry nitrogen and measuring

the liberated water by Karl Fischer titration. Three samplings per batch were

14
taken during the filling of the cans and four analyses per sample were per- formed (Table 6) . The uncertainty on these determinations does not exceed ±

150 yug/g of material.

Table 6. Moisture Content Material /ug Up O/g Material

031 2525 071 3038

194 2314

295 3000

446 822

4.6 Estimation of the Light Element Content

The weight losses observed during calcination consisted of water, oxygen, and a negligible amount of volatile substances. After rounding off and taking

a large uncertainty for the small contributions of volatile substances and hyperstoichiometry , a conservative estimate of the maximum content of light element impurities in the material is presented in Table 7.

Table 7 . Maximum Content of Impurities Z < 30 in U^Og

Material Impurities Z < 30

(jxg/g material)

031 < 4100

071 < 4600

194 < 4000

295 < 5000

446 < 2500

4.7 Estimation of the Heavy Element Content

The content of elements with Z>30 in the different materials does not exceed 10 /ig/g (see Table 5) . 4.8 Radioactive Impurities

The occurrence of spurious gamma-lines (other than from ^38^ daughters) was tested on one reference sample per abundance by looking for other more

intense gamma lines of ^^^U, ^^U, 237^ an(j their respective daughters. NBS and CBNM used their respective gamma- counting system for all gamma spectrometry

measurements, including determining the radioactive impurities (Figure 2a for CBNM and Figure 2b for NBS).

15
4.8.1 232u and Daughter Products -CBNM Measurements

It was essential to determine the 232U impurity content of the materials

because spurious gamma- lines of 232U daughter products exist in the energy

range used by low resolution Nal(Tl) gamma spectrometers for 23 -*U abundance measurements

.

919 The major effect is caused by the gamma-line of ^LZ Pb at 238.5 keV belonging to an energy range used for background subtraction when measuring the 185.7 keV gamma- line. Test results in Table 8 show that the 232U impurities of the materials are small enough to allow the use of the reference samples with Nal detectors

.

However, the counting ratio (238.5/185.7) for the 031 material will grow from 0.006 in 1982 (three years after chemical separation) to about 0.01 in 1989 and introduce a significant bias effect. Table 8. 232U Content as Determined from the Gamma Peak Rate of the 212 Pb Daughter at 238.5 keV Relative to 235U at 185.7 keV Material Gamma Peak Ratio Isotope Ratio

(238.5)/(185.7)

xlO2 232U/235Uxl09

031 0.62 0.8 071 < 0.02 2 < 0.03

194 0.03 0.03

295 0.01 0.01

446 0.08 0.1

1 Date measured 1982

2 Detection level for 238.5 keV gamma- line measurement

4.8.2 233U, 237U and Daughter Products-CBNM Measurements

Spurious gamma- lines which might bias low resolution gamma measurements of the 23 ^U abundance, are known for ^33U and for 237Np from the fast decay of

237U after chemical separation. Tests for identifying such impurities were performed by looking for the lines of 237Np at 212 keV and of 233 Pa at 312 keV

( 233 Pa is a daughter product of 237Np) as well as for the most intense lines of

233U at 164.6 keV and 245 keV. Neither impurity was detected, with detection

limits of 233U/235U < 5xl0" 5 and 237Np/235U < lxlO -6

. As a result, bias

effects, if any, are smaller than 0.05%.

16
4.9 U3O8 Surface Sample Density

The infinite thickness performance of the U3O3 reference samples was tested in order to ensure that the counting rate from the 235jj gamma- radiation

at 185.7 keV will correspond to > 99.9% of the radiation from an infinitely

thick u^Og layer. For that purpose the following measurements were performed:

the gamma -counting rate of 235y at 185.7 keV as a function of the thickness of a U^Og layer, with different collimators (viewing

angles)

the uniformity of the U^Og surface density of the reference samples

by gamma- transmission measurements.

4.9.1 Gamma -Counting Rate Versus U3 0g Layer Thickness

Additional U^Og samples identical to the reference samples, were prepared

to test the degree of infinite sample thickness for gamma- radiation at 185.7

keV. These samples contained uranium oxide ranging from 100 to 220 grams, equivalent to area sample densities (ms

) from 2.6 to 5.7 g/cm^ at a sample

diameter of 7.0 cm. Samples were made from the 446 material. Measurements were performed with the gamma measurement equipment as shown

in Figure 2a. Different collimator dimensions were chosen, corresponding to length/diameter ratios (1/d) of 2.0, 1.33 and 0.50, to measure the variation of

the gamma- counting rates relative to the counting rate of a reference sample

(mso =5.2 g/cm^) with the same 235y abundance (see Figure 7). For comparison, the counting rate variation (dCp) for the 185.7 keV

gamma- radiation perpendicular to sample surface was calculated: dCp/Cp = [(exp-msus )-(exp-msous )]/[exp-msous ] (3) mso = surface density of reference samples, mso = 5.2 g/cm2 ms = surface density of samples with variable thickness

us = attenuation coefficient of u^Og

.

Figure 7 shows that the measured variations approach the curve dCp/Cp calculated with an attenuation coefficient of U^Og , us = 1.2 cm^/g when the solid angle for the gamma radiation decreases (increasing ratio of collimator

length, 1, to diameter, d. A comparison of the observed to the calculated dC/C value indicates that samples with a density of 5.2 g/cm^ have a minimum value

of 99.86% of infinite thickness. 4.9.2 Uniformity of the U3O3 Area Density of the Material

Gamma- transmission measurements were made on the pressed uranium oxide powder layers within the Al cans. These measurements were performed at the center of the sample surface and at 9 areas distributed over the sample

surface, for 8 samples of each of the 5 reference materials used (Figure 8).

17
Figure 7. Variation of Counting Rate (AC/C) of U3 0g Samples versus Areal Density (ms

) Relative to a Reference Sample (mso )

a Measurement Set Up A-A

661.6 KeV Ys ( ,37CS )

U3 Og Sample

Lead Collimator

HP Ge Detector

b. Areas tested Figure 8. Set-up for U3O3 Mass Distribution Uniformity Tests by Gamma Transmission Measurements

18
The transmission tests were conducted with a collimated 1 -5/ Cs gamma-beam

for the determination of the fluctuations of the counting rate of the transmit- ted gamma- line at 661.6 keV. These fluctuations are a measure of the non- uniformity of the uranium oxide sample mass perpendicular to the sample

surface. The measurements were calibrated with "well defined" uranium oxide

layers. An Al absorber was used as a reference for testing the stability of

the measured gamma -line counting rate. The following results have been obtained for the fluctuations (dmg

) of the sample surface density around the mean value of 5.20 g/cm^

.

Material batch 031 071 194 295 446

Fluctuations dms [g/cm] < ± 0.15 < ± 0.25 Conclusions

It can be concluded that the ^35^ counting rate of the reference samples

at 185.7 keV will be about 0.10 +0.06% less than for corresponding (theoretical) samples of infinite thickness. It has been shown that this value is not only influenced by the fluctuations of the uranium oxide surface density but

also by the collimator chosen. 4.10 Can Window Thickness

In order that the window thickness uniformity of the cans meets the close tolerances specified in Figure 9, ultrasonic thickness measurements were per- formed on all of the cans using a thickness reference window. This reference window was specially prepared from the same Al-alloy 6061-T6 base material

stock used in the manufacturing the reference material cans. Thickness values on this reference window were determined at four "well defined" measuring locations by means of a comparator gauge (precision

achieved: ±1 pm) which was calibrated with gauge blocks according to DIN 861 and NFE 11010 specifications (maximum deviation of nominal thickness ±0.10

/zm) . The surface of each calibrated location on the reference window is identical to the surface of each location involved in an ultrasonic determination. The ultrasonic meter was recalibrated at frequent intervals in the course of a day's operation in order to minimize errors caused by instrument

drift. Under these conditions an accuracy of ± 5 on the window thickness

determinations was achieved. The cans were selected in sets of 6 (5 reference samples and 1 empty can) in such a way that the maximum difference of the mean window thickness: x^3 between any two cans in each set is 0.015 mm (see Figure 9). An example of can window thickness measurements , as provided with each

container, is given in Figure 5. The window flatness was better than 0.1 mm for all fabricated reference

samples as measured with a precision dial indicator along 4 diameters of each window

.

19
1) 1.980 mm < x^3, xg , X5 , X4 , and xcentr < 2.020 mm

2) s 13 < 0.015 mm

S3, S5, < 0.010 mm

3) For each can

| xg - x^ | < 0.005 mm

4) For each set of 6 cans (5 abundances + 1 empty can)

(^5)max ' (^5)min < °- 010 TM*

and

(^13)max - (xi3)min < °- 015 mm

4>U2

x^3 : mean thickness all points 1 to 13 xg : mean thickness points 1 to 8 X5 : mean thickness points 9 to 13 x^ : mean thickness points 9 to 12 xc : thickness control points 13 S5 : standard deviation on a single measurement for 5 control points

s^ : standard deviation on a single measurement for 4 control points

s^3 : standard deviation on a single measurement for all points

Figure 9. Specification for Uniformity of Window Thickness

20
4.11 Evaluation of the Uncertainty Contributions of a

ZJ:5 U Abundance Determination by Gamma Counting at CBNM All uncertainty contributions specified in the following sections

correspond to a confidence level of > 95%. 4.11.1 235jj Abundance of Reference Samples

The stated uncertainties for the ^35^ i so tope abundances as determined by

mass spectrometry, dAr/Ar , are summarized in Table 9. The values given

contribute directly to the uncertainty of the ^35u abundance determination by gamma counting, whereas the maximum uncertainty on the isotopic inhomogeneities, dAh/Aj-, will be smaller, dependent on the material distribution within

the reference samples. Table 9. Uncertainties of 235U Abundances from UF6 Mass Spectrometry (CBNM)

Material 031 071 194 295 446 ±dAr/Ar 100 (%) 0.07 0.07 0.07 0.07 0.07 ±dAh/Ar 100(%) 0.023 0.013 0.014 0.006 0.012

4.11.2 Chemical and Physical Sample Properties

Properties of the reference samples which influence the overall uncertainty of an abundance measurement by gamma -counting were described

earlier in this chapter. The effect of properties other than the 235y abun- dance on the measurement of the 23f>U gamma radiation at 185.7 keV corresponds

to uncertainties (dK/K) of the correction factors, K, in equation (1) of section I. A. Chemical Properties Measurements on all five material batches and on several samples per batch as described in this section, have shown the maximum impurity content to be as follows:

Z< 30 : < 5000 /zg/g Z> 30 : < 10 /xg/g This corresponds to a gamma attenuation with an uncertainty of: (dKn/K^xlOO < - 0.051 %. The major contribution to the above value results from the water content

of the materials (< 3200 /zg/g U308

).

21
B. Physical Properties of the Samples The uncertainty of the U^Og surface density and can window thickness are described below: U3O3 surface density: the residual inhomogeneity of the U3O3 surface density measured over the sample surface, perpendicular to the can window, is < ± 5% for a mean surface density of 5.2 g/cm2

. This corresponds to an uncertainty of: (dKs/Ks )xlOO < ± 0.07 %. Aluminum can window thickness: an uncertainty of the effective can window thickness of < ± 0.015 mm within a set of reference samples

will introduce an uncertainty of the measured counting rate of: (dKc/Kc )xl00 < ± 0.10 %. In both cases the uncertainties are typical for the viewing angles

encountered in detector/collimator configurations corresponding to collimator

height to diameter ratios (h/d) to 0.3 (5). 4.11.3 Overall Gamma -Counting Uncertainty of Reference Samples

The gamma counting uncertainty contributions from reference sample

properties, outlined in the previous section, are maximum values. They are independent in origin and can be used as a means of calculating the maximum

overall gamma emission rate non-uniformity at 185.7 keV of the reference

samples from a given batch of material, via the following equation:

dC/C (calculated) < [ (dAH/AR) 2 + Z(dK/K) 2

] 1/ 2 (4a) K

with (dA^/AR) <± 0.023 % and the quadratic summation of the maximum

uncertainties for the correction factor, K, 2 (dK/K) 2 = (dl^/K^ 2 + (dKs /Ks ) 2 + (dKc /Kc ) 2 (4b) (see Equation 1), using the values: dK^/YL^ < 0.051%, dKs /Ks < 0.07%, and

dKc /Kc < 0.10% it follows for the maximum overall gamma counting uncertainty of samples with the same 2 ^^U abundance: (dC/C)-100 (calculated) <± [ (0 . 023) 2+(0 . 051) 2+(0 . 07) 2+(0 . 10) 2

] 1/2%

(dC/C)-100 (calculated) < ± 0.14%. This value is practically unaffected by dAH/AR

and is in good agreement

with results given by the CBNM Gamma Spectrometry Section. The overall uncertainty of the reference samples for the calibration of abundance measurements by gamma spectrometry can be derived from: dC/C (overall) = [(dAR

/A) 2 + (dC/C) 2 (calculated)] 1^.

22
With the maximum value of dAR

/AR

= ± 0.07% in Table 9 it follows:

(dC/C) • 100 ( overall )< ± [ (0 . 07) 2+(0 . 023) 2+(0 . 051) 2+(0 . 07) 2+(0 . 10) 2

] 1/2 (dC/C) -lOO(overall) < ± 0.16%. This is a maximum value which will reduce to < 0.14% for samples belonging

to the same set because of the smaller tolerance of the can window thickness (± 0.015 mm within a set compared to ± 0.020 mm, and dKc /Kc < 0.07 instead of <0.10)

.

23
5. 235U/U Characterization

The 235u/U abundances were obtained as 235u/238u

234U/238U + 235^238^ 236U/238U + 1 235U/U = (5a)

The 235^238^ ratios were characterized by two different isotope mass spectrometric techniques:

a) Thermal ionization mass spectrometry (ThIMS)- where a sample is thermally ionized and the ratios of the ion beam intensities of all isotopes are measured by scanning the ion beams over a single

collector. The 235^238^- ratios are calculated from measured ratios:

(235u/238u)mater . al batch = K5/8 x ( 235 u/238 u) measured (5b) where K is a correction factor for systematic errors and 5/8 denotes 235^238^

b) Uranium hexafluoride mass spectrometry (UFgMS)- where the 235y an(j 238-y ion beams are collected simultaneously in two different collectors of a mass spectrometer equipped with a permanent magnetic field and working

with a constant accelerating voltage. In the measurement circuitry the ratio of ion currents of the unknown is converted to a ratio of voltages

and is compared to the same ratio of a reference sample (in this case: a synthetic isotope mixture) . This yields a correction factor directly

expressed as an a-value. Hence, the a-values obtained

(235u/238u)mater . al batch = a

. (235u/238u)synthet . c (5c) through UFg mass spectrometry have better reproducibility than ThIMS. If these measurements can be performed directly against accurate synthetic

isotope mixtures, they have the potential for better accuracy.

The 234U/238U and 236U/238U ratios were obtained only from ThIMS.

5.1 Measurements Made at CBNM by Mass Spectrometry

All measurements were carried out after all precautions had been taken to correct or eliminate known systematic errors. Correction for additional sys- tematic errors, including mass discrimination in the ion sources, were considered separately for ThIMS and UFgMS

.

5.1.1 Thermal Ionization Mass Spectrometry

Correction from unknown sources of systematic error, including the isotope

fractionation in the ThIMS ion source (the most difficult but also the most

important error) , were determined by measuring synthetic mixtures of enriched

23^U and 238y isotopes, calculated from their preparation to be accurate to ± 0.01 %. Measuring the mixtures in the mass spectrometer, and correcting these measurements for all known errors yielded "observed" values R0b s f° r tnese

ratios. The difference between "prepared" and "observed" ratios for the

24
mixtures permitted the detection, quantification, and correction for isotope

fractionation by means of the correction factor K = Rp rep/Robs

n o c O "3 0 The accuracy of the z-->

U/ZJOU characterization was then determined by the precision of the K- factor and the accuracy of the isotope mixtures.

The mixtures were made over a range of ^35Uy238u rat j_ os that bracketed the samples (see Table 10) and demonstrated that:

a) the measurement procedures and instrumentation used were linearly

responding to ion beams to the precision indicated

b) the isotope fractionation observed was independent of isotope abundance or ratio and only dependent on isotope mass

c) a reading or transcription error in the preparation of any single

mixture had not occurred

d) a weighing error in the preparation on any single mixture was not

detected.

This approach has been used previously to produce Isotopic Reference Materials for boron (13) and lithium (14,15).

Since isotope fractionation, as determined over a span of 3 mass units

difference (235-238), was only 0.03%, or 0.01% per mass unit, a linear extrapolation could be made to the 2^4U/238U and 236Uy238u rati 0 measurements of 0.04% and 0.02% respectively.

5.1.2 UFg Mass Spectrometry

Remaining systematic errors as well as mass discrimination in the UFg ion source were cancelled out by measuring directly against synthetic mixtures of enriched 2 -^U and 238y These mixes were known from their preparation to 0.01% and simulated the isotope composition of the material batches (i.e., the candidate RM's) to within 1%. The chemical conversion of the mixtures to UFg normally requires 2-3 g of material per sample. It was obviously impossible to consume that large an amount of enriched isotopes, therefore, the entire procedure was miniaturized

until chemical conversions on a 100 mg scale could be achieved without loss of accuracy (10 , 18) . The accuracy of the 235yy238-Q characterizations was then determined from

the precision of the a-value and the accuracy of the isotope mixtures.

The working scheme followed by CBNM is illustrated schematically in Figure

10. The various stages are described below: preparation of the synthetic mixtures and determination of their

uncertainty components

25
mass spectrometric measurements and their calibration

computation the ^^U/U abundances and their uncertainties.

In each step of the characterization process of the ^^U/U abundances, the significant systematic errors were quantified, the measurement in that particular step was corrected, the uncertainty of the correction factor (s) was incor- porated and the combined uncertainty was transferred to the next step of the process. All uncertainties are be expressed as 2s, this corresponding to a confidence level of about 95%. Propagation of this uncertainty has been done on the basis of the square root of the sum of squares (independent determinations) except when otherwise indicated. This was the case when calculating the uncertainty on 235nyy abundances (Equation 8) . 5.1.3 Preparation of Synthetic Uranium Isotope Mixtures

Enriched isotopes were purified chemically and converted to the highly

stable U30g . On a weight basis they were mixed to achieve different ratio values (see Table 10) in the range 0.003-0.05 corresponding to ^^^U/U abundances of 0.003-0.045 (or 0.3-4.5 mass %)

.

The important steps in the preparation of the mixtures were: determination of the isotopic purity of the highly enriched isotopes

determination of the chemical purity of the enriched isotope

weighing of the enriched isotope oxide fractions and corrections for water uptake

dissolution of the weighed fractions. 5.1.4 Isotopic Purity of the Highly Enriched Isotopes

The isotopic purity of the isotopes used for the synthetic mixtures was accurately determined through isotope dilution mass spectrometry. The results

of these measurements are listed in Table 11. The wide dynamic range encountered in the isotope ratios of the materials

made it necessary to determine some of the small abundances by comparing them with a known amount of 233^ acjded as an "internal isotopic standard" (see the upper part of Figure 10)

.

The accuracy of the 235^238^ ratios in the mixtures is dominated by the accuracy of the 235y abundance in the 238y enriched material (16). This material was spiked with IRM 040-1 ( 233U) (17) to give a 235U/233U ratio

approximately equal to one and this ratio was measured. The 235^238^ ratio was then determined from:235U/238U = 235^233^^^/^^

where N233 is the number of atoms 233y added. Both N233 and N238 were determined from weighing the calcined oxide and correcting for oxygen content and isotopic purity.

26
Table 10. Prepared Isotope Mixtures, with Uuncertainties (2s) and

their Components; 100 parts per 10 6 = 0.01%

prepared isotope mixtures uncertainty

(2s) on ratio in mixture:

2s R /R

in parts per 106

uncertainty components of 2sR/Rin parts perl0°(2s) from differences in

Ident.

Nos.

235

u/ 238u atom ratio

Rprep

+ 2 S R purity ometry purity determination

82 0.003 210 1941 127 83 0.003 241 59

41 126 112 44 14 34 84 0.007 238 63

56 78 51 44 14 36 85 0.007 302 88

57 78 51 44 14 36 118 0.007 206 69

61 85 51 44 14 49

119 0.007 326 01

62 84 51 45 14 49

102 0.020 183 6

1 8 91 21 45 14 75 103 0.019 976 7 2 1 103 21 45 14 89

104 0.030 599 0

3 2 105 16 45 14 92 105 0.030 982 0

3 2 105 15 45 14 92

106 0.047 121 6 4 7 101 13 45 14 88

107 0.047 546 7 4 8 100 13 45 14

27
Table 11 Isotope Abundances of Enriched Isotope Material Used to Prepare the set of Synthetic Isotope Mixtures (Uncertainties +2s)

.

Isotope Ratios

233>U/U T 234U/U

Enriched 235T <0. 0000002 0.0003607 +0.0000026

Enriched 238T <0. 0000002 0.0000003 +0.0000002

235U/U 0.9994145 +0.0000060

0.0000234 +0.0000004

236U/U 0.0000953 +0.0000052

<0. 0000001

238U/U 0.0001295 +0.0000015

0.9999763 +0.0000004

The other minor isotopes were determined in a similar manner. Because of

the small ion currents encountered, the measurements were performed using an electron multiplier.

In arriving at the final abundances and uncertainties shown in Table 11 all possible sources of error were considered since the uncertainty of the enriched isotopes is an important component of the total uncertainty of the mixtures. The following sources of error were evaluated: Interference from unresolved molecular species: the spectrum was closely examined using the highest sensitivity available since the magnitude of these species depend on the ionizing filament temperature, the latter was lowered until their contributions to the isotope

ratios were negligible.

Uranium blank: the contribution to the uranium mass spectrum from impurity uranium originating from chemical processing was <0.1 ng and from the rhenium filament <0.03 ng. Since 10 /ig sized samples were

analyzed, errors from this source were negligible.

The use of an electron multiplier: a knowledge of the isotope

discrimination was required. This discrimination was corrected by a factor of 0.23+0.10% per atomic mass unit. Time dependent isotope fractionation: the magnitude of this effect

was very small. Even so, a correction was made and an uncertainty of +0.10% per mass unit was assigned to the correction.

Baseline irregularities: the assumption of a linear trend in the baseline is normally sufficient for estimating the baseline at the position of a peak.

However, for low abundance peaks adjacent to the highly enriched isotopes, more complex corrections were required. The isotope most affected was the

28
Highly Enriched Isotopes

( Base Materials )

233

u 0.996

1 —u/u 0.999 4 —u/u 0.999 97

These do not constitute RM's by themselves since they do not bear a certified value which can be used to calibrate

a measurement

0.003

Sol. Dil. Synthetic Mixtures of Highly Enriched Isotopes

UF

6 High Prec. Mass Spec. Th.I

High Precision Mass Spectrometry

Requires a very high precision in order

- to make full use of the small uncertainty on the isotopic mixtures

- to attain the required uncertainty on the RM batches Measurement samples must be converted to very pure Uranyl nitrate and UF,

031 194 446

071 295

Ox. Isotopic Reference Materials

Isotopic Reference Materials Materials to be distributed

Ox. = Oxides

Sol. Dil. = Dilution of Solution

Figure 10. Scheme for the Preparation of Synthetic Isotope Mixtures in order

to put "absolute" values (with uncertainties) on Isotopic Reference Materials

29
"bU in the ^-"U enriched material where it was necessary to allow for a 1.5%

(Is) uncertainty.

5.1.5 Chemical Purity of the Enriched Isotopes

Enriched isotopes always have a limited chemical purity, but, in this

case, similar purities for the two enriched isotope materials were sufficient. Chemical purification of the two enriched isotopes was, therefore, carried out using the same procedure. The isotopes were purified by cation exchange on Dowex 50X8 (100-200 mesh, H-form) resin using a 90% tetrahydrofuran/10% 6M HN03 solution as eluent, followed by calcination at 900 °C for 3 hours. The u^Og was redissolved in 6M HNO3 . A second purification step consisted of a peroxide

precipitation of the uranium followed by calcination to U^Og at 900 °C for 3 hours

The chemical purity of the products was about 99.9% before the purification (about 800 /ig/g impurities), and 99.99% (about 100 /tg/g impurities) after purification (see Table 12)

The purity check was made with the aid of a JEOL-01-BM- 2 double focussing

spark source mass spectrometer (SSMS) . Relative Sensitivity Factors (RSF) were used for the elements detected. Impurity determinations were performed with

both Au (99.999%, 200 mesh, Cominco American Inc.) and C (Specpure Graphite,

grade I, Johnson Matthey) electrodes. Isotopic compositions of CI and added

spike isotopes ^^Sr and °^Sr were used for internal calibration purposes. Remaining volatile impurities (difficult to determine by SSMS) were eliminated - if present - by calcination for 3 hours at 900 °C. To account for possible undetected impurities, 200±100 /ig/g, instead of

100 /ig/g, was added to the total uncertainty of the isotope mixtures. The effect of the purity of the enriched isotopes on the uncertainty of the isotope

mixtures is shown in Table 10. 5.1.6 Deviations from Stoichiometry-Water Uptake of the Enriched

Isotope Oxides U3O8 was chosen as the chemical form for the enriched isotopes because of

its very stable stoichiometry . In this case, an identical composition, not necessarily theoretical (stoichiometric) is needed. U^Og is known to be reproducible to < 0.01 % in composition when calcined for a minimum of 1 hour

at 900 °C. Both enriched isotope oxides were calcined simultaneously in quartz tubes for 16 h in a furnace having a temperature uniformity of + 2.5 °C, and a difference between two quartz tubes of 1° maximum at the 900 °C equilibrium

situation. This step eliminated volatile impurities.

The calcinations were performed in an air flow of about 7 mL per minute on portions of about 1 g. Subsequently the crucibles were cooled on a large

aluminum block and weighed after 24 hours. Another 2 -hour calcination followed

and the process was repeated until the mass differences were less than 20 /ig (error <20ppm) and hence negligible. All operations were performed in a glove -box with a controlled humidity of 10-50 /zg water/g dry air. Water uptake

30
was <5 fig on 1-g samples (possible error <5 ppm and hence negligible) during

the weighing periods of 1 h maximum. The weighing uncertainty was < 5 /ig in all cases

.

At this point, samples were taken to determine the isotopic compositions

given in Table 11.5.1.7 Isotope Mixtures and their Uncertainties A series of synthetic isotope oxide mixtures was prepared by weight, and

the abundances were calculated from the preparation ("Rprep")- Th© se values

are given in Table 10 with their uncertainties, 2s and 2sr/R, and with the various components of these uncertainties. Immediately after the weighing, the two isotope oxide fractions were dissolved in 6 M HNO3 . All mass determinations were substitution weighings using calibrated weights against the CBNM standard kilogram. The latter is directly calibrated against the standard kilogram of the Bureau International des Poids et Mesures (BIPM) in Sevres, France

.

The 235U/238U ratio (R5/8) in the mixture was obtained from the 235U/238U

ratios, (RA,Rg) in the enricned isotope material, A (enriched "235") and B (enriched "238") as follows:

[RA (1+ RB )NA

/NB + RB (1+RA)] R5/8 = (6) (1+ RB )NA

/NB + (1+ RA) where N is the number of atoms of uranium in the enriched isotope materials

.

Uncertainties on R5/8 were calculated by propagating the errors for all relevant variables in Equation 6 (20). These results are summarized in Table

10. 5.1.8 Mass Spectrometric Measurements and Their Calibration

A. UFg Mass Spectrometry

Samples of the material batches and synthetic mixtures following the standard procedure (10). For each isotopic composition, two synthetic mixtures were prepared "bracketing" the nominal value for each material to within 1%

.

235-jjy238jj vaiues for the material batches were then determined relative to each

of the mixtures using Equation 5c. Results are given in Table 13.

B. Thermal Ionization Mass Spectrometry

1. 235uy238u Ratio Determinations. Appropriate synthetic mixtures were

prepared, measured on the mass spectrometer, and corrected for known

instrumental bias, to produce observed values, RQb s. Remaining unknown systematic biases (mostly from isotope fractionation in

the ion source) were quantified and an appropriate correction factor, K, was determined, (Rprep/Robs) • T^e resul ts are given in Table 14.

31
Table 12. Spark Source Mass Spectrometric Analysis of the Enriched •^-J -) U and

2 -38U Materials

bianco 235,

• g" 1 (Mg.g"

1

) (fg.g"

l

) (1) (2) (3) (4) (1) (2) (3) (4) element C Au C-matrix C-matrix Au-matri x C-matrix C-matrix C-matri x Au-matri x C-matrix

B 0.6

FNa 0.06 20 56 3.7 4 130 47 6.5 8 Mg 0.11 < 0.05 0.9 0. 05 < 0.05 2.4 1.8 0.6 1.2

Al 1.2 3.2 0.

6

1.8 0.9 26 0.6 0.8 0.3

Si 1.1 0.54 9.8 6.

1

7.5 7 77 14.1 4.6 6.2

P < 0.05 18 6.

6

0.8 5 16 3.3 5.6 2.3

S 2.9 0.44 170 17.8 5.

1

14 110 12.1 6 10.7

CI 2.9 3.1 330 30 37 39.3 270 64 67.6 33.8

K 4.5 0.7 1.7 4.

7

1.5 6.2 13 7 0.6 8.8

Ca 1.1 0.44 19 1 .8 3. 2 1.4 9.5 1.5 10.1 1 Sc < 0.4 1.2 1 < 0.4 0.7 1.9 0.7 < 0.4 0.7

Ti < 0.3 51 1.5 < 0.3 0.8 75 1.4 0.5 0.5

V

- 1.1 71 7.6 0.5 3.2 160 8.4 0.8 3.4

Cr 0.34 < 0.3 < 0.3 2.1 < 0.3 < 0.3 < 0.3 1.9 1.7 Mn < 0.3 < 0.3 < 0.3 0.3 < 0.3 < 0.3 0.6 0.6

Fe 0.7 [15] 41 2.2 16 17.4 27 2.2 [ 131] 22.1

Co < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3

Ni 1.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3 < 0.3

Cu 6.7 1.8 < 0.3 0.5 < 0.3 < 0.3 < 0.3 < 0.3

Sn 8.4 < 0.6 < 0.6 < 0.6 1.1 1 < 0.6 < 0.6

Zn 0.33

Br

I 0.650.63

2 ~ 15 ~ 9 ~ 750 ~ 145 ~ 80 ~ 100 ~ 920 ~ 165 ~ 106 ~ 100

Note: 40 other elements not detectable (detection limit «1 fj,g g ) (1) before purification (base material, C-matrix)

(2) after ion exchange (base material, C-matrix)

2(THFH) +N03

- + [UQ2 (NQ 3 ) 4 ],« (THFH) 2+ [U0 2 (N0 3 ) 4 ] 900 °C + 2 NO3- *-U 3 0 8 (3) after peroxide precipitation (Au-matrix) +NH3 (pH2) 900 °C U02 (N0 3 ) 2 sol'n U04 U3 08 +H2 0 2 (4) after peroxide precipitation (C-matrix)

32
Table 13 Determination of 235u/238u Ratios Against Synthetic Isotope Mixtures

by UFg Mass Spectrometry. Uncertainties Indicated Under the Values

are 2s. The Last Column Contains the Results of the "Double Standard" Method, i.e., an Interpolation of the Measurement Between

the Two Measurements Against the Two Synthetic Mixtures

Synthetic Mixture Used "absolute" 235U/23bU

UFg value of material

Material sample preparation a -value number value "single "double standard" standard"

031 82

83

0.00321019 +0.00000041

0.00324159 +0.00000041

1.00171 +0.00056

0.99213 +0.00052

0.0032157 +0.0000019

0.0032161 +0.0000017

0.0032157 +0.0000016

071 118

119

0.00720669 +0.00000061

0.00732601 +0.00000062

1.00775 +0.00031

0.99127 +0.00031

0.0072625

0.0000023

0.0072620 +0.0000023

0.0072623 +0.0000017

194 102

103

0.0201836 +0.0000018

0.0199767 +0.0000021

0.99358 +0.00024

1.00397 +0.00024

0.0200540 +0.0000052

0.0200559 +0.0000053

0.0200552 +0.0000038

295 104

105

0.0305990 +0.0000032

0.0309820 +0.0000032

1.00560 +0.00021

0.99322 +0.00020

0.0307704 +0.0000073

0.0307720 +0.0000068

0.0307711 +0.0000051

446 106

107

0.0471216 +0.0000047

0.0475467 +0.0000048

1.00434 +0.00021

0.99530 +0.00025

0.047326 +0.000011

0.047323 +0.000013

0.0473248 +0.0000082

33
Table 14. ThIMS (1st series) Measurements: Correction Factor, K, Determined

with Synthetic Isotope Mixtures Over a Range, R5/8 > 0.0003 - 0.05. Uncertainties Indicated are 2s correction factor, K

235n/2 ^U Mixtures Rpreparation

Sample

number

^preparation ^observed ^observed

82 0 +0

.00321019 .00000041

0 +0 0032133 0000024

0 9990

83 0 ±°

.00324159 .00000041

0 +0 0032420

0000032

0 9999

84 0 +0 .00723863

.00000056

0 ±° 0072419 0000054

0 9996

85 0 +0 .00730288

.00000057

0 +0 0073068 0000034

0 9995

102 0 ±°

.0201836

.0000018

0 +0

020186 000010

0 9999

103 0 +0

.0199767 .0000021

0 +0019987 0000100 9995

104 0. +0. 0305990

0000032

0 ±° 030606 000004

0 9998

105 0. ±°- 0309820

0000032

0 +0 030990

000011

0 9997

106 0. +0. 0471216

0000047

0 ±°047137

000032

0 9997

107 0. +0. 0475467

0000048

0 +0 047529

000026

1 0004

0.9997

2s = +0.0007 (n=10)

The resulting correction factor, K, demonstrates that the measurement

procedure and instrumentation are under control. No significant contamination,

transcription, reading or weighing error occurred in any of the mixture

preparations, otherwise they would have shown up as "outliers".

The correction factor, K, for all remaining systematic errors, mainly

isotope fractionation is 0.9997 + 0.0007 (2s, n= 10). This correction factor,

K, was applied to all ^35^238^ therma;L ionization measurements shown in Table

15. A correction factor per mass unit was calculated [1 -(1 - 0.9997)/3] and K

factors derived for the other ratios:

34
K Isotope effect (%) for 235U/238U (3 mass units) 0.9997 0.03

for 234U/238U (4 mass units) 0.9996 0.04

for 236U/238U (2 mass units) 0.9998 0.02

Table 15. ThIMS Observed and Corrected 235U/238U Ratios Uncertainties

Indicated are 2s Material 235H/238U

observed

Correction Factor K

235U/238H

corrected

031 0.0032203 +0.0000040

0.9997 +0.0007

0.0032193 +0.0000046

071 0.0072704 +0.00000570.9997 +0.00070.0072682 +0.0000077

194 0.0209090 +0.0000016

0.9997 +0.0007 0.020084 +0.000021

295 0.030830 +0.000033

0.9997 +0.0007 0.030821 +0.000039

446 0.047405 +0.000039

0.9997 +0.0007 0.047391 +0.000051

The unsatisfactory agreement between the UFgMS and the ThIMS first series values led to a second series of ThIMS measurements, using the same synthetic

isotope mixtures, but a different mass spectrometer and analyst. The resulting K factors are given in Table 16. The observed and corrected 23 ^U/238U atom

ratios are given in Table 17. In Table 18 all the CBNM mass spectrometry measurements are summarized.

5.1.9 Determination of the Minor Isotope Abundances

These abundances were measured relative to the 235 isotope using an electron multiplier. These were then converted to ratios relative to 238U

e.g., 234U/235U x

23 5u/238U = 23V238U. An upper limit for the abundance of z--> -> U was estimated with a scan of the mass spectrum at high sensitivity. The other isotopes were measured by

peak- jumping in a sequence 234, 235, 236, 234 under computer control. The sources of error taken into consideration have already been identified

earlier in this section. For this series of measurements, 5-/j,g samples were used and a slightly higher ionizing filament temperature was employed. This resulted in a smaller value for the isotope fractionation correction applied.

Allowance was also made for the time dependence of the correction due to the minor isotopes being measured about 40 minutes after the majors. Results are given in Table 19

.

35
Table 16. ThIMS Measurements- Second Series: Correction Factor, K, Determined

with Synthetic Isotope Mixtures Over the range, R, 0.003 - 0.05. Uncertainties Indicated are 2s correction factor, K

235n/238n Mixtures preparation

Sample No. preparation Robserved Robserved

82 0.,00321019 0 .0032156 0,,99832 (n=5) +0

.

, UUUUUU41 +0 . UUUUUz

/

+0

,

, 00084

85 0.,00730288 0 .0073177 0..99797 (n=6) +0

,

, UUUUULo / +0 . (JUUUOoo +0

,

, 00112

102 0,,0201836 0 .020217 0,,99832 (n=6) +0

,

, (JUUUUlo +0 c\c\c\c\c\ c +0

,

, (JUL) jy 103 0.,0199767 0 .020011 0,,99832 (n=6)

+0,,0000021 ±° .000012 +0,.00039 104 0,,0305990 0 .030638 0,,99875 (n=5)

+0,,0000032 .000045 ±0 ,00096 105 0.,0309820 0 .031020 0,.99875 (n=5)

+0,.0000032 ±° .000028 ±° .00096 106 0,,0471216 0 .047219 0 .99788 (n=6) ±0 .0000047 +0 .000057 ±° .00080

107 0 .0475467 0 .047650 0 .99788 (n=6) +0 .0000048 +0 .000008 +0 .00080

Table 17. ThIMS Observed and Corrected Atom 235U/238U Ratios. Uncertainties

Indicated are 2s Material 235U/238U atom

ratios observed

Correction

factor, K

235U/238U atom

ratios corrected

031 0.0032202 +0.0000004

0.99832 +0.00084

0.0032146 +0.0000049

071 0.007276 +0.000009

0.99797 +0.00112

0.0072612 +0.0000122

194 0.020097 +0.000012

0.99832 +0.00039

0.020063 +0.000015

295 0.030827 +0.000031

0.99875 +0.00096

0.030788 +0.000044

446 0.047434 +0.0000500.99788 +0.000800.047333 +0.000063

36
5.1.10 Computing ZJ -) U/U Abundance Values

In Table 18 the 235U/238U ratios obtained from UFgMS and ThIMS are compared. The isotope ratios given in Tables 18 and 19 have been used to derive the 23 ^U/U abundances according to Equation 5a and these are given in Table 20. The 2s uncertainties given in Table 20, include those from the mass spectrometric measurements and from the isotope mixture preparation process. Table 18. 235U/238U Atom Ratio Values, in Percent as Determined by ThIMS and UFg Mass Spectrometry. Uncertainties Indicated are 2s and do Contain the Uncertainties of the Measurements as well as on the Synthetic Mixtures

Material 235u/238u Atom Ratios UF6MS ThIMS ThIMS ThIMS

(first series) (second series) (average)

031 0.,32157 0. 32193 0. 32146 0. 32170

+0.,00016 +0. 00046 +0. 00049 +0. 00048

071 0.,72623 0. 72682 0. 72612 0.,72652

+0.,00017 +0. 00077 +0. 00122 +0.,00100 194 2,,00552 2,,0084 2, 0063 2.,0074 +0 ,00038 +0,,0021 +0.,0015 ±0 ' ,0018 295 3 .07711 3,.0821 3,,0788 3 ,0805 +0 .00051 +0 .0039 +0,.0044 +° .0042 446 4 .73248 4,,7391 4.,7333 4,.7362 ±° .00082 +0,.0051 +0,,0063 +0,.0057

Table 19. 234U/238U and 236U/238U Ratio Determination by ThIMS. Uncertainties Indicated are 2s,(n=6)

Material

031

071

194

295

446

234U/238U atom ratio

0.0000202 +0.0000012

0.0000528 +0.0000031

0.000176 +0.000010

0.000289 +0.000016

0.00038 +0.00002

»U/238U atom ratio

0.0001465 +0.0000086

<0. 0000002

0.00000286 +0.00000053

0.0000336 +0.0000024

0.000072 +0.000004

37
Table 20. CBNM 23 5u/U Isotope Abundances. Uncertainties Indicated are 2s and Correspond to the 95% Confidence Level

Material

235U/U UF6 ThIMS atom % atom % abundance abundance

031 0.32049 0.32061 +0.00016 +0.00047

071 0.72096 0.72119 +0.00017 +0.00098

194 1.96575 1.96750 +0.00037 +0.00173

295 2.98432 2.98746 +0.00048 +0.00390

446 4.51668 4.52007 +0.00075 +0.00519

5.1.11 Computation of 234U/U, 236U/U, and 238U/U Abundance Values

The 234U/U, 236U/U and 238U/U abundances are given in Tables 21 through

23. For the conversion from atom abundance to mass abundance, following atomic masses were used (28)

:

234.0409474 ± 0.0000052

235.0439252 ± 0.0000052 236.0455629 ± 0.0000050 238.0507858 ± 0.0000048

5.1.12 Isotopic Homogeneity

The isotopic homogeneity of the RM batches was verified in several ways:

the isotopic composition of the samples taken from the batches as delivered (13 samples)

;

the isotopic composition of samples taken during the can filling process

;

the agreement of gamma measurements described in Section VI. The chemical preparation and the sampling procedure of the materials are summarized in Figure 11.

38
Isotopic

enrichment

(form: UF6 )

single batchUF6

hydrolysis and conversion to

U-jOo (at 450 °C)

delivery at CBNM and sampling

acceptance

and homogeneity

measurements

canning at CBNM and sampling

during

canning

homogeneity

measurements

each batch (expect natural U) was taken from

one withdrawal from the enrichment facility

natural U was directly converted to UFg

each batch processed in single lot and stored in a single container except for the 446 material

each batch was hydrolyzed and converted

separately to l^Og as one single lot

13 samples were taken from each batch of

30 kg for isotopic homogeneity checks

relative measurments were performed by UFc mass spectrometry on the c

J

JU//: ->0 >U/' U ratio

during the canning process 1 sample per

day was taken (one every 10 cans) yielding

about 15 samples per material

isotopic homogeneity was checked by UFg mass spectrometry on the 235^/238^ ratio and gamma spectrometry on the ^^^U/U ratio

Figure 11. Homogeneity Verification of Materials

39
Table 21.

Material

031

071

194

295 446

234U/U Isotope Abundances. Uncertainties Indicated are 2s

atom abundance 0.0000201 0.0000524

0.0001724

0.0002799

0.0003636

234U/U

mass abundance

0.0000198 0.0000515

0.0001695

0.0002753

0.0003576

uncertainty

(2s) 0.0000012

0.0000030

0.0000102

0.0000159

0.0000208

Table 22

Material

031

071 194

295 446

236U/U Isotope Abundances. Uncertainties Indicated are 2s 236U/U

atom

abundance 0.0001460

<0. 0000002 0.0000028 0.0000326

0.0000687

mass abundance

0.0001448

<0. 0000002 0.0000028 0.0000323

0.0000682

uncertainty

(2s) 0.0000090

0.0000005

0.0000023

0.0000042

Table 23

Material

031

071

194

295 446

238U/U Isotope Abundances. Uncertainties Indicated are 2s 238U/XJ atom abundance 0.9966290

0.9927379

0.9801673

0.9698443

0.9544009

mass abundance

0.9966709

0.9928292

0.9804137 0.9702149

0.9549521

uncertainty

(2s) 0.0000091

0.0000035

0.0000107

0.0000165

0.0000213

40
A. Requirements on the Supplier's Chemical Preparation Process Isotopic homogeneity of four batches of 30 kg U3O3 was specified by the requirement that each batch of material had to come from a single UFg with- drawal from the isotopic enrichment facility (except, of course, for the natural uranium). Subsequently, each batch was handled as one single lot and

was placed in one container to let the UFg in the gas/liquid phase homogenize

isotopically . Subsequent hydrolysis and calcination to 450 °C was performed in clean installations, one batch at a time, rigorously isolated from any other material (19). These precautions during the chemical preparation should guarantee

isotopic homogeneity at the UFg molecular (and hence U atomic) level of the base material for the production of U^Og. This requirement was not fulfilled

for material 446 which was reported (a long time after delivery) to have been

prepared by powder mixing (19)

.

B. Upon Delivery at CBNM

Each batch was sampled at 12 different positions according to the well- defined scheme given in Figure 12; four samples were taken from the top part, 4 samples from the middle part and 4 from the bottom part of each batch. A 13th sample was taken at random (designated sample "33") and served as the tie-point

for the relative measurements. The figures also indicate the sample sizes

taken, as well as the identification numbers of all samples in each batch. A check of isotopic homogeneity was then carried out by UFg gas mass spectrometry. After conversion to UFg (10), the ^35^/238^ ratio of each sample

(1 to 12) was compared to the 23 5^/23 8^ rat i Q Q f t^e 13th sample taken from the same batch (12) . Particular attention was given to the UFg sample vessels to ensure that there was no cross -contamination (11)

.

The results of checking the isotopic homogeneity of each material batch on delivery are given in Table 24. These measurements have been considered as acceptance measurements of the material as received and have not been taken

into account in the uncertainty of the final values

.

Table 24. Check of Isotopic Homogeneity by UFg MS of the Base Materials Upon

Acceptance. Measurements Performed on 12 Samples Versus a 13th

Sample, "33", Taken at Random

Material Variability (2s, %) 235U/238U sample "33" 031 071

194

295 446

1.00014

0.99988

0.99992

1.00000

0.99998

0.054

0.011

0.018

0.015 0.012

41
1) Samples

Top Middle Bottom

21 22 23

21 25 26

27 28 29

30 31 32

22- 25-28-31

23- 26-29-32

(25g each)

2) Samples taken and distribution CBNM -NBS

21 f —

N

->

24 Oxide 25 Oxide 26 Oxide

27 2.5g 28 25g 29

30 each 31 each 32 each

1 1 NBS

05g

CBNM

0 3g

Th I MS

UF6 MS

17g

CBNM

03g

UF6 MS

1.7g

1 1 NBS

05g

CBNM

0.3 g

Th. I MS

£CBNM^ rCBNM^k

UF6 MS UF6 MS

1.7g 1.7g

CBNM NBS

20g 20g

For elem ent assay and impurity determination

Figure 12. Sampling of U3 08 NDA RM 071 Material for Isotopic Measurements

42
Possible deviations from isotopic homogeneity were observed to be equal to or less than 0.054% (2s) for the 031 material batch and equal to or less than

0.025% for all others on the basis of 1.7 g size samples. The 0.054% uncer- tainty on the 031 material batch is caused by two of the 12 samples which

showed a deviation of about 0.06%in their 235^238^- ratios

.

5.1.13 Sample Collection During the Filling of the Al Containers

About 15 samples were taken at a rate of one per day i.e., one after each filling of 10 cans. All or part of these samples were analyzed and compared to each other for identity of the ^35^238^ rat i 0 by UFgMS . An example is given

in Table 25 for one material (the natural uranium, 071)

.

Table 25. Check of Isotopic Homogeneity by UFg Mass Spectrometry During the Filling of the 071 Material, "p" Indicates Samples Taken During the can Filling Process. Uncertainties Indicated are 2s 235U/238n sample

Sample 2s number 235U/238u sample ..33.. 21p* 1 00011 0 00034

22p 0 99996 0 00036

23p 1 00009 0 00037

24p 1 00005 0 00034

25p 1 00004 0 00035

26p 1 00014 0 00035

27p 1 00017 0 00030

28p 1 00008 0 00039

29p 1 00004 0 00034

30p 1 00014 0 00036

31p 1 00011 0 00036

32p 0 99999 0 00034

33p 1 00008 0 00034

34p 1 00002 0 00034

35p 0 99999 0 00034

36p 1 00003 0 00036

37p 1 00011 0 00038

38p 0 99994 0 00046 erage 1 00006 0 00012

Results for all materials are summarized in Table 26. On the basis of 0.5-g samples the isotopic homogeneity of the materials is confirmed to be within 0.025%. The values for the 031 material include one result for one sample which deviated by approximately 0.04% from the others. 5.1.14 Evaluation of the Overall Uncertainty of the ^^\J/\] Abundances

For this evaluation, the ^^^U/U in Equation 5a can be written as TM 235U/238H

2J5U/Xj = (5a) 234U/238H + 235H/238U + 236H/238U + 1

43
Table 26. UFgMS Check of Isotopic Homogeneity During Filling of the Cans on

the Basis of 0.5 Gram Sample Sizes

Material

( 235U/238U) sample

Variability (2s)

( 235U/238U) arbitrary ref. 031 1.00024 0. 00023 (n=8) 071 1.00006 0..00013 (n=18) 194 1.00006 0 .00014 (n=7) 295 1.00005 0 .00006 (n=8) 446 1.00001 0 .00012 (n=6)

Rq + Rr + 1

*5

N

(7)

where A5 = 235U/U and R4 , R5

, and R6 are 234U/238U, 235U/238U, and 236n/238U( respectively

In order to calculate the uncertainty on the abundance A5 , the following

quantities are needed: the uncertainties of the three measured ratios, their

correlation coefficients and the three partial derivatives of A5 with respect

to these ratios. The derivation and incorporation of correlation coefficients allows calculation of the Is -uncertainty on A5 from Equation 7 as follows:

• 2 - 2A (2 S 0 S.P0

.) + A

2 S S.S.P.

,

S

2 (A5 )

5

v r 2 i"2i

1 1 1 ij (8) (R4 + R5 + R6 +1 )

where i , j = 1,2,3 and

si = s(R4 ), s

2 = s(R5 ), s3 = s(R6 ) Pll = P22 " P33 = 1 P12 = P(R4> R5>' P13 = P(R4- R6)> P23 = P( R5> R6) Using ratios and their Is -values from Table 19 values for s(A5) were obtained as given in Table 20. Terms of p^^ when i ^ j were extremely small because at least one factor

of the term- either s^, s^ or pj4 was very small. This was due to the fact that 235U/238U ratios were measured independently by UF6MS from the 234U/238U

and 236uy238u ratios by ThIMS

All components of the overall uncertainty on the 23 ^U/U abundances,

expressed in parts per 10^ (2s) of the certified value, are summarized in Table

27.

44
Table 27. Components of the Uncertainty of the ^-'U/U Abundance Determination

and the Verification of Isotopic Homogeneity (Expressed in ppm of Value)

Uncertainty Source Material

031 071 194 295 446

1. Preparation synthetic

mixture s

- isotope purity 113 51 21 16 13

s to ichiome try 44 44 45 45 45

- chemical purity 14 14 14 14 14

- weighing 34 49 89 92 88

2. UFgMS a -measurements 467 210 160 126 135

3.

TO/, o 1 O 234u/238u

Mf* J5 Q11T~£i'm£»TTf~C 1 3 10 16 21

4. 236U/238U Measurements 9 <1 <1 2 4 Sub -total 484 227 190 165 170

5. Isotopic homogeneity 230 130 140 60 120 Overall 536 262 236 176 208

5.2 Measurements Made at NBS by Mass Spectrometry

5.2.1 Uranyl Nitrate Dihydrate procedure (Series: 031, 194, 295, and 446)

A. Filament Material

All analyses were performed using a rhenium triple filament ion source

(21). Standard high purity rhenium (nominal purity = 99.9%) was used. Differ- ent lots of rhenium vary considerably in the observed uranium fractionation

pattern and in the amount of U0+ ion signal generated. Zone- refined rhenium

generally produces less U0+

, however it also tends to give a less stable signal

with less internal precision than can be obtained with regular rhenium (22)

.

Therefore, zone -refined rhenium is not recommended for this procedure. A

U

+/U0+ ratio of 10 3 or greater after 20 minutes was chosen as a criterion for data collection.

B . Filament Outgassing

The rhenium filaments were outgassed prior to sample loading by heating in

a vacuum (less than 2 x 10 Pa) and under a potential field of approximately

45V, at a current of 3A through the sample filaments, and 4A through the ionizing filaments, for 1/2 hour. Ideally filaments should be used within 48 hours after removal from the vacuum outgasser to minimize air O2 absorption.

45
Most U0+ during the analysis appears to come from absorbed O2 on the ionizing

filament

.

C . Sample Loading

Five microliters of solution containing 1 mg U/mL in 0 . 8M HNO3 were loaded

onto each of two rhenium sample filaments, and dried as the uranyl nitrate

dihydrate for mass spectrometric analysis. Sample sizes from 10 /ig U to less than 1 fig U per filament are usable, however since the isotopic fractionation

correction is very dependent on sample size (Figure 13) , the amount of uranium

on the filament had to be very carefully controlled. Drying of the solution

onto the filaments was aided by a heat lamp placed above the filaments.

Reproducibility of the drying conditions was maintained through the use of a programmable sample dryer (23) which precisely controlled both the current through the filaments , and the drying time . The uranium samples were dried

under the following conditions: 40 °C for 10 minutes, 65 °C for 5 minutes, and

100 °C for 1 minute. These drying temperatures correspond to currents through

the filaments of 1A, 1 . 3A and 1 . 5A at an air flow rate across the filament of

60 meters per minute (200 ft/m) . Different air flow rates require readjustment

of the filament currents to maintain the proper temperature (Figure 14) . It is important that the filament temperature does not exceed 110 °C; at 120 °C the formation of UO3 may occur which will result in a different isotopic

fractionation pattern for those samples that have been partially converted to this oxide. The first step of the drying procedure deposits the uranium on the rhenium

filaments as uranyl nitrate hexahydrate and the second step melts the hexahydrate on the filament to ensure good sample- filament contact (melting point of uranyl nitrate hexahydrate is approximately 60 °C) . Thermodynamic data indi- cates that a relative humidity of at least 44% is needed to ensure melting of

the hexahydrate before conversion to the trihydrate (24,25). The final step in

the drying procedure converts the hexahydrate, through the trihydrate, to the dihydrate. Samples loaded in this manner have been shown to be stable and give

highly precise data for up to one year after being loaded on the filaments.

However, the samples probably pick up water of hydration and convert back to the tri- or hexahydrate, thus requiring more pump-down time in the mass spectrometer

.

D . Analysis Procedure

After the pressure in the mass spectrometer source had reached 2 x 10"-' Pa, liquid nitrogen was added to the source cold trap to further reduce the pressure to 2 x 10 "^ Pa. The ionizing filament was heated to 2140 °C for 5 minutes after which the sample filaments were also turned on with a current of 1.0A through the sample filaments for an additional 5 minutes. After a total time of 10 minutes, the filaments were turned off and the liquid nitrogen was removed from the cold trap. This initial heating step is to remove adsorbed oxygen from the filaments and source parts before the analysis.

After the source pressure had returned to 2 x 10 "^ Pa, liquid nitrogen was again added to the source cold trap. The ionizing filament was set at 2140 °C using an optical pyrometer (0.65 nm wavelength, not corrected for emissivity) and the sample filaments were set to a current of 1.50A. At 5 minutes into the analysis the sample filament currents were increased to produce a total uranium

46
47
400

0-1 1 1 r

0 12 3 CURRENT (A)

Figure 14. Effect of Air Flow on Rhenium Filament Temperature

48
ion current at the collector of 2 x 10 _iiA. The sample filament currents were increased at 10 and 15 minutes into the analysis to produce uranium ion currents of 4 x 10 "^A and 6 x lO'-^A, respectively. At 20 minutes the ionizing filament temperature was readjusted to 2140 °C and the sample fila- ments were adjusted, if necessary, to maintain a 6 x 10 "^A total uranium

signal. After baseline measurements, isotope ratio measurements were made starting at 30 minutes into the analysis. The 238-^/235^ ratj[ 0 measurements were made for twenty minutes, after which

the sample filaments were increased to obtain a total ion current at the collector of 6 x 10" ^A. The increased signal intensity was necessary to measure the minor isotopes. The 234^235^ ancj 236^235^ ratios were measured

by mass scanning and were bracketed by 238^/235-^ ratio measurements to allow corrections for isotopic fractionation. Observed isotopic ratios were corrected to absolute values by comparison with samples of SRM U-500 analyzed under identical conditions. SRM U-500 has a certified 235U/238U ratio of

0.9997. 5.2.2 U03 Analysis Procedure (071 series)

Due to the recertification of several uranium isotopic SRM's and a calibrated measurement of the isotopic composition of SRM 960 (currently an assay standard of nominal 0.71% 23 ^u abundance), synthetically prepared

calibrated mixes were available for determining the isotopic fractionation

corrections for the 071 series material. These calibration mixes were prepared

from chemically and isotopically pure ZJOU and ZJ -) U solutions which were assayed prior to mixing using an NBS modification to the NBL High Precision

Titrimetric Method which is based on the Davies-Gray technique.

A. Filament Material and Filament Outgassing

These procedures and materials were identical to those used for the uranyl

nitrate dihydrate analysis procedure discussed previously.

B. Sample Loading and Analysis Procedure

The major loading and analysis procedures used are discussed in detail in NBS Special Publication 260-27 (27). The same initial drying conditions used

for the uranyl nitrate dihydrate procedure were used, followed by heating of

the filaments in air to approximately 600 °C for 3 seconds to convert the sample to UO3 . The sample analysis procedure followed the same timing and

signal intensity levels as was employed for the uranyl nitrate dihydrate

procedure

.

5.2.3 Ins trumentat ion Isotope ratio measurements were made on an NBS designed thermal ionization

mass spectrometer with a 30 cm radius of curvature, 90° magnetic sector. The instrument was equipped with a thin lens "Z" focusing ion source, and a multi- component, deep -bucket Faraday cage collector (26). The remainder of the measurement circuitry consisted of a vibrating reed electrometer, voltage-tofrequency converter, scaler, and programmable calculator. Timing, magnetic

field switching, and data acquisition were controlled by the programmable

calculator

.

49
5.2.4 Sample Dissolution and Preparation of Calibrated Mixes A sampling scheme to accurately determine the level of enrichment and to demonstrate the degree of homogeneity was designed by the NBS Center for Applied Mathematics and supplied to CBNM for guidance during sampling and packaging of the NDA samples. From the sampling scheme, NBS received twelve

0.5 gram samples from each enrichment for isotopic analysis. An example of the actual sampling locations is given in Figure 12. Approximately 3mg of U3O3 was taken from each of the 12 sample vials for each of the four material lots. The samples were dissolved in 1-2 mL of HNO3 by heating. The sample solutions were evaporated to dryness and then diluted

with 0 . 8M HNO3 to give a solution concentration of lmg U/mL. The calibration mixes were prepared from 235y an(i238u separated isotopes. The two separated isotopes, in the form of U3O3 , were dissolved in HNO3 and filtered to remove small amounts of undissolved material. Impurities were determined by spark source isotope dilution mass spectrometry. No impurities were found at levels that would interfere with either the assay or mass spectrometric measurements. After determining the approximate concentration of the two separated isotopes from preliminary mixes, aliquants for the assay and calibrated mixes were taken by mass. Seven assay aliquants were taken first, then aliquants for the three calibration mixes and finally seven more assay

aliquants. The three calibration mixes were prepared to have one mix at approximately the same ^->°u/^-> -»u ratio as natural uranium, one to be approximately 0.5 percent more and the third to be approximately 0.5 percent less. Each calibration mix was evaporated to dryness, dissolved and diluted with 0 . 8M HNO3 to give a solution containing 1 mg U/g and submitted along with the NDA

sample solutions for mass spectrometric analysis. The assay samples were transferred to 180 mL tall form glass beakers and submitted for analysis. The NBL high precision titration for uranium requires gram amounts of uranium for precisions of > 0.005 percent. Features of this procedure have been adapted to the conditions of the standard NBL titration (approx. 100 mg U) to produce a hybrid version which has precision of better than 0.01 percent

compared to the usual 0.05 percent. All techniques and reagents are identical

to the published New Brunswick Laboratory methods (9). However, to achieve

reductions in the chemical blank and improvements in the sharpness of the potentiometric end point, both the apparatus and reagents have been scaled

down. The best precision was obtained when reagent volumes were scaled to one-half their normal size. At one- fourth scale, problems in adequate rinsing

(e.g., 1.3 mL reagent rinse for a 60 mL beaker) are probably responsible for a lack of improvement in precision. Consequently, all controls (SRM 960) and samples were aliquanted to contain approximately 80 mg of uranium in a 180 mL

tall form beaker. Titrations indicated essentially a fixed uncertainty over a sample range of 50-125 mg of uranium. Mechanical pipet aids were used to deliver the non-standard reagent volumes (e.g., 2.5 mL) required for the scaled down titration. The chemistry of the titration is as follows. An excess of ferrous sulfate is employed to reduce U(VI) to U(IV) in a concentrated phosphoric acid

solution containing sulfamic acid. The excess ferrous ion is selectively

50
oxidized by nitric acid in the presence of Mo (VI) as a catalyst. Cold water

and vanadyl sulfate quench the oxidation and prepare the sample for titration.

The U(IV) must be titrated within 2-3 minutes using standard SRM potassium

dichromate and a potentiometric end point. To improve titration precision, two weight burets were used to dispense a concentrated and a dilute dichromate solution into the sample to potentials of 480 and 550 mV, respectively. A dilute dichromate solution was added in increments of 50 uL by a calibrated repeating pipet and potential measurements

were made after each addition up to approximately 650 mV. The end point was calculated by means of second derivative determination and the equivalent

amount of uranium was computed.

Under these conditions, the assay of SRM 960 averaged approximately 99.99 percent, thus, displaying a relatively constant bias of -0.01 percent through- out all titrations. Since the modified titration required 5-7 minutes to complete, it is believed that air oxidation of U(IV) provided a slight competition for the dichromate ion. Nevertheless, both within and between set precisions were 0.01 percent or less for all of the samples and SRM 960 controls. Samples of the two isotopes and SRM 960 were run in batches in order

to average out small day-to-day variations in the apparent recoveries of SRM

960. Because of this, biases in the determined ratio of the concentrations of

the two major isotope solutions should be insignificant. All of the NBS mass spectrometry results are given in Tables 28-32. 5.2.5 Uncertainties in the Isotopic Ratios and Abundances

The Is uncertainties reported in Tables 28-32 and Table 37 represent the statistical uncertainty of replicate ratio measurements of the material and are thus a precision statement only, since they do not take into consideration

uncertainties associated with possible sources of systematic error. The 2s uncertainties reported in Tables 36 and 39 include terms for known

sources of systematic errors and other sources of random errors associated with

the measurements , and are thus reflective of the accuracy of the measurements

.

These uncertainties were calculated from the following formula:

2s = (A2 + B

2 + C2

) 1/2 where A is the 2s uncertainty on the measured isotopic ratio, B is the 2s uncertainty on the isotopic fractionation correction (K factor) used to place

the measurements on an absolute basis, and C is the 2s uncertainty on the standard or synthetic mixes used to obtain the isotopic correction factor. This term also includes components to cover the uncertainties in the isotopic

composition and assay of the separated isotopes used in the preparation of synthetic mixes. In all cases, the stated uncertainties reported in Tables 36 and 39 are at least as large as the uncertainties calculated by the NBS Statistical Engineering Division. In some cases, the uncertainties have been

expanded at the discretion of the analysts

.

51
Table 28. NBS Measured Isotope Abundances for the 031 Material

(atom percent)

Sample # 238 236 235 234

1 99. 66228 0. 014684 0. 321033 0. 002006

2 99. 66250 0. 014810 0. 320705 0. 001982

3 99. 66228 0. 014863 0. 320881 0. 001977

4 99.,66270 0.,014740 0. 320575 0.,001984

5 99,,66291 0,,014555 0. 320530 0,,002004

6 99,,66224 0,.014772 0. 321005 0,,001987

7 99 ,66250 0 .014693 0. 320808 0 ,002002

8 99 ,66292 0 .014558 0. 320536 0 .001984

9 99 .66260 0 .014906 0. 320501 0 .001994

10 99 .66275 0 .014755 0. 320492 0 .001999

11 99 .66197 0 .014817 0,,321208 0 .002004

12 99 .66235 0 .014822 0,,320832 0 .001999 Ave atom% 99 .66250 0 .014748 0,,320759 0 .001994

Std. Dev. 0 .00014 0 .000110 0,.000240 0 .000010 Ave mass% 99 .66669 0 .014624 0,,316721 0 .001960 Atomic Weight = 238.0408

Table 29. NBS Measured Isotope Abundances for the 071 Material

(atom percent)

Sample # 238 235 234 22-1 99.,27421 0. 72050 0,,00529 22-2 99..27392 0. 72077 0.,00531 22-3 99.,27384 0. 72079 0,,00536 22-4 99..27391 0. 72080 0,,00529 27-col-l* 99..27379 0. 72086 0,,00535 27-col-2 99,,27409 0. 72056 0,,00535

28 99,,27392 0. 72076 0,.00532

32 99,,27406 0. 72060 0 .00534 32-col 99,,27411 0. 72054 0 .00535 Ave atom% 99,.27398 0.,72069 0 .00533

Std. Dev. 0 .00014 0,,00014 0 .00003 Ave mass% 99 .28311 0,,71165 0 .00524 Atomic Weight = 238.0289

Initial measurements on this material, using the uranyl nitrate dihydrate

sample loading procedure, produced abnormal mass specrometric behavior,

indicating possible interferences from chemical impurities. To ensure that

this problem would not bias the measurements using the UO3 sample loading

procedure, three samples (designated by -col) were further purified with an anion exchange column.

52
Table 30. NBS Measured Isotope Abundances for the 194 Material

(atom percent)

Sample # 238 236 235 234

41 98

.

,01538 0. 00031 1. 96651 0. 01780

42 98,,01538 0. 00031 1. 96664 0. 01766

43 98.,01573 0, 00029 1, 96647 0, 01750

44 98

.

,01572 0. 00029 1, 96641 0, 01758

45 98 ,01529 0.,00033 1, 96674 0.,01764 46 98 ,01578 0,,00031 1. 96629 0,,01762 47 98 ,01558 0,,00033 1,.96644 0.,01764

48 98 ,01620 0 ,00029 1,,96595 0 ,01755 49 98 .01542 0 ,00030 1,,96656 0 .01772

50 98 .01531 0 .00035 1.,96648 0 .01785

51 98 .01579 0 .00028 1 ,96606 0 .01787

52 98 .01619 0 .00031 1 .96567 0 .01783 Ave atom% 98 .01565 0 .00031 1 .96635 0 .01769

Std. Dev. 0 .00017 0 .00002 0 .00031 0 .00012 Ave mass% 98 .04029 0 .00031 1 .94200 0 .01740 Atomic weight = 237.9909

Table 31. NBS Measured Isotope Abundances for the 295 Material

(atom percent)

Sample 238 236 235 234

61 96 .98113 0..00323 2 ,98670 0 .02894

62 96 .98067 0 .00329 2 .98715 0 .02888

63 96 .98154 0 .00343 2 .98621 0 .02882 64 96 .97945 0,.00320 2,.98817 0 .02868

65 96 .98058 0,.00332 2,.98725 0 ,02886

66 96 .98082 0 .00332 2,,98689 0 .02897

67 96 .98102 0,.00326 2,,98705 0 .02867

68 96 .98127 0,,00326 2,,98671 0 .02876

69 96 .98153 0.,00323 2,,98667 0 .02858

70 96 .98140 0,,00332 2,,98638 0 .02891

71 96 .98151 0,,00329 2,,98636 0 .02885

72 96 .98145 0.,00326 2,,98675 0 .02855 Ave atom% 96 .98103 0.,00328 2,,98686 0 .02879

Std. Dev. 0 .00033 0,,00006 0.,00052 0 .00014 Ave mass* 97 .01817 0 ,00325 2,,95026 0 .02832 Atomic Weight = 237.9597

53
Table 32. NBS Measured Isotope Abundances for the 446 Material

(atom percent)

Sample # 238 234

81 95. 44199 0

.

00695 4

.

51435 0

.

03670

82 95. 44130 0

.

00700 4

.

51464 0

.

03707

O 0 oj 95.,44302 aU . uubyy cl OQO Dizyy a 03710

84 95.,44400 A(J

.

uuby D 4

.

51248 0 . 03655

O £

o_>

/. /. n o a au . AA"7 AA U(J /UU 4

.

C 1 /, 1 Q a mini uoby z 86 95,.44359 0. 00695 4. 51257 0. 03673

87 95 .44235 0. 00695 4.,51418 0. 03652

88 95 .44292 A uuby j , Di 5 jy Au

,

A Q £ ~7 /.

, U JO /<-+ 89 95 .44173 A uubyD 4

,

, 51434 ft ,03697

90 95 .44222 0 . , 00695 4

,

. 51413 0 ,03670

91 95 .44201 0.,00691 4..51420 0 ,03688

92 95 .44206 0,,00670 4 .51442 0 .03652 Ave atom% 95 .44242 0 .00694 4 .51381 0 .03678

Std. Dev. 0 .00076 0 .00008 0 .00077 0 .00020 Ave mass* 95 .49756 0 .00688 4 .45937 0 .03618 Atomic Weight = 237.9133

54
5.3 Measurements Made at NBL by Mass Spectrometry

Four 0.025-g samples of each material were weighed into beakers. The oxide was prepared by NBL's standard mass spectrometry procedure (9). Each

sample loading was considered a separate determination, two filaments per analysis using a triple filament ion source. The analysis plan was as follows: Analysis No. Material Determinations

1 NBS SRM 005 1 & 2

2 U 0315 1 & 2

3 U 0315 3 & 4

4 NBS SRM 005 3 & 4

5 U 0715 1 & 2 6 U 0715 3 & 4

7 NBS SRM 010 1 & 2

8 NBS SRM 020 1 & 2 9 U 195 1 & 2 10 U 195 3 & 4

11 NBS SRM 020 3 & 4 12 NBS SRM 030 1 & 2 13 U 295 1 & 2 14 U 295 3 & 4

15 NBS SRM 030 3 & 4

16 NBS SRM 050 1 & 2 17 U 446* 1 & 2 18 U 446* 3 & 4 19 NBS SRM 050 3 & 4 20 U 446* 5 & 6 21 U 446* 7 & 8 22 NBS SRM 050 5 & 6

The asterisk (*) indicates that series 445 samples were not available at the same time that the other samples were analyzed. However, they were analyzed

later in a similar manner and bracketed with NBS SRM 050.

The results of these analyses are given in the following Table 33. 5.4 CBNM Gamma Spectrometry Measurements A high resolution gamma spectrometry system with a full width at half

maximum (FWHM) of 0.83 keV at 185.7 keV, Figure 2., was used to determine the overall uniformity of the entire 140 sets of reference materials.

5.4.1 Measurement of all of the Canned Reference Materials

All 140 of the fabrication samples from each of the five materials were tested by gamma counting the ^35^ 185.7 keV gamma- ray, using a Pb collimator 25 mm in diameter and 40 mm in height. These gamma measurements were all low precision with 1 relative standard deviation (RSD) between +0.5% and +0.9%, and were made relative to a calibration sample with a ^35^ i so tope abundance of

55
Table 33. Uranium Isotopic Abundances Determined by NBL

Sample (n) 234 235 236 238 Rel. At. Wt. 031 (4) mass% 0. 0022 0,,3163 0. 0145 99 , ,6670 238. 0408

sd 0. 0002 0,,0002 0. 0002 0,,0002 atom% 0. 0022 0,,3203 0. 0147 99,,6628 071 (4) mass% 0. 0053 0. 7116 0. 0000 99

,

,2831 238. 0289

sd 0. 0002 0. 0002 0.,0003 atom% 0. 0054 0. 7207 0. 0000 99,,2739 194 (4) mass% 0. 0175 1, 9414 0. 0003 98

.

0407 237. 9910 sd 0. 0004 0. 0005 0. 0003 0, 0009 atom% 0. 0178 1. 9657 0. 0003 98. 0161

295 (4) mass% 0. 0280 2. 9477 0. 0030 97. 0213 237. 9599 sd 0. 0002 0. 0012 0. 0004 0, 0015 atom% 0. 0284 2. 9843 0. 0030 96,,9843 446 (8) mass% 0. 0361 4,,4617 0. 0063 95.,4958 237. 9134

sd 0. 0003 0,,0015 0. 0004 0,,0014 atom% 0. 0367 4,,5162 0.,0064 95,,4407 SRM U-005 mass% 0,,0024 0,.4835 0,,0045 99 .5096 238,,0359

(5) sd 0,.0002 0 .0002 0,,0003 0 .0003 atom% 0 ,0024 0 .4897 0 ,0046 99 .5033 SRM U-010 mass% 0 ,0056 0 .9911 0 .0067 98 .9967 238 ,0203

(2) sd 0 .0001 0 .0002 0 .0003 0 .0003 atom% 0 .0057 1 .0036 0 .0067 98 .9840 SRM U-020 mass% 0 .0123 2 .0122 0 .0166 97 .9590 237 .9887

(4) sd 0 .0002 0 .0005 0 .0002 0 .0004 atom% 0 .0125 2 .0374 0 .0167 97 .9334 SRM U-030 mass% 0 .0189 3 .0085 0 .0203 96 .9524 237 .9580

(4) sd 0 .0003 0 .0007 0 .0003 0 .0007 atom% 0 .0192 3 .0458 0 .0205 96 .9146 SRM U-050 mass% 0 .0275 4 .9497 0 .0475 94 .9753 237 .8981

(6) sd 0 .0005 0 .0016 0 .0005 0 .0020 atom% 0 .0280 5 .0098 0 .0479 94 .9194

2.98 atom %, to minimize the effect of detector efficiency shifts. Figures 15a

- 19a illustrate the statistical distribution of the counting rates of all the cans within a given abundance. In addition, the agreement shown between the distribution of the mean counting rate value and the counting precision of single measurements, represented by the shaded area, confirmed the statistical nature of the measured distribution, as well as monitored the uniformity of the samples during the canning process.

56
Two types of samples were selected from the low precision measurements for confirmation of sample uniformity and for high precision measurements:

samples with counting rates approximately equal to the mean counting

rate

samples with counting rates at the high and low ends of the distribution.

Some of these low precision samples comprised the 24 samples per material

that were sent to NBS and to another Joint Research Center within the European

Community, JRC-Ispra Establishment, for gamma spectrometry measurements.

5.4.2 High Precision Measurements of the 15 Selected Samples

The 15 samples from each of the five materials were selected from the center and tails of the counting rate distributions presented in Figures 15a - 19a. These cans were measured under the same conditions as before but with

precisions (RSD) between +0.16% and +0.29%. The results of the high precision

measurements, as seen in Figures 15b - 19b, confirm the can to can sample

uniformity observed in the low precision measurements.

5.4.3 High Precision Measurements of the 446 Material

The counting rates of all 140 samples from the 446 material were measured

relative to one of the samples in the series . These samples were remeasured

under conditions described earlier except for a Pb collimator 30 mm in diameter and 40 mm in height, and a 1 RSD measurement precision of +0.14%. The measurement distribution curve in Figure 20 compares the statistical

distribution to the counting precision of a single measurement. The shaded area also confirms the sample to sample uniformity that had been discussed

above, in sections 1 and 2. The results of these measurements indicate the sample to sample

non-uniformity was less than the calculated value of +0.14% which was projected

in section IV. K. 3.5.4.4 Determining the Linearity of the Sets High precision gamma measurements, using the same conditions as in 3, were made on the 50 CBNM sets fitted with ultrasonic seals. These measurements were

o 6 c made with a 1 RSD counting precision proportional to the "JU isotope abundance

in atom percent, (A^ of each material, such that 1 RSD % < ± 0.5 x (0.25+

0.1/V).

The observed counting rate for each material (n) was normalized to its 235u/U abundance [ (Av) n ] and corrected for the interference (Ky) from the ^34pa and 234mpa daugthers of 238U

cn • Kyn

Cn

= (9) (Vn

57
-3-2-1 0 1 2 3 No of RSD's

a. Low precision test of samples (RSD = 0.87%)

1 hR • 30 1 58 37 1 • 69 • 50 | 85 A 71 1 • 92

A 8k A 99 97 I • 105 A 107 | A 119

-1,65 0 1,65 No of RSD's

b. High precision test of samples (RSD = 0.28%)

as identified above Figure 15. Gamma Counting Rate Uniformity of U3 0g Reference Samples at 1857 keV (corrected for window thickness)

- 235U/U = 0.003207

- Germanium detector 0 : 50 mm; h: 15 mm with resolution (FWHM) of 0.83 keV

- Collimator 0: 25 mm; h: 40 mm

58
] : to Ispra

I > : to NBS

Selected samples for intercomparison tests

at NBS and Ispra

Sample identification No

Statistical distribution due to the counting precision of

a single measurement

RSD = 0.76%)

2 3 No of RSD's

a. Low precision test of samples (RSD = 0.76%)

No of RSD'S

b. High precision test of samples (RSD = 0.29%)

as identified above Figure 16. Gamma Counting Rate Uniformity of U3O3 Reference Samples at 185.7 keV (corrected for window thickness)

- 235 U/U = 0.07208

- Germanium detector 0: 50 mm; h: 16 mm with resolution (FWHM) of 0.83 keV

- Colliminator 0: 25 mm; h: 40 mm

59
2 3 No of RSD '

Low precision test of samples (RSD = 0.56%)

1

l • 10 A 41k8 • 59 19 61 39

A 78 | A ko A 81 | • 55 • 110 I • 70 A 124

I 1 117

1,83 No of RSD's

b. High precision test of samples (RSD = 0.19%)

as identified above Figure 17. Gamma Counting Rate Uniformity of U3O3 Reference Samples at 185.7 keV (corrected for window thickness)

- 235 U/U = 0.01967

- Germanium detector £3 : 50 mm; h: 16 mm with resolution (FWHM) of 0.83 keV

- Colliminator 0: 25 mm; h: 40 mm

60
a. Low precision test of samples (RSD = 0.51%)

1 21 10

J

< » 36

75 . » 38

95 1 I k 60 • ^^h \ [ 72 117 |

< » 73

131 | I k 93

133 |

< » 110

09 0 2.23

No of RSD'

s

b. High precision test of samples (RSD = 0.21%)

as identified above Figure 18. Gamma Counting Rate Uniformity of U3O3 Reference Samples at 185.7 keV (corrected for window thickness)

- 235 U/U = 0.02985

- Germanium detector 50 mm; h: 16 mm with resolution (FWHM) of 0.83 keV

- Colliminator 0: 25 mm; h: 40 mm

61
-3-2-1 0 1 2 3 No of RSD'S

a. Low precision test of samples (RSD = 0.51%)

0 22

if8 o 28

^9 A 38

52 0 59 64 A 72

78 73 o 100 A 81 • 121 85 T

-1.56 0 1.56 No of RSD's

b. High precision test of samples (RSD = 0.16%)

as identified above Figure 19. Gamma Counting Rate Uniformity of U3O3 Reference Samples at 185.7 keV (corrected for window thickness)

- 235 U/U = 0.04518

- Germanium detector 0: 50 mm; h: 16 mm with resolution (FWHM) of 0.83 keV

- Colliminator 0": 25 mm; h: 40 mm

62
No of RSD's

No° or samples : 140 Detector : Germanium, 0:50 mm; h :16mm Detector resolution (FWHM) : 0.83 keV for 185.7 keV Collimator : 0:30 mm; h:40 mm

Figure 20. High Precision Test of the Gamma Counting Rate Uniformity of U3O3 Reference Samples, 235U/U = 0.04518, at 185.7 keV

63
where C„ is the corrected counting rate for each of the five materials and

C^ is the weighted average of the observed counting rates (uncorrected) for the individual samples of material n. The deviation (dCn) for each material compared to the average value for all five materials (C) was calculated and normalized to C and these values are plotted in Figure 21.

(10)

This figure illustrates the linearity of the calibration curve for the particular instrumentation system used as well as the agreement between ratios

of the ^35^ i so tope abundance values obtained from the mass spectrometry measurements and the corresponding ratios of the measured gamma counting rates.

5 . 5 NBS Gamma Spectrometry Measurements

All of the NBS verification and isotope abundance measurements by gamma spectrometry were made on a counting system that consisted of an Ortec Gamma X

type high purity germanium detector, a Canberra 3105 high voltage supply, a Canberra 2021 S amplifier and a Nuclear Data ND 6620 computer-based analyzer

system. The amplifier time constant was set at 6 us, and an ADC conversion

gain of 8192 channels was used. At low counting rates, this system has a resolution of 1.69 keV full width at half maximum (FWHM) for the 1332 keV gamma

ray of 60Co and an efficiency of 7.99% relative to a 3"x3" Nal(Tl) detector at 25cm. The resolution observed in this work for the 185.7 keV gamma ray of ^35^ varied from 0.95 keV (FWHM) for the 031 sample samples (15% dead time) to 1.00 keV for the 446 sample series (22% dead time). Separation of the 182.57 and

185.7 keV gamma rays was such that the channel between these peaks contained

approximately 0.02% (above background) of the net 186 keV peak region counts. The 071, 194, 295 and 446 sample series were counted for times sufficient to accumulate approximately 5 x 10^ net counts in the 186 keV peak region (2-12 hours live time) . The counting statistics observed for the 186 keV peak region were typically 0.04-0.06%. The 031 samples were counted overnight which

produced approximately 2.5 x 10^ net counts in the 186 keV peak region.

Typically the observed counting statistics for the 031 samples were 0.10%. In order to ensure reproducibility in geometric positioning of the cans

for counting, a collimator assembly was fabricated from lead for this work (see

Figure 2a). A U-shape configuration with a diameter of 81 mm and a depth of 35mm was machined into the top surface of the lead to allow a rigid resting

place for the aluminum cans , and to maintain an identical counting geometry

from one sample to another. The collimator opening was then machined to a 50mm diameter with a 15mm thickness. A copper plate (12cm x 12cm x 6 mm) with a 70 mm diameter opening, for the high purity germanium detector endcap, was then affixed to the base of the collimator. This entire assembly was then fitted

with four mounting screws, allowing the assembly to be supported by the floor

of the detector shield and not by the detector.

The counting standards used in this work consisted of a set of three prototype 200g U^Og samples which had previously been developed to determine

the feasibility of the program. These counting standards were prepared by CBNM

64
[•/.]

1

' rl y—' ' H ; 1 —

1 2 3 A 5 [at%

A v

Figure 21. Average Variation of Normalized Gamma Counting 4ates at 185.7 keV versus 235U Abundance, Av , for the CBNM 50 Sets of Samples

(Uncertainies are 2s and Include the Mass Spectrometry

Uncertainties of Av

of 0.07%

65
(4) in the same manner as the reference materials with the exception of the aluminum containers. The prototype cans were fabricated with flat bottoms,

while the reference material cans were produced with 1mm recessed bottoms to minimize scratching of the can windows. The isotope measurements of these samples were performed and certified by CBNM to have ^^U/U atom percent abundances of 2.9788 + 0.0090, 0.7201 + 0.0021 and 0.02978 + 0.0009, respectively, and were identified as SD-524, SD-528, and SD-521. In addition, veri- fying gamma spectrometry measurements were also performed on the samples by NBS and CBNM. Although all three counting standards were used during the course of this work, the primary counting standard was considered to be the 2.9788 atom percent sample (SD-524). 5.5.1 Peak Integration Methods

Three different methods were used to integrate the 185.7 keV 235y

p eak : a Gaussian fitting routine; a channel by channel doublet summation of the com- bined 183 (small) and 186 keV peak regions, and a channel by channel singlet

summation of the 186 keV peak region alone. The channel between the 183 and

186 keV peak regions (containing 0.02-0.03% of the net counts in the 186 keV peak region) was included as part of the 186 peak region for the singlet

summation. Both peak and background regions for the two summation methods were selected symmetrically around the 185.7 keV peak centroid. See Figure 22. Of the three methods used for peak integration (see Table 34 for examples

of the results for a single series using all three methods) , it was expected

that the summation methods would be better than the Gaussian fitting for a number of reasons. Gaussian fitting introduces added uncertainties since gamma-ray peaks are not fully Gaussian. In addition, the integration routine can choose different peak boundaries for the samples and standards further increasing the uncertainty. The singlet method is somewhat sensitive to changes in the width of the 186 kev peak region between sample and standards

.

The results obtained for the five series using the three methods are listed in Table 35. However, since the set of five samples were all compared to the same standard and some differences were observed in peak width, the doublet summa- tion method is considered to be the most accurate. However, within a single

set of samples the singlet method provides the best measure of precision since

the doublet method includes a number of background channels as part of the peak

which contributes to poorer counting statistics.

5.5.2 Corrections Applied to Gamma Measurements

Several corrections were applied to the observed counting rate of the 186 keV peak region. These included corrections for pulse pileup, ^34pa an(i 234mpa interference , differences in window thickness of the sample containers , and differences in container geometry for the samples compared to the standards.

66
67
Table 34. ZJ:3U Abundances Measured at 185.72 kev Using the Three Integration Methods (atom percent with 1 sigma)

C3.T1 Nurnb © it Doublet3 Singletb Gaussian0 NBS-295-117 2. 97941 2. 97909 2. 97970 CBN-295-005 2. 98201 2. 98266 2. 98248 CBN-295-024 2. 98297 2. 98305 2. 98137 NBS-295-079 2. 98465 2. 98475 2. 98483 NBS-295-075 2. 98085 2. 98154 2. 98200 CBN-295-022 2. 98151 2. 98102 2. 98119 CBN-295-023 2. 98195 2. 98151 2. 98006 NBS-295-071 2. 97809 2. 97839 2. 97908

CBN- 295 -031 2. 98327 2. 98289 2. 98010 CBN-295-026 2. 98498- 2. 98398 2.,98653 CBN-295-036 2. 98325 2. 98261 2.,99087 NRS-99S-1 J.N J_J D J ~) J_ 09V 2. 98652 2. 98524 2,.98379 wrc 1NU J - QQS Z_ _/ - 11 OSV J 2. 98059 2,,98153 2 ,98179

Ll JJ lj £-Zt JL JL 17 2. 98490 2,,98434 2 .99165 NBS-295-084 2,,98352 2.,98281 2 .98724 CRN-99S-01

0

2 . 98300 2 . 98179 2 . 98984 NBS-295-083 2 .98374 2 .98515 2 .99325 CBN-295-012 2 .98591 2 .98585 2 .99317 NBS-295-133 2 .98179 2 .98211 2 .98654 NBS-295-076 2 .98229 2 .98227 2 .98691 NBS-295-132 2 .98452 2 .98455 2 .98344 CBN-295-019 2 .98364 2 .98166 2 .99037 NBS-295-087 2 .98481 2 .98491 2 .99344 CBN-295-062 2 .98345 2 .98291 2 .98220 Mean 2 .98298 2 .98277 2 .98549 Sample STD DEV 0 .00199 0 .00187 0 .00476

individual Uncertainties ±0.00155 individual Uncertainties ±0.00144 individual Uncertainties ±0.00115

68
Table 37. Summary of Uranium Isotopic Abundance Measurements by

ThIMS for 234U, 236U, and 238U

Material 234 236 238 At. Wt

.

031 CBNM-TblMS

071

194

295

446

atom% 0 00201 0 01460 99 66273 238 04087

o ^ AU 00012 0 AAAO A uuuyu 0 00091 NBL

atom% AU 0022 0 0147 99 6628 238 0408

OZs AU AAAO UUUZ Au AAA O UUUZ AU AAAO UUUZ NBS atom% AU AA1 O O uuiy y Au 01475 y y DOZ_> Zoo AAAO U4Uo

Is 0 00001 0 00011 0 0001

n M"NT urSrUNI -ThIMS atom% AU

A A C O A UUDZ4 <0 A AAAO UUUUZ 99 27389 238 02919

Zs AU AAAQ A UUU JU 0 00035 NBL

atom% AU 0054 <0 AAAAO UUUUZ 99 2739 238 0289

9/sc A

U UUUZ AU UUU J Nine atom% AU AAUU jcnj J <0 AAAAO UUUUZ 99 27398 238 0289

Is 0 00003 0 00014

CBNM -ThIMS atom% AU

ni 7i/, Ul / Z4- Au UUUZo y o UlDU _> 15 1 Q O 1 7

y y 1 / 9 o AU UU1UZ 001 09 Au AAA A R UUUUJ Au OOI UU 1U07/ JNrSLi atom% AU Ul /o Au UUU j Q Q y o Ulbl O "3 "7 Z j / yy iu

9z so AU OOO/i UUU*+ Au 0007 UUU J Au OOOQ uuuy NBS atom% AU

AI ~7£ Q Ul /by Au UUUJ1 y o m c; c ^ UlDbD O *3 "7 Z j / QQi n yy iu

Is 0 00012 0 00002 0 00017 CBNM -ThIMS atom% A

u uz /yy A

u AAO Of UUJZb y o y o u y d

9 "3.7 Z J / y buy

9zso AU OOI uui jy Au AAAO "3 UUUZ J Au OOI =1 MTJT JN-Dli atom% AU 09 R/i AU 00^0 UU jU 7 0 yo L¥ j 9 "3.7 Z J / 9zsc Q 0002 o 0004 o 0015

IN DO atom% AU AOuzoQ7Q/ y AU AAOOO UUjZo y D

QP.1 O^ 9^7 Z J /

1±se AU OOOl A UUUJ.£+ Au OOOOfi UUUUD u 00033 CBNM -ThIMS atom% 0 03636 0 00687 95 43622 237 9150

2s 0 00208 0 00042 0 00213 NBL

atom% 0 0367 0 0064 95 4407 237 9134

2s 0 0003 0 0004 0 0014

NBS atom% 0 03678 0 00694 95 44242 237 9133

Is 0 00020 0 00008 0 00076

73
Table 38. Values of Uranium Isotopes, ZiU\J , ^ i6U and Zi8\J , in percent, given on the certificate.

Material Uranium Isotopes

234u 236u 238u

031 atom 0 .0020 0 .0147 99.,6627

2s ±0

.

, UUUz ±0

,

. 0003 ±0

.

0004

mass U , . uuzu U , . U14o yy

.

6668 071 atom 0,.0053 <0..00002 99. 2738

Zs ILL)

,

. UUUZ UUU4

mass V

.

. UU DZ <u , uuuuz Q Q 0 0 O Q ZoZo

194 atom 0 .0174 0 .0003 98.,0159 2s ±0 .0002 ±0 .0001 ±0. 0018 mass 0 .0171 0 .0003 98.,0406 295 atom 0 .0284 0 .0033 96,.9826

2s ±0 .0004 ±0 .0002 ±0..0029 mass 0 .0279 0 .0033 97 .0196 446 atom 0 .0365 0 .0069 95 .4398

2s ±0 .0003 ±0 .0002 ±0 .0032 mass 0 .0359 0 .0068 95 .4950

Table 39. Summary of ^-"U/U Isotope Abundance Measurements, in Atom Percent with 2s Uncertainties, by Method and Laboratory

Laboratory/Method Material

03O Q71 194 295 446 CBNM-UF6 0.32049 ±0.00016

0.72096 ±0.000171.96575 ±0.00037

2.98432 ±0.000484.51668 ±0.00075 CBNM-ThlMS 0.32061 ±0.00047

0.72119 ±0.00098

1.96750 ±0.001732.98746 ±0.00390

4.52007 ±0.00519 NBS-ThlMS 0.32076 ±0.00052

0.72069 ±0.000681.9664 ±0.0017

2.9869 ±0.00224.5138 ±0.0033 NBL-ThlMS 0.3203 ±0.0007

0.7207 ±0.0007

1.9657 ±0.0015

2.9843 ±0.0039

4.5162 ±0.0039

NBS- Gamma 0.32053 ±0.00026

0.72081 ±0.00058

1.9660 ±0.0016

2.9830 ±0.0024

4.5158 ±0.0036

74
Table 40. Summary of the Certified Isotope Abundance Values,

(in percent) and uncertainties (±0.07% on all values)

Material

071 194 295 446 Atom

±0.07%

Mass

031 0.3206 ±0.0002

0.3166

0.7209 ±0.0005

0.7119

1.9664 ±0.0014

1.9420

2.9857 ±0.0021

2.9492

4.5168 ±0.0032

4.4623

The overall uncertainty for each of the Zi -> U/U certified values was estimated by combining the uncertainties of the mass spectrometry measurements with any possible deviation of the ^^^U/U ratio due to inhomogeneity . The resulting uncertainty for each abundance was enlarged to 0.07%, which

corresponds to the 95% confidence limit. The NBS gamma spectrometry measurements which were not a component of the certified value were found to be within 0.09% of the certified value for the 295 material and 0.02% for the 031, 071, 194 and 446 materials as can be seen

in Figure 23. These measurements demonstrate the accuracy that is achievable

for 235tJ/U i so tope abundance determination by gamma spectrometry.

75
enjBA pez||eiujoN

76
7. Acknowledgment: We are indebted to the Office of Standards Development, U. S. Nuclear Regulatory Commission and the former Office of Measurements for Nuclear

Technology, National Bureau of Standards as well as H. Thomas Yolken, the former Office Chief, for providing the NBS group with the initial support for this program in fiscal year 1978. We also are indebted to E. Bouwmeester, R. Damen, W. De Bolle, B. Dyckmans-Van Hout, R. Eyckens , M. Gallet, F. Hendrickx,

W. Lycke, J. Mast, A. Michiels , G. Muschenborn, W. Nagel, M. Parengh, F. Quik,

H. Ruts, J. Tr iffaux, A. Verbruggen, and V. Verdingh, members of the CBNM

staff, as well as K. Rosman (on leave to CBNM from the Western Australian

Institute of Technology) for their contributions to the development and certi- fication of this SRM, as well as to the European Safeguards Research and Development Association Working Group on Techniques and Standards for Non- Destructive Analysis for initiating this nuclear reference material project in Europe, providing advise on technical specifications, and participating in the execution of parts of this project. We also extend a note of thanks to the European Communities Nuclear Certification Group and a specical note of grati- tude to R. Lesser, CBNM, for all the effort that he devoted to this project.

Appreciation is also extended to W. Liggett, Jr., Center for Applied Mathematics, NBS, for developing the sampling plan, to R. Werz", CBNM, for the statistical assessment of the data used for the certification of this SRM, and

to J.R. Moody, NBS, for assaying the separated isotope solutions used to calibrate the mixes of the 071 material. Finally, a special note of thanks to T.E. Gills who coordinated the technical support aspects concerning the preparation, certification, and issuance of this reference material for the Office of Standard Reference Materials.

77
8. References

1. Commission of the European Communities Certification Report COM 4153,

"^^Uranium Isotope Abundance Certified Reference Material for Gamma Spectrometry," EC Nuclear Reference Material 171, 1985.

2. Harry, R.J.S., "Enrichment Standards for Gamma Spectrometry," Proc. 6th ESARDA Symposium on Safeguards and Nuclear Material Management, Venice,

Italy, May 1984, pp. 187-193, (ESARDA Rep. No. 17).

3. Kull, L.U. and Ginaven, R.O., Guidelines for Gamma-ray Spectroscopy Measurements of 235U Enrichment, BNL-504/4 (1974).

4. De Bievre, P., Meyer, H. , Van Audenhove, J., and Carpenter, B.S., Gramlich, J.W., Progress Report on the Establishment of Uranium Isotope

Abundance RM's for NDA, Proc. of the 4th Annual ESARDA Symposium, Petten, Netherlands, April 1982, pp. 65-74, (ESARDA Report No. 15).

5. Matussek, P., Accurate Determination of 23 ^u isotope Abundance by Gamma Spectrometry, A User's Manual for the Certified Reference Material, EC-NRM

171/NBS SRM 969, KfK Report 3752, 1984.

6. Ottmar, H., Primarbericht 07 . 0103p09A, KfK, February 1980.

7. Vaninbroukx, R. and Denecke, B., Determination of Gamma-Ray Emission

Probabilities in the Decay of 235U and 231Th, Nucl. Instr. and Methods 193 (1982), pp. 191.

8. Debeir, R. P., Violin, F. , Denis, R. , and Crutzen, S., Experience Gained

with the Unique Identification of 640 Standards of Uranium Enrichment, 5th ESARDA Symp., April 1983, (ESARDA Report No. 16).

9. U.S. Department of Energy New Brunswick Laboratory, Progress Report for

the Period July 1975 through September 1977, NBL-289, p. A- 5, January

1979.

10. De Bolle, W. , Conversion of Uranium Oxide to UFg for Isotopic Measurements by UFg Mass Spectrometry, EUR Report to be published, Internal Report

CBNM-MS-R-44-83.

11. De Bolle, W. , Technical Note on the Cleaning and Re-assembling of UFg

Containers, EUR Report to be published, Internal Report CBNM-MS-R-81/82

.

12. Muchenborn, G., and De Bolle, W. , 235U/238U Isotope Ratio Measurements on Double Collector UFg Mass Spectrometrs , EUR Report to be published, Internal Report CBNM-MS-R-74-83

.

13. De Bievre, P.J. and Debus, G.H., Absolute Isotope Ratio Determination of a Natural Boron Standard, Intern. J Mass Spec. Ion Phys . 2, 15 (1969).

14. Michiels, E. and De Bievre, P., A Precise Method for the Mass Spectrometric Determination of Lithium Isotope Ratios, Proc. 29th Annual Meeting

of the American Society for Mass Spectrometry, Minneapolis, MN(USA) , p. 240ff (1981).

78
15. Michiels, E. and De Bievre, P., Absolute Isotopic Composition and the Atomic Weight of a Natural Sample of Lithium, Intern. J. Mass Spec. Ion

Phys. 49, 265 (1983).

16. Werz, R. and De Bievre, P. How Enriched Must Separated Isotopes be to Prepare Synthetic Mixtures of a Given Accuracy?, Proc . 9th Intern. Mass

Spec. Conf., Wien 1982, Intern. J. Mass Spec. Ion Phys. 48, 365 (1983).

17. Certificate CBNM-IRM- 040/1 ( 233U)

.

18. De Bolle, W. and Muschenborn, G., Conversion of U308 to UFg on Small Samples for UFg Isotopic Measurements, EUR Report to be published, Internal Report CBNM-MS -R- 95 - 84

.

19. De Bievre, P., Private Communication, A. Sawyer, British Nuclear Fuels,

Limited, Springfields , UK (17.12.1979).

20. Werz, R. and De Bievre, P., Orthodox Evaluation of Uncertainties of Synthetic Isotope Mixtures, Internal Report CBNM-MS -R- 30- 83

.

21. Shields, W.R. , Ed., Analytical Mass Spectrometry Section: Instrumentation

and Procedures for Isotopic Analysis, Tech, Note 277, National Bureau of Standards, Washington, DC, July 1966, p. 99. 22. Barnes, I.L., Murphy, T.J., Gramlich, J.W., and Shields, W.R., Separation

by Anodic Deposition and Isotope Ratio Mass Spectrometry of Microgram and Smaller Samples, Anal. Chem. 45, 1881-1884 (1973).

23. Gramlich, J.W. and Shideler, R.W. , A Programmable Sample Dryer for Thermal Ionization Mass Spectrometry, NBS Tech. Note 1154, National Bureau of

Standards, Washington, DC, January 1982, pp. 15. 24. Wendolkowski , W.S. and Kirslis, S.S., Thermal Decomposition of Uranyl

Nitrate Hexahydrate , Report No. K- 1086, K-25 Plant, Union Carbide Co., Oak Ridge, Tennessee, February 1954, pp. 23. 25. Heald, E.F., Thiel College, Greenville, PA, personal communication.

26. Shields, W.R. , Ed., Analytical Mass Spectrometry Section: Summary of Activities, NBS Tech. Note 426, National Bureau of Standards, Washington,

DC, September 1967, pp. 53.

27. Garner, E.L., Machlan, L.A. , and Shields, W.R. , Uranium Isotopic Standard Reference Materials, NBS Spec. Publ. 260-27, National Bureau of Standards,

Washington, DC, April 1971, pp. 150.

28. Wapstra, A.H. and Bos, K. , Atomic Data and Nuclear Data Tables, 19, 175, 297 (1977).

79
Appendix

U. S. Department of Commerce

Malcolm Baldngc

Secretary

1 ->»-" -

Nilioiul Buraau oTStandardi

Erncil Ambler. Director ^National ££uraw of js>tattrtartte

Olerltftcat^

Standard Reference Material 969

Uranium Isotopic Standard Reference Material

for Gamma Spectrometry Measurements

(In Cooperation with the Commission of the European

Communities, Central Bureau for Nuclear Measurements, GeeL, Belgium, and the U.S. Department of Energy, New Brunswick

Laboratory, Argonne, Illinois.) This Standard Reference Material (SRM) is intended for use in the calibration and evaluation of gamma-ray counting

procedures for the nondestructive determination of the

235 U/U isotope abundance in uranium bulk materials. SRM 969 consists of a set of five different U3O8 powders, with nominal

2 5 U abundances of 0.31, 0.71, 1.94, 295, and

4.46 mass percent, encased in aluminum cans that have been manufactured to rigid specifications (See attachments) and

thoroughly tested. In addition an empty can is provided for use when measuring uranium materials of unknown U

abundances. SRM 969 was prepared as a set to permit measurement of materials containing uranium by using the

theoretically expected linear relationship between U abundance and the counting rate of the 18S.7 keV gamma-ray of

U. Each SRM subunit is made up of 200 g of U3O8 powder. Since SRM 969 consists of 5 different containers, and

each is unique in dimensions, the attached data sheets should be used with the specified container number. Individual data sheets are provided for each set of SRM's along with the certificate. The certified

235 U/U isotope abundances are shown in Table 1. The isotope abundances for

234 U/U, 236U/U, and

238 U/U are given in Table 2. Table S Certified

235 U/U Abundances in SRM 969

Material ID: 03

S

,071,. 194 295 446_ Atom Percent: 0.3206 0.7209 1.9664 2.9857 4.5168 ±0.0002 ±0.0005 ±0.0014 ±0.0021 ±0.0032 Mass Percent 6.3166 0.7119 1.9420 2.9492 4.4623 ±0.0002 ±0.0005 ±0.0014 ±0.0021 ±0.0032

Statement of Uncertainty The overall uncertainty of the

235 U/U abundance of each individual reference sample was estimated by combining the

different uncertainty components from the mass spectrometry measurements and the U/U homogeneity. The result- ing values were conservatively enlarged to 0.07% to include other possible measurement errors. Isotope Certification: The uranium isotopic abundances were determined by thermal ionization mass spectrometry (ThIMS) at the National Bureau of Standards (NBS) and by uranium hexafluoride mass spectrometry (UF6 MS) and ThIMS at the Central Bureau of Nuclear Measurements (CBNM). These measurements were corrected for mass

discrimination effects relative to NBS uranium isotopic SRM's or synthetic isotope mixtures. Additional measurements supporting the certification were made by the U.S. Department of Energy, New Brunswick

Laboratory (NBL), using ThIMS and by NBS using gamma spectrometry (see summary of the final results in Table 3). Measurements by Gamma Spectrometry: The isotope abundance measurements and verification by gamma spectro- metry were performed at NBS and CBNM utilizing the 185.7 keV gamma-ray of

235 U. All measurements were made

using a high-resolution germanium detector. The heterogeneity of the

235 U/U in each SRM subunit is <0. 05% relative.

Gaithersburg, MD 20899 Stanley D. Rasberry, Chief October 15, 1985 Office of Standard Reference Materials (Revision of Certificate dated 6-27-85) 80
Tabic 2. "4 U/U, "*U/U and "8 U/U,

Abundances in SRM 969

Material Uranium Isotopes

234 u/u 236u/u 238 u/u

031 atom 0.0020 0.0147 99.6627

2s ±0.0002 +0.0003 mass 0.0020 0.0146 77.DOUO 071 atom 0.0053 <0.00002 99.2738

2s ±0.0002 ± 0.0004 mass 0.0052 <0.00002 99.2828

194 atom 0.0174 0.0003 98.0159

2s ±0.0002 ±0.0001 ± 0.0018 mass 0.0171 0.0003 98.0406 295 atom 0.0284 0.0033 96.9826

2s ±0.0004 ±0.0002 ± 0.0029 mass 0.0279 0.0033 97.0196 446 atom 0.0365 0.0069 95.4398

2s ±0.0003 ±0.0002 ± 0.0032 mass 0.0359 0.0068 95.4950

Measurements leading to the development and certification of this SRM were made at NBS in the Inorganic Analytical Research Division by B.S. Carpenter, J.W. Gramlich, R.R. Greenberg, and L.A. Machlan; at CBNM by

E. Bouwmeester, R. Damen, P. De Bievre, W. De Bolle, H.L. Eschbach, R. Eyckens, M. Gallet, W. Lycke, H. Meyer,

G. Miischenborn, W. Nagel, F. Quik, and J. Van Audenhove; at U.S. Department of Energy New Brunswick Laboratory (NBL). by V.E. Connolly and A.C. Zook in the Safeguards Assessment and Reference Materials Branch. The sampling plan used for the certification measurements was developed by W. Liggett, NBS Center for Applied Mathematics. The statistical assessment of the data used for the certification of this SRM was performed by R. Werz, CBNM.

The overall scope and coordination of the technical measurements leading to certification were performed by

B.S. Carpenter, NBS, and P. De Bievre, CBNM.

The technical and support aspects concerning the preparation, certification, and issuance of this Standard Reference

Material were coordinated through the Office of Standard Reference Materials by T.E. Gills.

81
Supplemental Information

Description of SRM 969 The uranium oxides used for this SRM are from the same lots of material used and jointly certified by CBNM and NBS

to produce this SRM and the Certified Nuclear Reference Material (EC NRM 171) for the Commission of European

Communities. For unique identification and for checking the integrity of the subunit, the plugs used for sealing the cans are equipped

with ultrasonic seals, each having a unique ultrasonic spectrum. Parameters that are of special interest for abundance measurements by gamma spectrometry are summarized below: Chemical Purity of the U3O8

Material: U3O8 powder

Total impurities: <0.05 mass% of U3O8 content Moisture content: <0.3 mass% of U3O8 content Uranium Minor Isotopes (that could interfere with gamma spectrometry measurements)

(Note: Data is presented as a ratio of the mass of the minor isotope to the mass of

235 U.)

Material ID

232 U/ 23S U233 u/ 235 u (237U +

237Np)/

235 U

Chemical

Separation Date

031

071

194

SxlO"

10 <0. 3x10"

10 0.3x10"'°

<5xl0"5 <5xl0

-5 <5xlO"

5

<3xI0"6 <3xI0

-6 <3.10"*

1977

1977 1977 295 0.1x10-'° <5xlO

-5 <3xl0

-6 1977 446 1x10-° <5xl0"5 <3xl0"* 1979 IhOx Filling Information

Material ID

Mass

(g) Filling Height (mm)

Diameter (mm)U3O8 Density (g/cm )

031 200.1 ±0.2 20.8 ± 0.5 70.00 5.2 ± 0.3 071 200.1 ±0.2 20.8 ± 0.5 70.00 5.2 ± 0.3 194 200.1 ±0.2 20.8 ± 0.5 70.00 5.2 ±0.3"

295 200.1 ±0.2 20.8 ± 0.5 70.00 5.2 ± 0.3 446 200.1 ±0.2 15.8 ± 0.5 70.00 5.2 ± 0.3

82
Container Material Characteristics and Specifications Container Material: Aluminum type 6061 (ASTM-GS T6) (AH containers manufactured from the same base material) Constituent Elements: Mg 0.8 -1.2 Zn <^0.25

(in Base Material, Wt.%) Si 0.4 -0.8 Ti <0. 15 Cu 0.15-0.4 Fe <0.7

U <0.00025 Mn<0.15

Cr 0.04-0.35 Total other Elements <0.15

Container dimensions: Sec attached specifications Use of SRM 969

Ideally, physical materials used for the evaluation of nondestructive measurements should be representative of the unknown samples with respect to all parameters that influence the measurement. One of the most crucial factors in gamma spectrometry is the strong attenuation of the gamma ray in the sample material itself and in the sample container. This attenuation is generally influenced by parameters such as sample size, shape, material density, and matrix composition. In addition, characteristics such as container material, wall thicknesses and container size can also influence attenuation. SRM 969 is ideally suited for use with IhOs materials contained in aluminum cans with 2mm bottom wall thickness. To

be useful in the calibration of assay systems using other types of uranium samples, correction factors are needed. These

factors are intended to normalize the gamma-ray response with respect to differences in both the matrix composition and

the container. Typical correction factors are given in a special user's manual (Report KfK 3752(1984)) that has been

prepared to facilitate the correct use of SRM 969 and EC NRM 171. Note: The attenuation correction factors given are based on theoretical values for photon cross sections. They represent a possible source of systematic error for those

cases where the required corrections are large. Therefore, the correction given and the range of application should be experimentally validated to eliminate possible systematic errors from the gamma spectrometry abundance

measurements.

83
Because the enrichment meter principle is based on the assumption that a sample is "quasi-infinitely thick" for the

185.7 keV gamma ray, the application of the method is inherently restricted to relatively large samples. About 200 g of unknown material is required when standard containers, with 7cm diameter, are used. For many applications it may be

desirable to have reference samples that both physically and chemically differ from SRM 969. These reference samples

can be calibrated against SRM 969.

Notice and Warnings

Container: The aluminum cans for SRM 969 have numbers engraved in the cylindrical wall. The numbering system on

each can reflects the nominal isotope abundance of U in mass percent and a sequence number. The cans within a set should have the same sequence number. SRM 969 should be handled with great care to avoid any damage or deformation to the bottoms of the cans, since the bottoms serve as a window for the emitted gamma radiation. A special transport and storage case is supplied with

this SRM.

Identification: A unique tamperproof system, making use of an ultrasonic "fingerprint" identification device, has been

placed into the plug of each can. Documentation An NBS Special Publication, 260-96, and a European Commission Publication COM 4153, have been issued describing

the preparation and characterization of this SRM, and should be used in conjunction with SRM 969/ECNRM 171. An

additional User's Guide has been prepared and published as report KfK 3752. Storage The SRM subunits should be stored above -10 °C and below 40 °C.

Table 3. Summary of U/U Isotope Abundance Measurements,

in atom percent with 2s uncertainties, by Method and

Laboratory

Material ID 031 071 J94_ 795 446

Laboratory/ Method CBNM-UF6 0.32049 0.72096 1.96575 2.98432 4.51668 ±0.00016 ±0.00017 ±0.00037 ±0.00048 ±0.00075 CBNM-ThlMS 0.32061 0.72119 1.9675 2.9875 4.5201 ±0.00047 ±0.00098 ±0.0017 ±0.0039 ±0.0052 NBS-ThlMS 0.32076 0.72069 1.9664 2.9869 4.5138 ±0.00052 ±0.00068 ±0.0017 ±0.0022 ±0.0033 NBL-ThlMS 0.3203 0.7207 1.9657 2.9843 4.5162 ±0.0007 ±0.0007 ±0.0015 ±0.0039 ±0.0039 NBS-Gamma 0.32053 0.72081 1.9660 19830 4.5158 ±0.00026 ±0.00058 ±0.0016 ±0.0024 ±0.0036

84
NBS-114A (REV. 2-8C) U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No. 3. Publication Date BIBLIOGRAPHIC DATA REPORT NO. SHEET (See instructions) NBS/SP-260/96 September 1986

4. TITLE AND SUBTITLE Standard Reference Materials: Uranium-235 Isotope Abundance Standard Reference Materials for Gamma Spectrometry Measurements

5. AUTHOR(S) B# s> Carpenter, J. W. Gramlich, R. R. Greenberg, L. A. Machlan, P. DeBievre,

H. L. Eschbach, H. Meyer, J. Van Audenhove, V. E. Connolly, N. M. Trahey, A. C. Zook

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE GAITHERSBURG, MD 20899

7. Contract/Grant No.

8. Type of Report & Period Covered Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State, ZIP) National Bureau of Standards Department of Commerce

Gaithersburg, MD 20899

10. SUPPLEMENTARY NOTES Library of Congress Catalog Card No: 86-600585

Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant bibliography or literature survey, mention it here) The accurate determination of isotope abundances by any method requires

that the measuring systems be calibrated using well characterized isotope reference materials. The National Bureau of Standards (NBS) and the Central Bureau for Nuclear Measurements (CBNM) have jointly produced and certified

U„0g nondestructive assay (NDA) reference 'samples to be used for calibrating

gamma measurements. Five different uranium abundances have been certified

(0.31, 0.71, 1.95, 2.95, and 4.46 nominal mass percent, U/U). In this 260 series publication, the material fabrication and certification are described

and a discussion of the measurement results affecting the accuracy of gamma spectrometry is given. These certified standards represent the first example

of an international effort that establishes traceability to NBS, CBNM, and

the basic SI units.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons) gamma energy; gamma spectrometry; homogeneity; mass spectrometry; nondestructive

assay; reference material; specifications aluminum; uncertainty; U/U isotope abundances

13. AVAILABILITY 14. NO. OF § Unlimited PRINTED PAGES

|

| For Official Distribution. Do Not Release to NTIS 96

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