Isotopic Analysis of 

Uranium in Natural Waters 

by Alpha Spectrometry

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1696-F
Isotopic Analysis of 

Uranium in Natural Waters 

by Alpha Spectrometry

By KENNETH W. EDWARDS

RADIOCHEMICAL ANALYSIS OF WATER 

GEOLOGICAL SURVEY WATER-SUPPLY PAPER 1696-F

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1968
UNITED STATES DEPARTMENT OF THE INTERIOR 

STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY 

William T. Pecora, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402 - Price 15 cents (paper cover)
CONTENTS

Abstract- _____________________________________________

Introduction._ _________________________________________

Alpha spectrometry_____________________________________

Electrodeposition. ______________________________________

Concentration and purification of uranium in natural waters. 

Reagents._____________________________________________

Special apparatus______________________________________

Preparation of ion-exchange columns._____________________

Procedure for analysis._________________________________

Procedure for counting samples__________________________

Analysis of the spectra.__-___-_____-.________-__--__.____.

Results__ _ __________________________________________

Discussion.____________________________________________

Acknowledgment. _______________________________________

References _ ___________________________________________

Page

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9

10

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14

17

18

18

22

25

25

ILLUSTRATIONS

FIGURE 1. Graph showing alpha spectrum of uranium from Clear Creek, 

near Golden, Colo__________________________--__------_

2. Schematic diagram of alpha spectrometer_________________

3. Photograph of alpha spectrometer-________________________

4. Sketch of ion-exchange column.___________________________

5. Photograph of equipment for electrodeposition._____________

6. Graph showing effect of source thickness on resolution of 

uranium alpha spectra.__-______-__________--_-------_-

Page

F6

7

8

12

13

20

TABLES

TABLE 1. Relative statistical error in U234/U238 ratios for various interference-free counts.____^-____-____________-_--__-_---_-

2. Absolute statistical error in U234/U238 ratios for various interference-free counts._ ___________________________________

3. Energies of alpha particles emitted by naturally occurring 

radionuclides_ _ _________________________________________

4. Maximum, minimum, and recommended sample volumes for 

isotopic uranium analyses.______________________---__---

5. Recovery of uranium standards carried through entire chemical 

and electrochemical procedure._______---_______---_-----

in

Page 

F3

4

5

14

18
IV CONTENTS

Page 

TABLE 6. Isotopic uranium blanks run on distilled water samples._____ F19

7. Efficiency and resolution as a function of number of counts.

Sample 183-61_____________--_------_-_-______-----_-__ 21

8. U234/U238 activity ratios as determined by mass spectrometry

and alpha spectrometry_________________________________ 22

9. U234/U238 ratios of eastern slope streams in Colorado._________ 22
RADIOCHEMICAL ANALYSIS OF WATER

ISOTOPIC ANALYSIS OF URANIUM IN NATURAL WATERS 

BY ALPHA SPECTROMETRY

By KENNETH W. EDWARDS

ABSTRACT

A method is described for the determination of U234/U238 activity ratios for 

uranium present in natural waters. The uranium is coprecipitated from solution 

with aluminum phosphate, extracted into ethyl acetate, further purified by ion 

exchange, and finally electroplated on a titanium disc for counting. The individual 

isotopes are determined by measurement of the alpha-particle energy spectrum 

using a high resolution low-background alpha spectrometer. Overall chemical 

recovery of about 90 percent and a counting efficiency of 25 percent allow analyses 

of water samples containing as little as 0.10 Mg/1 of uranium. The accuracy of the 

method is limited, on most samples, primarily by counting statistics.

INTRODUCTION

Three isotopes of uranium occur in nature. These isotopes have 

atomic masses of 238, 235, and 234. U238 is by far the most abundant, 

accounting for 99.28 weight percent of all natural uranium. U235 

accounts for most of the balance (0.711 percent), and U234 has a very 

low chemical abundance (0.006 percent). In terms of their activities 

(disintegration rates), however, U238 and U234 are equally abundant, 

whereas U235 has only 4.6 percent of the abundance of U238 .

It is generally accepted that the abundance ratio of U235 to U238 is 

virtually constant (within 0.1 percent) throughout nature (Atomics, 

1964). Each of these isotopes is a parent of a naturally occurring 

radioactive decay series: U238 is the parent nuclide of the "uranium 

series," and U235 is the parent of the "actinium series." Since these 

isotopes bear no genetic relationship to each other, they are expected 

to be in the same chemical state and in constant ratio everywhere 

they occur.

Fl
F2 RADIOCHEMICAL ANALYSIS OF WATER

Uranium-234 is not an independently occurring radionuclide, but 

is rather a third generation decay product of U238.

"JJ238 U234

4.51 XWg y / 2.48X105 ?/

Pa234 

ia / 1.18m ia

1^234 /? 1^230

24.1 d 7.6X104 ?/ i

etc.

It is evident that, because of this genetic relationship, U234 owes its 

existence to the parent isotope U238, Furthermore, since the half life 

of U234 is short with respect to both the half life of U238 and the age 

of the earth, the total activity (that is, the total number of atoms 

which decay per unit time) of U234 in the earth must be equal to the 

total activity of U238. This follows directly from the laws of radioactive 

decay and growth.

Despite the large-scale equilibrium between these isotopes, local 

disequilibria may exist and, in fact, are very common. Numerous 

studies of uranium isotopes in nature have disclosed substantial 

disequilibria in natural waters (Chalov and others, 1964; Sakanoue 

and Hashimoto, 1964; Cherdyntsev and others, 1963; Thurber, 1962; 

Ancarani and Bettinali, 1960; Isabaev and others, 1960), in soils 

(Rosholt, Doe, Tatsumoto, 1966; Cherdyntsev and others, 1963), 

and in a wide variety of rocks and minerals (Cherdyntsev and others, 

1965, 1963, 1961; Rosholt, Butler, and others, 1965; Rosholt, Tatsumoto and Dooley, 1965; Syromyatnikov, 1965; Dooley and others, 

1964; Rosholt, Harshman, and others, 1964; Sakanoue and Hashimoto, 

1964; Rosholt, Shields, and others, 1963; Syromyatnikov and Tolmachev, 1962; Thurber, 1962; Isabaev and others, 1960). These disequilibria arise as a result of the physical and chemical changes which 

occur in the decay process. The energy released in the alpha decay 

of U238 to Th234 provides sufficient recoil energy to the atom to cause 

the breaking of chemical bonds. The "hot" atoms may immediately 

recombine with the matrix material or may be transported by water, 

with which they come in contact, to variable distances from the sites 

of the original decay process. In addition to the physical recoil process, 

Th234 and Pa234 are chemically different from uranium, and chemical 

differentiation may occur. Formation of U234 in a higher oxidation 

state than the surrounding U238 atoms may also occur as a natural 

consequence of the mechanics of the decay process. Chemical separation of isotopes is obviously possible when their oxidation states are 

not the same. These phenomena have been discussed by a number of 

investigators (Cherdyntsev and others, 1965, 1963, 1961; Rosholt,
ISOTOPIC ANALYSIS OF URANIUM IN NATURAL WATERS F3

Butler, and others, 1965; Rosholt, Tatsumoto, and Dooley, 1965; 

Syromyatnikov, 1965; Chalov and others, 1964; Rosholt, Harshman, 

and others, 1964; Rosholt, Shields, and others, 1963).

Uranium occurs in low concentrations in many natural waters. 

Sea water has an average uranium content of about 2 jug/1 (micrograms 

per liter) (Katz and Rabinowitch, 1951, p. 71), and most fresh waters 

have concentrations below 10/ig/l (Barker and others, 1965). Some 

ground-water sources, however, have uranium concentrations as high 

as several milligrams per liter. The present investigation was initiated 

to establish the methodology, sensitivity, and accuracy with which 

the relative abundances of Uranium-238 and uranium-234 can be 

determined in natural water. The method herein reported is intended 

primarily for analyses of natural waters, but it may also be used on 

other aqueous systems which are not appreciably enriched in uranium235. The method is based on spectral analysis of the alpha particles 

emitted by uranium isolated from the water sample. The number of 

counts necessary to obtain a desired accuracy in the U234/U238 ratio 

under interference-free conditions is shown for various ratios in tables 

1 and 2. Because of the low specific activity of uranium, 0.739 disTABLE 1. Relative statistical error in U234/TJ238 ratios for various interference-free

counts

Relative standard deviation in ratio for U234/TJ238 values of Total counts - 

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

200

300------

500

600

700

800

900

1, 000_ - _ _

l,200-___

1,500

2|OOOI___

2, 500_ ___

3, 000- ___

4, 000____

5,000_-__

6, 000_ _-_

8, 000_ ___

10,000-.-

12, 000- __

15, 000_ __

20, 000_ - -

30, 000_ __

40, 000- _ _

50,000--_

0. 157

. 128

. 110

. 099

. 090

. 084

. 078

. 074

.070

. 064

.057

. 050

. 044

. 040

. 035

. 031

. 029

. 025

. 022

. 020

. 018

. 016

. 013

. Oil

. 010

0. 146

. 1 J. 1QJ. 17

. 103

. 092

. 084

. 078

. 073

. 069

. 065

. 060

. 053

. 046

. 041

. 038

. 033

. 029

. 027

. 023

. 021

. 019

. 017

. 015

. 012

. 010

. 009

0. 142

. 116

. 101

. 090

. 082

. 076

.071

. 067

. 064

. 058

. 052

. 045

. 040

.037

. 032

. 029

. 026

. 023

. 020

. 018

. 016

. 014

. 012

. 010

. 009

0. 141

. 115

. 100

. 089

.082

.076

. 071

. 067

. 063

. 058

. 052

. 045

. 040

. 037

. 032

. 028

. 026

. 022

.020

. 018

. 016

. 014

. 012

. 010

. 009

0. 142

. 116

. 101

. 090

.082

.076

. 071

. 067

.063

. 058

. 052

. 045

. 040

. 037

. 032

. 028

. 026

. 022

. 020

. 018

. 016

. 014

. 012

.010

. 009

0. 144

. 117

. 101

. V/Z7 OQ1J.

. 083

. 077

. 072

. 068

.064

. 059

. 052

. 045

. 041

. 037

. 032

. 029

. 026

. 023

. 020

. 019

. 017

. 014

. 012

. 010

. 009

0. 145

. 118

. 103

. 092

. 084

. 078

. 073

. 069

.065

.059

. 053

. 046

. 041

. 038

. 033

. 029

. 027

.023

.021

. 019

.017

. 015

. 012

.010

. 009

0. 148

. 120

. 104

. 093

. 085

. 079

. 074

. 070

. 066

. 060

. 054

. 047

. 042

. 038

. 033

. 030

. 027

. 023

. 021

. 019

. 017

. 015

. 012

. 010

. 009

0. 150

. 122

. 106

. 095

.087

.080

. 075

. 071

.067

. 061

.055

. 047

. 042

. 039

. 033

. 030

. 027

. 024.

. 021

. 019

. 017

. 015

. 012

.011

. 010
F4 RADIOCHEMICAL ANALYSIS OF WATER

TABLE 2. Absolute statistical error in U234/U238 ratios for various interferencefree counts

Total counts

(U234+U238) -

200__-_--

300___--_

400_-_--.

500__-__-

600____-_

700_____-

800___ 

900__-___

1,000... _.

1,200.. 

1,500. .

2,000.__._

2,500.. __.

3,000 

4,000.____

5,000. 

6,000.. 

8,000... __

10,000____ 

12,000 

15,000 

20, GOO30, 000 

40, GOO50, 000 

Absolute standard deviation in ratio for ~U2u/~Uw values of 

0.4

0. 063 

. 051 

. 044 

.040 

. 036

. 034 

. 031 

.030 

. 028 

. 026

. 023 

. 020 

. 018 

. 016 

. 014

. 012 

. 012 

.010 

. 009 

. 008

.007 

. 006 

. 005 

. 004 

. 004

0.6

0. 088 

. 071 

.062 

. 055 

. 050

. 047 

.044 

. 041 

.039 

. 036

. 032 

. 028 

. 025 

. 023 

. 020

. 017 

.016 

. 014 

. 013 

. Oil

. 010 

.009 

.007 

. 006 

. 005

0.8

0. 114 

. 093 

. 081 

. 072 

.066

. 061 

.057 

. 054 

. 051 

. 046

. 042 

.036 

. 032 

. 030 

. 026

.023 

. 021 

. 018 

. 016 

. 014

. 013 

.011 

. 010 

. 008 

.007

1.0

0. 141 

. 115 

. 100 

. 089 

.082

. 076 

.071 

. 067 

.063 

. 058

. 052 

. 045 

. 040 

. 037 

.032

. 028 

.026 

. 022 

. 020 

. 018

. 016 

. 014 

. 012 

. 010 

.009

1.2

0. 170 

. 139 

. 121 

. 108 

. 098

. 091 

. 085 

.080 

. 076 

.070

. 062 

. 054 

. 048 

. 044 

. 038

. 034 

. 031 

. 026 

. 024 

. 022

. 019 

. 017 

. 014 

. 012 

. Oil

1.4

0. 202 

. 164 

. 141 

. 127 

. 116

. 108 

. 101 

.095 

. 090 

. 083

. 073 

. 063 

. 057 

. 052 

. 045

. 041 

. 036 

.032 

.028 

. 027

. 024 

. 020 

. 017 

. 014 

.013

1.6

0. 232 

. 189 

. 165 

. 147 

. 134

. 125 

. 117 

. 110 

. 104 

. 094

. 085 

. 074 

. 066 

. 061 

.053

.046 

.043 

. 037 

. 034 

. 030

. 027 

. 024 

. 019 

. 016 

. 014

1.8

0. 266 

. 216 

. 187 

. 167 

. 153

. 142 

. 133 

. 126 

. 119 

. 108

. 097 

. 085 

.076 

. 068 

. 059

. 054 

.049 

. 041 

. 038 

. 034

.031 

.027 

. 022 

.018 

. 016

2.0

0.300

. 244 

. 212 

. 190 

. 174

. 160 

. 150 

. 142 

. 134 

. 122

. 110 

. 094 

. 084 

. 078 

.066

. 060 

. 054 

. 048 

. 042 

. 038

. 034 

. 030 

. 024 

. 022 

. 020

integration per minute of U238 per jug, it is evident that it is often 

necessary to concentrate the uranium from several liters of water 

from a natural source to make possible a practical determination of 

the relative isotopic abundances. Principles and procedures of concentrating, electroplating, and counting the uranium isotopes are 

described in the following sections.

ALPHA SPECTROMETRY

Alpha particles are emitted from nuclei with discrete energies 

which are characteristic of the source. A particular nuclide may emit 

alpha particles of a single discrete energy or of several different 

energies, depending upon its nuclear properties. In either case, however, the energies of the alpha particles are well defined and may 

be used to characterize the emitting nuclide. Both qualitative and 

quantitative analyses may be made by measurements of the alpha 

energy spectra of suitable sources.

The introduction of semiconductor detectors into the field of nuclear radiation spectrometry has substantially simplified and improved 

both charged particle and gamma-ray energy spectrum measure-
ISOTOPIC ANALYSIS OF URANIUM IN NATURAL WATERS F5

ments. This is especially true in alpha spectrometry where silicon 

surface-barrier semiconductor detectors have the advantages of excellent linearity, stability, and resolution, very low background, fast 

response, moderate cost, and are easy to use and relatively troublefree. The insensitivity of alpha and beta semiconductor detectors 

to gamma radiation makes shielding unnecessary in most work. The 

principle of operation and modern aspects of these detectors are 

discussed in many publications (Nucleonics, 1964, 1962; Donovan, 

1962; Cathey, 1961).

The applicability of alpha spectrometry to quantitative analysis 

of alpha-emitting nuclides in natural water depends on (a) the concentration, chemistry, and decay characteristics of the nuclides 

present and on (b) the properties of the detection system.

The decay characteristics of natural alpha emitters are summarized 

in table 3. Several of these emitters (notably the polonium isotopes)

TABLE 3. Energies of alpha particles emitted by naturally occurring radionuclides 

[Data from Strominger and others, 1958]

Nuclide Half life Alpha energies, Mev; figures in parentheses are in percent

U238 ________ 4.51X109 y 4.195 (77), 4.147 (23)

U235 ________ 7.1X108 y 4.559 (6.7), 4.520 (2.7), 4.370 (25) 4.354 (35), 4.333

(14), 4.318 (8), 4.117 (5.8)

U234 _-_____- 2.48X105 y 4.768 (72), 4.717 (28) 

Pa231 ______- 3.43X10*y 5.406 (10), 5.017 (23), 5.001 (24), 4.971 (2.3), 4.938

(22), 4.921 (2.8), 4.839 (1.4), 4.722 (11), 4.696 (1.4),

4.666 (2.1) 

Th232_______ 1.39X1010 ?/ 4.007 (76), 3.948 (24)

Th230_______ 7.6X104 y 4.682 (76), 4.615 (24)

Th228 __.____ 1.91X10* y 5.421 (71), 5.338 (28)

Th227___.___ 18.2 d 6.036 (23), 6.007 (2.8), 5.976 (24), 5.958 (3.5), 5.914

(0.9), 5.865 (3.0), 5.805 (1.0), 5.793 (0.3), 5.761

(0.3), 5.755 (21), 5.712 (5.0), 5.708 (8.7), 5.699

(4.0), 5.692 (1.5), 5.667 (1.9)

*Ac227 ______ 21.6 ?/ 4.942(100)

Ra226-..,--- 1.622X103 y 4.777 (94), 4.589 (5.7)

Ra224_______ 3.64 d 5.681 (95), 5.445 (4.9)

Ra223 ____ 11.68 d 5.867 (1.0), 5.742 (10.5), 5.712 (50), 5.602 (24), 5.534

	(10.3), 5.497 (0.9), 5.429 (2.4) 

Rn222_______ 3.83 d 5.486(99+)

Rn22<>_______ 51.5s 6.282(99 + )

Rn219 _______ 3.92 s 6.813 (82), 6.547 (13), 6.419 (5)

Po218 _______ 3.05m 5.998 (~100)

Po21(L______ 0.158s 6.775 (~100)

Po215 _______ 1.83X10~3 s 7.36 (~100)

Po214 _______ 1.64X10-*s 7.680(100)

Po212 _______ 3.04X10-7 s 8.780(100)

Po211 _____._ 0.52s 7.442(100)

Po210 _______ 138.4 d 5.305 (~100)

*Bi212 ______ 60.5 m 6.086 (27.2), 6.047 (69.9), 5.765 (1.7), 5.603 (1.1)

*Bi211 ______ 2.16m 6.617 (83), 6.272 (17)

*Ac227 , Bi2 ' 2 , and Bi2 » decay only 1.2, 63.8, and 99.7 percent, respectively, by alpha emission. The balance 

of the decay is by beta emission.

293-H121 0 68
F6 RADIOCHEMICAL ANALYSIS OF WATER

decay with the emission of monoenergetic alpha particles, whereas 

many others decay with the emission of alpha particles of two or 

more discrete energies. Since these energies are discrete, the energy 

spectra of infinitely thin alpha sources deviate from line spectra only 

due to the limitations of the detection system. Further deviations 

from line spectra result from partial loss of energy of alpha particles 

in collisions with atoms within the source, for sources of finite thickness, and in the medium between source and detector. Since all the 

alpha energies fall within the energy range of 4 to 9 Mev (million 

electron volts), it is desirable to use samples and detector systems 

which will afford resolution adequate to avoid or minimize overlap of 

spectral peaks. Alpha particles lose energy very rapidly in passing 

through matter, so sources for spectral analysis should be kept as 

thin as possible and should be counted in a vacuum chamber.

Figure 1 shows the characteristic alpha spectrum of natural uranium. U238 and U234 each emit alpha particles of two distinct, but only 

slightly separated, energies. As shown in table 3, U238 has alpha 

energies of 4.195 Mev (77 percent) and 4.147 Mev (23 percent). 

These peaks are not completely resolved, but the minor peak is 

evidenced by the inflection on the low energy side of the major peak. 

Similarly, U234 has alpha energies of 4.768 (72 percent) and 4.717 

(28 percent) which are not completely resolved. The less abundant

50 70 90 110 130 150 

CHANNEL NUMBER

170

FIGURE 1. Alpha spectrum of uranium from Clear Creek at Golden, Colo. 

Sample collected Mar. 3, 1966.
ISOTOPIC ANALYSIS OF URANIUM IN NATURAL WATERS F7

(in activity) U235 has many different energies, but most of these are 

close together (82 percent 4.32-4.37 Mev).

A spectrum such as figure 1 is obtained by allowing alpha particles 

to react with matter in such a way as to generate electric pulses 

which are proportional in current and (or) voltage to the energy of 

the particle. These pulses are sorted by suitable electronic equipment 

into a pulse height, or energy, spectrum. Many types of detectors 

can be used for charged particle spectrometry, but gas ionization 

chambers or solid-state semiconductor detectors are most suitable. 

The latter have many advantages as noted above.

The solid-state detectors used are in effect silicon diodes consisting 

of a very thin sensitive surface of p-type silicon over a much thicker 

wafer of n-type silicon. A reverse bias of about 50 volts is applied 

across the diode from a stable power supply. Alpha particles striking 

the sensitive surface penetrate into the n-type layer and produce 

electron-hole pairs in direct proportion to the energy of the particle. 

The pairs are accelerated to the electrodes and produce a pulse which 

is picked up by a charge sensitive preamplifier. The pulse is amplified 

by a suitable low-noise amplifier and fed into a multichannel pulse 

height analyzer. The latter is simply a device for sorting the incoming 

pulses on the basis of their amplitude and storing them in appropriate 

bins or channels that are linearly related to the energy. After accumulation and storage of the pulses, the memory of the analyzer may be 

interrogated and the data read out as a spectrum. This spectrum may 

be in any one of a variety of forms: digital data typed or printed by a 

parallel printer, punched tape, magnetic tape, visual display on an 

oscilloscope, or graphical representation on an X-Y plot. A schematic 

representation of a complete alpha spectrometer is shown in figure 2. 

The system used for part of the work presented in this report is shown 

in figure 3.

Resolution of solid-state detectors is expressed as the ratio of the 

full width at half maximum (FWHM) of a monoenergetic peak to

FIGURE 2. Schematic diagram of alpha spectrometer.
F8 RADIOCHEMICAL ANALYSIS OF WATER

FIGURE 3. Alpha spectrometer.

the peak energy. Both the FWHM and source energy must be expressed in the same units (Mev), and the ratio is usually multiplied by 

100 to express it as a percent. The resolution varies with the size of 

the detector, the temperature of operation, and other factors, but it 

is nearly independent of the energy of the radiation measured. The 

detector used for much of the present work had a resolution of 0.7 

percent for 5.305 Mev alpha particles from polonium-210 under our 

experimental conditions. The apparent resolution of uranium peaks 

was 1.2 percent or larger because the isotopes are not monoenergetic 

sources, and the thickness and diameter of the deposits are much 

greater than those of the polonium-210 standard.

ELECTRODEPOSITION

The most desirable form of a radioactive material for alpha counting is an electroplated deposit of the pure substance on a small diameter, metal disc. As noted previously, alpha particles lose energy very 

rapidly in passing through matter, and this loss of energy has several 

undesirable consequences. A thick deposit will give a nongaussian 

distribution of particle energies measured by the detector, resulting in 

a peak which is skewed to the left; that is, a peak which appears fairly 

sharp on the high energy edge, but is broadened on the low energy
ISOTOPIC ANALYSIS OF URANIUM IN NATURAL WATERS F9

side. The peak is therefore more difficult to integrate with accuracy, 

and the chance of overlap with other peaks is increased.

It is normally desirable, for maximum resolution, for the source to 

be small in diameter with respect to the detector in addition to being 

thin. Other factors being equal, a small diameter deposit will give 

both maximum counting efficiency and the best resolution. In the 

special case of uranium, however, the specific activity (disintegration 

rate per unit weight) is so small that a small diameter deposit is either 

too thick or contains so little uranium that the counting rate is very 

low. It has therefore been found preferable in this work to use a 

deposit having an area approximately equal to that of the detector.

A simple and direct procedure for obtaining a thin, uniform deposit 

of uranium is electrodeposition of the oxide, U3O8 , on a metal-disc 

cathode. This deposition takes place when a weakly acidic solution of 

a uranyl salt is electrolyzed under moderately forcing conditions. 

Reduction of hydrogen ion at the cathode generates the basic condition at the electrode surface which is necessary for the deposition 

to take place. A variety of metals (platinum, titanium, stainless steel, 

nickel) was found to be satisfactory for cathodes; but titanium was 

used because of slightly higher recovery than on stainless steel, less 

corrosion than with nickel, and it is much more economical than 

platinum. Some other metallic oxides and free metals will codeposit 

with uranium if they are present in the final plating solution. Since 

codeposition of any material will adversely affect the resolution and 

counting efficiency, it is necessary that the uranium be virtually free 

of impurities when the electrodeposition is performed.

CONCENTRATION AND PURIFICATION OF URANIUM IN 

NATURAL WATERS

Uranium can be conveniently concentrated from dilute aqueous 

solutions by coprecipitation with aluminum phosphate under weakly 

acidic conditions. This precipitation gives virtually quantitative 

carry of uranium and has the advantage of affording a residue which 

is free of appreciable quantities of the major cations found in natural 

waters. The precipitate can be easily dissolved in acid, and the 

uranium selectively extracted into ethyl acetate using magnesium 

nitrate as a salting agent. Traces of thorium, if present in the water 

sample, may be coprecipitated and partially coextracted. The ethyl 

acetate layer is therefore evaporated to dryness; the residue is taken 

up with 8 N HC1, and the solution placed on an anion-exchange 

column. Thorium is removed from the column by washing with 8 N 

HC1, and the uranium is then eluted with 0.1 N HC1. The uranium 

fraction is evaporated to near dryness, taken up with 2 M NH4C1 

(adjusted to pH 2.6-3.0 with HC1), and electroplated.
F10 RADIOCHEMICAL ANALYSIS OF WATERl

REAGENTS

Reagent-grade chemicals should be used throughout.

Hydrochloric acid: 8 N and 0.1 N.

Nitric acid: Concentrated.

Ammonium hydroxide: Concentrated.

Ethyl acetate.

Methyl-red indicator solution.

Titanium metal discs: 1.25 inch diameter.

Ion-exchange resin: Bio-Rad AG 1-X8, 50-100 mesh.

Ammonium chloride reagent (2.00 M, pH 2.8):

Dissolve 107 g of NH4C1 in approximately 800 ml distilled water. 

Using a pH meter, add dilute HC1 dropwise until the pH reaches 

2.7. Dilute the solution to 1 liter using a volumetric flask. 

Store in a polyethylene bottle. 

Magnesium nitrate (3.5 M)-nitric acid (1 M) reagent:

Fill a 500 ml volumetric flask approximately 3/4 full with distilled 

water and add 32 ml of concentrated nitric acid. Mix thoroughly 

and add 449 g of Mg(NO3) 2 -6 H2O from a freshly opened 1- 

pound jar. Warm gently until completely dissolved. Bring the 

liquid level to the mark with distilled water. Cool to room 

temperature and adjust the liquid volume to 500 ml. 

Aluminum nitrate solution (0.2 M).

Dissolve 7.5 g of high purity Al(NO3)3-9 H2O in distilled water.

Add a drop of concentrated HNOs, and dilute to 100 ml. 

Diammonium hydrogen phosphate solution (0.11 M):

Dissolve 12 g of diammonium hydrogen phosphate in distilled

water and dilute to 1 liter. 

Ammonium nitrate solution (1 percent, w/v):

Dissolve 10 g of ammonium nitrate in distilled water and dilute 

to 1 liter.

SPECIAL APPARATUS

Beakers: Glass, 1.51.

Beakers: Teflon, 100 ml and 50 ml.

Funnels: Separatory, cylindrical, 60 ml capacity with Teflon stopcocks.

Columns: Ion-exchange, with teflon stopcocks. (Seefig. 4.)

Centrifuge.

Centrifuge tubes: 40 ml or 50 ml capacity.

Steam bath or low temperature hotplate.

Alpha spectrometer:

The alpha spectrometer used in most of the work consisted of the 

following components:

Ortec silicon surface-barrier detector, Model SBFJ450-60, 

450 mm2 sensitive area, 65-micron depletion depth at 50 

volts reverse bias.
ISOTOPIC ANALYSIS OF URANIUM IN NATURAL WATERS Fll

Ortec Model 804 vacuum chamber with Hasting DV-3M

thermocouple gauge tube. 

W. M. Welch Duo-Seal vacuum pump capable of reducing

pressure in counting chamber to less than 0.1 torr. 

Liquid nitrogen cold trap. (NOTE. In subsequent work a 

molecular sieve foreline trap (Ion Technology, Inc., Model 

TR-101) was found to be simpler to use and just as effective as the liquid nitrogen trap). 

Veeco vacuum gauge, No. T. E. 6, with 5 position switching

unit.

Ortec Model 101-201 low-noise amplifier system. 

Multichannel pulse-height analyzer. Both a Nuclear Data 

Model 180D 512 channel analyzer and a Technical Measurements Corp. Model 404 400 channel analyzer were used. 

Each was equipped with an IBM typewriter output, and 

the TMC analyzer also had a Tally punched tape readout. 

The alpha spectrometer with the Nuclear Data analyzer is

shown in figure 3. 

Electroplating apparatus:

The equipment used for electrodeposition of the uranium oxide is 

shown in figure 5. The cell consists of a stainless steel base with 

an 0.125-inch deep, 1.25-inch diameter depression for holding 

the titanium-disc cathode on which the uranium deposits. The 

vertical column is of teflon and has an outside diameter of 1.25 

inches and an inside diameter of 0.875 inch. The top of the cell is 

an 0.5-inch thick lucite disc with a 1.25-inch diameter by 0.125- 

inch deep depression to hold the column, and a 0.94-inch diameter hole through the center for insertion of the anode. The 

latter is a platinum wire helix suspended at a distance of approximately 0.875 inch above the titanium cathode from a 

polyethylene stopper. The stopper also holds a small funnel 

through which gases generated in the electrolysis escape, and 

which allows ammonium hydroxide to be added on completion 

of the electrodeposition. The teflon cylinder is held tightly in 

place and sealed both top and bottom by tightening the thumb 

nuts on the stainless steel rods that are attached to the base and 

pass through the Incite top. No gaskets are required. The pressure applied to the cathode disc by the column ensures good 

electrical contact between the disc and the stainless steel base. 

Power to the cell is provided by an Electro Products Laboratories Model D-612T filtered d-c power supply. Electrical connection to the cathode is made by a wire to a thumb screw in 

the base. A clip lead connects to the anode. Current is measured 

by a 2 amp ammeter connected in series with the cell.
F12 RADIOCHEMICAL ANALYSIS OF WATER

40 milliliter centrifuge tube 

fused to top of columm

1 centimeter inside' 

diameter

14 centimeters

Teflon stopcock

FIGURE 4. Ion-exchange column.
ISOTOPIC ANALYSIS OF URANIUM IN NATURAL WATERS' F13

FIGURE 5. Equipment for electrodeposition.

PREPARATION OF ION-EXCHANGE COLUMNS

Weigh out 6 g of AG 1-X8 resin into a 50 or 100 ml beaker. Add 

15 ml of 8AT" HCI, swirl, cover with a watch glass, and allow to stand 

for 15 minutes or longer. Fill the lower tip of the column with water 

and close the stopcock. Add approx 10 ml of 8 N HCI to the column 

and add a small plug of glass wool as a resin support, using a glass 

rod to push it down to the bottom of the column. Slurry the resin 

in the beaker and pour onto the column, keeping the stopcock closed. 

Use an additional 5 ml of 8 AT" HCI to rinse the remaining resin out 

of the beaker. Allow the resin to settle by gravity. When it appears 

to be completely settled, draw off the HCI to 1 or 2 cm above the 

resin bed. Use a jet of 8 N HCI from a wash bottle to wash down 

any resin adhering to the column above the resin bed and allow to 

settle. Add a small glass-wool plug to the top of the resin bed, being 

careful not to apply any pressure to the resin itself. Rinse the column 

with 50 ml 8 AT HCI, then 100 ml H2O, and finally another 50 ml 

of 8 A7" HCI all at a flow rate of approx 60 drops per minute. (NOTE. 

It will be necessary to readjust the flow rate each time the eluent is 

changed.) Draw down the level in the column to the top of the glasswool plug. Discard the washings. The column is now ready for use.
F14 RADIOCHEMICAL ANALYSIS OF WATER

PROCEDURE FOR ANALYSIS

1. Determine the approximate concentration of uranium in the 

sample by performing a fluorimetric analysis as described in 

Water-Supply Paper 1696-C. If the 7 ml volume of water 

called for in this procedure does not give a measurable amount 

of uranium, use a larger volume for the determination (70 ml 

max).

2. Kefer to table 4 to determine the sample volume to be used for 

the isotopic analysis. (This is based on a maximum weight of 

uranium in the sample of 220 /xg and a minimum weight of 

2.5 /xg- See notes 1 and 2, p. F16)

3. If the sample volume used is greater than 1 liter, reduce the 

volume by evaporation on a hotplate as follows (note 3, p. F17):

Sample volume (V), in liters Reduce volume to 

1<V<5__________________________ 1 liter

V>5___-----_______________________ V/5 liters

If the sample volume taken is 1 liter or less, no concentration 

by evaporation is required.

4. If the (reduced) volume is now 1 liter or less, proceed with step 

5. If the reduced volume is greater than 1 liter, divide it into 

two or more approximately 1-liter subsamples in 1.5-liter 

beakers and perform the procedure through step 14 as if these 

were separate samples. After completion of the extraction of

TABLE 4. Maximum, minimum, and recommended sample volumes for isotopic

uranium analyses

U concentration,

Mg/1

0. 10

. 2

.3

.4

.5

. 6

. 8

1.0

1. 2

1.5

2.0

3. 0

5. 0

7.0

10

12

15

20

30

50

70

100

Maximum 

(note 1, p. F16)

25

25

25

25

25

25

25

25

25

25

25

25

25

25

22

18

14

11

7.3

4.4

3. 1

2. 2

Volume,

Minimum 

(note 2, p. F16)

25

12. 5

8.4

6.3

5. 0

4. 2

3. 2

2.5

2. 1

1. 7

1. 25

0. 84

.50

.36

. 25

. 21

. 17

. 13

. 084

. 050

. 036

. 025

in liters

Recommended

25

25

25

20

20

20

20

20

20

20

15

10

10

5

5

5

5

2

2

2

1

1

Reduce recommended to 

5

5

5

4

4

4

4

4

4

4

3

2

2

1

1

1

1

1

1

1

1

1
ISOTOPIC ANALYSIS OF URANIUM IN NATURAL WATERS F15

step 14, the ethyl acetate layers are combined and evaporated 

together.

5. To each sample (in a beaker having a volume at least 1.5 times 

that of the sample) add 3 ml of concentrated HN03 , 2 ml of 

0.2 M aluminum nitrate solution, and 5 ml of diammonium 

hydrogen phosphate solution. Heat to boiling to remove any 

carbon dioxide.

6. Add a few drops of methyl-red indicator and neutralize to the 

yellow endpoint by dropwise additions of ammonium hydroxide. If, on addition of the indicator, a pink color forms and then 

disappears, the water probably contains excessive iodide or 

bromide ions. In that event, add ammonium hydroxide, 2 or 

3 drops at a time; then add 1 drop of indicator. Repeat this 

procedure until the indicator does not turn pink upon hitting 

the solution but instantly exhibits the yellow color.

7. Digest the precipitate on a steam bath for 30 minutes. Allow the 

precipitates to cool and settle for 1 hour or longer.

8. Using a small pipet or capillary tube connected to an aspirator, 

draw off as much of the supernatant liquid as possible without 

disturbing the precipitate.

9. Transfer the precipitate to a 50-ml pyrex centrifuge tube. Police 

down the beaker and the stirring rod with 1 percent ammonium nitrate solution, adding the washings to the centrifuge 

tube. Centrifuge and discard the supernate.

10. Add 6 ml of 3.5 M magnesium nitrate reagent and warm gently to 

dissolve the precipitate. Pour this solution into a 60 ml cylindrical separatory funnel containing 10 ml of ethyl acetate. Rinse 

the centrifuge tube three times with 3 ml portions of magnesium 

nitrate solution, adding these rinse solutions to the separatory 

funnel.

11. Shake the separatory funnel vigorously for 2 minutes and allow 

15 minutes for the layers to separate.

12. Using a pipet, draw off as much as possible of the ethyl acetate 

layer and transfer to a clean 50 ml teflon beaker.

13. Add another 10 ml of ethyl acetate to the solution in the separatory 

funnel and repeat step 11.

14. Draw off the ethyl acetate layer using the same pipet as used in 

step 12 and transfer to the teflon beaker. NOTE. If the sample 

was divided in step 4, the ethyl acetate extracts should now be 

combined so that there is only one beaker for each sample.

15. Evaporate the solution in the teflon beaker to dryness on a steam 

bath. Add 10 ml of 8 N HC1 to the beaker, cover with a watch 

glass, and warm on the steam bath for 10 minutes.
F16 RADIOCHEMICAL ANALYSIS OF WATER

16. Allow the beaker to cool to room temperature, then pour the 

solution on the ion-exchange column prepared as previously 

described. Rinse the beaker with an additional 5 ml of 8 N 

HC1 and add the rinse to the column.

17. Allow the liquid to flow through the column at a rate of 20-30 

drops per minute. When the level reaches the top of the resin 

bed, rinse the upper column with about 5 ml of 8 N HC1, 

again allowing the level to drop to the top of the resin bed. Add 

50 ml of 8 N HC1 to the column and elute at the same flow 

rate. Discard the eluate.

18. Place a clean 100 ml teflon beaker under the column and rinse 

the upper column with 5 ml of 0.1 N HC1 from a wash bottle. 

When the liquid level reaches the resin bed, add 60 ml of 0.1 

N HC1 to the column and collect the eluate at a rate of 20-30 

drops per minute.

19. Evaporate the eluate to dryness on a hotplate. Dissolve the 

residue in a few drops of concentrated HNO3 and evaporate 

on a steam bath or low temperature hotplate to about 1 drop.

20. Add 10 ml of 2 M NH4C1 solution, cover the beaker, and warm 

on the steam bath for 10 minutes.

21. Prepare a clean titanium disc by washing first with acetone, then 

with concentrated HNO3 , and finally with distilled water. 

Assemble the electroplating cell.

22. Pour the solution from step 20 into the plating cell. Rinse the 

beaker twice with 5 ml portions of plating solution and add 

these to the cell.

23. Electroplate the solution tor 100 minutes at lib 0.1 amp in a 

fume hood.

24. Loosen the stopper and cautiously pour 2 ml of concentrated 

NH4OH into the cell through the funnel. Allow the plating to 

continue for 1 minute.

25. Remove the anode from the cell with voltage still applied. Disconnect the electrodes and turn the power supply off. Pour off 

the electrolyte, disassemble the cell, and rinse the plated disc 

thoroughly with distilled water. Flame gently till dry.

26. Count the disc for 1,000 minutes (or less if the count rate is high) 

on the alpha spectrometer as described in the following section.

NOTE 1. A sample containing 220 jug of uranium will give a deposit 

thickness of 50 jug uranium per cm2 on the approximately 4 cm2 

plating area, assuming a recovery of 90 percent. Sample volumes 

larger than 25 liters may be used, but are not recommended.

NOTE 2. The sample volume used should contain at least 2.5/xg of uranium 

if a relative standard deviation of less than 10 percent in the 

U234/U238 ratio is desired using a 1,000-minute counting time. The
ISOTOPIC ANALYSIS OF URANIUM IN NATURAL WATERS F17

uranium concentration should therefore be 0.10 /ig/1 or greater, 

using a 25-liter sample, in order to obtain this accuracy. Sample 

volumes containing less than 2.5 /zg of uranium can, of course, 

be used if either a longer counting time or poorer accuracy can be 

accomodated. The 2.5 /zg value is calculated for an extreme case 

where the U238 activity is 2.5 times that of U234 , the recovery is 

90 percent, and the counting efficiency is 25 percent. 

NOTE 3. If the samples were not previously acidified, add 2 ml of 

concentrated HNO3 per liter of water before starting the evaporation.

PROCEDURE FOR COUNTING SAMPLES

All components of the alpha spectrometer are turned on and 

allowed to warm up for at least 30 minutes before use. The detector 

bias control on the amplifier should always be at zero when the 

amplifier is turned on or off. After the amplifier has warmed up, the 

bias voltage is slowly turned up to the detector operating voltage. 

This slow adjustment prevents a large surge of reverse current through 

the detector which might damage it. An operating bias of 50 volts was 

used in all work reported here, but slightly higher or lower voltages 

may be required with different detectors.

Liquid nitrogen is placed in the 2-liter Dewar flask around the 

cold trap. The main function of the trap is to condense out oil and 

water vapors to which the detector is rather sensitive.

The electroplated disc is placed in the vacuum chamber and centered 

as close as possible to the detector without touching it; the chamber 

is then evacuated to a pressure of 100/z (10" 1 torr) or less. The main 

amplifier gain, post amplifier gain, and post amp bias are preadjusted 

for the desired energy range of the spectrum using the mercury pulser 

on the amplifier or known energy alpha sources. In the work herein 

reported, an energy range of about 3.8-5.3 Mev was used for most 

uranium analyses, and 200 or 256 channels of analyzer memory were 

normally used for accumulation of the spectra. It is usually not 

desirable to use less than 100 or 128 channels of memory for accumulation of uranium spectra because of the possibility of increased errors 

in integration of a small number of channels.

The analyzers used are constructed so that the memory can be 

divided into as many as four equal channel memory groups. It is 

possible, then, to accumulate as many as four spectra simultaneously if 

external detector and amplification circuitry is adequate. By so doing, 

maximum utilization of the rather expensive analyzer is possible, and 

the cost of analyses is minimized.

Because of the low uranium content of most water samples, the 

activity of the electroplated deposits is low and a long counting time 

is necessary to obtain good accuracy in U234/U238 ratios (tables 1, 2).
F18 RADIOCHEMICAL ANALYSIS OF WATER

A preset counting time of 1,000 minutes is therefore recommended 

for most samples. Where such accuracy is not required or the sample 

is of high activity, shorter counting times may be used. After accumulation of the spectrum, the data are printed out on an IBM typewriter or other data transcriber.

ANALYSIS OF THE SPECTRA

The digital data from the IBM typewriter readout are point 

plotted on linear coordinate paper, and a smooth curve through the 

data is hand drawn. Experimental peak heights are obtained from the 

graphs, and the boundaries of integration of each peak are determined 

by the points at which the curves cross 10 percent of the number of 

counts at the peak maxima. Channels within these boundaries are 

then summed to determine the peak areas and hence the number of 

counts of each isotope. Channels outside the graphical peak boundary 

having greater than 10 percent of the peak maximum counts are not 

included in the summation, but channels within the boundaries with 

counts less than this number are included.

If resolution appears poor or marginal, the resolution of the U234 

peak should be determined by dividing the FWHM in Mev by the 

energy of the peak. If the ratio is greater than 0.02, the data should 

be discarded and a thinner deposit plated.

RESULTS

The efficiency of recovery of uranium by this method is shown in 

table 5. The recoveries are very satisfactory for volumes of 1 liter or 

less, but they are noticeably lower for volumes greater than 1 liter.

TABLE 5. Recovery of uranium standards carried through entire chemical and

electrochemical procedure

[Volume: Samples 274 and 275, in milliliters; remainder, in liters] 

Sample Volume Recovery 

274___.___________ _______ ________ 250 0.942

275__-_________________________________ 250 .887

Average.______________________________ . 914

233_______ ______ _ 1 . 916

234_______ _______ ________________ 1 .934

279____ 1 . 894

280________ _________________________ 1 .911

Average.______________________________ . 914

288____ ______ _ 1. 5 . 782

289. ________ _ 1. 5 . 836

290_______________ ______________ 1.5 .675

291___ _______________________________ 1.5 .673

Average._______________________________ . 742

253_--.___________ ____ _____ 2 .746

254_______________ ____ 2 .660

262_____ ______ ___ 2 .808

264__ ______________________________ 2 .738

Average,_______________________________ . 738
ISOTOPIC ANALYSIS OF URANIUM IN NATURAL WATERS F19

Sample volumes should therefore be reduced to 1 liter before precipitation. It is generally not desirable to reduce sample volumes to less 

than 20 percent of the original volume. A greater reduction in volume 

frequently leads to formation of undesirable precipitates which must 

be removed by filtration or centrifugation. When sample volumes 

greater than 5 liters are used, the evaporation is carried out to obtain 

two or more 1-liter sample concent/rates. Duplicate precipitations are 

carried out on these, but they are later recombined for electrodeposition. Better recovery and lower reagent blanks are achieved this way 

over those obtained by precipitating aluminum phosphate from larger 

volumes.

The reagent blanks are very small, as shown in table 6, but for lowactivity samples a correction should be made. Contribution of the 

blank to the statistical error of the method is covered in the followingsection.

TABLE 6. Isotopic uranium blanks run on distilled water samples

[Volume: Samples 276 and 277, in milliliters; remainder, in liters] 

Sample Volume V Cpm IT-'3* Cpm Gross IT Cpm

276______-_________ ___

277______________________

265 _ - -

278__-----_______________

Average

286__--------______-__-__

287______________________

255_______.______________

256_ _---__._______

______ 250

______ 250

______ 1

______ 1

______ 1. 5

______ 1.5

______ 2

______ 2

0. 012

. 012

. 020

. 003

. 012

. 008

. 070

. 031

. 181

0. Oil

. 008

. 019

. 003

. 010

. 022

. 035

. 023

. 099

0. 023

. 020

. 039

. 006

. 022

. 031

. 105

. 054

. 290

Thorium isotopes (Th230 and Th232) are of major concern from the 

standpoint of spectral interference in uranium analyses. The procedure 

must therefore allow for a clean separation of this element. Using ] 

Th228 as a tracer, the thorium recovery was found to average 0.22 

percent for the complete procedure. Since thorium activities are much 

lower than those of uranium in most natural waters, this degree of 

purification is generally satisfactory.

Because of the low specific activity of uranium, either a fairly heavy 

deposit or a long counting time will be required to obtain good statistical accuracy. Generally speaking, the uranium deposit should be as 

thick as possible without causing poor resolution due to loss of energy 

or alpha particles within the sample before they strike the detector. 

The variation of resolution with sample thickness is shown in figure 6. 

Optimum resolution is obtained with samples having a thickness of 

24 jug of uranium per cm2 , or less. A sample of this thickness would
F20

3.5

a 3.0

Q_

I 2.5

u_ 

o

z 2.0 

o

I 1.5

CO

cci L0

O

cc

LLJ °- 0.5

RADIOCHEMICAL ANALYSIS OF WATER

 i rn r

1 2 3 4 5 6 7 8 910 20 30 40 50 607080 100 200 

THICKNESS, IN MICROGRAMS PER SQUARE CENTIMETER

FIGURE 6. Effect of source thickness on resolution of uranium alpha spectra.

give a count rate of 17.7 cpm (counts per minute) for each isotope 

(assuming equilibrium) with a 4 cm2 disk and a typical counting 

efficiency of 0.25 cpm/dpm. This would require a counting time of at 

least 11.3 minutes to obtain a relative standard deviation of 10 percent 

in the U234/U238 ratio, at least 47 minutes for 5-percent error, and at 

least 1,130 minutes for a 1-percent standard deviation. Since the time 

required for good accuracy is rather long, it is sometimes desirable to 

use a thicker deposit and sacrifice somewhat on the resolution. It has 

been found that the U234/U238 ratio is reliable when the U234 resolution 

is numerically less than about 2 percent. The thickness of the final 

deposit is therefore limited to a maximum of 50 Mg of uranium per 

cm2 . At greater thicknesses it is either impossible to integrate the 

spectral peaks as described, or a somewhat low value for the U234/U238 

ratio is obtained. Use of different integration boundaries (that is, 

channels where the curve crosses 20 percent of the peak maximum) 

also results in erroneous values of the ratio for thick deposits. The 

upper limit of the amount of uranium which can be plated is actually 

of little concern on most natural waters because of the low concentrations of uranium present.

Resolution is also affected by factors other than sample thickness. 

The pressure in the vacuum counting chamber must be considered 

because the alpha particles traverse a small air gap between the 

source and the detector. In the present counting system this distance 

is approximately 3 mm. Under these conditions, no change in resolution was detected at pressures as high as 500/i (0.5 torr), although a 

slight shift in the position of uranium peaks to lower channels was 

noted between 1000 and 500;*. At approximately 620 mm pressure, 

the peak width (FWHM) was found to be about three times as great
ISOTOPIC ANALYSIS OF URANIUM IN NATURAL WATERS F21

as at 500ju for the same source. Thus, some evacuation of the counting 

chamber is necessary, but an expensive vacuum system is not required. 

A standard low cost mechanical pump was found sufficient to reduce 

pressures to 10ju-20ju, much lower than is actually needed.

Resolution is also dependent upon the size of the detector. For 

optimum resolution and minimum cost, the detector should be as 

small as consistent with the activity (and therefore the area) of the 

uranium deposits. Optimum resolution is seldom required, however, 

and a detector having a sensitive area of 450 mm2 has been found to 

be a good general purpose device. With this size detector, uranium 

deposits as much as 200 jug (at 50 jug/cm2) can be used.

The counting efficiency, and hence the U234/!!238 isotopic ratio, and 

the average resolution were found to be virtually independent of the 

total number of counts providing resolution is below 2 percent. This 

is shown in table 7 for different counting times on a single sample. 

Small variations in resolution were noted from one count to another, 

but this did not affect the counting efficiency.

A comparison of U234/U238 ratios obtained by alpha spectrometry 

with values obtained by mass spectrographic analysis is shown in 

table 8. Agreement was generally found to be within one standard

TABLE 7. Efficiency and resolution as a function of number of counts. Sample 183-61

Spectrum

Counts Efficiency Resolution

TJ234 TJ238 U234 TJ238

97__-___

122

87__.___

89______

91_.____

93_____.

95______

102 .___

107 -._-

110__-_-

113_____

116_--._

123 ...

99_____.

100_--._

101_____

103 ____

104__.__

105_____

108 ____

109 ____

112_---.

115_.___

____ 11,237

.___ 10, 181

_.__ 1,610

____ 1,599

____ 1,613

..__ 1,590

__ __ 1,593

____ 621

. _ _ _ 649

.___ 673

_ _ . _ 600

.___ 631

____ 618

___. 183

____ 150

___ _ 157

____ 163

____ 162

_.-. 157

____ 168

__._ 156

-___ 171

____ 162

11, 693

10, 382

1,630 

1,624 

1,685 

1,616 

1,632

640 

643 

627 

662 

649 

663

155 

162 

162 

187 

162 

161 

163 

170 

156 

171

23. 0) 

22. 7J

23. 4^ 

23. 2 

23.4 

23. 1 

23. 2)

22. 6 

23. 6 

24. 4 

21. 8 

22. 9 

22. 1

26. 6 

21. 8 

22. 8 

23. 7 

23. 5 

22. 8 

24. 4 

22. 7 

24.9 

23. 6

 22. 8 

>23. 3 <

 22. 9 « 

23. 7

r 23. 31 

,22. 5j

'23. T\ 

23. 0 

23. 9 

22. 9 

<23. ij

22. 7 

22. 8 

22. 2 

23. 4 

23. 0 

23. 5

f21. 2 

22.9 

22. 7 

26. 5 

22. 9 

22. 8 

23. 1 

24. 1 

22. 1 

^24. 2

22. 9 {|- 

^23. 2 ( L

ll:

i!

 22. 9 }'

L 

1

eo} 1- 62

48^ 

71 

57 }l. 60

48 

78j

60s! 

60

45 1 1 c 7 64 1-57

64 

5lJ

f 1. 67] 

U 85]

(1. 99^ 

1.81 

{1. 71 

1.76

U. 7lJ

(L 82 

1.93 

Jl. 74 

U. 90 

1.81 

U. 78

1. 76

a. so

 1.83

23. 3 -"
F22 RADIOCHEMICAL ANALYSIS OF WATER

deviation of the statistical counting error. The errors in the mass 

spectrographic data should be very small, and the ratios obtained by 

this method are taken to be the true values. Duplicate analyses of 

most samples by alpha spectrometry show good reproducibility.

Isotopic ratios for several eastern slope Colorado streams are 

shown in table 9. These streams were all found to have an excess of 

U234 but quite a large range of ratios were observed.

TABLE 8. U234/U238 activity ratios as determined by mass spectrometry and alpha

spectrometry

Source

Congo standard.

Ore 301711_ _____________________

301712___________________

301713________________________

Unknown origin

XJ234/U236

Mass 

spectrometry

_________ 1. 000

________ .4197

________ .4123

________ .4552

_________ 1. 39

ratios

Alpha 

spectrometry

0. 994 ±0 

1. 008 ± 

1. 002 ± 

. 411± 

. 419± 

. 403± 

. 444 ± 

. 467 ± 

1. 46 ± 

1.39 ± 

1. 37 ± 

1. 42 ±

02 

03 

02 

010 

006 

Oil 

009 

008 

03 

03 

03 

05

TABLE 9. U234/U238 ratios of eastern slope streams in Colorado

Source

Big Thompson River. _ _ ______

South St. Vrain Creek

Left Hand Creek

Clear Creek

Bear Creek

Date sampled

1966 

Apr. 7_-__- 

Mar. 24___-

.____do_____

Mar. 3---__

Mar. ll--_-

Volume 

collected 

Qiters)

20 

30

20

50

40 

20

U23S concentration

M2/1

0. 9 

1. 1

7. 2

1. 8

2. 1 

2. 4

U234/U238

1. 38±0. 10 

1. 67 ± . 07

1. 05± . 01

1. 14± . 02

1. 18± . 02 

1. 53± . 05

DISCUSSION

Alpha spectrometry appears to be a highly satisfactory method for 

the isotopic analysis of uranium present in natural waters. The 

procedure described separates uranium from bulk impurities which add 

to the thickness of the final deposit causing poor resolution and 

from radiochemical impurities which cause spectral interference and 

resultant counting errors.

Of the radionuclides which have alpha energies sufficiently close to 

those of the uranium isotopes to cause errors, only thorium appears 

to be of consequence in natural waters. Radium-226 has alpha energies
ISOTOPIC ANALYSIS OF URANIUM IN NATURAL WATERS F23

which would interfere if this nuclide were present in the final deposit, 

but radium is an active metal and does not electroplate under the 

conditions described. Protactinium-231 emits alphas of several 

different energies and is a potential interference in the analysis. 

Although detailed interference tests have not been made, no natural 

waters so far analyzed have been found to give detectable amounts of 

this nuclide in the final deposit. Protactinium is expected to behave 

similarly to uranium in the chemical separations of this procedure, 

and its absence in the final electrodeposits prepared from natural 

water samples is probably an indication of its absence in the original 

sample. Eighty-four percent of the Pa231 alpha-particle energies are 

in the range 4.92 to 5.05 Mev. These are sufficiently above the U234 

energies to be clearly distinguishable, although some overlap will 

occur. Where slight Pa231 activity is found, the spectral overlap may be 

approximated and a correction made. If relatively high interference 

occurs, a chemical separation may be necessary.

Actinium-227 also has an alpha energy (4.94 Mev) sufficiently 

close to that of U234 to cause interference if present in large amounts 

in the final deposit. Separation of this nuclide should be good, however, 

since actinium is not absorbed on the ion-exchange column. Furthermore, Ac227 decays only 1.2 percent by alpha emission; interference, 

therefore, should be negligibly small.

The method of plotting and integrating the spectra is somewhat 

tedious, but the time involved is small compared to that of the 

chemical procedure and improved results justify its use. Point plotting 

of the data may be done by either hand or an X-Y plotter, but the 

spectrum should be hand drawn. A smooth curve through the data 

allows a more accurate determination of the peak height and the 

boundaries of integration. The accuracy of the integrals, and hence 

the isotopic ratio, are therefore improved. Use of predetermined 

channel intervals for integration was found to be unsatisfactory 

because of drift in the detection and analysis system during long 

counting periods and because of variation in the resolution from one 

sample to another.

Tables 1 and 2 give the statistical error in U234/U238 ratios in the 

final deposit under interference-free conditions. These data were 

calculated from the equation:

g'fl__/0'234 I 0238 V/2 QS 

-ft \C234 ^2387

where <TR is the standard deviation of the activity ratio, R, of U234/U238 , 

0-234 and 0-238 are the standard deviations of the number of 

counts, (7234 and C"238, of the two isotopes. With the assumption that
F24 RADIOCHEMICAL ANALYSIS OF WATER

the standard deviation of each count is equal to the square root of 

the number of counts, equation 1 reduces to

. 1 Y/2 R~\C_ 

'234

This equation is valid if the number of counts of each isotope is 100 

or greater, but it leads to errors for lower total counts. The assumption 

is also made that the detector background is negligibly small. The 

gross uranium background for the detector was found to average 

0.004 cpm (0.002 cpm for each isotope), and neglect of this quantity 

is therefore quite acceptable.

The isotopic ratio as measured by the alpha spectrometer is correct, 

within the limitations of counting statistics, for the uranium on the 

disc. This is not necessarily the true value for the uranium in the 

original sample because of uranium contributed by reagents in the 

chemical procedure. Blank runs, as shown in table 6, gave an average 

uranium count rate of 0.022 cpm for sample volumes of 1 liter or less. 

Although this is small, it may be sufficient to cause error in the 

isotopic ratio for low-activity samples. The statistical error should 

therefore be larger than the values shown in tables 1 and 2.

If the gross counts for the sample are designated by A, the mean 

reagent blank plus detector background by B, and the net ("true") 

sample counts by C,

C=A-B 

and

so that, from equation 1

A ff2 3i

^23

If it is now assumed that the number of observed counts of each 

isotope is greater than 100,

Since the blanks are small CB<100) and B^=B238=B, the error in 

this count is approximated by
ISOTOPIC ANALYSIS OF URANIUM IN NATURAL WATERS F25

Then the relative standard deviation in the ratio is

and the absolute standard deviation is given by

When A^>^>B, equation 3 reduces to equation 2.

Equations 3 and 4 should normally be used for calculation of the 

error in isotopic ratios unless the sample activity is high or the blank 

is very small. Blanks should be redetermined each time any of the 

reagent batches are changed. Care should also be exercised to note 

that the total blank is the blank per precipitation multiplied by the 

number of precipitates for each sample which are combined, after 

extraction, to give the final uranium deposit.

ACKNOWLEDGMENT

The author wishes to express his appreciation to Dr. John Rosholt, 

of the Isotope Geology Branch, Geologic Division, U.S. Geological 

Survey, for use of equipment in part of this study and for furnishing 

standards and to Mrs. Marjorie Rickard for performance of much 

of the chemical laboratory work.

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U.S. GOVERNMENT PRINTING OFFICE : 1967 O-293-421