Gamma-Ray Absorptiometer for Determination of Uranium in Aqueous and Organic Solvent Solutions S. J. BRODERICK and J. C, WHITMER
U. S. Atomic
Energy Commission, New Brunswick,
F A F r a y absorptiometer has been constructed containing 250 mg. of americium-241 source, an ionization chamber filled with xenon gas, a vibrating-reed electrometer as amplifier, and a recorder to record the measurements. The absorption of y rays by aqueous and organic solutions of uranium in the range 1 to 550 grams of uranium per liter was measured. The sensitivity of the method is better than 1% in the higher ranges of uranium content and less than 0.5 gram of uranium per liter can be detected.
S
of the early work on y-ray absorptiometry was done by Miller and Connolly (d), who constructed an in-line apparatus for the determination of plutonium and uranium. Thulium-170 was used as a source, a stainless steel container fitted with fluorothene windows as a sample cell, a lead-lined ionization chamber filled with air or gas as the detector, and a modified Beckman micromicroammeter as the current-measuring device. The same procedure was described later by Miller (I), who substituted a 50-mg. Am241 source for TmliO with an improvement in the sensitivity by a factor of 2. Thurnau (3) built a somewhat different w a y absorptiometer system for the continuous analysis of solutions of heavy-metal salts, such as uranium, in concentrations of 300 to 420 grams of uranium per liter, using a weak source of 10 mg. of Am241. Instead of an ionization chamber, a scintillation detector was used, and the signals were recorded on a ratio recorder. The system was different, because two sources of Am241 were used: One beam passed through the sample, the other was unhindered; the two beams were permitted alternately to reach the detector by a motor-driven rotary shutter impinged on a radiation detector. The prime purpose of the dual-beam system was to provide stabilization against instrumental drift. Whittaker (4) discussed the theory of the method, y-ray sources, detectors, and amplifiers as applied to the analysis of uranium and plutonium solutions in static and flowing systems. OME
13 14
ANALYTICAL CHEMISTRY
N. 1.
Woodman et a2. (6) reported on the Whittaker assembly consisting of either 200 mg. of 129-day Tm170 or 100 mg. of 470-year Amz4'as a source. A practicable system for in-plant control was desired for the determination of uranium in aqueous solutions in concentrations of 0.5 to 550 grams of uranium per liter and in tributyl phosphate-hexane mixtures in concentrations of 1 to 120 grams of uranium per liter. The system selected was essentially that of Woodman et al., with some modification. A 250-mg. Amz4' source enclosed in a cover slide container was used instead of 100 mg. of AmZ4l,or TrnIi0 as recommended by Woodman. A stainless steel ionization chamber filled with xenon gas a t a pressure of 0.575 mm. of Hg was substituted for the aluminum chamber and the dimensions were somewhat changed. Whereas the Woodman apparatus was designed for in-plant use with a 30mm. sample tube, the present system has an adjustable sample holder to accommodate sample tubes of diameters up to 31 mm. The vibrating-reed electrometer and the recording equipment are similar. Some parts of the apparatus were built, while other comCARI ELECTROMETER
HIGH VOLTAGE
5oov
RECORDER
2: ,,
COPPER TUBE.
/KOVAR
SEAL
FOR FILLING -GUARD
ELE(;TROD€
INSULATORS-
CHAMBER STEEL OUARD CASINO
TUBE
-SLIDE ,SOURCE
COVER AM-241
Figure 1. Schematic diagram of y-ray absorptiometer
ponents were bought. This same setup with minor modifications can readily be adapted for in-plant use. EXPERIMENTAL
Apparatus. X schematic diagram of the absorption apparatus is shown in Figure 1. SAMPLETUBE. The tube is made from precision glass tubing about 6 inches long, one end of which is sealed. A side arm of 8-mm. tubing about 3 inches long is connected to the sealed end. The side arm allows bubbles to escape when the tube is filled with the test solution; the open end, which is used for filling, is closed with a rubber stopper. SOURCE.The source selected was 250 mg. of Am241,the maiimum quantity that may be used in a l/=inchdiameter tube without appreciable selfabsorption. The long half life of 470 years makes it attractive as compared to the 129-day half life of Tm170. The low energy 60-k.e.v. rays of Amzr1 are more readily adsorbed by the uranium solutions than are the 120-k.e.v. energy rays of Tm17O. The Am241was procured from the Oak Ridge National Laboratory, which converted the americium chloride to the hydroxide in a cylindrical fluorothene container. This container had an inside diameter of inch, a vvall thickness of /* inch, and a bottom thickness of '/IS inch. After precipitation and thorough washing, the precipitate was dried a t about 80" C. and cemented in place a t the bottom of the fluorothene cylinder with plastic cement. The cylinder was then placed in a contamination-free Lucite cylinder fitted with a Lucite cap sealed airtight. The bottom thickness of the Lucite cylinder was '/I6 inch, which, when added to the fluorothene container thickness, gwve a total thickneSs of '/* inch. The thickness of the plastic materials reduces the intensity of the 60-k.e.v. photons by about 15% and the 15-k.e.v. photons by about 60%. The Lucite container is next placed in a stainless steel holder with a sliding cover top. Jl'lwn the source is not in use, the dide cover is closed to protect personnc'l against stray radiation, The present source unit has been operating satkfactorily for about a year, but since some organic containers in use have ruptured Iiccause of gas formation caused by radiologic action, it will be returned and encapsulation
will be made in a stainless steel button. The buttons are 1 inch in diametcr and 1 ' 2 inch thick, with a 10-mil stainless steel window across the front. 1screw cap in the back holds the source in place. The steel buttons are much safer and easier to handle than the organic. They may be safely picked up with the fingers and placed in suitable containers, as long as the face of the button is always pointed away from the body. IONIZATIONCHAMBER. An ionization chamber (Figure 1) was selected over the scintillation counter because of its greater stability and lesser maintenance requirements in plant use. Ionization chambers may be made of different metals, such as aluminum, brass, or stainless steel. Stainless steel was chosen because it is considered to provide greater sensitivity, especially with xenon gas. The superiority of xenon gas over argon can be seen from the responses shown in Figure 2 . The chamber used consists of a stainless steel shell 4 inches high and 2.00 inches in inside diameter, made from 2.5-incth tubing. Across the bottom t o serve as the window was fastened a l/gp-inch aluminum plate. A 1/8-inchJi:imeter stainless steel rod was the collector electrode, passing through a Teflon insulator a t the top and extending to within l / 2 inch of the bottom. A guard electrode connected to a ground dissipated extraneous currents. The chamber was supplied with a tube connector and valve, so it could be evacuated. When a pressure of IO-' mm. of Hg could be held for several minutes, the chamber was filled with xenon gas of normal pressure (purity: nitrogen, 0.007 mole %; krypton, 0.007 mole %) from a 1-liter flask and then the valve was closed. Since the volume of the system, including the chamber, was about 350 ml., the xenon gas pressure within the chamber would then be only 575 mm. of Hg. A voltage of 500 volts is applied to the chamber t o collect the ions. The chamber must be insulated from the outside cover shield. MEASURING SYSTEM.A Cary vibrating-reed electrometer, Model 31, was used. The current flowing through the ionization chamber by action of the Am241source is of a small order of magnitude, 10-9 to 10-11 ampere. When the electrometer is coupled to a Hrown or a Sargent recorder, the variations in solution concentrations over a period of time can be recorded permanently. SAMPLE HOLDER. The sample holder consists of a base plate of aluminum 5'/2 X 4'12 X '/e inches thick, to which is attached, in the center, an aluminum vertical plate, 1 inch thick by 3 x 51/* inches wide, so as to form an inverted T. -4horizontal V is cut across the entire width of the vertical plate 2 inches above the base plate. The V-notch positions the round sample tube, which is held in place by a spring clamp. The base plate is movable backward and forward for positioning of differentsized sample tubes with respect to the
Figure 2. Response
of xenon and argon in ionization chamber
6
12
18
24
30
36
92
411
G R I M S U R I N Y L N I T R I T E I N 100 ML H20
7-ray source. Above the V-notch, grooves are cut so that stainless steel calibration plates, 2 inches wide and of various thicknesses, may be slipped in over the sample tube position. When the tube is removed, the calibrated plates may be used instead of solutions. These plates are standardized against known solutions of uranium. Slit size is 1 x inch. Procedure. Known solutions of uranyl nitrate were prepared by weighing the requisite amount of the salt, dissolving in distilled water, and making u p t o desired volume in volumetric flasks. The uranyl nitrate used was 47.56% uranium by analysis. The sample tube was filled with solution, making certain there were no air bubbles in the tube, and then placed in the holder. The source cover was apened and the unabsorbed photons passed into the ionization chamber; a potential of 500 volts was applied to the ionization chamber. The current flowing operated the recorder and the swing of the recorder pen was adjusted until the maximum concentration of solution gave a sizeable reading. The diameter of the sample tubes used varied from 15.5 to 22 mm., which was necessary to cover the wide range of uranium concentration and to obtain a sensitivity of a t least 1% in the higher ranges. The absorption formula is usually written as I = Ioe-vx, where X is the thickness or areal density of the absorbing medium and U is the mass absorption coefficient. U is variable and changes with the density of solution. Thus there are two variables in the equation. T o increase the sensitivity it is necessary to select the optimum diameter tube so as to reduce as much as possible the Compton type of absorption by scattering. IN-LINE TRIAL. A 1-liter solution containing 350 grams of uranium per liter was continuously circulated through the sample tube by a pump for 2 hours. The recorder gave a constant reading. When the solution concentration was varied by 1%, there was a 11/2division shift in the reading. The present arrangement gave evidence of satisfactory operation for inline control.
EFFECT OF IMPURITIES ON -/-RAY ABSORPTION OF URANIUM SOLUTIONS
The next portion of the study involved the application of the y-ray absorptiometer to uranium slurries to determine the magnitude of the effect of impurities in the slurries on absorption. The amounts of the impurities studied are those likely to be found in commercial slurries. A typical specification for a uranium concentrate is shown in Table I. At this point two components in the measuring circuit were changed. The Model 30 vibrating-reed electrometer was replaced by the new Model No. 31 and the older Brown recorder was replaced by a new Sargent recorder. The effect of impurities was determined in the range of 350 to 550 grams of uranium per liter as this range wm of most interest to the manufacturer. The absorptiometer sample tube was
Table I. Desirable Specification of Impurities for Uranium Concentrate
Element
Per Cent (of U I O ~Content) 75 .O min. of dry wt. of
UaOs
concentrate
2 . 0 max. Not t o exceed sum of I .31 X Fe 2% of
V206
PO4
+
content; total PO4 content not to exceed 6% of U308 content u308
Mo B C1, Br, I
F As
co. us08
c1 0 . 1 max. 2 . 0 max. 4.0 max.
SO;
Ca
0 . 6 max. 0 . 2 max. 0 . 1 max. expressed as
insoluble
€320
Si02 Ti, Cr, Cu, Mn, Ni, and Pb
10.0 max. 1 . 5 max. 0 . 1 max. 10.0 max. 10 max.
In small concn.
VOL 33, NO. 10, SEPTEMBER 1961
1315
.I g
'UBE
e w
8% w
26
I350
10 L
20
60
40
100
80
G R A M S U R A N I U M PER L I T E R ( T E P - H E X A N E
'0°
GRAMS
U R A N I ~ VLPI TEE RR
500
550
Figure 4. Effect of 5% iron on y-ray absorption of aqueows uranium solutions
420
5V-501
Figure 3. Effect of 4.2 and 10% sulfate on y-ray absorption of aqueous solutions STABILITY OF SYSTEM AND ACCURACY
made of precision tubing of 0.500 f 0.001-inch inside diameter. All of the impurities likely to be present in the slurries were not tried in the absorptiometer, because some of the available salts were difficult to retain in solution with the uranium salt. Others were not soluble in nitric acid. I n some cases it was necessary to make up the uranium solutions in 2M nitric acid. The acid radicals, impurities, and concentration on which data were collected are shown in Table 11. The impurity compounds in solid form were not added directly to the uranium solutions because of slow solubility of the compounds. Instead, it was necessary to dissolve the compounds separately in water and then add them to the uranium solution. In some cases the uranium solution was made acid with nitric acid. The procedure found satisfactory was to weigh out the correct amount of the uranyl nitrate solid, transfer it to a 100-ml. volumetric flask, add sufficient water (50 to 75 ml.) to dissolve the solid, and heat, if neces-
-S
The impurity in aqueous solution was then added to the cooled flask and the solution made to volume. A sample with the impurity was measured and immediately a uranium solution of the same concentration was measured.
spry.
RESULTS
The effect of added impurities on the absorption of the 7-rays is shown in Table 11. Typical curves are shown in Figures 3 to 6. The values given in the last column of Table I1 were obtained from the horizontal displacement of the impurity curve from the impurity-free curve. The equivalence values are given in absolute unitsi.e., grams per liter of uranium per gram per liter of impurity. There & a range given for some equivalence values. Part of the difference may be real and part due to how closely the curves are drawn through the plotted points.
.
Concn., G./L. U 350 400 450 500 550 S.D. (abs.) 0.880.51 0.58 0.24 0.30 S.D.(rel.),yo 1.0 0.7 1.0 0 . 5 0 . i
There is an obvious increase in recorder readings with time.
-m
URANYL N ' T R A T E
.__ 0 .
To evaluate the reproducibility of the measurements and to note the drift due to the electronic components, it was necessary to collect data over a period of time. Known solutions of 350, 400, 450, 500, and 550 grams of uranium per liter were made up in 1-liter quantities. These stock solutions were then run in the absorptiometer over a period of 18 days, three scans made each day. The standard deviation within days seems fairly constant, regardless of the concentration. It is estimated a t 0.13 gram of uranium per liter. The standard deviation seems to vary with concentration, the greatest being l.Oyo. The standard deviation calculations are:
PLUS 5%
URANYL N I T R A T E PLUS 5%
(RON
\
Y)
THORIUN
*
I?
D
400
Figure 5. Effect of uranium solutions
1316
450
G R A M S URANIUM PER L I T E R
500
556
5% thorium on the y-ray absorption of
ANALYTICAL CHEMISTRY
I350
4w
4 50 G R A M S URANIUM PER L I T E R
500
550
Figure 6. Effect of chromium, copper, manganese, cobalt, and nickel on y-ray absorption of uranium solutions
c 4 CU. U j O e 0ASIS
I H P U R ~ T I E SC O N S I S T OF A S O X I D E S ON
MN,CO,
a mi
The light elements such as silicon, sodium, calcium, and magnesium have a greater effect on the absorption than do the anions mentioned. iis the atomic number of the element increases, the absorption effect increases, as one would expect. Molybdenum in 3M nitric acid shows a marked increase in absorption. Lead, of high atomic number, naturally will have a large effect upon the absorption of the y-rays. Thorium and lead should be equal in their effect upon absorption, but the data in Table I1 are incomplete, ae only a 501, thorium concentration was compared to a less than 1% lead concentration. Since the atomic numbers of chromium, copper, manganese, cobalt, and nickel are of the same order of magnitude as that of iron, the effect on the absorption should be about the same, as seen in Table 11.
CALCULATED
a 400 GRAMS URANIUM PER L I T E R
500
550
Figure 7. y-Ray absorption in concentrations of grams of uranium per liter in organic solvent
0 to 120
TBP and hexane
To appraise the accuracy of the method, the following experiment was performed. Three solutions containing 365, 421, and 481 grams of uranium per liter were prepared by weighing the requisite amounts of uranyl nitrate (47.56%) into definite quantities of water. These solutions were called “Prepared” solutions. Each of these solutions was run in the absorptiometer and four scans were made for each solution. The scale divisions from the recorder scans were translated to grams of uranium per liter from the standard curve. This standard curve was constructed from five known solutions of uranium made similarly to the “Prepared” solutions. The data from this experiment are given in Table 111.
Table II.
Effect of Impurities (Usually Found in Uranium Slurries) on y-Ray Absorption of Uranium Solutions
Concn. (Abs.), G/G. U
Impurity SO,
0.05 0.10 0.06 0.12 0.10
PO,
Na caJ
Mg
0 0 , 1 4 4, 15 0
0 0.14-O,08 0.24-0.24
NaCl CaClz MgSO,. 7Hz0 Fe(NOa).9Hz0 .2H90 NaPMOA Pb(CzH;O?)z:3Hz0
0.03 each
Fe
Effect on C AbsorptionD
Compound Na2S04
0.05
0.29-0.33 1.34-1.39 2.09-2.13 1.07-1.12
Th( N03)3 9Hz0 Cr( ’ 9Hz0
CU(NO,)?.3Hz0 MnS04.4&0 COC12.6HaO
E J NOa
0.30-0,38
Ni(NHr)2(S0,)?.6H?0 HNOa
0.53
0.u2
Equivalence in g./L U per g./L of impurity. Range 350-550 g./L U. DISCUSSION
The variations in uranium concentrations affect the shape of the absorption curves. By selecting the optimum sample tube diameter and narrowing the range of solution concentration, nearly straight-line relations may be obtained. The curvilinear behavior is mostly due to the exponential character of the absorption and to the type of interaction taking place between photons and the atom species in solution. The sensitivity of the method in the range of 0 to 25 grams of uranium per liter is about 0.5 gram of uranium per liter and the sensitivity in the range of 300 to 550 grams of uranium per liter is better than 1%. By increasing the amplification the sensitivity can be increased to about 0.5 of the amount present. Figure 7 shows the effect of different-sized sample tubes on the sensitivity, where the 22-mm. tube shows a steeper slope of the curve than the 15.5-mm. tube.
ACKNOWLEDGMENT
Table 111. Measurements for Accuracy of Method on Three Prepared Solutions
(Solution prepared from 47.56% U analyzed salt) Grams of U per Liter Estimate In prefrom pared std. Av solns. curve Av. diff. % 365
366 364 365 364
364.8
0.06
421
421 420 421 42 1
420.8
0.05
481
483 479 478 479
479.8
0.25
The apparatus was constructed by Austin Padgett. LITERATURE CITED
( 1 ) Miller D. G., General Electric Corp.,
Hanford Atomic Products Operation, Rept. HW-39971 (November 1955). ( 2 ) Miller, D. G., Connolly, R. E., Zbid.; ReDt. HW-36788 (June 1. 1955). (3) Thurnau, D. H., ANA. C ~ M29, . 1772-3 (1957). ( 4 ) Whittaker, A., U. K. At. Energy Authorit , Symp. on Instrumentation in
Chemicay Analysis, Capenhurst Works, March 20, 1958. (5) Woodman, F. J., Clinton, T. G., Fletcher, W., Welch, G. A., Proc. 2nd International Conference for Peaceful Uses of Atomic Energy, United Nations, Geneva, Vol. 28,p. 423,1958.
RECEIVED for review February 9, 1961. Accepted June 30, 1961.
VOL 33, NO. 10, SEPTEMBER 1961
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