Radiometric Analysis of Leached Uranium-Thorium O r e Samples with the Beta-Gamma-Gamma Method P. W. DE LANGE Radioactivity Division, National Physical Research laboratory, Council for Scientific and Industrial Research, Pretoria, South Africa
b A radiometric method for the quantitative analysis of naturally leached uranium-thorium ore samples is presented. Ordinary p-y method calculations are done after the thorium oxide, ThOz, concentraton has been determined, utilizing the 2.6-m.e.v. y-photopeak from ThC” (T1208). Radiometric results of 24 samples are tabulated and agree well with chemical values for the uranium oxide, U308, and Tho2 contents. When 18-gram samples are used, drift of the photopeak of the 2.6-m.e.v. y has been less than 0.5% per week. Variations of 5’ C. in a room temperature of 21 O C. had no effect on the stability of the apparatus.
A
the p-7 equilibrium method (4, 7 ) as used in practice (1-3) has given excellent results for leached uranium ores and unleached uranium-thorium ores, the results for naturally leached uranium-thorium ores have not been satisfactory. Reports have appeared on the rather artificial extension of the P-7 method to include such cases and, without doubt! qroduced excellent results. However, similar conditions could not be reproduced in this laboratory ( 1 ) . This paper presents an extension of the equilibrium method that can be reproduced in any laboratory where reliable drift-free electronic equipment is available. LTHOUGH
EXTENSION OF THEORY INVOLVED IN EQUILIBRIUM METHOD TO P-y’-y”-METHOD
Consider the case where the uranium series has been naturally leached or redeposited in the presence of thorium minerals (1). KO current evidence exists that natural leaching of thoriumminerals takes place. By grouping the uranium series into a uranium and a radium group, the folloiving expression can be obtained for the beta counting rate:
+
+
CP = U ( P u P B ~ ) TBt (1) Similarly, for the gamma counting rate, C;, obtained a t the lowest possible discrimination level-100 k.e.v.- integral counted-there is Cl, =
Wr:
+ prr’) + Tr:
(2)
A third equation might be obtained 812
ANALYTICAL CHEMISTRY
by screening some of the p-rays. It will involve weight corrections, because a large amount of y-rays will be slowed down and counted by the Geiger tube. A more straightforward possibility is to utilize the 2.6-m.e.v. 7-photopeak from T h C ” in the thorium series. If the rvindow of a differential discriminator is set over this photopeak, a small amount of the low intensity 2.42-m.e.v. ?-photopeak from RaC(Bi214)will also be registered. The following expression for this gamma counting rate will be obtained: C”y - CPY: T Y ; (3)
+
The solving of Equations 1, 2, and 3 for T gives the expression: T = cp + c:’ . !+ -I c;. B“
(< r r
(0: - r:$)
4)
Yu
-
-1r
y‘, Yi
$ (?:- r: %)
(4)
To obtain the U value, a similar formula can be deduced or the % T h o l value obtained can be utilized to calculate the number of betas and gammas resulting from elements in the thorium232 series. The amount of betas and gammas due to elements in the uranium238 series can then be obtained by subtraction. The nominal formulas are used, for the determination of the % UsOs and the p-value ( 1 ) :
L‘
=
Uj(l
+ A ) - u;A
(5)
and
P =
L1;A/a - U i A Li
+ l/x
(6)
This procedure proved the most easily applicable in an assay laboratory. APPARATUS
For the routine application of gammadiscrimination it is necessary to have well stabilized counting conditions. Of the locally available equipment, the follon-ing arrangement m s selected. Gamma-Counting Unit. High voltage supply. Modified Tracerlab RLI-7 (Tracerlab, Inc.) with a measured drift of 0.004% per 9 hours. Linear amplifier. Tracerlab RLA-1 (improved) m-ith t h e proper 6BN6 discriminator. A window widener was available b u t was not used because its response vias frequencydependent above 1000 counts per second.
Multiplier phototube, Radio Corporation of iimerica 6655 (unselected) with a 2 x 13/&inch o sodium iodide Harshaw crystal (Harsha\y Scientific). Scaler. Locally designed, using Philips EIT tubes. With this arrangement, the drift of the 2.6-m.e.17. y-photopeak of the ThC” has been less than 0.5% per week and could be daily compensated for with a 11-step attenuator ranging from 1.00 to 1.125. A drift of f 5 ” C. a t a room temperature of 21” C. had no effect on the peak position. Beta-Counting Unit. High voltage supply. Dynatron Type 1082 A. Preamplifier. Dynatron 1014 A set a t a quenching time of 300 psec. Geiger tube. Tracerlab TGC-2 with window thickness of 2.1 mg. per sq. cm. Scaler. Dynatron Type 200. The arrangement of the apparatus is similar to that described ( 1 ) . The sample pan containing the mineral sample to be analyzed is placed between the window of the Geiger tube and the sodium iodide crystal, all mounted inside a lead castle. TECHNIQUE
Using about 18-gram samples, the three counting rates Cp, C;, and C; were obtained. Only background correction was made on the p-count after the dead time correction was carried out. ( T h e , sensitivity of the Geiger tube is neghgible.) For the two y-counts, background and sample-weight corrections were necessary and were made ( 1 ) . No daily corrections vere made on the results of the y-counter, b u t a n enclosed 10-gram 427, Us08 sample served as a reference source. Standardization was done lyith a 0.512% U30s standard obtained from the Canadian Department of Mines and Technical Surveys. The U,08 concentration in this sample as well as that in a 5.4y0 r 3 0 8 (pure uranium separated from daughter products) sample 11-as obtained on a mass spectrometer at the National Physical Research‘ Laboratory. The pure uranium sample is more than 3 years old and therefore contains its immediate daughter elements. -4uranothorianite sample, T249/11, which gave the best checks with chemical values on the P-y apparatus was used to obtain the constants for thorium. The chemical values are 0.4537, Tho2 and 0.1427, u308.
r-
The necessary sample preparations and the influence of nonuniformity have been described (1). The same sample treatment used before-methanol mixing of samples-gave good results.
Table
Radiometric and Chemical Values for Leached Mixed O r e Samples
Original
SO.
KO.
EXPERIMENTAL RESULTS
The total counting times involved in obtaining the results tabulated in Table I amounted to SO, 20, and 60 minutes, for Cp, C;] and Ct, respectively. The first four samples are Dominion reef-outcrop samples and are probably not in equilibrium. From the p-values listed it seems that uranium redeposit (or radium group loss) took place-more in one case than in the other. More important, however, are the good checks obtained for the T h o 2 values with the listed chemical values. To a lesser extent it is also the case for the UsOs values, although slightly high. The two sets of chemical values were obtained by two different analytical laboratories. Applying the ordinary p-y method ( I , 3 ) calculations (where p is assumed to be 1) gave negative results for the ThOz contents. Applying the ordinary y’-y” method (8)gave results for T h o z not much different from those obtained with the proposed p-y’-y” method, but the L-308 values mere below the chemical values. Samples 5 and 6 are composite samples and also showed a slight difference in the equilibrium condition of the uranium series. Samples 7 to 15 and 16 to 24 were obtained by sampling succmsively across the Dominion reef in two different cases. The p-7 method gave confusing results, but n-ith the p-7’-y” method, there is only one serious anomaly in the U I O ~ values (assuming t h a t the chemical figures are all in order)-sample 11-and one anomaly in the Tho2 wluessample 18. KOexplanation can be given for these differences other than that i t is perhaps failure of initial assumptions. Radiochemical analysis of disequilibrium ores b y Rosholt seems to prove that the generalized grouping of uranium and radium, is not specific enough (5, 6 ) . The errors involved in these tabulated radiometric values are best illustrated with a specific example-for sample 7 the results are: %Tho2 = 0.039 =k 0.003%, %UaOs = O.OS5 i 0.002%, and p = 0.81 i 0.05. The error to sample value ratio will naturally become smaller for higher concentration material. K h e n only %minute counting is carried out on t n o repeat samples, the minimum concentrations of us08 and T h o n that can he determined with the P - y ’ - y ’ ’ method are 0.17, for both unknom-ns n-hen a n 18-gram material is used. Hon ever, the grades of some samples nere rather low and the amounts
I.
NPRL Sample
Radiometric Values p-value ThOn,.~% U308, 76
1
A418
0.15
0 16
0.53
2
A419
0.14
0 35
0.37
3
A426
0.25
0 ’io
0.61
4
A432
0.024
0 028
0.58
5
KCGI
0,093
0 32
0.66
6
KCGIV
0.23
0 62
0.77
7
D622A
8 9
B C
16
D540A
23 24
H I
0.039 =k. 003
0.046 0.093 0,057 0.064 0.19 0.12 0.18 0.20 0.16 0.072 0.11 0.12 0.19 0.18 0.22 0.021 0.12
0 085 i 002 0 14 0 51 0 19 0 17 0 67 0 22 0 89 0 29 0 54 0 33 0 61 0 41 1 11 0 65 0 55 0 069 0 25
were less than 35 grams, so that a 90gram sample container which is normally used for low grade ore, could not be utilized. NOMENCLATURE
c
=
concentration of uranium in yo u308
concentration of thorium in % ThOs = equilibrium value of uranium P series uranium-radium ratio in - leached or enriched ore uranium-radium ratio in equilibrium ore = counting rate of p-particles Pu originating in the uranium group per 1% U308 = counting rate of ?-rays origiY nating in the uranium group per 1% U3O8 pe.p 18 grams with integral discriminator on 100 k.e.v. = counting rate of 8-particles Pr,Pt originating in the radium group per 1% u308 (equivalent) and for 1% Tho*, respectively = similar to r: but for radium * / r , Yt group and thorium Eeries, resoectivelv , = count‘ing rat; of -/-rays orig-/r , Y t inating in radium group and thorium series per 1% U108 (equivalent) and 1% Tho2, respectively, per 18 grams, falling in the 2.6-m.e.v. -/, , photopeak of ThC“ Up, Cy = apparent uranium concentratjons derived from the thorium corrected p and -/ measurementsT
=
Y
I
I,
A
,
0.81 i.05 0.76 0.85 0.92 1.oo 1.06 1.28 0.92 1.04 0.70 0.62 0.87 1.26 1.09 0.97 0.87 1.00 1.12
Chemical Values, -
70
ThOz
u308
0.15, 0.15 0.12, 0.13 0.27, 0.29 0.022, 0 025 0.008, 0.22 0.20, 0.20 0.050
0.11, 0.10 0.22, 0.19 0.59, 0.53 0.022, 0,037
0,055 0.098 0.065 0,055 0.18 0.13 0.18 0.22 0.19 0.085 0.25 0.14 0.17 0.16 0.25 0.020 0.13
0.13 0.54 0.21 0.066 0.75 0.27 0.89 0.33 0.46 0.30 0.47 0.48 1.12 0.62 0.47 0.074 0.26
...
0.23
...
0.53 0.080
ACKNOWLEDGMENT
The assistance given by Douglas Simpson in obtaining some of the samples is acknowledged. The author is also indebted to Peter Laxen and Trevor Steele of the Government Metallurgical Laboratory for supplying the two series of reef section samples, as d ell as their chemical values. Franz Strelow of the Xational Chemical Research Laboratory is thanked for some duplicate chemical values. LITERATURE CITED
(1) de Lange, P. IT.,Trans. Proc. Geol. SOC.S. Africa 59, 259 (1956). (2) Eichholz, G. G., Hilbron, J. W., McMahon, C., Can. J . PhzJs. 31, 613 (1953). (3) Golbek, G. R., Matvejev, V. V., Sliapnikov, R. S., A/Conf. 8/p/630 Geneva (1955). (4) LaPointe, C. VI., Williamson, D.,
Canadian >fines Branch Topical Report, TR-101/52 (1952). (5) Rosholt, J. K., ANAL. CHEX. 29, 1398 (1957). (6) Rosholt, J . N., Proc. 2nd Nuclear
Sci. Eng. Conf. “Advances in Nuclear Engineering,” Dunning, J. R., Prentice, B. R., eds., Vol. 2, Pergamon Press,
h-ew York, 1958. (7) Thommeret, J., J . phys. radium 10, 249 (1949). (8) Whitham, K., Ph.D. thesis, University of Toronto, Toronto, Canada, 1951.
RECEITEDfor review June 9, 1958. Accepted December 11, 1958. Work completed as part of a project undertaken by the National Physical Research Laboratory on behalf of the South African Atomic Energy Board, and published with the perniission of these two organizations. VOL. 31, NO. 5, MAY 1959
813
