Analysis of Naturally Leached Uranium-Thorium O r e Samples Application of Pure Gamma Spectrometry P. W. DE LANGEI Radioactivity Division, National Physical Research laboratory, Pretoria, South Africa
All the radiometric methods in use a t present are reviewed and their applications in the analysis of leached ores are discussed. Possible errors encountered in radiometric analysis are summarized. Pure y-spectrometry gives the best results for the concentration of Us08 and Tho2 in the worst cases of natural leaching.
T
of the concentration of uranium and thorium in mixed orc samples by radiometric methods is straightforward when the ore lias not experienced leaching of some oi the radioactive daughter products of uranium and thorium. If the equilibrium has been disturbed in the past, the determination of the relative amount of radiation emitted by the differcnt daughtw elements will be affectrd to different extents. The (Yrays emitted by E238 and Th23? can be measured directly ( 1 4 ) , but a-rays are very easily absorbed and thin sources are necessary. All the radiometric methods reviewed below utilize the p- and or y-rays cmitted by the various daughter products. The possibility of disturbance of equilibrium in the uranium series necessitates the use of a t least three independent equations to determine the different contents of c3Os and Tho2 as well as the disequilibrium value of the uranium series rvhen it is assumed that the potassium content is negligible. The worst possible disturbance of the equilibrium of the thorium series will be restored a t a masimum time of -7 x 6.7 years [the half life of Ta225(MsTh2)I -Le., 47 years. Therefore, the thorium series is usually taken to be in equilibrium with the mother element, Th232. Either p-absorption techniques can be uscd or some of the y-radiation can be absorbed n i t h the alternative of clcctronic discrimination of certain HE DETERXINATION
Present address, Service Radio&ments, C.E.N, Saclay, B.P. KO.2, Gif-eurYvette (S. et O . ) , France.
y-energies. K i t h modern electronic detection techniques it is more elegant and efficient to select some of the gamma photopeaks obtained with a NaI(T1) scintillation unit to obtain independent equations. A method using only y-spectrometry for the analysis of leached uraniumthorium ore samples is presented. Results are compared with those obtained by other radiometric, chemical, and mass spectrometric methods. PRESENT STATUS OF RADIOMETRIC ANALYSIS OF LEACHED ORE SAMPLES
p-y Method. For t h e application of t h e p-y method a beta count is obtained with a Geiger counter and a gamma count of t h e same sample with a scintillation counter or n i t h a thickwalled Geiger counter. Two equations with two unknowns, per cent U30s and per cent T h o 2 , are obtained ( 5 ) . This method, however, is not rigorously applicable to the analysis of leached mixed ore. Eichholz ( 7 ) presented evidence that this so-called equilibrium method (11, 23) is applicable to the analysis of leached ores under special conditions. But, unfortunately, these instrumental conditions could not be duplicated by the present author (3, 4). y-y Method. TS’hitham (24) intrcduced the use of tu-o integral measurements of the y-spectrum of mixed uranium-thorium ore samples. T h e y-spectra of t h e sample of uranothorianite are given in Figure 1 as measured a t t h e Radioactivity Division n i t h a 2 X 13, inch diameter SaI(T1) crystal. From these y-spectra i t is clear t h a t t h e y-y method as proposed by Whitham can be used mainly to measure the gamma activities of the thorium series and t h a t of the radium group in the uranium series when the activity due to the uranium group is small. The artificial grouping (5) of the radiations emitted by uranium series into a uranium group [U238to Th230(Io)]and a radium group (RaZz6to Pb206)is perhaps a generaliza-
tion but it proved to be a reasonable simplification. Strominger, Hollander, and Seaborg (22) as well as Senftle, Farley, and Lazar (19) were consulted in establishing the energy values of the gamma photopeaks. ,4more specific attitude \vas taken by Hurley (10) in using a differential discriminator. He could assay for thorium and uranium with a direct measurement of the counting rate in two energy bands, -239 and 180 k.e.17. Hurley suggests, however, that B separate uranium analysis, probably chemical, should be made when accurate values for the Tho2 concentration are desired, even for ores that are in equilibrium. A slightly different attitude was taken by the author (4)by selecting and counting first the number of gammas in the 2.62-m.e.v. peak of T1208 (ThC”) and second the total integral count above 100 k.e.v. I n the latter case the percentage of the y-rays originating in the uranium group amounts to 5% of the total y-intensity measured with a uranium sample that is in equilibrium. This percentage can be increased to more than 8% by decreasing the discriminator due to the presence of the 95 t o 100 k.e.v. y-transitions of Th234 and Pa2a4 (UX2) (Figure 2, left). Damon and Feely (2) used a 2-mm. thick NaI(T1) crystal in a scintillation spectrometer to enhance the effect of the 100-k.e.v. gamma photopeak, the crystal being transparent for y-energies above 100 k.e.v. They did not apply their technique to leached mixed ores. With all these modifications to the original y-y method it is still not possible t o obtain correct results for the U308 values in mi-ied leached ores, although the order of the concentration is correct. The T h o z values agree with the chemical values for samples which experienced slight leaching (3, 4). p-y-y Method. This is t h e first radiometric method specifically designed for t h e analysis of leached uranium-thorium ore samples, other t h a n t h e method of Peirson (14) using a-peak counting. VOL. 32, NO. 8, JULY 1960
1013
right order for the LT&&conceiitrations in leached ore. But a need n-as still felt for better analytical values. y O - y ’ - y ’ ’ Method. The fact t h a t the y-y method gave T h o ? values not much different from that of the 3-y-y method, as well as reasonable values for t h e U308content, iiirlicatecl t h a t t h e analytical problem iiiiglit be solved by applying pure ganiniii ;pectroinetry only. I n the y-spectrum of uranium-thorium ores o n l ~ %:. fen- possibilities exist where speiific peak selection can be applied ,uccessfully. The selection of the gaiiiiiirl pliotopeak a t 2.62 m.e.v. of T1’0’ p r o ~ w l;atisfactory for the determinatioii of ThO, concentration, if the thorium series is in equilibrium. This qualification. however, is necessary for an>- $- or ?radiometric analysis of thorium. The selection of the 239-k.e.~-.peak due to the y-ray of Pb212 (ThBj 3 s suggestd by Hurlry (10) is not as specific as the 2.62-m.e.v. peak of T120jvhere the detection efficiency can be increased
m
E! X w
ka (3
zI-
z
2u
1
0
1
200
1
1
1
400
1
1
G A M M A ENERGY Figure 1.
l
6 0 0 k.c.v.
0 0.5
1.0 1.5
GAMMA
2 . 0 2.5 m.e.v.
ENERGY
Gamma spectra of uranium-thorium ore
u&,
0.74 7 0 1.28 % ThOz Elements underlined a r e members of the thorium series. The rest a r e members of the uranium series. leff. Intermediate energy spectrum obtained with a window width of 14 k.e.v. Righf. High energy spectrum. X1, X4 obtained with a window width of 28.5 k.e.v., X10, X20 with a width of 57 k.e.v.
Golbek, Matveev, and Shlyapnikov (8) \yere the first to apply the method to mixed ores using only Geiger counters and different absorbers. They did not mention any analysis carried out on naturally leached samples. Serdyukova and Kapitanov (20) worked out a method for the siniultaneous separate determination of uranium, thorium, radium, and potassium in acid igneous rocks. They used an a-count combined with the 0-7-7 method to obtain four equations, but their results are only for artificial mixtures. Prosperi and Sciuti (15) used a y-scintillation unit to count all the y-rays (the minimum energy is not mentioned but it can roughly be taken as 200 k.e.v.). Apart from the 0-count with a Geiger counter, a third equation
1014
ANALYTICAL CHEMISTRY
was dbtained by the integral counting of all the y-rays above 2.4 m.c.v. The analysis of only one sample of a mixed ore is girrn where the uranium-radium ratio has been disturbed. It is not clear whether the particular sample is an artificial mixture or of natural origin. 4 similar technique has been developed at this laboratory ( 3 , 4), independent of the work of Prosperi and Sciuti, with this difference, that an integral ?-count is obtained a t 100 k.e.17. and that the second y-count is obtained as a differential count of the photopeak of the 2.62 n1.e.v. y-ray of T1208. A considerable decrease (about three times) in the background is obtained compared with an integral background count a t this energy. With this technique applied to naturally leached ores results were obtained that gave the
ing of the 100-k.e.v. peak of the y-transitions present in the decay oi T11234 (UX1) and Pa234m ( T S 2 ) .Figure 2 ) . I n the y-spectrum of a uraniuni mineral sample (Figure 2. left) this 100k.e.1.. peak is included in the complex peak a t 89 k.e.v. n-hich also coiisists of a range of K-L x-rays, due t o the spread in atomic weights of the *,-emitting isotopes in the uranium series. There is not’ a strong or niediuiii intense y-transition of 186 k.e.v. present in the (UX2) decay of Th234(UXl) and Pa23am (17, 22). Nel-erthrless, as {vas originally reported by Hahn and IIeitner (9) and others ( I S ) , a gamma photopeak of this energy can be seen in the ?-;;pectrum of U308(88% purity) (Figure 2, left) as was also reported -1, Daiiion mid Feely ( 2 ) . The pure uraniuni prepared 3 years ago mainly consist. of the follon-ing isotopes (the half-life d u e s are given in parenthesei for each isotope): (4.51 X lo9 years), ‘I’h234 (24.1 days), Pa?31n1(1.175 minutes), U234 (2.48 x lo5 y a i (8.0 x lo4 years) together with 5% activity of U”3j ( i . 1 X 10’ :-ea Th231 (25.64 hours) \Tit11 Pa’:” [ lo4 years). The abundance.; o and PaZ31\\-ill be nPgligiblJ- ~ m n l l . The only other st’rong gamma emitter, n.ith Th234 and Pa234mbeing rxcluded, is L?35 which is known (18. 2 2 ) to emit a 55% 186 + 2-k.e.v. y-ray. THEORY INVOLVED I N RADIOMETRIC ANALYSIS OF ORE SAMPLES
The theory of the 8-7 or equilibrium method and the y-y method ( 4 , 24) has been extensively discussed (5. 7 , 25‘).
The P-y-y method has also been prescnted by different authors (3, 8,I S ) . For the y-y-y method as developed in this laboratory, a brief summary of the necessary formulas will be given. As in the outline of the theory previously giT-en (3) for the application of tlie 0-y-y method, the yo-y’-y” method is actually a generalization of the original work of Thommeret (23). Threc equations are iieeded, with the assumption that, if natural leaching of :i particular sample under discussion liad occurred during the previous 50 years, Th!33” and its daughter elements have b e c ~ unleached. Brooke, Picciotto, and Poulaert ( 2 ) applied a three[quation method t o artificial mixtures of urmium and thorium as an extension of tlie 7-7 method of Hurley (10). The outline, of th(?theory is not given. The three equations required are obtaincd in the usual manner for a particular sample as follon-s: The total number of integral counts, obtained above 140 k.e.v. can be cqrrsscri
279
8 9 (COMPLEX
1
(Pal2)
X-RAYS)
A I
It
ii
i
u
1
i
e-;,
=
(’;,
L-(y’“
+ P-,’~.) + T*/’t
[The nomenclature includes only the definitions of t,hose constants not given in (S)]. For t’he results in later paragraphs the discriminator was set a t 100 k.e.1.. because it as particularly desired to compare the. results obtained with those previously reported from the application of the P-y’-y” method ( 3 ) . It would, however, be desirable to utilize a slightly higher level-140 k.e.1.. -to get away from the 89-k.e.v. coniplex peak. This was actually done with the second group of results reported in Table I. The t;;tal number of differential counts, C,. obtained over the 2.62-m.e.v. photopcnk of can be written:
c;.’ =
Upy”,,
+ T’”’,
(2)
l ‘ h ~total number of differential counts, C!; obtained in the 89-k.e.v. eoniplex peak in thv y-spectrum of a uraniumthorium niineral sample can be given as : C;
=
V(r;
+ p-&j - TY:
i,
(1)
(3)
Solving Equations I! 2 , and 3 in the sanir manner as done previously (3, 4) the following formulas are obtained for per ccnt U30sand per ccnt Tho?:
\
te
*,
i
i
8
PURE URANIUM POWDER
I
1
1
1
1
1
~
100
200
3 0 0 k.e.v.
100
0
Left. Righf.
Low energy gamma spectra pure uranium powder containing only
Uranium mineral powder with no thorium present; elements of the uranium group Monazite sample (3.28% Tho? f 0.07% UaOal
where S I =
S?=
-,p, x yr: x
*,;/y,: y:/yv:
3 0 0 .c.v.
GAMMA ENERGY
GAMMA ENERGY Figure 2.
200
- -,I
Apparatus and Standardization. T h e same well stabilized gamma counting apparatus used before in the 6-y-y analysis have been used in this investigation ( 3 ) . The scintillation head and sample pan for 18 grams of material are shielded by lead blocks, similar to the Eichholz (6) arrangement. K i t h the same standards used pwviously (3) the following values of the
constants were obtained in counts per minute per 1% U308or Tho2 per 18 grams of materials: =
7:
7;
=
7;=
7.59 X 10’;
-,E
=
1.038 X l o 3
1.878 X lo4; *,pa = 4.409 X lo3; = 9.4
-,,:
8.285 X 101;
-f:
=
2.276 X 103; y: = 89.6
The values for y’”, 7 a . were obtained iyith the integral discriminator set a t 100 k.e.17. All samples that iyere analyzed have been reasonably low grade ore. Emanation leakage on uranothorianite samples could not be detected previously and sealing of the samples mas not considered necessary. Furtheimoie, the backgiound count stayed reasonably constant for weeks, irrespective of thP grade of the ore samples that were analyzed. Procedure for y0-y’-y’’ Method. C; is obtained after background (about 500 c.p.m.) and neight corrections ( 6 ) h a r e been applied t o t h e VOL. 32, NO. 8, JULY 1960
1015
actual counting rate measured with t h e integral discriminator set a t 100 k.e.v. For the second group of results in Table I, the integral discriminator was set at 140 k.e.v. A weight correction is introduced to normalize the observed counting rate to that of a sample of 18 giams. The number of counts per minute in the 2.62-m.e.v. photopeak, C"y, is averaged over 20 minutes. The background correction, 2.7 counts per minute, is made and is then followed by the weight correction. The counting rate in the 89-k.e.v. peak, C;, is obtained with the gain setting increased 8 X and the discriminator set a t 70 k.e.v. with a windo\v width of 55 k.e.v. covering the half widths of the x-ray peak and that of the 100-k.e.v. photopeak (Figure 2, left). A background correction of 85 c.p.m. is made follom-ed by the weight correction.
,
0.51
+ I ,05
0.3 .01
,02
I
I
I
0.1
0.2
0.5
1.0
Errors Associated with Radiometric Assays. I n agreement n ith Sciuti and Prosperi (18) a general figure for the gross error associated n ith radiometric analysis n as found t o be -7%. This n-as the case for mixed ore containing 0.1% each of VsOs and T h o s . Counting times of 5 minutes n e r e used, except for t h e counting of the TlZo5 decay ?-peak, n hich was usually taken for 20 minutes. T h e folloning errors are possible in radiometric analysis: STATISTICAL ERRORS.These errors can be minimized by extended counting times. This was necessary for sample A432, but it is not always possible and optimum times need to be chosen for low grade ore. ERRORSDUE TO CALIBRATIOX OF THE rlPPARATUS BY STANDARD SAhIPLCS. The chemical values of U308 samples assayed as standards and used as such have caused many difficulties in the past. I n this laboratory the uranium
f I I
.02
I
I
.os
0.1
u s 0,CONCENTRATION (AVERAGE
IN
I
I
0.2
0.5
1.0
%
CHEMICAL VALUE)
Figure 3. Comparison of results of the UaOs content obtained with different methods
ERRORSDLX TO THE n H I F T I S T H L concentration in standard samples has been measured by the Mass SpectromIMPULSE AJIPLIFICATIOX.13y doing an integral count on a strong t - 3 0 6 etry Division. ERRORS DUE TO NONHOMOGENEITP sample above 2.5 m.e.v. at a fen succesO F THE STAXDBRD AXD U S K N O W N SAXsive near-lying discriminator values, a graph of the reference against count PLES. By methanol mixing of the samdiscriminator voltage can be obtained. ples (6) to be assayed this error can be By repeating a 5-minute count on this greatly diminished. Vnfortunately, this strong reference sample mery 2 to 4 uniform mixing refers only to the hours, the drift oi the puloc aniplificasample sent for analysis and not to tion can be constantly observed. Gain the head sample. alterations in steps of 0.0125 can be made although this n a s very seldom, if ever, necessary. d fen times during this investigation small changes of 0.2 to 0.5% !!-ere required in the disTable I. Results of Different Analytical Methods criminator setting for the 2.62-i11.e.v. ;Class peak. Y'-+"' ?+lethod" Chemical Assay SpectroERRORSA4SSOCIATED J\ ITH S O R J I A L I No. Sample I I1 Lab. 1 Lab. 2 metric ZATIOX OF THE COUSTISG RATES 'ro A Per cent 'hO, REFERESCESAMPLEKEIGHT. I t was A418 0.15 0.15 0.15 0.16 unfortunately not possible to study the 0.12 0.13 0.15 0.13 r2419 method of correction for sclf-absorption 0.39 0.25 A4426 0.27 0.25 proposed by Prosperi and Pciuti (15) 0.025 0.024 A432 0,022 0.031 0.11 0,108 0.093 KCGI 0.11 0 . 13b beforehand. Average nright correc0,208 0.23b 0.20 KCGIV 0.23 0.24 tion factors have been obtained from a series of measurements of diffrrcnt types Per cent ;,OS of samples ( 5 ) . An effort should be 1 A418 0 11 0 12 0 11 0 10 made to keep the sample neight to 2 A419 0 29 0 33 0 22 0 23 3 A426 0 49 0 52 0 59 0 53 \\ithin & 207, of the normalization 4 A432 0 038 0 022(?) 0 037 weight of 18 grams. 5 KCGI 0 22 0 25 0 25b 0 23 0 23 ERRORSI X P O ~ EBY D THE BASIC 6 KCGIV 0 52 0 61 0 586 0 53 0 61 ASXMPTIONS. The exact period and Values in column I obtained with the Cc taken a t 100 k.e.v. and those in column I1 nature of natural leaching are uncertain. with C; taken at 140 k.e.v. Even the pattern of natural lcaching of Values obtained after a reassay was requested from the particular laboratory. uranium-bearing ore is not alnays the 1016
ANALYTICAL CHEMISTRY
same, as recently indicated by Rosholt (16). This implies that the main assumption of uranium and radium grouping of the uranium series can only refer to a generalization of the leaching pattern. I t cannot be excluded that in a period less than 4T years [i X 6.7 years, t 1 1 2 of RaZZg(AIsThl)] and more strictly 13 ycars [i X 1.9 ycars, t 1 1 2 of ThZzS (RdTIi) 1, leaching occurred especially in the cnbc of outcrop samples. It is possible that selective leaching of Pb210 ( R a n , tl of 22 yrars) could h a r e occurred, but that nil1 not affect the results obtained n i t h the y O-y’-y” method ducl to the lon energy or very low intensity y-rays from Pb2Io,BialO, and Po2lU. Excess Rn::? absorption due to its I I 1 I 1 0 5 cscapc from some other strong uranium 01 02 05 01 0.2 0 5 Tho, CONCENTRATION IN % deposit in nature 11ill introduce a radium (AVERAGE CHEMICAL VALUE) group e x r ~ > b .n liich will resemble a Figure 4. Comparison of results of the fictitious uranium depleted sample. ThOz content obtained with different methods
To get ‘1 better impression of the succc- of the yo-y’-y‘’ method conipared n it11 the othcr radiometric methods in its application t o leached mixed o m , the r e d t s quoted in Table I h a w been plotted in Figures 3 and 4 against the ratio of the radiometric value to the average clicmicnl value. Discussion of Results. I n spite of the small nunihcr of samples, i t is clear from Figure 3 that the best agreement hetnccn tlic rewlts obtained for the L-308 concentration n ith a radiometric method nriti that obtained n ith chemical anal! qis I> rc~aclied by applying the yo-y’-’y” metliod. A discrepancy, although elicelied repeatcdly, exists only in the uranium concentration of -4419. From the &?’-y” and yo-”-?‘’ results it secms that a Ion C; integral count is obtairictl. One can speculate on the implication. ot suc~lia discrepancy in the uraniiini 1 alucq and no virtual discrepancy in the thorium valucs. I t is possible that the disequilibrium is of type 2 , or t! pe 3, according to Rosholt’s classification, but a detailed radiochemoi tlie daughter products is actu:ill> iequircd.
ACKNOWLEDGMENT
The chemical assay values supplied by the Sational Chemical Research Laboratory and the Government Metallurgical Laboratory are gratefully acknowledged. The author is indebted to A. J. Burger of the Kational Physical Research Laboratory who did the mass spectrometric assays. LITERATURE CITED
(1) Brooke, C., Picciotto, E., Poulaert, G.. B u l l . sac. belae aeol.. waldontol. et hykril. 67,315 (1958):
/.
( 2 ) Damon, P. E., Feely, H. K., U. S. At. Energy Comm. RME-3153 (1959). (3) de Lanae, P. IT.,-4x-a~.c ~ E a r .31,
812 (1959s. (4) de Lange, P. K., J . S . Afrzcan Znst. Xznmg and M e t . 59, 641 (1959). its) A- -P L a n w . P. IT.. Trans. Geol. Soc. S Africa 599 259 (19k6). (6) Eichholz, G. G., Canadian Mines Branch, TR-107/53 (1935). ( 7 ) Eichholz, G. G., Hilborn, J. K., RIchIahon. C., Can. J . Phzls. 31, 613 \ - I
RESULTS
Tlic results ohtained for the Thos and PaOs content nith different radiometric method-via., p-y’, y’-y’’, p-?’-r”, and yo-y’-y’’ methods-are given in T‘iblcs I and 11. These samples are $0 iai tlie best examples that could bt. obtamctl locally of mixed ore that ha> tiwn drastically leached in riatur e For comparison purposes mass ay values arc given
the differential discriminator covering the 89-&e.v. complex peak. yi, yP = similar to -it but obtained for the radium group and the thorium series, respectively.
The randomness of the spread of the ratios of the T h o B concentration in Figure 4 can be due to sampling effects or due to averaging effects in weight corrections or even due to slight voltage drifts during the actual samplc counting procedure. The valucs for T h o 2 obtained with the ?’-”’, @-?’-?”, and yo-”-?” methods are virtually the same, because of t l y selectiveness of the measurement of C, for Thos. NOMENCLATURE
-E, = counting rate of sprays originating in the uranium group per 1% U& per 18 grams of material with the window of
Table II. RadiometricValuesObtained with p-7,y-y and p-7-y Methods
p-; ‘ “/I-“, pr Method Method Method 11
SamKO. ple 1
2 3 4 5 6
5
6
(4)
Per cent U I O s 0 13 0.08 0.28 0.27 4426 0.61 0.45 A432 0.023 0.015 KCGI 0.28 0.22 KCGIV 0 . 5 i 0.48
1 A418 2 A419
3 4
(4)
Per cent Tho? A418 0 025 0 15 A419 negative 0 14 A426 negative 0 25 A432 0 005 0 026 KCGI negative 0 092 KCGIV 0 009 0 22
(3) 0 0 0 0
15
14 25
024 0 093 0 23 0.16 0.35 0.iO
0 028
0.32 0.62
249 (1949). (24) Khitham, I
