ANALYTICAL CHEMISTRY
1884 The indicated precision is & l o % for the vanadium and +2.3% for the nickel. ACKNOWLEDGMENT
The authors express their thanks to Ralph GrifRth and E. I. Bradshaw for their aid and suggestions in the preparation of this paper and to ?IT. J. Schlesser for sample preparations. LITERATURE CITED
pp. 363-75, Interscience, Sew York. 1950. W.T., “X-Rays in Practice,” pp. 668--71, XfcGraxHill, New York, 1946. (12) Wrightson, F. 31.: ANAL. CHEM., 21, 1543-5 (1949). (11) Sproull,
(1) Birks, L. S.,Brooks, E. J.. and Friedman, H.. ANAL.CHEM..25, 692-7 (1953). (2) Davis, E. N., and Van Kordstrand, R. d., Ibid., 26, 973-7 (1954) . (2) Dsroff, G. V., Hansen, J., and Hodgkins, C. R., Ibid., 25, 1898905
(4) Drroff, G. V., and Skiba. P., Ibid., 26, 1774-8 (1954). ( 5 ) Friedman, H., and Birks, L. S..Rev. Sci. Insfr., 19, 323-30 (1 948). (6) Gamble. L. W., and Jones, I\-. H., ASAL. C H E h r . . in press. (7) Glass, J. R., Kirchner, J . P.. llilner, 0. I., and Yurick, -1.S . , Ax.4~.CHEM.,24, 1728-32 (1952). (8) Karchmer, J. H., and Gunn, E. L., Ibid., 24, 1733-41 (1952). (9) Kev. C. W.. and Honean. -- G. D.. Ibid.. 25. 1Gi3-6 (1953). , , (10) Sandell, E. B., “Colorimetric Dktermiiiatkii of Trace Metals,”
(1953).
RECEIVED for review Yay 26. 1955. Accepted August 22, 1955. Presented before the American Petroleum Institute, 8t. Louis, Mo.. 1955.
Combined Radiometric and Fluorescent X-Ray Spectrographic Method of Analyzing for Uranium and Thorium WILLIAM J. CAMPBELL and HOWARD F. CARL Eastern Experiment Station,
U. S. Bureau o f Mines,
College Park,
Employing radioactivity measurements and fluorescent x-ray spectroscopy, a rapid method of analyzing ores for uranium and thorium has been developed. The total radioactivity of uranium and thorium is determined and expressed as per cent equivalent uranium. From the line intensity ratio ULalThLcu measured by fluorescent x-ray spectroscopy the weight ratio of uranium to thorium is calculated. As the relative radioactivity of uranium to thorium is Imown, the weight per cent of uranium and thorium can be calculated. The time required is approximately 20 minutes per analysis for both elements. The accuracy is +lo% of the amount present in samples containing more than 0.5% of the elements and the lower limit of detection is 0.01 to 0.03% of either element.
A
Md.
The sample is positioned in a holder at a constant distance from the Geiger tube window. The radioactivity is counted for a fixed time, usually 8 minutes, and the intensity is recorded as counts per minute above background. These values are compared directly with calibration curves prepared from radioactivity standards. A series of calibration curves is prepared for the various distances from the sample to the detector window. By choosing a suitable distance, it is possible to remain within the linear response range of the Geiger tube. Regardless of the uranium-thorium ratio, the radioactivity is expressed as “per cent equivalent uranium,” that amount of uranium in pitchblende in radioactive equilibrium necessary to give an equal activity. DETERMINATIOY OF THORIUM-URANIUM RATIO
After the radioactivity measurements are completed, the samples are placed in the fluorescent x-ray spectrograph to determine the thorium-uranium ratio. These elements have atomic numbers 90 and 92, so that a s a first approximation the line-intensity ratio of ThLa to U L a equals the weight-per cent
S A public service, Ihe Bureau of Mines, College Park, Md.,
examines a large number of mineralogical samples submitted for identification and for determining possible commercial value. This service includes approximately 2000 tests for radioactivity per year. Samples with measurable activity are studied further to determine the approximate concentration of uranium and thorium. For various reasons, principally the time and cost of analyses, chemical methods are not satisfactory. Optical spectroscopic techniques have only limited application. Preliminary investigations indicated that a combined technique for determining total radioactivity, with a subsequent uranium--thorium ratio measurement by fluorescent x-ray spectrography, offered excellent possibilities.
ThL9,
,
XENOTIME
l 1
U.125 WT Yo Th = 2 75 W T Yo
~
NaCI 24.564 A 5Ohv.llm0
1
OUARTZ
26 = 3 6 4 P 50 k v . 4 5 m o
1 ;
,
u Ld,aZ ’
,
I f
‘
, ’
,
8
u LU,
’,
~
,
IIVSTRUMENTATION
Radioactivity is measured with standard commercial Geigercounter equipment, with provisions for accurately positioning powdered samples. The x-ray spectrograph used is a modified Sorelco 90’ spectrometer employing commercially available parts. Any one of the commercial units of the North American Philips Co., the General Electric Co.. X-Rav Deuartment. or the hudied Research Laboratories, should be saiisfactory. ’ * A
RADIOACTIVITY MEASUREMENTS
The samples, ground to -325 mesh, are placed in suitable holders and packed to a constant level with the aid of a spatula.
I
1
’
uhap
4 -4
1’6
18
9
2b -22 1 DEGREES
Figure 1.
k
I
’v,d
A 29 30 31
28
, $$
i*vp,v~,ic.‘. 32
>:
28
Uranium La and thorium La spectra from xenotime
Showing relationship between line intensities and concentrations of respective elements
V O L U M E 27, NO. 1 2 , D E C E M B E R 1 9 5 5
1885
ratio of thoiiuni to uranium, as shown in Figure 1. Aimore exact value is obtained from the folloxing relationship: T h L a l U L a = C (\vt. % Th/wt.
% U)
(1)
where C is deterniined for a particular set of inrtrumental arid operating conditions from samples of knoxvn thorium and uranium content. Detail.. in instrumentation and sample preparation have been dewibed in the literature (2-4 1. CALCULATION OF URANIUM AND THORIUM CONTEhT
The uranium and thorium content, is cnlculnted from the following eqiiations.
z + 21 k
=
% equiv. uranium
x = w t . yo uranium !/ = wt,. 70thorinm/wt. "/a uranium X: = relative radioactivity of uranium to thorium
(There may be some variation in this constant between minerals. For this n-ork and the equipment employed, a value of 5.0 was ii.qed.) Equation 2 c:in lic solved f o i z,as the teinis 21 niid percentage of eqiiivnlent ur:iniuni h a w been determined. The corresponding thorium conteiit is o1)taiiled from Equation 3. =
li
Interfering Emission Lines and Absorption Edges
Comparison lines T h L a r = 12.966 k.e.v. U L a I = 13.613 k.e.1,.
Comparison edges ThLIII = 16.296 k.e.v. U L n r = 17 161 k.e.r.
Emission Lines
Absorption Edge Br K I r Lr Pt L r r Pb Lrir I3i L ~ r i
K.E.V. 17 011 l6.73G
Y Kli?
Y KB. S b Kai S b Ka2 P a LO, U LE?
16.614 16,520
16.700 16.425
K.E.V. I ? 47.5 13,413 13.2G8
13.044 13,424
(2)
where
JTt. cothorium
Table 11.
( % equiv. U - n?. yo GI
(3)
but this iiiaj not be true for chemical separation products. An alternative method has been suggested by Frank Grimaldi of the U. S. Geological Survey in which the uranium content is rapidly determined fluorimetrically (8). Radioactive equilibrium is not nece.ssary TI hen using this combined chemical and x-my spectiographic procedure.
APATITE
CARNOTITE
U s 0 0 3 WT % Th.017 W T %
uno14
w-
7-
ULU,
I QUARTZ 2 d * 364 A
.4CCURACY ASD SENSITIVITY
To check this nizthod n number of chemically analyzed samples run as unlrl:on.ns; the results are shown in Table I. They indicate the method is accurate to & l o % of the amount present escept for samples Containing leps than about 0.5% uranium or thorium. The technique is independent of the mineral type or nintris of the snniple. .Is shown in Figure 2, the lower limit of
ThLUl
rrerc
5 0 k v , 45 ma MO TARGET X-RAY TLBE
Table I. Comparison of Uranium and Thorium Analyses hp Chemical versus Physical Methods 5 Sample IIonazite 1 2 3
1 ti
Seniitime Aescliynite Thorianite 1
2
Pbosphate rock Pitchblende Carnotite 1 2
Thorite Apatite
Uranium
l:
