DETERMINING AND

ITS

AND

ROCKS

THE MANNER

CONCENTRATION OF

OF

DISTRIBUTION

IN

URANIUM MINERALS

I.G. Berzina, I.B. Berman, M.Yu. Gurvich, G.N. Flerov, and Yu.S. Shimelevich

UDC 546.791:539.173+539.173.7

The concentration of uranium, determined on the basis of chemical, radiographic or luminescence methods, is c h a r a c t e r i z e d in t e r m s of the average amount of this element per unit a r e a or volume f o r the m a t e r i a l under study [1]. In this paper, we describe a method for determining the m a n n e r of distribution of uranium in natural m i n e r a l s and rocks, as well as its concentration. The method is as follows: A nuclear particle which is released spontaneously or under bombarcknent leaves a defect area (or track) in the surrounding material which may be detected using selective chemical etching. The disturbed area is made apparent by the etching, so that it indicates the track of the nuclear particle undermicroscopic examination. It is then possible to differentiate between areas with different concentrations of uranium, thorium, and other radioactive elements by the magnitudes of the tracks; that is, in tens of microns. The method permits relative determinations of the concentrations of uranium and thorium in heavy mineral inclusions, as well as the "fabric" of concentration. Determining Tracks

the

Concentration

of Nuclear

Particles

of U r a n i u m Spontaneously

from

the

Emitted

in Micas

It has been shown [2] that with a t r a c k density Pl, the fundamental contribution to t r a c k s for particles spontaneously emitted by heavy elements in the m i n e r a l s is f r o m U238. The value for Pl is determined from the e x p r e s s i o n

9t = CNI23S~Ti~e, [,9, CI'EI"

250---

fSO

(

/--

100

5o/ O

]

I 40

85

I f20

fgOm, ~tg/cm 2

Fig. 1. Relationship of the density of tracks from emitted p a r t[eles to the m a s s of uranium on the target.

(1)

where CN is the number of atoms of uranium p e r cc of material, 1238 is the isotopic fraction of U238, k is the e m i s s i o n constant for U238, T is the time during which the m i n e r a l has existed, R describes the efficiency of t r a c k generation by the emitted p a r t i c l e s , and e d e s cribes the effectiveness in developing the t r a c k s caused by an emitted particle. Equation (1) is applicable in those cases in which the mine r a l s under study have not been subjected to t h e r m a l m e t a m o r p h i s m (inasmuch as t h e r m a l effects may d e s t r o y the t r a c k s left by emitted particles) and in which no m i g r a t i o n of uranium has taken p l a c e . On the basis of Eq. (1), we m a y write a f o r m u l a for d e t e r m i n ing the concentration: C=

~'

= A p'

~'2)

where A is a p a r a m e t e r , constant for a specific mineral. Thus, for determining the concentration of uranium in a sample, it is n e c e s s a r y to determine the density of t r a c k s left by spontaneously emitted p a r ticles. T o d o t h i s , the m i n e r a l under study is p r e p a r e d as a thin s e c tion and etched with a chemical reagent, which develops the surface

T r a n s l a t e d from Atomnaya l~nergiya, Vol. 23, No.6, pp.520-527, December, 1967. Original article submitted March 31, 1967.

1288

Fig. 2

Fig. 3

Fig. 2. Tracks in muscovite irradiated by particles emitted from a uranium target with a mass of 30 ~g/cm 2 (n = 2"10II neutrons/cm2)~ 1) tracks from particles emitted spontaneously by uranium; 2) tracks from particles emitted by uranium under bombardment. Fig. 3. Tracks on glass irradiated by neutrons in contact with the sample under study (n = 2-i012 neutrons/cm2): treatment: I-IF 2.5%, t= 20~ time = 20 rain.

e x p r e s s i o n of the t r a c k s caused by the nuclear p a r t i c l e s . The form and appearance of the developed t r a c k s f r o m the n u c l e a r p a r t i c l e s allows the formation of a judgment concerning the p r e s e n c e or absence of the effects of t h e r m a l m e t a m o r p h i s m [3, 4] and t h e r e f o r e , the applicability of the method. In the absence of t h e r m a l phenomena, the e r r o r in determining the concentration depends on the e r r o r made in determining the age, which is found by some independent method, and m e a s u r e m e n t statistics. if the age of the m i n e r a l is known, the method described here allows the determination of the concentration of uranium in a sample with great sensitivity. This method m a y be used in studying the distribution of uranium in m i n e r a l s . The data which are obtained m a y be used in the solution of a number of important p r o b l e m s , as for example, questions concerning the nature of o c c u r r e n c e of uranium in m i n e r a l s , the m i g r a t i o n of uranium f r o m surrounding rock, and others. The advantage of the method described consists of the possibility of determining the concentration of uranium and its distribution in m i c r o s c o p i c quantities of the m a t e r i a l under study; m o r e o v e r , the m e thod is c h a r a c t e r i z e d by high sensitivity and specificity. However, the number of m i n e r a l s for which the t r a c k s left by emitted p a r t i c l e s can be etched s a t i s f a c t o r i l y is limited. N e c e s s a r y conditions for the development of s a t i s f a c t o r y t r a c k s include the ability to p r e p a r e the m i n e r a l as a thin section and the absence of a significant number of defects of other types which would interfere with recognition of t r a c k s on the surface under study. _Determining Tracks

of

the Particles

Concentration

of

Emitted

under

Uranium

in

Minerals

from

Bombardment

The basis for this method in its general form has been described in reference [5], which suggested that the concentration of uranium in a mineral could be determined from the density P2 of tracks from nuclear particles emitted under bombardment, using the formula:

(3)

92 -= C N I 2 3 ~ ( ~ n R e ,

where 1235 is the isotopic fraction of U 235, (9" is the capture cross n is the integral flux of thermal neutrons.

section of U 235 for thermal

neutrons

and

1289

TABLE 1. The Concentration of Uranium in Micas, Determined from Spontaneous and Induced Radioactivity of the Uranium Atoms Sample number t7 21 32 84 t5 10 t3

Concentration, • ~0"iI atoms Catom

Type of mica spontaneous induced radioactiv- radioactivity i t y . M~scovite Muscovite Muscovite Muscovite Muscovite Phlogopite Vermiculite

6 40 2

19 36 36

7 36 2

45 36 41

Fig.4. Tracks on a Lawson detector, irradiated by particles emitted from a uranium target of mass 30gg/cm 2 (n = 6"101~ neutrons/cm 2) : treatment: KOH 40%, t = 60~ time = 30 mh~,

F o r the determination of the concentration of uranium, e x p r e s s e d in g/g, with consideration of the s p e c t r u m for neutrons emitted by cadmium with energies in the range E 0 to El, Eq. (3) m a y be written in the f o r m : E l

92 = C No I23~d ~- e ~ n (E) (~(E) dE, t~u 2 -d

(4)

E0

where N o is A v o g a d r o ' s number, a (E) is the differential capture c r o s s section, n(E) is the integral neutron flux,/~U is the m o l e c u l a r weight of u r a n i u m , a n d d is the density of the m a t e r i a l . The concentration of uranium, determined on the basis of Eq. (4), m a y be e x p r e s s e d by the relation C = B ~z

02

,

(5)

I "(E) ~ (6) dE Eo

where B is a p a r a m e t e r which is constant for a specific c r y s t a l , and which depends on the p r o p e r t i e s of the c r y s t a l . The value of the integral which enters into Eq. (5) is determined by the following method. Uranium targets with various m a s s e s , along with detectors immediately adjacent to the t a r g e t s , a r e placed in the collimated beam f r o m the r e a c t o r along with the sample under study during irradiation, and the t r a c k s f r o m the t a r g e t s are r e c o r d e d [6]. The detector is selected so that the particles emitted will cause defects in its s t r u c t u r e which m a y be developed as t r a c k s on selective etching. After irradiation, the detector is separated from the target, and is analyzed by etching to develop the t r a c k s caused by the emitted p a r t i c l e s . As a r e s u l t of the irradiation, the p a r t i c l e s emitted by the uranium in the t a r g e t will be r e g i s t e r e d by the detector. The density, Pt, of t r a c k s on the detector will be given by the e x p r e s s i o n El

o~9 - m--L ,ui :V0/2~el ~,) a (E) n (E) dE,

(6)

E0

where m i is the mass of uranium in the target in g/cm 2, and c i is the efficiency of the detector in recording tracks from emitted particles. With the solution of Eqs. (4) and (6) for C, we find

(] ~ =

V2~1

, R

Pl

kt~2el ,r

Pl

'

(7)

lit i

The value for m i / P i is found f r o m a graph for the relationship of the density of t r a c k s caused by emitted particles on the detector, Pi, to the m a s s of the deposited l a y e r of uranium on the target, m i (see Fig. 1).

1290

iLY

I\

C'

c'

_.

C"

a

~-

c

I\

C'

c'

'

b

c"

c"-'

F i g . 5. P o s s i b l e d i s t r i b u t i o n s f o r u r a n i u m c o n t e n t s : a) removal of uranium from minerals; b) introduction of uraninto into minerals; i) spontaneous emission; 2) induced emission; C) average value for distributed uranium; C') average value for the concentration of uranium in inclusions.

It is desirable that there be no uranium impurities in the best of detectors, inasmuch as the disintegration of uranium in the detector would lead to a significant error in counting the tracks caused by particles emitted by uranium in the material. A Lawson detector works well, inasmuch as contamination with heavy elements is excluded in the preparation of such detectors. If mica or glass is used in the detector, the concentration of uranium in these materials must be determined first. Figure 2 shows a photomicrograph demonstrating the dual development of tracks in mica before and after irradiation in contact with a uranium target. The coarser tracks are tracks developed by spontaneously emitted particles, while the finer tracks are those from particles emitted by the uranium target under bombardment. The difference in appearance of the tracks is explained by the dual nature of the tracks on the surface after irradiation. Figs. 3 and 4 show photomicrographs of tracks developed on the surfaces of glass and a Lawson detector. The value of ~, taken from the specific results given in references optical glass, 76% for a Lawson detector and 100% for muscovite [2].

[7, 8], is accepted as 42% for

Thus, the concentration of uranium in a sample is a function only of the density of tracks from particles released under irradiation, as recorded by the detector, and the density of tracks from particles released under irradiation, considered directly at the sample. The method of determining the concentration of uraninm from particles released under irradiation is as follows. The sample is prepared by chemical etching to develop the tracks caused by particles emitted spontaneously by uranium. Then, a freshly cleaned sample is formed into a packet with a Lawson detector, and irradiated with a collimated beam of neutrons (thermal) from a reactor. After irradiation, the sample is etched a second time, permitting the recognition of tracks developed on the surface by particles released by the uranium under bombardment. The tracks which appear after the second etching as the result of particles released under irradiation differ in appearance markedly from the tracks caused by spontaneous emission, primarily in that they cover less area. Depending on the manner in which the etching was done, the characteristic features of the etched patterns depend on the duration of the etching [9]. E x p e r i m e n t a l d a t a o n t h e d e t e r m i n a t i o n of the c o n c e n t r a t i o n of u r a n i u m in m i c a u s i n g the d e n s i t y of t r a c k s f r o m s p o n t a n e o u s and i n d u c e d r a d i a t i o n a r e l i s t e d in T a b l e 1. I t m a y b e s e e n f r o m the t a b l e t h a t the v a l u e s f o r t h e c o n c e n t r a t i o n of u r a n i u m o b t a i n e d w i t h the t-vr methods correspond.

129[

Fig. 6

Fig. 7

Fig. 6. Concentration of tracks from induced emission from uranium on the surface of a biotite flake: treatment: HF 13~, t=20~ time= 10 rain. Fig. 7. Concentration of tracks from induced emission from uranium in muscovite: treatment: HF 54%, t = 20~ time 3 hours.

Sample 15 is an exception, in that the concentration of uranium determined from the tracks from spontaneously emitted particles is less then the concentration obtained from the induced radioactivity by a factor of 2.5. This discrepancy is apparently explained by the fact that this particular sample of muscovite had been subjected to thermal metamorphism, as indicated by evidence indicating alteration of the etched tracks [3, I0]. We will now consider the interpretation of data obtained with both techniques. Most important is the possibility that the migration of uranium since the origin of the mineral might be detected. Tracks from particles emitted under bombardment indicate the present concentration of the uranium, while tracks from spontaneous emission indicate the presence of uranium over the entire time from the origin of the mineral. The appearance of tracks after irradiation in areas where there were no tracks from spontaneous radioactivity is evidence for the migration of uranium (see Fig. 5a). On the other hand, frequently it is possible to find uranium only in the nature of local concentrations (see Fig. 6). In this case, one may assume the introduction of uranium into the mineral after its formation, since there is no non-uniformity of the density of tracks developed in the sample from spontaneous emission. The curves for the distribution of uranium in the sample differ somewhat before and after irradiation. There is one maximum on the curve for the distribution of uranium based on spontaneous emissions, corresponding to the uniform dispersion of uranium through the mica (see curve 1 in Fig, 5b). The same maximum is apparent when the curve is compiled from tracks developed by irradiation. However, there are other maximums on this curve which correspond to uranium concentrations (see curve 2, Fig. 5b). In some cases, it }s possible to establish the age of such uraniferous inclusions. For example, in one sample, the muscovite had uranium concentrations amounting to C = 10-2%, for which we determined an age using the track method (2, 4) of 20 million years. On the basis of the analysis of a great number of samples of mica, it is possible to conclude that migration of uranium is most common in micas from granites. In these micas, uranium concentrations may be found as minerals or inclusions having specific geometric forms (Fig. 7). We may suppose that the formation of these minerals is related to the flow of hydrothermal solutions and the subsequent deposition of uranium in the mica. Frequently, the increased concentrations of uranium are found along cracks. A photomicrograph of etched mica with such cracks is shown in Fig~8 as an example. The absence of tracks from spontaneous emissions along the cracks is evidence here for the deposition of the uranium at a much later time than that at which the mica was formed. In some crystals, such as NaCl, KCI, CaCO3, and others, other types of defects are observed along with the tracks caused by the emitted particles, which have an appearance similar to that of the true tracks after treatment with the chemical reagents. Some particular experiments [11] have indicated the possibility of identifying tracks from emitted particles and dislocations in ionic crystals. In cases where the density of dislocations does not exceed 10G/cm2, it is possible to determine the density of tracks from particles emitted under bombardment, using a method of two-stage etching of the crystals, similar to the technique described above. Results of such measurements, obtained for ionic crystals are given in Table 2.

1292

F i g . 8. M o v e m e n t of u r a n i u m along n a t u r a l m a c r o s c o p i c f r a c t u r e s in m i c a : a) b i o t i t e No. 79; b) b i o t i t e No. 18: t r e a t m e n t , HF = 13%: t = 20~ t i m e = 10 rain.

TABLE 2. D e t e r m i n a t i o n of the Conc e n t r a t i o n of U r a n i u m in Ionic C r y s t a l s f r o m B o m b a r d m e n t of the U r a n into

30

o

C~ystaI

c,

•

atoms/atom

to

k

z

8,8 i2,2 7,6

NaCl KC! CaCO3

104

105

logp

F i g . 9. D i s t r i b u t i o n of the d e n s i t y of t r a c k s f r o m p a r t i c l e s e m i t t e d into a Lawson d e t e c t o r , f o r a p r e s s e d s a m p l e of g r a n i t e .

The efficiency of developing tracks, created by particles emitted in ionic crystals, by the means described in reference [12].

is determined

The error in determining the concentration of uranium by this technique with the method for determining neutron flux described above depends on the statistics of measurement and may amount to 15-50% (depending on the distribution of the uranium). The possible recognition of effects of uranium concentrations and the migration of uranium the use of this method in the solution of important geologic problems, such as the differentiation mary uranium from that deposited as a consequence of hydrothermal processes and hypergenetic the possibility of recognizing mechanisms leading to the differentiation of uranium in the earth's its transport into the upper reaches of the crust, and so on.

permits of priprocesses, crust and

The advantages of this method are the same as those enumerated for the method of determining uraninr~ concentration from spontaneous emissions, but it should be noted in addition that the determination of uranium content with this method is not subject to errors caused by the effects of thermal metamorphism after formation of the mineral. This allows the satisfactory determination of the fabric of the concentration, as well as the concentration of uranium in different inclusions. A disadvantage of the method, as also in the first case, is the limited range of minerals which may be subjected to chemical etching. Determination Rocks,

of the

Minerals

and

Particles

from

Selective

Detectors

method

Induced

Concentration Materials Emission

of from from

Uranium Tracks Uranium

in Various Caused

by

Using

The basis for the possible existence of such a method has been shown in references [5, 13]. This consists of the registration with special detectors of the effects of induced emissions which are

1293

TABLE 3. Concentration of Uranium in Various M a t e r ials, Determined Using Lawson Detectors

Cx'IO_6

Material

C~

Material

weight % NaCI (natural) NaC1 (artificial) KC1 (natural) KC1 (artificial) CaCO3 (coarse grained) LiF (artificial)

x 10-4 weight %

Quartz sand

0.2

Granite

1.8

Clay tectolite

1.6

Limestone

1.4

Light-colored eassiterite

3.8

Dark-colored cassiterite

12.9

8.0 0.6 5.2 0.6 1.0 4.0

generated in the surficial l a y e r s of the m a t e r i a l under study as the r e s u l t of the action of a neutron flux. Use of the method we have developed p e r m i t s determination of the concentration of uranium in thin sections, single c r y s t a l s , specific rock fragments, liquids and other m a t e r i a l s . At the p r e s e n t time, this method is the only method which p e r m i t s the determination of uranium concentration over a b r o a d range of concentrations in a v a r i e t y of m a t e r i a l s . In evaluating the concentration, we use the f o r m u l a C=

p~u d

n (8).

(8) de

Eo

where p is the density of t r a c k s f r o m particles emitted under b o m b a r d e m e n t in the detector. The integral

In

(E) Cr (E)

dE

is evaluated by the method described e a r l i e r , using uraniferous t a r g e t s of various m a s s e s .

Eo

The resulting expression for determining the concentration of uranium has the f o r m m~

C =: k p - ~ ,

(9)

where C is the concentration of uranium in g/g, k is a p a r a m e t e r with a value c h a r a c t e r i s t i c of the m a t e r ial under study, p is the density of t r a c k s in the detector, r e m o v e d f r o m the sample, Pi is the density of t r a c k s in the detector, removed f r o m the target, and m i is the m a s s of uranium deposited on the t a r g e t in g/cm 2. The experimental method is as follows. A Lawson detector is applied to the sample as a coating. Then the material under study is subjected to a thermal neutron flux from a reactor. After irradiation, the Lawson detector is removed from the sample and prepared. In cases where particles have been emitted from irradiated uranium in the surface layers of the sample, the emitted particles leave tracks in the Lawson detector which may be developed by chemical etching. There is a unique relationship between the number of particles emitted in the material and the number of tracks in the Lawson detector, with the efficiency of the detector being taken into consideration. For a sufficiently high uranium concentration (10-4%) and neutron flux (10i5 to 10i6 neutrons/cm2), it is possible to evaluate the distribution of uranium on a m a c r o scopic scale with the Lawson detector. In cases where the preparation of the coatings is difficult or considered to be unsatisfactory, the sample under study is ground uniformly, so that it may be sieved without a residue. The prepared sample is pressed in a special mold in the form of a tablet with a diameter of 6 mm and a height of 3 ram. One side of the tablet is attached to Plexiglas, while the other side is coated with a Lawson detector. This

1294

Fig. I0 Fig. i0. Prepared Lawson of a granite sample.

Fig. II detector showing the distribution of uranium

on the surface

Fig. ii. A treated film of a Lawson detector (b) from a grain of cassiterite (a); the darkened areas on the Lawson detector correspond to higher track densities caused by emitted particles (• 3).

p a c k e t is i n s e r t e d in the r e a c t o r for i r r a d i a t i o n with t h e r m a l n e u t r o n s . Then the Lawson d e t e c t o r is s t r i p ped f r o m the p a c k e t and p r e p a r e d . The Lawson d e t e c t o r p r e p a r e d in this m a n n e r is e x a m i n e d u n d e r the microscope. The d e n s i t y of t r a c k s , p , on the d e t e c t o r , n e e d e d in the e v a l u a t i o n of Eq. (9), i s d e t e r m i n e d as f o l lows. One e v a l u a t e s the d e n s i t y of t r a c k s on the d e t e c t o r f r o m e m i t t e d p a r t i c l e s and d e t e r m i n e s s o m e " e f f e c t i v e " a r e a f o r the total s u r f a c e of the s a m p l e , which is a s e c o n d a r y c h a r a c t e r i s t i c of the d i s t r i b u tion of u r a n i u m for the m a t e r i a l under study. Then, one would c o n s t r u c t a c u r v e f o r the d i s t r i b u t i o n of t r a c k s in the s a m p l e for the " e f f e c t i v e " a r e a so d e t e r m i n e d . F i g u r e 9 shows an e x a m p l e of a curve f o r the d i s t r i b u t i o n of v a l u e s for t r a c k d e n s i t y , r e c o r d e d on a Lawson d e t e c t o r with an i r r a d i a t e d s a m p l e of g r a n i t e . The f i r s t m a x i m u m c o r r e s p o n d s to a u n i f o r m d i s t r i b u t i o n of u r a n i u m in the g r a n i t e , while the s e c o n d m a x i m u m c o r r e s p o n d s to the u r a n i u m in i n c l u s i o n s , with the p r e p a r e d Lawson d e t e c t o r being shown in F i g . 10. C u r v e s f o r other m a t e r i a l s with u r a n i u m in i n c l u s i o n s show a s i m i l a r f o r m . In all c a s e s , the value f o r Puniform c o r r e s p o n d i n g to the f i r s t m a x i m u m on the d i s t r i b u t i o n c u r v e was taken for the e v a l u a tion of the u n i f o r m d i s t r i b u t i o n of u r a n i u m . V a l u e s f o r the u n i f o r m d i s t r i b u t i o n of u r a n i u m for s e v e r a l m i n e r a l s and r o c k s which w e r e obtained with the m e t h o d d e s c r i b e d h e r e a r e l i s t e d in Table 3. The c o n c e n t r a t i o n s of u r a n i u m in g r a n i t e s , l i m e s t o n e s , and c l a y s w e r e d e t e r m i n e d with p r e s s e d s a m p l e s . C o m p a r i s o n of r e s u l t s obtained with p r e s s e d s a m p l e s and with thin s e c t i o n s showed e x c e l l e n t a g r e e m e n t . The v a l u e s f o r the u n i f o r m c o n c e n t r a t i o n of u r a n i u m in c a s s i t e r l t e [14] w e r e obtained with thin s e c t i o n s . C o n c e n t r a t i o n s of u r a n i u m in a r t i f i c i a l ionic c r y s t a l s w e r e d e t e r m i n e d d i r e c t l y on the c r y s t a l s using a chemical etching p r o c e s s . The e r r o r in d e t e r m i n i n g the c o n c e n t r a t i o n of u r a n i u m by the m e t h o d d e s c r i b e d h e r e c o n s i s t s of e r r o r s m a d e in e v a l u a t i n g the d e n s i t y of t r a c k s c a u s e d by the e m i t t e d p a r t i c l e s , in m e a s u r i n g the e f f i c i e n cy of a p p e a r a n c e of the t r a c k s f r o m the e m i t t e d p a r t i c l e s , and in the a c c u r a c y in d e t e r m i n i n g the m a s s on the u r a n i u m b e a r i n g t a r g e t s . Depending on the type of s a m p l e s a n a l y z e d , t h e s e e r r o r s m a y amount to 15 to 50 %. The d e s i r e d s e n s i t i v i t y of the m e t h o d is 10 -8 weight p e r c e n t f o r an i n t e g r a l t h e r m a l n e u t r o n flux of 10 t6 n e u t r o n s / c m 2. P r a c t i c a l r e c o m m e n d a t i o n s f o r the u s e of v a r i o u s d e t e c t o r s , the n e c e s s a r y n e u t r o n flux l e v e l s and the u r a n i n m - b e a r i n g t a r g e t s f o r m e a s u r i n g the n e u t r o n flux in r e l a t i o n to the s u p p o s e d c o n c e n t r a t i o n s of u r a n i u m to be m e a s u r e d a r e l i s t e d in T a b l e 4.

1295

TABLE 4. Radiation Sources, Detectors, Integral Neutron Fluxes and Uranium Target Masses for Determining the Neutron Flux for Various Values of the Concentration of Uranium in the Sample under Study Supposed concentration of uranium in weight % t0 1 10-r iO-Z

Radiation source

Integral neutron flux, n e u t r o n s /cm2

1

I Ampule source, neutron I generator lSame ]Neutron generator, re[actor with moderate ]power ,Reactor with moderate

~0TM

10n t0 n tOr~

power iO-s

t0-~ t0-5 lO-6_lO-a

S~me ,,

1013 101~ 101~ 1016

Recommended detector Glass, Lawson detector, mica Same Lawson detector, mica Same '9

Lawson detector Same

Mass of uranium on the ~arget, Pg/cm ~ natural mixture U235 of uranium isotopes to--100 0,t~1 t0-2_t0--1 10-3_10-~

10-4_t0-8 t0-5_10-4 t0-6_t0-5

tO--iO0 .10--10o [--iO

O,t--t

T h e high s e n s i t i v i t y in d e t e r m i n i n g u r a n i u m c o n c e n t r a t i o n s a l l o w s the u s e of t h i s m e t h o d in i n v e s t i g a t i n g the d i s t r i b u t i o n of i m p u r i t i e s in s e m i c o n d u c t o r s , m e t a l s , s u r f a c e c o a t i n g s , b i o l o g i c a l m a t e r i a l s , v a r i o u s l i q u i d s (oil, w a t e r ) and s o on. F o r e x a m p l e , the m e t h o d d e s c r i b e d h e r e h a s b e e n u s e d in s t u d y i n g the d i s t r i b u t i o n of u r a n i u m in c a s s i t e r i t e (SnO2). A t y p i c a l p i c t u r e of the t r a c k s d e v e l o p e d on a L a w s o n d e t e c t o r b y p a r t i c l e s e m i t t e d f r o m u r a n i u m in c a s s i t e r i t e on i r r a d i a t i o n is shown in F i g . 11. I n a s m u c h a s the m e t h o d d e s c r i b e d h e r e p e r m i t s the i d e n t i f i c a t i o n of c o n c e n t r a t i o n s of u r a n i u m in m a t e r i a l s on t h e b a s i s of the e f f e c t s of p a r t i c l e s e m i t t e d to a d e t e c t o r , it m i g h t b e t e r m e d a n e m i t t e d p a r ticle radiography method. I t is known t h a t the n u c l e u s of t h o r i u m is s u b j e c t to d i s i n t e g r a t i o n a s the r e s u l t of i n t e r a c t i o n w i t h a f a s t n e u t r o n flux. T h i s p r o p e r t y a l l o w s the d e t e r m i n a t i o n of the t h o r i u m c o n c e n t r a t i o n in v a r i o u s m a t e r i a l s a l o n g w i t h the d e t e r m i n a t i o n of u r a n i u m , if t h e s a m p l e is i r r a d i a t e d w i t h a dual e n e r g y n e u t r o n flux: i n s i d e a c a d m i u m c o n t a i n e r to p r o v i d e t h e r m a l n e u t r o n s , and t h e n o u t s i d e it. E v a l u a t i n g the d e n s i t y of t r a c k s d e v e l o p e d b y e m i t t e d p a r t i c l e s in b o t h c a s e s , and knowing the i s o t o p i c r a t i o of U 235 to U 238, one m a y e a s i l y c a l c u l a t e the c o n c e n t r a t i o n of t h o r i u m in the s a m p l e . (The d e t e r m i n a t i o n of t h e t h o r i u m c o n c e n t r a t i o n a n d the n a t u r e of i t s d i s t r i b u t i o n in s a m p l e s w i l l be the s u b j e c t of a s e p a r a t e p a p e r . ) A t the p r e s e n t t i m e , m e t h o d s of i d e n t i f y i n g c h a r g e d p a r t i c l e s a r e b e i n g i n v e s t i g a t e d i n t e n s i v e l y , a n d t h e r e is a r e a s o n a b l e p r o b a b i l i t y t h a t the d i s t r i b u t i o n n o t only of u r a n i u m b u t a l s o of o t h e r e l e m e n t s in the s a m p l e s m a y b e s t u d i e d u s i n g t h e r e a c t i o n (n, a ) a s w e l l a s the r e a c t i o n (n,f). T h e a u t h o r s e x p r e s s t h e i r t h a n k s to V. P . P e r e l i g i n f o r the u s e of e x p e r i m e n t a l r e s u l t s f r o m h i s d i s s e r t a t i o n a n d to E. I. D o l o m a n o v a f o r c a r r y i n g out the e v a l u a t i o n s of u r a n i u m c o n t e n t in c a s s i t e r i t e .

LITERATURE I. 2. 3. 4. 5. 6. 7. 8. 9. i0.

1296

CITED

V.K. Markov et al., Uranium, Methods for Its Determination [in Russian], Atomtzdat, Moscow (1964). P. Price and R. Walker, J. Geophys~ Res., 68, 4847 (1963). Ya. E. Geguzin, I.G. Berzina, and I.V. Vorob'eva, Izv. AN SSSR, Ser. Geol., No.6, 21 (1966). I~ G. Berzina, I. B. Berman, and I.M. Zlotova, Izv. AN SSSR, Set. Geol., 9, i0 (1966). P. Price and R. Walker, Appl. Phys. Letters, 2, 23 (1963). I.G. Berzina et al., In the book: Proceedings of the Second Coordination Meeting on Dosimetry of Large Doses [in Russian], Izd. FAN UzbSSR, Tashkent (1966), p. 144. A. Kapustzik, V. P. Pereligin, and S. P. Tret'yakova, Equipment and Technique for Experiments [in Russian], Vol. 5 (1964) p. 72. Kh. Abdullaev et al., Preprint OIYaI PZ-2961, Dubna (1966). I.G. Berzina, Dokl. Akad. Nauk SSSR, 170, 681 (1966). I.G. Berzina et al., Dokl. Akad. Nauk SSSR, 171, 1313 (1966).

11. 12. 13. 14.

I.G. I.G. R.L. I.G.

Berzina and I. B. Berman, Dokl. Akad. Nauk SSSR, 174, 48 (1967). Berzina, I. B. Berman, and M. Yu. Gurvieh, Atomnaya ]~nergiya, 2_~2, 504 (1967). Fleiseher et al., Science, 14___99383(1965). Berzina and E. I. Dolomanova, Dokl. Akad. Nauk SSSR, 17__55,171 (1967).

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