D E T E R M I N A T I O N OF S T A B L E N E O N I S O T O P E S IN

R A D I O A C T I V E M I N E R A L S A N D N A T U R A L G A S E S

Y u . A. S h u k o [ y u k o v , G. Sh . A s h k i n a d z e , a n d V. B . S h a r i f - Z a d e

UDC 621.039.1

Radioact ive minera l s contain the isotopes Ne 21 and Ne 22 which are formed in (~, n), (~, p), and other nuc lear reac t ions [1-4] in amounts f rom ~10 -9 to =10 -6 cm3/g. In thei r migrat ion, the isotopes Ne 21 and Ne 22, like the radiogenic isotopes of xenon, krypton, argon, and helium [5, 6], leave the c rys t a l s t ruc tu re of the minera l s and appear as a const i tuent of the natural gases . In natura l gases , the content of rad io- genic Ne ~i and Ne 22 v a r i e s f rom 10 -8 to 10 -6 volume percent [7, 8].

The determinat ion of mic roamount s of the s table isotopes Ne zl and Ne 22 in na tura l objects is of in te r - es t in studies of the phys ieochemical conditions for the exis tence of radioact ive minera l s , in studies of natural nuclear p r o c e s s e s , and of the genes is of na tura l hel ium-containing gases , and in working out cer ta in p rob l ems in prospec t ing for rad ioac t ive e lements .

S e p a r a t i o n of N e o n f r o m M i n e r a l s a n d G a s e s f o r

I s o t o p i c A n a l y s i s

The equipment used was const ructed with the help of I. N. Tolst ikhin. To separa te neon, mine ra l samples were fused in a vacuum furnace by means of a tungsten sp i ra l (Fig. la) . F i r s t , the furnace was baked out at 200-250~ for seve ra l hours . In the determinat ion of neon isotopes in gases , the gas container was opened by means of the device sketched in Fig. lb .

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7--]_

Fig. 1.

o o

o o

o

o

o

a

Apparatus for extract ion of gases f r o m samples : a) vacuum furnace: 1) fo revacuum line; 2) rubber gasket ring; 3) vacuum gasket; 4) copper gasket; 5) vacuum seal for electrode; 6) molybdenum electrode; 7) wa te r cooling; 8) h i - chrome hea te r winding; 9) crushed quartz; 10) tungsten spiral ; 11) minera l s a m - ple; 12) h igh-vacuum line: b) container for ext rac t ion of neon f r o m samples of na tura l gases : 1) iron core for smashing g lass gas containers; 2) e l ec t romagne t winding; 3) knife-edge vacuum seal; 4 ) v a c u u m line; 5) g lass gas containers ; 6) seal ing wa te r column; 7) center ing jig.

Trans la ted f rom Atomnaya l~nergiya, Vol. 34, No. 6, pp. 461-464, June, 1973. Original ar t ic le

submitted July 31, 1972.

�9 1973 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West ]7th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced [or any purpose whatsoever without permission of the publisher. A copy of this article is available [rom the publisher [or $15.00.

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Fig. 2. Diagram of sys tem for neon purification: 1,7, 8, 12) metal sylphon valves; 2, 21) m e r c u r y seals; 3) sealable glass ampul; 4) diffusion cell; 5) metallic ca l - cium getter; 6, 9, 13, 15) mercu ry seals controlled by sylphon level regulators; 10) Vacuum furnace; 11) glass cold trap; 14, 16) tube containing activated cha r - coal; 17) measur ing capi l lary of compress ion mano- meters ; 18, 20) electromagnetic valves; 19) m e r c u r y compress ion manometer pump; 22) me rcu ry Toeppler pump.

As a rule, the volume of contaminating gases was 107-108 t imes g rea te r than the volume of neon and subsequent operations amounted to separation of the neon f rom all other gases.

A glass sys tem was used which was f i rs t pumped down to a p re s su re of (1-3) �9 10 -7 to r r by means of a DRN-10 mer cu ry diffusion pump (Fig. 2). in the pumping lines, Du-6 metal sylphon valves were used. Par t s of the sys tem where gases remained a comparat ively long time during the course of an experiment were provided with U-shaped mercu ry seals controlled by metal sylphons.

After the completion of fusion with mercu ry seals closed, water vapor and carbon dioxide was frozen out in a trap cooled to -196~ Then the temperature of a titanium get ter was raised to 1000~ the main portion of the chemical ly active gases was absorbed, and the gases were t rans fe r red into a manometer - pump with electromagnetic valve [10] by opening the seals controlled by a sylphon level regulator . Fu r the r - more, the heavy noble gases (argon, krypton, and xenon) were adsorbed on activated charcoal held in a glass tube at -196~ Having shut the seals, the gases were re leased f rom the manometer pump for final purification in a calc ium absorber . For this purpose, it was heated to 650~ and then the tempera ture was reduced to 300~ As a result , a mixture of two gases remained: helium and neon in a ratio of approxi- mately 106 : 1.

By means of the manometer pump, me rcu ry seal, and electromagnet ic valve, the he l i um-neon mix- ture was t rans fe r red into a diffusion cell which was a quartz bulb 2 cm in diameter having a wall 50 # thick. For a temperature T, wall thickness l, and p res su re p, an amount of gas diffuses through S cm 2 of a quartz walI per unit time which is given by the following expression:

e---r = K ( r ) - PlS. dt

The coefficient K(T) depends on the kind of gas for a given temperature; the permeabil i ty of quartz for helium is considerably g rea te r than the permeabi l i ty for neon.

A temperature of 1000~ was chosen as the working temperature . The main portion of the helium migrated through the wail of the diffusion cel[ in 4-6 h. The amount remaining was usually no more than (2-3) �9 10 -4 cm ~. During the period when the diffusion cell was in operation, the neon loss did not exceed 3-5% (Fig. 3). After removal of the main portion of the helium, the remaining gas was t ransfer red by a Toeppler pump into a sealable glass ampul with a closed mercu ry seal.

567

10 ~

~0 s

io 4

iOy

iO 2

iO ~

iO o

0

1 I 1 1 1 2 J 4

I 5 %Ne

Fig. 3. Di f fe rence in diffusion r a t e s through a quar tz m e m b r a n e for he l ium and neon: He 0 is the or ig ina l amount of he l ium in dif- fusion cell; He t is the amount of he l ium at the t ime t; % Ne is the neon f r ac t ion which d i f fuses out of the ce l l up to the t ime t.

I s o t o p i c A n a l y s i s o f N e o n

The total amount of neon, including ins t rumenta l background , was ~10 -6 e m 3. The background was a tmosphe r i c neon (Ne 2~ 90.50%; Ne 21, 0.269%; Ne 22, 9.23%) [8], which was p r e sen t in the s y s t e m because of incomple te vacuum condi t ions resu l t ing f r o m desorp t ion f r o m internal s u r f a c e s of the s y s t e m and f r o m the su r face of the sample . To m e a s u r e such an amount of neon, a s t andard MV2302 m a s s s p e c t r o m e t e r (acce le ra t ing vol tage, 3 kV; reso lu t ion , 4000-5000; e lec t ron mul t ip l i e r at output; nonun i fo rm 180 ~ m a g - net ic field) was modif ied [10].

The m e a s u r e m e n t s w e r e made in a quas i s t a t i c v a c u u m mode in which the h igh -vacuum va lves w e r e shut. Gases escap ing f r o m internal s u r f a c e s w e r e absorbed by a t i tan ium ge t t e r pump connec ted to the a n a l y z e r c h a m b e r .

The amount of ion c u r r e n t f r o m (Ne 2~ + was d i s to r ted by the p r e s e n c e of a line f r o m the ion (Ar4~ ++ in the m a s s s p e c t r o m e t e r . To e l imina te this effect , and the effect of the superpos i t ion of the (CO2) ++ line on the (Ne 22) + line as well , the ionizing vol tage was kept below 38 V. To reduce the effect of the (Ne2~ l) + ion on m e a s u r e m e n t s of (Ne2i) +, the t i tan ium ge t t e r was thoroughly ou tgassed be fo re each expe r imen t until there was s igni f icant reduc t ion in the hydrogen background . As spec ia l expe r imen t s d e m o n s t r a t e d , the ions (H~O 18) +, (DIO l?) +, (DIO 18) +, etc. played no impor tan t ro l e s ince w a t e r vapor was eff ic ient ly f rozen out in the cold t rap of the feed s y s t e m and was d e c o m p o s e d in the t i tanium ge t t e r in the a n a l y z e r c h a m b e r . The re w e r e no not iceable l ines in the s p e c t r u m of o rganic r a d i c a l s unreso lved f r o m neon l ines.

Because of d i s c r i m i na t i on effects , the m e a s u r e d isotopic compos i t ion of a t m o s p h e r i c neon usual ly dif- fe red by 1 - 3 ~ f r o m the isotopic compos i t ion de te rmined with m a s s s p e c t r o m e t e r s p r e v i o u s l y ca l ib ra ted by means of mix tu res of s epa ra t ed neon isotopes [7, 11]. The appropr i a t e c o r r e c t i o n was made to the neon r e su l t s obtained f r o m the s a m p l e s s tudied.

The total amount of neon in the s a m p l e s was de t e rmined by the peak-he igh t method. F o r this p u r - pose, a definite amount of a neon s tandard was admit ted into the a n a l y z e r c h a m b e r and the dependence of the output s ignal f r o m the m a s s s p e c t r o m e t e r on the amount of s t andard de t e rmined . Using this depen- dence, one could d e t e r m i n e the total volume of neon in a sample f r o m the magni tude of the signal .

To admit neon into the a n a l y z e r c h a m b e r and to se lec t a given volume of a neon s tandard , a spec ia l feed s y s t e m was used (Fig. 4). The neon sample under invest igat ion, which was in a g l a s s ampul, was placed in a sylphon a m p u l - c r u s h e r . Af te r obtaining a vacuum of ~10 -7 to r r , the ampul was c rushed and the neon d i s c h a r g e d through a sylphon valve into the a n a l y z e r c h a m b e r , the h i g h - v a c u u m valves of which had been c losed in advance. Af te r the m e a s u r e m e n t s , the neon was pumped out of the c h a m b e r through the h igh -vacuum va lves .

The r e q u i r e d amount of neon s tandard was wi thdrawn f r o m a g l a s s bulb into a m e a s u r i n g cap i l l a ry by means of va lves and s m e r c u r y seal . The vo lume of gas in the c a p i l l a r y was m e a s u r e d by the c o m p r e s - sion method. The ra t io between the vo lumes of the sphere of the c o m p r e s s i o n m a n o m e t e r and the cap i l l a ry tube (Vsph/V c = 500) wa s de t e rmined in advance. The re fo re , knowing the or ig ina l vo lume of gas in the m e a s u r i n g c a p i l l a r y and d i s t r ibu t ing it between the sphe re of the m a n o m e t e r and the c a p i l l a r y tube, one can ca lcu la te the volume of gas r each ing the cap i l l a ry tube. In this way, the r equ i r ed vo lume of neon (from 10 -s to 10 -6 c m 3) was m e a s u r e d out. The a c c u r a c y in de t e rmin ing m i c r o a m o u n t s of neon by this method was •

The quant i ty of the rad iogenic i so topes Ne 21 and Ne 22 in rad ioac t ive m i n e r a l s o r na tu ra l ga se s was ca lcula ted f r o m the e x p r e s s i o n s

568

. 71 17

8

Fig. 4. D iag ram of m a s s - s p e c t r o m e t e r feed s y s t e m for m e a s u r e m e n t of m[croamounts of neon: 1) bulb containing neon standard; 2) m e - tal va lves for withdrawing neon Standard; 3, 5, 17) cord t raps; 4, 6) manomete r tubes; 7, 8, 9, 11, 16) meta l sylphon valves; 10) sylphon a m - pu l - c rushe r ; 12) m e r c u r y seal; 13) m a s s - s p e c t r o m e t e r chamber ; 14) compres s ion m e r - cu ry m a n o m e t e r pump; 15) measur ing cap i l - lary; 18) m e r c u r y diffusion pump; 19) fo r e - vacuum line; 20) h igh-vacuum sylphon valves; 21) cap i l l a ry tube for sampl ing neon.

~1 _ T~I - - / N e 2 1 ~ .NerO . 1Nera d - iN m e a s ~ N e 2 ~ m e a s

22 N e 2 ~ _ [ N e ~~ ~ N e r o Nerad =~" meas kNe2~latm" meas

By means of the method descr ibed, neon isotopes were de te rmined in pitchblende, s m a r s k i t e , be t a - fite, monazi te , br i thol i te , and lovchorr t te , in na tura l gases f rom regions where there a re uranium de- posi ts and f rom shot holes tn sal t s t ra ta , etc. [3, 4, 8].

The authors thank t~. K. Gerl tng for d iscuss ions of the work and A. B. Verkhovski i for help tn p e r - fo rming the exper iments .

L I T E R A T U R E C I T E D

1. G. Wetheri l l , Phys. Rev . , 96, No. 3, 679 (1954). 2. D. Bogard et a l . , J. Geoph~-. R e s . , 70, No. 3, 307 (1965). 3. V . B . Shar t f -Zade et a l . , Geokhimiya, No. 3, 314 (1972). 4. Yu. A. Shukolyukov et a l . , At. Ene rg . , 3 1 , 5 3 0 (1971). 5. Yu. A. Shukolyukov and G. Sh. Ashkinadze, Geokhimiya, No. 10, 1083 (1967). 6. I . M . Morozova and G. Sh. Ashktnadze, Mingration of Noble Gas Atoms tn Minera ls [in Russian],

Nauka, Leningrad (1971). 7. D. Emer son et a l . , Internat . J. of Mass Spec t romet ry and Ion Phys . , 1, No. 1, 105 (1968). 8. Yu. A. Shukolyukov and V. B. Sharff -Zade, and G. Sh. Ashkinadze, Ge-okhimLya, No. 3, 568 (1973). 9. I . N . Tolst ikhin, Author ' s Abst rac t of Candidate ' s Disser ta t ion , Leningrad (1966).

10. Yu. A. Shukolyukov, Fiss ion of Uranium Nuclei in Nature [in Russian], Atomizdat, Moscow (1971). 11. J. Walton and A. Cameron , Z. Na tu r fo r sch . , 21a, No. 1/2, 115 (1966).

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