Determination of bromine content and isotopic composition of bromine in stony meteorites by neutron activation

Oeochimica etCosmochimiea Acta1965,Vol.29,pp.467to 474.Pergamon PressLtd. Printed inNorthern Ireland

Determination of bromine content and isotopic composition of bromine in stony meteorites by neutron activation A. WYTTENBACH, H. R. VON GUNTEN and W. SCHERLE Eidg. Institut fiir Reaktorforschung, Wiirenlingen, Switzerland (Received

1964)

5 November

Abstract-Activation analysis was used to determine the bromine content of eleven stony meteorites. The values found by this method are appreciably lower than those obtained up to now by conventional chemical analysis. The highest value found is 1.56 ppm and the lowest 35 ppb. Hot water leaches removed considerable amounts of bromine in two samples tested The bromine to chlorine ratio in the leach water is essentially (31 and 88 per cent respectively). this seems to exclude the possibility of surface contamination the same as in the meteorite; of the samples. The ratio of chlorine to bromine content in eleven meteorites varies over two orders of magnitude and depends on the absolute chlorine content. Within the limits of accuracy of the method used, the isotopic composition of bromine in the 5 samples tested seems to be the same as in the case of terrestrial bromine.

1. INTRODUCTION RECENT investigations by neutron activation of the iodine content (GOLES and ANDERS, 1960) and on the chlorine content of stony meteorites (v. GTJNTEN, WYTTENBACH and SCHERLE, 1964) have raised considerable doubt as to the reliability of earlier determinations of halogens carried out by conventional chemical techniques. Therefore we decided to determine the bromine content of some stony This seems to be desirable particularly in view meteorites by neutron activation. of the fact that bromine determinations have been reported only three times (v. FELLENBERG, 1927; SELIVANOV, 1940; BEHNE, 1953), and that none of them have been made by neutron activation. Bromine determination by neutron activation analysis has several interesting aspects; one of them is due to the fact that both stable bromine isotopes are activated to products which can be distinguished from one another quite easily. Therefore this method furnishes a check on the isotopic composition of extraterrestrial bromine. 2. PRINCIPLE OP METHOD Bromine

is activated

by thermal neutrons

according

7gBr(n, y)s0~801nBr

to (1)

and 81Br(n, Y)8zBr Interfering reactions which may give rise to radioactive lives greater than 1 hour are listed below:

(2) bromine isotopes with half-

8oKr(n, p)80~80mBr

(3)

*2Kr(n, p)*2Br

(4)

85Rb(n, tC)*2Br

(5)

235U(n, f)*3Br

(6)

467

468

A. WYTTENBACH,H. R. VON GUNTENand W. SCHERLE

Reactions (3) and (4) have thresholds above thermal neutron energy. The cross section of reaction (5) at thermal, neutron energy is too small for a serious interference and the uranium content in meteorites as determined by GOLES and ANDERS (1962) is not high enough to produce interfering amounts of 83Br. We therefore conclude that the amount of radioactive bromine produced by irradiation with thermal neutrons is an exact measure of the bromine content of the samples. The radioactive bromine is extracted from the samples by the procedure given in section 3. The yield of this separation is determined by comparing the weight of the resulting AgBr with the amount of bromine added as carrier after the irradiation. The count rate of the extracted bromine is compared with the count rate of a monitor that has been irradiated simultaneously under identical conditions. This eliminates the necessity of absolute counting and flux monitoring. Three pieces of filter paper, each impregnated with 535 x 1O-3 mg bromine (as NH,Br) were A correction had to be applied for the self-shielding in the used as monitors. (REYNOLDS and meteorite samples. This correction was estimated theoretically MULLINS, 1963) to be approximately 4 per cent; however it is af?licted with a high degree of uncertainty. In our work on chlorine activation (VON GUNTEN, WYTTEWBACH and SCHERLE, 1964) we found experimentally under identical conditions a mean self-shielding correction factor of 8 per cent. A correction of 6 per cent with an uncertainty of 14 per cent is therefore applied. 3. EXPERIMENTAL Preparation

of samples

In order to get a clean sample, great care was exercised in selecting samples from the interior of meteorites, and in grinding them.* The samples used in this work were aliquots from the meteorite powder used in our work on chlorine activation (VON GUNTEN et al., 1964); for more details concerning the preparation of the samples, the reader is referred to this publication. Irradiations The samples (300-500 mg) were irradiated together with the monitors in the DIORIT reactor at a position where the flux is essentially pure thermal (approx. 1012n/sec cm2). The samples where both *OmBr and s2Br were analysed were irradiated for 24 hr, cooled for 24 hr, and again irradiated for 8 hr. This scheme was chosen to enhance the production of the 36 hr *2Br without increasing the total activity to an undesirably high level (which is caused mainly by isotopes with shorter half lives, such as 3%, 56Mn, 65Ni and 2*Na). Those samples which were analysed for s2Br only were irradiated for 40 hr and then cooled for 24 hr. Chemical analysis The irradiated meteorite powder was poured from its plastic bag directly into a zirconium crucible containing 20 mg of bromine carrier as KBr, and was covered * We are indebted to Prof. GEISSand Dr. GR~GLER of the Physikalisches Institut University of Berne (Switzerland) for the selection and grinding of the samples.

at the

Content and composition of bromine in meteorites

469

The mixture was fused, the resulting fusion with 2.5 g each of NaOH and Na,O,. cake dissolved in water, transferred to a distillation apparatus and acidified with 10 ml of a Ti,(SO,),-solution (15% w/v) were added to ensure that all H,SO,. bromine is reduced to the oxidation state of -1. The distillation was carried to The distilling HBr was trapped in NaOH. The the appearance of SO,-fumes. major contaminating impurities at this point were 36C1, 12*1, 56Mn and 24Na. Further purification was therefore necessary and was accomplished by the following procedure: The NaOH-solution in the trap was acidified and sufficient 0.1 M KMnO, was added to oxidize all Br’ to Bra, which was extracted twice with 10 ml of Ccl,, The organic phase was washed with 20 ml of water and the Br, stripped from the Ccl,-phase with 10 ml of water and 3 drops of a 1 M NaHSO, solution. The whole extraction cycle was repeated once. The resulting water-phase was heated to boiling to expel any excess of SO,, then the bromide was precipitated as AgBr. The precipitate was filtered on a filter paper disk, washed with H,O and acetone, dried and weighed. The chemical yield of the separation was usually between 60 and 75 per cent. No special step for the decontamination from iodine was introduced, since lzaI(7 = 25 m) has decayed by the time counting was begun. The filter paper monitors had to be processed also, since their 24Na activity was high enough to interfere with counting. The filter paper was destroyed by gentle heating with NaOH and H,O, in an Erlenmeyer flask containing 20 mg of bromine carrier as KBr. Thereafter the same extraction procedure as for the meteorite samples was applied. In order to measure the amount of leachable bromine, 2 irradiated meteorite samples were boiled with water (containing 20 mg of Br’) for 4 hour. The meteorite powder was then filtered off, dried and analysed as usual. The water was analysed for bromine by the extraction procedure given above. Counting 80n’Br was counted on a B-flow counter through 100 mg Al/cm2 via its daughter 60Br(/3: 1.99 and 1.38 MeV). The absorber reduced the count rate of BOmBrby a factor of 1.6 when compared with the count rate without absorber, whereas the count rate of szBr(@: 0.44 MeV) was reduced under identical conditions by a factor of 25. The eficiency of the counter for 80mBr under these conditions was 16 per cent. Counting was started approx. 5 hr after the end of the irradiation; counts were taken every hour for the first 12 hr and later at intervals of 8 hours. 82Br was counted for its y-rays by two different methods: (1) counting on a single channel y-counter, with the lower cut-off set at 0.45 MeV. (2) counting on a y-y-coincidence counter, with one window set at 0.55 MeV (the most prominent peak of the spectrum), and the other window covering the range between 0.5 and O-9 MeV. Coincidence counting reduced the count rate approximately by a factor of 10 when compared with single channel y-counting, but gave a considerably better count-to-background ratio. However the final results were the same for both methods of counting. Counting for 82Br was started 36 hr after the end of irradiation and was continued for 6 days.

470 Decay curve

A.

WYTTENBACH, H. R. VONGUNTENand W. SCHERLE

analysis

Decay curve analysis was done by a least square computer program. The best fit was always reached with T( 8’JmBr) = 4-42 hr and r(szBr) = 36 hr. The value of T(~O~B~) is in disagreement with the value of 4-71 hr as recommended by the Nuclear Data Sheets (1959), but agrees well with the values found by other workers: 4.38 & O-02 hr (KING and VOIGT, 1957) and 4-40 -& 0.05 hr (SCHMIDT et al., 1960). The decay curve analysis gave no indication whatever of any activity arising from isotopes other than from 80mBr and S2Br. 4.

The fractionation

RESULTS

factor u, defined by

E = [~]

_j

[~~

st&;n_.&

is given in Table 1. NH,Br (AnalaR) was used as standard. The standard deviation in the fractionation factor as determined by the analysis of 9 standards is estimated Table

1. Fractionation

a

sample Potter Mocs II Benton Pultusk Norton County I Table

factors a

1.03 1.03 1.02 1.00 0.99

2. Results of bromine and chlorine analyses

Sample Hypersthene chondrites 1 MOW Ia 2 MOW IIa 3 Kunashak 4 Potter 5 Dhurmsala 6 Bruderheim 1” 7 Bruderheim II” E~t~t~te chro~dr~te 8 Hvittis Soko-Banja-ly;pe 9 Benton Bronzite chondrite 10 Pultusk Enstatite achondrite 11 Norton County I

ppm Br

ppm Cl

0.97 1.56

266 110 89

2828 2507 123 132 403 113 57

1.04

222

213

0.18

57

317

0.17

97

570

0435 0.069 0.22 0.34 0.66

0.067

99 173 27 45

2.1

-

Cl/Br

31

a Mocs I was a sample containing visible veins; material with a minimum of veins was selected for Mocs II. b Bruderheim I and II were ground from two different pieces of the meteorite, which showed no visible differences.

Content and composition

471

of bromide in meteorites

to be &@O2. This error arises from the counting data and the radiochemical purity of the sample; however, it is not influenced by the determination of the chemical yield and only negligibly by self-shielding. The results of the quantitative bromine analysis are given in Table 2, together with the values for chlorine (from VON GUNTEN et al., 1964). Due to a limited supply in meteorite powder most bromine data were based on a single determination only. However, from the analyses of 9 monitors the standard deviation of a single determination was estimated to be f 2 per cent. Since the procedure used for the meteorite samples was slightly more complicated (due to the fusion step) we consider the precision to be f3 per cent. In order to check this point, two samples with extreme bromine values (Mocs II and Bruderheim II) were analysed a second time; for these analyses the monitor on the filter papers was changed from NH,Br to KBr, and KBrO, was used instead of KBr as carrier in the fusion step. The values obtained by these cross-cheek analyses are O-072 and 1.48 ppm respectively, thus sustaining the assumed precision of f3 per cent. Furthermore it could be shown that exchange problems do not lead to difficulties in our procedure. The accuracy of the results will be mainly influenced by the precision and the self-shielding correction (as discussed in section 2). We therefore assume an overall error of &6 per cent for the bromine content. Table 3 gives the results of the leach experiment, together with those for leaehable chlorine, which have been measured independently (VON GUXTEENet al., 1964). Table

Sample Hvittis Dhurmsala

3. Results of leach experiments

o/0 of total amount leachable BX? Cl 30.8 87.8

21 81

Halogen ratio in the leach Y0 Cl/y&Br Cl/BP 0.68 O*Q2

146 373

5. DISCUSSION To the best of our knowledge, information on the bromine content of stony meteorites is very scant. Mocs has been analysed twice before by purely chemical methods, giving bromine contents of 20 ppm (v. FELLERBERG, 1927) and 11 ppm (BEHNE, 1953). Our values of O-035 and 0.07 ppm for the same meteorite are considerably lower. Although our values for Mocs and for Bruderheim point to an inhomogeneous distribution of chlorine and bromine, we feel that the former analyses were apparently affected by serious problems of contamination of reagents, which problem is of no importance in the neutron activation technique. It is of interest to note, that v. FELLENBERG’S (1927) values for the iodine content in chondrites have been found to be systematically high by one or two orders of magnitude (GOLES and ANDERS, 1962); these values have been obtained by essentially the same procedure as in the case of bromine. On the other hand, our values agree better with the value of O-47 ppm quoted for the meteorite Saratov (SELIVANOV, 1940). Table 2 shows that the fluctuation of the bromine content in the 5 hypersthene chondrites is much more pronounced than in the case of chlorine. The iodine

A.

472

WYTTENBACH,H. R. VON GUNTEN

and

W.

SCHERLE

content too does not seem to vary so considerably. GOLES and ANDERS (1962) report the following iodine contents for ~e~eo~~s analysed in this work: Bruderheim I6 f 6 ppb, Noes 50 f 30 ppb. The ratio of chlorine to bromine is not constant, but varies over two orders of magnitude, the limits being 31 and 2828. If the ratio Cl/Br is plotted against the chlorine content (Fig. I), it seems that the ratio grows rapidly with increasing chlorine content. If one is willing to accept such a relationship in spite of the small! number of experimental points, several t~~pt~ng explanations might be put forward. IIowever, we feel that more experimental data should first be supplied,

10 0

I 200

I $00

I

300

wmCl

Fig.

1. ChlLvrineto bromins ratio as a function of chfarine content. The numbers refer to Table 2.

The experimental determination of the chlorine to bromine ratio is of considerable interest in view of the fact that it has been used by SUESS and UREY (1956) in the construction of their table of the abundances of the elements, Susss and UREY used the chlorine to bromine ratio as derived froru sea water, which is 292, ALLER (1961) however has expressed doubts as to the correctness of accepting the sea water value as “cosmically” valid, and our results seem to sustain this view at least for the meteorites. In the two samples that were leached, considerable amounts of the total bromine am extractable with water (Table 3). This would indicate that bromine forms part of a water soluble compound. Since there is no big ~~erentiatio~l between chlorine and bromine in these experiments, the two elements probably form part of the same compounds. Furthermore, our leach results seem to exclude the possibility that the meteorites have been contaminated by chlorine and bromine, since

Content and composition

of bromine in meteorites

473

these contaminations would be required to have the same chlorine to bromine ratio as the meteorite, which is a rather unlikely assumption. Further experiments will clarify the influence of the grain size of the analysed samples on the leachable amount of halogens. For the time being, we can only say that the grain size of the Dhurmsala-sample was bigger than that of the Hvittissample, and that, contrary to expectations, the amount of leachable halogen was A preliminary search for cations accompanying the found bigger in Dhurmsala. halogens in the leach experiments revealed the presence of Na, K, Co, Mn, Fe, SC, Cr and possibly rare earths (of course, elements giving rise to shorter lived isotopes have been missed due to the cooling time of several hours). The fractionation factors IXgiven in Table 1 do not lie significantly outside our accuracy. We therefore conclude that the isotopic composition of bromine in meteorites and in terrestrial samples is the same within f 2 per cent. Since the halflife of *lKr(2 x lo5 yr, decaying into ‘nBr) is not long enough for any slKr to survive the time necessary for the bodies to cool to temperatures low enough to retain krypton (approximately lo* yr (FISH et al., 1960)), the decay of *lKr is not a likely process for altering the isotopic composition of bromine in meteorites. This is in agreement with our results. The accuracy obtained in the determination of the isotopic composition is clearly one order of magnitude lower than what is usually obtained by mass-spectrometry. The neutron activation technique however has two advantages over mass-spectrometry: Firstly it can be applied to very small meteorite samples (500 mg meteorite corresponding to approximately 0.05 ,ug Br in our case) ; and secondly there is no need to carry out chemical operations on these very small quantities of material, thus avoiding possible sources of contamination. Acknowledgements-The authors wish to acknowledge the helpful discussions with Prof. J. GEISS and Dr. 1~. ANDERS (University of Berne). The assistance of Dr. P. TEMPUS and Mr. B. KELLER Meteoritic specimens were obtained in carrying out the irradiations is deeply appreciated. through the courtesy of Prof. R. E. FOLINSBEE, Edmonton (Canada), Prof. J. GEISS, Borne (Switzerland), Prof. E. L. K~INOV, Moscow and of Prof. C. B. MOORE, Tempe (U.S.A.). REFERENCES ALLER L. H. (1961) The Abundance of the Elements. Interscience, New York. BEHNE W. (1953) Untersuchungen zur Geochemie des Chlor und Brom. Geochim. et Cosmochim. Acta 3, 186. v. FELLENBERG TH. (1927) Untersuchungen tiber das Vorkommen von $Jod in der Natur. Biochem. 2. 187,1. FISH R. a., GOLES G. G. and ANDERS E. (1960) The record in the meteorites, III. Astrophys. J. 132, 243. GOLES G. G. and ANDERS E. (1960) Iodine content of meteorites and their 112g-Xe*2s ages J. Geophys. Res. 65, 4181. GOLES G. G. and ANDERS E. (1962) Abundances of iodine, tellurium and uranium in meteorites. Geochim. et Cosmochim. Acta 26, 723. VON G~JNTEN H. R., WYTTENBACH A. and SCHERLE W. (1965) Determination of chlorine in stony meteorites by neutron activation analysis. Geochim. et Cosmochin Acta 29, 475-480. KING A. M. and VOIGT A. F. (1957) Relative yields of nuclear isomers of Br”’ produced by a (y, n) reaction. Phyls. Rev. 105, 1310. Nuclear Data Sheets (1959) National Research Council-National Academy of Sciences, Washington D.C.

A. WYTTENBACH, H. R. VON GUNTEN and W. SCHERLE

474

REYNOLDS S. A. and MULLINSW. T. (1963) Neutron flux perturbation in activation analysis. Int. J. Appl. Rad. Isotopes 14,421. SCHMIDT-OTTW. D., HOF~MANN K. W. and KRAUSE I. Y. (1960) Die Faktoren der inneren Umwandlung beim s5Brsom(4.4 Std). 2. Phyls. 158,242. SELIVANOVL. S. (1940) Chemical analysis of the Saratov meteorite. C.R. Acad. Sci. U.S.S.R.

26,388. SIJESS

H. E. and UREY H. C. (1956) Abundances

of the elements.

Rev.

Mod.

Phys.

28, 53.