Radiochemical measurements of 10Be and 7Be formation cross sections in oxygen by 135 and 550 MeV protons

2.A.I

J

Nuclear Physics A195 (1972) 311 - 320; (~0 North-Holland Publishin9 Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

RADIOCHEMICAL

MEASUREMENTS

OF

1°Be AND VBe F O R M A T I O N

CROSS SECTIONS IN OXYGEN BY 135 AND 550 MeV P R O T O N S B. S. A M I N , S. BISWAS, D. L A L and B. L. K. S O M A Y A J U L U

Tata Institute of Fundamental Research, Bombay-5, India Received 25 July 1972 Abstract: Radiochemical m e a s u r e m e n t s o f the activities o f ~°Be a n d rBe in oxygen due to the b o m b a r d m e n t o f water s a m p l e s by p r o t o n s o f energies 135 and 550 McV are reported. The ratios a(l°Be)/cr(VBe) o f the cross sections in oxygen are found to be 0 . 1 9 4 0 . 0 1 and 0.32==0.02 for 135 a n d 550 MeV protons, respectively, based on t~ (1°Be)=-2.7 "~ 106 y and t,] ( 7 B e ) - 5 3 . 6 d. U s i n g the best estimate o f the excitation function o f 7Be in oxygen, the formation cross sections for 1°Be in oxygen are estimated to be 1.0_-0.06 a n d 2.9 :-0.2 m b for p r o t o n s o f energies 135 a n d 550 MeV respectively. Cross sections for ~°Be in oxygen, averaged over the energy s p e c t r u m o f cosmic rays, a n d based on the observations o f its activity in meteorites and deepsea sediments are discussed. The implication o f the present m e a s u r e m e n t s for the use o f the J°Be isotope as a " c l o c k " to estimate the " a g e " of cosmic rays is briefly noted.

E I

NUCLEAR

R E A C T I O N S 160(P' a ( X)7" l d°e ld°Be' uBc e deEa( )i:OBe). 135' / I 550 c MeV; ~ ( m7e a sBu r eCd

)

;

1. Introduction

The long-lived radionuclide ~°Be (half-life = 2.7 x 106 y ) is suitable as a radioactive tracer for studying the details of cosmic-ray propagation ~), cosmic-ray prehistory based on observations of nuclear interactions in extraterrestrial samples 2), and the chronology of marine sediments 3). For these varied applications it becomes necessary to know the formation cross sections of ~°Be from C, N, O and heavier nuclei. In view of the importance of the knowledge of the cross sections for ~°Be and light elements (Li, Be, B) in general, several groups have developed new techniques for determining these formation cross sections: however, these cross-section data are sparse, primarily because these measurements involve special problems and are time consuming. Three techniques have been used for measurements of ~°Be cross sections: radiochemical 4), mass-spectrometric 5.6) and nuclear emulsion kinematic 7). We are currently engaged in the measurement of cross sections for the formation of t°Be (and other long-lived nuclides) from C, O, AI, Mg, Si and Fe using the radiochemical method. In the case of the radiochemical method, a measurement of the amount of 10Be formed in a given irradiation is difficult due to the fact that the activity of the short-lived 7Be (half-life = 53.6 d) interferes with the measurement ofthe/J-activity 311

312

B.S. AMIN etal.

of 10Be (E,,~x = 0.55 MeV); the interference is appreciable in spite of the fact that 7Be is a "f-emitter. Therefore, it becomes necessary to wait for several years after an irradiation to allow the 7Be activity to decay to a level where its presence does not contribute any significant counting in the/3-counter employed for detection of 1°Be. In this paper, we report data on ~°Be cross sections in oxygen due to the bombardment of water samples by protons of 135 and 550 MeV energy. The samples studied by us are the same concentrates analysed by Yiou et al. 6) using the mass-spectrometric technique; the samples were irradiated in January-March 1967. (The results of measurements of 10Be in other targets will be reported later; 7Be activities in some of these cases are still too large to permit a measurement of the t°Be activity.)

2. Experimental procedure Details of the bombardment of the water targets and the methods used to obtain concentrated deposits for their mass-spectrometric analysis have been discussed by Yiou et al. 6). Briefly, about 30 g pure water was irradiated with external proton beams of 135 and 550 MeV using the Orsay and CERN synchrocyclotrons, respectively. By the courtesy of the late Professor R. Bernas, we received three deposited sample plates of the water concentrates after irradiation and mass-spectrometric analysis. In table 1, the relevant details of these targets are given. [Since samples A and B (see table 1) were irradiated on the same date with 135 MeV protons and the expected TABLt: 1 Irradiation details a n d observed 7Be activities in p r o t o n - I : o m b a r d e d H z O samples Orsay target code ~)

Energy (MeV)

Date o f irradiation

Approximate number of incident protons

A B C

135 135 550

21 Jan. 1967 21 Jan. 1967 2 M a r c h 1967

~ 101~'1 1.4/.1017[ ~ 10 t7

7 Be activity net counts/rain disintegrations/rain observed on day o f irradiation 6 Oct. 1968

264.4 95.7

2.18×10 a 4.45×107

~) W e received the p l a n c h e t s used by Prof. Bernas a n d his g r o u p for mass-spectrometric d e t e r m i n a t i o n s o f Li a n d Be isotopes. T h e two Orsay s a m p l e s A and B were c o m b i n e d by us a n d this c o m b i n e d sample is d e n o t e d as A + B.

~°Be activity was small in each case, we combined these targets.] The deposits were at first counted for 7Be activity and then brought into solution with 10 M HCi and a few drops of 16 M HNO3. The sample holders were counted again for 7Be activity; it was concluded that more than 99 ~ of the 7Be activity was brought into solution by the HCI treatment. A beryllium carrier, as BeSO4, equivalent to 33.6 mg BeO was added and the solution was dehydrated. Then a few drops of HF were added and the

7.1°Be CROSS SECTIONS

313

sample was repeatedly fumed with HESO 4. The beryllium solution was taken in l0 M HCI and passed through an ion-exchange resin (Dowex l-X8). Be(OH)2 was precipitated from the effluent in the presence of EDTA. The sample was ignited, and BeO was deposited in the cavity of a flat rectangular lucite holder of cross section 4. l cm 2. Beta counting was carried out using a flow-gas-type low-level Geiger counter 8) of background about 5 counts/h and having a 38 % counting efficiency for 4°K B-rays. Gamma counting of the lucite planchets was carried out using a flat 5.08 cm diameter NaI(TI) crystal coupled to a 128-channel pulse-height analyser. The ~-spectrometer has a background of 6 counts/rain for the 20-channel envelope that contained the 0.47 MeV 7Be peak and a detection efficiency of 4.6 % for 7Be ~-rays. Both B- and ,/-countings were carried out at regular intervals for about a year until most of the 7Be activity had decayed. 3. Results

In table 1, we have summarized the observed 7Be activities in the samples A + B and C; the results are given separately for the date of counting as well as the date of irradiation. The 7Be 7-activities as well as the total activity monitored by the gas-flow B-counter decreased with a half-life of 54 d in agreement with the expectations for 7Be (t~ = 53.6 d) [ref. 9)]. The observed B-counting rate corresponds to a counting efficiency of about 0.1% for 7Be ?-rays. When the counting rates observed with the B-counter began to level off (fig. I), the samples were subjected to a chemical recycling; the time elapsed since irradiation was about 30 months. BeO was dissolved in HF + HCI and fumed with H2SO4. Radiochemical purification was then carried out according to the procedure given by Amin et al. lo) which consists essentially of a selectix.e ionexchange separation followed by sulphide and sulphate precipitations after the addition of hold-back carriers, Cu +÷ and Sr + +, and finally an extraction with TTA. The beryllium extracts were then counted for their B-activity over a further period of about 3 y. In between, absorption measurements were also carried out to determine the characteristic energy of the B-activity and the samples were subjected to a second radiochemical purification. The net B-activities observed in samples A + B and C are shown in fig. 1 for the samples obtained after first chemical extraction as well as subsequent first and second chemical recyclings. The reciprocals of the chemical efficienciesf~ and f2 for the first and second recycles are shown in fig. 1. In order to compare the activity levels corrected for chemical efficiency, the results obtained after the first and second recyclings should be multiplied by f l andflJ2, respectively. The counting data indicate that about 200 d after the first chemical extraction, the 7Be activity had already decayed sufficiently so that its contribution to the counting rate in the B-counter had become negligibly small (fig. 1). The activities in the recycled samples, after both the first and second recycles are in good agreement with the net

314

B.S. AMIN etal.

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fl-activities between 200 and 360 d in the original chemical extraction, if due account is taken of the chemical efficiencies. Absorption measurements for the samples A + B and C yielded half-thickness values of 23 _+3 and 24_+4 rag" c m - 2 polyethylene, respectively. The corresponding values, measured under identical conditions of counting for synthetic ~1) and natural 1°Be sources are 21.2_+0.3 and 21 + 1 mg - cm -2 polyethylene, respectively. The absorption measurements for the samples A + B and C are plotted in fig. 2. In view of the observed levelling off of the fl-activities as expected and recycling to constant specific activity levels (' °Be disintegrations/min per mg BeO), we conclude that the beryllium extracts were pure, at least in as much as the measured net flactivity 360 d after the first chemical extraction was not due to 7Be ,,,-rays but was primarily due to the radionuclide t°Be. In table 2, we have summarized our best estimates for the net '°Be and 7Be activities in the two samples A + B and C. The corresponding calculated cross-section ratio ~(10Be)/~(VBe) is also given in table 2: for this calculation it was assumed that the half-lives of 7Be and '°Be are 53.6 d and

~" ~°Be CROSS SECTIONS

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ABSORBER THICKNESS, t(rng c~ 2) Fig. 2. Net activity in the fl-counter as a function of the absorber (polyethylene) thickness for the two samples A -}- B and C. The lines represent the least-squaresfit. TABLE 2 7Bc and I°Be activities in oxygen targets Energy (MeV)

Orsay target code

135 550

A tB C

TBe activity net counts/ disintegrations/ rain rain on day of observed irradiation (6 Oct. 1968) 264.4 95.7

2.18;<10 s~) 4.45 × 107 b)

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2.3 -0.11 0.78_-'_0.06

0.194-0.01 0.32-L0.02

a) 21 January 1967. ") 2 March 1967. 2.7 X 10 6 y respectively. T h e yield for 477 keV 7-rays in 7Be d e c a y is t a k e n to be 10.3 °/,,, [ref. 9)]. 4. Diseussion of results In table 3, we h a v e s u m m a r i z e d all c r o s s - s e c t i o n d a t a a v a i l a b l e to d a t e for 1°Be a n d 7Be in p r o t o n - i n d u c e d r e a c t i o n s in c a r b o n , n i t r o g e n a n d o x y g e n . In a d d i t i o n to the three m e t h o d s r e f e r r e d to in sect. 1, we h a v e also listed the e s t i m a t e d cross section for 10Be based on o b s e r v a t i o n s o f its activity in m e t e o r i t e s and s e d i m e n t s . T h e m e t e o ritic m e t h o d is discussed by G o e l 12). T h e g e o p h y s i c a l m e t h o d is based on o b s e r v a -

316

B.S. AMIN etal. TABLE 3

J°Be and 7Be cross sections in C, N and O Target

Proton energy

Method

o'(1°Be)/a(TBe)

(MeV)

carbo,1

oxygen

125 150 220 600 135 135 550 550 400-3000

a(7Be) ~) nuclear emulsion kinematic mass-spectrometric 0.09 --0.03 ~) radiochemical 0.23 ±0.06 c) mass-spectrometric 0.25 4-0.02 b) mass-spectrometric radiochemical mass-spectrometric radiochemical meteoritic

0.0674-0.018 d) 0.19 !.0.01 ") 0.088±0.01 a) 0.32 5_0.02 c)

nitrogen

125

nuclear emulsion kinematic

CN30

220

radiochemical

0.44 -.'0.2

geophysical

0.3 t)

Atmosphere

(50 500:

Absolute cross section (rob)

13.0 11.0 9.0 11.0 5.5 5.5 9.0 9.0

or(l°Bc) 5~(0.4 g) 1.0 ±0.3 1.9 _-0.6 2.8 : 0.3 0.37--0.12 1.0 _-_0.06~) 0.8 --0.1 2.9 -0.2 ~) 2.0 ~-0.5 h) 1.6 : 1.6~)

~)

4

Radiochemical values in this table are based on t.t(l°Be) --~ 2.7 .: 106y and tt(TBe) •~) cr(VBe) values arc based on excitation functions in figs. 3 and 4. ") Ref. 24). ') Rcf. ~1. a) Rcf. '~). c) Present work. f) Refs. ~3.,.~). ~) Ref. 7). h) Ref. 1.,).

:'2

53.6 d.

tions of ~°Be activity in sediments whose rate of a c c u m u l a t i o n could be estimated using radioactive methods a n d / o r magnetic reversal technique ,3. ~4). The estimation of absolute ~°Be cross sections entails additional errors in the present work or that reported earlier ~5) because the absolute 7Be cross sections reported in the literature show a finite scatter ~5 - : 2 ) . All available data on the absolute 7Be cross sections for p r o t o n irradiations in oxygen are plotted as a function of energy in fig. 3; a smooth curve d r a w n by us t h r o u g h the data is taken to represent the excitation function for 7Be in oxygen, in the case of c a r b o n , the excitation function given in fig. 4 is that adopted by T o b a i l e m 23) as the best curve for p r o t o n b o m b a r d m e n t s of carbon. Based on the 7Be cross-section values gives in figs. 3 and 4, we have listed the absolute ~°Be cross sections in table 3; the quoted errors in the t°Be cross sections, however, do not take into account uncertainties in the 7Be cross sections. C o m p a r i n g our present results for the a( ~°Be)/a(VBe) ratios with those reported by Yiou et al. ' s) which in fact refer to the same samples analysed by them (table 1), we note a discrepancy (table 3) of a b o u t a factor of 3-4 in the results for both A + B a n d C targets, i.e. for 135 a n d 550 MeV p r o t o n irradiations. It seems worthwhile m a k i n g a similar c o m p a r i s o n in the case of c a r b o n where both radiochemicai 4) a n d mass-spectrometric estimates 24) are available. In this case, one notes a very satisfactory agreement and it is therefore clear that there does not necessarily exist a sys-

v.t°Be CROSS SECTIONS

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(GeV)

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Fig. 4. Excitation function for ~Be in carbon for proton bombardment, as adopted by Tobailem et al. 23).

t e m a t i c d i f f e r e n c e w h i c h c a n b e a t t r i b u t e d to t h e t e c h n i q u e used o r p o s s i b l y t o a n e r r o r in t h e half-life o f 1°Be, a p a r a m e t e r w h i c h e n t e r s o n l y in t h e c a s e o f t h e r a d i o c h e m i c a l m e t h o d (see b e l o w f o r f u r t h e r d i s c u s s i o n s ) . It s e e m s i m p o r t a n t t o n o t e t h a t t h e m e a s u r e m e n t s o f a(~°Be)/a(TBe) r a t i o s in c a r b o n b y F o n t e s et al. 24) w e r e c a r r i e d o t t w i t h t h e h e l p o f a n i m p r o v e d v e r s i o n o f t h e m a s s s p e c t r o m e t e r 3,,) o r i g i n a l l y u t i l i z e d b y Y i o u et

al. 6).

However, we cannot comment whether the discrepancy between our

p r e s e n t e s t i m a t e s a n d t h o s e o f Y i o u et

al. 6.15)

for an oxygen target can be ascribed

318

B.S. A M I N e t al.

to any technical problems associated with the original mass spectrometer employed. We feel that we have made all the necessary checks on the purity of the beryllium samples. In addition to the fact that the/:~-activities were followed over a period of about 3 y after allowing 7Be activity to decay (fig. 1), we were also able to confirm the Em~x value ot the/J-radiation counted. This could be done with high precision because ol the large fl-activities. The result, as discussed earlier, is in accordance with ex~.ectations tor a pure 10Be samlzle. In view of these considerations, we feel confident of the cross-section valves reported here. As commented above, in the radiochemical method, the 1°Be cross section has to be based on an assumed value of its halt-life, whereas this is not the case tor the massspectrometric method. In fact, if we assume that our determinations of the 10Be and 7Be activities as well as those of Yiou et al. 15) on the relative numbers of I °Be and 7Be atoms for the oxygen targets A, B and C are correct, then we can determine the half-life of ~°Be using the following expression: t~(l°Be) _-_ n(l°Be)/n(VBe) t.t(VBe)

(1)

A(I°Be)/A(VBe) '

where n and A are the number of atoms and the activity (disintegration rate) due to 7Be or 1°Be at t = 0 (irradiation time) respectively; t~ = half-life. Another way of expressing eq. (1) would be: a.('°Be-----~)= -A.('°Be) t½(l°Be) a(TBe) A(TBe) ti(TBe)"

(2)

We have estimated relative l OBe/TBe cross sections based on eq. (2) and assuming t~(1°Be) = 2.7 x 106 y and t½(TBe) --: 53.6 d. It then immediately follows that if we accept the mass-spectrometric data of Yiou et al. ~5) for 135 and 550 MeV irradiations of oxygen, the calculated values of the half-lives of I °Be will be 2.7 x 10 ° x (0.067,,'0.19) = 0.96x 106 y and 2.7 × 106× (0.088/0.32) = 0.74>< 106 y respectively, for the two cases. Similar calculations in the case of ~arbon, however, yield a value close to 2 . 7 x 106 y since the radiochemical estimates for ~°Be cross sections at 220 MeV [ref. 4)] are not inconsistent with the value based on an interpretation of mass-spectrometric estimations 2,)) at 150 and 600 MeV. Yiou and Raisbeck z6) recently measured the ~°Be and 7Be activities in carbon targets bombarded with 150 and 600 MeV protons and suggested a revision of the half-life of ~°Be to (1.5__+0.3)x I0 ° y, based on the radiochemical and mass-spectrometric measurements of i OBe" This is a circular argument because a revision of the half-life essentially implies a disagreement between the radiochemical and massspectrometric determinations of ~°Be cross sections. At the present moment, we feel that it is not possible to unambiguously decide whether the Orsay and Bombay group results imply a shorter half-life for ~°Be or an error in the experimental determination of the ratio n( I °Be),'n(TBe) by the Orsay group, or of A( ) °Be) by the Bombay group.

7. lOBe CROSS SECTIONS

319

Pending the settlement of this issue, we prefer to retain the present procedure, t, iz. calculating cross-section values based on t,( 1°Be) = 2.7 × 106 y. It seems useful to make a few comments on the behaviour of the i 0Be cross section with energy for carbon, oxygen and nitrogen. In the case of nitrogen, a cross-section valuc can be deduced on the basis of the reported value 4) for semicarbazide (CN30). However, this estimate has a large uncertainty because of large errors in t:~B : osssection values in carbon and oxygen. Of the two indirect methods of estimating the 10Be cross section, viz. geophysical [ref. 13)] and meteoritic 12), the former is straightforward because: (i) nitrogen is abundant in the atmosphere (N3.70) and (ii) the two targets N and O are light nuclei so that one can safely assume that the slopes of the excitation functions for i 0Be production may not be different in these nuclei. Assuming a mean value of 10 mb for 7Be in oxygen in the energy interval 50-500 MeV, where most of the isotope production occurs due to cosmic radiation in the earth's atmosphere 25), a value of 3 mb is c>timated which should be taken to approximate closely the value for the ~°Be cross section in nitrogen (see table 3). The meteoritic method is based on the measurements of t°Be activity in chondrites. Here the contribution to 1°Be production comes not only from O but also AI, Si, etc. Although the assumptions made by Goel 12) in obtaining a( t °Be)/tr(TBe) in O from the meteorite data are not too extreme, of course, one cannot be sure of this in view of the uncertainties due to differences in the excitation functions in different targets as elaborated by Raisbeck and Yiou 27). Lastly we wish to draw attention to the implication of the present results for the problem of "age" of cosmic rays using the isotope 10Be as a "clock" 1). The production of ~°Be in cosmic rays will be largely dominated by the spallation reactions with neighbouring elements, carbon and oxygen, whose abundances are nearly equal and which are the most abundant elements next to helium [e.g. see Cartright e t al. 29)]. A number of calculations have been made to estinaate the expected ratios of Be/B (or i OBe/Be) in cosmic rays for the survival and the decay of 10Be during the propagation phase, and these have been compared with available experimental ratios for estimating the mean lifetime of cosmic rays 30-33). In all these calculations the value of o( 1°Be) in oxygen used was about a factor of 3 to 4 lower than those measured in the present work. The value of 0"(I °Be) in oxygen was taken as 1/3.5 of the value in carbon, whereas the present work yields a( 1°Be) in oxygen as nearly equal to or slightly higher than a(~°Be) in carbon. Therefore if the present cross sections are accepted it is necessary to re-evaluate the estimates of the lifetime of cosmic rays based on this method.

We are extremely grateful to the late Prof. R. Bernas for providing the concentrates of the irradiated water targets, and to Dr. F. Yiou for comments and discussions. We are also beholden to Prof. Bernas for extensive discussions during various phases of this work until his sudden death in July 1971.

320

B.S. A M I N et al.

References 1) B. Peters, Pontificiae Acad. Sci. Scripts Varia 25 (1963) 1; S. Hayakawa, K. Ito and Y. Terashima, Prog. Theor. Phys. suppl. 6 (1958) 1 2) J. R. Arnold, M. Honda and D. Lal, J. Geophys. Res. 66 (1961) 3519 3) B. Peters, Proc. Ind. Acad. Sci. A41 (1955) 67 4) M. Honda and D. Lal, Nucl. Phys. 51 (1964) 363 5) F. Yiou, Ann. of Phys. 3 (1968) 169 6) F. Yiou, M. Baril, J. Dufaure de Citrea, P. Fontes, E. Gradstajn and R. Bernas, Phys. Rev. 166 (1968) 968 7) M. Jung, C. Jacquot, C. Baixeras-Aigualeela, R. Schmiff, H. Braun and L. Girardin, Phys. Roy. 188 (1969) 1517 8) D. Lal and D. R. Schink, Rev. Sci. Instr. 31 (1960) 395 9) C. M. Lederer, M. Hollander and 1. Perlman, Table of isotopes (Wiley, New York, 1967) p. 594 10) B. S. Amin, D. P. Kharkar and D. Lal, Deep Sea Rcs. 13 (1966) 805 11) J. R. Arnold, Science 124 (1956) 584 12) P. S. Goel, Nature 223 (1969) 1263 13) B. S. Amin, M.Sc. thesis, Bombay University, 1970 14) B. S. Amin, D. Lal and B. L. K. Somayajulu, to be published 15) F. Yiou, R. Siede and. R. Bernas, J. Geophys. Res. 74 (1969) 2447 16) P. A. Benioff, Phys. Rev. 119 (1960) 316 17) M. Honda and D. Lal, Phys. Rev. 118 (1960) 1618 18) G. Albouy, J. P. Cohen, M. Gusakow, N. Poffe, H. Sergolle and L. Valentin, Phys. Lett. 2 (1962) 306 19) L. Valentin, G. Albouy, J. P. Cohen and M. Gusako~, Phys. Lett. 7 (1963) 163 20) G. V. S. Rayudu, Can. J. Chem. 42 (1964) 1149 21) G. V. S. Rayudu, J. lnorg. Nucl. Chem. 30 (1968) 2311 22) P. L. Reeder, J. lnorg. Nucl. Chem. 27 (1965) 1879 23) J. Tobailem, C. H. Lassus St. Genies and L. LeVeque, CEA-N-1466 (1971) 24) P. Fontes, C. Perron, J. Lestringuez, F. Yiou and R. Bernas, Nucl. Phys. A165 11971) 405 25) D. Lal and. B. Peters, Handbuch der Phys. 46 (1967) 551 26) F. Yiou and G. M. Raisbeck, prcprint, 1972 27) G. M. Raisbeck and F. Yiou, Nature 233 (1971) 73 28) J. B. Cumming, Ann. Rev. Nucl. Sci 13 (1963) 261 29) B. G. Cartwright, M. Garcia-Munoz and J. A. Simpson, Proc. 12th Int. Conf. on cosmic rays, Hobart, vol. 1 (1971) p. 215 30) M. M. Shapiro and R. Silbcrberg, Ann. Rex'. Nucl. Sci. 20 (1970) 323 31) M. M. Shapiro and R. Silberberg, Acta Phys. Hung. 29, suppl. I (1970) 485 32) F. W. O'Dell, M. M. Shapiro, R. Silberberg and C. H. Tsao, Proc. 12th Int. Conf. on cosmic rays, Hobart, vol. 1 (1971) p. 197 33) G. M. Raisbeck and F. Yiou, Phys. Rev. Lett. 27 (1971) 875 34) E. Gradsztajn and M. Salomc, to bc published