- Email: [email protected]
Hyperfragment production and decay and the non-mesic absorption rate for stopping K− mesons in neon
Volume 27B, number
THE
8
PHYSICS
HYPERFRAGMENT NON-MESIC ABSORPTION
LETTERS
PRODUCTION RATE FOR
16 September
AND DECAY STOPPING K-
AND MESONS
1968
IN NEON*
H. 0. COHN Oak Ridge
National
Laboratory,
Oak Ridge,
Tennesse,
USA
R. D. McCULLOCH Computing
Technology
Center,
Union Carbide
Nuclear
Division,
Oak Ridge,
Tennessee,
USA
and W. M. BUGG r, G. T. CONDO f, N. GARRETT and L. M. TUCKER University of Tennessee, Knoxville, Tennessee, USA Received
17 August 1968
we have measured the hyperfragment prodIn a bubble chamber study of stopping K- + neon interactions, uction probability (trapped A events) following K- absorption to be 9.5 l 3%. The mesic decay probability for these hvnerfrazments was less than 9% and the multi-nucleon or non-mesic K- absorption probability was 22 f 4%:
The formation of hyperfragments following the absorption of K- mesons by complex nuclei has long been known to be one of the principal final states arising from nuclear K- absorption. However, because nearly all experiments utilizing nuclei more massive than helium have been performed either in nuclear emulsion or propanefreon bubble chambers, the determination of the rate of hyperfragment formation for specific nuclei is possible only by delineating either the fraction of K- captures occurring on each nuclear species present or by distinguishing the nucleus involved in each individual K- absorption. Since these procedures are inexact and since experiments with similar objectives [l-3] have occasionally led to diverse results, we present here our data concerning the rate of hyperfragment formation and the mesic (r-) decay probability for those hyperfragments following the absorption of stopping K- mesons in a neon-hydrogen mixture. The presence of hydrogen in the target causes little difficulty since K- p interactions, at rest, have been exhaustively studied [4] and since in about t of the hydrogen captures the unique signature of a collinear S*nr pair is produced. The data were obtained from an exposure of * Research jointly sponsored by the U.S.Atomic Energy Commission under contract with the Union Carbide Corporation and by the University of Tennessee. p Also Oak Ridge National Laboratory, Oak Ridge, Tennessee.
the Columbia-BNL 30 inch bubble chamber filled with = 24 mole per cent neon-hydrogen mixture. Stopping K- mesons have been distinguished by curvature measurements on the incident K- track. A study of curvature measurements on collinear C 71events indicates that a nearly pure sample of stopping K- interactions can be achieved by restricting our sample to events where the curvature of the incident track is less than 34.5 cm but greater than 25.5 cm as measured on the last 15 cm of track length. The lower limit was imposed to reject events where the incident Ksuffered scattering sufficient to render a considerable uncertainty in its momentum. Elimination of the K-p interactions yields a sample of ‘762 stopping K- interactions in neon. The hyperfragment production probability, P, folowing Ii- absorption by complex nuclei is determined in this experiment by noting that the final state must contain a hyperon (2 or A) or a hyperfragmbnt. Thus, if a hyperon is not emitted from a K- absorption, it must have been trapped in the residual nucleus or a fragment thereof. Because of the inferior resolving power of a bubble chamber relative to nuclear emulsion, all such trapped A events are referred to as hyperfragments. This is convenient since the nuclear absorption of a stopping K- is very unlikely to yield a hyperfragment of sufficient momentum to travel a discernible distance from the primary K- interaction. (No unambiguous example of 527
Volume 27B, number 8
PHYSICS LETTERS Table 1
Multinueleon rate for various pion reabsorption prob-
ability. Pion reabsorption probability
Multinucleon Kabsorption probability
0
0.27
5%
0.23
7%
0.22
10%
0.19
h y p e r f r a g m e n t p r o d u c t i o n h a s been o b s e r v e d in this e x p e r i m e n t . ) F r o m the 762 stopping K- a b s o r p t i o n s by neon, we have o b s e r v e d the following n u m b e r s of h y p e r ons: 315 A ~ p n - , 42 ~,+ ~ n+n, 60 D - ~ n - n and 2 ~ - s t a r s with a A. T h e s e n u m b e r s m u s t be ~orrected for: (a) finite s i z e of the c h a m b e r (b) ~ - i n t e r a c t i o n s (c) z e r o length d e c a y s of A and ~ ± , (d) o t h e r decay m o d e s of A and D+. We e s t i m a t e that (a), through a c o n s e r v a t i v e c h o i c e of fiducial v o l u m e , would add at m o s t a s i n g l e A to the s a m p l e and should p r o p e r l y be i g n o r e d . C o r r e c t i o n (b), deduced f r o m an independent study of ~'.- i n t e r a c t i o n s , c o n t r i b u t e s 16 2;-. A study of the length d i s t r i b u t i o n s of the and A d e c a y s i n d i c a t e s the following l o s s e s : 7% f or A d e c a y s , 15% f o r Z + d e c a y s , and 15% f o r Y.- d e c a y s . In all i m p o r t a n t detail, t h e s e c o r r e c ti ons a r e in e s s e n t i a l a g r e e m e n t with p r e v i o u s bubble c h a m b e r s t u d i e s [5,6]. F i n a l l y , c o r r e c t i n g f o r the ~ + ~ p~O and the A ~ nlr ° d e c a y m o d e s y i e l d s a total of 506 A, 99 E+, and 86 E- h y p e r 9ns e m i t t e d f r o m o u r 762 events. The p r o b a b il it y of A e m i s s i o n is 66% ± 4% which c o m p a r e s f a v o r ably to the 60 ± 3% r e p o r t e d by B u r a n et al. [6] in a f r e o n bubble c h a m b e r , e s p e c i a l l y when account is taken of the h e a v i e r liquid used t h e r e and the c o n c o m i t a n t i n c r e a s e in the A t r a p p i n g p r o b a b i l i ty e x p e c t e d f o r h e a v i e r nuclei. U n d e r the r e a s o n a b l e a s s u m p t i o n that all e v e n t s not containing a A o r ~ hyperon r e p r e s e n t h y p e r f r a g m e n t f o r m a t i o n , it is found that P = 9.5 ± 3~o. T h i s f i g u r e is in good a c c o r d with the value of 8 + 2% r e p o r t e d by L e m o n n e et al. [7] f o r K- capt u r e s in light e m u l s i o n nuclei (CNO) but is s o m e what s m a l l e r than the 17 ± 3~0 deduced by Knight et al. [5] f o r K- cap t u r e by c a r b o n and f l u o r i n e in a propane-freon chamber. 528
16 September 1968
It is a l s o p o s s i b l e to e s t i m a t e the f r a c t i o n of the h y p e r f r a g m e n t s f o r m e d by K- a b s o r p t i o n in neon which decay m e s i c a l l y (into the n- mode). Since h y p e r f r a g m e n t decay in our c h a m b e r app e a r s to o c c u r at the p r i m a r y K- i n t e r a c t i o n v e r tex, mesically_ (Tr-) d e c a y i n g h y p e r f r a g m e n t s can give r i s e to K s t a r s which a p p e a r to yield two ~- m e s o n s . This s i g n a t u r e can a l s o a r i s e f r o m z e r o length A(-~ pTr-) decay when the A production was a c c o m p a n i e d by a 7r meson. Our s a m p l e contained six double n- e v e n t s of which 6 + 3 w e r e e s t i m a t e d to a r i s e f r o m z e r o length A d ecay. Thus no m o r e than t h r e e m e s i c a l l y d e c a y ing h y p e r f r a g m e n t s a r e p r o d u c e d in conjunction with ~- m e s o n s at the p r i m a r y K- i n t e r a c t i o n . E m u l s i o n w o r k [7] has indicated that about half of the h y p e r f r a g m e n t s p r o d u c e d in light nuclei a r e a c c o m p a n i e d by a 7r-. Adopting this f i g u r e we can st at e that the m e s i c decay p r o b a b i l i t y f o r h y p e r f r a g m e n t s (of any m a s s ) p r o d u c e d by Ka b s o r p t i o n by neon is ~ ~ = 9%. T h i s low value s u g g e s t s that m o s t of the h y p e r f r a g m e n t s p r o d uced in this e x p e r i m e n t a r e quite m a s s i v e (A 15 - 20). The m u l t i n u c l e o n o r n o n - m e s i c (K- + N + N --' Y + N) K- a b s o r p t i o n p r o b a b i l i t y can a l s o be deduced f r o m the above data. Using the z e r o length E and A d ecay c o r r e c t i o n s d e s c r i b e d above and a s s u m i n g c h a r g e independence, we find that 555 of the 762 e v e n t s (73 + 4%) give r i s e to the e m i s s i o n of a pion. Thus the upper l i m i t f o r the n o n - m e s i c K- a b s o r p t i o n p r o b a b i l i t y in neon is 27 + 4%. Any pion r e a b s o r p t i o n in the nucleus in which it was c r e a t e d would r e d u c e this f i g u r e as shown in table 1. It is c o m m o n l y a s s u m e d that the pion r e a b s o r p t i o n p r o b ab i l i t y for n u c l e a r e m u l s i o n e x p e r i m e n t s is ~ 10% [8]. H o w e v e r , the r e c e n t data of D av i s et al. [9] i m p l y that n- r e a b s o r p t i o n f o r c a p t u r e s in heavy n u c l e i o c c u r s with about twice the f r e q u e n c y it does f o r light n u cl eu s capture. It would, t h e r e f o r e , s e e m m o r e prudent to a s s u m e a pion r e a b s o r p t i o n p r o b ab i l i t y of ~ 7% in neon. Under this a s s u m p t i o n , we find that the n o n - m e s i c K- a b s o r p t i o n p r o b ab i l i t y in neon is 22 ± 4%. This value is c o n s i s t e n t with the 9 - 30% r e p o r t e d by J o n e s et al. [10] for an a v e r a g e o v e r the e l e m e n t s of n u c l e a r e m u l s i o n . It is a l s o c o m p a r a b l e with the 17 ± 4% r e p o r t e d by the H e l i u m Bubble C h a m b e r C o l l a b o r a t i o n group [1 ] ], w e r e pion r e a b s o r p t i o n was ignored. N e v e r t h e l e s s the s i m i l a r i t y of the h el i u m , neon, and e m u l s i o n data indicate that f o r nuclei h e a v i e r than A = 3, no m a r k e d v a r i a t i o n of the m u l t i n u cleon cap t u r e r a t e with A is to be expected.
Volume 27B, n u m b e r 8
PHYSICS
W e w i s h to t h a n k D r . A . G . P r o d e l l a n d D r . J . R . S a n f o r d a s w e l l a s t h e 30 i n c h b u b b l e c h a m ber crew for a successful run, particularly inasmuch as this was the first run with such a high c o n c e n t r a t i o n of n e o n . W e a r e a l s o g r a t e f u l to D r . F. R. H u s o n f o r h e l p i n g u s to m o d i f y o u r p r o grams for a neon-hydrogen mixture.
References 1. D.H. Davis, M. C s e j t h e y - B a r t h , J. Sacton, B . D . J o n e s . B. Sanjeevaiah and J. Z a k r z e w s k i , Nuovo Cimento 22 (1961) 275. 2. R. C e s t e r . G. Ciocchetti, A. Debenedetti. A. M a r z a r i C h i e s a , G, Rinaudo, C. Deney, K. Gottstein and W. W. P u s c h e l , N u o v o C i m e n t o 22 (1961) ]069.
LETTERS
16 S e p t e m b e r 1968
3. A. Filipkowski, E. Marquit, E. Skrzypczak and A . W r o b l e w s k i , Nuovo Cimento 25 (1962) 1. 4. W . E . H u m p h r e y and R . R . R o s s , P h y s . Rev. 127 (1962) 1305. 5. W . L . K n i g h t , F . R . Stannard. F. O p p e n h e i m e r . B. Rickey and R. Wilson. Nuovo Cimento 32 (1964) 598. 6. T. Buran~ A . G . F r o d e s e n , O, Skjeggestad. H. T~fte and I. Vegge, Nucl. P h y s i c s 87 (1967) 532. 7. J. L e m o n n e , C. Mayeur. J. Sacton, D.H. Davis. D. A. Garbutt and J. Allen, Nuovo Cimento 34 (1964) 529. 8. K- European Collaboration, Nuovo Cimento 14 (1959) 315. 9. D . H . D a v i s , S . P . Lovell. M. C s e j t h e y - B a r t h , J. Sacton, G. Schorochoff, M.O. 'Reilly, Nucl. P h y s i c s B1 (1967) 434. 10. K- European Collaboration, Nuovo Cimento 19 (1961) 1077. 11. Helium Bubble C h a m b e r Collaboration Group; P r o c . 1960 Annual Intern. Conf. on High e n e r g y p h y s i c s , R o c h e s t e r ( I n t e r s c i e n c e P u b l i s h e r s , Inc.. New York) p. 426.
529
THE
8
PHYSICS
HYPERFRAGMENT NON-MESIC ABSORPTION
LETTERS
PRODUCTION RATE FOR
16 September
AND DECAY STOPPING K-
AND MESONS
1968
IN NEON*
H. 0. COHN Oak Ridge
National
Laboratory,
Oak Ridge,
Tennesse,
USA
R. D. McCULLOCH Computing
Technology
Center,
Union Carbide
Nuclear
Division,
Oak Ridge,
Tennessee,
USA
and W. M. BUGG r, G. T. CONDO f, N. GARRETT and L. M. TUCKER University of Tennessee, Knoxville, Tennessee, USA Received
17 August 1968
we have measured the hyperfragment prodIn a bubble chamber study of stopping K- + neon interactions, uction probability (trapped A events) following K- absorption to be 9.5 l 3%. The mesic decay probability for these hvnerfrazments was less than 9% and the multi-nucleon or non-mesic K- absorption probability was 22 f 4%:
The formation of hyperfragments following the absorption of K- mesons by complex nuclei has long been known to be one of the principal final states arising from nuclear K- absorption. However, because nearly all experiments utilizing nuclei more massive than helium have been performed either in nuclear emulsion or propanefreon bubble chambers, the determination of the rate of hyperfragment formation for specific nuclei is possible only by delineating either the fraction of K- captures occurring on each nuclear species present or by distinguishing the nucleus involved in each individual K- absorption. Since these procedures are inexact and since experiments with similar objectives [l-3] have occasionally led to diverse results, we present here our data concerning the rate of hyperfragment formation and the mesic (r-) decay probability for those hyperfragments following the absorption of stopping K- mesons in a neon-hydrogen mixture. The presence of hydrogen in the target causes little difficulty since K- p interactions, at rest, have been exhaustively studied [4] and since in about t of the hydrogen captures the unique signature of a collinear S*nr pair is produced. The data were obtained from an exposure of * Research jointly sponsored by the U.S.Atomic Energy Commission under contract with the Union Carbide Corporation and by the University of Tennessee. p Also Oak Ridge National Laboratory, Oak Ridge, Tennessee.
the Columbia-BNL 30 inch bubble chamber filled with = 24 mole per cent neon-hydrogen mixture. Stopping K- mesons have been distinguished by curvature measurements on the incident K- track. A study of curvature measurements on collinear C 71events indicates that a nearly pure sample of stopping K- interactions can be achieved by restricting our sample to events where the curvature of the incident track is less than 34.5 cm but greater than 25.5 cm as measured on the last 15 cm of track length. The lower limit was imposed to reject events where the incident Ksuffered scattering sufficient to render a considerable uncertainty in its momentum. Elimination of the K-p interactions yields a sample of ‘762 stopping K- interactions in neon. The hyperfragment production probability, P, folowing Ii- absorption by complex nuclei is determined in this experiment by noting that the final state must contain a hyperon (2 or A) or a hyperfragmbnt. Thus, if a hyperon is not emitted from a K- absorption, it must have been trapped in the residual nucleus or a fragment thereof. Because of the inferior resolving power of a bubble chamber relative to nuclear emulsion, all such trapped A events are referred to as hyperfragments. This is convenient since the nuclear absorption of a stopping K- is very unlikely to yield a hyperfragment of sufficient momentum to travel a discernible distance from the primary K- interaction. (No unambiguous example of 527
Volume 27B, number 8
PHYSICS LETTERS Table 1
Multinueleon rate for various pion reabsorption prob-
ability. Pion reabsorption probability
Multinucleon Kabsorption probability
0
0.27
5%
0.23
7%
0.22
10%
0.19
h y p e r f r a g m e n t p r o d u c t i o n h a s been o b s e r v e d in this e x p e r i m e n t . ) F r o m the 762 stopping K- a b s o r p t i o n s by neon, we have o b s e r v e d the following n u m b e r s of h y p e r ons: 315 A ~ p n - , 42 ~,+ ~ n+n, 60 D - ~ n - n and 2 ~ - s t a r s with a A. T h e s e n u m b e r s m u s t be ~orrected for: (a) finite s i z e of the c h a m b e r (b) ~ - i n t e r a c t i o n s (c) z e r o length d e c a y s of A and ~ ± , (d) o t h e r decay m o d e s of A and D+. We e s t i m a t e that (a), through a c o n s e r v a t i v e c h o i c e of fiducial v o l u m e , would add at m o s t a s i n g l e A to the s a m p l e and should p r o p e r l y be i g n o r e d . C o r r e c t i o n (b), deduced f r o m an independent study of ~'.- i n t e r a c t i o n s , c o n t r i b u t e s 16 2;-. A study of the length d i s t r i b u t i o n s of the and A d e c a y s i n d i c a t e s the following l o s s e s : 7% f or A d e c a y s , 15% f o r Z + d e c a y s , and 15% f o r Y.- d e c a y s . In all i m p o r t a n t detail, t h e s e c o r r e c ti ons a r e in e s s e n t i a l a g r e e m e n t with p r e v i o u s bubble c h a m b e r s t u d i e s [5,6]. F i n a l l y , c o r r e c t i n g f o r the ~ + ~ p~O and the A ~ nlr ° d e c a y m o d e s y i e l d s a total of 506 A, 99 E+, and 86 E- h y p e r 9ns e m i t t e d f r o m o u r 762 events. The p r o b a b il it y of A e m i s s i o n is 66% ± 4% which c o m p a r e s f a v o r ably to the 60 ± 3% r e p o r t e d by B u r a n et al. [6] in a f r e o n bubble c h a m b e r , e s p e c i a l l y when account is taken of the h e a v i e r liquid used t h e r e and the c o n c o m i t a n t i n c r e a s e in the A t r a p p i n g p r o b a b i l i ty e x p e c t e d f o r h e a v i e r nuclei. U n d e r the r e a s o n a b l e a s s u m p t i o n that all e v e n t s not containing a A o r ~ hyperon r e p r e s e n t h y p e r f r a g m e n t f o r m a t i o n , it is found that P = 9.5 ± 3~o. T h i s f i g u r e is in good a c c o r d with the value of 8 + 2% r e p o r t e d by L e m o n n e et al. [7] f o r K- capt u r e s in light e m u l s i o n nuclei (CNO) but is s o m e what s m a l l e r than the 17 ± 3~0 deduced by Knight et al. [5] f o r K- cap t u r e by c a r b o n and f l u o r i n e in a propane-freon chamber. 528
16 September 1968
It is a l s o p o s s i b l e to e s t i m a t e the f r a c t i o n of the h y p e r f r a g m e n t s f o r m e d by K- a b s o r p t i o n in neon which decay m e s i c a l l y (into the n- mode). Since h y p e r f r a g m e n t decay in our c h a m b e r app e a r s to o c c u r at the p r i m a r y K- i n t e r a c t i o n v e r tex, mesically_ (Tr-) d e c a y i n g h y p e r f r a g m e n t s can give r i s e to K s t a r s which a p p e a r to yield two ~- m e s o n s . This s i g n a t u r e can a l s o a r i s e f r o m z e r o length A(-~ pTr-) decay when the A production was a c c o m p a n i e d by a 7r meson. Our s a m p l e contained six double n- e v e n t s of which 6 + 3 w e r e e s t i m a t e d to a r i s e f r o m z e r o length A d ecay. Thus no m o r e than t h r e e m e s i c a l l y d e c a y ing h y p e r f r a g m e n t s a r e p r o d u c e d in conjunction with ~- m e s o n s at the p r i m a r y K- i n t e r a c t i o n . E m u l s i o n w o r k [7] has indicated that about half of the h y p e r f r a g m e n t s p r o d u c e d in light nuclei a r e a c c o m p a n i e d by a 7r-. Adopting this f i g u r e we can st at e that the m e s i c decay p r o b a b i l i t y f o r h y p e r f r a g m e n t s (of any m a s s ) p r o d u c e d by Ka b s o r p t i o n by neon is ~ ~ = 9%. T h i s low value s u g g e s t s that m o s t of the h y p e r f r a g m e n t s p r o d uced in this e x p e r i m e n t a r e quite m a s s i v e (A 15 - 20). The m u l t i n u c l e o n o r n o n - m e s i c (K- + N + N --' Y + N) K- a b s o r p t i o n p r o b a b i l i t y can a l s o be deduced f r o m the above data. Using the z e r o length E and A d ecay c o r r e c t i o n s d e s c r i b e d above and a s s u m i n g c h a r g e independence, we find that 555 of the 762 e v e n t s (73 + 4%) give r i s e to the e m i s s i o n of a pion. Thus the upper l i m i t f o r the n o n - m e s i c K- a b s o r p t i o n p r o b a b i l i t y in neon is 27 + 4%. Any pion r e a b s o r p t i o n in the nucleus in which it was c r e a t e d would r e d u c e this f i g u r e as shown in table 1. It is c o m m o n l y a s s u m e d that the pion r e a b s o r p t i o n p r o b ab i l i t y for n u c l e a r e m u l s i o n e x p e r i m e n t s is ~ 10% [8]. H o w e v e r , the r e c e n t data of D av i s et al. [9] i m p l y that n- r e a b s o r p t i o n f o r c a p t u r e s in heavy n u c l e i o c c u r s with about twice the f r e q u e n c y it does f o r light n u cl eu s capture. It would, t h e r e f o r e , s e e m m o r e prudent to a s s u m e a pion r e a b s o r p t i o n p r o b ab i l i t y of ~ 7% in neon. Under this a s s u m p t i o n , we find that the n o n - m e s i c K- a b s o r p t i o n p r o b ab i l i t y in neon is 22 ± 4%. This value is c o n s i s t e n t with the 9 - 30% r e p o r t e d by J o n e s et al. [10] for an a v e r a g e o v e r the e l e m e n t s of n u c l e a r e m u l s i o n . It is a l s o c o m p a r a b l e with the 17 ± 4% r e p o r t e d by the H e l i u m Bubble C h a m b e r C o l l a b o r a t i o n group [1 ] ], w e r e pion r e a b s o r p t i o n was ignored. N e v e r t h e l e s s the s i m i l a r i t y of the h el i u m , neon, and e m u l s i o n data indicate that f o r nuclei h e a v i e r than A = 3, no m a r k e d v a r i a t i o n of the m u l t i n u cleon cap t u r e r a t e with A is to be expected.
Volume 27B, n u m b e r 8
PHYSICS
W e w i s h to t h a n k D r . A . G . P r o d e l l a n d D r . J . R . S a n f o r d a s w e l l a s t h e 30 i n c h b u b b l e c h a m ber crew for a successful run, particularly inasmuch as this was the first run with such a high c o n c e n t r a t i o n of n e o n . W e a r e a l s o g r a t e f u l to D r . F. R. H u s o n f o r h e l p i n g u s to m o d i f y o u r p r o grams for a neon-hydrogen mixture.
References 1. D.H. Davis, M. C s e j t h e y - B a r t h , J. Sacton, B . D . J o n e s . B. Sanjeevaiah and J. Z a k r z e w s k i , Nuovo Cimento 22 (1961) 275. 2. R. C e s t e r . G. Ciocchetti, A. Debenedetti. A. M a r z a r i C h i e s a , G, Rinaudo, C. Deney, K. Gottstein and W. W. P u s c h e l , N u o v o C i m e n t o 22 (1961) ]069.
LETTERS
16 S e p t e m b e r 1968
3. A. Filipkowski, E. Marquit, E. Skrzypczak and A . W r o b l e w s k i , Nuovo Cimento 25 (1962) 1. 4. W . E . H u m p h r e y and R . R . R o s s , P h y s . Rev. 127 (1962) 1305. 5. W . L . K n i g h t , F . R . Stannard. F. O p p e n h e i m e r . B. Rickey and R. Wilson. Nuovo Cimento 32 (1964) 598. 6. T. Buran~ A . G . F r o d e s e n , O, Skjeggestad. H. T~fte and I. Vegge, Nucl. P h y s i c s 87 (1967) 532. 7. J. L e m o n n e , C. Mayeur. J. Sacton, D.H. Davis. D. A. Garbutt and J. Allen, Nuovo Cimento 34 (1964) 529. 8. K- European Collaboration, Nuovo Cimento 14 (1959) 315. 9. D . H . D a v i s , S . P . Lovell. M. C s e j t h e y - B a r t h , J. Sacton, G. Schorochoff, M.O. 'Reilly, Nucl. P h y s i c s B1 (1967) 434. 10. K- European Collaboration, Nuovo Cimento 19 (1961) 1077. 11. Helium Bubble C h a m b e r Collaboration Group; P r o c . 1960 Annual Intern. Conf. on High e n e r g y p h y s i c s , R o c h e s t e r ( I n t e r s c i e n c e P u b l i s h e r s , Inc.. New York) p. 426.
529