c protons with heavy nuclei in emulsion

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Nuclear Phystcs 70 (1965) 567--573; ( ~ North-Holland Publishing Co., Amsterdam

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Not to be reproduced by photopnnt

or microfilm

without written permisston from the publisher

EMISSION OF FAST DEUTERONS IN INTERACTIONS OF 24 GeV/c PROTONS WITH HEAVY NUCLEI IN EMULSION T. SANIEWSKA, E. S K R Z Y P C Z A K and P. ZIELIbISKI

Instttute of Experimental Phystcs, University of Warsaw and

Institute of Nuclear Research, Warsaw Received 5 February 1965 In mteracttons of 24 GeV/c protons wLth heavy nuclei m emulsion, the frequency of emlsstun and the angular and momentum dlstributmns of fast deuterons m the momentum interval 400-900 MeV/c are mveshgated. A very copious ermssmn of fast deuterons m large stars (1.8=[=0.3 per star) ks observed. The experimental results are discussed from the pomt of view of the model presented by Butler and Pearson.

Abstract:

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N U C L E A R REACTIONS Ag, Br(p), E = 24 GeV; measured deuteron productton (0, Ed), proton production (0, Ep).

1.

Introduction

A copious emission of deuterons with energies of several hundreds of MeV in cosmic ray interactions has been observed in the early emulsion work in Bristol 1). A number of counter experiments 2-8) in the accelerator region has recently shown that deuterons with energies up to several GeV are frequently emittdd in nucleon - nucleus and nucleon - nucleon collisions. Several suggestions 9-14) have been proposed in order to explain why a high-energy proton and neutron frequently leave a nucleus in the form of such a loosely bound nuclear structure. In particular Butler and Pearson 14) have recently introduced a model which is in quantitative agreement with the results of the CERN and Brookhaven experiments concerning nucleon-nucleus interactions 3-7). It seems that this model can be extended by considering various classes of interactions with a given species of target nucleus. Experimentally, new information on the fast deuteron emission could be obtained along the same lines using techniques which permit observation of individual events. The aim of the present work is to investigate the emission of 420-860 MeV/c deuterons in interactions of 24 GeV/c protons with heavy nuclei in emulsion and in particular to observe how the emission of deuterons depends on the energy transfer to the target nucleus. The experimental results are discussed in the light of the ButlerPearson model. 567

T. SANIEWSKA et al.

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2. Experimental Procedure

A stack of Ilford G5 emulsion exposed to the beam of 24 GeV/c protons has been used in this experiment. An area scan of the central pellicles of the stack has been performed for proton-induced stars. After an examination of the secondary particles a sample of 794 interactions with N h > 8 has been collected. Here N h denotes the number of heavily ionizing secondary particles, defined by the condition g > 1.4gplateau corresponding to particle velocities fl < 0.7. The sample represents interactions of incident protons wlth the nuclei Ag and Br, the interactions being characterized by large energy transfers to the target nuclei. For each star the number of heavily ionized particles has been determined. The secondary particles giving flat tracks (dip angle less than 6 °) characterized by ranges exceeding 2500/am and ionization exceeding 3.5 times the plateau value have been analysed. Geometrical corrections resulting from acceptance of tracks only with dip angle less than 6 degrees are introduced later in the analysis of the results. The sample of secondary particles accepted consists of 372 particles. For each analysed particle coming to rest in the emulsion the following data are determined: (a) the angle of emission with respect to the direction of the primary proton, (b) the range in emulsion, (c) the ionization determined by gap counting on the track segment from the residual range of 1000 pm up to the parent star or up to the residual range of 5000 pm. A sample of 24 particles has been analysed additionally by the constant sagitta method in order to determine their masses independently. The results are consistent with the identification based on gap counting. A few e-particles present among the analysed tracks are easily identified by ray counting. The sample of 30 tracks which do not come to rest in the emulsion (left the stack or interacted m flight) has been analysed by applying other methods such as measurements of ionization changes along the track and/or multiple Coulomb scattering. This information allows the identification of non-stopping particles in most of the cases. As a result of the analysis the following information is available for each of the analysed secondary particles: (i) the number of heavily ionizing particles in the parent star, (ii) the nature of the particles (in ~ 92 ~ of the particles the identification was unambiguous), (iii) the energy and momentum at the emission, (iv) the angle of emission and the value of the corresponding geometrical correction factor. 3. Results

3.1. FREQUENCY OF EMISSION The following (corrected)yields of particles with momenta (0.21-0.43) GeV/c per nucleon have been obtained in 794 stars with N h > 8:0.9-t-0.1 deuterons per

HEAVY NUCLEI

569

star, 3.0+ 0.2 protons, 0.15 +0.04 helium nuclei, the rough estimate of the number of tritons being 0.04 per star. The deuteron-to-proton rato is 0.30_ 0.05. The ratio is in approximate agreement with the early emulsion work ~) and the new results from the counter experiments 3- 7). The comparison of the absolute cross section is hardly possible due to differences in the m o m e n t u m intervals under consideration, the differences in the kind of target nuclei used and the uncertainty of the target efficiency in some of the counter experiments. Number

1

of stors 60

~?hTrs N#h obsorved

m1

protons

i

Starm wd,s, observed

I

deuterons

xo i

Number OPSt~FS

zo

20]

¢o

(a)

5

ro

m

m

m

m

m

4o %

~0

i m

s5

(b)

m

2~

m

35

40 ,%

Fig. 1. The distribution o f the n u m b e r of heavily ionizing particles Nl~ in stars. The shaded areas correspond to the stars m which b o t h deuteron and p r o t o n are observed, a) deuteron emission, b) p r o t o n emission.

The distributions of the number of heavily ionizing particles N h in the stars containing identified deuterons or identified protons are shown in fig. 1a and b, respectively. F o r the samples of stars characterized by condition 8 < Nh < 17 and 17 < N h we obtain the deuteron and proton yields given in table 1. A striking feature of the copious deuteron emission is the fact that the deuterons are emitted mainly from stars with a large number of heavily ionizing particles correspondmg to a large energy transfer to the target nuclei. In fact about 60 ~ of the deuterons are emitted in stars with N h > 17. These stars is) constitute about 30 of the total number of stars with N h > 8. TABL~ 1 The average n u m b e r o f deuterons and p r o t o n s per star in stars of different size 8
0.4 -t- 0.1 2 4~0.3

Nh>

17

I. 8 4- 0.3 4.2zk0.4

570

T. SANIEWSKA et al.

This feature of the deuteron emission appears both for the deuterons under consideration and heavier fragments, such as 8Li fragments, observed by Gajewski et al. 16), but the tendency is stronger in the case of deuterons. Neglecting the contribution to the deuteron emission in the interactions with Ag and Br nuclei from stars with N b < 8 and taking into account the fraction of stars with N h < 8 in emulsion and the composition of the emulsion, we obtain the value Pfotoos

Number of parhdes

F ~ = 20 ± Q3

300,

Deuterons Number of parttcles

2~.

F =22-*05

200

200-

loo.

~oo-

(b)

(a) -7.o

-0~

do

,d5

,1o

cosoc

-~o

-as

do

,d~

,to

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F i g . 2. The d~stnbution of the angle of emission: a) deuterons, b) protons. Here and m the following figure the histograms correspond to the corrected numbers; the shaded areas represent the actually measured numbers of particles.

5 0 0 + 5 0 mb as an estimate of the cross section of the deuteron emission in the momentum interval here considered in the interactions of the 24 GeV/c protons with A ~ 100 nuclei. 3.2. A N G U L A R DISTRIBUTIONS

The angular distributions o f the deuterons and protons are shown in fig. 2a and b, respectively. Both kinds of particle are forward collimated. The degree of this collimation - the forward-to-backward ratio - is similar for both kinds of particle within the limits of errors, the deuteron-to-proton ratio being approximately constant in all angular intervals.

HEAVY NUCLEI 3.3. M O M E N T U M

571

DISTRIBUTIONS

The momentum distributions of the deuterons and protons are shown in fig. 3a and b, respectively. While the distribution of the protons is approximately constant over the momentum interval under consideration, the frequency of the deuterons decreases with the increase of the momentum. Dividing the deuterons into two angular intervals we observe that this decrease is more pronounced for large angles of emission. Number of parttcles

Protons

5OO

450 4O0

Deuterons Number of parhcles 300

35O

250.

25o

2oo-

2oo

trio.

150.

I

3o0

Io0.

I

L_

50.

02.~

O.30

Q35

04O

fa)

Q45

Ply1

50.

[Gee/c]

025

03o

Fig. 3. The dlstrxbutlons of the momentum per nucleon,

O~

O4o

045

P/nucl~GeVT~

a) deuterons, b) protons.

4. Discussion We compare the experimental results with the predictions of the model presented by Butler and Pearson 14). The model connects the yields of deuterons and protons (ref. 14), formula (34))

no (p) = CF(p)I(R o)n~ (p),

(l)

where no and np are the yields of deuterons and protons with momentum p per nucleon, C is a constant (whose value is given in ref. 14)), F(p) = ~

1(p2 )~t m~cc2 + 1 ,

(2)

m is the nucleon mass and I(Ro) denotes a function (evaluated in ref. 14)) characterizing the target nucleus.

572

T. SANIEWSKA et al.

Our sample represents the interactions of protons with nuclei of average mass number A ~ 100 (bromine and silver nuclei). The spectrum of protons is approximately constant over the momentum interval here considered (see fig. 3b). Thus the formula (1) is reduced here to a proportionality of the yield of the deuterons to the function F(p). The function F(p) decreases with momentum, so does the deuteron yield (fig. 3a) showing a quantitative agreement with the prediction of the model. The Butler-Pearson model in its present form applies to an average interaction of the incoming particle with a target nucleus. In this form the model has been used for the interpretation of the aforementioned counter experiments. In emulsion, individual events are subject to observation and the stars can be divided into classes corresponding to various types of interactions. Our stars have been divided into two classes characterized by the conditions 8 < N~ < 17 and 17 < N h, respectively. Extending the vahdity of the Butler-Pearson model by assuming that the formula (1) holds for different classes of interactions in a given species of nucleus, we should expect a quadratic increase of the deuteron yield with the rise of the proton yield for different classes of interactions. The data given in table 1 are in agreement with this prediction of the extended Butler-Pearson model. There is no angular dependence in the formula (1). The observed momentum spectrum of the deuterons, however, is steeper at larger angles than at the smaller ones while the proton spectrum remains approximately constant. This observation may serve as an indication of contributions of other processes to the yield of the deuterons under consideration. There could be some contributions from the tail of the evaporation, from the direct and md~rect pick-up processes and from the knock-on of preformed deuteron clusters by cascade particles 9-13). It is also perhaps worthwhile to have in mind that the very high deuteron yield in the large stars, m which the target nuclei are splat anto many droplets, could be related to specific properties of heavy nuclei at large energy transfers comparable with the total binding energy of the nuclei. It might be e.g. that at large energy transfers a lowering of the average nuclear density favours the formation and ejection of the deuteron clusters m the whole body of a disintegrating nucleus. 5. Conclusions

A copious emission of fast (420-860 MeV/c) deuterons in interactions of 24 GeV/c protons with heavy nuclei in emulsion is observed. A very high yield of the deuterons (1.8 +0.3 per star) is found in large stars corresponding to large energy transfers to the target nuclei. Some features of emission of the deuterons can be described in terms of the model presented by Butler and Pearson. The authors are greatly indebted to Dr. K. Hansen, Copenhagen University, for lending the stack of emulsion. We would like to express our gratitude to Professor M. Danysz for his valuable comments. We appreciate the efficient help of Mrs. M. Pazdanowska, Mrs. B. Cleglak and Mrs. H. Ginter for scanning and measurements.

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References 1) C. F. Powell, P. H. Fowler and D. H Perkins, The study of elementary particles by the photographic method (Pergamon Press, London, 1959) chapt. XIII 2) L. S. Azhglrej et aL, JETP 33 (1957) 1185 3) V. T. Coccom et a l , Phys. Rev. Lett. 5 (1960) 19 4) G. yon Dardel, R. M. Mermod, G. Weber and K. Winter, Proc. 1960 Int. Conf. on High-Energy Physics, p. 837 5) L. Gllly et aL, Proc. 1960 Int. Conf. on High-Energy Physics, p. 808 6) V. L. Fitch, S. L. Meyer and P. A. Plroue, Phys. Rev 126 (1962) 1849 7) A. Schwarzschlld and C. Zupan616, Phys. Rev. 129 (1963) 854 8) G. Cocconl et a l , Phys. Lett. 7 (1963) 222 and references therein 9) B. H. Bransden, Proc. Phys. Soc. A65 (1952) 738 10) D. I. Blokhmtsev, JETP 33 (1957) 1295 11) I. S. Shapiro and V. M. Kolybasov, Nuclear Physics 49 (1963) 515 12) V. V. Balashov, A. N. Bojarkma and 1. Rotter, Nuclear Physics 59 (1965) 417 13) R. Hagedorn, Phys. Rev. Lett. 5 (1960) 276; N. Clm. 25 (1962) 1017 14) S. T. Butler and C. A. Pearson, Phys. Rev. 129 (1963) 836 15) J Bogdanowlcz et a l , Nuclear Physics 40 (1963) 270 16) W. Gajewskl et a l , Nuclear Physics 45 (1963) 27, N. A. Perfilov, O. V. Lozhkm and V. I. Ostroumov, Yadernye reaktsu pod delstvlem chastits vysoklkh energu (Acad. Sci. USSR, Moscow-Leningrad, 1962)