Negative pion capture on 4He with emission of neutron-proton, neutron-deuteron and neutron-triton pairs

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Nuclear Physics A216 (1973) 145--156; (~) North-HollandPublishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

NEGATIVE PION CAPTURE ON 4He WITH EMISSION OF NEUTRON-PROTON, NEUTRON-DEUTERON AND NEUTRON-TRITON PAIRS R. J. BARRETT t, j. McCARTHY, R. C. M I N E H A R T and K. ZIOCK

Physics Department, University of Virginia, Charlottesville, Virginia 22901, USA tt Received 23 July 1973 Abstract: The energies and opening angles for neutron-proton, and neutron-deuteron pairs emitted

after ~z- capture in 4He have been measured. The data are compared to available theoretical calculations. Both energetic protons and deuterons are strongly correlated in direction with an energetic neutron, being emitted at approximately 180 ° to each other. A strong enhancement at the high-energy end o f the deuteron spectrum is attributed to a strong final state interaction

between the remaining two neutrons. NUCLEAR REACTIONS 4He(n-, np), 4He(~z-, nd), E = 0 MeV; measured a(E., Ep, 0,, 0p). Liquid target. 1. Introduction

Absorption of negative pions on nuclei may be a sensitive mechanism for the study of clustering effects. For example, Kolybasov ~' 2) has calculated the pion capture process in several nuclei from the point of view of a simple pole model involving absorption of the pion on an c~-cluster. Although hampered by rather incomplete experimental data for absorption on a free a-particle, the calculations seemed to have some qualitative success. Recent experimental work 3-6) has tended to confirm the importance of clustering effects in pion capture. In the work of Lee e t al. 5, 6) who measured the energies and opening angles for (n, p), (n, d) and (n, t) pairs emitted after n - capture in 12C, the neutron energy distributions were in good agreement with Kolybasov's results, and the angular distributions were in at least qualitative agreement. These results have created renewed interest in a detailed experimental study of pion absorption in free helium nuclei which - one might hope - will ultimately lead to a better understanding of the role of a-clusters in more complex nuclei. Besides it relevance to a-particle clustering effects in nuclei, pion capture in 4He is of course interesting in its own right. Attempts to calculate n - capture on 4He have been made by Eckstein 7) who assumed point capture on two nucleons and by Koltun and Reitan 8) who employed a single nucleon capture model with strong "rescattering" effects. t Present address: Los Alamos Scientific Laboratory, Los Alamos, New Mexico, 87544, USA. ** Woik partially supported by the US Atomic Energy Commission and by the Research Corporation. 145

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The capture of negative pions in 4He can lead to three non-radiative final states: n- +4He ~ 3H+n (a) zH+2n

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Branching ratios for these three modes have been determined by Block et al. 9), and the energy spectra of the charged particles have been measured by Calligaris et al. l o), and Ziock et al. 11). Calligaris also measured the neutron energy but did not make a coincidence measurement of neutron-charged particle pairs from this reaction. In this experiment we have obtained distributions in opening angle and relative momentum for correlated neutron-charged particle pairs resulting from all three final states.

2. Experimental techniques A schematic drawing of the experimental layout is shown in fig. 1. Negative pions produced in the 600 MeV proton Synchrocyclotron at SREL were degraded and brought to rest in a liquid helium target. The stopping of a pion in the target was signified by a C1, C2, C3, C4, C5 coincidence. The charged particle emitted after the absorption of the pion was identified by combining measurements of range and time of flight. On the other side of the target a neutron time-of-flight spectrometer, covering an angular range of about 70 °, was used to determine the energy and angle of one of the neutrons emitted in coincidence with the charged particle. The charged-particle time of flight was measured between the scintillation counters C6 and C7 using constant fraction timing. Frequent calibrations were run during the experiment using the back-to-back y-rays from Z2Na. Despite the size of the two scintillators (C6 was 15 cm × 20 cm and C7 was 40 cm × 45 cm) the system produced unambiguous separation between protons and deuterons (fig. 2). The range was determined in a range chamber consisting of 32 planar proportional counters of area 45 cm x 45 cm and 0.95 cm thickness separated by aluminum sheets of increasing thickness to cover the desired stopping range. Once the identity of the charged particle was ascertained, the range was used to determine its energy. Two sonic spark chambers SC1 and SC2 were used to monitor the charged particle trajectory. This information was used: (a) To determine whether or not the particle had originated in the target volume. (b) To correct the time-of-flight information for the differences in path length of the charged particle as well as for differences in travel time of the light in the two plastic scintillators. (c) To correct the range information for the angle under which the charged particle had penetrated the range chamber. The neutron spectrometer was similar to that employed by Lee et al. 5). Eight 12.7 cm diameter NE213 liquid scintillators were placed so as to detect neutrons over an angular range of 120 ° to 180 ° relative to the direction of the charged particle. A

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width of the peak corresponds to a time-of-flight resolution of about 1.5 nsec (FWHM) which is consistent with the properties of the detectors and electronic circuits. All estimates of neutron energy resolution were based on the assumption of 1.5 nsec time resolution. Background from material in the vicinity of the target accounted for about 20 % of the proton data and 10 % of the deuteron data. This background was measured by taking several runs with an empty target, and was subsequently subtracted from the data. An IBM 360-44 computer equipped with the IBM-Yale Scientific Interfacing System was used for data acquisition and on-line analysis. 3. Results 3.1. D E U T E R O N

FINAL STATE

The deuteron energy (momentum) spectrum exhibits a peak at the kinematic limit of about 56 MeV (461 MeV/c) in clear disagreement with the calculations of Eckstein 7) (fig. 4). A juxtaposition of our data with those of Ziock et al. 11) (fig. 5) reveals that both experiments exhibit the 56 MeV peak. The increased prominence of the peak in our measurement is due to the fact that both of the neutrons associated with an end-point deuteron are constrained to move in the direction of the neutron detectors, and thus the probability of detecting such a deuteron is effectively doubled in a coincidence experiment like ours. E NE RGY(M eV )

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The two neutrons in this case have about the same energy (28 MeV) and exit from the target in nearly the same direction. We believe therefore that the enhancement at the high energy end of the deuteron spectrum is due to a strong final state interaction between the two neutrons, reflecting the well-known virtual state observed in low energy nucleon-nucleon scattering ~2). The angular spectrum (fig. 6) shows strong peaking in the back-to-back direction, but it is important to remember that the deuteron energy threshold is high. It is expected that for lower energy deuterons there would be a much smaller correlation with the neutron direction. The energy spectrum of the deuteron-correlated neutrons (fig. 7) and the relative momentum spectrum for deuteron-neutron pairs (fig. 8) were also obtained. Theoretical calculations are not available for comparison. 3.2. P R O T O N F I N A L S T A T E

The proton energy (.momentum) spectrum agrees quite well with both the calculations of Eckstein (fig. 9) and the measurement of Ziock et aL (fig. 10), but not with the calculation of Koltun and Reitan s). The relative momentum spectrum (fig. 1 I)

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and the angular correlations (fig. 12) show a similar lack of agreement with Koltun and Reitan's calculation. This situation is due in large part to the use of three-body kinematics in their calculations. This is tantamount to the assumption that the two

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which the two undetected neutrons have less than 20 MeV in their two-particle c.m. system. undetected neutrons act as a single particle. Our missing mass spectrum (fig. 13) shows that these two neutrons can have as much as 60 MeV of internal energy. If, however, we look only at the data which conform fairly closely to the assumed three-body kinematics, as shown in fig. 14, we find reasonably good agreement with the theory. To obtain this relative m o m e n t u m spectrum we arbitrarily selected only those events for which the two undetected neutrons had less than 20 MeV of internal energy. 3.3. T R I T O N F I N A L S T A T E

With our apparatus, it was not possible for tritons to reach the range chamber. However, 22 % of them did reach the second scintillation counter, C7, and they could be identified by their time of flight. The m o m e n t u m distribution of the neutrons associated with the tritons has already been mentioned and is shown in fig. 3.

4. Conclusions F o r the large majority of the spectra presented in this paper, no satisfactory theoretical description has yet been published. However there are some comparisons which can be made to existing calculations. Eckstein's predictions come close to describing our charged particle energy spectra,

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R . J . B A R R E T T e t al.

even though she has used incorrect values for the n-nucleon coupling constants 13), assumed hydrogenic wave functions for the pion, and neglected final state scattering. Her omission of the final state scattering would account for her failure to predict the 56 MeV peak in the deuteron energy spectrum, since this peak appears to be a direct consequence of the final state neutron-neutron scattering. The proton data are explained qualitatively by the theory of Koltun and Reitan but their use of three-body kinematics makes it difficult for us to make a meaningful comparison. Our data demonstrate that for 4He as for other nuclei there is a strong back-toback correlation in the direction of the neutron-charged particle pairs. When our data are combined with previous experiments, the experimental knowledge of pion capture in +He is now complete enough to allow detailed comparison with future theories. Development of more realistic models for the effects of ~clustering in nuclei using a phenomenological approach such as that of Kolybasov should also be possible. The authors wish to thank Dr. S. T. Thornton for lending us some of the required cryogenic equipment. We are indebted to Dr. S. Sobottka for the use of his neutron time-of-flight system and for advice on its operation. We also thank Drs. J. M. Eisenberg, H. J. Weber and J. Noble for illuminating discussions about some of the theoretical points. Mr. R. H. Cole and Mr. Thomas Meyer helped with collecting data. Finally we acknowledge the willing cooperation and assistance of the staff of the Space Radiation Effects Laboratory. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)

V. M. Kolybasov, Sov. J. Nucl. Phys. 3 (1966) 535 V. M. Kolybasov, Sov. J. Nucl. Phys. 3 (1966) 704 A. O. Vaisenberg, E. D. Kolganova and N. V. Rabin, JETP (Sov. Phys.) 20 (1965) 84 P. J. Castleberry, L. Coulson, R. C. Minehart and K. O. H. Ziock, Phys. Lett. 3413 (197!) 57 D. M. Lee, R. C. Minehart, S. E. Sobottka and K. O. H. Ziock, Nucl. Phys. A182 (1972) 20 D. M. Lee, R. C. Minehart, S. E. Sobottka and K. O. H. Ziock, Nucl. Phys. A197 (1972) 106 S. G. Eckstein, Phys. Rev. 129 (1962) 413 D. S. Koltun and A. Reitan, Nucl. Phys. B4 (1968) 629 M. M. Block, T. K. Kuchi, D. Koetke, J. Kopelman, C. R. Sun, R. Walker, G. Culligan, V. L. Telegdi and R. Winston, Phys. Rev. Lett. 11 (1963) 301 F. Calligaris, C. Cernigoi, I. Gabrielli and F. Pellegrini, High energy physics and nuclear structure, ed. S. Devons (Plenum Press, New York, 1970) p. 367 K. O. H. Ziock, C. Cernigoi and G. Gorini, Phys. Lett. 33B (1970) 471 J. M. Blatt and V. F. Weisskopf, Theoretical nuclear physics (Wiley, New York, 1952) ch. 11 A. Figureau and M. Ericson, Nucl. Phys. BI0 (1969) 349