Charged Particle Production in Stopped Pion Absorption by Nuclei
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CLEI Experiment
Charged Particle Production in Stopped Pion Absorption by Nuclei Yu. B. Gurov1) , V. S. Karpukhin1), S. V. Lapushkin1), T. I. Leonova1), R. V. Pritula1), B. A. Chernyshev1)* , and V. G. Sandukovsky1) Received December 25, 2018; revised December 25, 2018; accepted December 25, 2018
Abstract—The spectra of 3,4 He charged particles produced in stopped pion absorption by nuclei were measured for 17 nuclei in the mass-number range of 6 ≤ A ≤ 209 by means of a semiconductor spectrometer. These data were analyzed on the basis of a phenomenological model where the absorption reaction in question is treated as a three-stage process: primary absorption by intranuclear clusters, preequilibrium stage, and evaporation stage. It is shown that a dominant contribution to the production of helium isotopes with energies not less than 20 MeV comes from the preequilibrium stage. The contribution of the knockout and pickup mechanisms to 3,4 He production is estimated. DOI: 10.1134/S1063778819030074
1. INTRODUCTION Pion absorption by nuclei is a many-body problem. By virtue of the energy- and momentumconservation laws, pion absorption on a free nucleon is forbidden, while pion absorption on an intranuclear nucleon is strongly suppressed. In view of this, pion absorption occurs on intranuclear clusters. The stopped pion energy mπ c2 is dynamically distributed among nucleons of the cluster involved. The remaining nucleons receive a recoil momentum as a residual nucleus. Absorption on an intranuclear proton–neutron (pn) pair carrying the deuteron quantum numbers of J P = 1+ , lnp = 0, and I = 0 is a dominant mechanism for stopped negatively charged pions. However, this process (π − + d → n + n) does not lead to the production of charged particles. It follows that, in order to determine the nature of the formation of charged particles, it is necessary to examine secondary interactions of primary neutrons or less probable processes, including absorption on intranuclear proton–proton (pp) pairs and on heavier clusters. A quantitative description of the yields and spectra of charged particles has so far remained an open question in all cases, with the exception of extremely light nuclei of 3,4 He [1–3]. Theoretical models that take into account absorption on clusters and secondary interactions provide 1)
National Research Nuclear University MEPhI, Moscow, 115409 Russia. * Email: [email protected]
only a qualitative description of experimental data— in many cases, distinctions may become as large as several tens of percent [2, 4, 5]. At the same time, it was shown in the studies of our group [6– 11] that some quantitative conclusions on the pionabsorption process can be drawn from a phenomenological analysis of experimental data. Within the proposed phenomenological model, the yields of p, d, and t particles off medium-heavy and heavy nuclei were described satisfactorily in [6–10]. In the recent study reported in [11], it was shown that, within this model, the spectra of singly charged particles can be described satisfactori
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