Some special features of the fragmentation of a relativistic nucleus 11 B in photoemulsion
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EMENTARY PARTICLES AND FIELDS Experiment
Some Special Features of the Fragmentation of a Relativistic Nucleus 11 B in Photoemulsion F. G. Lepekhin Petersburg Nuclear Physics Institute, Russian Academy of Sciences, Gatchina, 188350 Russia Received July 17, 2006; in final form, December 5, 2006
Abstract—Events where two doubly charged fragments are directly formed from a fragmenting nucleus 11 В of momentum 2.75 GeV/с per nucleon are separated, the cascade production of such fragments via the fragmentation channel 8 Ве → 2α being avoided. Where possible, the mass numbers of these doubly charged fragments are determined by using a signal from Coulomb scattering in photoemulsion. It is found that the measured fraction of the isotope 6 Не is (9.6 ± 1.5)%, while its calculated probability is about 12%. The transverse-momentum distributions obtained experimentally for the isotopes 3 Не and 4 Не are compatible with a Rayleigh distribution characterized by constant values of 110.4 ± 6.0 and 127.5 ± 6.0 MeV/с, respectively. These features of the distributions agree with a purely statistical mechanism of the fragmentation of 11 В nuclei. PACS numbers: 25.10.+s DOI: 10.1134/S1063778807060099
1. INTRODUCTION A procedure for separating individual fragmentation events in which 10 B nuclei of momentum 1.7 GeV/с per nucleon fragment through the channel involving a 8 Be nucleus was tested in [1]. The analysis performed there revealed that the cascade mechanism of the fragmentation of relativistic light nuclei does indeed exist and that the fraction of experimentally observed doubly charged fragments emitted by their unstable combinations, referred to as prefragments [2], is not small. In order to study the mechanism of relativistic-nucleus fragmentation, it is therefore necessary to separate channels involving the cascade fragmentation of the primary nucleus from channels where direct particle emission by this nucleus is the most probable. It is obvious that, while nuclei of the isotopes 3 He and 4 He can be emitted not only by a prefragment such as 8 Be but also by 5 Li and 5 He nuclei, nuclei of the isotope 6 He are most likely to be emitted directly from a relativistic nucleus 11 B. The problem of classifying helium isotopes according to their mass numbers is not new. In a number of studies, including that which was reported in [3], the classification was performed on the basis of assessing the momentum of a particle by its multiple scattering in emulsion. However, this is a very cumbersome procedure. Moreover, the momentum of relativistic-nucleus fragments is only used to separate particles in mass numbers. As was shown in [4],
the mass-number classification of particles alone can be reliably performed by using a much smaller volume of measurements along particle tracks. It is precisely this circumstance that made it possible to carry out the analysis reported here. The entire body of information about the experiment being discussed is contained in [5]. Therefore, many details concerning the measurements and experimental-data tre
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