Some special features of the fragmentation of a relativistic nucleus 11B in photoemulsion

Events where two doubly charged fragments are directly formed from a fragmenting nucleus ¹¹B of momentum 2.75 GeV/c per nucleon are separated, the cascade production of such fragments via the fragmentation channel ⁸Be → 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 ⁶He is (9.6 ± 1.5)%, while its calculated probability is about 12%. The transverse-momentum distributions obtained experimentally for the isotopes ³He and ⁴He are compatible with a Rayleigh distribution characterized by constant values of 110.4 ± 6.0 and 127.5 ± 6.0 MeV/c, respectively. These features of the distributions agree with a purely statistical mechanism of the fragmentation of ¹¹B nuclei.