The pre-equilibrium process of the 165Ho( α, xn yp) reaction at Eγ = 109 MeV studied by particle-gamma coincidence measurements

Abstract

Exclusive proton spectra for individual reaction channels of the 165Ho( α, p xn) reaction at E α = 109 MeV were measured in coincidence with discrete γ-rays characteristic of the residual nuclei. The proton spectra for individual reaction channels of the 165Ho( α, p xn) reaction with small x clearly show two proton groups. One is the proton group appearing as a peak at the highest-energy region of the spectrum. It arises from proton emission at the pre-equilibrium (PEQ) stage, being followed by the evaporating neutrons at the equilibrium (EQ) stage. The other is the proton group seen as a bump (or tail) at the lower-energy side of the peak. It is due to proton emission at the PEQ stage and at least one-neutron emission at the PEQ stage as well. This shows that there exists a threshold energy for the PEQ particle emission. The exclusive spectra as well as the inclusive ones are compared with exciton model calculations where all particles emitted at the PEQ stage of the reaction are properly taken into account. The calculation starting with the initial 4p1h exciton state was found to reproduce well the observed proton spectra. The escape probability Γ↑/( Γ↑ + Γ↓) was found to be about 30% for the doorway state (initial exciton state) with about 100 MeV excitation energy produced by α-particle bombardment on 165Ho.