Positron Nondestructive Testing of Gamma-Neutron Irradiated Boron Nitride Ceramics

Resumo

The effect of pulse neutron gamma-radiation on material made from sintered ceramics of hexagonal boron nitride α-BN is studied using positron annihilation spectroscopy (PAS). The radiation vacancy defects of the α-BN crystal lattice are effective traps of the positrons diffusing into the bulk particles of the material. The positrons’ lifetime (LT) spectra and angular correlation curves of annihilation gamma-ray quanta (ACAR) in ceramic specimens irradiated by neutrons with a cumulative dose of the neutron flow of 10¹³ to 10¹⁴ neutron/cm² (the average energy of the neutrons is 1.7 MeV) and gamma radiation of 10³ to 10⁵ Р (E_γ = 1 MeV) with the subsequent annealing of the irradiated specimens at 600°C are measured. The observed changes of the annihilation characteristics of the LT and ACAR in the irradiated and sintered specimens allow us to conclude that there is an effective capture of the alpha particles formed upon neutron irradiation via the nuclear reaction B¹⁰(n, α)Li⁷ with vacancy defects. Helium atoms are released from vacancies to the surface of the α-BN plates upon annealing of the irradiated specimens at 600°C. This mechanism explains the high level of radiation stability of α-BN ceramic dielectrics in ionizing radiation fields.