The accurate acquisition and correlation coincidence of positron annihilation signal form the basis of the lifetime spectrum sensitive characterization of microscopic defects in materials. The complex radiation background interferes with the acquisition of positron annihilation signals, particularly in the study of neutron radiation damage of nuclear structural materials. The γ ray background generated by radionuclides induced by neutron activation affects the measurement results of positron lifetime spectrometer.

This study aims to investigate the effect of γ background on positron annihilation lifetime measurement.

First of all, the positron lifetime measurement system is built in a "fast-fast coincidence" manner, and radiation background simulation experiments are designed by selecting two typical nuclides, ⁶⁰Co and ¹³⁷Cs sources, with nearby feature γ photon energy for measuring positron annihilation lifetime. Then, the spectra under two typical activity ratios are compared with the activated neutron-irradiated samples.

The simulation results indicate that the double high energy γ rays generated by ⁶⁰Co sources are the primary factors affecting the spectrum shape and lifetime components. When the ⁶⁰Co/²²Na activity ratio is relatively low, 1.9, the peak-to-valley ratio of the spectrum significantly degrades, with the increase of random coincidence probability caused by radiation background. Further, at high activity ratio of 3.3, besides random coincidence, the false coincidence probability increases sharply, and the spectral shape is evidently distorted. For neutron-irradiated RPV steel, the lifetime value is reduced by 17% and 46% at low and high activity, respectively, compared with the non-irradiated samples.

Using the simulation method of radiation background sources and the influence rules of interference γ in this study, new techniques for eliminating γ background could be further explored in positron annihilation lifetime measurement.