Neutron-induced point defects and luminescence properties of enriched Zn82Se crystals

Abstract

Enriched Zn82Se is a very promising material for the detection of the neutrionoless double beta decay. The 82Se isotope acts as the source of the decay, and ZnSe detects the scintillation and bolometric events. Due to the high cost in the preparation of the Zn82Se material, the optimization of its properties, especially those related to the scintillation process, is mandatory. It is well known that excellent scintillation efficiency in ZnSe is obtained when the material presents close donor–acceptor pairs, as, for example, the A-centers. In this work, neutron irradiation was used to study formation of point defects in enriched and non-enriched ZnSe samples. Applying instrumental neutron activation analysis (INAA) and inductively coupled plasma mass spectroscopy, the enrichment of Zn82Se samples was determined to ∼95%, in addition, trace impurities such as Cu, I, and Na were quantified by INAA at the μg/g level. Based on electron paramagnetic resonance (EPR) and photo-EPR measurements, we show that irradiation with fast neutrons induced point defects such as the isolated zinc vacancy (VZn) and a new low-symmetric donor center called NC1, which we tentatively assign to Zn di-interstitials. Low-temperature photoluminescence measurements of the neutron-irradiated Zn82Se revealed three broad PL bands centered in the visible spectral range. Based on the INAA analysis shown in this work, we attribute the previously unidentified 540 nm PL band in the Zn82Se sample to recombination between shallow donors and deep copper acceptors. The formation of point defects by neutron irradiation and their stability are discussed.