Using Bulk Zinc Oxide as a Neutron Radiation Detector

Abstract

Zinc oxide (ZnO) has long been described as a radiation hard material, but there was not enough interest taken in previous researches to study the effect of radiation on the ZnO, especially the irradiation at room temperature. This study was designed to test the radiation hardness properties of ZnO by using different ionising radiation types. The main interest is in investigating the effect of neutrons on the optical properties of the zinc oxide. Two types of samples were studied: pure ZnO single crystal bulk and ZnO crystal bulk with gadolinium (Gd) implanted into its Zn-polar face (Gd:ZnO). Each type of sample was exposed to slow and fast neutron radiation for a time interval that varied between minutes and days depending on the sample type. Slow neutron irradiation was carried out using 241Am-Be sources supplied by the University of Canterbury, which were placed under a 4.5 cm thick wax moderator, with a resultant flux of 1.5 × 106 n/s.cm2. The clinical neutron therapy system (CNTS) at University of Washington, USA was used as the source of the fast neutrons with a flux of 1.4 × 108 n/s.cm2. Usually, the irradiation was carried out on the Zn-polar face of the samples. The damage introduced by irradiating the Zn-polar face of the pure ZnO single crystal bulk showed a change in the optical properties of the ZnO bulk, due to the nuclear reaction of both slow and fast neutrons with the Zn isotopes causing a transmutation of Zn into gallium (Ga). This was observed via an increase in the photoluminescence intensity of the Ga exciton line of up to 80% compared to the un- irradiated (reference) sample. It was also found that application of radiation improved the spectral line width of implanted samples in a manner similar to annealing. This was particularly well observed in Gd:ZnO lengthy slow neutron irradiation. In these samples, the narrowness of the spectral lines in the donor bound exciton (DoX) region reflected a nonimplanted reference sample. This improvement is due to the energy delivered by the neutron absorption repairing the damage. The existence of Gd in the crystal appears to assist this improvement due to its high neutron capture cross-section. Zinc oxide is a radiation hard material within a limit, depending on the type of ionising radiation it is exposed to. This study showed that changing the optical properties of zinc oxide is possible if the right polar face was chosen and sufficient time of irradiation was given to allow the effect of the nuclear reaction to appear.