Effects of Gamma-ray and neutron irradiation on infrared optical properties of ZnSe and ZnS for ITER divertor thermography

Abstract

Gamma-ray and neutron irradiation tests were performed by using antireflective-coated and non-antireflective-coated samples of ZnSe and ZnS to examine the impact on their infrared optical properties. In neutron irradiation tests, ZnSe and ZnS samples showed minimal transmittance reduction even up to 1.44 × 1016 n/cm2 (evaluated by dpa value, the values are 5.77 × 10−6, 2.07 × 10−5 for ZnSe and ZnS, respectively), about 5.8 times higher than the expected neutron fluence in ITER. Gamma irradiation at 100 kGy did not affect any of the samples, however, at 5.6 MGy (2.6x the expected dose of ITER), non-antireflective-coated ZnSe and ZnS exhibited induced absorption bands. Antireflective coating did partially mitigate this effect, but ZnS, even with antireflective coating, showed up to 2.5 % degradation over the wide wavelength range of 1.5–4.5 µm, whereas ZnSe was less affected. Both ZnSe and ZnS exhibited a modest 2 % absorption around 3 µm. Surface analysis by SEM, laser microscope, and XPS linked the 3-µm degradation to gamma-irradiation-induced Zn(OH)2 formation that was confined to the surface. Thus, transmittance degradation of ZnSe over the lifespan of ITER was estimated to be a maximum of approximately 7.8 %. The practical use of ZnSe in the IRTh system requires further evaluation with Si and CaF2 lenses and for the optical design to be finalized. Nevertheless, these findings suggest that ZnSe is a promising candidate for the IRTh system's lenses.