A study of the effects of gamma radiation on optical components used in specially constructed hot cell laser-induced breakdown spectroscopy (LIBS) instruments

Abstract

Two innovative hot cell Laser-Induced Breakdown Spectroscopy (LIBS) instruments, designed and built by Applied Photonics Limited, are set to be deployed for the purpose of material characterisation in a high radiation environment. One LIBS instrument uses a ‘through-wall’ optical periscope, where the laser and optical spectrograph are located outside of the hot cell. The second instrument uses a remote probe located within the hot cell, containing a compact Nd:YAG (neodymium-doped yttrium aluminium garnet) passively Q-switched laser and plasma light collection optics.The design of both LIBS instruments means that some optical components (e.g. prisms, lenses, fibre optic cables and, in the case of the in-cell remote probe, a Nd:YAG laser head custom-designed and manufactured by Applied Photonics Ltd) will be exposed to high levels of gamma radiation. This entails that these components must be suitably reliable and gamma radiation resistant to last the duration of the planned work. In this article, we report on the results of a limited study on the effect of gamma irradiation (5 kGy, 25 kGy and 50 kGy absorbed dose using a cobalt-60 gamma source) on the optical properties of BK7 (Crown glass) and Ultra-Violet Fused Silica (UVFS) lenses, both with and without high-energy V-coat anti-reflection coating, and UVFS fibre optic cables. The effect of gamma irradiation on the pulsed laser beam energy output of three passively Q-switched Nd:YAG laser heads was also evaluated. The performance of each of the laser heads was tested before and after gamma irradiation by using an energy meter to measure the laser energy output of each of the three laser heads. The optical transmission properties of the components used for plasma light collection and transmission (i.e. lenses, fibre-optics) were tested using a deuterium-halogen lamp in combination with an optical spectrometer. In this limited study, we found that the BK7 lenses were clearly affected by the gamma radiation in that they became light to dark brown in appearance with increasing gamma irradiation whereas the UVFS optics (coated and uncoated) appeared visually, at least, to be unaffected. The optical transmission tests showed that the BK7 lenses became increasingly opaque to UV-VIS wavelengths with increasing gamma irradiation whereas the UVFS lenses and prisms were not so affected. The high-energy V-coat anti-reflection coatings appeared also to be unaffected by the gamma irradiation levels used in this study, at least within the limits of this investigation. However, the UVFS fibre-optic cables did show a reduction in transmission of UV light with increasing gamma irradiation.