Investigations on electron beam irradiated rare-earth doped SrF2 for application as low fading dosimeter material: evidence for and DFT simulation of a radiation-induced phase

Abstract

A recent approach to measure electron radiation doses in the kGy range is the use of phosphors with an irradiation dose-dependent luminescence decay time. However, the applicability of the previously investigated material NaYF₄:Yb³⁺,Er³⁺ is limited as it shows pronounced fading. Therefore, in this work, a modified SrF₂ synthesis is presented that results in SrF₂ nanoparticles codoped with Yb and either Er, Hm, or Tm. To assess their suitability as dosimeter material, dose response, as well as its degree of fading over 50 up to 140 days after irradiation were measured. Fading rates as small as 5% in SrF₂:Er,Yb and 4% in SrF₂:Ho,Yb were derived, which are comparable to established dosimeter materials. A combination of spectroscopy, diffraction and DFT calculations was used to elucidate the effect of irradiation, pointing towards the formation of a secondary phase of Yb²⁺ that we predict could be Yb₂OF₂. This irreversible formation of a secondary phase is considered to be the explanation for the low fading behavior in SrF₂-based phosphors compared to NaYF₄:Yb, Er, a highly attractive feature for electron beam dosimetry.