Valency conversion of samarium ions under high dose synchrotron generated X-ray radiation

Microbeam Radiation Therapy (MRT) is a synchrotron based technique for cancer treatment. During MRT tumours are irradiated with micro-planar arrays of synchrotron-generated X-rays. The quality assurance of such a technique depends on the accurate measurement of very high dose levels (>1000 Gy) together with a precise measurement of the dose distribution over a scale of a few microns. For such applications we need to have a high dynamic range, high-dose detector with very high spatial resolution. As part of the work towards the development of such a detector we have examined the X-ray radiation induced conversion of rare-earth ions, especially Sm3+ to Sm2+, embedded in suitable media. We have exposed a variety of Sm3+ doped media, from single crystals to glass-ceramics, to a high dose radiation at the Canadian Light Source to examine whether we could convert the rare-earth ion valency. The irradiation was carried out at a bending magnet beamline using filtered white beam with a spectrum peaking at 50 keV. We have used the distinctly different signatures in the photoluminescence emission spectra of the two valency states to identify the rare-earth ions and whether a valency conversion has taken place. We report the results from these experiments, which show that the conversion of Sm3+ to Sm2+ is not universal and depends very much on the host material. We discuss possible reasons for the presence and lack of radiation induced valency conversion of Smions, and identify the challenges that must be overcome to develop a high-resolution, high-dose detector. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim