Thermoluminescent response of beta-irradiated silica beads

Abstract

Research into the potential for silica based materials as TLDs continues to increase, harnessing the various advantages that the materials have to offer as a dosimeter, including; affordability, chemical inertness, geometry and size, while producing suitable TL yields. Studies into the possibility of beta source dosimetry open another range of applications for this type of dosimeter, including procedures involving the possibility of in vivo dosimetry (Morino et al., 2002). This work aims to further investigate the properties of a much-studied batch of Mill Hill silica beads of various colours, irradiated with a Sr90/Y90 source yielding a dose rate of 0.02 Gy/s, activity 37 MBq and maximum energy of 2.27 MeV, facilitated by the Risø TL/OSL reader. The TL yield is compared to that in previous work, with the same set of beads irradiated by a 6 MV photon beam provided by a Varian electron linac (Varian Medical Systems, Palo Alto, CA) X-ray source, delivering a dose of 10 Gy. Present study shows a decrease in the mass normalised TL yield from X-ray to beta source by a factor of 10, assumed to be accounted for by the estimated mean penetration depth of the beta particles in the doped silica beads. Investigating the potential for doped silica beads to be used as a beta source dosimeter a linearity study is reported with a dose range of 5–20 Gy. The studies show that all coloured beads except green show a strong conformity to a linear fit of the data over the tested range of doses. Fading studies conducted with clear petite beads (producing the greatest TL yield/mg) for doses of 5 Gy and 20 Gy have shown that the majority of fading occurs within the first hour following irradiation, with a plateau in the fading to approximately 17% and 22% loss of signal for 5 Gy and 20 Gy respectively in the interval of 24–70 h after irradiation. The decay of the TL yield/mg was fitted to a logarithmic trend of reduction for both the 5 Gy and 20 Gy irradiation, with R2 values of 0.9883 and 0.9705 respectively.