Mechanism of nonlinearity in the response characteristics of thermoluminescent dosimeters

A new model is proposed for explaining the nonlinearity in the response characteristics of three common thermoluminescent phosphors (CaSO4:Dy, CaF2:Dy and Mg2SiO4:Tb). The increased trapping of charge carriers in the dosimetry traps during irradiation, due to a reduction in trapping efficiency of deep traps caused by radiation damage, was found responsible for the supralinearity in TL response of dosimetry peaks and sublinearity in TL reponse of high temperature peaks of these phosphors at high γ ray doses. The radiation damage of TL traps is attributed to the structural changes in the TL material at high γ ray doses, as a result of the displacement of atoms from the lattice sites possibly through excitation or ionisation processes. As a result of high spatial excitation density and consequent energy wastage, α particles possibly damage the TL traps less efficiently than γ rays. The reduced movement of positive ions due to interstitials created by direct collisions with α particles could also explain this phenomenon. Deep traps are more prone to radiation damage than shallow traps, because it is the energy of excitation or ionisation that is being converted into defect formation. (author)