32 MEASUREMENT OF PARASITIC NEUTRON FIELD IN THE TREATMENT ENVIRONMENT OF A VARIAN CLINAC 2100 MEDICAL LINAC USING SUPERHEATED BUBBLE DETECTORS

Abstract

TLDs drops with increasing LET due the track structure of interacting densely ionising radiations. This phenomenon is known as quench effect (QE). Evidently the QF averts the application of tiny TLD chips in “in field” dosimetry relevant to hadron therapy. The light conversion efficiency (LCE) of Lithium Fluoride thermoluminescent dosimeters TLD-700 (LiF:Mg,Ti) decreases with the increasing linear energy transfer (LET) of the interacting radiation particles. On the contrary, the LCE of Beryllium-Oxide increases with increasing LET of the particles impinging on it. By utilizing these two phenomena we have developed a simple technique to estimate the “quench corrected” LET of interacting particles using a pair of thin TLD-700 and BeO chips. Furthermore, the self-attenuation characteristics of both types of TLD were estimated using a spectrophotometer. The TLDs were calibrated using alpha particles from a Am source attenuated using Mylar foils of different thicknesses for the LETTissue: 165, 178 and 195keV/m. The predicted LET estimation range was found to be 1-200 keV/m, encompassing a wide area of interest from Plateau-toBragg Peak region of a therapeutic proton beam to hadron-therapy, based on Carbon ions as well as fast neutrons of different energies.