Response of a Plate-type Thermoluminescence Dosimeter to a Therapeutic Carbon Beam

In order to measure the three-dimensional dose distribution in X-ray radiotherapy, we developed a plate-type thermoluminescence dosimeter (TLD) using the TL phosphor Li₃B₇O₁₂:Cu, which has Z_eff = 7.42 and a density of 1.01 gcm⁻³. This detector was expected on useful application in advanced X-ray radiotherapies such as intensity-modulated ratiation therapy (IMRT) etc., because it can obtain 3D dose measurements simply and quickly with a single exposure. In this paper, we report the response of this plate-type TLD to a therapeutic carbon beam at the Heavy Ion Medical Accelerator in Chiba (HIMAC). Using a plate heater and charge coupled device (CCD) camera, we captured thermoluminescence images of plate-type TLDs irradiated by a therapeutic carbon beam. The TL intensity was measured at each water equivalent depth and compared with output of the ionization chamber. The relative TL efficiency of this TLD as ⁶⁰Co gamma-ray equivalent decreased gradually with increasing dose-averaged linear-energy-transfer (LET), reaching about 20% at the Bragg peak of 290 MeV/u carbon ions.