Assessment of Cerenkov optical noise in a brachytherapy scintillating fibre dosimeter with an air-core Ag-PTFE light guide

Abstract

Plastic scintillating fibre dosimeters have been the subject of multiple studies in the field of medical dosimetry, due to their notable dosimetric properties, including water equivalence, small size and absence of energy and dose rate dependence. The main drawback of this dosimeter type in high dose-rate brachytherapy is the presence of Cerenkov photons produced by electrons with velocities exceeding the speed of light in the fibre plastic medium. In this work, aimed at minimizing Cerenkov noise at its source in a prototype scintillation fibre dosimeter, the plastic light guide exposed to the radiation field was replaced by an air-core Ag-PTFE light guide of miniature size. Cerenkov-to-signal ratio was first assessed in fibre bundles using a dedicated experimental setup and simulations. This ratio was found of about $\sim 1$ $\text{\%}$ for scintillating fibres when exposed to radiation in the energy range 1–2 MeV. The performance of the air-core Ag-PTFE light guide dosimeter was then studied, resulting in a decrease of the Cerenkov light in the total signal from $\sim 50$ $\text{\%}$ to less than 0.3%, compared to the standard dosimeter with a plastic optical light guide. The counterpart of this substantial reduction of optical noise is a reduction of 40% in the dosimeter light collection efficiency. However, this is not a limiting feature of this Cerenkov-free dosimeter, since further optical optimizations are possible, in addition to the use of a high-gain and high-sensitivity photodetector for its readout.