Test of a Fiber Optic Scintillation Dosimeter with BGO Tip in a Co Irradiation Chamber

Following a previous feasibility study, a photon dosimeter have been constructed by attaching a small piece of BGO scintillator to a low attenuation plastic optical fiber for which the light output is measured with a current-type PMT. The dosimeter model is tested in a Co irradiation chamber in the dose rate range varying from about 50 to 1200 Gy/h. The dose rates are measured at a number of predetermined positions in the form of the net PMT current. MCNPX simulations are carried out to calculate the energy being deposited in the scintillator piece at the same positions. The normalized PMT current distribution is compared with that of the deposited energy distribution to evaluate how accurately the dosimeter model can follow the dose rate change. In addition, commercially available alanine pellet dosimeters are used to measure the absorbed dose rates at the same positions. The normalized PMT current distribution is compared with that of the alanine-measured dose rate. A dose conversion factor is obtained at each position by dividing the alanine-measured dose rate by the PMT current and then an average dose conversion factor is calculated from the point values. When the average conversion factor is used, the maximum deviation between the dose rates measured with the dosimeter model and alanine dosimeters, respectively is estimated to be less than 10%. The scintillator piece is irradiated to a dose of 100 kGy with a dose rate of about 4,430 Gy/h. The reduction of the light output immediately and one day after the irradiation is estimated. The light output decreased to 83% just after the irradiation and recovered to 95% one day after the irradiation.