Evaluation of the neutron spectra in a medical accelerator facility using a modified Bonner Sphere Spectrometer
The energy spectra of neutrons generated by a 15 MV Varian Clinac CX LINAC for radiation therapy were evaluated at three locations using a modified Bonner Sphere Spectrometer (mBSS) developed by the Philippine Nuclear Research Institute (PNRI). The mBSS system consists of several high-density polyethylene shells with an indium foil central detector. Indium foil exploits the detection reaction 115In(n,γ)116mIn, emitting gamma rays from 116mIn which were then counted using an NaI(Tl) scintillator. From the gamma spectrum, the saturation activity at each point was calculated, and the neutron spectra were derived using the FRUIT code and the precalculated mBSS response functions. Results show that fast neutrons are the major contributor in the total neutron fluence at the points along the patient couch. Thermal neutrons dominate at maze-room junction due to the thermalization of the fast neutrons with the low-Z materials surrounding the treatment room. The calculated ambient dose equivalents at the three measurement locations were 2.24 mSv, 0.72 mSv and 0.076 mSv, per 1 Gy photon dose. Results of this work can be used in the evaluation of the radiation protection protocols of a LINAC facility against neutron contamination and shielding.