SU‐GG‐I‐64: Photoneutron Intensity Variation with Field Size Around Radiotherapy Linear

Purpose: CR‐39 NTDs were used to study the variation of fast and thermal neutron relative intensities around an 18 MeV linear accelerator x‐ray beam with the field size. Method and Materials: The linear accelerator (Varian 2300 CD) was used with maximum x‐ray energy : 18 MeV Fast neutron detection used Bare NTDs. Method and Materials: NTDs were covered with lithium tetra borate(Li2B4O7) converter. Results: Fast neutron relative intensity is symmetrical about the beam axis and exhibits almost an exponential‐like drop with distance from the isocenter of the accelerator for all the field sizes. Neutron intensities do not change significantly with beam size outside the primary beam. At the primary beam (isocenter), the relative fast neutron intensity is highest for 40×40 cm2 field size and decreases linearly with the decrease in the field size. Conclusion: The fast neutron relative intensity was found to be symmetrical about the beam axis and exhibits an exponential‐like drop with distance from the isocenter of the accelerator for all the field sizes. At the primary beam (isocenter), the relative fast neutron intensity is highest for 40×40 cm2 field size and decreases linearly with the decrease in the field size. Outside the primary beam, the fast neutron intensity does not change significantly with the field size. The thermal neutron intensity was found to be almost uniform for all the field sizes.