Thermal and fast neutron dose equivalent distribution measurement of 15-MV linear accelerator using a CR-39 nuclear track detectors

The main purpose of this study is to measure the contribution of the thermal and fast neutron dose along the central axis of the 15 MV Elekta Precise linac in a tissue equivalent phantom. In order to achieve this purpose, different points were selected in three field sizes of 5 x 5 cm², 10 x 10 cm² and 15 x 15 cm². Fast and thermal neutrons were measured using CR-39 nuclear track detectors with and without thermal neutron converter of ¹⁰B, respectively. According to the results, the fast neutron dose equivalent was decreased as the depth increased (field size 5 x 5, 10 x 10 and 15 x 15 cm² fall from 0.35 to 0.15, 0.5 to 0.3 and 0.5 to 0.3, respectively). Thermal dose equivalent was increased as the depth increased in the tissue equivalent phantom (field size 5 x 5, 10 x 10 and 15 x 15 cm² rise from 0.1 to 0.4, 0.4 to 0.8 and 0.4 to 0.9, respectively). In conclusion, at depth <3 cm, most existing neutrons are fast and CR-39 films are sensitive to fast neutrons; therefore, they are more appropriate than thermoluminescent dosemeters in measuring neutron dose equivalent. (authors)