⁶⁰Co ENERGY SPECTRUM AND OUTPUT CALCULATION BY USING MCNP MONTE CARLO CODE

Abstract

In a field of the recent medical care, linear accelerators are mainly used as an external beam radiotherapy device. On the other hand, cobalt-60 irradiation apparatuses are used for dose calibration of absolute dosimeters such as ionization chambers because of its good stability and clearly known energy spectrum comparing with those of linear accelerators. At the Research Institute for Radiation Biology and Medicine, Hiroshima University, a cobalt-60 apparatus (111 TBq) has been installed for the use of radiation biology study. We calculated basic output characteristics of this cobalt-60 irradiation apparatus using the Monte Carlo MCNP code. The energy spectrum of the cobalt-60 was used as an input of the MCNP code and, output factor (OPF) and percentage depth dose (PDD) were obtained as basic characteristics. We compared calculation with the measured results using a tertiary standard ionization chamber. The agreement between the calculation and measurements were less than 4.8%. Also in the case of this cobalt-60 apparatus low dose-rate experiment sometimes performed to irradiate cells, mice and etc. To reduce the dose rate, Pb plates were used. In this paper we calculated such spectra after absorption of the Pb plates to obtain the aspects of the scattering gamma rays. Actual energy spectra in the phantom etc. are not easy to measure. However sometimes it is necessary to know energy spectra to understand dose distribution or such characteristics precisely. In future this method will be widely used for the output calculation and for quality assurance (QA) and quality control (QC) of radiotherapy treatment planning system.