In this investigation, neutron capture therapy (NCT) through high energy neutrons using Monte Carlo method has been studied. In this study a new method of NCT for a sample liver phantom has been defined, and interaction of 12？MeV neutrons with a multilayer spherical phantom is considered. In order to reach the desirable energy range of neutrons in accord with required energy in absence of eligible clinical neutron source for NCT, this model of phantom might be utilized. The neutron flux and the deposited dose in the all components and different layers of the mentioned phantom are computed by Monte Carlo simulation. The results of Monte Carlo method are compared with analytical method results so that by using a computer program in Turbo-Pascal programming, the deposited dose in the liver phantom has been computed. 1. Introduction Neutron capture therapy (NCT) has been one of the most important methods for treatment of cancers in recent years. This method of radiation therapy is applicable in treatment of liver cancer. During clinical practice, it is always essential to stop absorption of additional dose by normal tissue. On the other hand, measurement and assessment of the absorbed dose and its calibration is an important matter [1, 2]. Thus computation and modeling of the deposited dose by Monte Carlo method before practical treatment is recommended. An appropriate software tool for this purpose is MCNP4C code. It is a particular engineering solution when BNCT facilities such as low energy neutron source are not available. In this paper for simulation by MCNP4C code, a phantom is considered so that it has been encased by polyethylene sphere with 20？cm radius. This sphere is covered with a layer of cadmium which has 100？ m thickness [3, 4]. The cadmium layer has high absorption cross-section for thermal neutrons and helps to reentrance the scattered neutrons from surface of the sphere to the phantom [5]. The polyethylene sphere is surrounded by a graphite shell which has 25？cm radius and 5？mm thickness (according to moderation ratio: ). This layer serves as a reflector to reduce escaping the fast neutrons [6]. In the present work, neutrons are emitted from an external source, and after passing through polyethylene and slowing down, their deposited energy in the phantom’s materials is computed by the MCNP4C code. The F6 tally in the MCNP4C code is applied. The absorbed energy in the liver is computed through analytical computations as well. It includes generation of random numbers along with using the neutron diffusion equation [7]. The outcomes of two
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