الفهرس 
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Abstract
When silicon is irradiated the objective is to produce number of phosphorus atoms in the target sample in order to obtain a given resistivity after the treatment. The resistivity of the sample is decreased by the transmutation of the silicon, by neutrons to phosphorus. Irradiation is carried out by thermal neutrons. The irradiation of silicon ingot large diameter has been carried out in heavy water research reactor since the thermal neutron flux to the fast neutron flux in order of 1000:1. The neutron spectrum is highly thermalized and some of these neutrons can reach the center of the silicon ingot and gives the radial resistivity gradient in accept range. Due to the disadvantages of heavy water research reactor such as tritium generation as a result of the neutron capture by deuterium. The tritium is radioactive emitting beta particles with a half life of 12.3 years so the heavy water research reactor is closed to avoid the intake of bete particles. The new trend in light water research reactor to design a neutron filter from heavy water or graphite to moderate the neutron to offer neutron spectrum like heavy water reactors, and keep the advantages of light water research reactors such as open pool. In this work we try to use graphite, heavy water and light water to design a neutron filter using the MCNP for different silicon ingot diameter. Silicon ingots up to 10 inches diameter can be irradiated in the light water research reactors with accepted radial resistivity gradient (RRG). Graphite is the best filter in case of 10 inch with maximum radial variation (MRV) 7.564%; light water is the best filter in case of 6 and 8 inch with MRV 2.197% and 4.85% respectively. In case of 6 and 10 inch Heavy water is the second choice.