Neutron transmutation doping of silicon: optimization of length of a silicon ingot in HS-3018 radial beam hole of Dhruva and radioactivity in the silicon ingot

The neutron transmutation doping of silicon (NTD), as a method to produce a high quality semiconductor, utilizes the transmutation of a silicon element into phosphorus by neutron absorption in a silicon single crystal. In this paper, we present the design of a Si ingot irradiation in the 300 mm diameter radial beam hole of Dhruva reactor and formation of radioactivity. The goal of the design is to achieve an even flat axial distribution of the resistivity, or 30Si(n, g)31Si reaction rate, in the irradiated Si ingot. We used the MCNP code to calculate the thermal neutron flux distribution along the radial beam hole to optimize the length of silicon ingot or to calculate the reaction rate distribution in the Si ingot. The fluctuations in the axial distribution were estimated to be within ±5.0%. The doping of semiconductor materials by nuclear transmutation is associated with the formation of radioactivity. In the case of silicon, 31Si activity, surface contamination and 32P activity from the silicon are of significance. While the 31Si activity will decay to zero after about 4 days, the surface contamination can be removed by an etching treatment applied to the silicon. The formation of 32P activity can be controlled to a certain extent by a suitable choice of neutron flux density. (author)