Design and characterization of a facility for fast neutron irradiation of semiconductors at Penn State

This paper describes the design and characterization of a fast neutron irradiation facility (FNI) at the Penn State Breazeale Reactor. The facility was designed to provide a hard neutron spectrum with a minimum of contamination by thermal neutrons and gamma rays, and to accommodate semiconductor wafers of up to 8 inches (similar to 20 cm) in diameter. The design was effected through two-step Monte Carlo simulations that included (i) core criticality simulations, and (ii) FNI shielding analysis. A rectangular FNI shape was selected to improve the neutronic coupling between the FNI and the core. Analysis was performed to determine an optimum combination of materials and their dimensions. The FNI was constructed and it has been in use for about two years now. A set of activation foils was irradiated to obtain the FNI neutron spectrum and evaluate spatial flux distribution. The neutron spectrum was unfolded using the SAND-II code. Good agreement was observed between the calculated and measured data.