Enhancement of a 252Cf-based neutron beam via subcritical multiplication for neutron capture therapy

Previous studies indicated that an epithermal-neutron beam based on bare Cf-252 is not feasible for neutron capture therapy (NCT). It was reported that a clinically useful epithermal-neutron beam requires a minimum of 1.0 g of Cf-252, which is more than twice the US current annual supply. However, it was reasoned that the required quantity of Cf-252 could be dramatically reduced when used with a subcritical multiplying assembly (SMA), This reasoning is based on the assumption that the epithermal-neutron beam intensity for NCT is directly proportional to the fission neutron population, and that the neutron multiplying factor of the SMA can be estimated by 1/(1 - k(eff)). We have performed detailed Monte Carlo calculations to investigate the validity of the above reasoning. Our results show that 1/(1 - k(eff)) grossly overestimates the beam enhancement factor for NCT. For example, Monte Carlo calculations predict a beam enhancement factor of 6.0 for an optimized SMA geometry with k(eff) = 0.968. This factor is much less than 31 predicted by 1/(1 - k(eff)). The overestimation is due to the fact that most of the neutrons produced in the SMA are self-shielded, whereas self-shielding is negligible in a bare Cf-252 source. Since the beam intensity of a 0.1 g Cf-252 With the optimized SMA enhancement is still more than an order of magnitude too low compared to the existing reactor beams, we conclude that the enhancement via an SMA for a Cf-252-based epithermal-neutron beam is inadequate for NCT. (C) 2000 Elsevier Science Ltd. All rights reserved.