Enhancement of a Cf-252-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 tha t a clinically useful epithermal-neutron beam requires a minimum of 1.0 g o f Cf-252, which is more than twice the US current annual supply. However, i t was reasoned that the required quantity of Cf-252 could be dramatically r educed when used with a subcritical multiplying assembly (SMA), This reason ing is based on the assumption that the epithermal-neutron beam intensity f or 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)) gross ly overestimates the beam enhancement factor for NCT. For example, Monte Ca rlo 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 predicte d 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 magni tude too low compared to the existing reactor beams, we conclude that the e nhancement via an SMA for a Cf-252-based epithermal-neutron beam is inadequ ate for NCT. (C) 2000 Elsevier Science Ltd. All rights reserved.