The replacement of cadmium as a thermal neutron filter

Cadmium is widely used where it is desirable to eliminate the thermal component of a neutron spectrum. However, with the restrictions on the use of cadmium in the European Union due to the recent European Union Directive 2002/95/EC (RoHS), the inclusion of cadmium in instrumentation is becoming untenable. The work presented here examines a number of candidate materials in order to find a suitable replacement for cadmium. It is shown through the examination of cross-section data, MCNP modelling and experimental evaluation that flex-boron is suitable in some applications while gadolinium has absorption properties which are broadly similar to cadmium. However, it is clear that none of the candidates can quite equal to the performance of cadmium.