Accurate Low-Cost Determination of ²⁵²Cf Neutron Source Activity: Benchmarking with MCNP and GEANT4

Accurate determination of radioactive sources is critical for institutions that maintain any radioactive sources. Common techniques – manganese bath activation, coincidence detection, or shadow cone technique – require either extensive equipment or precalibrated rem-meters. We have developed a method for determining activity of a ²⁵²Cf neutron source that is simple, inexpensive, and requires no additional calibrated instruments. This method relies upon a collection of Nuclear Instrumentation Modules (NIM) electronic units with a ³He detector in a custom-built neutronically shielded box. We review the implementation of NIM modules to correct for a noisy discriminator, and show how this device can be used to determine neutron activity in our source. The geometry is modeled with MCNP and GEANT4. By adding borated polyethylene attenuators in the experiment and the model, we are able to employ an optimization routine to determine both the source activity and the concentration of boron in the attenuators. This approach allows for a determination of the source activity to a precision of 2.8% at the k=1 level. The GEANT4 and MCNP models agree within their uncertainties.