Studies on the efficiency of the neutron shielding for the SIMPLE dark matter search

The SIMPLE project for direct dark matter search is located in a deep underground laboratory, where non-WIMP signals are expected due to neutrons and alpha particles naturally occurring in the facility. This work presents a first study on the efficiency of the neutron shielding for SIMPLE and possible routes for its optimization. The evaluation of the neutron component considers spontaneous fission and (α,n) neutrons originating from the 238U and 232Th present in the experiment materials. Using recently published data on (α,n) yields and spectra, a Monte Carlo model using the MCNP code is employed to simulate the transport of both spontaneous fission and (α,n) neutrons. The application of MCNP offers an alternative method to the SOURCES code used systematically by others for the evaluation of the (α,n) component. Results supporting the optimization of the neutron shield for SIMPLE are described and the feasibility of reducing the event rate to less than 1 evt/kgd is demonstrated.