Characterization of the internal background for thermal and fast neutron detection with CLLB

We report on a set of experiments conducted to determine what effects, if any, the internal background in the CLLB scintillation detector has on the thermal neutron detection performance. We conducted source measurements using an unmoderated and moderated 252Cf neutron/γ-ray source and long (48-h), unshielded and shielded, background measurements to characterize the internal background with and without a source present. These measurements allowed us to determine the 2-d event selections needed to isolate the thermal neutron peak observed in pulse shape vs. energy space and apply those selections to our background measurements. Our results indicate that the thermal neutron detection capabilities of the CLLB are marginally affected by the presence of internal background. An unmoderated 113-µCi 252Cf source at 15 cm from the detector yields a thermal neutron rate of 8×10−2/s cm3, while moderating the source with 5 cm of polyethylene yields a thermal neutron rate of 5.5×10−1/s cm3. The measured background rate for events that fall within the selected thermal neutron region is 1.2×10−3/s cm3. Lastly, the potential for CLLB for detecting fast neutrons was investigated.