To find voids in concrete, when we used a radioisotope backscatter detector, the thermal neutron measurement results included the effect from the sides of the detector in our prior studies. This is caused by scattered fast neutron in all directions from the source. To solve this problem, two types of experiments were conducted. In the first, the characteristics of different moderators were investigated to choose the applicable material. In the second, when the moderator was attached to the void detecting system, movement of radio-isotope count on mock specimen were examined experimentally. The experiments in this paper revealed that attached moderator to this device improves the void detection capability.
