the disclosure is directed to an apparatus and method for determining the content and distribution of a thermal neutron absorbing material within an object . neutrons having an energy higher than thermal neutrons are generated and thermalized . the thermal neutrons are detected and counted . the object is placed between the neutron generator and the neutron detector . the reduction in the neutron flux corresponds to the amount of thermal neutron absorbing material in the object . the object is advanced past the neutron generator and neutron detector to obtain neutron flux data for each segment of the object . the object may comprise a space reactor heat pipe and the thermal neutron absorbing material may comprise lithium .