High Energy Gamma Radiation Induced by Fast Neutrons from Californium Source in NaI(Tl) Detector in Close Geometry Measurements

A paper deals with the measurement of the effect of fast neutrons from the Cf-252 neutron source on the muon background detection with the NaI(Tl) scintillation detector. Previous research at TAMU has shown a possibility to measure high-energy cosmic ray particles, or secondarily produced muons, to predict the natural neutron background in the search for artificial neutron sources by mobile systems. One of the problems encountered in testing with Cf-252 was the increased contribution of high-energy gamma rays at small distances between the neutron source and the detector, which affects muon measurements. A primary source of a prompt gamma radiation, was found in the capsule of neutron source and AmBe material itself. More gamma photons are produced by neutrons in the detector crystal. If there is a moderator between the source and the detector, the contribution of this radiation increases due to neutron capture reactions. For higher neutron energies are crucial inelastic scattering, proton emission, alpha particle emission or multiple neutron emission accompanied by prompt gamma emission. On the other hand, experiments have not confirmed the effect of a steel plate between the detector and the neutron source. The solution may be to increase the lower discrimination during measurement with NaI(Tl) or to analyze the region between 8–14 MeV for the unusual increase in the number of pulses.