Cosmic-Ray Contribution to the Background of NaI Scintillation Spectrometers

The cosmic-ray contribution to the background of a 3-in. by 3-in. NaI detector in an iron room has been studied by coincidence technique with a 4π plastic scintillator covering former detector. The NaI detector was surrounded by various material inside the 4π guard detector in order to find the effect of the surrounding material. The observed counting rate due to the cosmic-ray component was about 19% of the total background in 0.1–0.4 MeV and 31% in 0.6–0.9 MeV when the NaI detector was surrounded by 5 cm lead. The cosmic-ray contribution did not critically depend upon the atomic number of the surroundings in the energy of 0.6–0.9 MeV and in the annihilation peak but it decreased rapidly with increasing atomic number in 0.1–0.4 MeV.