Absolute Efficiency Measurements of NE-213 Organic Phosphors for Detecting 14.5- and 2.7-MeV Neutrons

Abstract

The absolute efficiency of two NE‐213 organic phosphors for detecting 14.5‐ and 2.7‐MeV neutrons has been determined by the associated particle technique with the recoil particles from the T(d,n)⁴He and D(d,n)³He reactions. The output of the neutron detector was placed in coincidence with the helium recoil pulses and the efficiency of the detector was obtained as the ratio of the coincidence counts to the helium recoil counts. The scintillators were 12 cm in diameter and had thicknesses of 2.61 and 6.10 cm, and each was preceded by a 5‐mm thick NE‐102 plastic scintillator and followed by a 3.3‐cm (average thickness) Lucite light pipe. The detection efficiency, for neutrons striking the central 25% of the detector area, was measured as a function of the bias level on a tunnel diode driven by the current pulse from dynode 14 of a 58AVP multiplier phototube and also as a function of a bias threshold on the total light output. The bias level was calibrated in terms of the pulse‐height spectra resulting from the interactions in the phosphors of gamma rays of several energies. Comparisons were made between experimental efficiencies and those obtained from Monte Carlo calculations. For a threshold of 40°±5 keV electron total light equivalent, the agreement is within 7% for the smaller detector and 1.5% for the larger.