Design of an energy-independent spherical-type neutron dose monitor

Abstract

The thickness of a spherical-type neutron dose monitor has been optimized in order to obtain improved dose response compared to that of existing instruments. The monitor structure includes four layers of the spherical shell. The outermost layer is composed of a poly-methyl methacrylate (PMMA) shell. A boron nitride shell is used as the second layer while two polyethylene shells are used as the third and the core layers. Twelve radial directional thermo-luminescent detectors (TLD) detectors are arranged between the layers at two different depths and one in the center. Considering the reaction rate of 6Li(n,α) T for the TLD detectors, a series of calculations were performed using MCNP5 by varying the thickness of the layers. Neutron cross-section libraries based on JENDL-3.3 were applied for the calculations. To evaluate the dose response of the monitor, the spectrum of the D 2O-moderated 252Cf source was used as mentioned in ISO 8529. The monitor is sensitive to measurements of three different energy groups at three depths of the moderator. The neutron dose was calculated from the responses of a linear combination of TLD group detectors. The dose response was improved and well agree with expected dose, H*(10), compared to that of existing instruments, especially for intermediate energies of neutrons. The total ambient dose equivalent was varied only 3% for two different incident directions, hence the direction distribution was found small for dose measurement.