Development of a neutron detector for the ambient dose equivalent measurement

In recent years, Vietnam has experienced an increase in the use of radioactive sources, especially the applications of neutrons in industry, medicine and thus radiation protection monitoring has been of wide interest. Besides the main sources of neutrons detected in sealed radio-isotopic sources, nuclear reactors and neutron generators, neutrons are also encountered in high energy particle accelerator systems. The radiation field outside the shielding of accelerators is frequently dominated by the neutron component, which has complex and non-uniform energy distributions. Exposure to free neutrons can be hazardous since the interaction of neutrons with molecules in the human body can lead to chromosome damage and adversely affect human health. According to the recommendations of the International Commission on Radiological Protection (ICRP) for area monitoring, the ambient dose equivalent, H*(10), is used as an approximation of the protection quantity in radiation measurements of external exposure. The ambient dose equivalent measurement will provide necessary information for controlling the radiation at workplaces and defining restricted or prohibited areas. However, the precise measurement and assessment of neutron dose remains difficult due to the intrinsic characteristics of neutron radiation, for example, no-charge, continuous neutron fluence spectrum, etc.

Based on the results of the project entitled: “Optimization of the energy response function of a neutron survey meter using a Helium proportional counter”, a team of researchers of the Institute for Nuclear Science and Technology (INST) have developed a neutron detector for measuring the ambient dose equivalent H*(10) rate in the energy range from thermal to 15 MeV with high sensitivity and fully compliant with the recommendations of the international standard IEC 61005-2014 for neutron survey meters.

The developed instrument consists of a 3He proportional counter embedded in a multi-layer moderator made of high-density polyethylene (HDPE) and Cadmium (Cd). It is designed with the outer dimensions of 20.5 cm in diameter and 24.5 cm in length, and about 6 kg in weight. The sensitivity, the linearity as well as the performance of the instrument in the workplace was measured at the neutron calibration laboratory of the INST using a standard neutron field of 241Am-Be and several simulated workplace neutron fields.

The obtained results show that the neutron sensitivity of this detector reaches 2.84 cps/µSv.h-1. The relative H*(10) response of the instrument has a similar tendency to that of other commercial neutron survey meters (e.g. Aloka TPS 451C, Fuji electric NSN1, NDN1, etc.).