A high-sensitivity GEM-based TEPC as an area neutron dose monitor - 113

As recommended by the International Commission on Radiation Units and Measurements (ICRU), the operational quantities for area monitoring of external penetrating radiations is called the ambient dose equivalent, H*(10), defined as the dose equivalent measured at the depth 10 mm inside a tissue-equivalent phantom. Several techniques have been developed to measure H*(10) for gamma rays satisfactorily. However, the measurement of H*(10) for neutrons has not been satisfactory. Traditionally, a spherical tissue equivalent proportional counter (TEPC) has been used to measure the dose distribution in lineal energy D(y), which can then be used to obtain the H* (10) via the following relationships: H = Q-bar D (1) where Q-bar = 1/D∫ D(y)Q(y)dy (2) and Q(y) is the radiation quality factor expressed as a function of y. A neutron survey meter based on a traditional TEPC, however, suffers from a low neutron sensitivity - 48 counts μSv^-1. Recently, a new plate-like TEPC based on gas electron multiplier (GEM) was developed. Several of this plate-like TEPCs can be stacked together as a device to increase its sensitivity to neutrons. This paper presents the design configuration of a multilayer GEM-based TEPC and the computational results of its performance as an area neutron dose monitor. The design configuration of the GEM-based TEPC for area neutron dose monitoring consists of three identical GEM-based TEPC stacked together. In practice, however, one may stack as many as ten TEPCs together without significantly increasing the total volume and weight of the device. Each GEM-based TEPC, in turn, consists of three layers of materials - the cathode made of A-150 plastic, the GEM foil, and the anode made of a copper-coated printed circuit board. The size of each TEPC is 10 cm x 10 cm x 0.7 cm. For a stack of ten TEPCs, the total volume of the detector would be 10 cm x 10 cm x 7 cm, which is quite reasonable for an area monitoring device. A Monte Carlo program has been written to determine the energy deposition distribution in a single-layer (i.e. 10 cm x 10 cm x 0.7 cm) GEM-based TEPC for ²⁵²Cf fission neutrons. Based on this energy deposition distribution, one may obtain D, Q, and H. The results thus obtained, in turn, translate to 190 counts μSv^-1. Accordingly, a stack of ten GEM-based TEPCs would give 1900 counts μSv^-1, a 40-time increase over the existing TEPC-based neutron survey meter