A GEM-Based TEPC for Neutron Protection Dosimetry

C.K. Wang and M. Seidaliev (Georgia Institute of Technology)

As recommended by the International Commission on Radiation Units and Measurements, the operational quantity for individual monitoring of external penetrating radiations (i.e. neutrons and gamma rays) Hp(10), is defined as the dose equivalent measured at the depth 10 mm inside a tissue-equivalent phantom. While TLDs have been used successfully to measure Hp(10) for gamma rays, no satisfactory technique has been developed to measure Hp(10) for neutrons. The traditional 0.5"-dia tissue equivalent proportional counter (TEPC) is too bulky for measuring Hp(10). Recently a new type of TEPC based on the gas electron multiplier (GEM) was developed. The GEM-based system eliminates the need of a central wire as anode, and thus makes it possible to build a high efficiency miniature TEPC, ideal for measuring Hp(10) for both neutrons and gamma rays. We have built a prototype GEM-based TEPC. It is a 1"x1" square made by three layers glued together - the cathod (made of A-150), the GEM, and the anode (made of copper). The GEM is sandwiched between A-150 and copper. Two rexolite guard rings are used as the insulator that separates the GEM from A-150 and Copper. The region between A-150 and GEM is the gas cavity, which is 2-mm thick and filled with propane-based tissue-equivalent gas. The region below the GEM is the charge collection region. During operation the A-150 is held at the ground potential and the copper is held at a positive voltage. We have used the protoype TEPC and obtained a pulse height spectrum for a californium-252 source. This pulse height spectrum can be used to obtain the lineal energy distribution, which in turn, can be converted to Hp(10).

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