Study of new substrate THGEMs with low neutron scattering and low radioactivity

Abstract

New types of Thick Gaseous Electron Multipliers (THGEMs) made of PCB-type substrates, including Ceramic, Kapton, PTFE, and in addition to FR-4, were developed for applications requiring low neutron absorption, scattering, and low natural radioactivity, such as the THGEM-based neutron detector, and the THGEM-based gaseous multiplier (GPM). Using Geant4 simulation, the result of the total neutron absorption and scattering ratio of bare substrate is Kapton(high) > FR-4 > Ceramic > PTFE (low). The Ceramic and PTFE substrates have lower ratios about 0.707% and 0.635% per layer(0.2 mm thick) respectively. The gamma-induced electron background is the reverse, Ceramic > PTFE > FR-4 > Kapton, from 1.5% to 0.7% at 1.0 MeV, which induces highest electron background in the range of 0∼7 MeV. The natural radioactivity background was measured for these types of substrate samples. The Ceramic substrate has lower radioactivity, 232Th = 8.8±0.9 Bq/kg, 238U = 6.3±0.9 Bq/kg, and 40K is too low to be detected. Some THGEM samples were produced by these four types of substrates in the same shape: 0.2-mm thickness, 0.2-mm hole diameter, 0.6-mm pitch, and 70-μm rim. The performances of these new substrate THGEMs are promising. The effective gain is at the 104 level and the gain was stable during more than 100 hours continuous test. The energy resolution @ Ar+IsoB = 97:3 is at the level of 20% to 27%. These new substrate THGEMs are available in 200×200 mm2 size and even larger, some of them are applied to neutron flux monitor detector and gaseous photomultipliers (GPM).