Optimization of a Micromegas-based fission chamber for application to intensive thermal neutron measurement.

Abstract

The use of Micromegas to construct fission chambers offers advantages of wide range and high sensitivity, providing significant application value in high-intensity thermal neutron measurements, such as reactor neutron flux rate monitoring. However, the pad array readout method of detectors can lead to multiple-triggering from a single signal, which increases the burden on the electronics and limiting the detector's counting capacity. Therefore, it is necessary to optimize the detector structure and operating conditions to restrict the transverse migration and diffusion of charged particles. It is crucial to reduce the multi-channel response and improve the count range of the detector. This study utilized Monte Carlo simulations to investigate detector performance, focusing on conversion efficiency, detection efficiency, emitted energy spectrum, and average energy deposition, as well as the transverse migration and diffusion characteristics of charged particles. The results indicate that the drift region width significantly impacts the spatial distribution of primary electrons. Using a noble gas with a higher atomic number, such as Xe, or increasing gas pressure can effectively reduce the spread of primary electron positions. With a gas mixture of 70 % Xe and 30 % CF₄ and a drift region electric field of 1000 V/cm, the transverse diffusion of electrons is minimized, with a full width at half maximum (FWHM) of 0.13 mm. Experiments on neutron count rate and repeated count ratio with different drift region widths confirmed that limiting the drift region width could effectively reduce repeated counts caused by multi-channel responses. With a conversion layer thickness of 1.4 mg/cm² and a drift width of 2 mm, the mean neutron count rate is 0.079, and the repeated count ratio is only 21.09 %. This study provides theoretical and reference foundations for enhancing the count rate range of this new fission chamber and developing it into a high-count-rate detector for measuring high-intensity thermal neutrons.