Optimization Of Fast Neutron Convertors For A Bulk-Micromegas-based Neutron Detector With Geant4

As a kind of new generation micro-pattern gaseous detectors, the Micro-Mesh Gaseous Structure(Micromegas)detector has been widely studied and used in many different fields over the last ten years. In2006,1.Giomataris and others developed the Bulk-Micromegas. The detector is based on a printed circuit board (PCB) technology to produce the amplification/avalanch area of the detector. This manufacturing process is simpler and low cost. Bulk-Micromegas based neutron detectors is mainly composed of the drift plate (with the aluminized mylar film or a metal mesh), polyethylene conversion layer, an avalanche cathode, the readout anode (made by printingcircuit board etching is made). The three plates divide the detector into two regions:a drift gap and an amplification gap. The drift gap is generally a few millimeters, and its electric field is about100V/cm. The amplification gap is about50-100μm with the electric field of tens kV/cm. When neutrons enter the detector, they interacts with the conversion layer made of polyethylene, and then produce the recoiled protons. These recoiled protons can ionize the gas inside the detector. Electrons in the drift electric field drift to the anode, and pass through grid mesh into the avalanche zone, and then are multified by the strong electric field. These avalanche electrons induce electrical signal on the readout anode. The signals are collected and processed by the multichannel readout electronics. The energy and position of incident neutron can be obtained.In a fast-Neutron Imaging system based on a Bulk-Micromegas, the low neutron conversion efficiency of convertor is a bottleneck. With a Geant4-based Monte Carlo simulation program, the affect of thickness on the conversion efficiency for different energy neutron from Am-Be source and14MeV neutron source were simulated. The simulation data were analyzed with a ROOT program. The results show that the conversion efficiencies are saturated for Am-Be neutrons and14MeV neutrons when the thicknesses of convertors are400μm and1600fμm respectively. Additionally, three new structures of converters have been proposed and simulated. Their conversion efficiencies are about3times higher than the traditional convertor’s.