Study On The Sealed GEM Neutron Detector

With the development of the neutron science technology and the neutron source,the ability to explore the micro world is enhanced.The wire structure neutron detector based on ~3He is still the mainstream detectors of the neutron instruments.The counting rate of these detectors is less than 10~4Hz/mm~2 and the price is high,which limits the development of neutron science and technology.The boron coated GEM（Gas Electron Multiplier）neutron detectors can offer excellent rate capability,good spatial resolution,and good time properties.However,most of the GEM neutron detectors work in flow-gas mode.The stability of detector is affected by the variation of the atmospheric pressure.In addition,the application of detector in the vacuum environment is limited by the gas system.The gas system also occupies space of the neutron instruments.The difficulty of sealed GEM neutron detector is that the substrate of GEM is organic,which would release impurity gases.The working gas would be polluted,and these gases would be prone to polymerization,forming deposits and adhering to the surface of GEM.The detector cannot work stably for a long time.In this paper,the existing problems of the sealed GEM neutron detector were studied.Development of the sealed GEM neutron detector is based on the ceramic GEM with small neutron scattering and absorption,less outgassing and heating.The neutron detection efficiency and drift region thickness of the detector were simulated and optimized by Geant4.The aging process of working gases was analyzed,and the properties of working gases were simulated and compared by Garfield++.The gains of detector with different drift region electric field,GEM voltage and induction region electric field were simulated,and the relevant experiments were carried out,which was consistent with the simulation results.The experiments show that the properties of ceramic GEM were not affected by multiple high temperature heating.The method of two-dimensional digital coincidence was adopted in fast readout electronics.Neutron events were filtered and reconstructed by the correlation coincidence of signals in the time and space domain.The gas chamber was sealed with aluminum wire,and the purity of the working gas in the detector was achieved by using the vacuum material with low outgassing and heating degassing at 80°C for 7 days.The sealed GEM neutron detector with a 100 mm×100 mm sensitive area was fabricated,and the performance of the detector was investigated by the neutron beam and ²⁴¹Am radioactive source.The spatial resolution of the detector was better than 3 mm（FWHM）,and the detector exhibited good energy/wavelength selective neutron imaging ability.The instantaneous counting rate of the detector could reach 1.2 MHz.The stability of the sealed and the flow-gas GEM detector was compared.The counting rate fluctuation of the former was only 0.25%（RSD）in 658 h measurement,and that of the latter was 0.77%（RSD）in478 h measurement.The periodic fluctuation of counting rate could be observed in the flow-gas GEM detector measurement.The difference between the sealed and the flow-gas GEM detector would further expand with the extension of time scale.The detector showed reliable and stable performance in sealed mode.The first sealed GEM neutron detector in the world has been successfully developed.The improvement of neutron detection technology would promote the progress of the neutron science and technology.