Experimental Research On Domestically Produced THGEM-based Thermal Neutron Detector

As an ideal tool for detecting mesoscale structures of materials, the neutron scattering technology has been widely used in many scientific fields. A thermal neutron detector is one of the key components in neutron scattering facilities. Due to the development of high flux, new generation neutron sources, such as the spallation neutron source, the traditional3He-based neutron detector facing a big challenge. Additionally, because of the global shortage of3He supply, there is an urgent demand for next generation thermal neutron detectors.A GEM(Gas Electron Multiplier) based neutron detector consists of cascade boron-coated GEM foils as neutron convertors, a single GEM or multi-GEMs for electron multiplication, and the readout board for collecting hits produced by neutrons. With its very good performances, such as a good spatial resolution, a high counting rate(10MHz/mm2), radiation resistance, flexible detector shape and readout patterns, good n/gamma discrimination, GEMs are widely considered as important alternatives to3He neutron detectors. For a long time, the used of standard GEM foils has heavily relied on the import from CERN with a very high price and long order time, which has become the development bottleneck of this next generation neutron detector in China. In2011, IHEP and UCAS firstly successfully developed THGEMs (THick Gaseous Electron Multipliers), manufactured economically by standard printed-circuit drilling and etching technologies in China. Compared with standard GEMs, THGEMs have a higher gain, sub-millimeter spatial resolution and the possibility of industrial production capability of large-area robust detectors.In this paper, the experimental research on a thermal neutron detector based on the domestically produced THGEM is presented. Firstly, in order to study its basic characteristics of the two kinds of THGEMs from IHEP and UCAS, their counting rate plateaus, energy resolutions and gains were measured in the variety(90/1080/20、70/30) Ar/CO2gas mixtures with different high voltages. According to the experiments, the opertimized operating parameters or THGEMs have been obtained. Secondly, to realize the THGEM for thermal neutron detections, a prototype of such a neutron detector with two-dimensional position sensitive readout system was developed and tested with55Fe X-ray,252Cf neutrons and reactor neutrons.The detection uniformity across the sensitive area tested with55Fe X-rays is better than95%. For the test of the counting rate ability, an X-ray machine was used. The results show that the detector can stand as high as high counting rate of above1MHz, and has a good linearity. For obtaining its key operating parameters for neutron detection, the preliminary performance tests were carried out by using252Cf neutrons, which acted as preparation for the subsequent neutron beam experiment. Finally, the detector was tested on the reactor neutron beam line designed for the neutron powder diffraction spectrometer in Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics in Mianyang. The neutron detection efficiency was characterized as (4.9+0.1)%for neutrons with a wavelength1.59A, but the theoretically predicted value was4.2%. The estimated spatial resolution was better than3mm in FWHM. Such a prototype detection system can be used to measure the neutron beam profile online and has a good2D imaging capability, which has reached all of the technical design specifications.This work paved a road to the successful research and development of the next generation, high detection efficiency THGEM based neutron detector for imaging applications.