Fast Neutrons and Gamma-Rays Imaging with a LXe/THGEM-GPM Detector

We present the concept and performances of a novel detector developed for simultaneous imaging and spectroscopy of fast neutrons (by TOF) and gamma-rays (by pulse-height analysis). The project has been motivated by the need of efficient radiographic means for the detection of concealed explosives and high-Z fissile materials in cargo and containers. The detector combines a liquid-xenon (LXe) converter/scintillator coupled to a UV-sensitive gaseous imaging photomultiplier (GPM); the latter comprises a cascaded gas-avalanche electron multiplier, based on Thick Gas Electron Multiplier (THGEM) elements, with the first electrode coated by a CsI photocathode. Imaging of gamma and neutrons relies on their induced UV scintillation-light localization in the LXe converter by the position-sensitive GPM.

We shall present and discuss the basic properties of a 4” diameter detector prototype comprising a capillary-filled LXe converter, coupled to a triple-THGEM imaging-GPM operated in Ne/CH4 mixtures at cryogenic temperatures. The localization properties of ⁶⁰Co gamma-rays and a mixed fast-neutron/gamma-ray field from an AmBe neutron source were derived from irradiation of a Pb edge absorber. Spatial resolutions of 12±2 mm and 10±2 mm (FWHM) were reached with ⁶⁰Co and AmBe sources, respectively, in the present detector geometry. The experimental results are in good agreement with GEANT4 simulations – for the present geometry.

The calculated ultimate expected resolutions, in the energy ranges foreseen for the gamma/neutron radiography project, e.g. 4.4 and 15.1 MeV gamma and 1-15MeV neutrons, are 2-4mm and ~2mm (FWHM), respectively.

The results indicate that the novel mixed radiation-field detection concept has the potential of use in fast-neutron resonant transmission (FNRT) radiography and dual discrete gamma-ray (DDGR) radiography.