  Recently, a new detector concept, for combined imaging and spectroscopy of
fast-neutrons and gamma was presented. It encompasses a liquid-xenon (LXe)
converter-scintillator coupled to a UV-sensitive gaseous Thick Gas Electron
Multiplier (THGEM)-based imaging photomultiplier (GPM). In this work we present
and discuss the results of a systematic computer-simulation study aiming at
optimizing the type and performance of LXe converter. We have evaluated the
detector spectral response, detection efficiency and spatial resolution for
gamma-rays and neutrons in the energy range of 2-15 MeV for 50 mm thick
converters consisting of plain LXe volume and LXe-filled capillaries, of
Teflon, Polyethylene or hydrogen-containing Teflon (Tefzel). Neutron detection
efficiencies for plain LXe, Teflon-capillaries and Tefzel-capillaries
converters were about 20% over the entire energy range. In polyethylene
capillaries converters the neutron detection efficiency was about 10% at 2 MeV
and increased up to about 20% at 14 MeV. Detection efficiencies of gammas in
Teflon, Tefzel and polyethylene converters were ~35%. The plain-LXe converter
provided the highest gamma-ray detection efficiency, of ~40-50% for 2-15 MeV
energy range. Optimization of LXe-filled Tefzel capillary dimensions resulted
in spatial resolution of ~1.5mm (FWHM) for neutrons and up to 3.5 mm (FWHM) for
gamma-rays. Simulations of radiographic images of various materials using two
discrete energy gamma-rays (4.4 MeV and 15.1 MeV) and neutrons in broad energy
range (2-10 MeV) were performed in order to evaluate the potential of elemental
discrimination.
