A novel detector concept is under development for an efficient combined imaging and spectroscopy of fast-neutrons and gamma rays. It encompasses a fast LXe scintillator, of high neutron and gamma-ray conversion efficiency, contained within “fiber-like” capillaries. The scintillation-light within the capillaries, induced by neutron or gamma interactions with Xe atoms (resulting in nucleon or electron recoils, respectively), propagates along the capillaries by total internal reflection; it is detected by a cryogenic position-sensitive gaseous photomultiplier (GPM) through a UV-transparent window. The photoelectrons, induced by photon conversion on a CsI photocathode deposited on the top electrode of the GPM, are multiplied by successive Thick Gas Electron Multipliers (THGEM). The localization of the interacting radiation within the LXe capillary converter is derived from the center-of-gravity (COG) of the event-emitted UV scintillation photons detected by the GPM.