  Novel high efficiency fast-neutron detectors were suggested for fan-beam
tomography applications. They combine multi-layer polymer converters in gas
medium, coupled to thick gaseous electron multipliers (THGEM). In this work we
discuss the results of a systematic study of the electron transport inside a
narrow gap between successive converter foils, which affects the performance of
the detector, both in terms of detection efficiency and localization
properties. The efficiency of transporting ionization electrons was measured
along a 0.6 mm wide gas gap in 6 and 10 mm wide polymer converters Computer
simulations provided conceptual understanding of the observations. For a drift
lengths of 6 mm electrons were efficiently transported along the narrow gas
gap, with minimal diffusion-induced losses; an average collection efficiency of
95% was achieved for the ionization electrons induced by a primary electron of
a few keV initial energy. The 10 mm height converter yielded considerably lower
efficiency due to electrical and mechanical flaws of the converter foils. The
results indicate that detection efficiencies of around 7% can be expected for
2.5 MeV neutrons with 300 foils converters, of 6 mm height, 0.4 mm thick foils
and 0.6 mm gas gap.
