  In view of a possible application as a charge-particle track readout for an
Active Target Time Projection Chamber (AT-TPC), the operating properties of
THick Gaseous Electron Multipliers (THGEM) in pure low-pressure Helium were
investigated. This paper includes the effective gain dependence on pressure for
different detector configurations (single-, double-, triple-cascade setup),
long-term gain stability and energy resolution from tracks of 5.5 MeV alpha
particles. Stable operational conditions and maximum detector gains of
10^4-10^7 have been achieved in pure Helium at pressure ranging from 100 torr
up to 760 torr. Energy resolution of 6.65% (FWHM) for 690 keV of energy
deposited by 5.5 MeV alpha particles at 350 torr was measured. The expected
energy resolution for the full track is around 2.4% (FWHM). These results,
together with the robustness of THGEM electrodes against spark damage, make
THGEM structures highly competitive compared to other technologies considered
for TPC applications in an active target operating with pure noble gases,
requiring a high dynamic range and a wide operating pressure range down to few
hundred torr.
