  The operating principle and performances of the Multi-layer Thick Gaseous
Electron Multiplier (M-THGEM) is presented. The M-THGEM is a novel hole-type
gaseous electron multiplier produced by multi-layer printed circuit board
technology; it consists of a densely perforated assembly of multiple insulating
substrate sheets (e.g., FR-4), sandwiched between thin metallic-electrode
layers. The electron avalanche processes occur along the successive
multiplication stages within the M-THGEM holes, under the action of strong
dipole fields resulting from the application of suitable potential differences
between the electrodes. The present work focuses on investigation of two
different geometries: a two-layer M-THGEM (either as single or double-cascade
detector) and a single three-layer M-THGEM element, tested in various
low-pressure He-based gas mixtures. The intrinsically robust confinement of the
avalanche volume within the M-THGEM holes provides an efficient suppression of
the photon-induced secondary effects, resulting in a high-gain operation over a
broad pressure range, even in pure noble gas. The operational principle, main
properties (maximum achievable gain, long-term stability, energy resolution,
etc.) under different irradiation conditions, as well as capabilities and
potential applications are discussed.
