Study of relevant parameters of GEM-based detectors

The Gas Electron Multiplier consist of a thin Kapton insulating (50 $\mu$m) foil copper-clad on both sides and perforated by a high density, regular matrix of holes (around 100 per square millimeter). Typically the distance between holes (pitch) is 140 $\mu$m and diameters of about 70 $\mu$m. The mesh is realised by conventional photolitographic methods as used for the fabrication of multi-layer board. Upon application of a potential difference between the GEM electrodes, a high dipole field develops in the holes focusing the field lines between the drift electrode and the readout element. Electron drift along the channel and the charge is amplified by a factor that depends on the field density and the length of the channel. Owing to their excellent position resolution and rate capability GEM-based detector are very suitable to be used in different applications: from the high energy physics to the medical field. The GEM temporal and rate gain stability was studied and it was discovered that the gain variations are not more than 20%. Two main phenomena are responsible for the gain variation: the charges movement inside the Kapton and the effect of the external radiation. The charges movement within the Kapton imply a global gain increase in all the GEM area, while the radiation imply a local gain decrease. The first effect is visible measuring the gain immediately after supplying HV to the detector, while the second one is visible only a fter the GEM has been powered for some hours. Different kinds of GEM geometries (different type of etched GEMs) were tested but the gain variation was always present. It was found out that triple and single GEM structures behave differently: some physical phenomena present in the triple GEM do not show up in the single GEM. A new kind of GEM-based detector for future On Beam-PET applications was tested and developed. The objective of this new kind of detector is the detection and localization of high energy photons making use of an appropriate mesh converter sheet in front of a GEM device used for charge amplification. To compensate for the low conversion efficiency of a single element, multi-layer structures will be developed, with stacks of converter-GEM modules. Each elementary cell of this detector is composed by a converter followed by a GEM foil. To ensure that the charge collected does not depend on the position of interaction each cell has to have high local gain but a very low mesh electronic transparency ($\epsilon$) achieving the global cell gain of one (GEM-Gain = 1/$\epsilon$). It means that the GEM should have the highest possible gain to be able to detect also MIPs and that the charge entering each cell must be similar to the charge released by the photoelectron coming from photon conversion. The three coordinates of photon conversion point are measured in two different ways. The depth of interaction is measured knowing which co nverter is the first to give a signal. The other two coordinates are given by the final readout pads. The test detector used to prove the operating principle was composed by only two cells of the final detector and it was tested only with s X-Rays (8.9 keV). The mesh (50 $\mu$m hole diameter, 400 $\mu$m pitch, 1.2% optical transparent) was not used as converter. The condition of unitary cell gain was discovered and the principle was proven measuring separately the GEMs gain and the mesh electronic transparency. The present best unitary cell gain condition achieve a GEM gain of more or less 350 and a mesh electronic transparency around 0.0035