Abstract
A triple GEM detector was tested at the CERF facility at CERN as an on-line beam imaging monitor and as a counting reference device. It was exposed to a 120 GeV/c positively charged hadron beam (approximately 2/3 pions and 1/3 protons), which hits a copper target generating a wide spectrum of different kinds of particles used for various experiments. The flux of beam particles ranged over three orders of magnitude, from 8·104 s−1 to 8·107 s−1. The profile of the beam acquired with the GEM was compared to the one measured with a MWPC and no saturation was observed. In addition, the count rate measured with the GEM was compared to the one measured with an Ionization Chamber, which is routinely used for monitoring the beam intensity. Another way of monitoring the intensity of the beam was also explored, which is based on the total current driven from the GEM foils. The digital readout allows making a 2D online image of the beam for the alignment with the copper target in the CERF facility. A low residual activation of the detector was observed shortly after irradiation.
Highlights
A triple GEM detector was tested at the CERF facility at CERN as an on-line beam imaging monitor and as a counting reference device
The beam profile at the CERF facility was measured with the Triple GEM detector at different beam intensities and was compared to the one obtained by the routinely used Multi Wire Proportional Chambers (MWPC)
The GEM detector showed no saturation at high beam intensity
Summary
The triple GEM detector consists of three 50 μm thick insulating kapton foils [7], clad on each side with a thin copper layer of 5 μm and chemically perforated with a high density of holes with a biconical structure of 70 μm external and 50 μm internal diameter and a pitch of 140 μm. Drift region: here the detected particles ionize the gas, due to the drift field, ions move to the cathode and electrons to the first GEM foil. Two Transfer regions of 1 mm and 2 mm, respectively, where the field guides the electrons to the following GEM foils in order to achieve a higher gain. The first Transfer region should be as narrow as possible in order to minimize the primary ionization from background particles producing a detectable signal. The reason for choosing a low gain is because of the relatively high particle flux, in order to lower the efficiency of the detector and avoid discharge and saturation effects. The high voltage system used to power the GEM foils is the HVGEM NIM module [11] Each channel is equipped with a high sensitivity current meter for the detection of possible discharges and the current driven by the detector with 10 nA precision
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