Detector Operation and DAQ Setup
GEM detector operation determines the its signal multiplication, it may contain a one or more one preamplifier. Operating GEM detector mainly depends on the detector structure, The applied voltage on preamplifier's sides determines the its multiplication, so as the voltage increases the multiplication increases. However, there is a limit for voltage applied on each preamlifier to avoid the discharge that may damage the cards. If a particle signal needs more amplification, more cards will be added to the detector's cavity. This type of detectors has many applications that demands a specific number of cards to attain a successful application. In our case, the detector's structure is based on a triple GEM preamplifiers; it allows the maximum voltage to 350 V to detect the the alpha particle and the fission fragment signal, its multiplication relationship to the applied voltage is shown in the figure below.
Operating the detector occurs by providing a voltage to the HV-divider circuit that is connected to the detector's cards. Using CAEN N470, a four channel digital power supply, allows to increase the voltage between the sides of the first GEM preamplifier to reach up to 350 V; then voltage gradually decreases to be the least on the sides of the third GEM preamplifier to reach 265V, as the circuit board is directly provided by a voltage of 2.8 kV. Another channel is responsible for increasing the voltage of the cathode; When the cathode voltage is 3.1 kV, it creates a drift voltage of 300V in the drift area.
The power supply is connected to a discharge protection circuit. CAEN N470 has a signal Lemo 00 input to activate a kill option automatically when a discharge signal occurs through the detector operation, the kill input is connected to the detector charge collector output, so as a discharge happens and its signal wider than 15 us, kill option state turns true, so it will trip all the power supply channels.
The detector distinguish between the neutron signal from the other particles' signal by analyzing the data as the detector's shutter is open and as it is close. GEM detector has the ability to detect most of the signal created by the particles if they cause ionization in the gas in a rate of 5 MHz, so the detector detects the signal of electrons, Bremsrahlung radiation, and neutrons. The neutrons' signal is distinguished in comparing two runs; one run records the detector signal as the detector's shutter is open, and the second run records its signal as the shutter is close. Within a common known time period, the difference between the two signal represents the neutrons signal.Testing the ability of the shutter to stop the fission fragments is confirmed by being able to stop the alpha particles produced by U-233 that is installed inside the detector and showing a difference in the Q-ADC spectrum between the shutter positions.
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