A GEM Detector System for an Upgrade of the CMS Muon Endcaps

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where we have a higher percentage of CF4 than CO2. A more detailed study is underway for the effect of CF4 gas in GEMs and other detectors. 4.4 Avalanche simulation The studies done here use “single-electron” avalanches as also described in the paper[19]. We generate a stack of single electrons randomly at a distance of 250 µm in the z-direction, from the GEM. The plane lies in the x-y direction. The electron is given an initial energy of 0.1 eV and a random initial direction of motion. Then one of the processes listed above is selected at random, weighed by their relative probability considering the electron energy at the time of the collision. An additional electron is added to the stack in case of Penning transfer or ionization; the electron is removed from the stack in case it is attached or attempts to leave the drift medium. The process is repeated until the stack is exhausted. The total gain, Gtot which is the total number of electrons produced in the avalanche, differs from the measured or effective gain Geff because the latter is derived solely from the current in the anode plane. Avalanche electrons which terminate on the GEM electrodes or insulator do not contribute to the anode current and henceGeff is always smaller than Gtot. The “single-electron” avalanche simulation has been repeated at least 1000 times for varying GEM voltages and Penning parameters. All simulations were performed at standard temperature and pressure, i.e. T0 = 293.15 K, P0 = 1atm = 760 Torr. Some of the primary electrons are lost due to attachment to the gas molecules (attachment loss) or to the metal (geometric loss). The losses due to attachment and due to geometric losses can be seen in Fig. 39. The attachment loss as expected is independent of the GEM potential difference, while the geometric losses decrease with increasing GEM potential (or higher electric field), as electrons are accelerated and sucked into the GEM hole faster with increasing electric fields. This leads to lower geometric losses. The attachment loss is∼ 12% in this gas mixture. Figure 39: Loss rate for primary electrons for rp = 0.6. The losses occur due to attachment to the quencher gas molecules (left) and due to the electrons hitting the surface of copper metal or polyimide walls (right) The electrons which make it into the GEM hole give rise to the avalanche. All of these secondary electrons do not contribute to the signal as some are lost on hitting the walls of the polyimide or hitting the copper metal and some are lost due to attachment to the gas molecules. The losses due to attachment and due to geometric losses can be seen in Fig. 40. The geometric loss increases with increasing GEM potential because the electrons exiting the hole are attracted back by the lower copper metal which is positive with respect to the upper copper metal plate. The secondary attachment loss reduces to ∼ 8% as the electrons gain energy inside the GEM hole due to the high electric field inside the hole. 39
Figure 40: Loss rate for secondary electrons. The losses occur due to attachment to the quencher gas molecules (left) and due to the electrons hitting the surface of copper metal or polyimide (right) The overall loss of the primary and the secondary electrons can be seen in the Fig. 41. Figure 41: Overall Loss rate for primary (left) and secondary (right) electrons for penning parameter rp = 0.6 If lp and ls are the primary and secondary electron losses, then we can define the collection efficiency (which is the efficiency of the electrons to make it into the hole), asǫcoll = 1−lp, and the extraction efficiency (which is the efficiency of secondary electrons to make it out of the GEM hole), as ǫextr = 1−ls. The effective gain (Geff ) is then defined as : Geff = ǫcoll ×ǫextr ×Gtot The effective gain and total gain can be seen in Fig. 42. It is shown for different values of the Penning parameter. 40
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Page 3 and 4: Abstract This technical proposal ai
Page 5 and 6: 5.3 Simulated chamber properties .
Page 7 and 8: List of Figures 1 CMS transverse se
Page 9 and 10: 49 Simulated total L1 GEM+RPC trigg
Page 11 and 12: List of Tables 1 Performance requir
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A GEM Detector System for an Upgrade of the CMS Muon Endcaps

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