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Gas gain in a single GEM: Charging-up effect at different voltages
15 June 2012
Speaker: Pedro Correia, University of Aveiro
Co-Authors: Carlos Oliveira, University of AveiroJoão Veloso, University of AveiroRob Veenhof, CERN
RD 51 Mini Week
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IMPORTANT TOPICS
Electron and Ion charging-up effect on electric field and on
gain - differences on gain
Simulations for Vgem=200V, 300V and 400V – effective gain
and ionization position on z axis
Conclusions and Future Work
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First study about the initial deposition of charges on kapton surface
The kapton surface goes from -25µm to +25µm
FEBRUARY IMPORTANT RESULTSCHARGES DEPOSITION ON KAPTON
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Considering the previous deposition histograms, we decided cut the kapton surface again
We cut the lower half part of the kapton intro 20 slices and the upper part in 4 slices.
FEBRUARY IMPORTANT RESULTSNEW UNITARY CELL
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FEBRUARY IMPORTANT RESULTSMETHOD
Calculate field map (no
charging-up)
Simulate avalanches (Garfield)
Calculate new field map (cumulative charging-up)
Analyse final position of electrons and ions
10.000 primary electrons per iteration
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RECENT WORKINITIAL CONDITIONS
Standard GEM: Hexagonal Distribution, Edge=140µm, kapton=50µm and metal
electrodes=5 µm each
Gas Mixture: Argon - 70% ; CO2 - 30%
Vgem=200,300 and 400 V
P=760 torr
T=293 K
Drift Field=2kV/cm
Induction Field=3kV/cm
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TOTAL CHARGES ON KAPTON VGEM=400V
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Net deposited charge (ions-electrons) added to the new field maps.
For later iterations, we see the compensation between electron and ion deposition in almost all regions of the hole.
Iter 0 iter 100 iter 200
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GAS GAIN
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At 200V, the effective gain (left) and the ionization position (right)
Gain increase Ionizations position decrease Simulated gain start to decrease at some point
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GAS GAIN
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At 300V, the effective gain (left) and the ionization position (right)
Gain increase Ionizations position decrease Simulated gain start to decrease at some point
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GAS GAIN
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At 400V, the effective gain (left) and the ionization position (right)
Initial decrease on gain, correspondent with increase on ionization position - statistical fluctuation or something else?
Stabilization on gain(same as experimental data) and ionization positions
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VARIATION OF ELECTRIC FIELD
Considering ions and electrons deposition, the simulated gain will tend to equilibrium. The variation on Electric Field was represented on an animation (see attach file .wmv)
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On the firsts iterations, we see fast changes on the Electric field.
With aniquilation of the electrons and ions contribution on the kapton surface, the Electric field will tend to an equilibrium stage – the gain will stabilize
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ACTUAL AND FUTURE WORK
Try to reduce the statistical variation of gain, simulating more primary electrons for each field map (this will increase the time of the simulation)
The inclusion of the charges movement on the kapton surfaces will give a more correct approach for our simulation, we expect stop seeing the small drop on gain for later iterations, at 200V and 300V
Due to electron mobility, the electrons deposited near the bottom electrode can be collected faster, and therefor the continuously increasing of negatives charges on that surface will not occur.
We will try soon to apply this to other Micro-Pattern Gaseous Detectors (Korean GEM, THGEM, MHSP, etc)
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END
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Any suggestions are welcome.
Thank you for your attention.