1 pixel readout of gems motivation the setup results next steps a. bamberger, k. desch, j. ludwig,...
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Pixel Readout of GEMs
MotivationThe SetupResultsNext Steps
A. Bamberger, K. Desch, J. Ludwig, M. Titov, U. Renz, N. Vlasov , P. Wienemann, A. ZwergerUniversity of Freiburg
ECFA LC Workshop Vienna 16/11/2005
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Motivation
Idea: use a Pixel readout chip (w/o Si sensor) as integrated device hosting pads + readout electronics for a TPC (pioneered at NIKHEF)
Potential advantages very small (50x50 m2) Pads potentially very good point+momentum resolution dE/dx via cluster counting frontend electronics automatically integrated (‘active endplate’)
Potential concerns diffusion will limit resolution (gas!): how small is necessary? cost? stable operation possible?
Freiburg initial goals: - demonstrate operability using triple GEM + Medipix2 chip - measure & understand (details of) charge cloud
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The Setup
drift electrode
readout electrode
GEM 1 GEM 2GEM 3
• drift volume 6 mm
• transfer gaps 2 mm
• induction gap 1 mm
typ. potentials/fields:
ED= 1.1 kV/cm
ΔVGEM= 404 V
ET = 3.2 kV/cm
EI = 4.0 kV/cm
6mm
2mm
2mm1mm
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The Setup
in total 25 square pads of 2x2 cm2 with readout electronics
β- source Ru106, 3.5 MeV from daughter Rh106 crosses 4 pads with MediPix2 in between
GEM on top of MediPix2 and pads
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The Setup
55 μm
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Parameters
- lower threshold at typically ~1000e-
- upper threshold used for estimating the total charge of a cluster
- HV 4200 V, 404 V across GEMs gain 6x104
- Use collimated 55Fe source for calibration
- β´s from the 106Ru source used for ~MIP signals
- low trigger level (<¼ of the average pulse height from
β´s crossing ~ 8 cm of Ar/CO2).
- four-fold trigger coincidence to readout MediPix2
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Gain determination
r
q
qmax
lower thr. qlow
upper thr. qhigh
Fe55 quanta of 5.9 keV produce ~220 primary electrons Estimate total charge on the pixels (need to assume shape of cloud (not very critical)
Reconstruct total chargeusing (rlow,, qlow) and (rhigh,qhigh)
FeFe5555 conversions: conversions:
14 mm14 mm
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Gain determination
default point of operation: gain = 6x104
can even do‘spectroscopy’ withcluster radii-method:
Ar escapeAr escape
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Threshold Calibration of MediPix2
acceptable noise level low THL =155 equ. to 960e-
qTHL > 960 e-
from charge injection + calibration of injector with sources+ threshold equalization
upper threshold (THH)
lower threshold (THL)
large number=lower thr. value!
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Tracks from 106Ru
using lower thresh.only
using upper and lowerthreshold
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Cluster charge and size
cluster distribution has a Laundau-ish taildepletion at low number of primary electrons seen
3.5 pixels correspond to 190 μm ↔ σgaussian = 165 μm
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Preliminary track fitting
isolated tracks selectedtypically 6-8 clustersstandard deviation of straight line fit:
very preliminarymore sophisticatedcluster findingand classificationneeded(handle overlapping clusters, ’s etc.)
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Hot news
He(70%)/CO2 70/30 tracks
smaller clusters? better separated? quantitative results to come…operated at higher voltage, need new gain determination
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- triple-GEM – MediPix2 chip setup successfully operated at a gain of ~6 104 threshold/pixel below 1000 e-
- use of low/high threshold allows an estimate of the total charge and cluster size- gain calibration with 55Fe
- sensitivity to MIPs (o(10) clusters/cm), not yet fully efficient for single electrons (ways out: larger gain, larger pixels? or live with it)- point resolution clearly better than 100 µm – room for improvement- stable and safe operation in long term running
Summary
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Next Steps
- simulation ->M.Hauschild next talk
- study feasibility of huge TPC (cost,cooling,mechanics,readout)
- join TimePix project (within EUDET)
- construct TPC endplate to work with large prototype at DESY
- mechanical aspects
- readout system for TimePix/testbeam
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Backup
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low rate of the source (100 Hz on first pad)
fourfold coincidence adjusted for gitter due to drift time difference and low trigger levelmin. ionizing electrons at1.25 keV/cm ↔ 46 e-
at the peak calibrated through Fe55 ecape peak on a pad next to MediPix2 chip
readout of MediPix2 with trigger
observe: FWHM/peak ~100% !
Trigger
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Summed up per trackaccepted clusters include also „filled“ blobs (charge estimated by extrapolated r½)
the average charge is about 50 e-
the expected value from experiment is 46•1.4 ~ 65 e-
some loss expected due to lower threshold setting at 4200 V
Charge determination