timing in thick silicon detectors andrej studen, university of michigan, cima collaboration
DESCRIPTION
Motivation Thick silicon detectors improve efficiency for gamma-ray detection. In a coincident setup (PET, Compton camera) good timing resolution is required. Experimental data not promising [1]. Could it be compensated by different readout strategy and bias conditions? [1] N. Clinthorne et al. Timing in Silicon Pad Detectors for Compton Cameras and High Resolution PET; IEEE NSS/MIC, Portoriko, 2006TRANSCRIPT
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Timing in Thick Silicon DetectorsAndrej Studen, University of Michigan, CIMA collaboration
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Outline Motivation Timing in pad detectors Two intuitive solutions Comparison to measured data Where to go from here
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Motivation Thick silicon detectors improve efficiency for gamma-ray detection. In a coincident setup (PET, Compton camera) good timing resolution is required. Experimental data not promising [1]. Could it be compensated by different readout strategy and bias conditions?
[1] N. Clinthorne et al. Timing in Silicon Pad Detectors for ComptonCameras and High Resolution PET; IEEE NSS/MIC, Portoriko, 2006
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Model application Silicon pad sensors used in Compton & silicon PET experiments at UofM: p+-n-n+, 256/512 pads Pad size 1.4 x 1.4 mm2, Thickness: 1 mm, FDV: 150 V (!), ASIC: VATAGP3:
Charge sensitive pre-amplifier CR-RC shaper with 200 ns shaping time. Leading edge discriminator.
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Signal formation in a pad detector
Charge q moving in electric field induces current pulse on readout electrode:
P-side
N-sideelectrons
holes
+-
Compton scattering or photo-absorption
~100 um (E gamma)
gamma-rayRecoil electron
Readout electrode
)( EEvE wqw qqI Signal shape depends on:
Electric fieldRamo fieldInteraction depth
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Electric field Thickness (1mm) « lateral dimension (12/24 mm by 48 mm).
P-n junction. Large field
region.Charges are
fast.
Low field region. Charges move slowly.
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Ramo Field Pad size ~ depth. Asymmetry.
Large Ramo field. Large
contribution to current pulse.
Low field region. Charges contribute less.
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Interaction depth Two regions: Near region:
Large E field, Large Ramo, Fast rise-time.
Far region: Small E field, Small Ramo, Slow rise-time.
Very sensitive because of short electron path.
Asymmetry of both fields works against us.
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Example: single e-h pair at pad edge, 1.4 VFD
Far region fZ=0.9
Near region fZ=0.1
Detector
Trigger time shift
Preamp, CR-RC; t=200 ns
Leading edge trigger
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Solution 1:Adding adjacent pads
Reducing Ramo asymmetry. Noise of 9 pads added – jitter increased 3 x
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Solution 2:Increasing bias
Much shorter times w/ higher bias Often unpractical
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Simulation overview GEANT4 used to generate “true” paths of recoil electrons 661 keV photons; 137-Cs (also measured) Voltages from 200 V -> 400 V Both single and summed pads
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Results overview
Threshold: 15 keV (experiment). Time-walk: Dominates below ~ 100 keV: Could be compensated by appropriate
readout strategy. Three levels assumed for illustrative
purposes.
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Comparison to measurements
Measured in Compton mode (PMT start, silicon stop; PMT timing resolution ~ 10 ns)
Sharp edge
Blunt edge
Spurious tail
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Comparison, U=400 V Simulation marginally better, measurement data more symmetric. Spurious tail gone.
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Solution simulation
RAMO9 pads200 V
BIAS400 V1 pad
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Latest greatestDo both!
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Conclusions Shape of Ramo field has a significant influence on timing in thick silicon detectors. Solutions: Multi-pad readout (noise!), Different detector geometries (strips?) Different trigger strategies. Operate at higher bias voltages.
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Backup slidessubtitle
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Illustration of depth-related trigger time variation
15 % trigger, 1.4 VFD
70 ns60 ns50 ns40 ns30 ns20 ns
Single pad 9 pad sum
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Illustration (cont’d) 15% threshold, 1.4 VFD
Single Pad 9 pad sum