Timing in Thick Silicon Detectors Andrej Studen, University of Michigan, CIMA collaboration.
Post on 18-Jan-2018
223 Views
Preview:
DESCRIPTION
Transcript
Timing in Thick Silicon DetectorsAndrej Studen, University of Michigan, CIMA collaboration
Outline Motivation Timing in pad detectors Two intuitive solutions Comparison to measured data Where to go from here
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
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.
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
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.
Ramo Field Pad size ~ depth. Asymmetry.
Large Ramo field. Large
contribution to current pulse.
Low field region. Charges contribute less.
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.
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
Solution 1:Adding adjacent pads
Reducing Ramo asymmetry. Noise of 9 pads added – jitter increased 3 x
Solution 2:Increasing bias
Much shorter times w/ higher bias Often unpractical
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
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.
Comparison to measurements
Measured in Compton mode (PMT start, silicon stop; PMT timing resolution ~ 10 ns)
Sharp edge
Blunt edge
Spurious tail
Comparison, U=400 V Simulation marginally better, measurement data more symmetric. Spurious tail gone.
Solution simulation
RAMO9 pads200 V
BIAS400 V1 pad
Latest greatestDo both!
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.
Backup slidessubtitle
Illustration of depth-related trigger time variation
15 % trigger, 1.4 VFD
70 ns60 ns50 ns40 ns30 ns20 ns
Single pad 9 pad sum
Illustration (cont’d) 15% threshold, 1.4 VFD
Single Pad 9 pad sum
top related