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