Digital Signal Processing for HPGe Detectors

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James Lyons
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1 Digital Signal Processing for HPGe Detectors David Radford ORNL Physics Division July 28, 2012

2 HPGe Detectors Hyper-Pure Ge (HPGe) detectors are the gold standard for gamma-ray spectroscopy Unsurpassed energy resolution Indispensible to in-beam nuclear structure studies for many decades Made from a single large crystal pulled from molten hyper-pure Ge Operated as a large reverse-biased diode; up to 5 kv bias No current flows until a gamma ray interacts with an electron in the Ge, lifting its energy above the band gap of the Ge semiconductor This electron scatters off other electrons, creating many electron-hole pairs; each pair takes ~ 3 ev in energy The electrons and holes separate in the strong electric field and are collected at the electrodes The resulting charge pulse is proportional to the deposited gamma-ray energy, and is amplified and digitized Operated at cryogenic temperatures to prevent thermal generation of electron-hole pairs 2 Managed by UT-Battelle for the U.S. Department of Energy

3 HPGe Detectors Historically, there have been two designs, both cylindrical: Coaxial and Planar. Later we will discuss a new design, Point Contact detectors n+ p p+ p+ p n+ Closed-end coaxial Planar 3 Managed by UT-Battelle for the U.S. Department of Energy

4 HPGe Detectors Historically, there have been two designs, both cylindrical: Coaxial and Planar. n+ p p+ p+ p n+ ~ 3 kv + Closed-end coaxial Planar 4 Managed by UT-Battelle for the U.S. Department of Energy

5 HPGe Detectors Drift paths of electrons and holes Charge-integrating pre-amp Fast ADC Closed-end coaxial Electric potential 5 Managed by UT-Battelle for the U.S. Department of Energy

6 Signal Processing Once the preamplifier signal/waveform has been digitized, need to process the data to extract interesting numbers, e.g. Amplitude Gives energy, so we want it to be as accurate as possible Timing Pulse-shape analysis to select specific types of events Noise filtering and/or reduction Pile-up rejection and/or correction

7 Extracting Signal Amplitude Preamp signal Decays exponentially with ~ 50µs time constant Voltage Height of signal is proportional to detected energy Time

8 Extracting Signal Amplitude But the signal has noise Need to integrate signal for several microseconds to determine amplitude more precisely Use Trapezoid Filter

9 Extracting Signal Amplitude Output of Trapezoid Filter Undershoot due to exponential decay of signal

10 Extracting Signal Amplitude Need to correct for signal decay time Pole-zero correction

11 Trapezoid and P/Z Correction Need to know the baseline (asymptote) and signal decay time constant tau E ~ S2 S1 S2 S2 S1 S1 Baseline

12 Point Contact Detectors No deep hole; small point-like central contact Length is shorter than standard coaxial detector Excellent discrimination between single-interaction and multipleinteraction events Excellent resolution at low energies n+ n+ p p+ p+ p P-type Coaxial P-type Point Contact 14 Managed by UT-Battelle for the U.S. Department of Energy

13 Pulse-Shape Response Point Contact detectors are ideal for discrimination between single-site and multi-site events (or determining the number of interactions) Charge Signal a) Single-site 400 b) Multi-site Current Signal Time (ns) Time (ns)

14 Pulse-Shape Response 95% efficiency for double-escape peak = single-site events 99% rejection of single-escape peak = multi-site events Th source Red: all events Blue: PSA-selected events 9/30/09 Radford, RedTeam Review 16 Managed by UT-Battelle for the U.S. Department of Energy

Inverted-Coaxial Point-Contact Detector Designed

radically different from a normal coaxial detector

prototype has recently been produced by Canberra

15 Inverted-Coaxial Point-Contact Detector Designed and developed here at ORNL Drift of charges is radically different from a normal coaxial detector Very long drift times, ~ 2 µs A segmented prototype has recently been produced by Canberra France 17 Managed by UT-Battelle for the U.S. Department of Energy

16 Am SuperPulses Finely collimated Am source, directed at known location on the detector surface Select events with 60 kev in a single hit segment Use PSA to select only single-site events Time-align events to a common time Take average signal to reduce noise to negligible level Charge Single event PC Seg1 Seg2 Seg3 Superpulse 19 Managed by UT-Battelle for the U.S. Department of Energy Time ( s)

ORTEC. Research Applications. Pulse-Height, Charge, or Energy Spectroscopy. Detectors. Processing Electronics

ORTEC. Research Applications. Pulse-Height, Charge, or Energy Spectroscopy. Detectors. Processing Electronics ORTEC Spectroscopy systems for ORTEC instrumentation produce pulse height distributions of gamma ray or alpha energies. MAESTRO-32 (model A65-B32) is the software included with most spectroscopy systems