Digital Signal Processing of Scintillator Pulses

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Digital Signal Processing of Scintillator Pulses

• Saba Zuberi, Wojtek Skulski, Frank Wolfs
• University of Rochester

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Outline

• Description of the DDC-1 digital pulse processor.
• Response to scintillator pulses.
• Gamma-ray spectra obtained with DDC-1
• Pulse Shape Discrimination and Particle ID
• Conclusion

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Single Channel Prototype Digital Pulse Processor

• 12-bit sampling ADC, operating at 48MHz sampling
  rate
• USB interface processor, 8K internal memory
• Output reconstruction channel for development and
  diagnostic

JTAG connector
ADC 65 MHz 12 bits
FPGA
Variable gain amp
USB processor connector
Signal IN
Signal OUT
Fast reconstruction DAC 65 MHz 12 bits
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DDC-1 Digital Pulse Processor
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Response to Scintillator Pulses

• Fast Plastic Scintillator BC-404
• Original decay time 1.8ns
• Nyquist filter fc20 MHz
• Good response to very fast pulse

• Slower Scintillator Pulse
• Signal from Bicron NaI(Tl)
• Effective Decay time 0.23ms
• Good response to slower pulse

1 sample 20.8 ns
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Response to scintillator pulses Phoswich Detector

• Fast plastic pulse clearly separated from slower
  decay in CsI(Tl)

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Response to scintillator pulses CsI(Tl)

• natThorium source
• a-particle
• High ionization density
• Overall decay time 0.425ms
• g-ray
• Low ionization density
• Longer overall decay time than a-particle
  (0.695ms for electron)
• Clear pulse shape dependence on type of radiation

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Gamma Ray Spectra
60Co

• Signals obtained from Bicron 2 x 2 NaI(Tl)
• X-rays from excitation of Pb casing of detector
• Low energy region
• 56Ba characteristic x-ray, 33keV, from 137Cs
  decay measured
• FWHM 23.2keV
• High energy region
• FWHM of 662keV 137Cs 7.1

137Cs
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Pulse Shape Discrimination Phoswich

• Thick natTh source used with 1cm3 CsI(Tl) 1cm3
  Plastic detector
• Select events by leading-edge discriminator
  programmed in PC GUI
• Cut signals in plastic determined by FAST/SLOW
• Discard ADC overflow

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Particle ID Cs-137 Co-60

• PID TAIL/TOTAL

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Particle ID in CsI(Tl) phototube

• Distinct bands obtained for a-particles and
• g-rays
• Cosmics passing through CsI(Tl) look like g-rays.
  
• Energy independent PID
• FOM 1.85, constant for 1 to 4 MeV
• FOM drops to 0.78 for 0.5 to 1 MeV
• Not as good as FOMECsI(Tl) photodiode
• PID windows not yet optimized.
• Digital smoothing filter not yet applied.
• FOM peak separation/ SFWHM

1 W. Skulski et al, Nucl. Instr. and Meth. A
458 (2001) 759
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Conclusion

• Wide range of signals handled by DDC-1, including
  fast plastic signals.
• Nyquist filter is crucial for fast pulses.
• NaI(Tl) g-ray spectra also show X-ray peaks at
  33keV.
• Pulse shape discrimination demonstrated with
  CsI(Tl).
• Energy independent PID obtained.
• PID not as good as CsIphotodiode.
• PID algorithms will be optimized.
• Applications of the DDC-1
• Algorithm development, student projects.