Transcript Digital Signal Processing of Scintillator Pulses

Digital Signal Processing of Scintillator Pulses
Saba Zuberi, Wojtek Skulski, Frank Wolfs
University of Rochester
S. Zuberi, University of Rochester
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
S. Zuberi, University of Rochester
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
S. Zuberi, University of Rochester
Signal IN
Signal OUT
Fast reconstruction DAC 65 MHz * 12 bits
DDC-1 Digital Pulse Processor
S. Zuberi, University of Rochester
Response to Scintillator Pulses
• Fast Plastic Scintillator BC-404 • Slower Scintillator Pulse:
– Original decay time: 1.8ns
– Nyquist filter fc=20 MHz
• Good response to very fast
pulse
Sample value
–Signal from Bicron NaI(Tl)
–Effective Decay time: 0.23ms
• Good response to slower
pulse
ADC trace
2200.0
2.0E+3
1800.0
Samples
1600.0
1400.0
20.0
30.0
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40.0
Sample number
50.0
60.0
1 sample = 20.8 ns
Response to scintillator pulses: Phoswich Detector
• Fast plastic pulse clearly
separated from slower decay
in CsI(Tl)
Sample val ue
ADC trace
cosmic ray
SLOW
CsI(Tl)
crystal
Samples
2100.0
SLOW
2.0E+3
FAST
Bicron
BC-404
1900.0
FAST
1800.0
phototube
1700.0
0.0
50.0
100.0
150.0
Sample number
200.0
teflon
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Response to scintillator pulses: CsI(Tl)
Sample value
ADC trace
•
natThorium
source:
a-particle
2100.0
alpha-particle
– High ionization density
– Overall decay time: 0.425ms
2050.0
Samples
2.0E+3
50.0
100.0
150.0
Sample number
Sample value
200.0
ADC trace
2100.0
2050.0
1950.0
Samples
1900.0
50.0
100.0
150.0
Sample number
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– Low ionization density
– Longer overall decay time than aparticle (0.695ms for electron)
• Clear pulse shape dependence
on type of radiation
gamma-ray
2.0E+3
g-ray
200.0
Gamma Ray Spectra
• Signals obtained from Bicron 2” x
2” NaI(Tl)
• X-rays from excitation of Pb casing
of detector
• Low energy region:
–
60Co
56Ba
characteristic x-ray, 33keV,
from 137Cs decay measured
– FWHM = 23.2keV
• High energy region :
– FWHM of 662keV 137Cs: 7.1%
S. Zuberi, University of Rochester
137Cs
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
Compton
Scattering
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662keV
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 FOME<1MeV = 1.89 obtained [1] for
CsI(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
S. Zuberi, University of Rochester
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 CsI+photodiode.
– PID algorithms will be optimized.
• Applications of the DDC-1:
– Algorithm development, student projects.
S. Zuberi, University of Rochester