S. Zuberi, University of Rochester
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
USBprocessor connector
FPGA
JTAG connector
Fast reconstruction DAC 65 MHz * 12 bits
Signal OUT
Signal IN
Variablegain amp
ADC 65 MHz * 12 bits
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
S. Zuberi, University of Rochester
DDC-1 Digital Pulse Processor
S. Zuberi, University of Rochester
Response to Scintillator Pulses
• Fast Plastic Scintillator BC-404– Original decay time: 1.8ns
– Nyquist filter fc=20 MHz
• Good response to very fast pulse
ADC trace Sample value
Sample number20.0 30.0 40.0 50.0 60.0
1400.0
1600.0
1800.0
2.0E+3
2200.0
Samples
1 sample = 20.8 ns
• Slower Scintillator Pulse:–Signal from Bicron NaI(Tl)
–Effective Decay time: 0.23s
• Good response to slower pulse
S. Zuberi, University of Rochester
Response to scintillator pulses: Phoswich Detector
CsI(Tl) crystal
cosmic ray
phototube
teflon
Bicron BC-404FAST
SLOWADC trace Sample value
Sample number0.0 50.0 100.0 150.0 200.0
1700.0
1800.0
1900.0
2.0E+3
2100.0
SLOW
FAST
Samples
• Fast plastic pulse clearly separated from slower decay in CsI(Tl)
S. Zuberi, University of Rochester
Response to scintillator pulses: CsI(Tl)
• natThorium source:
-particle– High ionization density
– Overall decay time: 0.425s
-ray– Low ionization density
– Longer overall decay time than -particle (0.695s for electron)
• Clear pulse shape dependence on type of radiation
ADC trace Sample value
Sample number50.0 100.0 150.0 200.0
1900.0
1950.0
2.0E+3
2050.0
2100.0
gamma-ray
Samples
ADC trace Sample value
Sample number50.0 100.0 150.0 200.0
2.0E+3
2050.0
2100.0
alpha-particle
Samples
S. Zuberi, University of Rochester
Gamma Ray Spectra
• 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%
60Co
137Cs
S. Zuberi, University of Rochester
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
S. Zuberi, University of Rochester
Particle ID: Cs-137 & Co-60
• PID = TAIL/TOTAL
Compton Scattering
662keV
S. Zuberi, University of Rochester
Particle ID in CsI(Tl) + phototube
• Distinct bands obtained for -particles and rays• Cosmics passing through CsI(Tl) look like 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/ FWHM
[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) -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.