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.