Digital Signal Processing of Scintillator Pulses

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.