Digital Signal Processing of Scintillator Pulses

1. Digital Signal Processing of Scintillator Pulses Saba Zuberi, Wojtek Skulski, Frank Wolfs University of Rochester

2. 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

3. 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

4. DDC-1 Digital Pulse Processor

5. 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 • Slower Scintillator Pulse: • Signal from Bicron NaI(Tl) • Effective Decay time: 0.23ms • Good response to slower pulse 1 sample = 20.8 ns

6. cosmic ray CsI(Tl) crystal SLOW Bicron BC-404 FAST phototube teflon Response to scintillator pulses: Phoswich Detector • Fast plastic pulse clearly separated from slower decay in CsI(Tl)

7. 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

8. 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

9. 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

10. Compton Scattering 662keV Particle ID: Cs-137 & Co-60 • PID = TAIL/TOTAL

11. 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

12. 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.