Digital pulse processing for NaI(Tl) detectors

Abstract

We apply two different post-processing techniques to digital pulses induced by photons in NaI(Tl) detector and compare the obtained energy resolution to the standard analog approach. Our digital acquisition approach is performed using a single-stage acquisition with a fast digitizer. Both the postprocessing techniques we propose rely on signal integration. In the first, the pulse integral is calculated by directly numerically integrating the pulse digital samples, while in the second the pulse integral is estimated by a model-based fitting of the pulse. Our study used a 7.62 cm x 7.62 cm cylindrical NaI(Tl) detector that gave a 7.60% energy resolution (at 662 keV), using the standard analog acquisition approach, based on pulse shaping amplifier. The new direct numerical integration yielded a 6.52% energy resolution. The fitting approach yielded a 6.55% energy resolution, and, although computationally heavier than numerical integration, is preferable when only the early samples of the pulse are available. We also evaluated the timing performance of a fast-slow detection system, encompassing an EJ-309 and a NaI(Tl) scintillator. We use two techniques to determine the pulse start time: constant fraction discrimination (CFD) and adaptive noise threshold timing (ANT), for both the analog and digital acquisition approach. With the analog acquisition approach, we found a system time resolution of 5.8 ns and 7.3 ns, using the constant fraction discrimination and adaptive noise threshold timing, respectively. With the digital acquisition approach, a time resolution of 1.2 ns was achieved using the ANT method and 3.3 ns using CFD at 50% of the maximum, to select the pulse start time. The proposed direct digital readout and post-processing techniques can improve the application of NaI(Tl) detectors, traditionally considered 'slow', for fast counting and correlation measurements, while maintaining a good measurement of the energy resolution.2

Performance comparison between signal digitizers and low-cost digital oscilloscopes: spectroscopic, pulse shape discrimination and timing capabilities for nuclear detectors

“…Digitizers normally do not require preamplifiers or particular signal processing [1,2], greatly simplifying the DAQ schematics, even though in some cases a preamplifier is needed. The performances can be comparable or better than the traditional analog counterparts, in terms of energy and time resolution and of acquisition rate [1,3]. Depending on the desired performances, digitizers may be costly as the cost per acquisition channel could be of the order of 1 k$.…”

High-resolution energy and fast timing measurements for nuclear spectroscopy using digital signal processing

“…The performance of LaBr 3 :Ce detectors, in terms of both energy and timing resolution, utilizing digital techniques, was studied [14]. The use of digital pulse processing for scintillation detectors was reported [15]. The application of a digital system for lifetime measurement was described [16].…”

Digital pulse deconvolution method for current tails of NaI(Tl) detectors

“…Due to its slow time response, the problem of pulse pileup still cannot be resolved effectively for high count rate applications [2]. With the development of high-speed sampling techniques, the directly digitized current signals of NaI(Tl) detectors can be implemented to obtain a narrow pulse [3]. However, this means the detector should be equipped with a high-speed analog-to-digital converter (ADC) and corresponding high-speed signal processing circuits, all of which will increase the cost of the system.…”