Speaker
Dr
Mohammad Nakhostin
(University of Surrey)
Description
Fast scintillation detectors such as LaBr3(Ce) and CeBr3 have appeared as a good candidate for timing and energy measurements in nuclear physics experiments and also for use in medical imaging modalities such as positron emission tomography (PET) systems. Since in such applications the number of data channels is generally large, a proper multi-channel data acquisition system is required to exploit the excellent properties of the detectors. In recent years, digital signal processing techniques have gained popularity for the development of multi-channel data acquisition systems due to their high degree of flexibility and stability and also the possibility of offline analysis. In this paper, we present the results of timing and energy measurements using digital signal processing of LaBr3(Ce) detectors. The photomultiplier (PMT) output signals from two identical LaBr3(Ce) detectors with a cylindrical shape (1.5" diameter and 2" tall) are directly digitized using an ultrafast digitizer with sampling rates adjustable up to 4 GSample/s and with 10-bits resolution. To extract energy information, the sampled signals were processed using different algorithms such as digital trapezoidal and integration-differentiation (CR-RC) pulse filters, as well as, with a simple integration of PMT current signals. The results show that the energy resolution does not critically depend on the type of pulse filter, and therefore, a simple integration of pulses is sufficient to produce an energy resolution comparable with the analog results (3.5% at 662 keV energy). The effect of pulse sampling rate on the energy resolution was also studied, indicating a degradation of the resolution with reducing the pulse sampling rate. In regard to the timing measurements, a digital version of constant-fraction discrimination (CFD) method was used. A time resolution of 240 ps (FWHM) was achieved using the energy lines of 60Co with a pulse sampling rate of 4 GS/s. This time resolution is superior to the result obtained with the standard NIM modules (305 ps) due to the possibility of a fine adjustment of the time pickoff parameters. However, our studies show that at lower sampling rates (below 1 GS/s), the digital timing can be limited by the aliasing error. By using an anti-aliasing filter before the pulse sampling process, a time resolution of 375 ps (FWHM) and an energy resolution of 3.5 % at 662 keV was achieved with a sampling rate of 500 MS/s. Pulses with such sampling rate can be conveniently processed with the available FPGA (Field programmable gate arrays) technology making it possible to build compact digital LaBr3(Ce) detection systems.
Authors
Dr
Mohammad Nakhostin
(University of Surrey)
Prof.
Paddy Regan
(Surrey University)
Prof.
Zsolt Podolyak
(University of Surrey)
