A new positron annihilation lifetime spectrometer based on DRS4 waveform digitizing board

Abstract

A new simple digital positron lifetime spectrometer has been developed. It includes a DRS4 waveform digitizing board and two scintillation detectors based on the XP2020Q photomultiplier tubes and LaBr3 scintillators. The DRS4 waveform digitizing can handle small pulses, down to few tens of millivolts, and its time scale linearity and stability are very good. The new system has reached a 206 ps time resolution, which is better than the conventional analog apparatus using the same detectors. These improvements make this spectrometer more simple and convenient in comparison with other spectrometers, and it can be applied to the other scintillation timing measurements with picosecond accuracy.

Study of time resolution by digital methods with a DRS4 module

“…The pulse area between the digitized waveform and the baseline is related to γ ray energy. The value of the pulse area is simply to sum the amplitude value of each channel over the whole waveform range by a digital integration method [4,9]. The obtained energy resolutions by the DRS4 module at 511 keV are 3.42% and 4.10% respectively for the two detectors.…”

“…At present, various kinds of DPP modules have been realized using switched capacitor circuits, for example MAGIC [1] and MEG [2]. They can also be useful for PET (positron emission tomography) scanners and portable oscilloscopes [3,4]. DPPbased systems are more simple and convenient than other systems.…”

“…The Paul Scherrer Institute, in Switzerland, has designed a new DPP module, the DRS4 chip [5,6], which is based on a switched capacitor array (SCA) [3−7]. Its characteristics of high channel density, high analog bandwidth of 950 MHz, and low noise of 0.35 mV make this chip ideally suited for high precision waveform digitization [4,7]. In order to upgrade the read-out circuits of TOF-PET (time-of-flight PET) systems, a new type of readout and acquisition system based on the DRS4 chip has been developed by the Nuclear Electronics Group in the Institute of Modern Physics, Chinese Academy of Sciences.…”

“…The module consists of a DRS4 chip, an ADC chip, a FPGA and other devices. It is capable of performing eight channel waveform sampling and digitization at a maximum sampling rate of 5.12 gigabit samples per second (GSPS) with 66 dB input dynamic range [4,7]. The internal time resolution between channels is about 52 ps as tested by the cable delay method [7].…”

“…The DRS4 module has been employed with fast scintillators and PMTs for coincidence timing measurements. During the experiments, the arrival time difference of two coincident 22 Na γ-rays were detected by two LaBr 3 scintillators coupled with XP20D0s [4,9], by means of various DPP techniques, such as leading edge discrimination (LED) [9][10][11][12][13], digital constant fraction zero-crossing discrimination (CFD zero-crossing) [9−12] and digital constant fraction discrimination (dCFD) [9][10][11]13]. Each method was repeated a large number of times to optimize the parameters for the best resolution and accuracy.…”