Speaker
Description
The selective sampling method (SESAM) was proposed by Müller from the BIPM, as an alternative to the β-γ coincidence counting. The SESAM does not need to introduce a coincidence resolving time during which accidental coincidences could be recorded. Correcting of these accidental coincidences is the trickiest part of β-γ coincidence counting. Compare to coincidence or anticoincidence approaches, the principle of the SESAM is simple and does not require sophisticated corrections. However, since its invention, the SESAM has not been widely used due to the too long dwell time of the commercially available MCA. In recent years, high-frequency digitizers have appeared on the market, enabling the SESAM to be implemented digitally. For the first time, digital SESAM was implemented on a primary measurement system consisting of a proportional counter (PC) for the beta channel and a NaI(Tl) scintillator (NaI) for the gamma channel, named by the acronym 4πβ(PC)-γ(NaI). The signal processing is performed by a CAEN DT5730 digitizer. The shaped analog signals from PC and NaI detectors are sent as inputs to the digitizer. The digitizer processes the digitized pulse signals from the detectors using the DPP-QDC real-time firmware embedded in the FPGA. The output data formed by the {timestamp, energy} doublet of each pulse is stored in list-mode files (CSV format). Dedicated software (D-SESAM) has been developed for offline implementation of digital SESAM. D-SESAM consists of three modules, namely “software-based circuits module”, “dead time and background correction module” and “efficiency extrapolation module”. The "software based circuit module" performs an extendable dead-time on the beta channel, a delay time on the gamma channel and builds the time distribution between gamma and beta events. The β efficiency (ε) can be easily derived from the time distribution spectrum using the ratio of the counts of the region of interest preceding the beta event (g) and the region of interest following the beta event (G), such as ε=1-g/G. The “dead time and background correction module” is used to perform the dead-time and background corrections on the counts (g and G) involved in the calculation of the β efficiency, and to correct counting losses caused by dead time in the β channel. The “efficiency extrapolation module” is applied to measure and correct the γ sensitivity in the β channel. To demonstrate the performance of the 4πβ(PC)-γ(NaI) digital SESAM, the technique was applied to a standard solution of Co-60 standardized using the conventional 4πβ(PC)-γ coincidence counting method. The result from SESAM is in good agreement with the one obtained by 4πβ(PC)-γ coincidence counting, with a relative deviation of 0.3%. The digital version of SESAM, through the use of a digitizer, overcomes the limitations that existed in analogue architectures and brings this primary calibration approach back to the fore. Corresponding author e-mail address: liuhr@nim.ac.cn
