Compton Suppression System with Simplified Slow Anti-Coincidence Circuit

Slow coincidence circuits for the anti-coincidence measurements have been considered for use in Compton suppression technique. The simplified version of the slow circuit has been found to be fast enough, satisfactory and allows an easy system setup, particularly with the advantage of the automatic threshold setting of the low level discrimination. A well-type NaI detector as the main detector surrounded by plastic guard detector have been arranged to investigate the performance of the Compton suppression spectrometer using the simplified slow circuit. The system has been tested to observe the improvement in the energy spectra for medium to high-energy gamma-ray photons from terrestrial and environmental samples. INTRODUCTION In gamma-ray spectrometry measurements, some of the photons from the sample under investigation (placed on the top of a flat-detector or inside a well-type detector) are scattered within the radiation detector itself depositing part of their energy within the detector and escape. This leads to the generation of Compton associated background representing incomplete energy deposition of the incident photons, which leads to the distortion of the obtained spectrum from the actual energy distribution. It is possible to detect such (escaping) scattered photons by the use of a larger detector made of a less expensive material such as Plastic or NaI surrounding the main detector. By correlating events in the main detector and the surrounding “shield” detector with timing electronics, events counted in the shield detector can be used to reject simultaneous events in the main detector (veto signal). Although fast coincidence circuits are important for the ultimate in timing measurements and also commonly used in Compton suppression techniques (Hasinoff et al., 1974; Parus et al., 2003), the use of a slow timing circuit has been found to be satisfactory in similar measurements for low-level studies and was recommended due to its simpler electronic setup (Bikit et al., 1999). Furthermore, it has been reported that anti-coincidence measurements for the Compton suppression technique do not require fast coincidence circuits to provide good suppression. Equivalent results may be provided with "simple" instrumental arrangements without the need for special setup procedures or adjustments (). MATERIALS AND METHOD Some of the energy amplifiers (i.e. Canberra 2025 and 2026) are provided with an extra feature in which logic pulses are produced for each signal input from the detector “incoming event” (known as: Incoming Count Rate "ICR"), thus the need for additional electronic components (NIM units) such as T-SCA’s as used in standard Slow Circuits that produce the logic pulses for the timing analysis is omitted in this case. This arrangement is a simplified version of the slow coincidence circuit and results in a further reduction in the required number of electronic components (NIM units) compared to other circuits (fast and 1 Compton Suppression, Made Easy. Application Note AN-D-8901, Canberra, USA standard slow) as well as an easier system setup, particularly, due to the automatic adjustment of the low level discriminators, Figure 1. The main detector is a well-type NaI(Tl) of a size of 7.6 cm X 7.6 cm with a well size to accommodate vials of capacity of 20 ml. The surrounding active shield is a plastic detector of a well-type shape of size 18.4 cm diameter x 15.2 cm long with a well cavity that can accommodate the NaI(Tl) detector. The detector wall-thickness and bottom-thickness are of 5 cm thick, Figure 1. Both detectors (main well-NaI detector and surrounding guard detector) have been supplied with standard PM-tubes. Figure 1 The "Simplified Slow Coincidence circuit" is based on the use of the ICR outputs "logic pulses" available with the energy amplifiers (Canberra 2025/2026) for timing analysis. The main detector is the well-type NaI(Tl) detector where the sample contained in a vial is placed inside its cavity. The larger well-type detector is the plastic active shield. RESULTS AND DISCUSSION Pulse Gating The threshold of the discrimination level against the system noise can be examined by making the logic pulse from the discriminator to open the electronic gate for the same corresponding energy signal from the detector to pass through. Figure 2 shows the lower part of the energy spectra acquired with different amplifier gains using the Canberra amplifier (either model 2025 or 2026) that are provided with the additional feature of producing a logic pulse “ICR” (Incoming Count Rate) output for each preamplified input signal. As observed, the Auto-Discrimination Level is always at a fixed position regardless the amplifier gain setting when using the ICR logic outputs. However, increasing the amplifier gain will allow the discrimination level to be set at lower energy threshold values, if desired. Time Spectra for the Standard Slow and Simplified Slow coincidence circuits Evaluation/Comparison of the two different coincidence circuits (Standard Slow using Ortec T-SCA's and Simplified Slow using ICR outputs) requires the study of their time resolution. So, the two detectors were arranged opposite to each other for this measurement. Using Na source, the time resolution FWHM is 53.7 ns and 56 ns for the Canberra ICR and Ortec T-SCA circuits respectively, whereas the FW1/10M values are 152 ns and 128.5 ns for the Canberra ICR and Ortec T-SCA circuits respectively. Generally, both systems of Simplified Slow and Standard Slow circuits show a similar timing performance. Pre Amp Amp