Real time digital assay of mixed radiation fields

• Main Author: Aspinall, Michael Douglas
• Published: Lancaster University 2008
• Subjects: 539.722
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.578245

This thesis reports on the development of several digital techniques for the use in neutron metrol- ogy. These include, a neutron/γ discriminator algorithm for organic liquid scintillators, a method for characterizing these detectors empirically, a technique for unraveling piled-up events and a Sample-Interpolation Timing (SIT) pick-off method for high resolution neutron Time-of-Flight (TOF) measurements. All of these techniques were developed specifically for use in the digital domain, thus exploiting inherent advantages of processing signals digitally. Pulses in the time domain, arising from the interaction of photons and neutrons in an or- ganic liquid scintillator, were recorded digitally from several mixed-field sources; including ameri- cium/beryllium, californium-252 and a pulsed neutron source. Neutrons and γ rays were suc- cessfully distinguished in all of these data using a novel digital discrimination technique known as Pulse Gradient Analysis (PGA). The PGA method was verified against the discrimination of event types based on their flight time over a known distance afforded by the digital capture of events pulsed in time. Both qualitative and quantitative comparisons are reported and the discrimination afforded by PGA is observed to be consistent with that achieved by digital TOF. PGA is simple and exploits samples early in the life of the pulse, thus it is compatible with current embedded system technologies, offers a degree of immunity to pulse pile-up and heralds a real-time means for neutron/γdiscrimination that is fundamental to many potential industrial applications. < The digital recording and discrimination of mixed radiation fields has afforded the devel- opment of an empirical method for the characterization of scintillators, in terms of their pulse shape, which is quick and easy. It provides generic neutron and γ(-ray pulse shapes for the detec- tor that can be used to derive analytical descriptions of each pulse via an accepted six-parameter formulism. PGAs capability to discriminate between event types early in the life of the pulse offers the ability to discriminate signals disrupted by pulse pile-up. The identification and disentanglement of synthesized piled-up data is demonstrated . A unique, digital time pick-off method, known as SIT is described. This method demonstrates the possibility of improved timing resolution for the digital measurement of TOF compared with digital replicas of analogue time pick-off methods for signals sampled at relatively low rates. Three analogue timing methods were replicated in the digital domain (leading-edge, crossover and constant-fraction timing). SIT was developed solely for the digital domain and thus per- forms more efficiently on digital signals compared with analogue time pick-off methods replicated digitally, especially for fast signals that are sampled at rates that current affordable and portable devices can achieve. SIT can be applied to any analogue timing method replicated digitally and thus also has the potential to exploit the generic capabilities of analogue techniques with the benefits of operating in the digital domain. Finally, this thesis reports on the design and implementation of a fast digitizer and embedded- processor for on-the-fly signal processing of events from scintillators. PGA was deployed on this bespoke hardware and demonstrates remarkable improvement over analogue methods for the assay of mixed fields and the real-time discrimination of neutrons and γ rays.