Michael Aspinall

A Digital Method for the Discrimination of Neutrons and

A digital method for the discrimination of neutron and γ-ray events from an organic scintillator has been investigated by using frequency gradient analysis (FGA) based on the Fourier transform. Since the scintillation process and the photomultiplier tube (PMT) anode signal are often very noisy, most pulse-shape discrimination methods in a scintillation detection system (e.g., the charge comparison (CC) method or pulse gradient analysis (PGA)) using time-domain features of the signal depend greatly on the associated de-noising algorithm. In this research, the performance of the new FGA method and the PGA method have been studied and compared on a theoretical basis and then verified by time-of-flight (TOF). The frequency-domain features extracted by the FGA method exhibit a strong insensitivity to the variation in pulse response of the photomultiplier tube (PMT) and can be used to discriminate neutron and γ-ray events in a mixed radiation field. It is shown that the FGA method results in an increased figure of merit (FOM) which corresponds to a reduction in the area of overlap between neutron and γ-ray events. The FGA method has the potential to be implemented in current embedded electronic systems to provide real-time discrimination in standalone instruments.

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A Simplified Digital Charge Collection (SDCC) method of discrimination between neutron and gamma pulses in an organic scintillator is presented and compared to the Pulse Gradient Analysis (PGA) discrimination method. Data used in this research were gathered from events arising from the 7Li(p,n)7Be reaction detected by an EJ-301 organic liquid scintillator recorded with a fast digital oscilloscope. Time-of-Flight (TOF) data were also recorded and used as a second means of identification. The SDCC method is found to improve on the figure of merit (FOM) given by PGA method at the equivalent sampling rate.

Rays With Organic Scintillation Detectors Using Frequency Gradient Analysis

The design, build and test of a digital analyzer for mixed radiation fields is described. This instrument has been developed to provide portable, real-time discrimination of hard mixed fields comprising both neutrons and γ rays with energies typically above 0.5 MeV. The instrument in its standard form comprises a sensor head and a system unit, and affords the flexibility to provide processed data in the form of the traditional scatter-plot representation separating neutron and γ-ray components, or the full, sampled pulse data itself. The instrument has been tested with an americium-beryllium source in three different shielding arrangements to replicate the case in which there are only neutrons, only γ rays and where both neutrons and γ-rays are present. The instrument is observed to return consistent results.

A Wavelet Packet Transform Inspired Method of Neutron-Gamma Discrimination

The design, principle of operation and the results of measurements made with a four-channel organic scintillator system are described. The system comprises four detectors and a multiplexed analyzer for the real-time parallel processing of fast neutron events. The function of the real-time, digital multiple-channel pulse-shape discrimination analyzer is described together with the results of laboratory-based measurements with 252Cf, 241Am-Li and plutonium. The analyzer is based on a single-board solution with integrated high-voltage supplies and graphical user interface. It has been developed to meet the requirements of nuclear materials assay of relevance to safeguards and security. Data are presented for the real-time coincidence assay of plutonium in terms of doubles count rate versus mass. This includes an assessment of the limiting mass uncertainty for coincidence assay based on a 100 s measurement period and samples in the range 0-50 g. Measurements of count rate versus order of multiplicity for 252Cf and 241Am-Li and combinations of both are also presented.

The design, build and test of a digital analyzer for mixed radiation fields

The IAEA, in collaboration with the Joint Research Center (Ispra, IT) and Hybrid Instruments (UK), is developing a liquid scintillator-based neutron coincidence counting system to address a number of safeguards applications. Interest in this technology is increasing with the advent of high-flash point, nonhazardous scintillating fluids coupled with significant advances in signal processing electronics. Together, these developments have provided the enabling technologies to allow liquid scintillators to be implemented outside of a laboratory environment. Another important aspect of this detector technology is that it can be used with the current installed infrastructure of safeguards assay instruments and data acquisition electronics. It is also an excellent candidate for the replacement of 3He-based systems in many applications. As such, a comparison to an existing 3He-based system will be presented to contrast the differences and benefits for several applications. This paper will describe the experiments and associated modeling activities engaged to carefully characterize the detection system and refine the models. The latest version of MCNPX-PoliMi Monte Carlo modeling code was used to address the specific requirements of liquid scintillators. Additionally, this development activity has driven the collaborative development with Hybrid Instruments of a high-performance pulse shape discriminator (PSD) unit. Specific applications will be described with particular emphasis on those in which liquid scintillators provide immediate benefit over traditional detection methods.

Real-Time, Fast Neutron Coincidence Assay of Plutonium With a 4-Channel Multiplexed Analyzer and Organic Scintillators

The design, principle of operation and the results of the first measurements made with a 4-channel multiplexed analyzer for real-time parallel processing of fast scintillation detectors is described. Recent advancements in the performance of organic scintillation media for the detection of fast neutrons, not least the reduction in hazard and the recent advent of plastic scintillation materials exhibiting Pulse Shape Discrimination (PSD), has highlighted the possibility of using multiple detectors in systems based on detectors exploiting these media. In this paper a real-time, digital multiple-channel PSD analyzer is described based on a single-board solution with an integrated high-voltage supply and graphical user interface. It has been developed to meet the requirements of nuclear materials assay of relevance to safeguards and security, and tested in a variety of related laboratory-based environments.

Liquid scintillator-based neutron detector development

A unique, digital time pick-off method, known as sample-interpolation timing (SIT) is described. This method demonstrates the possibility of improved timing resolution for the digital measurement of time of flight compared with digital replica-analogue time pick-off methods for signals sampled at relatively low rates. Three analogue timing methods have been replicated in the digital domain (leading-edge, crossover and constant-fraction timing) for pulse data sampled at 8 GSa s−1. Events arising from the 7Li(p, n)7Be reaction have been detected with an EJ-301 organic liquid scintillator and recorded with a fast digital sampling oscilloscope. Sample-interpolation timing was developed solely for the digital domain and thus performs 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. Sample interpolation 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. A threshold in sampling rate with respect to the signal pulse width is observed beyond which further improvements in timing resolution are not attained. This advance is relevant to many applications in which time-of-flight measurement is essential.

A 4-channel multiplex analyzer for real-time, parallel processing of fast scintillators

The IAEA, in collaboration with the Joint Research Center (Ispra, IT) and Hybrid Instruments (Lancaster, UK), has developed a full scale, liquid scintillator-based active interrogation system to determine uranium (U) mass in fresh fuel assemblies. The system implements an array of moderate volume (~1000ml) liquid scintillator detectors, a multichannel pulse shape discrimination (PSD) system, and a high-speed data acquisition and signal processing system to assess the U content of fresh fuel assemblies. Extensive MCNPX-PoliMi modelling has been carried out to refine the system design and optimize the detector performance. These measurements, traditionally performed with 3He-based assay systems (e.g., Uranium Neutron Coincidence Collar [UNCL], Active Well Coincidence Collar [AWCC]), can now be performed with higher precision in a fraction of the acquisition time. The system uses a high-flashpoint, non-hazardous scintillating fluid (EJ309) enabling their use in commercial nuclear facilities and achieves significantly enhanced performance and capabilities through the combination of extremely short gate times, adjustable energy detection threshold, real-time PSD electronics, and high-speed, FPGA-based data acquisition. Given the possible applications, this technology is also an excellent candidate for the replacement of select 3He-based systems. Comparisons to existing 3He-based active interrogation systems are presented where possible to provide a baseline performance reference. This paper will describe the laboratory experiments and associated modelling activities undertaken to develop and initially test the prototype detection system.

Sample-interpolation timing: an optimized technique for the digital measurement of time of flight for γ rays and neutrons at relatively low sampling rates

Scintillation detectors offer a single-step detection method for fast neutrons and necessitate real-time acquisition, whereas this is redundant in two-stage thermal detection systems using helium-3 and lithium-6, where the fast neutrons need to be thermalized prior to detection. The relative affordability of scintillation detectors and the associated fast digital acquisition systems have enabled entirely new measurement setups that can consist of sizeable detector arrays. These detectors in most cases rely on photomultiplier tubes, which have significant tolerances and result in variations in detector response functions. The detector tolerances and other environmental instabilities must be accounted for in measurements that depend on matched detector performance. This paper presents recent advances made to a high-speed FPGA-based digitizer. The technology described offers a complete solution for fast-neutron scintillation detectors by integrating multichannel high-speed data acquisition technology with dedicated detector high-voltage supplies. This configuration has significant advantages for large detector arrays that require uniform detector responses. We report on bespoke control software and firmware techniques that exploit real-time functionality to reduce setup and acquisition time, increase repeatability, and reduce statistical uncertainties.

Development of a liquid scintillator-based active interrogation system for LEU fuel assemblies

A detector system for the characterization of radiation fields of both fast neutrons and γ rays is described comprising of a gated photomultiplier tube (PMT), an EJ299-33 solid organic scintillator detector, and an external trigger circuit. The objective of this development was to conceive a means by which the PMT in such a system can be actuated remotely during the high-intensity bursts of pulsed γ-ray contamination that can arise during active interrogation procedures. The system is used to detect neutrons and γ rays using established pulse-shape discrimination (PSD) techniques. The gating circuit enables the PMT to be switched off remotely. This is compatible with use during intense radiation transients to avoid saturation and the disruption of the operation of the PMT during the burst. Data are presented in the form of pulse-height spectra and PSD scatter plots for the system triggered with a strobed light source. These confirm that the gain of the system and the throughput for both triggered and un-triggered scenarios are as expected, given the duty cycle of the stimulating radiation. This demonstrates that the triggering function does not perturb the system response of the detector.