Marek Flaska

Correlated neutron emissions from 252Cf

Abstract This paper presents new experimental results of correlated, prompt neutron emission from the spontaneous fission of 252Cf. Specifically, we present correlated-neutron emission probabilities and average energies for two detected neutrons as a function of the angle between the two neutrons. Experimental results are compared to several Monte Carlo models that include the number, energy, and angular distributions of prompt neutrons from fission.

Dual-particle imager for standoff detection of special nuclear material

An advanced dual-particle imaging system is being developed for standoff, passive detection of special nuclear material. This system consists of three detector planes and will be capable of imaging both photons and fast neutrons. The ability of the system to detect fast neutrons makes it more difficult to effectively shield a threat source. This feature has an advantage over the commonly used Compton-camera systems, which are only sensitive to photons. Additionally, the detection of fast neutrons will allow for increased performance in regions with high levels of photon background radiation. The first two planes of the system consist of EJ-309 liquid scintillators and the third plane consists of NaI scintillators. This detector/plane combination allows image reconstruction using both neutrons and photons. In the liquid scintillators, neutron interactions are distinguished from photon interactions using an optimized pulse shape discrimination technique. The Monte Carlo transport code MCNPX-PoliMi has been used for the initial studies of this system due to its ability to track detailed information on interactions of interest and time-correlated particle production. This information has been used to optimize system parameters and has also allowed for investigation of image reconstruction techniques including simple backprojection and maximum likelihood expectation maximization (MLEM). A small-scale prototype is being developed for testing and validation of the simulations. This paper will analyze preliminary measurements and will also discuss simulations of several shielded source scenarios.

Use of an LGB detector in nuclear nonproliferation applications

Capture-gated detectors have attracted the attention of several researchers because of their promise for neutron spectroscopy in mixed neutron-gamma-ray fields. Examples are the boron-loaded detector (loaded with either natural B or B enriched in B-10), the Li-6-glass/plastic scintillation detector, and the lithium-gadolinium-borate (LGB) detector (loaded with Li-6, natural Gd, and B-10). In the work presented in this paper, the LGB plastic scintillation detector is investigated. This detector is sensitive to not only fast neutrons and gamma rays, but also to thermal neutrons. Some efforts have been made in the past to characterize this detector for several neutron and gamma-ray energies. In this paper, we compare measurements and Monte Carlo simulations for the LGB detector. In the comparisons, very good agreement is achieved, which indicates accurate Monte Carlo modeling. This work has applications in fields such as nuclear safeguards, nuclear nonproliferation, and national security.

Detection of Special Nuclear Material by means of promptly emitted radiation following photonuclear stimulation

Techniques have been developed to exploit abundant prompt emissions from photonuclear reactions for the identification of special nuclear material (SNM). These enhancements are designed to reduce inspections times and delivered dose in systems which have, historically, relied solely on delayed emissions. Experimental evidence is presented for prompt neutron time-of-flight measurements, neutron/photon correlations in multiple detectors, and novel detector development, specifically LaBr3 scintillators with new gating and buffering circuits to identify prompt gamma signatures. Significant and specific signatures indicative of the presence of SNM can be distinguished for the prompt neutron time-of-flight experiment and the neutron/photon correlations in multiple detectors.

Warhead verification as inverse problem: Applications of neutron spectrum unfolding from organic-scintillator measurements

Verification of future warhead-dismantlement treaties will require detection of certain warhead attributes without the disclosure of sensitive design information, and this presents an unusual measurement challenge. Neutron spectroscopy—commonly eschewed as an ill-posed inverse problem—may hold special advantages for warhead verification by virtue of its insensitivity to certain neutron-source parameters like plutonium isotopics. In this article, we investigate the usefulness of unfolded neutron spectra obtained from organic-scintillator data for verifying a particular treaty-relevant warhead attribute: the presence of high-explosive and neutron-reflecting materials. Toward this end, several improvements on current unfolding capabilities are demonstrated: deuterated detectors are shown to have superior response-matrix condition to that of standard hydrogen-base scintintillators; a novel data-discretization scheme is proposed which removes important detector nonlinearities; and a technique is described for ...

A Data Processing System for Real-Time Pulse Processing and Timing Enhancement for Nuclear Particle Detection Systems

This paper presents a digital data processing system that allows real-time processing of neutron and gamma-ray pulses. The captured data are processed in real time on a field programmable gate array device. A number of pulse processing algorithms are implemented on this system including correlation-based triggering and on-the-fly pulse shape discrimination. Moreover, a timing enhancement technique using correlations is presented that allows sub-sample timing accuracy of the time-of-arrival of pulses. Finally, simulation and measurement results are presented and discussed in detail to show the systems potential.

Digital data acquisition and processing for a neutron-gamma-ray imaging system

A digital, data-acquisition system for use with a large number of detectors was set up at the University of Michigan. Fast waveform digitizers from CAEN Technologies with 8 channels were synchronized to create a fully scalable system, with a current set-up of 32 channels. While some of the systems limitations are still being investigated, the excellent time resolution of the system enabled accurate time-of-flight measurements at the Los Alamos Neutron Science Center (LANSCE).

Requirements and quantitative comparison of fast waveform digitizers for data-acquisition systems designed for nuclear nonproliferation applications

Accurate pulse shape discrimination (PSD) is essential for organic scintillators such as EJ-301s (or EJ-309s with higher flash point) because they are sensitive to both neutrons and gamma rays. Because of the background gamma-ray presence the accurate neutron detection requires accurate discrimination of neutrons from gamma rays. This is especially important for applications where fast and robust systems are paramount, such as nuclear nonproliferation and safeguards. For nuclear nonproliferation and safeguards applications, accurate discrimination of neutrons from gamma rays significantly influences the outcome of material identification/characterization. Specifically, particle misclassification can lead to longer measurement times or even to false identification/misclassification of measured material. In this paper, various fast waveform digitizers are compared from the PSD-performance point of view. Specifically, a 12-bit, 250-MHz digitizer is compared to a 12-bit, 500-MHz digitizer and a 10-bit 1-GHz/2-GHz digitizer. The results presented in this paper indicate that the 12-bit-500-MHz resolution combination leads to the best PSD results. The results also show that a 10-bit digitizer can perform better than a 12-bit digitizer, if the 10-bit digitizer has a significantly better time resolution.

Characterization of a Mixed Multiplicity Counter Based on Liquid Organic Scintillators

A measurement system capable of multiplicity measurements for both neutrons and gamma rays has been developed. The benefit of such an approach is in the increased number of available observables. A pure neutron assay results in three observables for third-order multiples, while a combined neutron/gamma-ray assay results in nine observables for the same order of multiples. The idea is to use the additional observables to achieve greater accuracy when determining unknown parameters of the sample such as the fissile mass. The measurement system is based on liquid scintillation detectors (EJ-309s) which feed detected pulses to a digital data-acquisition system. The excellent pulse shape discrimination capabilities of the EJ-309s allow for accurate differentiation between gamma-ray pulses and neutron pulses. The PSD is vital to correctly identify the different multiples up to the third order: n, γ , nn, nγ , γγ , nnn, nnγ , nγγ and γγγ . Previous investigation of the measurement system showed that good counting statistics can be achieved within minutes for spontaneous-fission sources such as 252Cf. In this paper, we present new measurement results and corresponding Monte Carlo simulations aimed at characterizing the measurement system. Comparison of the measured and simulated multiples is discussed in detail and a relatively good agreement is found.

Time-of-flight measurement for energy-dependent intrinsic neutron detection efficiency

Shortage in the current 3He supply has prompted a search for potential alternatives to the neutron detectors currently used in many nuclear nonproliferation and safeguards applications. An alternative detector must be efficient in detecting fission neutrons, and in rejecting or discriminating against gamma-ray radiation. For characterization of numerous detector types, it is helpful to have a technique for evaluating these two characteristics which is relatively fast and easy to perform. Here, a bench-top time-of-flight technique is presented which is based on a coincidence measurement with two ‘independent’ liquid scintillators (no direct source tagging is employed). The neutron source used is 252Cf. The technique can be used to measure energy-dependent intrinsic neutron detection efficiency for incident neutron energies of 0.5–5 MeV, as well as gamma-neutron discrimination efficiency. Measurement results are presented for three 2×2-inch cylindrical liquid scintillation detectors: EJ309, EJ315, and an additional EJ315 with naphthalene added.