Detection Of Special Nuclear Material Research Articles

Mixed Source Interrogation of Steel Shielded Special Nuclear Material Using an Intense Pulsed Source

This paper explores the benefits of using a mixed photon and neutron radiation source for active detection of special nuclear material. More than fifty irradiations were performed using an 8 MV electron accelerator employing and induction voltage adder (IVA). The experiments used a high atomic number converter to produce a Bremsstrahlung photon spectrum which was then used to create a neutron source via a nuclear interaction with heavy water (deuterium oxide, D2O). This mixed particle source was used to irradiate a depleted uranium (DU) sample, inducing fission in the sample. Several thicknesses of steel shielding were tested in order to compare the performance of the mixed photon and neutron source to a Bremsstrahlung-only source. An array of detectors were fielded to record both photons and neutrons emitted by the fission reactions. A correlation between steel shielding and a detection figure-of-merit can be seen in all cases where the Bremsstrahlung-only source was used. The same relationship for the mixed photon-neutron source is less consistent. The data collected from the fielded detectors is compared to MCNP6 calculations and good agreement is found.

Development of a Time-tagged Neutron Source for SNM Detection

Associated particle imaging (API) is a powerful technique for special nuclear material (SNM) detection and characterization of fissile material configurations. A sealed-tube neutron generator has been under development by Lawrence Berkeley National Laboratory to reduce the beam spot size on the neutron production target to 1mm in diameter for a several-fold increase in directional resolution and simultaneously increases the maximum attainable neutron flux. A permanent magnet 2.45GHz microwave-driven ion source has been adopted in this time-tagged neutron source. This type of ion source provides a high plasma density that allows the use of a sub-millimeter aperture for the extraction of a sufficient ion beam current and lets us achieve a much reduced beam spot size on target without employing active focusing. The design of this API generator uses a custom-made radial high voltage insulator to minimize source to neutron production target distance and to provide for a simple ion source cooling arrangement. Preliminary experimental results showed that more than 100 μA of deuterium ions have been extracted, and the beam diameter on the neutron production target is around 1 mm.

Investigation of Active Background From Photofission in Depleted Uranium Using Cherenkov Detectors and Gamma Ray Time-of-Flight Analysis

Prompt gammas from induced fission could potentially be a valuable fission signature to use for detection of special nuclear materials like highly enriched uranium. Detectability requires an understanding of both signal and background, and the background from highly enriched uranium can be understood using depleted uranium as a surrogate. In order to study the prompt gamma active backgrounds from special nuclear materials induced through photofission, a combination of measurements using the Idaho Accelerator Center 44 MeV Electron Linear Accelerator and Monte Carlo N-Particle simulations were used. In this paper, we discuss the prompt, time-gated glass Cherenkov detector response to gamma emissions resulting from short pulse 6, 10, and 25 MeV bremsstrahlung beams incident on a depleted uranium target. Considering any threshold used with the Cherenkov detectors, the prompt gamma signal from depleted uranium is buried in the active background from the interrogating photon source. This paper describes the contributions to this prominent prompt active background observed in our measurements.

Passive and Active Correlation Techniques for the Detection of Nuclear Materials

In the frame of the French trans-governmental R&D program against CBRN-E threats, CEA (French Alternative Energies and Atomic Energy Commission) is studying the detection of Special Nuclear Materials (SNM) by neutron interrogation with the Associated Particle Technique (APT). Coincidences including at least 3 fission neutrons or gamma rays induced by tagged neutrons are used to detect and distinguish SNM from benign materials in which lower multiplicity events of 1 or 2 particles are produced by (n,2n) or (n,n' ) reactions. Coincidences are detected by fast plastic scintillators and correlated with tagged neutrons to improve the signal-to-noise ratio. Dedicated data acquisition electronics (DAQ) have been developed with independent FPGA cards associated with each detector, so that the acquisition window can be opened by any of the plastic scintillators. DAQ tests in passive mode are presented, in which acquisition is triggered by the sum signal of all detectors. The system time and energy calibration and resolution are reported, as well as the qualification of numerical simulations thanks to experimental data acquired with simple setups using a source. Numerical studies for the design and performance of cargo container inspection are also performed with the MCNP-PoliMi computer code and the ROOT data analysis package. SNM detection in iron matrix is quite straightforward, but in organic matrix, data processing will need to combine more information to evidence SNM.

Gamma/neutron time-correlation for special nuclear material detection – Active stimulation of highly enriched uranium

The time-correlated pulse-height technique can distinguish multiplying (special nuclear material) from non-multiplying sources. The technique relies upon the measurement of correlated photon–neutron pairs using organic liquid scintillation detectors. For such interactions, the distribution of measured neutron recoil energy versus the time-of-flight difference between correlated photons and neutrons are imprinted with the fission chain dynamics of the source. The theoretical time-of-arrival assuming the photons and neutrons are created in the same fission is calculated. Correlated pairs with longer time-of-arrival indicate delays caused by self-induced fission chains in a multiplying source. For the specific circumstances of simulated measurements of 25.4kg of highly enriched uranium at 50cm source to detector distance, correlated pairs from fission chains can arrive upwards of 40ns later than correlated pairs with the same neutron energies from non-multiplying sources like 252Cf at the same source detector distance. The use of detectors with ns scale time resolution and the use of pulse digitization allows for the distinction of these events.This method has been used successfully in the past to measure a variety of plutonium-bearing samples. The particle transport code MCNPX-PoliMi has been used to simulate and validate these measurements as well. Due to the much lower signature emission rate of 235U, this technique has not yet been used to measure the presence of highly enriched uranium. In this work we therefore explore the use of the time-correlated pulse-height technique with the introduction of an interrogating neutron source to stimulate fission. The applicability of 252Cf, AmLi and a DD generator neutron sources is explored in a series of simulations. All three sources are viable options with their own pros and cons with the choice of appropriate source depending upon the intended application. The TCPH technique is envisioned as a viable measurement solution of special nuclear material in situations in which the presence of shielding material disqualifies the use of passive gamma spectroscopy or gamma spectroscopy reveals classified information on the special nuclear material’s isotopic composition.

A response function methodology to calculate induced fission neutron flux distribution in an active interrogation system

A hybrid deterministic and Monte Carlo radiation transport methodology is being developed for modeling an active interrogation system for detection of special nuclear material (SNM) in a container. This methodology includes four steps: i) (n, γ) interaction and subcritical multiplication in cargo; ii) (n, γ) interaction due to fission neutrons throughout the cargo; iii) gamma transport to the detector window; iv) detection of gamma-rays by the detector. This methodology enables accurate determination of neutron/gamma fluxes in real times for comparison with measurements, and examination of various materials compositions. In this paper, we will address the first step of the methodology. We have developed a response-function formulation to calculate the subcritical multiplication and the neutron flux distribution. Using the MCNP Monte Carlo code, the response coefficients are pre-calculated for various cargo materials and SNM combinations. We have demonstrated that the new response function methodology yields fission neutron rates which are in excellent agreement with the full Monte Carlo calculation predictions (within 10% for all sampled locations including those near the wall), while achieving significant speedups of several orders of magnitude.

A binned clustering algorithm to detect high-Z material using cosmic muons

We present a novel approach to the detection of special nuclear material using cosmic rays. Muon Scattering Tomography (MST) is a method for using cosmic muons to scan cargo containers and vehicles for special nuclear material. Cosmic muons are abundant, highly penetrating, not harmful for organic tissue, cannot be screened against, and can easily be detected, which makes them highly suited to the use of cargo scanning.Muons undergo multiple Coulomb scattering when passing through material, and the amount of scattering is roughly proportional to the square of the atomic number Z of the material. By reconstructing incoming and outgoing tracks, we can obtain variables to identify high-Z material. In a real life application, this has to happen on a timescale of 1 min and thus with small numbers of muons.We have built a detector system using resistive plate chambers (RPCs): 12 layers of RPCs allow for the readout of 6 x and 6 y positions, by which we can reconstruct incoming and outgoing tracks. In this work we detail the performance of an algorithm by which we separate high-Z targets from low-Z background, both for real data from our prototype setup and for MC simulation of a cargo container-sized setup. (c) British Crown Owned Copyright 2013/AWE

Pressurised xenon as scintillator for gamma spectroscopy

Detectors based on liquid or gas xenon have been used and are in use for a number of applications, in particular for the detection of gamma rays.Xenon is a well-suited medium for gamma spectroscopy thanks to itshigh atomic number and, consequently, large cross-section forphoto-electric absorption. This paper presents experimental studiesof high pressure xenon as a scintillator, with the aim of developing agamma ray detector for the detection of Special Nuclear Materials(SNM). The first goal was to study the dependence of the light yieldand of the energy resolution on the thermodynamic conditions. Wepresent preliminary results from an optimised version of thedetector.

Differential Time of Flight Technique for the Detection of Special Nuclear Materials

An adaptation of the differential die away (DDA) technique, which enhances the detection of fissile materials, has been developed and demonstrated in the laboratory. The differential time-of-flight (DTOF) technique applies to situations where special nuclear material (SNM) is located some distance (up to a few meters) away, is accessible from only one side, and is either bare or embedded in an environment that is slightly neutron moderating, and where the more conventional DDA pulsed neutron source technique is less effective. The technique is based on the fission reaction of thermal neutrons with fissile material, if present. The thermal neutrons are furnished by the moderation of neutrons from an electronic neutron generator (ENG). Fast neutrons are generated in relatively narrow pulses of tens to hundreds of microseconds. They are slowed down in a suitable moderator surrounding the source. The time of flight (TOF) of these neutrons, before arrival at the SNM, spreads over orders of magnitude due to the wide spread in their velocities, which lead to a time spread of the neutron-induced fission in the SNM. Neutron signatures resulting from the fissions, e.g., prompt fast neutrons (as well as few delayed neutrons) can then be detected by fast neutron detectors well after the original source neutrons have died away. The fast neutron detectors must be insensitive to the numerous source-related thermal neutrons and are a critical component of the technique, since they can differentiate, by time as well as energy, between the fission signatures and the source-related background. The choice of source moderator is also important to the success of the technique. Two moderators were designed and studied: one made of polyethylene and the other made of beryllium (Be). The latter is superior delivering a higher flux and longer neutron die-away times. The feasibility of the DTOF technique was demonstrated by detecting a sample of 350 g 235U (in the form of 19.9% enriched uranium) at a range of distances and under a variety of conditions employing a commercial (d, T) pulsed neutron generator and the two moderators.

MCNP Simulation Benchmarks for a Portable Inspection System for Narcotics, Explosives, and Nuclear Material Detection

MCNPX simulations have been used to guide the development of a portable inspection system for narcotics, explosives, and special nuclear material (SNM) detection. The system seeks to address these threats to national security by utilizing a high-yield, compact neutron source to actively interrogate the threats and produce characteristic signatures that can then be detected by radiation detectors. The portability of the system enables rapid deployment and proximity to threats concealed in small spaces. Both dD and dT electronic neutron generators (ENG) were used to interrogate ammonium nitrate fuel oil (ANFO) and cocaine hydrochloride, and the detector response of NaI, CsI, and LaBr3 were compared. The effect of tungsten shielding on the neutron flux in the gamma ray detectors was investigated, while carbon, beryllium, and polyethylene ENG moderator materials were optimized by determining the reaction rate density in the threats. In order to benchmark the modeling results, experimental measurements are compared with MCNPX simulations. In addition, the efficiency and die-away time of a portable differential die-away analysis (DDAA) detector using 3He proportional counters for SNM detection has been determined.

SNM detection with an optimized water Cherenkov neutron detector

Special Nuclear Material (SNM) can either spontaneously fission or be induced to do so: either case results in neutron emission. For this reason, neutron detection performs a crucial role in the functionality of Radiation Portal Monitoring (RPM) devices. Since neutrons are highly penetrating and difficult to shield, they could potentially be detected escaping even a well-shielded cargo container. If the shielding were sophisticated, detecting escaping neutrons would require a highly efficient detector with close to full solid angle coverage. In 2008, we reported the successful detection of neutrons with a 250 liter (l) gadolinium doped water Cherenkov prototype [1]—a technology that could potentially be employed cost effectively with full solid angle coverage. More recently we have built and tested both 1-kl and 3.5-kl versions [2], demonstrating that very large, cost effective, non-flammable and environmentally benign neutron detectors can be operated efficiently without being overwhelmed by background. In this paper, we present a new design for a modular system of water-based neutron detectors that could be deployed as a real RPM. The modules contain a number of optimizations that have not previously been combined within a single system. We present simulations of the new system, based on the performance of our previous detectors. Our simulations indicate that an optimized system such as is presented here could achieve SNM sensitivity competitive with a large 3He-based system. Moreover, the realization of large, cost effective neutron detectors could, for the first time, enable the detection of multiple neutrons per fission from within a large object such as a cargo container. Such a signal would provide a robust indication of the presence of fissioning material, reducing the frequency of false alarms while increasing sensitivity.

SNM detection by means of thermal neutron interrogation and a liquid scintillation detector

The feasibility of using a pulsed neutron generator in a graphite assembly together with a single liquid scintillation detector for the detection of special nuclear materials is investigated. Thermal source neutrons induce fission in fissile material present in the sample. By means of pulse shape discrimination the detector signals from fast fission neutrons are easily identified among the signals from gamma rays and the interrogating thermal neutrons. The method has potential in applications for detection of special nuclear materials in shielded containers.

Event-by-event study of neutron observables in spontaneous and thermal fission

The event-by-event fission model freya is extended to spontaneous fission of actinides and a variety of neutron observables are studied for spontaneous fission and fission induced by thermal neutrons with a view toward possible applications for detection of special nuclear materials.

Superheated emulsions for the detection of special nuclear material

Abstract A novel solution for the detection and smuggling interdiction of special nuclear materials is presented here consisting of large detector modules which contain superheated emulsions and which are readout with an optical approach. The detectors can be produced to be fully sensitive to prompt fission neutrons and totally insensitive to the interrogation beam, whether X-rays or neutrons below a chosen energy threshold. Therefore, the detectors are able to operate while the selected interrogation beam is on and they will only pick up the signal from fission neutrons. A position-sensitive readout mechanism is used in our design, relying on the scattering of light by neutron-induced bubbles. A beam of coherent light crosses the active area of the detector, and local variations in scattered light due to the presence of bubbles are detected in real time by arrays of silicon planar photodiodes affixed along the whole length of the detector. The system may offer a variety of advantages compared to current approaches, such as the possibility of simultaneous irradiation and detection, i.e. a 100% duty cycle, without requiring complex signal analysis, and high signal-to-noise ratio, minimizing costly nuisance alarms, thanks to its inherent insensitivity to photons.

Special nuclear material detection with a mobile multi-detector system

The detection of special nuclear material has been studied with a mobile inspection system used both as a high sensitivity passive neutron/gamma spectroscopic tool and as an active inspection device using tagged neutrons. The detection of plutonium samples seems to be possible with passive interrogation, even for small samples, thanks to the yield of gamma ray and neutrons. Moreover the gamma ray spectrum shows clear signatures related to 239Pu. The passive detection of uranium is much more difficult because of the low neutron yield and of the easiness of shielding the gamma ray yield of highly enriched U samples. However, we show that active interrogation with tagged neutrons is able to provide signatures for the discrimination of uranium against other heavy metals.

Review of Hybrid (Deterministic/Monte Carlo) Radiation Transport Methods, Codes, and Applications at Oak Ridge National Laboratory

** This paper provides a review of the hybrid (Monte Carlo/deterministic) radiation transport methods and codes used at the Oak Ridge National Laboratory and examples of their application for increasing the efficiency of real-world, fixed-source Monte Carlo analyses. The two principal hybrid methods are (1) Consistent Adjoint Driven Importance Sampling (CADIS) for optimization of a localized detector (tally) region (e.g., flux, dose, or reaction rate at a particular location) and (2) Forward Weighted CADIS (FW-CADIS) for optimizing distributions (e.g., mesh tallies over all or part of the problem space) or multiple localized detector regions (e.g., simultaneous optimization of two or more localized tally regions). The two methods have been implemented and automated in both the MAVRIC sequence of SCALE 6 and ADVANTG, a code that works with the MCNP code. As implemented, the methods utilize the results of approximate, fast-running 3-D discrete ordinates transport calculations (with the Denovo code) to generate consistent space- and energy-dependent source and transport (weight windows) biasing parameters. These methods and codes have been applied to many relevant and challenging problems, including calculations of PWR ex-core thermal detector response, dose rates throughout an entire PWR facility, site boundary dose from arrays of commercial spent fuel storage casks, radiation fields for criticality accident alarm system placement, and detector response for special nuclear material detection scenarios and nuclear well-logging tools. Substantial computational speed-ups, generally O(10 2-4 ), have been realized for all applications to date. This paper provides a brief review of the methods, their implementation, results of their application, and current development activities, as well as a considerable list of references for readers seeking more information about the methods and/or their applications.

An Analysis of Intense Pulsed Active Detection (IPAD) for the Detection of Special Nuclear Materials

We propose and analyze computationally the Intense Pulsed Active Detection (IPAD) of fissile materials. This approach employs bremsstrahlung photons from a single intense 100-ns interrogating pulse produced by a pulsed-power generator. The IPAD system requires only 3 s to detect delayed neutrons. In contrast, a multiple-pulse-train LINAC system requires several minutes. Because of the much smaller interrogation time, the IPAD system offers a much greater contrast between the induced signal and the cosmic-ray neutron background. For a similar signal-to-noise ratio as the LINAC system the IPAD offers a greatly reduced tissue dose. For a similar dose the IPAD offers a greater signal-to-noise with a much smaller false alarm rate. Our detailed numerical analysis employs the ITS and MCNPX codes, from which we have calculated critical quantities such as the number of delayed neutrons per unit dose and the number of prompt fission neutrons that can be distinguished from photoneutrons created in benign materials such as would arise from 207Pb. Some fundamental scaling relations have also been developed from our computations. For bremsstrahlung endpoint energies (Eend) well below the giant dipole resonance, the neutrons per dose increase as the 9th power of the Eend which transitions to the 2nd power as Eend increases beyond the giant dipole resonance. The tissue dose scales as the 2nd power of Eend throughout the range. Therefore, for any applications where the total dose is limited, no additional photo-fission neutrons are produced at high bremsstrahlung endpoints.

Tensioned Metastable Fluid Detectors in Nuclear Security for Passively Monitoring of Special Nuclear Materials―Part A

This paper (constituting Part A) describes the transformational Tensioned Metastable Fluid Detector (TMFD) based method for “passive” detection of Special Nuclear Materials (SNMs) as related to nuclear security. Purdue University is developing novel, multi-purpose tension metastable fluid nuclear particle detectors by which multiple types of nuclear particles can be detected with high (90%+) intrinsic efficiency, spectroscopic capability, directional information, rapid response, large standoff and significant cost-savings compared with state-of-the-art systems. This paper focuses specifically on recent advances in the use of these novel detector systems for neutron spectroscopy. These techniques will then be discussed and evaluated in the context of area monitoring in waste processing applications with a focus on passive monitoring of radioactive source particles from SNMs. The companion paper (Part B) addresses TMFD technology as it pertains to active interrogation.

Linac based photofission inspection system employing novel detection concepts

Abstract Rapiscan Systems is developing a LINAC based cargo inspection system for detection of special nuclear material (SNM) in cargo containers. The system, called Photofission Based Alarm Resolution (PBAR) is being developed under a DHD/DNDO Advanced Technology Demonstration (ATD) program. The PBAR system is based on the Rapiscan Eagle P9000 X-ray system, which is a portal system with a commercial 9 MeV LINAC X-ray source. For the purposes of the DNDO ATD program, a conveyor system was installed in the portal to allow scanning and precise positioning of 20 ft ISO cargo containers. The system uses a two step inspection process. In the first step, the basic scan, the container is quickly and completely inspected using two independent radiography arrays: the conventional primary array with high spatial resolution and a lower resolution spectroscopic array employing the novel Z-Spec method. The primary array uses cadmium tungstate (CdWO4) detectors with conventional current mode readouts using photodiodes. The Z-Spec array uses small plastic scintillators capable of performing very fast (up to 108 cps) gamma-ray spectroscopy. The two radiography arrays are used to locate high-Z objects in the image such as lead, tungsten, uranium, which could be potential shielding materials as well as SNM itself. In the current system, the Z-Spec works by measuring the energy spectrum of transmitted X-rays. For high-Z materials the higher end of the energy spectrum is more attenuated than for low-Z materials and thus has a lower mean energy and a narrower width than low- and medium-Z materials. The second step in the inspection process is the direct scan or alarm clearing scan. In this step, areas of the container image, which were identified as high Z, are re-inspected. This is done by precisely repositioning the container to the location of the high-Z object and performing a stationary irradiation of the area with X-ray beam. Since there are a large number of photons in the 9 MV Bremsstrahlung spectrum above the photofission “threshold” of about 6 MeV, the X-ray beam induces numerous fissions if nuclear material is present. The PBAR system looks for the two most prolific fission signatures to confirm the presence of special nuclear materials (SNM). These are prompt neutrons and delayed gamma rays. The PBAR system uses arrays of two types of fast and highly efficient gamma ray detectors: plastic and fluorocarbon scintillators. The latter serves as a detector of fission prompt neutrons using the novel threshold activation detector (TAD) concept as well as a very efficient delayed gamma ray detector. The major advantage of TAD for detecting the prompt neutrons is its insensitivity to the intense source related backgrounds. The current status of the system and experimental results will be shown and discussed.

Tension metastable fluid detection systems for special nuclear material detection and monitoring

Tension metastable fluid states offer unique potential for radical transformation in radiation detection capabilities. States of tension metastability may be obtained in tailored resonant acoustic systems such as the acoustic tension metastable fluid detector (ATMFD) system or via centrifugal force based systems such as the centrifugal tension metastable fluid detector (CTMFD) system; both under development at Purdue University. In this paper we describe research results with CTMFD systems for use in the detection of key actinide isotopes constituting special nuclear materials (SNMs) in spent fuel. Tests in a CTMFD system demonstrate the ability to detect alpha activity (at ∼100% efficiency) of U-isotopes at concentrations of ∼100 ppb (which is unprecedented and about ×100–1000 more sensitive than from conventional liquid scintillation spectroscopy). An inherent capability of TMFD systems concerns on demand tailoring of fluid tension levels allowing for energy discrimination and spectroscopy. This appears especially useful to detect the key isotopes of U and other transuranic isotopes of Pu, Np, Am, and Cm that are at different stages of nuclear fuel reprocessing (i.e., UREX+).