Detection Of Fissile Materials Research Articles

Determination of storage history of fissile materials based on activation effect of concrete floors

In the fields of arms control verification, homeland security, and antiterrorism, the detection and tracing of fissile materials is an extremely important work. In this paper, a method for determining the storage history of fissile materials based on the activation effect of concrete floors is proposed. This method is expected to be applied in the detection and tracing of fissile materials. First, a Monte Carlo simulation program was developed using the JMCT software capable of simulating the neutron activation effect of materials. Second, the activation effect of concrete floors under the irradiation of fission neutrons from fissile materials was studied using the numerical simulation program; it was found that 7 radionuclides, including 24Na, 27Mg, and 42K, can be used to determine the storage history of the fissile materials. Finally, the yields and numbers of the activation products caused by natural neutron background in concrete floors were calculated, as well as the detection limits of the fissile materials based on the above method.

Detection of special nuclear materials (SNM) using passive and pulsed neutron source based active coincidence methods

Detection of fissile and fertile materials and their accountability is important for nuclear safeguards. This paper reports detection of 238U and 235U in the same experimental setup using passive and pulsed neutron source based active coincidence methods respectively. The coincidence analysis has been carried out using a time stamp data acquisition system. Furthermore, the paper reports the fast ever detection of 235U using D-T pulsed neutron source based active coincidence method along with time stamp data acquisition system. In this study, 142.86 gm 238U and 1 gm 235U have been detected successfully using passive and pulsed neutron source based active coincidence methods with 6000 s and 400 s time stamp data respectively acquired using only eight 3He detectors of 45 cm active length. Furthermore, the present research methodology can be extended for the detection of Plutonium. This work will also be of importance towards detection of small quantity of SNM in cellulose waste from nuclear material processing facilities.

Comparison of the single and double count using MCNP6 and ONMS Geant4 software

The quantification and detection of fissile material are valuable for safeguards, nuclear security, and fuel management. Non-Destructive Assay (NDA) (i.e. using neutron and gamma measurements) provide reliable means to detect and estimate the fissile content (i.e. mass) in a vast range of material in different forms found across various facilities in the nuclear fuel cycle. Neutron measurements are used either for total counting (i.e. pure metal fissile samples) or coincidences/multiplicity counting (i.e. impure fissile samples). These various flavors of neutron measurement are routinely used to quantify fissile material mass and inventory lists. Monte Carlo codes have evolved considerably and are used routinely for modeling and simulation of diverse physics. MCNP6 (Monte Carlo N Particle) and ONMS-Geant4 (Open Neutron Multiplicity Software) are used in this work to generate single and double count (i.e. based on the accidental and real plus accidental histograms) for two fresh fuel sources with the assumption that the neutron emission process is solely due to spontaneous fission. Previous work has shown that for these fresh fuel sources, about 93% of the neutrons emitted is due to spontaneous fission. ONMS was designed to provide an open-source platform that will facilitate researchers in the nuclear community to perform neutron counting simulations. ONMS has been shown to give good results for detectors with high efficiencies, strong (α, n) sources, strong spontaneous fission sources and sources baring plutonium. Detailed model simulations result from ONMS and MCNP6 are compared for a weak californium source and uranium-bearing fresh fuel assembly using a detector set up with relatively low efficiency. The detector response is characterized to compare the single and double counts obtained from both code packages. For the weak californium source, the neutron triplets are also evaluated from both code packages.

Autonomous Neutron Facility for Detecting Fissile Nuclear Materials

The prospect of creating an autonomous neutron facility for the detection of fissile nuclear materials in samples, including those in confined volume, is discussed. It is proposed to obtain a reference field of thermal neutrons on the basis of a polyethylene moderator ball and a portable fast neutrons source, developed at NRC “Accelerator” NSC KIPT based of a continuous electrostatic accelerator of deuterons. The developed source of thermal neutrons is planned to be used to activate small objects and goods in order to identify substances prohibited for movement containing to find 233U, 235U and 239Pu in their composition. The prompt finding of fissile elements will indicate about an attempt to transport them illegally. A more thorough inspection can be carried out using special equipment after the detention of suspicious goods, citizens or vehicles. The possibility of detecting prompt fission neutrons is considered not only in the traditional way using a polyethylene moderator and proportional 3He detector, but also without application of any moderator using oxide or semiconductor scintillators. For detection fissile materials the method based on using the high-energy part of the γ-spectrum of fission fragments (greater than 4900 keV), as well, as the approach applied in the passive non-destructive analysis by the γ-line with Eg = 185.7 keV from 235U, are substantiated. It is shown that the proposed facility for the detection of fissile nuclear materials is able to determine the presence of isotopes 233U, 235U and 239Pu in tested objects and goods with the using non-destructive testing method.

Design of 500 MHz RFQ linear accelerator for a compact neutron source, RANS-III

A compact neutron source using a particle accelerator is a promising tool for material analysis, infrastructural diagnostics, fissile material detection, and medical applications. For non-destructive, on-site inspection of the degradation of aged concrete and steel reinforcement it is essential to develop a light and compact neutron source system. To design a more compact RFQ linac for a transportable neutron source, we developed a 500 MHz RFQ linac.In this paper the design of a 500 MHz RFQ linac for a compact neutron source is described. First, using beam tracking simulation software, we designed the cell parameters of the 500 MHz RFQ linac, which can accelerate a proton beam from 30 keV to 2.49 MeV. Then, using three-dimensional electromagnetic simulation software and multiphysics simulation software, we optimized the cavity design of the 500 MHz RFQ linac, and investigated the thermal properties of the cavity with the cooling channels.

Feasibility study of fissile mass detection in 870 L radioactive waste drums using delayed gamma rays from neutron-induced fission

The measurement of delayed gamma rays following neutron-induced fission is simulated with MCNP 6.1 to investigate the feasibility of fissile material detection in long-lived, medium activity radioactive waste in 870 L drums. The signal from homogeneously distributed fissile material in the drum is several hundred counts in the main delayed gamma peaks of interest. In a peripheral position or in the drum center, the signal is however too small to allow for a reliable measurement.

A detector for shielded fission sources

Abstract In recent years, a great deal of attention has been given to the passive detection of fissile material by detecting spontaneously emitted neutrons. Neutron detection schemes generally rely on low-density detection media with high neutron-gamma discrimination capability. However, such detectors generally suffer from low efficiency for high energy (>1.00 MeV) neutrons and usually are constructed with moderating material built into the detector package to compensate for that. But when the source is highly shielded by hydrogenous materials, the moderating material surrounding the detector actually decreases the detector efficiency (Rees and Czirr, 2012). This problem is further compounded if the source is shielded with borated material, significantly reducing the number of emerging neutrons. We have built, tested, and modeled a simple neutron detector that uses a large block of plastic scintillator fitted with a removable cadmium foil. This detector is sensitive to fission gammas, gammas emitted from neutron capture in shielding, gammas emitted from neutron capture in the plastic scintillator, and gammas produced by neutron capture in the cadmium foil. We demonstrate that by applying suitable data analysis, we can detect a shielded californium source with a very small chance of false positives from typical gamma sources . When boron is added to the shielding, we also use the 477 keV gammas emitted when neutrons capture in boron. This process is enhanced by using an auxiliary NaI gamma detector . Although the detection method flies in the face of currently promoted detectors, it is a simple technique that could be applied to a variety of detection applications.

Fissile material detection using neutron time-correlations from photofission

The detection of special nuclear materials (SNM) in commercial cargoes is a major objective in the field of nuclear security. In this work we investigate the use of two-neutron time-correlations from photo-fission using the Prompt Neutrons from Photofission (PNPF) detectors in Passport Systems Inc.’s (PSI) Shielded Nuclear Alarm Resolution (SNAR) platform for the purpose of detecting ∼5 kg quantities of fissionable materials in seconds. The goal of this effort was to extend the secondary scan mode of this system to differentiate fissile materials, such as highly enriched uranium, from fissionable materials, such as low enriched and depleted uranium (LEU and DU). Experiments were performed using a variety of material samples, and data were analyzed using the variance-over-mean technique referred to as Y2F or Feynman-α. Results were compared to computational models to improve our ability to predict system performance for distinguishing fissile materials. Simulations were then combined with empirical formulas to generate receiver operating characteristics (ROC) curves for a variety of shielding scenarios. We show that a 10 second screening with a 200 μA 9 MeV X-ray beam is sufficient to differentiate kilogram quantities of HEU from DU in various shielding scenarios in a standard cargo container.

Detection of tiny amounts of fissile materials in large-sized containers with radioactive waste

The paper is devoted to non-destructive control of tiny amounts of fissile materials in large-sized containers filled with radioactive waste (RAW). The aim of this work is to model an active neutron interrogation facility for detection of fissile ma-terials inside NZK type containers with RAW and determine the minimal detectable mass of U-235 as a function of various param-eters: matrix type, nonuniformity of container filling, neutron gen-erator parameters (flux, pulse frequency, pulse duration), meas-urement time. As a result the dependence of minimal detectable mass on fissile materials location inside container is shown. Nonu-niformity of the thermal neutron flux inside a container is the main reason of the space-heterogeneity of minimal detectable mass in-side a large-sized container. Our experiments with tiny amounts of uranium-235 (<1 g) confirm the detection of fissile materials in NZK containers by using active neutron interrogation technique.

Neutron detection using a water Cherenkov detector with pure water and a single PMT

We present the performance of a novel neutron detector based on a water Cherenkov detector (WCD) employing pure water and a single photomultiplier tube (PMT). The experiments presented in this work were performed using 241AmBe and 252Cf neutron sources in different neutron moderator and shielding configurations. We show that fast neutrons from the 241AmBe and 241Cf sources, as well as thermal neutrons from a neutron moderator, despite having different spectral characteristics, produce essentially the same pulse histogram shape. This characteristic pulse-height histogram shapes are recorded as a clear signature of neutrons with energies lower than ≃11MeV. This is verified in different experimental conditions. Our estimation of the neutron detection efficiency is at the level of (15±5)%, for fast neutrons. Since water is the material employed as active volume, the results of this study are of interest for the construction of low cost and large active volume neutron detectors for various applications. Of special importance are those related with space weather phenomena monitoring as well as those for the detection of fissile special nuclear material, including uranium or plutonium.

Growth and device processing of hexagonal boron nitride epilayers for thermal neutron and deep ultraviolet detectors

Solid-state neutron detectors with high performance are highly sought after for the detection of fissile materials. However, direct-conversion neutron detectors based on semiconductors with a measureable efficiency have not been realized. We report here the first successful demonstration of a direct-conversion semiconductor neutron detector with an overall detection efficiency for thermal neutrons of 4% and a charge collection efficiency as high as 83%. The detector is based on a 2.7 μm thick 10B-enriched hexagonal boron nitride (h-BN) epitaxial layer. The results represent a significant step towards the realization of practical neutron detectors based on h-BN epilayers. Neutron detectors based on h-BN are expected to possess all the advantages of semiconductor devices including wafer-scale processing, compact size, light weight, and ability to integrate with other functional devices.

Liquid-Xe detector for contraband detection

We describe progress made with a liquid-Xe (LXe) detector coupled to a gaseous photomultiplier (GPM), for combined imaging and spectroscopy of fast neutrons and gamma-rays in the MeV range. The purpose of this detector is to enable the detection of hidden explosives and fissile materials in cargo and containers. The expected position resolution is about 2m and 3.5mm for fast neutrons and gamma-rays, respectively. Experimental results obtained using an 241Am source yielded energy and time resolutions of 11% and 1.2ns RMS, respectively. Initial results obtained with the position-sensitive GPM are presented.

Generation of fluorescent silver nanoclusters in reverse micelles using gamma irradiation: low vs. high dosages and spectral evolution with time

Reverse micelles (RMs) containing aqueous solutions of Ag ? ions in their core produce fluorescent Ag nanoclusters (NCs), upon exposure to gamma irradiation. The fluorescence spectra of the NCs evolve over days to weeks after the exposure, and usually show large increases in intensity. Responses of as high as 2.8 9 10 4 CPS/Gy were reached. A dosage as low as 0.5 Gy (10 % of the lethal dosage for humans) produces NCs having fluores- cence intensities higher than background. The RMs can be employed in novel gamma radiation detectors with appearance of fluorescence indicating that radiation was once present. In applications involving detection and tracking of fissile materials, the evolution of the fluores- cence spectra over time may provide additional informa- tion about the radiation source. A two-phase liquid system is used for RM formation in a simple procedure. It is likely that this synthesis method may be adapted to produce NCs from other metal ions.

High-power, photofission-inducing bremsstrahlung source for intense pulsed active detection of fissile material

Intense pulsed active detection (IPAD) is a promising technique for detecting fissile material to prevent the proliferation of special nuclear materials. With IPAD, fissions are induced in a brief, intense radiation burst and the resulting gamma ray or neutron signals are acquired during a short period of elevated signal-to-noise ratio. The 8 MV, 200 kA Mercury pulsed-power generator at the Naval Research Laboratory coupled to a high-power vacuum diode produces an intense 30 ns bremsstrahlung beam to study this approach. The work presented here reports on Mercury experiments designed to maximize the photofission yield in a depleted-uranium (DU) object in the bremsstrahlung far field by varying the anode-cathode (AK) diode gap spacing and by adding an inner-diameter-reducing insert in the outer conductor wall. An extensive suite of diagnostics was fielded to measure the bremsstrahlung beam and DU fission yield as functions of diode geometry. Delayed fission neutrons from the DU proved to be a valuable diagnostic for measuring bremsstrahlung photons above 5 MeV. The measurements are in broad agreement with particle-in-cell and Monte Carlo simulations of electron dynamics and radiation transport. These show that with increasing AK gap, electron losses to the insert and outer conductor wall increase and that the electron angles impacting the bremsstrahlung converter approach normal incidence. The diode conditions for maximum fission yield occur when the gap is large enough to produce electron angles close to normal, yet small enough to limit electron losses.

Bright perspectives for nuclear photonics

With the advent of new high-power, short-pulse laser facilities in combination with novel technologies for the production of highly brilliant, intense γ beams (like, e.g., Extreme Light Infrastructure – Nuclear Physics (ELI-NP) in Bucharest, MEGaRay in Livermore or a planned upgrade of the HIγS facility at Duke University), unprecedented perspectives will open up in the coming years for photonuclear physics both in basic sciences as in various fields of applications. Ultra-high sensitivity will be enabled by an envisaged increase of the γ-beam spectral density from the presently typical 102γ/eVs to about 104γ/eVs, thus enabling a new quality of nuclear photonics [1], assisted by new γ-optical elements [2]. Photonuclear reactions with highly brilliant γ beams will allow to produce radioisotopes for nuclear medicine with much higher specific activity and/or more economically than with conventional methods. This will open the door for completely new clinical applications of radioisotopes [3]. The isotopic, state-selective sensitivity of the well-established technique of nuclear resonance fluorescence (NRF) will be boosted by the drastically reduced energy bandwidth (<0.1%) of the novel γ beams. Together with a much higher intensity of these beams, this will pave the road towards a γ-beam based non-invasive tomography and microscopy, assisting the management of nuclear materials, such as radioactive waste management, the detection of nuclear fissile material in the recycling process or the detection of clandestine fissile materials. Moreover, also secondary sources like low-energy, pulsed, polarized neutron beams of high intensity and high brilliance [4] or a new type of positron source with significantly increased brilliance, for the first time fully polarized [5], can be realized and lead to new applications in solid state physics or material sciences.

FISSILE MATERIALS DETECTION VIA NEUTRON DIFFERENTIAL DIE-AWAY TECHNIQUE

This work is devoted to the differential die-away technique that is widely used for active detection of fissile materials via pulsed neutron generators. The technique allows direct detection of milligram quantities of uranium-235 and plutonium-239 in objects with volumes up to several cubic meters. Our group has demonstrated this technique, creating a special installation based on the commercially produced ING-07T pulsed neutron generator. The installation includes eight proportional 3 He -counters mounted inside a polyethylene moderator with a cadmium filter, as well as a polyethylene chamber into which a 70-liter container is loaded for inspection. Preliminary testing showed that the minimum detectable mass of unshielded uranium-235 is ∼3 mg, using a 5.108 n/s neutron yield and 8 min measurement time. When the container is filled with neutron absorbing materials, e.g., iron, the minimum detectable mass increases to ∼30 mg. Use of borated screens further increases the minimum mass that can be detected. The tested installation and/or its modifications can be used for control and detection of fissile materials in various applications from luggage inspection to control containers with nuclear fuel cycle radioactive wastes.

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.

High-pressure 4He drift tubes for fissile material detection

A detector efficiency model based on energy extraction from neutrons is described and used to compare 4He detectors with liquid scintillators (EJ301/NE-213). Detector efficiency can be divided into three regimes: single neutron scattering, multiple neutron scattering, and a transition regime in-between. For an average fission neutron of 2MeV, the amount of 4He needed would be about 1/4 of the amount of the mass of EJ301/NE-213 in the single-scattering regime. For about 50% neutron energy extraction (1MeV out of 2MeV), the two types of detectors (4He in the transition regime, EJ301 still in the single-scattering regime) have comparable mass, but 4He detectors can be much larger depending on the number density. A six-tube 11-bar-pressure 4He detector prototype is built and tested. Individual electrical pulses from the detector are recorded using a 12-bit digitizer. Differences in pulse rise time and amplitudes, due to different energy loss of neutrons and gamma rays, are used for neutron/gamma separation. Several energy spectra are also obtained and analyzed.

Detection of fissile material using cold neutron interrogation combined with neutron coincidence counting

A variety of neutron interrogation techniques exists for the identification and characterization of fissile materials. Typically the sample is irradiated continuously or in pulses, and the neutron or gamma response is used for the characterization of the sample. Active neutron coincidence counting is one of these techniques, based on the detection of correlated prompt neutrons from induced fission in 235U, in order to quantify uranium. Typically AmLi neutron sources are used for the interrogation so far. In this study we used for the first time a guided cold neutron beam for interrogation, which is a far more intense source of uncorrelated neutrons. A pilot neutron coincidence setup was installed at a neutron beam of the Budapest Neutron Centre, and samples of small volumes and various uranium contents were measured. It was proven that the detection of fission events (Doubles) is feasible even for micrograms of 235U and the detector response is proportional to the fissile content of the sample.

A note on neutron capture correlation signals, backgrounds, and efficiencies

A wide variety of detection applications exploit the timing correlations that result from the slowing and eventual capture of neutrons. These include capture-gated neutron spectrometry, multiple neutron counting for fissile material detection and identification, and antineutrino detection. There are several distinct processes that result in correlated signals in these applications. Depending on the application, one class of correlated events can be a background that is difficult to distinguish from the class that is of interest. Furthermore, the correlation timing distribution depends on the neutron capture agent and detector geometry. Here, we explain the important characteristics of the neutron capture timing distribution, making reference to simulations and data from a number of detectors currently in use or under development. We point out several features that may assist in background discrimination, and that must be carefully accounted for if accurate detection efficiencies are to be quoted.