Detection Of Fissile Materials Research Articles

A single-shot nanosecond neutron pulsed technique for the detection of fissile materials

A novel technique with the potential of detecting hidden fissile materials is presented utilizing the interaction of a single powerful and nanosecond wide neutron pulse with matter. The experimental system is based on a Dense Plasma Focus (DPF) device as a neutron source generating pulses of almost mono-energetic 2.45 MeV and/or 14.0 MeV neutrons, a few nanoseconds in width. Fissile materials, consisting of heavy nuclei, are detected utilizing two signatures: firstly by measuring those secondary fission neutrons which are faster than the elastically scattered 2.45 MeV neutrons of the D-D reaction in the DPF; secondly by measuring the pulses of the slower secondary fission neutrons following the pulse of the fast 14 MeV neutrons from the D-T reaction. In both cases it is important to compare the measured spectrum of the fission neutrons induced by the 2.45 MeV or 14 MeV neutron pulse of the DPF with theoretical spectra obtained by mathematical simulation. Therefore, results of numerical modelling of the proposed system, using the MCNP5 and the FLUKA codes are presented and compared with experimental data.

Towards high efficiency solid-state thermal and fast neutron detectors

Variety of applications of fast neutron detection utilize thermal neutron detectors and moderators. Examples include homeland security applications such as portal monitors and nuclear safeguards which employ passive systems for detection of fissile materials. These applications mostly rely on gas filled detectors such as 3He, BF3 or plastic scintillators and require high voltage for operation. Recently there was considerable progress in the development of solid-state neutron detectors. These operate by detection of charged particles emitted from neutron interactions with a converter material. In order to increase neutron detection efficiency to a usable level, the thickness of the converter material must exceed the range of the charged particles in the converter, which limits the efficiency of planar detectors to several percent. To overcome this limitation three dimensional structured solid-state devices are considered where the converter can be thicker but still allow the charged particles to escape into the semiconductor. In the research described here this was accomplished by a semiconductor device that resembles a honeycomb with hexagonal holes and thin silicon walls filled with the converter material. Such design can theoretically achieve about 45% thermal neutron detection efficiency, experimentally about 21% was observed with a partially filled detector. Such detectors can be fabricated in variety of sizes enabling designs of directional fast neutron detectors. Other converter materials that allow direct detection of fast neutrons were also considered by both simulation and experiments. Because the semiconductor thickness is less than a few hundred microns, the efficiency of these detectors to γ-ray(s) is very low.With further developments these new solid-state neutron detectors can replace gas ionization based detectors in most 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.

A method for decreasing the loading on scintillation channels in facilities with pulsed neutron sources

A method is proposed for decreasing the loading on scintillation channels in nuclear material detection and monitoring facilities comprising pulsed neutron sources, neutron moderators, and scintillators with a system for pulse shape discrimination between neutrons and photons. This method is based on the use of composite scintillators containing cylindrical shells of a thermal-neutron absorbing material. Selecting the sizes of zones in a composite scintillator and the absorber type (cadmium and lithium carbonate with different 6 Li content), it is possible to considerably increase the decay time constant for thermal neutrons in a composite scintillator and thereby reduce its load as compared to a homogeneous scintillator over the same time period after a pulse from the neutron source. The sizes of the scintillator component parts and the absorber material are optimized, which provides a means for decreasing the load on the scintillation channel several-fold while maintaining constant detection efficiency for fast fission neutrons. The capabilities of this method for decreasing the load are demonstrated by the example of the operational prototype of the fissile material detection and monitoring facility with a graphite moderator and LS-13 scintillators. The efficiency of the facility used in this method is compared to that of a facility with a deuterium-containing scintillator, in which no radiative capture photons are produced.

Inexpensive and practical sealed drift-tube neutron detector

The design, construction, and performance of a type of sealed 3He drift tubes for neutron detection are presented. Because the 3He pressure is in the 25–300 mbar range, the detector costs are not dominated by the 3He gas. Intrinsic neutron detection efficiencies up to 5% have been observed by using high-density polyethylene moderation. Sensitive measurements of the detector lifetime are achieved by monitoring the full-energy peak of the 3He(n, p) 3H reaction as a function of time. The neutron peak position shows a 24-h cycle that may be explained by the physical adsorption of gases onto the wall. The estimated lifetimes of the detectors are sufficiently long and therefore, the design and the construction are robust and practical for applications such as fissile material detection.

Finding the parameter of digital discrimination between scintillation responses to neutrons and photons in radioactive and fissile material detection systems

Use of a digital technique for discriminating between neutron and photon responses of high-efficiency scintillation detectors in fissile material detection systems is proposed. Software-controlled processing of scintillation signals makes it possible to separate the time and energy dependences of the neutron and photon responses of the detection system and obtain various physical parameters indicating the presence of fissile materials in the subject of inspection.

Optimization of the extraction facilities of an electron accelerator – bremsstrahlung source for the pulsed photonuclear method

The objective of the present work is to select the material and thickness of an electron-beam converter and filter and a bremsstrahlung collimator for designing the extraction facilities of an accelerator in an experimental setup for studying the photonuclear method. Numerical modeling of the conversion of electrons into bremsstrahlung in the electron-beam converter and filter and passage of the bremsstrahlung in the collimator is performed. The following main criteria are optimized: the neutron yield from the fissile material under bremsstrahlung, the radiation loads on the content of the irradiated container, and the radiation background. The data obtained are used to design and build the target block at the exit of the electron accelerator, including the beam converter and filter, and the bremsstrahlung collimator for an experimental model of a fissile-material detection facility based on the U-28 accelerator.

Fast neutron inspection of sea containers for the presence of “dirty bomb”

The possibility of the detection of “dirty bomb” presence inside sea containers is evaluated. The method proposed for explosive and fissile material detection makes use of two sensors (X-rays and neutrons). A commercial imaging device based on the X-ray radiography performs a fast scan of the container, identifies a “suspect” region and provides its coordinates to the neutron based device for the final “confirmatory” inspection. In this two sensor system a 14MeV neutron beam defined by the detection of associated alpha particles is used for interrogation of only volume elements marked by X-ray sensor. The object’s nature is determined from passive and neutron induced, gamma energy spectra measurements. Experimental results (time-of-flight and gamma energy spectra) obtained for the irradiation 30kg of TNT, depleted uranium and other materials hidden inside the container are presented.

Observation of /sup 238/U photofission products

Sophisticated solutions are being developed by researchers to improve detection of clandestine fissile material. In this work, a simpler approach is investigated as an interim solution. Using the existing 9-MeV X-ray radiography units deployed for cargo inspection, delayed fission product gamma rays from induced photofission are seen minutes after probing a small sample of /sup 238/U. By acquiring both time and energy information, a relatively large number of usable fission products can be identified. Using the radiographic images along with an efficient portable scintillator capable of detecting gamma rays above 3 MeV may provide a cost-effective improvement to existing inspection systems.

On the possibility of detecting fissile nuclear materials using coincident gamma rays

Almost every radiation monitor installed at the customs and border control points designed for detection of nuclear fissile materials uses gas neutron counters tracing decay neutrons. They have a low background level, but at the same time small efficiency. This work describes the nuclear fissile materials detection methodology based on the instantaneous gamma-radiation accompanying the decay. The tests results of designed and produced radiation monitors based on this principle to control pedestrian and vehicles are presented.

Application of neutron generators for high explosives, toxic agents and fissile material detection

The portable pulsed neutron generators developed and designed by VNIIA are presented. The neutron methods are good tools for non-destructive assay of dangerous materials. The samples of various systems for the detection of high explosives in luggage, identification of toxic agents in chemical munitions, and also for the prevention of illicit trafficking of fissile elements in passengers’ luggage are discussed.

Large area imaging detector for long-range, passive detection of fissile material

Recent events highlight the increased risk of a terrorist attack using either a nuclear or a radiological weapon. One of the key needs to counter such a threat is long-range detection of nuclear material. Theoretically, gamma-ray emissions from such material should allow passive detection to distances greater than 100 m. However, detection at this range has long been thought impractical due to spatially fluctuating levels of natural background radiation. These fluctuations are the major source of uncertainty in detection and mean that sensitivity cannot be increased simply by increasing detector size. Recent work has shown that this problem can be overcome through the use of imaging techniques. In this paper we describe the background problems, the advantages of imaging and the construction of a prototype, large-area (0.57 m/sup 2/) gamma-ray imager to detect nuclear materials at distances of /spl sim/ 100 m.

Method and system based on pulsed neutron generator for fissile material detection in luggage

The paper discusses the problem of fissile material (FM) detection in passenger luggage. Different methods of control of unauthorized FM movement were analyzed. Application of differential die-away technique was substantiated. Experimental prototype with sensitivity of uranium-235 detection equal to 5 g during 5 s was described. A method for revealing deliberate FM masking by neutron-absorbing shields is suggested.

Development and applications of position-sensitive solid-state gamma ray detectors

The development of high-resolution position-sensitive, solid-state detectors will enable gamma ray detectors with improved sensitivity and imaging capabilities. The gamma ray astrophysics group at NRL has been developing germanium strip detectors for several years. We have shown that three-dimensional locations for gamma ray interactions can be determined with sub-millimeter accuracy, and have also demonstrated imaging capability within a single germanium strip detector. We have also initiated work on thick, silicon strip detectors. This was based on the fact that three sequential interactions can enable the energy and direction cone of the incident gamma ray to be determined, even without total energy deposition of the incident gamma ray. We are also working on low-power ASICs that are required to handle the large number of channels associated with arrays of strip detectors. Progress on this work will be presented, along with applications to high-energy astrophysics, medical imaging, nuclear physics, detection of fissile materials, and monitoring of environmental radioactivity.

Neutron tomographic fissile assay in spent fuel using the lead slowing down time spectrometer

A tomographical fissile assay technique for light water reactor spent fuels was developed based on the lead slowing down time spectrometer (LSDTS). The Monte Carlo method was used for system performance and for simulation of the sensitivity of detection of fissile materials using neutron emission tomography. A typical spent PWR fuel element was simulated and the spatial and mass sensitivity of the fissile components were investigated. From these studies, the LSDTS system is shown to be a very sensitive device for analyzing the spatial distribution of total fissile materials of a spent fuel assembly in a properly selected assay neutron energy range. This method is also applicable to nuclear waste assay.

Fissile material isotopic composition by γ-ray spectra

This work is a physical feasibility study of nondestructive detection and identification of fissile material. The key point of the technique is the extraction of other useful information through the measurement of characteristic γ radiation emitted spontaneously from the nuclear fissile material. Using numerical simulation, this method was tested and the results showed that the isotopic composition and other characteristic information can be extracted from the measured γ-ray spectra with an accuracy better than 2%. Also, the present study indicated that the sample self-absorption correction is a crucial factor to improve the accuracy of the calculations and we present a simplified semi-empirical expression for the sample self-absorption correction of some simple geometric configurations. The method discussed in this paper can be applied to the fissile material safeguarding techniques such as monitoring, verifying, accounting, etc.

Calculations of neutron transport through a simulated waste matrix

The assay of nuclear waste characterized as Remote-Handled Transuranic (RH-TRU) waste presents a formidable problem due to the waste's high neutron and γ-ray backgrounds. The differential die-away technique (DDT) that is currently employed as a method for the assay requires the incidence of a large fluence of thermal neutrons within the volume element that contains fissile material. To study the neutron moderation and transport through nuclear waste matrices of different densities, the simulation of neutron transport was initiated using the Monte Carlo N-Particle (MCNP) code. A series of calculations using the MCNP 4A computer code have taken place. The calculations were performed to examine neutron transport across a wide range of waste matrix densities and interrogating neutron source energies. A composite matrix based on elemental analysis of RH-TRU legacy waste at ORNL was simulated and the density of this composite was varied between 200 and 2000 kg/m3. An isotropic interrogating neutron source was varied from 0.01 to 15 MeV to examine the transport of neutrons across these various densities. Fast (0.01–15 MeV), epithermal (10−7–0.01 MeV), and thermal (10−8–10−7 MeV) neutron transport was examined. The moderation of fast neutrons into thermal neutrons and their subsequent transport to the center of the drum was studied. A table with predictions of lower limits of detection of fissile material in a RH-TRU waste drum under certain conditions has been prepared.

Gamma-ray spectroscopic system for remote detection and monitoring of fissile materials

The scientific and methodical aspects of system design for radiation monitoring of nuclear weapons for treaty verification and for customs monitoring to prevent contraband of the fissile materials are considered in this paper. The /spl gamma/-ray spectroscopic systems for remote detection of nuclear weapons and for the monitoring of the number of warheads within a missile nose cone are considered, and the results of experimental test and computer simulations of this system are given.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Fissile material detection by active neutron interrogation with radioactive neutron sources

Different radioactive neutron sources (Am-Li, Am-Be, Cf-252) were considered and tested for active fissile material detection. The low energy Am-Li neutron source has outstanding features. The low dose rate permits full access to the measuring site. The low energy (thermal) neutron flux at the source position is high compared to the high energy neutrons that reach the counter position. In a graphite matrix, of small size, 7 g U-235 led to a doubling of the neutron count rate whereas 200 g graphite changes this count rate by only 2%. The system is in addition operable in the presence of 1012 Bq of fission products. The detection of fissile material down to 1 g and below has been shown to be possible with this instrument in small samples.