Fission Signatures for Nuclear Material Detection

As part of its R&xD activities in the fields of radioactivewaste drum storage and homeland security, the NuclearMeasurement Laboratory of CEA Cadarache has started studiesrelated to the detection of induced delayed fission gamma rays asa signature of U/Pu presence either in radioactive wastes or incargo containers and luggage. The study described in the presentpaper explores the feasibility of detecting fission delayed gammarays of nuclear materials interrogated by a pulsed neutrongenerator. For this purpose, Monte Carlo simulations have beenperformed with ACT, the MNCP6 Activation Control Card.Simulated results have been compared with experimental data tovalidate the numerical model. Samples of uranium andplutonium have been irradiated for 2 hours with a pulsed D-Tneutron generator delivering 14 MeV neutrons with an averageemission of 8.107n/s, which are thermalised in a graphite cellcalled REGAIN. At the end of irradiation, activated nuclearmaterials were placed in a low-background, high-resolutiongamma spectroscopy station in order to detect delayed gammarays emitted by fission products. Anomalies have been observedin the calculated time decay curve of fission delayed gamma rayswith MCNP6 ACT card, but the time behavior is correct for non-fission activated materials like aluminum or copper. On the otherhand, the number of counts recorded in the main simulatedgamma ray lines from activated nuclear material fission productsis consistent with the experimental results, thus validating thesimulation scheme in view of further studies on thecharacterization of radioactive waste drums or special nuclearmaterial detection in cargo containers.

Innovative systems with increased sensitivity and resolution are in great demand to detect diversion and to prevent misuse in support of nuclear materials management for the U.S. fuel cycle. Nuclear fission is the most important multiplicative process involved in non-destructive active interrogation. This process produces the most easily recognizable signature for nuclear materials. High-energy gamma rays can also excite a nucleus and cause fission through a process known as photofission. After photofission reactions, delayed signals are easily distinguishable from the interrogating radiation. LINAC-based, advanced inspection techniques utilizing the fission signals after photofission have been extensively studied for homeland security applications. Previous research also showed that a unique delayed gamma ray energy spectrum exists for each fissionable isotope. Isotopic composition measurement methods based on delayed gamma ray spectroscopy will be the primary focus of this work.

The effective point of measurement for spherical ionization chambers has been determined for 60Co and 137Cs gamma rays and for 300 kV X rays inside a water phantom. For reasons of comparison, measurements were also performed with a thimble-type Baldwin-Farmer ion chamber. For spherical ion chambers the displacement correction factors were 1-(0.37 +/- 0.04) r. 10(-2) for 60Co gamma rays and 1-(0.22 +/- 0.05) r. 10(-2) for 137 Cs gamma rays, where r is expressed in mm, whereas no displacement was observed for X rays. The differences in displacement obtained for the various types of radiation can be attributed to differences in attenuation and scatter characteristics of the various radiation qualities.

To study the detailed electronic structures of carbon‐related materials at an energy resolution better than 25 meV, we have developed a monochromatic low‐voltage analytical electron microscope under a project “Triple‐C phase‐2”. The developed microscope is equipped with a double Wien‐filter monochromator system [1] and delta‐type aberration correctors [2]. It works at an accelerating voltage range from 15 kV to 60 kV. The two Wien‐filters for the monochromator, which is located just below the extraction anode of Schottky source, enables us to obtain an achromatic probe at an exit of the monochromator, since the second filter cancels out the energy dispersion generated by the first filter. The energy spread of electrons (ΔE) is controllable by choosing the width of the slits, which are located between two filters, independently on the probe size at the specimen. The delta‐type aberration corrector consists of three dodeca‐poles to correct geometrical aberrations up to the fifth‐order including six‐fold astigmatism. In addition, so as to obtain high energy resolution spectra in electron energy‐loss spectroscopy (EELS), the microscope is equipped with a high energy resolution spectrometer (Quantum‐ERS from Gatan Inc.), which incorporates with the highly sensitive detection system at lower accelerating voltage and the highly stabilized power supplies for the prism and the lens system. The ultimate energy resolution was acquired to be 14 meV at an accelerating voltage of 30 kV with an acquisition time of 2 ms, as shown in Fig. 1. And at the longer acquisition time of 0.5 seconds, the energy resolution was measured to be 20 meV. These results exceeded the original target of 25 meV. Figures 2 (a) and (b) show raw ADF‐STEM images of a single‐layered graphene at 60 kV and 30 kV. These images were obtained with a monochromatic electron probe, whose ΔE was 228 meV (using a 4 μm slit). By using a monochromatic electron probe at 60 kV, C‐C dumbbells of single‐layered graphene were clearly resolved. And a single‐carbon atom on a graphene was successfully imaged at 30 kV. These results indicate that the developed microscope enables us to analyze materials with high energy and spatial resolutions at lower accelerating voltage. We tested a low‐loss EELS map using a hexagonal boron nitride (h‐BN) specimen with a monochromatic probe at 30 kV using a 0.1 μm slit. The experimental conditions are listed as probe size = 1 nm, probe current = 10 pA and acquisition time for each pixel = 0.3 seconds. Figure 2 (a) shows the ADF‐STEM image from the EELS mapping area. Fig. 2 (b) shows the extracted low‐loss spectrum from the edge of the specimen indicated with the framed yellow square in Fig. 2 (a). This spectrum, which was measured with an energy resolution of 22 meV, showed a sharp peak corresponding to an optical phonon at about 170 meV. Figure 2 (c) shows the EELS map at the phonon excitation energy, whose intensity is normalized with the zero‐loss peak intensity in each pixel. The phonon intensity was found to be strongly delocalized at the vacuum area beyond the edge of the specimen of more than 100 nm.

Passive detection of special nuclear material (SNM) at long range or under heavy shielding can only be achieved by observing the penetrating neutral particles that it emits: gamma rays and neutrons in the MeV energy range. The ultimate SNM standoff detector system would have sensitivity to both gamma and neutron radiation, a large area and high efficiency to capture as many signal particles as possible, and good discrimination against background particles via directional and energy information. Designing such a system is a daunting task. Using timemodulated collimators could be a transformative technique leading to practical gamma-neutron imaging detector systems that are highly efficient with the potential to exhibit simultaneously high angular and energy resolution. A new technique using time encoding to make a compact, high efficiency imaging detector was conceived. Design considerations using Monte Carlo modeling and the construction and demonstration of a prototype imager are described.

The Société Française D'Ophtalmologie has sponsored its most recent publication—<i>L'oeil et les Radiations Ionisantes</i>—which represents a superb, exhaustive investigation of the effect of ionizing radiation on normal and abnormal ocular tissue. The treatise is in French, well written, and has many illustrations and drawings to supplement the text. The first part of the two volume set is<i>Radiobiologie et Radio pathologie</i>. The general physics and biochemical and cellular effects of various types of radiation are described, as well as the ocular effects of atomic explosions. The second part discusses the effect of X and gamma radiations on the structures of the eye. The results of various levels of radiation on animals and humans are tabulated, as well as many phenomena connected with the radiation procedure. The third part, the treatment of tumors by radiation, deals with every conceivable type of ocular tumor and describes the effect of radiation

During the operation of linear accelerators, gamma and neutron radiation is produced. Gamma radiation causes semiconductor degradation. Therefore, it is necessary to replace electronic devices when the maximum allowed dose is reached to ensure reliable operation of the accelerator. Neutron radiation causes soft errors in memories and registers because of Single Event Upsets (SEUs). This may disturb the whole accelerator operation. Because of this, radiation monitoring in linear accelerator tunnels is recommended. A portable dosimeter reader was designed and build. Because gamma and neutron radiation can cause different effects in electronic devices and different way of protection against this two types of radiation are applied, this two types of radiation should be measured selectively. Three different types of dosimeters are used: gafchromic films (gamma radiation), SRAM-based dosimeters (neutron radiation) and LED-based dosimeters (neutron radiation) to ensure selective measurement. All methods need dedicated reading electronic circuitry. Nevertheless, a single readout device was designed. The device was calibrated and tested in the FLASH environment at DESY. Radiation doses, near the ACC2 module and bunch compressor were measured. The device is expected to be regularly used in currently built at DESY European X-Ray Free Electron Laser (XFEL).

Work is in progress at the Columbia Astrophysics Laboratory on the development of a liquid xenon imaging chamber for gamma‐ray astronomy. Such an instrument will combine high detection efficiency, good energy and position resolution with a large effective area. These properties, together with the excellent background rejection capability provided by 3‐dimensional imaging, will allow orders of magnitude better sensitivity than currently used instruments, for the detection of weak gamma‐ray sources in space.We have started with studies of the maximum energy resolution achievable with noble liquid ionization chambers. A value of 26 keV FWHM for 1 MeV electrons has been measured in liquid argon, dominated mostly by recombination straggling on highly ionizing delta electrons produced in large number along the primary ionizing track.We are presently working on liquid xenon to establish the most efficient gas purification scheme and the best spectroscopic performance. The initial result of about 38 keV FWHM for the energy resolution of the dominant 570 keV gamma rays from 207Bi, measured with a gridded ionization chamber, is very encouraging.Our final goal is the construction of a large area (∼1000 cm2) liquid xenon imaging chamber with 20 keV FWHM energy resolution at 1 MeV and a few millimeters spatial resolution, which will be capable of exploring a wide range of astrophysics phenomena expected to produce gamma‐rays in the energy range of 0.1 to 10 MeV. As a first application of this instrument we are planning on a spring 1990 balloon flight out of Australia to observe the 122 keV gamma‐ray line of 57Co produced by the supernova 1987A with a 3σ sensitivity of 2×10−5 ph cm−2 sec−1.

An essential feature of the Mossbauer effect is that the recoilless gamma rays have wavelengths in the same range, about 0,1mm, as the X rays used in the X-ray diffraction study of crystals. The Debye-Waller factor for X rays, exp - 1/2 , becomes the recoilless fraction in the Mossbauer case when the net momentum transfer Ḵ1-Ḵ0 for the X rays is replaced by the recoil momentum Ḵ for the gamma ray. The recoilless fraction of X-ray scattering is identified with the Laue spots and Bragg peaks of the X-ray diffraction pattern and the X-ray crystallographer uses resolution in momentum space to separate out the elastic part of the scattering. This separation is imperfect but can be radically improved if the X rays are replaced by Mossbauer gamma rays and the energy resolution of the Mossbauer experiment is combined with the momentum resolution of the X-ray experiment. The momentum resolution of this combination can be made to be of the same order as in X ray or neutron diffraction but the energy resolution, typically 10-8eV will be much better than for the neutron experiment, typically 10-4 eV. The high energy resolution of the Mossbauer experiment in fact practically prevents its use in the observation of single phonon lines, as in the neutron inelastic scattering experiments, since phonon lines are too broad, ~ 10-4eV. However, as we shall show below, there are situations in which the high energy resolution can be put to good use.

Liquefied-noble-gas (LNG) detectors offer, in principle, very good energy resolution for both neutrons and gamma rays, fast response time (hence high-count-rate capabilities), excellent discrimination between neutrons and gamma rays, and scalability to large volumes. They do, however, need cryogenics. LNG detectors in sizes of interest for fissionable material detection in cargo are reaching a certain level of maturity because of the ongoing extensive R&}D effort in high-energy physics regarding their use in the search for dark matter and neutrinoless double beta decay. The unique properties of LNG detectors, especially those using Liquid Argon (LAr) and Liquid Xenon (LXe), call for a study to determine their suitability for Non-Intrusive Inspection (NII) for Special Nuclear Materials (SNM) and possibly for other threats in cargo.Rapiscan Systems Laboratory, Yale University Physics Department, and Adelphi Technology are collaborating in the investigation of the suitability of LAr as a scintillation material for large size inspection systems for air and maritime containers and trucks. This program studies their suitability for NII, determines their potential uses, determines what improvements in performance they offer and recommends changes to their design to further enhance their suitability. An existing 3.1 liter LAr detector (microCLEAN) at Yale University, developed for R&}D on the detection of weakly interacting massive particles (WIMPs) was employed for testing. A larger version of this detector (15 liters), more suitable for the detection of higher energy gamma rays and neutrons is being built for experimental evaluation. Results of measurements and simulations of gamma ray and neutron detection in microCLEAN and a larger detector (326 liter CL38) are presented.

Nuclear resonance fluorescence and effective Z determination applied to detection and imaging of special nuclear material, explosives, toxic substances and contraband

The goal of this work is a detailed study of uranium detection by laser-induced breakdown spectroscopy (LIBS) for application to activities associated with environmental surveillance and detecting weapons of mass destruction (WMD). The study was used to assist development of LIBS instruments for standoff detection of bulk radiological and nuclear materials and these materials distributed as contaminants on surfaces. Uranium spectra were analyzed under a variety of different conditions at room pressure, reduced pressures, and in an argon atmosphere. All spectra displayed a high apparent background due to the high density of uranium lines. Time decay curves of selected uranium lines were monitored and compared to other elements in an attempt to maximize detection capabilities for each species in the complicated uranium spectrum. A survey of the LIBS uranium spectra was conducted and relative emission line strengths were determined over the range of 260 to 800 nm. These spectra provide a guide for selection of the strongest LIBS analytical lines for uranium detection in different spectral regions. A detection limit for uranium in soil of 0.26% w/w was obtained at close range and 0.5% w/w was achieved at a distance of 30 m. Surface detection limits were substrate dependent and ranged from 13 to 150 microg/cm2. Double-pulse experiments (both collinear and orthogonal arrangements) were shown to enhance the uranium signal in some cases. Based on the results of this work, a short critique is given of the applicability of LIBS for the detection of uranium residues on surfaces for environmental monitoring and WMD surveillance.

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.

We have been developed an electron tracking Compton camera (ETCC) using an SOI pixel semiconductor detector for observation sub MeV gamma rays. An electron tracking Compton camera can reconstruct arrival directions of gamma rays event by event as a point using the recoil directions of electron tracks , so this camera does not increase false spot comparing with a classical Compton camera. Also, an ETCC can discriminate between the gamma rays and backgrounds by detecting the track length and energy of recoil electrons. In addition, ETCC using a semiconductor detector is useful for detecting line gamma rays because of its high energy resolution. We focused on a n SOI pixel semiconductor detector, which has a small pixel pitch of a few ten um. The sensor has a trigger circuit mounted on each pixel, so we can select only candidate gamma ray events and readout only the selected pixels. We developed a prototype Compton camera using XRPIX2b sensor, which is an SOI pixel sensor developed by Kyoto University. We evaluated the detection capability of recoil electron due to 511 keV gamma rays while changing various conditions such as scattering angle and incident directions of gamma rays. From the results of the evaluation in the laboratory, we succeeded in detecting recoil electron tracks and estimating the recoil directions using the track image. Furthermore, we confirmed to be able to detect the radioisotope position using back projection method. We obtained sufficient quantitative detailed performance data of the current sensor to develop a novel sensor for ETCC.

Aloe vera-mediated green synthesis of bismuth-zinc-iron nanocomposite for radiation shielding applications