Nuclear Physics and Gamma-Ray Sources for Nuclear Security and Nonproliferation

The following sections are included:

• Preface
• Contents

The development of high power lasers and the combination of such novel devices with accelerator technology has enlarged the science reach of many research fields, in particular High Energy, Nuclear and Astrophysics as well as societal applications in Material Science, Nuclear Energy and Medicine. The European Strategic Forum for Research Infrastructures (ESFRI) has selected a proposal based on these new premises called “ELI” for Extreme Light Infrastructure. ELI will be built as a network of three complementary pillars at the frontier of laser technologies. The ELI-NP pillar (NP for Nuclear Physics) is under construction near Bucharest (Romania) and will develop a scientific program using two 10 PW class lasers and a Back Compton Scattering High Brilliance and Intense Low Energy Gamma Beam, a marriage of Laser and Accelerator technology at the frontier of knowledge. In the present paper, the technical and scientific status of the project as well as the applications of the gamma source will be discussed.

Comprehensive monitoring of the supply chain for nuclear materials has historically been hampered by non-intrusive inspection systems that have such large false alarm rates that they are impractical in the flow of commerce. Passport Systems, Inc. (Passport) has developed an active interrogation system which detects fissionable material, high Z material, and other contraband in land, sea and air cargo. Passport's design utilizes several detection modalities including high resolution imaging, passive radiation detection, effective-Z (EZ-3D™) anomaly detection, Prompt Neutrons from Photofission (PNPF), and Nuclear Resonance Fluorescence (NRF) isotopic identification. These technologies combine to: detect fissionable, high-Z, radioactive and contraband materials, differentiate fissionable materials from high-Z shielding materials, and isotopically identify actinides, Special Nuclear Materials (SNM), and other contraband (e.g. explosives, drugs, nerve agents). Passport's system generates a 3-D image of the scanned object which contains information such as effective-Z and density, as well as a 2-D image and isotopic and fissionable information for regions of interest.

Nondestructive detection and assay of nuclide is one of the promising applications of energy-tunable gamma-rays from laser Compton scattering. In JAEA, we are developing technologies relevant to the gamma-ray non-destructive assay, which include a high-brightness gamma-ray source based on advanced laser and accelerator technologies and gamma-ray measurement techniques optimized for highly radioactive samples. In this paper, the status of the above R&D's is reviewed.

Development of gamma-ray beam interrogation technologies for remote detection of special nuclear materials and isotope analysis requires comprehensive databases of nuclear structure information and gamma-ray induced nuclear reaction observables. Relevant nuclear structure details include the energy, spin and parity of excited states that have significant probability for electromagnetic transition from the ground state, i.e, the angular momentum transferred in the reaction is Δl ≤ 2. This talk will report recent Nuclear Resonance Fluorescence (NRF) measurements to identify and characterize new low-spin states in actinide nuclei at energies from 1 to 4 MeV, which is the energy range most important for remote analysis methods. These measurements are carried out using the nearly mono-energetic linearly polarized gamma-ray beam at the High Intensity Gamma-ray Source (HIγS) at the Triangle Universities Nuclear Laboratory. Also, studies of the (γ, n) reaction on a variety of nuclei with linearly polarized beams at HIγS indicate that this reaction might be used to discern between fissile and non-fissile materials. This work will be described. In addition, an overview will be given of a concept for a next generation laser Compton-backing scattering gamma-ray source to be implemented as an upgrade to increase the beam intensity at HIγS by more than an order of magnitude.

In the low-energy part of the electric dipole strength of atomic nuclei an addition structure has been identified beside the well-known Giant Dipole Resonance, which is usually denoted as Pygmy Dipole Resonance (PDR). The PDR has attracted strong interest in the last years in nuclear physics. Different experimental approaches have been used in the last decade in order to investigate this new interesting nuclear excitation mode. Among the different experiments the nuclear resonance fluorescence (NRF) method has played a pioneering role in the research on the PDR.

This is a brief report on recent studies of photonuclear reactions by using medium energy photons produced by laser photons scattered off GeV electrons. The medium energy photonuclear excitations of IAR (isobaric analogue resonance) are used to study neutrino nuclear responses relevant to double beta decays and astro-neutrinos. Gamma rays following medium-energy resonant photonuclear reactions are used for high-sensitivity non-destructive nuclear isotope detections.

Characterization and control of radioactive waste packages are important issues in the management of a radioactive waste repository. Therefore, Andra performs quality control inspection on radwaste package before disposal to ensure the compliance of the radwast characteristics with Andra waste disposal specifications and to check the consistency between Andra measurements results and producer declared properties. Objectives of this quality control are: assessment and improvement of producer radwaste packages quality mastery, guarantee of the radwaste disposal safety, maintain of the public confidence. To control radiological characteristics of radwaste package, non-destructive passive methods (gamma spectrometry and neutrons counting) are commonly used. These passive methods may not be sufficient, for instance to control the mass of fissile material contained inside radwaste package. This is particularly true for large concrete hull of heterogeneous radwaste containing several actinides mixed with fission products like 137Cs. Non-destructive active methods, like measurement of photofission delayed neutrons, allow to quantify the global mass of actinides and is a promising method to quantify mass of fissile material. Andra has performed different non-destructive measurements on concrete intermediate-level short lived nuclear waste (ILW-SL) package to control its nuclear material content. These tests have allowed Andra to have a first evaluation of the performance of photofission delayed neutron measurement and to identify development needed to have a reliable method, especially for fissile material mass control in intermediate-level long lived waste package.

On site categorization and collection of radioactive and nuclear materials are required at radiological and nuclear incident site. We are developing portable equipment and radiation protection for radiological emergency response team to carry out emergency missions safely at the incident sites. In this report, we review radiation monitoring system including wireless dosimeter system and neutron shield with water developed in our institute. Also the development of fast-neutron directional detector with a micro pattern gas detector is described.

An X-band (11.424GHz) electron linac as a neutron source for nuclear data study for the melted fuel debris analysis and nuclear security in Fukushima is under development. Originally we developed the linac for Compton scattering X-ray source. Quantitative material analysis and forensics for nuclear security will start several years later after the safe settlement of the accident is established. For the purpose, we should now accumulate more precise nuclear data of U, Pu, etc., especially in epithermal (0.1-10 eV) neutrons. Therefore, we have decided to modify and install the linac in the core space of the experimental nuclear reactor “Yayoi” which is now under the decommission procedure. Due to the compactness of the X-band linac, an electron gun, accelerating tube and other components can be installed in a small space in the core. First we plan to perform the time-of-flight (TOF) transmission measurement for study of total cross sections of the nuclei for 0.1-10 eV energy neutrons. Therefore, if we adopt a TOF line of less than 10m, the macro-pulse length of generated neutrons should be shorter than 100 ns. Electronenergy, macro-pulse length, power, and neutron yield are ~30 MeV, 100 ns – 1 micros, ~0.4 kW, and ~10¹¹ n/s (~10³ n/cm²/s at samples), respectively. Optimization of the design of a neutron target (Ta, W, ²³⁸U), TOF line and neutron detector (Ce:LiCAF) of high sensitivity and fast response is underway. We are upgrading the electron gun and a buncher to realize higher current and beam power with a reasonable beam size in order to avoid damage of the neutron target. Although the neutron flux is limited in case of the X-band electron linac based source, we take advantage of its short pulse aspect and availability for nuclear data measurement with a short TOF system. First, we form a tentative configuration in the current experimental room for Compton scattering in 2014. Then, after the decommissioning has been finished, we move it to the “Yayoi” room and perform the operation and measurement.

Particle accelerators are in widespread use as intense, precisely controllable photon sources, but many applications, including nuclear nonproliferation, are limited by size. Laser-driven plasma accelerators (LPAs) reduce accelerator size, but a compact system also requires addressing radiation hazards resulting from disposal of particle beam energy after photon production, typically requiring large and heavy “beam dumps”. In this paper, we investigate, through 3-D Particle-In-Cell simulations, an LPA stage demonstrating equal effectiveness at accelerating and decelerating an electron beam over a very short distance. This indicates that in addition to providing compact accelerators, such structures can effectively reduce beam energy after photon production and hence beam dump weight and volume. This is important to the development of compact photon source systems which can satisfy needs including transportable operation or operation in populated areas.

Highly-collimated, polarized, mono-energetic beams of tunable gamma-rays may be created via the optimized Compton scattering of pulsed lasers off of ultra-bright, relativistic electron beams. Above 2 MeV, the peak brilliance of such sources can exceed that of the world's largest synchrotrons by more than 15 orders of magnitude and can enable for the first time the efficient pursuit of nuclear science and applications with photon beams, i.e. Nuclear Photonics. Potential applications are numerous and include isotope-specific nuclear materials management, element-specific medical radiography and radiology, non-destructive, isotope-specific, material assay and imaging, precision spectroscopy of nuclear resonances and photon-induced fission. This review covers activities at the Lawrence Livermore National Laboratory related to the design and optimization of mono-energetic, laser-Compton gamma-ray systems and introduces isotope-specific nuclear materials detection and assay applications enabled by them.

A compact gamma-beam source dedicated to the development of the nuclear security technologies by use of the nuclear resonance fluorescence is described. Besides, such source is a very promising tool for novel technologies of the express cargoes inspection to prevent nuclear terrorism. Gamma-beam with the quanta energies from 0.3MeV to 7.2MeV is generated in the Compton scattering of the “green” laser photons on the electron beam with energies from 90MeV to 430MeV. The characteristic property of the proposed gammabeam source is a narrow spectrum (less than 1%) at high average gamma-yield (of 10¹³γ/s) due to special operation mode.

Optical resonators and optical recirculators are key elements of Compton X/γ ray machines. With regard to their use in laser physics or in time-frequency metrology, these devices have to obey severe constraints when implemented in the vaccum of an electron accelerator. Our group has developed both types of devices. In this proceedings an original recirculator design, that was developed within the European proposal to the ELI-NP γ ray source call for tender, is described. This is an aberration free device which allows reciculating 32 times a short and high intensity laser pulse. It also allows synchronizing each of the 32 passes with the electron RF cavities within 100 fs. The second topic of these proceedings is a description of our R & D on optical resonators dedicated to laser-electron interactions. We have locked two different picosecond laser oscillators to the highest cavity finesse F=30000 ever reached in pulsed regime. We also designed and build a new kind of non-planar cavity, tetrahedron shape, providing circularly polarized eigen modes. This cavity was installed in the ATF accelerator of KEK and successfully used to produce a high gamma ray flux. Thanks to an original fibre amplifier, we succeed in stacking 100 kW of average power inside the cavity.

Recent investigations into dipole resonances below the neutron separation threshold have focused on characterizing the properties of the so-called Pygmy Dipole Resonance. The amount of extra PDR strength on top of a GDR tail depends largely on the choice, or the method of extraction of photon strength functions. Whereas most experimental searches for the PDR were performed on spherical nuclei, the present work focuses on recent experiments on ⁷⁶Se and ⁷⁶Ge, on the virge of deformation.

Nuclear resonance fluorescence (NRF) is useful for nondestructive assay (NDA) of nuclear materials such as spent nuclear fuel. Counting precision of the NRF-based measurement system can be affected by background counts from self-activity of spent fuel and coherent scattering such as Rayleigh, nuclear Thomson, and Delbrück scattering. In this talk, the measurement principle and calculated uncertainties of the proposed detection system are presented.

Non-destructive assay (NDA) of ²³⁹Pu in spent nuclear fuel is possible using the isotope-specific nuclear resonance fluorescence (NRF) integral resonance transmission (IRT) method. The IRT method measures the absorption of photons from a quasi-monoenergetic γ-ray beam due to all resonances in the energy width of the beam. According to calculations the IRT method could greatly improve assay times for ²³⁹Pu in nuclear fuel. To demonstrate and verify the IRT method, the IRT signature was first measured in ¹⁸¹Ta, whose nuclear resonant properties are similar to those of ²³⁹Pu, and then measured in ²³⁹Pu. These measurements were done using the quasi-monoenergetic beam at the High Intensity γ-ray Source (HIγS) in Durham, NC, USA. The IRT signature was observed as a decrease in scattering strength when the same isotope material was placed upstream of the scattering target. The results confirm the validity of the IRT method in both ¹⁸¹Ta and ²³⁹Pu.

Laser Compton scattering gamma-ray beam source has been developed at the NewSUBARU synchrotron light facility. The available maximum Gamma-ray photon energy is 76 MeV. The flux of quasi-monochromatic gamma-ray photons (for example: 16.7 MeV, ΔE/E ~ 5%) is more than 10⁶photons/sec using a 35 W Nd:YVO₄ laser combined with the 1 GeV storage electron beam with an intensity of 300 mA. We used the electron beams at E_e = 0.55 ~ 1.47 GeV for changing the energy of quasi-monochromatic gamma-ray beam. Gamma-ray beams were used for application experiments, a nuclear physics research, a nondestructive inspection of thick material, a generation of positron by pair creation, a magnetic Compton scattering measurements, and a nuclear transmutation.

The absolute energies of the electron beam and the laser Compton-scattered (LCS) γ-ray beam provided at the NewSUBARU synchrotron radiation facility were calibrated within the accuracy of the order of 10^-4 by measuring the LCS γ-ray energies with a energy-calibrated high-purity germanium detector.

The practice of nondestructive assay (NDA) of nuclear materials has, until now, been focused primarily (1) on smaller objects (2) with less fissile material and (3) with less self-generated radiation. The transition to the application of NDA to spent fuel assemblies and similar large objects violates these three conditions, thereby bringing the assumptions and paradigm of traditional NDA practice into question for the new applications. In this paper, a new paradigm for these new applications is presented which is based on the fundamental principles of nuclear engineering. It is shown that the NDA of spent fuel assemblies is mostly a three-dimensional problem that requires the integration of three independent NDA measurements in order to achieve a unique and accurate assay. The only NDA techniques that can avoid this requirement are those that analyze signals that are characteristic to specific isotopes (such as those caused by characteristic resonance interactions), and that are neither distorted nor overly attenuated by the other surrounding material. Some photon-based NDA techniques fall into this exceptional category. Such exceptional NDA techniques become essential to employ when assaying large objects that, unlike spent fuel assemblies, do not have a consistent geometry. With this new NDA paradigm, the advanced photon-based NDA techniques can be put into their proper context, and their development can thereby be properly motivated.

The giant dipole resonance (GDR) in ⁶Li was investigated via the ⁶LI(γ, xn) reaction, where x = 1, 2 or 3 at an incident energy range of E_γ =5-55. The(γ, n) cross section was the most dominant cross section among them. The GDR in ⁶Li was found to consist of two components at E_x = 11 MeV and 33 MeV. The component at E_x = 11 MeV seems to be the intrinsic GDR in ⁶Li. The other at E_x = 33 MeV is inferred to be the GDR due to the α cluster excitation in ⁶Li, based on the comparison with the results in light ion reactions. The GDR in free ⁴He is known to locate at E_x= 26 MeV. However, the GDR excitation energy due tothe α cluster excitation in ⁶Li is found to be higher than that of the ⁴He. This fact suggests that the size of the α cluster in ⁶Li is smaller than that of ⁴He due to the nuclear medium effect.

Cosmic-ray-muon imaging is proposed to assess the damages to the Fukushima Daiichi reactors. Simulation studies showed capability of muon imaging to reveal the core conditions.The muon-imaging technique was demonstrated at Toshiba Nuclear Critical Assembly, where the uranium-dioxide fuel assembly was imaged with 3-cm spatial resolution after 1 month of measurement.

Compton inverse radiation is emitted in the process of backscattering of the laser pulses off the relativistic electrons. This radiation possesses high spectral density and high energy of photons—in hard x-ray up to gammaray energy range—with moderate electron energies (hundreds of MeV up to 1 GeV) due to short wavelength of the laser radiation. The Compton radiation is well collimated: emitting within a narrow cone along the electron beam. A distinct property of the Compton inverse radiation is a steep high-energy cutoff of the spectrum and the maximal intensity just below the cutoff. The Compton sources can attain: spectral density up to 10¹⁴ gammas/(s 0.1%bandwidth) in MeV range of energies, and spectral brightness up to 1020 gammas/(smm²mr² 0.1% bw).

Applicability of Compton sources for nuclear waste management and detection of radioisotopes and fissionable nuclides are discussed in the report. Also application limits of Compton gamma sources for transmutation of radioactive isotopes are estimated. A recently proposed subtracting method, in which two sets of data obtained by irradiating the object by the Compton beams with slightly different maximal energies are compared, will enhance resolution of detection radioactive elements at the 'atomic' (hundreds of keV) and the 'nuclear' (a few MeV) photon energies.

A neutron generator based on inertial electrostatic confinement (IEC) of fusion plasmas is being developed for a non-destructive inspection system of special nuclear materials hidden in sea containers. The new IEC device is equipped with a multistage feedthrough which was designed aiming at both capability of a high bias voltage and enhancement of ion recirculation by modification of electric fields in the IEC device. Experimental comparison was made with a conventional single-stage IEC device developed in an earlier work. As the results, both the increase in the applied voltage and the modified field symmetry by the new multistage scheme showed significant enhancement in the neutron output. As a consequence, neutron output per input discharge current was enhanced drastically by a factor of ~30 in total. Also, the first pulsing experiments of the newly developed IEC neutron generator showed pulsed neutron output with a rapid pulse fall-off of ~ 1 μsec successfully.

A non-destructive inspection system for special nuclear materials (SNMs) hidden in a sea cargo has been developed. The system consists of a fast screening system using neutron generated by inertial electrostatic confinement (IEC) device and an isotope identification system using nuclear resonance fluorescence (NRF) measurements with laser Compton backscattering (LCS) gamma-rays has been developed. The neutron flux of 10⁸ n/sec has been achieved by the IEC in static mode. We have developed a modified neutron reactor noise analysis method to detect fission neutron in a short time. The LCS gamma-rays has been generated by using a small racetrack microtoron accelerator and an intense sub-nano second laser colliding head-on to the electron beam. The gamma-ray flux has been achieved more than 10⁵ photons/s. The NRF gamma-rays will be measured using LaBr₃(Ce) scintillation detector array whose performance has been measured by NRF experiment of U-235 in HIGS facility. The whole inspection system has been designed to satisfy a demand from the sea port.

For detection of hidden special nuclear materials (SNMs), we have developed an active neutron-based interrogation system combined with a D-D fusion pulsed neutron source and a neutron detection system. In the detection scheme, we have adopted new measurement techniques simultaneously; neutron noise analysis and neutron energy spectrum analysis. The validity of neutron noise analysis method has been experimentally studied in the Kyoto University Critical Assembly (KUCA), and was applied to a cargo container inspection system by simulation.

Fission fragments play an important role in nuclear reactors evolution and safety. However, fragments yields are poorly known : data are essentially limited to mass yields from thermal neutron-induced fissions on a very few nuclei. SOFIA (Study On FIssion with Aladin) is an innovative experimental program on nuclear fission carried out at the GSI facility, which aims at providing isotopic yields on a broad range of fissioning systems. Relativistic secondary beams of actinides and pre-actinides are selected by the Fragment Separator (FRS) and their fission is triggered by electromagnetic interaction. The resulting excitation energy is comparable to the result of an interaction with a low-energy neutron, thus leading to useful data for reactor simulations. For the first time ever, both fission fragments are completely identified in charge and mass in a new recoil spectrometer, allowing for precise yields measurements. The yield of prompt neutrons can then be deduced, and the fission mechanism can be ascribed, providing new constraints for fission models. During the first experiment, all the technical challenges were matched : we have thus set new experimental standards in the measurements of relativistic heavy ions (time of flight, position, energy loss).This communication presents a first series of results obtained on the fission of 238U; many other fissioning systems have also been measured and are being analyzed presently. A second SOFIA experiment is planned in September 2014, and will be focused on the measurement of the fission of ²³⁶U, the analog of ²³⁵U+n.

The present status of nuclear data was reviewed from the viewpoint of the research and development of non-destructive inspection methods for international controlled materials such as ²³⁵U, ²³⁹Pu, ²⁴¹Pu, D₂O, ⁶LiD, and ⁹Be. The nuclear characteristics of these materials were discussed, and neutron- and photon-reaction cross section data were reviewed. It was found that the accuracy of neutron data was enough for the research and development but that of photon data was not enough.

Nuclear databases are important tools to apply nuclear phenomena to various fields of nuclear engineering. It is now recognized that the databases must be further developed for photo-nuclear reaction data for nuclear security, safety and nonproliferation applications. Hokkaido University Nuclear Reaction Data Centre (JCPRG) has contributed to the Experimental Nuclear Reaction Data Library (EXFOR) which is developed by the International Network of Nuclear Reaction Data Centres under coordination by IAEA. We report here on the recent compilation of the nuclear data files for the photonuclear reaction.

Laser Compton back scattering photon beams and other gamma-ray sources are discussed in frame of the nuclear nonproliferation problem. New facility of ILC MSU (International Laser Center of Lomonosov Moscow State University) is described. Measured characteristics of the electron and gamma radiation in dependence on the laser parameters including the peak power, pulse duration and others are presented.

Using a high-contrast (10¹⁰:1) and high-intensity (10²¹ W/ cm²) laser pulse with a duration of 40 fs from an optical parametric chirped-pulse amplification/Ti:sapphire hybrid laser system in JAEA, a 40 MeV proton bunch is obtained, which is a record for laser pulse with energy less than 10 J. The efficiency for generation of protons with kinetic energy above 15 MeV is 0.1%.

We have developed a new photonuclear data file, JENDL/PD-2014, in order to further increase the accuracy of cross sections and nuclide yields. It was evaluated by using the photoabsorption cross sections based on giant dipole resonance and the results calculated by nuclear reaction models. The number of nuclides is increased to 181 in order to take into account the use in many application fields. The photoneutron yields from some of shielding materials were examined in comparison with a experiment.

The advent of MeV photon beam facilities from the Compton scattering of laser beams off ultra-relativistic electrons at modern synchrotron light sources offers a unique tool for nuclear science and technology research. The distinct spectrum with high intensities at the endpoint energies suggests several interesting applications in addition to photo nuclear physics studies. We present some examples for photons up to 15 MeV energies.

This paper describes the scientific aims and potentials as well as the preliminary technical design of IRIDE, an innovative tool for multi-disciplinary investigations in a wide field of scientific, technological and industrial applications. IRIDE will be a high intensity “particles factory”, based on a combination of high duty cycle radio-frequency superconducting electron linacs and of high energy lasers. Conceived to provide unique research possibilities for particle physics, for condensed matter physics, chemistry and material science, for structural biology and industrial applications, IRIDE will open completely new research possibilities and advance our knowledge in many branches of science and technology. IRIDE is also supposed to be realized in subsequent stages of development depending on the assigned priorities.

We have been developing an active, non-destructive detection system based on nuclear resonance fluorescence (NRF) for inspecting special nuclear materials (SNMs) such as ²³⁵U in a container at a seaport. The study of the NRF yield dependence on the target thickness of SNMs is required to evaluate the performance of the inspection system. To this end, an NRF experiment has been performed using a laser Compton backscattering γ-ray beam line at New SUBARU in ²⁰⁸Pb. Cylindrical shaped natural lead targets with a 0.5 cm radius and varying thicknesses of 1.0, 1.44, and 3.05 cm were irradiated at a resonance energy of 7.332 MeV. The NRF yield was detected using two HPG detectors with relative efficiencies of 120% and 100% positioned at scattering angles of 90° and 130°, respectively, relative to the incident γ-ray beam. As a result, the NRF yield exhibited a saturation behavior for the thick lead target. An analytic treatment and Monte Carlo simulation using GEANT4 was performed to interpret the reaction yield (RY) of the NRF interaction. The simulation result is in good agreement with the experimental data for the target thickness dependence. The analytic treatment, the NRF RY model, is also in reasonable agreement.

Photoreactions are applied for a useful assay tool to detect hidden nuclear materials. If we can employ polarized γ-rays, the reactions would be used to detect hidden nuclear materials with a higher signal-to-noise ratio. In 1950's, Agodi predicted that the angular distribution of cross sections in (γ, n) reactions with a 100% linearly polarized γ-ray beam for dipole excitations should be anisotropic and universally described by a simple function of a + b sin(2ϕ) at polar angle ϕ = 90°. However, there is no experimental reaction data with linear polarized photons except some light nuclei such as deuteron. We have verified experimentally the fact that this anisotropic angular distribution is manifested on ¹⁹⁷Au, ¹²⁷I, and natural Cu using linearly polarized laser Compton scattering γ-rays at NewSUBARU. We have measured neutron energy using a Time-Offlight method. We have changed the angle of linear polarized plane of the incident laser. Neutron angular distributions on the three targets can be well reproduced by the formula predicted by Agodi. We have verified the Agodi's prediction over the wide range region for the first time.

A γ-ray diffraction property of a silicon single crystal was studied in the Laue geometry using 1.33 MeV and 1.17 MeV γ-ray of ⁶⁰Co. The thickness was chosen to maximize the reflectivity of (440) lattice plane in γ-ray energy ranges of 1 to 2 MeV. We measured diffracted γ-rays from the crystal by an NaI scintillator. A measured diffraction intensity of 1.33 MeV γ-rays was 18 counts/sec by using 3.0 TBq a ⁶⁰Coγ-ray radiation system. The measured intensity is 30% lower than an expected value.

A high intensity γ-ray source from the laser Compton scattering (LCS) by an electron beam in the energy recovery linac (ERL) is a very useful prove for a nondestructive assay to identify nuclear species. In order to demonstrate a high performance of the accelerator and laser required for the γ-ray source, an LCS experiment is planned at the Compact ERL (cERL) at High Energy Accelerator Research Organization (KEK). A mode-locked fiber laser, laser enhancement cavity, beamline, and experimental hatch are under construction for the LCS experiment.

A high-flux mono-energetic γ-ray beam can be generated via Compton scattering of high-power laser by high-brightness electron beam. We have developed a high-brightness and high-current electron gun for generation of the high-flux γ-ray beam. Recently we demonstrated 500 keV electron beam generation, which meets the high-brightness requirement, from our DC photocathode gun at Japan Atomic Energy Agency. The gun was transported to High Energy Accelerator Research Organization (KEK) and connected to the following accelerator system. The gun operational status at KEK and our plan to develop a multialkali photocathode with a long lifetime are presented.

We are proposing non-destructive assay system of nuclear materials with laser Compton scattering combined with an energy-recovery linac (ERL) and a laser. Since constructing accelerator system for nuclear safe guard and security requires small cavities, spoke cavities have many advantages such as shortening the distance between cavities, small frequency detune due to micro-phonics and easy adjustment of field distribution for strong cell coupling.

Calculations of optimized cavity shape and HOM coupler shape have been performed and rf properties with aluminum spoke cavity model have been also measured. Considering refrigerator system required for superconducting accelerator, we are planning to develop 325MHz spoke cavity which can be practically operated with 4K liquid helium. We have started to fabricate the niobium one-spoke cavity.

Muonic X-ray measurement by the use of cosmic muon has a potential to identify nuclear material in containers. We performed a feasibility study by using an iron target. Two plastic scintillators detected incoming cosmic-ray muons and a veto scintillator identified muons stopped in the target. Germanium detectors in coincidence with the scintillators measured muonic X-ray energies. We clearly observed muonic X-ray peaks in the photon spectrum, of which the energies were consistent with known muonic X-ray energies. By using the obtained spectrum, input parameters of the Monte-Carlo simulation were checked. The simulation for uranium target showed that this method is promising.

The detection of special nuclear materials (SNM) is an important issue for nuclear security. The interrogation systems used in a sea port and an airport are developed in the world. The active neutron-based interrogation system is the one of the candidates. We are developing the active neutron-based interrogation system with a D-D fusion neutron source for the nuclear security application. The D-D neutron source is a compact discharge-type fusion neutron source called IEC (Inertial-Electrostatic Confinement fusion) device which provides 2.45 MeV neutrons. The nuclear materials emit the highenergy neutrons by fission reaction. High-energy neutrons with energies over 2.45 MeV amount to 30% of all the fission neutrons. By using the D-D neutron source, the detection of SNMs is considered to be possible with the attention of fast neutrons if there is over 2.45 MeV. Ideally, neutrons at En>2.45 MeV do not exist if there is no nuclear materials. The detection of fission neutrons over 2.45 MeV are hopeful prospect for the detection of SNM with a high S/N ratio. In the future, the experiments combined with nuclear materials and a D-D neutron source will be conducted. Furthermore, the interrogation system will be numerically investigated by using nuclear materials, a D-D neutron source, and a steel container.

The Nuclear Physics and Gamma-ray Sources for Nuclear Security and Nonproliferation (NPNSNP) meeting held in Tokai-mura, Japan from January 28^th to 30^th, 2014 revealed both the rapid evolution and growth of monoenergetic, laser-Compton, gamma-ray source technology and the emergence of numerous important applications enabled by this technology. More than $500M of large-scale source and development activities were represented at the meeting, including all of the major projects in the United States, Europe and Japan. The meeting was both highly stimulating intellectually and provided an excellent venue for the exploration of new collaborations between groups…

The following sections are included:

• Program Schedule of the International Workshop
• List of Participants