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Compact mono-energetic photon sources are sought for active interrogation systems to detect shielded special nuclear materials in, for example, cargo containers, trucks and other vehicles. A prototype gamma interrogation source has been designed and built that utilizes the 11B(p,gamma)12C reaction to produce 12 MeV gamma-rays which are near the peak of the photofission cross section. In particular, the 11B(p,gamma)12C resonance at 163 kV allows the production of gammas at low proton acceleration voltages, thus keeping the design of a gamma generator comparatively small and simple. A coaxial design has been adopted with a toroidal-shaped plasma chamber surrounding a cylindrical gamma production target. The plasma discharge is driven by a 2 MHz rf-power supply (capable up to 50 kW) using a circular rf-antenna. Permanent magnets embedded in the walls of the plasma chamber generate a multi-cusp field that confines the plasma and allows higher plasma densities and lower gas pressures. About 100 proton beamlets are extracted through a slotted plasma electrode towards the target at the center of the device that is at a negative 180 kV. The target consists of LaB6 tiles that are brazed to a water-cooled cylindrical structure. The generator is designed to operate at 500 Hz with 20 mu s long pulses, and a 1percent duty factor by pulsing the ion source rf-power. A first-generation coaxial gamma source has been built for low duty factor experiments and testing.

Abstract The role of active-interrogation systems for nuclear security is to detect the presence of special nuclear material inside an object by observing the radiation emitted by that object when it has been exposed to known sources of external radiation. Because of the cost, complexity, and the need to avoid irradiating occupants, active-interrogation systems are intended for cargo applications where shielding can prevent detection by passive radiation detection systems. To ensure that active-interrogation systems for detection of special nuclear material are designed and tested to a consistent level of performance, technical standards are needed for evaluating such systems. This paper addresses the testing standards needed for active-interrogation systems to detect high atomic number materials, fissionable materials, and specific special nuclear materials. Because the use of special nuclear material for a testing standard is not practical, this work focuses on the determination of materials that could be used as surrogates in that they provide a similar response as targets of special nuclear materials. The results of this paper, determined through analytic calculations and radiation transport modeling, are based upon scenarios constructed and applied to specific active-interrogation modalities. [ABSTRACT FROM AUTHOR]

The delayed γ-ray and neutron fission signals utilized in active inspection techniques were measured simultaneously in order to directly compare their detection sensitivities. Fissionable and non-fissionable targets were irradiated by a 15-Hz pulsed bremsstrahlung beam operating at endpoint energies from 7 to 22 MeV. The fissionable mass detection limits for both these signals decreased approximately three orders of magnitude as the irradiation energy was increased with the delayed γ-ray limits 4.3–8.2 times smaller. The signals from the non-fissionable targets were consistent with the natural passive backgrounds for irradiation energies up to 16 MeV. At higher bremsstrahlung energies, there was a target independent active background in the delayed γ-ray signal that accounted for 35% of the gross yield. In addition, these higher irradiation energies resulted in products from 9 Be(γ,p) 8 Li and 18 O(γ,p) 17 N reactions interfering with the delayed γ-ray and neutron fission signals, respectively. [ABSTRACT FROM AUTHOR]

The fast neutron nuclear reactions, such as 19 F(n, α) 16 N and 19 F(n, p) 19 O, can be used to detect highly energetic neutrons due to their energy thresholds above which these activation reactions can occur. This was recently shown (Gozani et al., 2011 [2] ) as a means to detect concealed nuclear materials via the detection of the high energy (≈3 MeV) prompt neutrons emitted during the photofission process. Fluorine-loaded scintillation detectors, such as inorganic BaF 2 and CaF 2 , and non-hydrogenous fluorocarbon (FC) liquid scintillators, such as Saint-Gobain BC-509 and Eljen Technology EJ-313, are possible candidates. The latter was selected and implemented in the above mentioned reference. In our paper, we propose a new pentafluorostyrene-based plastic scintillator (F-plastic) which can be a good alternative to the abovementioned scintillators. The fluorine content of F-plastic is equal to 3.73×10 22 atoms/cm 3 , and the F / H ratio is 1.66. The fluorescence and radioluminescence spectra of the F-plastic display an emission maximum centered approximately at 420 nm. The light output measured for gamma rays is 3100±300 photons/MeV, which is approximately 30% of the light output of the standard EJ-200 plastic scintillator and is similar to EJ-313. The response of the F-plastic to neutrons and gamma rays is presented and compared to the EJ-200 scintillator. Additionally, the n/γ pulse shape discrimination (PSD) was measured and showed improvement of the discrimination at neutron energies as high as 3 MeV. [ABSTRACT FROM AUTHOR]

U photofission is studied in the region of nuclear excitation energies that extends up to 20 MeV. Independent photofission-fragment yields and cumulative photofission-fragment yields after the emission of fast neutrons are measured by gamma-spectroscopy methods. The mass distributions of photofission fragments are obtained at the endpoint bremsstrahlung energies of 19.5, 29.1, 48.3, and 67.7 MeV. A comparative analysis of the behavior of the symmetric and asymmetric modes of photoninduced fission as a function of the average excitation energy of the fissioning nucleus is performed. The integrated yield of U photofission is calculated, and the evaluated cross sections for photofission and for photoneutron reactions are discussed. [ABSTRACT FROM AUTHOR]

Data on cross sections for the reaction U( γ,F) in the giant-resonance region were analyzed in connection with the preparation of new experiments aimed at studying U photofission-in particular, in beams of photons from in-flight positron annihilation on internal targets of positron storage rings. These data were taken from measurements also performed with annihilation photons but from positron beams external to the accelerators used. The procedures applied in such measurements and based both on processing the multiplicity of detected neutrons and on detecting fission fragments were also analyzed. [ABSTRACT FROM AUTHOR]

The photofission cross section of 238U was measured at sub-barrier energies as a function of the γ-ray energy using a monochromatic, high-brilliance, Compton-backscattered γ-ray beam. The experiment was performed at the High Intensity γ-ray Source (HIγS) facility at beam energies between Eγ = 4.7 MeV and 6.0 MeV and with ~3% energy resolution. Indications of transmission resonances have been observed at γ-ray beam energies of Eγ = 5.1 MeV and 5.6 MeV with moderate amplitudes. The triple-humped fission barrier parameters of 238U have been determined by fitting EMPIRE-3.1 nuclear reaction code calculations to the experimental photofission cross section. [ABSTRACT FROM AUTHOR]

Abstract: A hybrid photoneutron target including natural uranium has been studied for a 20MeV linear electron accelerator (Linac) based Boron Neutron Capture Therapy (BNCT) facility. In this study the possibility of using uranium to increase the neutron intensity has been investigated by focusing on the time dependence behavior of the build-up and decay of the delayed gamma rays from fission fragments and activation products through photo-fission reactions in the BSA (Beam Shaping Assembly) configuration design. Delayed components of neutrons and photons were calculated. The obtained BSA parameters are in agreement with the IAEA recommendation and compared to the hybrid photoneutron target without U. The epithermal flux in the suggested design is 2.67E9 (n/cm2s/mA). [Copyright &y& Elsevier]

The ratio of the probabilities of population of the isomeric and ground states, so called the isomeric ratio is closely connected to the angular momentum of the initial fission fragments that is dissipated at the later stages of gamma ray cascade. This ratio also provides important information on the nuclear level structure as well as the nuclear reaction mechanism involved. In this work, the isomeric ratio in fission fragment Xe from photofission of U induced by 23.5 MeV bremsstrahlung has been determined by the method that uses inert gaseous flow. The results have been discussed and compared with that of other authors. [ABSTRACT FROM AUTHOR]

A new detector system dedicated for very/super heavy elements (VHE/SHE) detection that decay by spontaneous fission is presented. Such a decay mode of nuclei can be found e.g. in the region of Z≈100 (Fermium). Future experiments realized in the frame of French-Polish collaboration at the cyclotrons in GANIL and in the Heavy Ion Laboratory (HIL), University of Warsaw, is described. The results of test measurements made in the HIL for reactions 20Ne(8 A.MeV)+120Sn, 179Au are presented. [ABSTRACT FROM AUTHOR]

A theoretical analysis of the most detailed experimental data on the subbarrier fission of 232Th is performed. The energy dependences of the components of the cross sections for photofission through specific channels characterized by the quantum numbers J π K are obtained on the basis of previously measured total photofission cross sections and angular distributions of fission fragments. A theoretical analysis of the subbarrier photofission of 232Th is performed on the basis of the results obtained for these components. This analysis relies on the one-dimensional fission model within which one assumes a three-humped structure of the fission barrier and introduces damping in the second and third wells by including an imaginary part in the potential used. Values are selected for the parameters of the fission barriers and are compared with similar data from other studies. It is shown that the energy dependences of the photofission cross sections in question can be described adequately. [ABSTRACT FROM AUTHOR]

Abstract: The ALTO facility (Accélérateur Linéaire auprès du Tandem d’Orsay) has been built and is now under commissioning. The facility is intended for the production of low energy neutron-rich ion-beams by ISOL technique. This will open new perspectives in the study of nuclei very far from the valley of stability. Neutron-rich nuclei are produced by photofission in a thick uranium carbide target (UC x ) using a 10μA, 50MeV electron beam. The target is the same as that already had been used on the previous deuteron based fission ISOL setup (PARRNE []). The intended nominal fission rate is about 1011 fissions/s. We have studied the adequacy of a thick carbide uranium target to produce neutron-rich nuclei by photofission by means of Monte-Carlo simulations. We present the production rates in the target and after extraction and mass separation steps. The results from Monte-Carlo simulations are compared to experimental data either with the ALTO facility (in the first step of commissioning, i.e. 100nA of the electron beam current intensity), or with fast-neutron-induced fission generated from a 26MeV deuteron beam. The results obtained support the suitability of FLUKA simulation code used to describe all the photofission process with an electron beam energy of 50MeV. [Copyright &y& Elsevier]

Primordial nucleosynthesis is one of the three observational evidences for the Big--Bang model. Even though they span a range of nine orders of magnitude, there is indeed a good overall agreement between primordial abundances of 4He, D, ³He and 7Li either deduced from observation or primordial nucleosynthesis. This comparison was used for the determination of the baryonic density of the Universe. For this purpose, it is now superseded by the analysis of the CMB anisotropies. Big--Bang nucleosynthesis remains, nevertheless, a valuable tool to probe the physics of the early Universe. [ABSTRACT FROM AUTHOR]

This is a brief review of the progress made since the seminal contributions to the foundations of the theory of nucleosynthesis by M. Burbidge, G. Burbidge, Fowler and Hoyle, and by Cameron. The reviewed topics are (1) the nuclear physics input to the nucleosynthesis models (nuclear masses, fission, rates of β-decays, neutrino reactions, photoreactions, and nuclear charged particle-induced or neutron-induced reactions), (2) the nuclear physics and astrophysics aspects of the r-process, and (3) the same items for the p-process. [ABSTRACT FROM AUTHOR]

A method for determining the quadrupole deformation parameter of transuranium nuclei is presented. This method is based on giant dipole resonance splitting in the photofission cross section. The experimental setup containing an avalanche counter of fission fragments, as well as a method for reconstructing the cross section from the measured dependence of the photofission reaction yield on the endpoint energy of the bremsstrahlung spectrum, are described. The setup was tested on the bremsstrahlung beam of the MT-25 microtron at the Flerov Laboratory for Nuclear Reactions of the Joint Institute for Nuclear Research. The test results—in particular, the measured excitation function of the uranium photofission reaction—are presented. [ABSTRACT FROM AUTHOR]

We expand upon the idea of using γ-rays for nuclear photofission of 238U at the giant dipole resonance to generate rare neutron-reach nuclei. The SPIRAL II project proposes the employment of 10-20-MeV Bremsstrahlung γ-rays generated by a 45-MeV electron beam /http://ganinfo.in2p3.fr/research/developments/ spiral2/ index.html/. In this paper, we explore the possibility of using a Compton γ-ray source for such a process. The Collider Accelerator Department at Brookhaven National Laboratory is developing high-current (up to 1 A), high-brightness (down to 1-mm · mrad normalized emittance), and high-energy energy-recovery linacs (up to 20-GeV electron beam energy for eRHIC). These electron beams are perfectly suited for generating photon beams with tremendous average power, approaching the megawatt level. The range of photon's energy extends from subelectronvolts from free-electron lasers to 10 GeV from the Compton process. In this paper, we focus on a γ-ray source for producing rare isotopes. [ABSTRACT FROM AUTHOR]

The yields of products originating from 238U photofission are measured at the bremsstrahlung endpoint energies of 50 and 3500 MeV. Charge and mass distributions of fission fragments are obtained. Symmetric and asymmetric channels in 238U photofission are singled out on the basis of the model of multimode fission. This decomposition makes it possible to estimate the contributions of various fission components and to calculate the fissilities of 238U in the photon-energy regions under study. [ABSTRACT FROM AUTHOR]

Abstract: We describe new Monte Carlo codes for the simulation of coincident counts, both real and accidental, originating from photonuclear and photoatomic events in nuclear, and non-nuclear, materials. The codes perform fully analog simulation of all secondary neutrons, and photons, emitted by photonuclear, and photoatomic, events. Separate algorithms are then used to simulate the accidental coincidences. These new codes have been applied to a realistic measurement scenario. Two possible techniques for minimizing the detection of accidental coincidences are then analyzed: pulse-shape discrimination and detector lead shielding. The results showed that the use of lead-shielded liquid scintillation detectors with pulse-shape discrimination is a promising technique for the detection of actinides in shielded configurations by minimizing accidental counts. The data acquisition time for this approach would be on the order of 1min or less for the measurement scenario considered here. [Copyright &y& Elsevier]

Abstract: Quantifying the mass of actinides in large concrete waste packages using non-destructive methods is a major challenge for the management and the storage of packages in appropriate facilities. Since the beginning of 1990s, our team in CEA has been working on the development of a method based on interrogation with high-energy photons to assay actinides in large concrete waste package. The method consists in using photons of high energy (bremsstrahlung radiation) in order to induce photofission reactions on the fissile nuclei present in the wastes. The measurement of the delayed neutrons emitted by fission products allows us to quantify the actinides present in the wastes. The accuracy of the method can be deeply improved by carrying out a tomography, i.e. computing the three-dimensional spatial distribution of the actinide mass inside the nuclear waste package. This paper presents the first experimental results of a tomography carried out on a 1.2t real concrete waste package. Measurements associated with reconstruction algorithms and Monte Carlo simulations have allowed to locate an equivalent mass of 690mg of uranium 238 centered in a disc of 20–25cm of diameter at 75cm height. These measurements have been performed in the SAPHIR irradiation facility at CEA/Saclay. This facility houses a pulsed linear electron accelerator (energy range from 15to 30MeV, pulse duration of 2.5μs, peak current of 130mA). The located mass has been subsequently confirmed by a destructive analysis of the package. [Copyright &y& Elsevier]

Abstract: An examination of delayed neutron emission decay rates allows for the detection and identification of fissionable isotopes. The dependence of the decay rates on bremsstrahlung end-point energies and accelerator repetition rates was examined. Delayed neutrons emitted following photofission in a target were detected between bremsstrahlung pulses from a variable-energy linear electron accelerator. Bremsstrahlung end-point energies of 14, 16, 18, 20, and 22MeV are used to show that the decay rates have no obvious dependence on the energy of the irradiating photons. Accelerator repetition rates of , , 15, 30, and 60Hz demonstrate the expected frequency dependence of the decay rates. [Copyright &y& Elsevier]

Abstract: Recently a renewed interest in photonuclear reactions has appeared. It is motivated by a number of different applications where progress in reliable and, in some cases, very high-intensity electron accelerators was awaited. In particular, today''s interest is linked to the nuclear material interrogation and non-destructive nuclear waste characterization, both based either on prompt neutron, or delayed neutron, or delayed gamma detection following photofission. The knowledge of photonuclear reactions is also extremely important in the design of electron accelerators used for medical applications, nuclear physics, photoneutron sources, radioactive ion beam production, etc. In this paper we present the photonuclear activation file (PAF) under development to be included into any material depletion code. By now the PAF contains the photonuclear cross-sections for more than 600 isotopes, fission fragment distributions and delayed neutron evaluations for most of the actinides in the photon energy range from 0 up to 25MeV. Theoretical predictions, evaluated data files and available experimental data were used to construct the PAF, and some benchmarks are in progress to ensure its quality. The release of the first version of PAF is planned in 2006. [Copyright &y& Elsevier]

Results are presented that were obtained by measuring the independent yields of Kr (A = 89 – 93) and Xe (A = 135 – 142) appearing as fragments in the photofission of 237Np and 243Am odd nuclei. The respective experiments were performed in a beam of bremsstrahlung photons from electrons accelerated to an energy of 25 MeV at the microtron of the Laboratory of Nuclear Reactions at the Joint Institute for Nuclear Research (JINR, Dubna). Use was made of the procedure involving the transportation of fragments emitted from the target by a gas flow along a capillary and the condensation of inert gases in a cryostat at liquid-nitrogen temperature. The identification of Kr and Xe appearing as fragments was performed by the gamma spectra of their daughter products. The mass-number distributions of the independent yields of Kr and Xe isotopes were obtained, along with those for the complementary fragments (Y and La in the fission of 237Np and Nb and Pr in the fission of 243Am). © 2005 Pleiades Publishing, Inc. [ABSTRACT FROM AUTHOR]