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This work is devoted to calculating the rate of radiation damage production in a uranium target irradiated with high-energy electrons with an energy of 100 MeV. The Monte Carlo program MCNPX was used to perform a complete mathematical modeling of a complex of processes occurring in a uranium target when irradiated with high-energy electrons: the development of an electromagnetic shower, the production of photoneutrons, the transport of particles in the target and the creation of radiation damage in it. The analysis showed that fragments of U-238 photo-fission give the main input into the rate of damage production in a uranium target which reaches the value of 100 dpa/year. The expected service life of a uranium target under irradiation is 3 years of operation at full accelerator power. [ABSTRACT FROM AUTHOR]

In inverse kinematics, we explored Coulomb fission cross-sections and time scales. Heavier projectiles such as 238U, 132Xe, and 208Pb on lighter targets such as 16O 26Mg, 27Al, 32S, 35Cl, 40,48Ca, 45Sc, 48,50Ti, 56,58Fe, 64Ni, 89Y, 30Si and 65Zn were looked into. The photofission cross-sections and the number of equivalent photons impinge on the target nuclei are important parameters in cross-section assessment. The anticipated cross-sections increases with increase in atomic number of target nuclei. We also explored the time scale of projectile nuclei at various transition levels. The estimated time scales accord well with the available experimental value, which is in the order of picoseconds. In addition, we predicted time scales of 165Ho nuclei in the transition state from 19/2+→15/2+ to 29/2+→25/2+ for which experimental time scales are not available in literature. As a result, Coulomb fission is one of the causes that has severely hampered in the formation of compound nuclei. Prediction of time scales, in particular, would aid in understanding the structure of various energy levels.

The emerging experimental program with brilliant gamma beams at the Extreme Light Infrastructure - Nuclear Physics facility (ELI-NP) is presented with emphasis on the prepared day-one experiments. Ex- periments at ELI-NP will cover nuclear resonance fluorescence measurements, studies of large-amplitude motions in nuclei, photo-fission and photonuclear reactions of astrophysics interest, and measurements of photonuclear reaction cross sections. The physics cases of the flagship experiments at ELI-NP and the performance of the related instruments, which are under construction for their realization, are discussed.

An overview of the main directions of present-day studies with quasimonochromatic γ beams are discussed with an emphasis on the research opportunities which will be offered at the Extreme Light Infrastructure Nuclear Physics (ELI-NP) facility at Magurele near Bucharest in Romania. Experiments with γ beams at the extremes of high temperature and high neutron-to-proton ratios are outlined. Some of the experimental setups for nuclear structure, reaction and astrophysics studies, which are available at ELI-NP, are described. Perspectives for studies of exotic nuclei within the Gamma Factory experiment at CERN are discussed. The expected fission-fragment yields at the Gamma Factory are presented. The perspectives for nuclear structure studies with γ rays with orbital angular momentum are outlined. [ABSTRACT FROM AUTHOR]

In experiments for photofission of 238U with bremsstrahlung, the isomeric yield ratios for 133Xe at the end-point energies Ee = 6.25, 14.5 and 18 MeV and for 135Xe at Ee = 6.25, 12.5 and 14.5 MeV were measured. Isomeric yield ratios were obtained taking into account the contribution from beta-decay of isobaric nuclei to the yields of investigated isotopes. Average angular moments of investigated fission fragments were determined using the statistical model of decay.

The photofission fragment mass yields of actinides are evaluated using a systematic statistical scission point model. In this model, all energies at the scission point are presented as a linear function of the mass numbers of fission fragments. The mass yields are calculated with a new approximated relative probability for each complementary fragment. The agreement with the experimental data is quite good, especially with a collective temperature Tcol of 2 MeV at intermediate excitation energy and Tcol = 1 MeV for spontaneous fission. This indicates that the collective temperature is greater than the value obtained by the initial excitation energy. The generalized superfluid model is applied for calculating the fragment temperature. The deformation parameters of fission fragments have been obtained by fitting the calculated results with the experimental values. This indicates that the deformation parameters decrease with increasing excitation energy. Also, these parameters decrease for fissioning systems with odd mass numbers.

Based on 238U photofission experiments with the bremsstrahlung photons, the isomeric yield ratios were measured for 95Nb and 133Xe nuclei at the end-point energy Ee = 12.5 MeV, and 135Xe nuclei at Ee = 12.0 MeV. Experimental values of isomeric yield ratios were obtained taking into account the contribution from beta-decay of isobaric nuclei in yields of investigated isotopes. Average angular momenta of investigated fission fragments were determined using the statistical model of decay.

• Photofission is a promising technique for nuclear waste package characterization. • 9 MeV linear accelerator parameters and target materials impact the assay performance. • Monte Carlo simulations are conducted in PHITS and MCNP6 codes and compared. • Optimization of the photon flux and reduction of neutron background are conducted. Photofission-based active non-destructive assay is a promising technique for nuclear waste characterization. The latter is challenging due to the presence of actinides, fission products and non-homogeneous matrix. This paper presents an optimization study of a 9 MeV linear electron accelerator used as a source of Bremsstrahlung photons for such photofission applications. The primary focus is made on qualitative and quantitative assessment of the Bremsstrahlung flux, timing characteristics and reduction of background neutrons. Assessment is conducted using Monte Carlo simulations in particle transport codes PHITS and MCNP6 with an objective to compare the simulation results. Our results demonstrate a good agreement between the simulation results of both codes. However, for an effective application of the photofission technique for nuclear waste assay, a careful selection of the target material and introduction of parasitic neutron suppression techniques are necessary. Analysis routines and simulation procedures are described in detail and results are presented. [ABSTRACT FROM AUTHOR]

The total excitation energy as a function of fragment mass (TXE/A) are calculated for neutron induced fission of 235 U and photofission of 238 U by using the statistical model. With comparing the calculated results and the experimental data, the statistical model is modified. In new modified model, TXE are calculated by adding the neutron binding energy to the deformation energy. The calculated results using modified statistical model are in good agreement with the experimental data. Then, the total excitation energy as a function of fragment mass (TXE/A) are evaluated for photofission of Uranium isotopes. [ABSTRACT FROM AUTHOR]

To study the influence of the structural properties of UC x targets on their release properties, several types of targets using different precursors (carbon and uranium) were synthesized, characterized, irradiated and heated leading to the determination of the released fractions of eight elements. In this article, the production rates of these targets are estimated under the use conditions at ALTO, i.e. with targets bombarded by an electron beam (10 μA, 50 MeV). We have simulated the fission number produced using the FLUKA code. Then, we have determined the release efficiency as a function of the half-life of the isotopes using average diffusion coefficients deduced for the elements studied previously. Finally, we compare the production rates obtained from the various targets and conclude that the target must be adapted to the element studied. It is crucial to find in each case the best compromise between the target density and the release efficiency. [ABSTRACT FROM AUTHOR]

We use a combination of crossed laser-molecular beam scattering experiments and velocity map imaging experiments to investigate the three primary photodissociation channels of chloroacetone at 193 nm: C-Cl bond photofission yielding CH3C(O)CH2 radicals, C-C bond photofission yielding CH3CO and CH2Cl products, and C-CH3 bond photofission resulting in CH3 and C(O)CH2Cl products. Improved analysis of data previously reported by our group quantitatively identifies the contribution of this latter photodissociation channel. We introduce a forward convolution procedure to identify the portion of the signal, derived from the methyl image, which results from a two-step process in which C-Cl bond photofission is followed by the dissociation of the vibrationally excited CH3C(O)CH2 radicals to CH3 + COCH2. Subtracting this from the total methyl signal identifies the methyl photofragments that result from the CH3 + C(O)CH2Cl photofission channel. We find that about 89% of the chloroacetone molecules undergo C-Cl bond photofission to yield CH3C(O)CH2 and Cl products; approximately 8% result in C-C bond photofission to yield CH3CO and CH2Cl products, and the remaining 2.6% undergo C-CH3 bond photofission to yield CH3 and C(O)CH2Cl products. [ABSTRACT FROM AUTHOR]

This study initially characterizes the primary photodissociation processes of epichlorohydrin, c-(H2COCH)CH2Cl. The three dominant photoproduct channels analyzed are c-(H2COCH)CH2+Cl, c-(H2COCH)+CH2Cl, and C3H4O+HCl. In the second channel, the c-(H2COCH) photofission product is a higher energy intermediate on C2H3O global potential energy surface and has a small isomerization barrier to vinoxy. The resulting highly vibrationally excited vinoxy radicals likely dissociate to give the observed signal at the mass corresponding to ketene, H2CCO. The final primary photodissociation pathway HCl+C3H4O evidences a recoil kinetic energy distribution similar to that of four-center HCl elimination in chlorinated alkenes, so is assigned to production of c-(H2COC)=CH2; the epoxide product is formed with enough vibrational energy to isomerize to acrolein and dissociate. The paper then analyzes the dynamics of the C3H5O radical produced from C–Cl bond photofission. When the epoxide radical photoproduct undergoes facile ring opening, it is the radical intermediate formed in the O(3P)+allyl bimolecular reaction when the O atom adds to an end C atom. We focus on the HCO+C2H4 and H2CO+C2H3 product channels from this radical intermediate in this report. Analysis of the velocity distribution of the momentum-matched signals from the HCO+C2H4 products at m/e=29 and 28 shows that the dissociation of the radical intermediate imparts a high relative kinetic energy, peaking near 20 kcal/mol, between the products. Similarly, the energy imparted to relative kinetic energy in the H2CO+C2H3 product channel of the O(3P)+allyl radical intermediate also peaks at high-recoil kinetic energies, near 18 kcal/mol. The strongly forward-backward peaked angular distributions and the high kinetic energy release result from tangential recoil during the dissociation of highly rotationally excited nascent radicals formed photolytically in this experiment. The data also reveal substantial branching to an HCCH+H3CO product channel. We present a detailed statistical prediction for the dissociation of the radical intermediate on the C3H5O potential energy surface calculated with coupled cluster theory, accounting for the rotational and vibrational energy imparted to the radical intermediate and the resulting competition between the H+acrolein, HCO+C2H4, and H2CO+C2H3 product channels. We compare the results of the theoretical prediction with our measured branching ratios. We also report photoionization efficiency (PIE) curves extending from 9.25 to 12.75 eV for the signal from the HCO+C2H4 and H2CO+C2H3 product channels. Using the C2H4 bandwidth-averaged absolute photoionization cross section at 11.27 eV and our measured relative photoion signals of C2H4 and HCO yields a value of 11.6+1/-3 Mb for the photoionization cross section of HCO at 11.27 eV. This determination puts the PIE curve of HCO measured here on an absolute scale, allowing us to report the absolute photoionization efficiency of HCO over the entire range of photoionization energies. [ABSTRACT FROM AUTHOR]

In experiments for photofission of 238U with bremsstrahlung, the isomeric yield ratios for 133Xe at the end-point energies Ee = 6.25, 14.5 and 18 MeV and for 135Xe at Ee = 6.25, 12.5 and 14.5 MeV were measured. Isomeric yield ratios were obtained taking into account the contribution from beta-decay of isobaric nuclei to the yields of investigated isotopes. Average angular moments of investigated fission fragments were determined using the statistical model of decay. [ABSTRACT FROM AUTHOR]

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

The characterization of radioactive waste packages is mandatory for their transport, interim storage and final disposal. In this framework, the Nuclear Measurement Laboratory of CEA DES IRESNE Institute, at Cadarache, France, uses a high-energy electron linear accelerator (LINAC) to produce an interrogating bremsstrahlung beam with endpoint energies ranging from 9 to 21 MeV to perform X-ray imaging and high-energy photon interrogation on large concrete packages. In particular, highenergy photon beam induces photofission reactions in both fissile (235U, 239Pu, 241Pu) and fertile (238U, 240Pu, 232Th, etc.) actinides possibly present in the radioactive waste. In order to assess their mass, we use delayed gamma rays emitted by their photofission products, which are measured with a 50 % relative efficiency High-Purity Germanium (HPGe) detector. Actinide differentiation, which is important for the fissile mass estimation, is based on the ratios of gamma rays emitted by different photofission products and requires appropriate corrections for the gamma attenuation in concrete. To this aim, we report here a localization method of point-like nuclear materials in the concrete matrix, based on the differential attenuation of several gamma rays emitted by a same photofission product. We use here the 1435.9 and 2639.6 keV lines of 138Cs, with both experimental data and MCNP numerical simulations to determine the (r,θ) coordinates of nuclear materials. Then, the depth inside the concrete matrix, which is determined with a precision of a few percent, mainly depending on counting statistics on 1435.9 and 2639.6 keV net peak areas, is used to correct for the different gamma ratios used in the actinide identification method. Experimental tests with uranium samples have been performed to validate the localization method.

A radioactive isotope (RI) separator named ERIS (electron-beam-driven RI separator for SCRIT) was constructed at the SCRIT (Self-Confining Radioactive isotope Ion Target) electron scattering facility at RIKEN RI Beam Factory and has been in operation since 2013. Recently, ion stacking and pulse extraction were developed at ERIS to improve the dc-to-pulse conversion efficiency of a radio-frequency quadrupole cooler buncher. Using a surface-ionization ion source, long-time stacking inside ERIS was realized and a dc-to-pulse conversion efficiency of more than 50% was achieved with a relatively small amount of buffer gas. [ABSTRACT FROM AUTHOR]

Isomeric yield ratios for the fragments of photofission of 235U, 237Np and 239Pu by bremsstrahlung with end-point energy of 18 MeV are measured. New data for isomeric yield ratios of the fragments 84Se, 90Br, 131Te, 132Sb, 132I, 133Te, 134I, 135Xe are determined. The contributions to the isomeric yield ratios from β-decay of the nuclei of parent isobaric chains were removed. Mean angular momenta of studied nuclei are estimated within the framework of an extended Huizenga-Vandenbosh statistical model with using EMPIRE 3.2 code.

In this paper, test measurements of a fibered optically stimulated luminescent dosimeter (OSLD) system performed during a dedicated experimental phase in EOLE zero-power reactor are presented. The measurement setup consists of an OSLD crystal connected onto the extremity of an optical fiber and a laser stimulation system, manufactured by the CEA/LIST, Saclay, France. The OSL sensor is remotely stimulated via an optical fiber using a diode-pumped solid-state laser. The OSL light is collected and guided back along the same fiber to a photomultiplier tube through appropriate optical filters. OSL signals are corrected for background noise and integrated to provide the absorbed doses thanks to calibration coefficients. The light stimulation also fully bleaches the crystal (reusable for the next measurement). Results obtained using this system are compared to usual gamma-heating measurement protocol using OSLD pellets. Both measurement processes were calibrated to ensure the comparison of absolute doses in gray at the same positions in the core. The presence of induced radioluminescence in the OSLD during the irradiation was also observed and could be used to monitor the gamma flux. The feasibility of remote measurements is achieved, whereas further developments could be conducted to improve this technique since the readout procedure still requires to withdraw the OSLD off the gamma flux (hence from the core) on account of the dose rate (around a few Gy $\cdot \text{h}^{\mathrm {\mathbf {-1}}}$), and the readout time remains quite long for online applications. Several improvements are foreseen and will be tested in the forthcoming years. [ABSTRACT FROM AUTHOR]

This review paper describes the destructive and non-destructive measurements implemented or under development at CEA, in view to perform the most complete radioactive waste characterization. First, high-energy photon imaging (radiography, tomography) brings essential information on the waste packages, such as density, position and shape of the waste inside the container and in the possible binder, quality of coating and blocking matrices, presence of internal shields or structures, presence of cracks, voids, or other defects in the container or in the matrix, liquids or other forbidden materials, etc. Radiological assessment is then performed using a series of non-destructive techniques such as gamma-ray spectroscopy, which allows characterizing a wide range of radioactive and nuclear materials, passive neutron coincidence counting and active neutron interrogation with the differential die-away technique, or active photon interrogation with high-energy photons (photofission), to measure nuclear materials. Prompt gamma neutron activation analysis (PGNAA) can also be employed to detect toxic chemicals or elements which can greatly influence the above measurements, such as neutron moderators or absorbers. Digital auto-radiography can also be used to detect alpha and beta contaminated waste. These non-destructive assessments can be completed by gas measurements, to quantify the radioactive and radiolysis gas releases, and by destructive examinations such as coring homogeneous waste packages or cutting the heterogeneous ones, in view to perform visual examination and a series of physical, chemical, and radiochemical analyses on samples. These last allow for instance to check the mechanical and containment properties of the package envelop, or of the waste binder, to measure toxic chemicals, to assess the activity of long-lived radionuclides or pure beta emitters, to determine the isotopic composition of nuclear materials, etc. [ABSTRACT FROM AUTHOR]

Based on 238U photofission experiments with the bremsstrahlung photons, the isomeric yield ratios were measured for 95Nb and 133Xe nuclei at the end-point energy Ee = 12.5 MeV, and 135Xe nuclei at Ee = 12.0 MeV. Experimental values of isomeric yield ratios were obtained taking into account the contribution from beta-decay of isobaric nuclei in yields of investigated isotopes. Average angular momenta of investigated fission fragments were determined using the statistical model of decay. [ABSTRACT FROM AUTHOR]

Photofission of Th, U, Np and Pu isotopes are investigated. Modified version of Gorodisskiy approach that is developed to study the neutron-induced fission are employed to simulate fission fragment mass distribution for these isotopes in different energies. The effect of emitted neutron prior to scission point is studied. Peak to valley ratio is also extracted. Obtained results using this approach are compared with original Gorodisskiy model as well as available experimental data. Satisfactory agreement is achieved between theoretical and experimental data especially in medium and low -ray energies than original formalism of Gorodisskiy. [ABSTRACT FROM AUTHOR]

We report on the dedicated implementation of the blocking technique for a LaBr 3 (Ce) detector as well as associated electronics and data acquisition system for border protection applications. The detector is meant to perform delayed γ -ray spectroscopy of fission fragments produced via photofission induced by a high intensity pulsed photon beam. The gating technique avoids saturation of the detection chain during irradiation. The resulting setup allows us to successfully perform delayed γ -ray spectroscopy starting only 30 ns after the gating operation. The measured energy resolution ranges from 5% to 6.5% at 662 keV depending on the γ -ray detection time after the gating operation. [ABSTRACT FROM AUTHOR]

Nuclear materials characterization in 870 L cemented waste drums is challenging due to significant neutron and gamma ray attenuation. Among the existing non-destructive approaches, photon active interrogation is of particular interest. A LINAC accelerates electrons to energies greater than 10 MeV, leading to highly penetrating interrogating bremsstrahlung photons that can reach nuclear materials in the depth of the 870 L waste drum and allow the detection of delayed gamma rays emitted by photofission products. Because the interrogating photon and neutron fluxes are extremely high during irradiation, measurements with high purity germanium detectors are usually performed after a long irradiation time, for example by moving the detector or the waste drum. Therefore, such measurements are only sensitive to delayed gamma rays associated to photofission products having sufficient long half-life of at least tens of seconds, which cuts a valuable signal coming from fission products with a shorter half-life. In this paper, we report gamma rays measurements of very short lived photofission products using a 3 in × 3 in cylindrical LaBr 3 (Ce) fast scintillation detector. A depleted uranium sample was irradiated by a pulsed bremsstrahlung source with maximum energy 17.5 MeV in a macro pulsing mode (1 s irradiation, 2 s cooling). This allowed to acquire a strong delayed gamma signal within a time window of 2 s. A clear signature of nuclear materials was observed even in the depth of a concrete mock-up, with an excellent signal-to-noise ratio beyond 3.5 MeV where the background of nonnuclear activated materials is negligible. These results are compared to a semi analytical model that qualitatively agrees but underestimates the measurements by a constant factor 1.7, probably caused by a wrong beam intensity normalization. Indeed, in this work the LINAC pulse time structure (macro pulsing) was different from its nominal working point used to characterize the beam (usual pulsing mode). Since it allows direct calculations of the detector response to delayed gamma-ray photofission measurements, this model is a fast calculation alternative to time-consuming Monte Carlo simulations, in view of further studies that will follow this feasibility demonstration. [ABSTRACT FROM AUTHOR]

Isomeric yield ratios of 90Rb, 133Te, 134I and 135Xe fission fragments have been measured. Fragments have been created in photofission of 233U nuclei by bremmsstrahlung photons with end-point energy near 10.5 and 17 MeV and of 241Am nuclei irradiated by bremmsstrahlung photons with end-point energy near 9.8 MeV. Experimental values of isomeric yield ratios have been calculated with allowing for the contribution from beta-decay of isobar-ic nuclei in yields of investigated isotopes. Average angular moments of investigated fission fragments have been determined using the statistical model of decay.

The isomeric yield ratios of 133Te, 134I, 135Xe in the photofission of 235U with bremsstrahlung with the end-point energy 17 MeV are measured. The contributions to the isomeric yield ratios produced by β-decay from nuclei of parent isobaric chain are taken into account. The distribution and average value of angular mo-mentum of the primary fission fragments are determined by the statistical model analysis. The codes EMPIRE 3.2 and TALYS 1.4 are applied for calculations of gamma-transitions to the isomeric and ground states. Dependence of determined distributions and average values of angular momentum on shapes of the radiative strength func-tions and the nuclear level densities are studied.

We used crossed laser-molecular beam scattering to study the primary photodissociation channels of chloroacetaldehyde (CH2ClCHO) at 157 nm. In addition to the C-Cl bond fission primary photodissociation channel, the data evidence two other photodissociation channels: HCl photoelimination and C-C bond fission. This is the first direct evidence of the C-C bond fission channel in chloroacetaldehyde, and we found that it significantly competes with the C-Cl bond fission channel. We determined the total primary photodissociation branching fractions for C-Cl fission:HCl elimination:C-C fission to be 0.65:0.07:0.28. The branching between the primary channels suggests the presence of interesting excited state dynamics in chloroacetaldehyde. Some of the vinoxy radicals from C-Cl photofission and most of the ketene cofragments formed in HCl photoelimination have enough internal energy to undergo secondary dissociation. While our previous velocity map imaging study on the photodissociation of chloroacetaldehyde at 157 nm focused on the barrier for the unimolecular dissociation of vinoxy to H + ketene, this work shows that the HCl elimination channel contributed to the high kinetic energy portion of the m/z = 42 signal in that study. [ABSTRACT FROM AUTHOR]

The N = 52 Ga 83 β decay was studied at ALTO. The radioactive 83 Ga beam was produced through the ISOL photofission technique and collected on a movable tape for the measurement of γ -ray emission following β decay. While β -delayed neutron emission has been measured to be 56–85% of the decay path, in this experiment an unexpected high-energy 5–9 MeV γ -ray yield of 16(4)% was observed, coming from states several MeVs above the neutron separation threshold. This result is compared with cutting-edge QRPA calculations, which show that when neutrons deeply bound in the core of the nucleus decay into protons via a Gamow–Teller transition, they give rise to a dipolar oscillation of nuclear matter in the nucleus. This leads to large electromagnetic transition probabilities which can compete with neutron emission, thus affecting the β -decay path. This process is enhanced by an excess of neutrons on the nuclear surface and may thus be a common feature for very neutron-rich isotopes, challenging the present understanding of decay properties of exotic nuclei. [ABSTRACT FROM AUTHOR]

Photofission cross-section ratios of 235 U and 238 U have been measured using monoenergetic photon beams at the HI γ S facility of TUNL. These measurements have been performed in small energy steps between 9.0 and 16.6 MeV using a dual-fission ionization chamber. Measured cross-section ratios are compared with the previous experimental data as well as with the recent evaluated nuclear data library ENDF. [ABSTRACT FROM AUTHOR]

One of the experimental programs that will be carried out at the Extreme Light Infrastructure – Nuclear Physics (ELI-NP) facility is the production of exotic neutron-rich ion beams in an IGISOL facility via photofission in a stack of actinide targets placed at the center of a cell filled with He gas. Simulations with the Geant4 toolkit were done for the optimization of the target configuration that maximizes the rate of released photofission fragments. The cell geometry is established based on the stopping properties of these fragments. Studies, based on simulations with Geant4 and SIMION 8.1, of the space charge effect and its induced electric field in the gas cell are presented. Estimates of the extraction time and efficiency of the photofission fragments are derived. [ABSTRACT FROM AUTHOR]

Simulations providing estimates of the ELI-NP gamma-beam energy bandwidths and beam spot dimensions for the collimated beams, and expected fission rates with the narrow-width beams of bandwidth 0.3% and 0.5% on actinide targets, 238U and 232Th, are reported. These estimates support the feasibility of precise transmission resonance measurements with the narrow-width ELI-NP γ beams. The status of the design and construction of a state-of-the-art experimental array, consisting of four double-sided, Frisch-gridded Bragg ionization chambers, coupled to ΔE-E detectors, is reported. [ABSTRACT FROM AUTHOR]

The Extreme Light Infrastructure – Nuclear Physics (ELI-NP) laboratory in Bucharest, Romania, will host an IGISOL-type facility which will study exotic neutron-rich ion beams produced via photo-fission in actinide targets placed at the center of a cryogenic gas cell. Using a complex radio frequency quadrupole for radioactive ion beam formation, a powerful multiple-reflection time-of-flight mass spectrometer for selecting the refractory isotopes of interest, and several measurement stations, this facility will be an exciting new tool for nuclear scientists. The two mechanisms of ion transport inside the gas cell, namely drift by electric fields and drag by supersonic gas jets, are studied. A program to test the individual components of the stopping cell is discussed. [ABSTRACT FROM AUTHOR]

This paper reviews efforts on using high energy (25-30 MeV) and high power (10-20 kW) electron linacs and lower energy (7 MeV) proton linacs for medical radioisotope production. Using high energy x-rays from the electron linacs, PET (Positron Emission Tomography) radioisotopes are produced through photonuclear reactions such as 19F(γ,n)18F, which also allow production of other PET radionuclides 11C, 13N, and 15O. Other mostly used medical radionuclides 99mTc can also be obtained by using the electron linacs, through photofission or photonuclear reactions. Proton linacs for PET have also been recently developed and the product has been available in the market since 2005. The linacs have been tested for 18F production. As a proton accelerator, the target systems and nuclear reactions are similar to the ones used in PET cyclotrons

In this paper we present the prompt photofission neutron detection performance of undoped CaF2 scintillator using Threshold Activation Detection (TAD). The study is carried out in the frame of C-BORD Horizon 2020 project, during which an efficient toolbox for high volume freight non-intrusive inspection (NII) is under development. Technologies for radiation monitoring are the part of the project. Particularly, detection of various radiological threats on country borders plays an important significant role in Homeland Security applications. Detection of illegal transfer of Special Nuclear Material (SNM) - 235U, 233U and 239Pu - is particular due to the potential use for production of nuclear weapon as well as radiological dispersal device (RDD) V known also as a “dirty bomb”. This technique relies on activation of 19F nuclei in the scintillator medium by fast neutrons and registration of high-energy β particles and γ-rays from the decay of reaction products. The radiation from SNM is detected after irradiation in order to avoid detector blinding. Despite the low 19F(n,α)16N or 19F(n,p)19O reaction cross-section, the method could be a good solution for detection of shielded nuclear material. Results obtained with the CaF2 detector were compared with the previous study done for BaF2 and 3He detector. These experimental results were obtained using 252Cf source and 9 MeV Varian Linatron M9 linear accelerator (LINAC). Finally, performance of the prompt neutron detection system based on CaF2 will be validated at Rotterdam Seaport during field trails in 2018.

Semiconductor gamma-spectroscopy method has been used for measuring the cumulative yields of the 28 fragments of 237Np photofission for 21 mass chains at 12.5 MeV bremsstrahlung maximum energy. Resulted mass distribution of heavy fragments shows the higher yields in the mass region 133 - 134, 138 - 140 and 145 - 146.

The feasibility of fissile mass quantification in large, long-lived medium activity radioactive waste packages using photofission delayed gamma rays has been assessed with MCNPX. The detection limit achievable is lower than the expected uranium mass in these waste packages, but the important sensibility to the waste matrix density and sample localization imposes to get an accurate measurement of these parameters. An isotope discrimination method based on gamma-ray ratios has been evaluated showing that photofission delayed gamma rays can be used to measure the fissile mass as well as the total uranium mass. [ABSTRACT FROM AUTHOR]

There is a great demand to develop non-destructive techniques to identify and quantify Special Nuclear Materials (SNM) in homeland security and nuclear safeguards applications. Passive assay could be extremely challenging in some scenarios. Active interrogation technique based on photofission has been identified as one of the promising approaches. In radiation detection system design based on such technique, it is highly desired to have abilities to accurately and efficiently simulate delayed γ-rays emitted from photofission reactions. In this work, simulation results were compared with measurement outcomes to demonstrate the capabilities and limitations of the code MCNPX 2.7.0 in the simulation of delayed γ-rays from photofission of uranium and plutonium samples. First, high-energy delayed γ-rays (E γ ~2.7–4.5 MeV) from photofission of 238 U were simulated and validated against the energy spectra measured in between linac pulses. Second, low-energy delayed γ-ray spectra (E γ ~0.6–2.7 MeV) measured with a list-mode system after irradiation of 239 Pu were used in the validation. [ABSTRACT FROM AUTHOR]

In homeland security and nuclear safeguards applications, non-destructive techniques to identify and quantify special nuclear materials are in great demand. Although nuclear materials naturally emit characteristic radiation (e.g. neutrons, γ-rays), their intensity and energy are normally low. Furthermore, such radiation could be intentionally shielded with ease or buried in high-level background. Active interrogation techniques based on photofission have been identified as effective assay approaches to address this issue. In designing such assay systems, nuclear data, like photofission product yields, plays a crucial role. Although fission yields for neutron-induced reactions have been well studied and readily available in various nuclear databases, data on photofission product yields is rather scarce. This poses a great challenge to the application of photofission techniques. In this work, short-lived high-energy delayed γ-rays from photofission of 238 U were measured in between linac pulses. In addition, a list-mode system was developed to measure relatively long-lived delayed γ-rays from photofission of 238 U and 239 Pu after the irradiation. Time and energy information of each γ-ray event were simultaneously recorded by this system. Cumulative photofission product yields were then determined using the measured delayed γ-ray spectra. [ABSTRACT FROM AUTHOR]

An extensive experimental program for the study of photofission will take place at the Extreme Light Infrastructure – Nuclear Physics (ELI-NP) facility, where different actinide targets will be exposed to a brilliant gamma beam to produce fission fragments. We report on the implementation within the Geant4 simulation toolkit of the photofission process, of related background processes, and of extended ionic charge parameterization. These developments are used to evaluate the production rates of photofission fragments and their release efficiency from the actinide targets. [ABSTRACT FROM AUTHOR]

Pulsed photonuclear techniques are commonly used in homeland security and nuclear safeguards applications to achieve enhanced detection sensitivity. For example, photoneutrons generated by a pulse-mode linear accelerator (linac) are commonly utilized to produce characteristic capture gamma rays for the detection of nitrogen-rich explosives. Recently, in an effort to develop innovative systems with increased sensitivity to detect diversion and prevent misuse, the authors proposed to assay used nuclear fuel for its plutonium content using a photofission technique, in support of nuclear material management in the U.S. fuel cycle. Passive spectroscopy measurements in the presence of intense background from fission products could be very difficult. Focusing on high-energy delayed gamma rays emitted by short-lived products from photofission presents a much more promising solution. However, as discovered in this study, a commercially available standard high-purity germanium (HPGe) preamplifier can be easily saturated for tens of milliseconds after each linac pulse. This greatly reduces the live time of the system especially when the linac repetition rate is high. On the other hand, although significantly reduced by increasing the lower-level threshold, the input count rate can still easily reach 106 cps (counts per second). Developing a gamma spectroscopy system that can handle such a high count rate has been a major challenge. In this work, a commercial HPGe preamplifier was modified to reduce the saturation time and tail time to improve its high-rate performance in a pulsed photonuclear environment. Results of the modifications were evaluated via both simulations and experiments and proven to be effective without significant degradation of energy resolution. The field-effect transistor (FET) and feedback components were first moved to the warm side to enable the modifications. The saturation time of the preamplifier following a linac pulse was greatly reduced by decreasing the value of the feedback resistor. The effect of reducing the tail time of the output signal was also studied. A traditional trapezoidal shaping approach was then employed to study the impact of the modifications on energy resolution. [ABSTRACT FROM AUTHOR]

Photofission of actinides is studied in the region of nuclear excitation energies that covers the entire giant dipole resonance (GDR) region. The mass distributions of 238U photofission fragments have been explored theoretically for eight different endpoint bremsstrahlung energies from 11.5 to 67.7 MeV, which correspond to average photon energies of 9.09 to 15.90 MeV. Among these energies, 29.1 MeV corresponds to the average photon energy of 13.7 ± 0.3 MeV, which coincides with the GDR peak for 238U photofission. The integrated yield of 238U photofission, as well as charge distribution of photofission products, is calculated and its role in producing nuclei and their neutron-richness is investigated. [ABSTRACT FROM AUTHOR]

In the frame of a long-term research program on the characterization of large radioactive waste packages by photofission, the Nuclear Measurement Laboratory of CEA IRESNE has measured cumulative yields of 239Pu, 235U and 238U photofission products by using a Bremsstrahlung photon beam produced by a 17.5 MeV linear electron accelerator. A characterization of the energy of the Bremsstrahlung photon beam has been carried out by photon activation analysis with different samples of gold, nickel, uranium, zinc and zirconium. The contribution of neutron fission in the different samples has also been estimated by MCNP simulations in order to assess as precisely as possible the photofission yields. Finally, 26 cumulative photofission product yields are reported for 239Pu, 28 for 238U and 26 for 235U, with half-lives ranging from 14 min to more than 3 days, some of them being not recorded so far in the literature. Among these reported photofission product yields, 18 have been measured for all 3 actinides, which can thus be used for their discrimination. A differentiation criterion based on delayed gamma-ray ratios has been established to determine the most efficient photofission product couples to estimate the enrichment of a 235U/238U mixture or the fissile fraction (235U + 239Pu)/actinide mass in a mixture of uranium and plutonium. • Photofission is a promising technique to assay nuclear waste packages. • Photon activation analysis is used to characterize a 17.5 MeV Bremsstrahlung beam. • Photofission products cumulative yields have been determined for 239 Pu, 235 U and 238 U. • The contribution of neutron fission has been estimated with the MCNP code. • Photofission product yield difference makes actinides differentiation practicable. [ABSTRACT FROM AUTHOR]