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In this work, the rate of radiation damage production in a uranium target of the neutron source at NSC KIPT un-der electrons irradiation with an energy of 100 MeV was estimated. The contribution of elastic and inelastic proc-esses is determined: high-energy electrons and gamma quanta, photo-neutron production, fragments arising from photo-fission, damage from neutrons. It was found that the main input into damage production in a uranium target give fragments of photo-fission.

The total kinetic energy as a function of fragment mass (TKE/A) was calculated for photofission of $${}^{232}$$ Th, $${}^{234}$$ U, $${}^{238}$$ U, $${}^{240}$$ Pu, and $${}^{242}$$ Pu using a systematic statistical scission point model. In the proposed systematic method, all the energies involved in fission need to be expressed as a function of mass and atomic numbers of fission fragments. Therefore, the shell correction energy of fission fragments was approximated in terms of fission fragment mass numbers based on Ruben’s idea. The deformability parameter ( $$\alpha$$ ) of fission fragments was also formulated as a function of mass and atomic numbers of fission fragments by fitting the calculations and experimental data according to Terrell’s formulas. The calculated results are in good agreement with the experimental data, except for fragments with magic numbers, which their shell correction energy and deformability parameters change rapidly.

Experiments on measuring the energy spectra and time distributions of delayed neutrons are described. The neutrons are produced in 238U photofission under the action of bremsstrahlung photons in the LUE-8-5 pulsed electron accelerator at the Russian Academy of Sciences’ Institute for Nuclear Research. The kinetic energy of incident electrons is ≈10 MeV. Measurements are made in the intervals between beam pulses. Fast neutrons are registered by a scintillation spectrometer, with identification of background γ quanta according to the shape of the scintillation pulses.

The technique is described for measuring energy spectra and temporal distributions of delayed neutrons from photofission at the pulsed electron accelerator LUE-8-5 of the Institute for Nuclear Research of the Russian Academy of Sciences (INR RAS). The measurements were performed by the neutron scintillation spectrometer using pulse shape discrimination of background γ-quanta. The results of measurements of 238U-photofission with the kinetic energy of incident electrons ≈10 MeV are given.

Three neutron-induced models have been investigated for photofission phenomena. The fission fragment mass yields were calculated for actinide isotopes by three neutron induced models on a logarithm scale and compared with photofission experimental data. The peak-to-valley ratios for photofission of 232Th, 240Pu and 238U were calculated and compared with the photofission experimental works. There are considerable differences between the calculated results and experimental works for photofission of 232Th. Then, the yields of symmetric and asymmetric fission products for photofission of 232Th were calculated. Comparison of the calculation and experimental results showed that this nucleus should be separated from others in formalism. The average light mass (〈AL〉) and heavy mass (〈AH〉) were calculated for photofission of other Actinide isotopes at different excitation energies. At last, the fission fragment mass yields were studied with changing the number of emitted neutrons for those models with comparing to photofission experimental data. From the carried out calculations, it is to see that all models cannot describe the total set of the experimental data on the photofission of Actinide nuclei. This means that it is necessary to revise the basic theoretical representation about the nature of photofission processes in the dependence on the energy, at least.

Isomeric fission products, such as ${}^{134m}\mathrm{Te}$ (${T}_{1/2}=164$ ns) and ${}^{136m}\mathrm{Xe}$ (${T}_{1/2}=2.95\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{s}$), are proposed as a signature of special nuclear materials in active interrogation applications. To test this conjecture, monoenergetic 9, 11, and 13 MeV photons from the $\mathrm{HI}\ensuremath{\gamma}\mathrm{S}$ facility are used to induce fission of ${}^{235}\mathrm{U}$, ${}^{238}\mathrm{U}$, and ${}^{239}\mathrm{Pu}$ targets. The de-excitation $\ensuremath{\gamma}$ rays of the isomers are measured using time-gated spectroscopy with high-purity germanium detectors. The ${}^{134m}\mathrm{Te}$ and ${}^{136m}\mathrm{Xe}$ isomers are detected and identified by the energy and decay half-life of their characteristic $\ensuremath{\gamma}$-ray transitions. The ratio of yields for these two signature $\ensuremath{\gamma}$ rays, corresponding to ${}^{134m}\mathrm{Te}/{(}^{136m}\mathrm{Xe}{+}^{136}\mathrm{I})$, is found to be strongly correlated with the identity of the fissioning nuclei. These results show that fission-product isomers may be used in active interrogation to detect and identify special nuclear materials, even providing information on the isotopic enrichment. The feasibility of an active interrogation scenario using a bremsstrahlung beam is discussed.

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.

The scission point model has been used to calculate the total kinetic energy (TKE) as a function of fission fragments for photofission reactions. By comparing the calculated TKE values with the available experimental data, the deformation parameters of the fission fragments are obtained in the scission point model. The results show that the deformation parameters of the fission fragments have a large change near the symmetrical region. TKE distribution has been investigated for neutron-induced fission, spontaneous fission, and photofission to predict actinide TKE distribution. This indicates that the TKE distribution depends on the mass and atomic number of compound nuclei and, of course, the excitation energy. The odd-even effect plays an important role in predicting TKE distribution behavior. Also, TKE distribution has a similar trend for all isotopes and the TKE distribution of spontaneous fission is significantly different from the TKE distribution of photofission. At last, the total kinetic energy distributions for the photofission fragments of light actinides are evaluated in the scission point model.

Passive detection of special nuclear material (SNM) is challenging due to its inherently low rate of spontaneous emission of penetrating radiation, the relative ease of shielding, and the fluctuating and frequently overwhelming background. Active interrogation, the use of external radiation to increase the emission rate of characteristic radiation from SNM, has long been considered to be a promising method to overcome those challenges. Current active-interrogation systems that incorporate radiography tend to use bremsstrahlung beams, which can deliver high radiation doses. Low-energy ion-driven nuclear reactions that produce multiple monoenergetic photons may be used as an alternative. The ${}^{12}\mathrm{C}(\mathrm{p},{\mathrm{p}}^{\ensuremath{'}}{)}^{12}\mathrm{C}$ reaction is one such reaction that could produce large yields of highly penetrating 4.4- and 15.1-MeV gamma rays. This reaction does not directly produce neutrons below the approximately 19.7 MeV threshold, and the 15.1-MeV gamma-ray line is well matched to the photofission cross section of ${}^{235}\mathrm{U}$ and ${}^{238}\mathrm{U}$. We report the measurements of thick-target gamma-ray yields at 4.4 and 15.1 MeV from the ${}^{12}\mathrm{C}(\mathrm{p},{\mathrm{p}}^{\ensuremath{'}}{)}^{12}\mathrm{C}$ reaction at proton energies of 19.5, 25, and 30 MeV. Measurements are made with two $3$-in. EJ-309 cylindrical liquid scintillation detectors and thermoluminescent dosimeters placed at ${0}^{\ensuremath{\circ}}$ and ${90}^{\ensuremath{\circ}}$, with an additional $1.5$-in. $\mathrm{Na}\mathrm{I}$($\mathrm{Tl}$) cylindrical scintillation detector at ${0}^{\ensuremath{\circ}}$. We estimate the highest yields of the 4.4- and 15.1-MeV gamma rays of $1.65\ifmmode\times\else\texttimes\fi{}{10}^{10}$ and $4.47\ifmmode\times\else\texttimes\fi{}{10}^{8}\phantom{\rule{0.2em}{0ex}}{\mathrm{sr}}^{\ensuremath{-}1}$ $\ensuremath{\mu}{\mathrm{C}}^{\ensuremath{-}1}$ at a proton energy of 30 MeV, respectively. The yields in all experimental configurations are greater than in a comparable deuteron-driven reaction that produces the same gamma-ray energies---${}^{11}\mathrm{B}(\mathrm{d},\mathrm{n}\ensuremath{\gamma}{)}^{12}\mathrm{C}$. However, a significant increase of the neutron radiation dose accompanies the proton energy increase from 19.5 to 30 MeV.

A position-sensitive twin Frisch-grid ionization chamber has been constructed for future photofission experiments using nearly monochromatic, linearly polarized gamma-ray beams. By exchanging the anode plates in the standard ionization chamber on both sides by an array of grid- and strip-anodes, which are rotated by 90° relative to each other and read out by means of resistive charge division, a position sensitivity is achieved that allows the azimuthal fragment emission angle and hence the fission axis orientation to be determined. The performance of this gaseous detector has been studied using the well-known 252Cf spontaneous fission process. The fission axis orientation could be determined relative to an arbitrary axis in space with a resolution better than 7° FWHM. Measured pre-neutron mass and total kinetic energy distributions are consistent with literature, which ensures that the mass and energy resolution for fission fragments is not affected by the position-sensitive structure.

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.

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.

A method for evaluating the grade of uranium (U) ores through the analysis of β -delayed neutrons following ( γ , fission) is discussed. By measuring β -delayed fission neutrons stimulated from 238 U, the U-Ra disequilibrium problem, which limits the accuracy of traditional grade estimation methods that rely on measuring radioactivity , is addressed. In this research, 6 MV bremsstrahlung photons, delivered by a 6 MeV electron linear accelerator (e-LINAC), were used to irradiate U ore samples, and a moderator surrounded thermal neutron detector was designed to register the emitted neutrons after each X-ray pulse. Although the prompt fission neutrons were discarded due to the interference caused by the photoneutrons emitted from the matrix nuclides in the analytical environment, an excellent linear relationship was observed between the counts of β -delayed neutrons and U ore grades, demonstrating that this method is promising for the in-situ analysis of U ores.

The applicability of the photofission reaction ratio (PFRR) method to identify high-enriched uranium is studied by switching the photon source from the Gaussian spectrum to a bremsstrahlung spectrum. The combination of 6 and 11 MeV Gaussian photon energy was used in previous study for applying PFRR method because the photofission cross sections between 235U and 238U at these two energies differ significantly. A parametric study was conducted with the incident electron energy 7.0 MeV and the result showed that these electrons generate similar photofission reactions as that of 6.0 MeV Gaussian photons, while 13.5 MeV electrons is found to give even better results for PFRR considering the measurement noise contributed by multiple neutron emission, (γ, 2n) reaction. Bremsstrahlung photons were injected into a 1 mm thick uranium metal target with varying uranium enrichment. The PFRR increases linearly with uranium enrichment, and is found to provide a higher sensitivity than the Gaussian photons owing to higher total photofission reactions.

It is shown that the multiplicities and angular and energy distributions of neutrons and photons evaporated from thermalized fragments originating from the spontaneous and low-energy induced fission of nuclei, the relative yields of ground and isomeric states of final fragments, and the features of delayed neutrons emitted upon the beta decay of the above fragments can successfully be described by employing nonequilibrium distributions of spins and relative orbital angular momenta of fission fragments formed in the vicinity of the scission point for the fissile nucleus being studied. It is also shown that these distributions, which are characterized by large mean values of the spins and orbital angular momenta directed orthogonally to the symmetry axis of the fissioning nucleus are successfully constructed upon simultaneously taking into account zero-mode transverse wriggling and bending vibrations of a fissile compound nucleus in the vicinity of its scission point, the wriggling vibrations being dominant. It is confirmed that the zero-mode wriggling vibrations considered immediately above are directly involved in the formation of the angular distributions of fragments originating from the spontaneous and low-energy fission of nuclei. This makes it possible to describe successfully such distributions for photofission fragments.

It is confirmed that one source of the large relative orbital momenta L of fragments in spontaneous and stimulated low-energy nuclear fission is quantum transverse zero-point wriggling vibrations of the fissioning system near its scission point. The angular distributions of fragments of low-energy photofission of actinide nuclei, calculated using the quantum theory of fission, are compared. Vibrations are allowed for by using parameter C w determined by Nix and Swiatecki. Agreement between the experimental and theoretical angular distributions for 234U, 236U, 238U, 238Pu, 240Pu, 242Pu nuclei is observed. The strong sensitivity of the theoretical angular distributions for 238Pu, 240Pu, 242Pu nuclei toward the choice of parameters of transient fissioning states at the external and internal fission barriers is demonstrated.

This paper sets up a formalism that is sufficiently general to describe the effects of photofission, photonuclear, (n, xn), (n, n?x?), and (n, x?) reactions on the neutron-gamma Feynman-alpha variance-to-mean ratios. Such a formalism is obtained using the Chapman-Kolmogorov (master) forward equation for the above-mentioned set of nuclear reactions. Thereafter, the issue of estimating reaction intensities for gammas in the master equation is highlighted by the paper. As an example, a quantitative evaluation of reaction intensities is given for a case when (n, ?), photonuclear, and (n, 2n) reactions are relevant for the system. However, an evaluation of the influence of these types of reactions to the values of the Feynman variance-to-mean ratios is not within the scope of this paper. Overall, the results obtained in this paper are intended to give an extended systematic framework for the study of the neutron- and gamma-based nondestructive assay problems in nuclear reactor applications and materials control.

Active interrogation creates an environment that is particularly challenging from a radiation-detection standpoint: the elevated background levels from the source can mask the desired signatures from the SNM. Neutron based interrogation experiments have shown that nanosecond-level timing is required to discriminate induced-fission neutrons from the scattered source neutrons. Previous experiments using high-energy bremsstrahlung X-rays have demonstrated the ability to induce and detect prompt photofission neutrons from single target materials; however, a real-world application would require spectroscopic capability to discern between photofission neutrons emitted by SNM and neutrons emitted by other reactions in non-SNM. Using digital pulse-shape discrimination, organic liquid scintillators are capable of reliably detecting neutrons in an intense gamma-ray field. Photon misclassification rates as low as 1 in 106 have been achieved, which is approaching the level of gaseous neutron detectors such as 3He without the need for neutron moderation. These scintillators also possess nanosecond-timing resolution, making them candidates for both neutron-and photon-driven active interrogation systems. We have applied an array of liquid and NaI(Tl) scintillators to successfully image 13.7 kg of HEU interrogated by a DT neutron generator; the system was in the direct presence of the accelerator during the experiment.

Monte Carlo is one of the methods to simulate the generation and transport of radiation through matter. The most widely used radiation simulation codes are MCNP and Geant4. The simulation of fission production and transport by MCNP has been thoroughly benchmarked. There is an increasing number of users that prefer using Geant4 due to the flexibility of adding features. However, it has been found that Geant4 does not have the proper fission-production cross sections and does not produce the correct fission products. To achieve accurate results for studies in fissionable material applications, Geant4 was modified to correct these inaccuracies and to add new capabilities. The fission model developed by the Lawrence Livermore National Laboratory was integrated into the neutron-fission modeling package. The photofission simulation capability was enabled using the same neutron-fission library under the assumption that nuclei fission in the same way, independent of the excitation source. The modified fission code provides the correct multiplicity of prompt neutrons and gamma rays, and produces delayed gamma rays and neutrons with time and energy dependencies that are consistent with ENDF/B-VII. The delayed neutrons are now directly produced by a custom package that bypasses the fragment cascade model. The modifications were made for U-235, U-238 and Pu-239 isotopes; however, the new framework allows adding new isotopes easily. The SLAC nuclear data library is used for simulation of isotopes with an atomic number above 92 because it is not available in Geant4. Results of the modified Geant4.10.1 package of neutron-fission and photofission for prompt and delayed radiation are compared with ENDFB-VII and with results produced with the original package.

This paper reports recent developments in high-energy X-ray imaging, photofission, gamma-ray spectroscopy, passive neutron coincidence counting and active neutron interrogation at the Nuclear Measurement Laboratory of CEA DEN, and applications in support to the French nuclear industry.

International audience; Magnetic moment measurements of isomeric states in neutron-rich isotopes populated in photofission using γ-beam facility at the Extreme Light Infrastructure−Nuclear Physics (ELI-NP) laboratory aim at addressing many open questions related to the structure of the nuclei in A around 100 and 130 mass regions. The possibility of such measurements at ELI-NP is briefly discussed here using Geant4 Monte Carlo simulations. The present paper report the estimates of γ-beam profile at the target, fission fragment production rates and the possible target geometry.

The results of the study of gamma-transition description in fast neutron capture and photofission are presented. Recent experimental data were used, namely, the spectrum of prompt gamma-rays in the energy range 2÷18 MeV from 14-MeV neutron capture in natural Ni and isomeric ratios in primary fragments of photofission of the isotopes of U, Np and Pu by bremsstrahlung with end-point energies =10.5, 12 and 18 MeV. The data are compared with the theoretical calculations performed within EMPIRE 3.2 and TALYS 1.6 codes. The mean value of angular momenta and their distributions were determined in the primary fragments 84 Br, 97 Nb, 90 Rb, 131 ,133 Te, 132 Sb, 132,134 I, 135 Xe of photofission. An impact of the characteristics of nuclear excited states on the calculation results is studied using different models for photon strength function and nuclear level density

A recent set-up of a multi-cathode Frisch-grid ionization chamber and experimental results of a proof-of-principle experiment are presented. Preliminary data on the obtained polar angular and mass distributions as well as total kinetic energy of fission fragments produced in neutron-induced fission of 232Th and 238U are discussed that show that the chamber has been operated successfully. For the additional measurement of the azimuthal angular distribution, a design of segmented anodes has been tested. Preliminary data do not allow a suffcient reconstruction of the azimuthal angle.

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~\cite{pnpf} for the purpose of detecting $\sim$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 $Y_{2F}$ or Feynman-$\alpha$. 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~$\mu$A 9~MeV X-ray beam is sufficient to differentiate kilogram quantities of HEU from DU in various shielding scenarios in a standard cargo container., Comment: 16 pages, 12 figures; keywords: fissile material, active interrogation, neutron time-correlation, photofission

This paper explores the benefits of using a mixed photon and neutron radiation source for active detection of special nuclear material. More than fifty irradiations were performed using an 8 MV electron accelerator employing and induction voltage adder (IVA). The experiments used a high atomic number converter to produce a Bremsstrahlung photon spectrum which was then used to create a neutron source via a nuclear interaction with heavy water (deuterium oxide, D2O). This mixed particle source was used to irradiate a depleted uranium (DU) sample, inducing fission in the sample. Several thicknesses of steel shielding were tested in order to compare the performance of the mixed photon and neutron source to a Bremsstrahlung-only source. An array of detectors were fielded to record both photons and neutrons emitted by the fission reactions. A correlation between steel shielding and a detection figure-of-merit can be seen in all cases where the Bremsstrahlung-only source was used. The same relationship for the mixed photon-neutron source is less consistent. The data collected from the fielded detectors is compared to MCNP6 calculations and good agreement is found.

An empirical parametrization for the production cross sections of $^{238}\mathrm{U}$ photofission fragments at low energies (${E}_{\ensuremath{\gamma}}l30$ MeV) is developed. This parametrization, $\mathrm{GIF}^{238}\mathrm{U}$, consists of three parts, namely, the photofission cross section, the mass yield, and the isobaric charge distribution. The photofission cross section is parametrized to reproduce measured data over a wide energy range. The mass yield distribution is described by the energy-dependent multimodal fission model. The isobaric charge distribution is improved, relative to other parametrizations, to describe many experimental data sets over a broad mass range. Comparisons with different measured $^{238}\mathrm{U}$ photofission data sets at various energies reveal that $\mathrm{GIF}^{238}\mathrm{U}$ is in very good agreement with measured elemental and mass yields and can accurately reproduce experimental isotopic yields. Production cross sections (yields) of photofission fragments calculated by this parametrization indicate that many neutron-rich nuclei approaching the $r$-process path can be accessed via photofission of $^{238}\mathrm{U}$ at radioactive-beam facilities.

Author(s): Clarke, SD; Wieger, BM; Enqvist, A; Vogt, R; Randrup, J; Haight, RC; Lee, HY; Perdue, BA; Kwan, E; Wu, CY; Henderson, RA; Pozzi, SA | Abstract: For the first time, the complete neutron multiplicity distribution has been measured from the photofission of U235 induced by high-energy spallation γ rays arriving ahead of the neutron beam at the Los Alamos Neutron Science Center. The resulting average neutron multiplicity 3.80±0.08 (stat.) neutrons per photofission is in general agreement with previous measurements. In addition, unique measurements of the prompt fission energy spectrum of the neutrons from photofission and the angular correlation of two-neutron energies emitted in photofission also were made. The results are compared to calculations with the complete event fission model freya.

Measurements of fission product yields play an important role for the understanding of fundamental aspects of the fission process. Recently, neutron-induced fission product-yield data of $^{239}\mathrm{Pu}$ at energies below 4 MeV revealed an unexpected energy dependence of certain fission fragments. In order to investigate whether this observation is prerogative to neutron-induced fission, a program has been initiated to measure fission product yields in photoinduced fission. Here we report on the first ever photofission product yield measurement with monoenergetic photons produced by Compton back-scattering of FEL photons. The experiment was performed at the High-Intensity Gamma-ray Source at Triangle Universities Nuclear Laboratory on $^{239}\mathrm{Pu}$ at ${E}_{\ensuremath{\gamma}}=11$ MeV. In this exploratory study the yield of eight fission products ranging from $^{91}\mathrm{Sr}$ to $^{143}\mathrm{Ce}$ has been obtained.

International audience; The N=52 Ga83 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.

Although nuclear transmutation methods for fission have existed for decades, the focus has been on neutron-induced reactions. Recent novel concepts have sought to use both neutrons and photons for purposes such as active interrogation of cargo to detect the smuggling of highly enriched uranium, a concept that would require modeling the transmutation caused by both incident particles. As photonuclear transmutation has yet to be modeled alongside neutron-induced transmutation in a production code, new methods need to be developed. The CINDER2008 nuclear transmutation code from Los Alamos National Laboratory is extended from neutron applications to dual neutral particle applications, allowing both neutron- and photon-induced reactions for this modeling with a focus on fission. Following standard reaction modeling, the induced fission reaction is understood as a two-part reaction, with an entrance channel to the excited compound nucleus, and an exit channel from the excited compound nucleus to the fission fragmentation. Because photofission yield data—the exit channel from the compound nucleus—are sparse, neutron fission yield data are used in this work. With a different compound nucleus and excitation, the translation to the excited compound state is modified, as appropriate. A verification and validation of these methods and data has been performed. This has shown that the translation of neutron-induced fission product yield sets, and their use in photonuclear applications, is appropriate, and that the code has been extended correctly.

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.

A novel method of measuring the enrichment of special nuclear material is presented. Recent photofission measurements using a linearly polarized γ-ray beam were performed on samples of 232 Th, 233,235,238 U, 237 Np, and 239,240 Pu. Prompt neutron polarization asymmetries, defined to be the difference in the prompt neutron yields parallel and perpendicular to the plane of beam polarization divided by their sum, were measured. It was discovered that the prompt neutron polarization asymmetries differed significantly depending on the sample. Prompt neutrons from photofission of even–even (non-fissile) targets had significant polarization asymmetries (~0.2 to 0.5), while those from odd-A (generally fissile) targets had polarization asymmetries close to zero. This difference in the polarization asymmetries could be exploited to measure the fissile versus non-fissile content of special nuclear materials, and potentially to detect the presence of fissile material during active interrogation. The proposed technique, its expected performance, and its potential applicability are discussed.

238U 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 238U photofission is calculated, and the evaluated cross sections for photofission and for photoneutron reactions are discussed.

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.

The bremstrahlung induced fission of 234,238U and 232Th has been studied at the superconducting Darmstadt linear accelerator (S- DALINAC) in the excitation energy region close to the fission barrier. In this contribution results on the fission fragment mass, total kinetic energy (TKE) and angular distributions will be presented. Fission fragment mass and TKE distributions from 234U were studied for the first time in this energy region. The results have been analyzed in terms of fission modes and a dominant yield of the mass asymmetric standard-2 mode was found in all studied nuclei. No strong dependence of the fission mode weights on the excitation energy of the compound nucleus were found. Correlations between mass, TKE and angular distributions have been investigated in 234U and 232Th. A correlation in form of an increased anisotropy for far-asymmetric masses and low TKE were found in both fissioning systems. A possible interpretation of this correlation in terms of fission modes is discussed.

Photon-induced fission has been investigated as a method to detect and identify nuclear materials. Although high-energy delayed-fission γ-rays have been considered as a reliable signature for detection of fissionable materials, interference from γ-rays produced as secondary effects from other photonuclear reactions is inevitable. This effect has been studied in distinguishing fissionable materials from non-fissionable materials based on differential delayed γ-ray yields via both simulation and measurements. The energy spectra of delayed-photofission γ-rays carry isotopic information of the target materials. The feasibility of accurate spectrometry measurements in between intense linear accelerator pulses has been demonstrated using three independent spectroscopy systems. The measured delayed γ-rays spectra were then compared with MCNPX simulation results. Through the comparison, this article intends to show the capabilities of the current version of MCNPX in applications of simulating the photofission process.

The mass distribution of the 238U photofission fragments formed under γ-quanta action is analyzed in the range of excitation energies of a fissile nucleus from 5 to 20 MeV. The influence on the ratio of asymmetric and symmetric photofission of the 238U structure and excitation energy of a fissile nucleus is discussed. A combined analysis and the comparison of the behavior of the asymmetric and symmetric modes of photofission were conducted for the first time. The results we obtained are compared with the prediction of the multimode model of the dependence of fission modes on the nuclear excitation energy.

Photofission of 238U by bremsstrahlung photons is studied at four energies of an electron accelerator: 19.5, 29.1, 48.3, and 67.7 MeV. The yields of the fission fragments after the emission of prompt neutrons are obtained using the gamma-ray spectroscopic technique. The mass distributions of photofission are obtained at different upper energies of the bremsstrahlung spectrum. The ratio the symmetric-fission mode to the asymmetric mode is obtained from the mass distribution. The symmetric mode becomes 3–4 times greater than the asymmetric as the excitation energy of the 238U nucleus increases from 12 to 16 MeV.

A comparative study of fission of actinides, especially $^{238}\mathrm{U}$, by proton and bremsstrahlung photon is performed. The relative mass distribution of $^{238}\mathrm{U}$ fission fragments has been explored theoretically for both proton- and photon-induced fission. The integrated yield along with charge distribution of the products are calculated to find the neutron richness in comparison with the nuclei produced by the $r$ process in nucleosynthesis. Some $r$-process nuclei in the intermediate-mass range for symmetric fission mode are found to be produced almost two orders of magnitude more for proton-induced fission than for photofission, although the rest of the neutron-rich nuclei in the asymmetric mode are produced in comparable proportion for both processes.

Average angular momenta of the fission fragments 90Rb, 130Sb, 133Te, 134I, and 135Xe resulting from 237Np photofission induced by Bremsstrahlung γ-ray quanta with endpoint energies of 9.8 and 17 MeV, and from 241Am photofission induced by γ-ray quanta with an endpoint energy of 17 MeV, are found by measuring the isomeric yield ratios. The experimental values of the isomeric ratios are calculated with allowance for the contribution from the beta decay of isobaric nuclei to the yields of the investigated isotopes.

Successful detection of fissionable material contained in a variety of matrices was demonstrated by photon active interrogation of fissionable and inert target materials. Samples were irradiated with pulsed 15 MeV photons generated by a LINAC and tungsten electron/photon converter, operating at 15 Hz. Matrix materials included air (no matrix), wood, water, and lead. A unique dual mode gamma/neutron detector was used to acquire data from both fission product gamma and fission product neutron emission. Neutron emission was recorded by detecting the 478 keV capture gamma from the 10 B (n,α) 7 Li reaction, generating a photopeak in the recorded gamma spectrum. Two signatures were found to correctly differentiate between the fissionable target ( 238 U) and inert targets (lead, steel, air, and beryllium), with substantial differences in delayed gamma and neutron signatures for fissionable and inert materials in all cases. The signatures are simple to compute and are not significantly affected by system variations or interferences expected during cargo scanning.

The angular distribution of 237Np fission fragments was measured over the energy range 7.4 MeV–9.2 MeV in steps of 0.4 MeV of bremsstrahlung radiation. Lexan polycarbonate films (SSNTD) were used to detect the fission fragments. The photon intensity from the Microtron accelerator at a distance of 15 cm from the bremsstrahlung converter (tantalum target) facility was estimated to be 1010 photons s−1 using the code EGS-4. The photofission cross-section was calculated using the angular distribution of fission fragments and the results were compared with those obtained using the code EMPIRE-II and various barrier parameters of the RIPL-1, RIPL-2 libraries and with the new analytical fission barrier formula based on the Hugenholtz–Van Hove theorem. The results are in good agreement with the prediction of the RIPL-1 barrier parameter and analytical fission barrier formula.

For the on-line monitoring of high fast neutron fluxes in the presence of a strong thermal neutron component, SCK·CEN and CEA are jointly developing a Fast Neutron Detector System, based on 242Pu fission chambers as sensors and including dedicated electronics and data processing systems. Irradiation tests in the BR2 reactor of 242Pu fission chambers operating in current mode showed that in typical MTR conditions the fission chamber currents are dominated by the gamma contribution. In order to reduce the gamma contribution to the signal, it was proposed to use the fission chambers in Campbelling mode. An irradiation experiment in the BR2 reactor with a 242Pu and a 235U fission chamber, both equipped with a suitable cable for measurements in Campbelling mode, proved the effectiveness of the suppression of the gamma-induced signal component by the Campbelling mode: gamma contribution reduction factors of 26 for the 235U fission chamber and more than 80 for the 242Pu fission chamber were obtained. The experimental data also prove that photofission contributions are negligibly small. Consequently, in typical MTR conditions the gamma contribution to the fission chamber Campbelling signal can be neglected.

Fission of U-238 and U-234 induced by bremsstrahlung of 6.5-9.0 MeV endpoint energy has been investigated at the superconducting Darmstadt electron linear accelerator S-DALINAC. Using a twin Frisch grid ionization chamber, fission-fragment energy and mass distributions have been determined by means of the double kinetic-energy technique. Results on the fission-fragment characteristics from U-238(gamma, f) are in agreement with results from the literature. In addition fission-fragment mass and energy distributions from U-234(gamma, f) are presented for the first time in this energy region. An analysis of fission modes within the Brosa model has been performed. The relative yield of the S 1 mode was found to be (13 +/- 3)% in U-234 and (35 +/- 2)% in U-238.

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 135Xe from photofission of 233U 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.

Photofission of [Formula: see text]Th, [Formula: see text]U, [Formula: see text]Np and [Formula: see text]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 [Formula: see text]-ray energies than original formalism of Gorodisskiy.

The investigation of neutron-rich exotic nuclei is crucial not only for nuclear physics but also for nuclear astrophysics. Experimentally, only few neutron-rich nuclei near the stability have been studied, however, most neutron-rich nuclei have not been measured due to their small production cross sections as well as short half-lives. At ELI-NP, gamma beams with high intensities will open new opportunities to investigate very neutron-rich fragments produced by photofission of 238U targets in a gas cell. Based on some simulations, a novel gas cell has been designed to produce, stop and extract 238U photofission fragments. The extraction time and efficiency of photofission fragments have been optimized by using SIMION simulations. According to these simulations, a high extraction efficiency and a short extraction time can be achieved for 238U photofission fragments in the gas cell, which will allow one to measure very neutron-rich fragments with short half-lives by using the IGISOL facility proposed at ELI-NP.

Yields of fragments originating from 232Th photofission were measured at bremsstrahlungphoton endpoint energies of 50 and 3500 MeV. Charge and mass distributions of fission fragments were analyzed. On the basis of the model of multimode fission, symmetric and asymmetric channels are singled out in 232Th photofission at intermediate energies. This decomposition made it possible to estimate the contributions of various fission components and the fissility of 232Th.

Fission fragment angular distribution measurements of even-even (232Th) and odd mass (237Np) nuclei were carried out using bremsstrahlung radiation in the energy range 7.4 to 9.2 MeV from variable energy microtron at Mangalore University. In the present work SSNTD lexan polycarbonate films were used to detect the fission tracks from both even-even (232Th) and odd mass (237Np) nuclei. From the knowledge of track density formed from photofission reaction of 232Th and 237Np, angular distribution of fission fragments was measured. The results obtained are discussed on the basis of Age Bohr theory in this paper.

The detection and identification of delayed fission γ -rays emitted between intense bremsstrahlung pulses was investigated as a method to discriminate between fissionable and non-fissionable materials. Photofission and other photonuclear reactions were induced in U 238 , Pb, Fe and Be targets using a pulsed bremsstrahlung beam for several endpoint energies between 8 and 20 MeV . In the 33 ms between irradiating pulses, correlated photon energy and time spectra were collected using a high-purity germanium detector. When non-fissionable materials were irradiated, the high-energy γ -ray yield decreased a couple orders of magnitude within the first ∼ 12 ms after the pulse. In contrast, the high-energy γ -rays emitted from the decay of fission fragments were nearly constant throughout the period between bremsstrahlung pulses. Hence, a basic energy and time discrimination provides a unique high-energy delayed γ -ray signature for fissionable material detection.