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The non-destructive characterization of large, concrete radioactive waste packages is a major challenge for criticality safety aspects and long-term management. On the one hand, the fissile mass present inside must be estimated precisely for their transport and storage in the safest possible way. On the other hand, a precise knowledge of their alpha activity makes it possible to send them towards the most appropriate final repository, which potentially implies significant cost savings. The 870 L concrete packages produced by the CEA and stored at the Cadarache center cannot be characterized by gamma spectrometry or passive and active neutron measurements because of insufficient sensitivity and very penalizing uncertainties associated with the high attenuation of neutrons and gamma rays in the waste or in the surrounding concrete layer. Therefore, Active Photon Interrogation (API) is being investigated to locate and quantify fissile material in these bulky concrete waste drums. This technique is based on the photofission phenomenon when actinides are interrogated with photons of energy higher than 6 MeV. Since all actinides have a similar photofission cross section, the method does not allow a direct estimate of the mass of fissile material (e.g. 235U, 239Pu) or the alpha activity (mainly due to plutonium isotopes). It is therefore necessary to use a differentiation method to evaluate the proportions of the actinides of interest. To do this, differences in photofission product yields between actinides are exploited via the measurement of delayed gamma-ray ratios with a high purity germanium detector. New experimental data concerning the production rates of photofission products of 235U, 238U and 239Pu have been measured by irradiating uranium and plutonium samples with a Bremsstrahlung photon beam produced with a linear electron accelerator located in the CINPHONIE irradiation cell of the IRESNE institute, at CEA Cadarache. A preliminary characterization of the Bremsstrahlung photon beam and of the photoneutron flux produced by the accelerator was carried out via photon and neutron activation measurements of reference materials. From the cumulative photofission yields measurements, the most effective photofission product pairs for actinide differentiation were then identified for different mixtures of 235U, 238U and 239Pu isotopes. A method for localizing actinides inside a mock-up of a concrete waste package was also developed and tested, based on both experimental data (differential attenuation of delayed gamma rays emitted by 138Cs) and Monte-Carlo simulations with the MCNP6 code. Localization is an essential step to correct for matrix effects, since the differential attenuation of delayed gamma rays affects actinide discrimination, but also to estimate the uncertainty on the fissile mass evaluated by API. Finally, detection limits ranging from a few grams to tens of grams depending on the waste characteristics and the position of actinides in an 870 L waste package were estimated by simulation with the experimentally validated MCNP model. Ways of improving these detection limits are being studied, in particular by reducing the active background or by measuring high-energy delayed gamma rays with NaI(Tl) scintillation detectors.; La caractérisation non-destructive des colis de déchets radioactifs bétonnés et volumineux est un défi important pour des questions de sûreté-criticité et de gestion sur le long terme. D’une part, la masse fissile présente à l’intérieur doit être estimée précisément pour leur transport et leur entreposage de la façon la plus sûre possible. D’autre part, une connaissance précise de leur activité alpha permet de les envoyer vers le mode de stockage définitif le plus approprié, ce qui implique potentiellement des économies conséquentes. Les colis bétonnés de 870 L produits par le CEA et entreposés au centre de Cadarache ne peuvent être caractérisés par spectrométrie gamma ou mesures neutroniques passives et actives en raison d’une sensibilité insuffisante et d’incertitudes très pénalisantes associées à une forte atténuation des neutrons et des rayonnements gamma dans les déchets ou le béton. Par conséquent, l’Interrogation Photonique Active (IPA) est étudiée pour localiser et quantifier la matière fissile dans ces fûts de déchets bétonnés volumineux. Cette technique est basée sur le phénomène de photofission lorsque des actinides sont interrogés avec des photons d’énergie supérieure à 6 MeV. Puisque tous les actinides ont une section efficace de photofission similaire, la méthode ne permet pas d’estimer directement la masse de matière fissile (par exemple 235U, 239Pu) ou l’activité alpha (principalement due aux isotopes du plutonium). Il faut donc recourir à une méthode de différenciation pour évaluer les proportions des actinides d’intérêt. Pour ce faire, les différences dans les taux de production des produits de photofission selon l’actinide sont exploitées via la mesure de ratios de pics de gamma retardés avec un détecteur germanium haute pureté. De nouvelles données expérimentales concernant les taux de production des produits de photofission de l’235U, l’238U et du 239Pu ont été mesurées en irradiant des échantillons d’uranium et de plutonium avec un faisceau de photons de Bremsstrahlung issu d’un accélérateur linéaire d’électrons situé dans la cellule d’irradiation CINPHONIE de l’institut IRESNE, au CEA de Cadarache. Une caractérisation préalable du faisceau de photons de Bremsstrahlung et du flux de photoneutrons produits par l’accélérateur a été menée via des mesures d’activation photonique et neutronique de matériaux de référence. À partir des mesures des taux de production cumulatifs, les couples de produits de photofission les plus efficaces pour différencier les actinides ont ensuite été identifiés pour différents mélanges d’isotopes 235U, 238U et 239Pu. Une méthode de localisation des actinides dans une maquette de colis de déchets en béton a aussi été développée et testée, basée à la fois sur des données expérimentales (atténuation différentielle des rayonnements gamma retardés émis par le 138Cs) et des simulations Monte-Carlo avec le code MCNP6. La localisation est une étape essentielle tout d’abord pour corriger les effets de matrice, puisque l’atténuation différentielle des rayonnements gamma retardés affecte la discrimination des actinides, mais également pour estimer l’incertitude sur la masse fissile évaluée par IPA. Enfin, des limites de détection de l’ordre de quelques grammes à dizaines de grammes selon les caractéristiques du colis et la position de la matière nucléaire dans le fût de 870 L ont été estimées par simulation avec le modèle MCNP validé expérimentalement. Des pistes d’amélioration de ces limites de détection sont étudiées, notamment via la réduction du bruit de fond actif ou la mesure des rayonnements gamma retardés de haute énergie avec des détecteurs à scintillation de type NaI(Tl).

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.

International audience; Photofission is a promising technique for the radiological characterization of nuclear waste packages containing concrete matrices. The goal of this paper is two-fold. First, we assess the performance of the photofission technique and characterize radiologically a 220-L nuclear waste mock-up drum containing a concrete liner using a 9 MeV linear electron accelerator (linac) and uranium or plutonium samples. Delayed neutrons from photofission are detected in an interpulse mode thanks to 3He counters, which are deployed as reference detectors. The minimal masses obtained with the linac operated at 9 MeV and 100 Hz during 10 min are 0.336 grams for plutonium and 0.539 grams for uranium. Second, we build an MCNP6 numerical model of the measurement setup and validate it against the experimental data. Furthermore, using the validated MCNP6 numerical model at 9 MeV we assess the performance of the photofission technique at 7 MeV and with virtual cases representing different matrix materials. Results of this study indicate that a linac operated at 7 MeV could be sufficient for application with concrete, plastic and metallic waste matrices, which fill the entire volume of a 220-L drum.

The total excitation energy as a function of fragment mass (TXE/A) are calculated for neutron inducedfission of 235U and photo fission of 238U 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.

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.

International audience; Many developments are currently in progress towards new threat detection technologies for security-related applications. In particular, the photofission technique can be applied to detect Special Nuclear Material (SNM) hidden inside cargo containers, which can potentially be used to make nuclear weapons or dirty bombs. This technique relies on detecting prompt and delayed neutrons and gamma rays emitted by the photofission reaction for which the energy threshold is around 6 MeV for plutonium and uranium isotopes. In the frame of the H2020 ENTRANCE project and following the H2020 C-BORD project, CEA List is investigating the performance of the photofission technique using a linear electron accelerator (linac) operated at 7 MeV, i.e. delivering photons with energies not far above the threshold of the reaction of interest. One of the aim is to couple the photofission technique with compact linacs embeded in mobile X-ray scanning systems used by customs for control at borders. Moreover, we investigate the potentialities of triple-discriminating plastic scintillators for the detection of SNM with such systems. These detectors are manufactured by the laboratory and are based on $^6$Li-doped plastic scintillators which allows them to discriminate between gamma-rays, thermal neutrons and fast neutrons by Pulsed Shape Discrimination (PSD). Thus, they could potentially measure simultaneously both the neutron and gamma signatures from SNM and enable to overcome the challenges met when measuring the low signals expected due to the fact that the photofission cross-section values reach a few millibarns between 6 and 7 MeV. First, the PSD capabilities and scintillation yields of these new detectors are characterized in the laboratory environment using a $^{252}$Cf source. Then, we perform measurements at the SAPHIR platform located at CEA Paris-Saclay with the 9 MeV linac available and several SNM samples. Neutron sensitivity as well as detection limits are then compared to reference measurements carried out with a high-density polyethylene detection block housing five 150NH100 $^3$He detectors. In parallel, we implemented an MCNP6 Monte Carlo simulation model of the experimental setup at 9 MeV. Once validated, this model is used to assess the performance of triple-discriminating plastic scintillators for photofission measurements carried out at 7 MeV. In collaboration with Smiths Detection, field tests are planned in 2023 at the seaport of Rijeka, Croatia, with different cargo matrices and a 7 MeV mobile X-ray scanner equipped with triple-discriminating plastic scintillators.

International audience; 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 2 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.

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.

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 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.

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.

Background: Neutron-rich nuclei in the $\mathrm{A}\ensuremath{\approx}140--160$ mass region provide valuable information on nuclear structure such as quadrupole- and octupole-shape coexistence and the evolution of the collectivity. These nuclei have also a nuclear engineering interest because they contribute to the total decay heat after a fission burst. The information concerning $^{145}\mathrm{La}$ is very limited.Purpose: The study of low-spin states in $^{145}\mathrm{La}$ will provide a more detailed level scheme and enable the determination of the half-lives of the excited states.Methods: Low-spin excited states in $^{145}\mathrm{La}$ have been investigated from the $^{145}\mathrm{Ba}\phantom{\rule{4pt}{0ex}}{\ensuremath{\beta}}^{\ensuremath{-}}$ decay. The $^{145}\mathrm{Ba}$ nuclei were directly produced by photofission in the ALTO facility or obtained from the ${\ensuremath{\beta}}^{\ensuremath{-}}$ decay of $^{145}\mathrm{Cs}$ also produced by photofission. Gamma spectroscopy and fast-timing techniques were used.Results: A new level scheme was proposed including 67 excited levels up to about 3 MeV and 164 transitions. Half-lives in the few-nanosecond range were measured for the first excited states. Configurations for levels up to $\ensuremath{\approx}600$ keV were discussed.Conclusions: The available information on the low-spin states of $^{145}\mathrm{La}$ has been modified and considerably extended. The analysis of the properties of the first excited states, such as excitation energies, decay modes, log $\mathit{\text{ft}}$ values, reduced transition probabilities, and Weisskopf hindrance factors, has enabled the identification of the first members of the bands corresponding to the ${g}_{7/2},$ ${d}_{5/2},$ and ${h}_{11/2}$ proton configurations.

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 neutron rich $$^{123}$$ Ag nucleus was populated via induced photofission of UC $$_x$$ at the ALTO ISOL facility. Its $$\beta $$ -decay properties were studied by means of detecting $$\beta $$ -delayed $$\gamma $$ and neutron activities. The measured half-life T $$_{1/2}$$ = 0.350(20) s agreed with the earlier data; the $$\beta $$ -delayed neutron emission branching P $$_{1n}$$ = 1.01(24)% was reestablished using $$\gamma $$ and neutron counting.

International audience; 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, France, has measured cumulative photofission yields of $^{235}$U and $^{238}$U short-lived and long-lived fission products by using a Bremsstrahlung photon beam produced by a 16 MeV electron linear accelerator (LINAC). To this aim, a characterization of the Bremsstrahlung photon beam has been carried out by photon activation analysis with different samples of gold, nickel, uranium and zirconium. The residual neutron flux exiting the LINAC head (lead collimator, borated polyethylene and cadmium shield) has also been characterized by neutron activation analysis with indium samples to estimate the contribution of photoneutron fissions in the uranium samples used to assess the photofission yields. Finally 49 fission product yields are reported for $^{238}$U and 26 for $^{235}$U, with half-lives ranging from 64 s to more than 3 days, some of them not recorded so far in the literature Some photofission products cumulative yields show significant differences between $^{235}$U and $^{238}$U, which confirms the possibility of an isotopic discrimination method based on delayed gamma-ray ratios analysis for radioactive waste characterization

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.

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.

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.

To study the influence of the structural properties of UCx 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 {\mu}A, 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., Comment: 11 pages, 5 figures, 1 table

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.

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.

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.

The results of measuring the photofission reaction yield for actinide nuclei in the near-threshold energy region are presented. At the LUE-8-5 accelerator of the Institute for Nuclear Research of the Russian Academy of Sciences, the energy and time spectra of the delayed gamma quanta were obtained by irradiating fissile isotopes by bremsstrahlung photons. The conclusion is drawn about the possibility of identifying fissile isotopes by the behavior of the photofission cross section in the near-threshold region.

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 existing photofission product yield data sets are often based on nuclear models, and limited experimental measurements of these fission product yields have been performed. These experiments resulted in measured cumulative fission product yields (CFPYs) for 238U and 232 Th for the bremsstrahlung X-ray endpoint energies of 8, 14, and 20 MeV. The half-lives of the reported fission products range from 1.07–40.8 s. This work is motivated by a demonstrated need for novel isotope production methods for nuclear forensics and improved nuclear data to support the nonproliferation community. A high-purity germanium detector and a pneumatic transfer system were employed for measurements of short-lived fission products, collecting data between cycles of accelerator irradiation and measurement.

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.

An approach that allows to adapt the GEF- code to simulate the yields of the products of the photofission of actinide nuclei is proposed. A simulation of the bremsstrahlung spectrum of the microtron M-30 at electron energies of 8,5 ÷ 20 MeV was carried out. The calculations of the excitation energy have been made for the fissioning nuclei 232 Th, 235 U, 238 U, 237 Np, 239 Pu, 240 Pu, 242 Pu, 241 Am. The modeling of photofission product yields of nuclei 232 Th, 235 U, 238 U, 237 Np, 239 Pu, 240 Pu, 242 Pu, 241 Am with the use of GEF-code was carried out.

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.

Photofission cross-section ratios of 235U and 238 U have been measured using monoenergetic photon beams at the HI γS 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.

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.

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.

A x-ray inspection system utilizing a continuous-wave 9 MeV rhodotron x-ray source for scanning cargo containers is presented. This system scans for contraband, anomalies, stowaway passengers, and nuclear threats for trucks and towed cargo containers. A transmission image is generated concurrently with a 3D image of the cargo, the latter presenting material information in the form of atomic number and density. Neutrons from photofission are also detected during each scan. In addition, nuclear resonance fluorescence detectors are capable of identifying specific isotopes. This system has recently been deployed at the Port of Boston.

Special Nuclear Material (SNM) such as uranium and plutonium isotopes could be potentially used to make a nuclear bomb involved in a terrorist attack. To prevent illicit trafficking of SNM in Europe, inspection of cargo containers at borders is required. X-ray scanning is widely used by customs to control suspect containers. However, this technology is not the most adapted to bring to light SNM signature. Therefore, a second line technology based on the photofission reaction could enable to detect SNM in a non-destructive manner. In the frame of the C-BORD European project, several teams specialized in photofission have joined forces to design an active photon interrogation system enabling to detect SNM hidden in cargo containers. The system is based on the use of a linear electron accelerator (linac). First, high-energy photons generated by Bremsstrahlung in the conversion target of the linac irradiate a suspect area of the cargo container. Secondly, prompt and delayed particles emitted following photofission reactions are detected if SNM is present. The largest seaport in Europe (Maasvlakte, in the suburbs of Rotterdam, Netherlands), where 9 MeV linacs are used by Dutch customs for X-ray scanning, has been chosen as test site. In order to deploy a full system taking advantage of all photofission signatures, different subsystems have been developed. In this paper, first, we give an overview of the photofission system. Then, we show how the system has been deployed at the Rotterdam seaport in September and October 2018 and tested on mock-ups of cargo containers. Finally, we show that setting up a photon interrogation system on an industrial facility initially designed for X-ray scanning is a challenge which has been successfully met in the frame of the C-BORD project to deploy the first photofission measurement system on an industrial site in Europe. The outcomes of this first deployment will be exposed and the future challenges to be addressed will be discussed.

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.

With a half-life of 3.85(5) d, the fission product $^{127}\mathrm{Sb}$ is potentially useful for the characterization of fallout debris that would be collected for forensic analysis following a hypothetical nuclear event. The $\ensuremath{\beta}$ decay of $^{127}\mathrm{Sb}$ has many characteristic $\ensuremath{\gamma}$-ray lines, however, the majority of decay strength is concentrated in the three strongest lines near 686, 473, and 784 keV, based on low-precision spectroscopic measurements from decades ago. In a new measurement of $^{127}\mathrm{Sb}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ decay, obtained from radiochemical separations of U photofission products, we show that the energies of the 686 and 784 lines are actually shifted lower by 0.8 and 1.1 keV, respectively. Such large discrepancies in the nuclear data inhibit accurate isotopic analyses and have important implications, for example, on the assessment of fission debris that might be collected in the days following a nuclear event. Therefore, a new evaluation of $^{127}\mathrm{Sb}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ decay and appropriate revisions to the various nuclear databases are warranted. This paper reports a new set of $\ensuremath{\gamma}$-ray energies and relative intensities corresponding to $^{127}\mathrm{Sb}$ decay.

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

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 (Eγ~2.7–4.5 MeV) from photofission of 238U were simulated and validated against the energy spectra measured in between linac pulses. Second, low-energy delayed γ-ray spectra (Eγ~0.6–2.7 MeV) measured with a list-mode system after irradiation of 239Pu were used in the validation.

In this work we present the isomeric ratio of fission product 135Xe in the photo-fission of actinide elements 232Th, 233U and 237Np induced by end-point bremsstrahlung energies of 13.5, 23.5 and 25.0 MeV which were determined by the method of inert gaseous flow. The data were analyzed, discussed and compared with the similar data from literature to examine the role of energy separated in fission process, excitation energy and reaction channels effects.

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 238U 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 238U and 239Pu 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.

We have developed a new nondestructive assay technique based on the photonuclear reaction, which is aimed at measuring the isotopic composition of nuclear fuel materials without relying on their self-generated neutron information. This methodology enables measurement of the number of neutrons produced by the photofission reactions at different specific photon energies, and only information on the relative counts and not the absolute values or energies, which are considerably affected by backgrounds, are required for the enrichment induction according to a mathematical process. The present methodology estimated the 235U enrichment value with 8% accuracy when the cross section uncertainty is 5%.

Yields of Kr and Xe isotopes in photofission of 232Th, 238U, 237Np, 244Pu, 243Am, and 248Cm were tested for isoscaling dependence. Isoscaling for Kr is revealed. For Xe, isoscaling is found to be affected by the STI and STII fission modes governed by the N = 82 and N = 88 neutron shells. The work was performed at the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research (JINR).