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

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

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

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

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

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

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

The photonuclear reactions of actinides induced by bremsstrahlung γs are studied in the region of nuclear excitation energies that covers the entire giant dipole resonance region. A comparative analysis of the behavior of the symmetric and asymmetric modes of photon-induced fission as a function of the average excitation energy of the fissioning nucleus is performed. The mass distributions for photofission fragments are obtained. The integrated yield as well as charge distribution of photofission products is determined. Thus, the production cross sections of neutron-rich nuclei are calculated and their exoticity (neutron richness) is explored in comparison to the nuclei produced by rapid neutron capture process in nucleosynthesis. [ABSTRACT FROM AUTHOR]

Photon activation technique is used to obtain photofission mass distributions of 238U after irradiation in a bremsstrahlung beam with end-point energies 19.5, 29.1, 48.3, and 67.7 MeV. The weights of the symmetric (SL) and asymmetric (STI and STII) modes in the mass distributions of photofission of 238U and neutron-induced fission of 235,238U at the average excitation energies of up to 20 MeV are calculated on the basis of the obtained results and other available data. The weight of the SL mode increases by about an order of magnitude as the average excitation energy increases from 8.5 to 17 MeV, while the weight of the STI mode decreases by two times, and the weight of the STII mode slightly increases. It is shown that the peak-to-valley ratios in photofission of 238U and neutron-induced fission of 235U are very close across almost the whole studied energy range. A similar conclusion is made concerning the behavior of asymmetric and symmetric fission modes in photofission of 238U and neutron-induced fission of 235U and238U. [ABSTRACT FROM AUTHOR]

We determine the isomeric ratios of Xe in the photo-fission of Th and U induced by end-point bremsstrahlung energy of 13.5 MeV. The isomeric ratios were determined from the characteristic gamma ray activities of the isomeric and ground states of Xe. The induced gamma ray spectra of Xe were measured by a spectroscopic system, which consists of a HPGe detector connected to a PC based multichannel analyzer. In order to improve the accuracy of the experimental results, the method of inert gaseous flow was used to carry the fission product from the reaction camera to the collector. The isomeric ratios of Xe from the present work are the first time measurements. The present results were analyzed, discussed and compared with the similar data from literature to examine the role of excitation energy, energy separated in fission process and effect of reaction channels. [ABSTRACT FROM AUTHOR]

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

The isomeric yield ratios of 133Te, 134I, 135Xe in the photofission of 235U with bremsstrahlung with the end-point energy Ee= 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 momentum 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 functions and the nuclear level densities are studied. [ABSTRACT FROM AUTHOR]

In this work we present an analysis of the yields of fission fragments induced by bremsstrahlung photons with endpoint energies of 50 and 3500 MeV on 232Th and 238U targets using the simulation code CRISP. A multimodal fission option was added to this code and an extension of the calculation to the properties of the fission products is presented. By dividing the fissioning nuclei according to their fissionability, an approach is introduced which accounts for the contribution of symmetric and asymmetric fission. By adopting this procedure, it was possible to calculate the main parameters for the fission-fragment charge distribution such as the most-probable charge for a given fission product mass chain and its corresponding width parameter. Also, it was possible to reproduce features of fragment mass distribution and evaluate the fissility of fissioning nuclei for photon-induced fission of 232Th and 238U. [ABSTRACT FROM AUTHOR]

The mass-yield distributions of various fission products in the 45- and 80-MeV bremsstrahlung-induced fission of 232Th have been determined by using a recoil catcher and an offline γ-ray spectrometric technique in the electron linac at the Pohang Accelerator Laboratory, Korea. The mass-yield distributions were obtained from the fission-product yield data using charge-distribution corrections. The peak-to-valley (P/V) ratio, the average value of light mass ((AL)) and heavy mass ((AH)), and the average number of neutrons ((v)) in the bremsstrahlung-induced fission of 232Th at different excitation energies were obtained from the mass-yield data. From the present measurements and the existing data from the 232Th(γ,f) reaction and those from the 232Th(n,f) reaction at various energies, the following observations were obtained: (i) The mass-yield distributions in the 232Th(γ,f) reaction at various energies are triple humped, similar to those of the 232Th(n,f) reaction, (ii) The yields of fission products for A = 133-134, A = 138-139, and A = 143-144 and their complementary products in the 232Th(γ,f) reaction are higher than those of other fission products due to the nuclear structure effect, (iii) The yields of symmetric fission products for A = 133-134 and their complementary products in the 232Th(γ,f) reaction are lower than those in the 232Th(n,f) reaction, whereas those for A = 143-144 and their complementary products are reversed, (iv) The result of increasing of the symmetric product yield causes the decreasing of the peak-to-valley ratio with increasing the excitation energy. However, it is surprising to see that the increasing trends for the symmetric products yields and the decreasing trends for the P/V ratio in the 232Th(γ,f) and 232Th(n,f) reactions are not similar but those in the 238U(γ,f) and 238U(n,f) reactions are similar to each other, (v) The average values of (AL), (AH), and (v) at different excitation energies in the 232Th(γ,f) and 232Th(n,f) reactions are similar but those in the 238U(γ,f) and 238U(n,f) reactions are different. [ABSTRACT FROM AUTHOR]