Photofission Fragments Research Articles

A general framework for describing photofission observables of actinides at an average excitation energy below 30 MeV * *Supported by the National Natural Science Foundation of China (11675075), Independent research project of key laboratory of plasma physics, CAEP (JCKYS2021212009) and Hengyang Municipal Science and Technology Project (202150054076)

A reasonable prediction of photofission observables plays a paramount role in understanding the photofission process and guiding various photofission-induced applications, such as short-lived isotope production, nuclear waste disposal, and nuclear safeguards. However, the available experimental data for photofission observables are limited, and the existing models and programs have mainly been developed for neutron-induced fission processes. In this study, a general framework is proposed for characterizing the photofission observables of actinides, including the mass yield distributions (MYD) and isobaric charge distributions (ICD) of fission fragments and the multiplicity and energy distributions of prompt neutrons (np ) and prompt γ rays (γ p ). The framework encompasses various systematic neutron models and empirical models considering the Bohr hypothesis and does not rely on the experimental data as input. These models are then validated individually against experimental data at an average excitation energy below 30 MeV, which shows the reliability and robustness of the general framework. Finally, we employ this framework to predict the characteristics of photofission fragments and the emissions of prompt particles for typical actinides including 232Th, 235, 238U and 240Pu. It is found that the 238U(γ, f) reaction is more suitable for producing neutron-rich nuclei compared to the 232Th(γ, f) reaction. In addition, the average multiplicity number of both np and γ p increases with the average excitation energy.

Deformation Parameters and Collective Temperature Changes in Photofission Mass Yields of Actinides Within the Systematic Statistical Scission Point Model

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.

Kinetic energy distribution for photofission of light actinides

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.

The Perusal of Photofission Fragments Mass Yields for Actinide Isotopes by Systematics Neutron Models

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.

Simulation for Optimizing the Design of Cryogenic Stopping Cell for the IGISOL Facility at ELI-NP

The production of the exotic neutron-rich ion beams from photofission of the actinide targets in an IGISOL facility will be studied via an experimental program that will take place at the Extreme Light Infrastructure - Nuclear Physics (ELI-NP) facility. Geant4 simulation toolkit was used for optimizing the target configuration in order to maximize the rate of released photofission fragments from targets placed in a cell filled with He gas.

Product yields for the photofission of 232Th, 234,238U, 237Np, and 239,240,242Pu actinides at various incident photon energies

Photofission fragments mass yield for $$^{232}\hbox {Th}$$ , $$^{234,238}\hbox {U}$$ , $$^{237}\hbox {Np}$$ , and $$^{239,240,242}\hbox {Pu}$$ isotopes are investigated. The calculations are done using a developed approach based on Gorodisskiy’s phenomenological formalism. The Gorodisskiy’s method is developed to be applied for the neutron-induced fission. Here we revised it for application to photofission. The effect of emitted neutron prior to fission on the fission fragment mass yields has also been studied. The peak-to-valley ratio is extracted for the $$^{240}\hbox {Pu}$$ isotope as a function of energy. Obtained results of the present formalism are compared with the available experimental data. Satisfactory agreement is achieved between the results of present approach and the experimental data.

Study of photofission fragment mass distribution of 232Th, 238U, 237Np and 240Pu isotopes in various γ-ray energies

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.

Production of neutron-rich nuclei approaching r-process by gamma-induced fission of 238U at ELI-NP

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.

Empirical parametrization for production cross sections of neutron-rich nuclei by photofission of U238 at low energies

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.

Decisive role of wriggling vibrations in the formation of angular and spin distributions of products originating from binary and ternary fission of oriented nuclei

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.

Design of the gas cell for the IGISOL facility at ELI-NP

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.

Simulation of photofission experiments at the ELI-NP facility

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

Energy dependence of exotic nuclei production cross sections by photofission reaction in GDR range

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.

Yields of neutron-rich nuclei by actinide photofission in the giant dipole resonance region

Photofission of actinides is studied in the region of nuclear excitation energies that covers the entire giant dipole resonance (GDR) 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 of $^{238}$U photofission fragments are obtained at the endpoint bremsstrahlung energy of 29.1 MeV which corresponds to mean photon energy of 13.7$\pm$0.3 MeV that coincides with GDR peak for $^{238}$U photofission. The integrated yield of $^{238}$U photofission as well as charge distribution of photofission products are calculated and its role in the production of neutron-rich nuclei and their exoticity is explored.

Изомерные отношения и средние угловые моменты фрагментов фотоделения 235U, 237Np и 239Pu

Изомерные отношения и средние угловые моменты фрагментов фотоделения 235U, 237Np и 239Pu

238U photofission in the energy region of the giant dipole resonance

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.

Prompt neutron polarization asymmetries in photofission ofTh232,U233,235,238,Np237, andPu239,240

Photofission experiments were performed on targets of $^{232}\mathrm{Th}$, $^{233,235,238}\mathrm{U},$ $^{237}\mathrm{Np}$, and $^{239,240}\mathrm{Pu}$ using nearly 100% linearly polarized, high intensity ($\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{6}\ensuremath{\gamma}/$s), and nearly-monoenergetic $\ensuremath{\gamma}$-ray beams having energies between 5.3 and 7.6 MeV at the High Intensity $\ensuremath{\gamma}$-ray Source (HI$\ensuremath{\gamma}$S) located at Duke University and Triangle Universities Nuclear Laboratory. An array of 12--18 liquid scintillator detectors was used to measure prompt fission neutron yields parallel and perpendicular to the plane of beam polarization. Polarization asymmetries, the differences between the in-plane and out-of-plane yields divided by their sums, were measured. Asymmetries close to zero were found for ${}^{233,235}$U, $^{237}\mathrm{Np}$, and ${}^{239}$Pu while significant asymmetries ($\ensuremath{\sim}$0.2--0.5) were found for $^{232}\mathrm{Th}$, ${}^{238}$U, and ${}^{240}$Pu. Predictions based on previously measured photofission fragment angular distributions combined with a model of prompt neutron emission agree well with the experimental results.

Photofission Fragment Characteristics of 234,238U and 232Th in the Barrier Region

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.

The mass distribution of 238U photofission fragments

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.

Iodine- and xenon-isotope yield ratio in photofission fragments of heavy nuclei

The ratios of cumulative yields of the 135I isotope and the independent yield of 135Xe at photofission of 232Th, 238U, 237Np, 243Am, and 238Cm by bremsstrahlung radiation with a boundary energy of 25 MeV are measured. The selective gas transportation of fission fragments and a measurement of the spectra of their γ radiation were used. A large difference in the ratios of the yields for these targets was discovered: from 34 for 238U to 1.3 for 243Am. This difference is discussed based on the known ideas of the formation mechanism of fission fragments.