Total Excitation Energy Research Articles

Temperature ratio RT = TL/TH of fully accelerated complementary fragments (used for TXE partition) obtained independently of prompt emission model calculations

Several refined prompt emission model codes, nowadays employed, use the partition of total excitation energy (TXE) according to the temperature ratio RT = TL/TH of fully accelerated fragments. In such codes RT is given as input, either as an unique value for all fragmentations or as a function of AH; This temperature ratio RT being obtained by fitting the experimental ν(A) data with the respective prompt emission model code. This paper proposes a method for obtaining “experimental RT(AH)” (also based on experimental ν(A)), but without resorting to prompt emission model calculations for the fit of ν(A) data. So that this is an independent method providing RT(AH) which can be employed by any prompt emission model code in which the TXE partition is done at the full acceleration according to RT given as input. A procedure which facilitates the parameterization of RT(AH) is proposed, too. A prediction of the RT(AH) shape at very high excitation energies of the fissioning nucleus is also reported. The comparison of RT(AH) ratios obtained from any TXE partition based on modeling at scission with those provided by the present method can constitute a supplementary validation of the respective modeling at scission.

Total excitation energy distribution for neutron-induced fission and photo-fission of uranium isotopes

The fission fragment total excitation energy, TXE(A), is investigated for neutron-induced fission of uranium isotopes using three different methods. Different methods for calculations of the TXE(A) produced by the decay of low excited systems are analyzed and their results are compared with the available TXE values. The calculated TXE values have been compared with the results of other studies for the neutron induced fission of 233U, 235U as well as the photo-fission of 238U. The scission point method for evaluating TXE has been modified by comparing the available and calculated results. TXE(A) is evaluated for other uranium isotopes using three methods. It is found that TXE values in the neutron induced fission of 233U, 235U and the photo-fission of 238U are greater than the calculated TXE values of other uranium isotopes. The TXE values of light uranium isotopes decrease with increasing their mass numbers, while the TXE values of heavy uranium isotopes increase as their mass numbers increase. Also, the calculated TXE values for all isotopes increase sharply near the symmetric region, which is due to the formation of unstable fission fragment. On the other hand, a slight increase in neutron multiplicity and TXE values can be observed in heavy fission fragments (Ah>155), which could be due to the neutron excess. However, the sharp increase in neutron multiplicity and TXE values in some studies for the heavy fission fragments (Ah>155) is due to a missing correction in the data analysis.

Calculations of Positron Scattering from Boron, BH, BF, BF2, and BF3

The single-center convergent close-coupling (CCC) method is applied to calculate positron scattering from boron. A model potential approach is utilized to extract the positronium formation, direct ionization, and values between the positronium formation and ionization thresholds. We present results for total, electron loss, elastic, momentum transfer, total bound state excitation, positronium formation, direct ionization, stopping power, and mean excitation energy from 10−5 eV to 5000 eV. For boron, there is only one other set of theoretical positron calculations for elastic and momentum transfer above 500 eV, which is in excellent agreement with the current CCC results. Using the current results for boron atoms and previous CCC calculations for hydrogen and fluorine atoms, positron scattering from BF, BF2, BF3, and BH molecules is calculated for energies between 0.1 eV and 5000 eV with a modified independent atom approach.

Total kinetic, excitation energy and neutron multiplicity distribution of californium isotopes

The mean total kinetic energy as a function of fission fragments, <TKE>(A), is calculated for neutron-induced fission of 247,249,251,253,255Cf, as well as for spontaneous fission of 248,250,252,254Cf using the scission point model. A comparative analysis is performed between the calculated results and existing experimental data. Investigation into the behavior of <TKE>(A) is conducted, particularly focusing on intermediate and heavy actinides. While nuclear and Coulomb energy evaluations suffice for estimating <TKE> values in light actinides, this approximation proves inadequate for evaluating TKE values for intermediate and heavy actinides. Additionally, an evaluation is made of the total excitation energy as a function of fission fragments, <TXE>(A), concerning spontaneous fission in 247−254Cf. It is found that TXE values sharply increase near the symmetric region across all isotopes and exhibit a gradual increase in heavy fission fragments. Furthermore, an exploration of the total neutron multiplicity as a function of fission fragments, νT(A), is conducted for 252Cf fission using two methods. The νT(A) results obtained through these methods are compared with experimental data, and νT(A) is estimated for unmeasured californium isotopes. The average neutron emission per fission event, ν¯(Ai), is then calculated for spontaneous fission of 251Cf and 253Cf, yielding values of 3.46 and 3.59, respectively.

Two-dimensional total absorption spectroscopy with conditional generative adversarial networks

We explore the use of machine learning techniques to remove the response of large volume γ-ray detectors from experimental spectra. Segmented γ-ray total absorption spectrometers (TAS) allow for the simultaneous measurement of individual γ-ray energy (Eγ) and total excitation energy (Ex). Analysis of TAS detector data is complicated by the fact that the Ex and Eγ quantities are correlated, and therefore, techniques that simply unfold using Ex and Eγ response functions independently are not as accurate. In this work, we investigate the use of conditional generative adversarial networks (cGANs) to simultaneously unfold Ex and Eγ data in TAS detectors. Specifically, we employ a Pix2Pix cGAN, a generative modeling technique based on recent advances in deep learning, to treat (Ex,Eγ) matrix unfolding as an image-to-image translation problem. We present results for simulated and experimental matrices of single-γ and double-γ decay cascades. Our model demonstrates characterization capabilities within detector resolution limits for upwards of 93% of simulated test cases.

Momentum Distribution Studies of Projectile Fragments from Peripheral Collisions Below the Fermi Energy

This paper presents our recent studies of multinucleon transfer in peripheral collisions in reactions below the Fermi regime. Our current focus is the study of the mass, angular and momentum distributions of the projectile-like fragments from the reaction of an 86Kr beam at 15 MeV/nucleon with a target of 64Ni. Experimental data from our previous work with the MARS spectrometer at the Cyclotron Institute of Texas A&amp;M University were compared with model calculations. The dynamical stage of the reaction is described with either the Deep-Inelastic Transfer Model (DIT) or with the microscopic Constrained Molecular Dynamics model (CoMD). The de-excitation of the hot projectile-like fragments is performed with the GEMINI model. The momentum distributions are characterized by a quasi-elastic peak and a deep-inelastic peak. Two-body kinematics was employed to extract the total excitation energies of these regions. Through the thorough study of peripheral reactions in the Fermi energy regime we expect to gain valuable information that could lead to the understanding of how the rare isotopes in regions such as the r-process path and the neutron drip line are formed and the reaction mechanism(s) that take place.

Cascade Infrared Thermal Photon Emission

The later stages of cooling of molecules and clusters in the interstellar medium are dominated by emission of vibrational infrared radiation. With the development of cryogenic storage it has become possible to experimentally study these processes. Recent storage ring results demonstrate that intramolecular vibrational redistribution takes place within the cooling process, and an harmonic cascade model has been used to interpret the data. Here we analyze this model and show that the energy distributions and the photon emission rates develop into near-universal functions that can be characterized with only a few parameters, irrespective of the precise vibrational spectra and oscillator strengths of the systems. We show that the photon emission rate and emitted power vary linearly with total excitation energy with a small offset. The time developments of ensemble internal energy distributions are calculated with respect to their first two moments. The excitation energy decreases exponentially with a rate constant which is the average of all k1→0 Einstein coefficients, and the time development of the variance is also calculated.

Influence of energy partition in fission and pre-neutron fragment distributions on post-neutron fragment yields, application for 235U(nth,f)

The key role in the calculation of post-neutron fragment yields (independent FPY) is played by the pre-neutron fragment distributions Y(A,TKE) and the prompt neutron multiplicity (calculated in the frame of a prompt emission model). The knowledge of excitation energies of fully accelerated fragments is crucial in any prompt emission model. So that the influence on independent FPY of two methods of total excitation energy (TXE) partition, one based on modeling at scission and another on a parameterization at the full acceleration of fragments, and of two reliable Y(A,TKE) data of 235U(nth,f) is investigated by using the deterministic prompt emission model with a detailed treatment of sequential emission DSE. The independent FPY results Y(Ap) and Y(Z,Ap) obtained with both methods of TXE partition and both Y(A,TKE) distributions describe reasonably well the experimental data. The use of different TXE partitions and Y(A,TKE) data does not change the positions of the most pronounced peaks and dips in the Y(Ap) structure, only their magnitude is influenced by Y(A,TKE), while the TXE partition influences the position of less pronounced peaks and dips in the structures of Y(Ap) and Y(Np) (Np=Ap-Z). The influence of both the TXE partition and the Y(A,TKE) data on different kinetic energy distributions of post-neutron fragments is less pronounced than that on Y(Ap) and Y(Np)

Microscopic investigations into fission dynamics beyond the saddle point

The real-space density-constrained time-dependent Hartree-Fock-Bogoliubov method is exploited to obtain the dynamic potential energy and several experimental variables of fission fragments. The resultant predictions of collective variables, especially the total kinetic energy and total excitation energy of the fission fragments, finely match the experimental data. It is demonstrated that scission dynamics including the shell effects and pairing correlations should be the focus of further studies.

Total excitation energy distribution for neutron-induced fission of thorium isotopes

Total excitation energy distribution for neutron-induced fission of thorium isotopes

Preformation probability of light charged particles emitted equatorially in ternary fission of Cf252

We investigate the preformation probability of the light particle that formed in the preliminary stage of the ternary fission process of the $^{252}\mathrm{Cf}$ nucleus. Considering the equatorial cluster tripartition kinematics, the relative yields of the ternary fission channels with $^{4,6}\mathrm{He}, ^{10}\mathrm{Be}$ and $^{14}\mathrm{C}$ as light third nucleus, are calculated. We use the Wentzel-Kramers-Brillouin approximation to calculate the penetrability of the different nuclei involved in the ternary fission process, based on microscopic Skyrme potentials. The obtained results indicate the optimum total excitation energy of the heavy nuclei produced in the $\ensuremath{\alpha}$ ternary fission channels to be within the range of 3--4 MeV. The estimated $\ensuremath{\alpha}$-preformation probability corresponding to this range is comparable with that estimated for the spontaneous $\ensuremath{\alpha}$ decays of the participating heavy nuclei. While the extracted preformation probability at excitation energy less than its indicated optimum values exhibits unphysical large values, the calculations based on the higher excitation energies indicate extremely small values. In the ternary fission process, the preformation probability of the emitted light particles heavier than the $\ensuremath{\alpha}$ particle exponentially decreases with increasing its mass number. A phenomenological expression is suggested based on the obtained results to estimate the preformation probability of an emitted cluster of a given mass number.

Correlation studies of fission-fragment neutron multiplicities

We calculate neutron multiplicities from fission fragments with specified mass numbers for events having a specified total fragment kinetic energy. The shape evolution from the initial compound nucleus to the scission configurations is obtained with the Metropolis walk method on the five-dimensional potential-energy landscape, calculated with the macroscopic-microscopic method for the three-quadratic-surface shape family. Shape-dependent microscopic level densities are used to guide the random walk, to partition the intrinsic excitation energy between the two proto-fragments at scission, and to determine the spectrum of the neutrons evaporated from the fragments. The contributions to the total excitation energy of the resulting fragments from statistical excitation and shape distortion at scission is studied. Good agreement is obtained with available experimental data on neutron multiplicities in correlation with fission fragments from $^{235}$U(n$_{\rm th}$,f). At higher neutron energies a superlong fission mode appears which affects the dependence of the observables on the total fragment kinetic energy.

Overview of two deterministic modelings for prompt emission in fission

Two models with a deterministic treatment of prompt emission in fission were developed at the University of Bucharest. Both models work with the same ranges of initial fragments and total kinetic energy and they use the same partition of the total excitation energy at full acceleration based on modelling at scission. The main difference between these modelings regards the prompt emission treatment itself. I.e. the Point-by-Point (PbP) model uses a global treatment of sequential emission while the other modeling is based on an event-by-event treatment of sequential emission. Both models are submitted to a rigorous validation. This paper focuses on model results of different prompt γ-ray quantities, which describe very well the existing experimental data. A new method to calculate prompt γ-ray spectra, including a global treatment based on the distribution of prompt γ-ray energy per quanta, is proposed.

Role of the triplet metastable levels in ionization/excitation of palladium atom in glow discharge plasmas using several plasma gases

The major excitation mechanism of palladium ionic lines (Pd II) in glow discharge plasma was investigated when argon, neon, krypton, and argon‑helium mixed gas were employed as the plasma gas. Different types of Pd II lines were intensively excited depending on the kind of the plasma gases. Correspondences in the excitation energy were found for each plasma gas, the excited levels of 8.1–8.6 eV for argon, 13.1–14.4 eV for neon, and more than 15 eV for helium, while no intense Pd II lines were found in krypton. These phenomena are principally attributed to an asymmetric charge transfer collision with the plasma gas ion, as previous studies have pointed out for other metallic elements. However, the excitation scheme of the Pd II lines is somewhat different from the normal scheme for the other metallic elements. In particular, it is a unique feature that the triplet metastable levels, 4d95s 3D3,2,1, rather than the ground state level, 4d10 1S0, of the palladium atom play a critical role in excitations of the Pd II lines. This effect is principally because the total excitation energy follows the energy resonance condition, as well as the spin multiplicity follows the spin conservation rule between the colliding partners.

Investigation of neutron emission through the local odd-even effect as a function of the fission product kinetic energy

A recent experimental campaign was completed at the LOHENGRIN spectrometer. It was dedicated to the determination of the local odd-even effect as a function of the fission product kinetic energy for a given mass. We discuss here the mass $A=139$ produced from the thermal neutron induced fission of $^{241}\mathrm{Pu}$. A comparison with the Monte Carlo code fifrelin allows one to interpret these data in regards to the neutron emission process. The long term goal is to test and validate the phenomenological temperature ratio law used in fifrelin to split the total excitation energy between both fission fragments.

The calculation of total fragment excitation energy for photofission of Uranium isotopes

The calculation of total fragment excitation energy for photofission of Uranium isotopes

Ternary fission of α -structured nuclei with 12≤A≤60 : A three-body decay approach

We study the ternary fission of various even-even $\ensuremath{\alpha}$-structured parent nuclei with $12\ensuremath{\le}A\ensuremath{\le}60$, such as $^{12}\mathrm{C}$, $^{16}\mathrm{O}$, $^{20}\mathrm{Ne}$, $^{24}\mathrm{Mg}$, $^{28}\mathrm{Si}$, $^{32}\mathrm{S}$, $^{36}\mathrm{Ar}$, $^{40}\mathrm{Ca}$, $^{44}\mathrm{Ti}$, $^{48}\mathrm{Cr}$, $^{52}\mathrm{Fe}$, $^{56}\mathrm{Ni}$, and $^{60}\mathrm{Zn}$, within a three-body decay approach. In the present study, we consider the three spherical ternary fission fragments with the lightest fragment ${A}_{3}$ at the middle of the other two main fission fragments, ${A}_{1}$ and ${A}_{2}$. The effects of hyperradius and the hypermomentum quantum number in the ternary fission potential energy barrier are also studied. Further, the total excitation energy and the ternary fission relative yields are calculated for the possible ternary fragmentation of each parent nuclei considered. From the results obtained, we find the emission of $\ensuremath{\alpha}$-structured ternary fragments is more favored than the other ternary fragment combinations.

Scission configuration of U239 from yields and kinetic information of fission fragments

The simultaneous measurement of the isotopic fission-fragment yields and fission-fragment velocities of $^{239}$U has been performed for the first time. The $^{239}$U fissioning system was produced in one-neutron transfer reactions between a $^{238}$U beam at 5.88 MeV/nucleon and a $^{9}$Be target. The combination of inverse kinematics at low energy and the use of the VAMOS++ spectrometer at the GANIL facility allows the isotopic identification of the full fission-fragment distribution and their velocity in the reference frame of the fissioning system. The proton and neutron content of the fragments at scission, their total kinetic and total excitation energy, as well as the neutron multiplicity were determined. Information from the scission point configuration is obtained from these observables and the correlation between them. The role of the octupole-deformed proton and neutron shells in the fission-fragment production is discussed.

Calculation of the fission observables in the resolved resonance energy region of the 235U(n,f) reaction

Measurement of the fission fragments in coincidence with the emitted prompt neutrons was undertaken recently, at JRC-Geel institute, for the 235U(n,f) reaction in the resolved resonance energy region, up to 160 eV incident neutron energy. From this experimental work, fluctuations of several fission observables (mass yields, average total kinetic energy T̅K̅E̅, average prompt neutron multiplicity v̅P) were clearly observed. In the present work, these experimental pre-neutron fission fragment mass and kinetic energy distributions were used as input data for the FIFRELIN Monte Carlo code. By adopting the Hauser-Feshbach statistical model, the code simulates the de-excitation of the fission fragments. Four free parameters are available in the code: two of them (called RTmin and RTmax) govern at the scission point the sharing of the total available excitation energy between the two nascent fission fragments, while the two others (called σL and σH) assign the initial fission fragment spins. In this way, fission observables (prompt particles energy spectra and multiplicities, delayed neutrons multiplicity,. . . ) and correlations between them can be predicted and investigated. Here, these four free parameters were tuned in order to reproduce the average prompt neutron multiplicity at the resonance En=19.23 eV, resonance for which the experimental statistical uncertainty on v̅P is the lowest one. Then, the calculations were perfomed for all resonances by keeping the same set of free parameters. We show that the calculated fluctuations of v̅P in the resonances can rather be well reproduced by considering only the fluctuations of the pre-neutron mass yields and kinetic energy. In addition, from our calculation procedure, other fission observables fluctuations can also be predicted.

Fission fragments observables measured at the LOHENGRIN spectrometer

Nuclear fission yields are key data for reactor studies, such as spent fuel inventory or decay heat, and for understanding fission process. Despite a significant effort allocated to measure fission yields during the last decades, the recent evaluated libraries still need improvements in particular in the reduction of the uncertainties. Moreover, some discrepancies between these libraries must be explained. Additional measurements provide complementary information and estimations of experimental correlations, and new kinds of measurements enable to test the models used during the nuclear data evaluation process. A common effort by the CEA, the LPSC and the ILL aims at tackling these issues by providing precise measurements of isotopic and isobaric fission yields with the related variance-covariance matrices. Additionally, the experimental program involves a large range of observables requested by the evaluations, such as kinetic energy dependency of isotopic yields and odd-even effect in order to test the sharing of total excitation energy and the spin generation mechanism. Another example is the complete range of isotopic distribution per mass that allows the determination of the charge polarization, which has to be consistent for complementary masses (pre-neutron emission). For instance, this information is the key observable for the evaluation of isotopic yields. Finally, ionic charge distributions are indirect measurements of nanosecond isomeric ratios as a probe of the nuclear de-excitation path in the (E*, J, π) representation. Measurements for thermal neutron induced fission of 241 Pu have been carried out at the ILL in Grenoble, using the LOHENGRIN mass spectrometer. Methods, results and comparison to models calculations will be presented corresponding to a status on fission fragments observables reachable with this facility.