Fission Channels Research Articles

Fission dynamics with systems of intermediate fissility

A 4π light charged particle spectrometer, called 8π LP, is in operation at the Laboratori Nazionali di Legnaro, Italy, for studying reaction mechanisms in low-energy heavy-ion reactions. Besides about 300 telescopes to detect light charged particles, the spectrometer is also equipped with an anular PPAC system to detect evaporation residues and a two-arm time-of-flight spectrometer to detect fission fragments. The spectrometer has been used in several fission dynamics studies using as a probe light charged particles in the fission and evaporation residues (ER) channels. This paper proposes a journey within some open questions about the fission dynamics and a review of the main results concerning nuclear dissipation and fission time-scale obtained from several of these studies. In particular, the advantages of using systems of intermediate fissility will be discussed.

Formation of heavy neutron-deficient nuclides in 3He-induced reactions

Reactions of 3He with actinide nuclei are a promising tool for the production of heavy neutrondeficient nuclei, which are of great interest as tracers in environmental studies. In this work, highly enriched 235U targets 1 mg cm–2 thick were irradiated using 3He ions with energies of 20.4–42.0 MeV at the Accelerator Laboratory of University of Jyvaskyla, Finland. The irradiated targets were analyzed with gamma and alpha spectrometers and then dissolved in order to chemically separate Pu and Np from the fission products and the target material. The chemical yields of 234Np and 236Pu were determined by measuring their activities prior to and after chemical separation. Previously unknown excitation functions of the nuclides 234, 236, 237Pu and 234, 235, 236m Np were thus found. The obtained experimental data were analyzed using the direct reaction theory for a model with nuclear friction included in the fission channel and due consideration of the preequilibrium processes.

Nuclear data for fusion: Validation of typical pre-processing methods for radiation transport calculations

Nuclear data form the basis of the radiation transport codes used to design and simulate the behaviour of nuclear facilities, such as the ITER and DEMO fusion reactors. Typically these data and codes are biased towards fission and high-energy physics applications yet are still applied to fusion problems. With increasing interest in fusion applications, the lack of fusion specific codes and relevant data libraries is becoming increasingly apparent. Industry standard radiation transport codes require pre-processing of the evaluated data libraries prior to use in simulation. Historically these methods focus on speed of simulation at the cost of accurate data representation. For legacy applications this has not been a major concern, but current fusion needs differ significantly. Pre-processing reconstructs the differential and double differential interaction cross sections with a coarse binned structure, or more recently as a tabulated cumulative distribution function. This work looks at the validity of applying these processing methods to data used in fusion specific calculations in comparison to fission. The relative effects of applying this pre-processing mechanism, to both fission and fusion relevant reaction channels are demonstrated, and as such the poor representation of these distributions for the fusion energy regime. For the natC(n,el) reaction at 2.0MeV, the binned differential cross section deviates from the original data by 0.6% on average. For the 56Fe(n,el) reaction at 14.1MeV, the deviation increases to 11.0%. We show how this discrepancy propagates through to varying levels of simulation complexity. Simulations were run with Turnip-MC and the ENDF-B/VII.1 library in an effort to define a new systematic error for this range of applications. Alternative representations of differential and double differential distributions are explored in addition to their impact on computational efficiency and relevant simulation results.

Prompt fission neutron spectra of n+235U above the (n,nf) fission threshold**Supported by National Natural Science Foundation of China (11205246, 91126010, U1230127, 91226102), IAEA CRP (15905), and Defense Industrial Technology Development Program (B0120110034)

Calculations of prompt fission neutron spectra (PFNS) from the 235U(n, f) reaction were performed with a semi-empirical method for En = 7.0 and 14.7 MeV neutron energies. The total PFNS were obtained as a superposition of (n,xnf) pre-fission neutron spectra and post-fission spectra of neutrons which were evaporated from fission fragments, and these two kinds of spectra were taken as an expression of the evaporation spectrum. The contributions of (n,xnf) fission neutron spectra on the calculated PFNS were discussed. The results show that emission of one or two neutrons in the (n,nf) or (n,2nf) reactions influences the PFNS shape, and the neutron spectra of the (n,xnf) fission-channel are soft compared with the neutron spectra of the (n,f) fission channel. In addition, analysis of the multiple-chance fission component showed that second-chance fission dominates the PFNS with an incident neutron energy of 14.7 MeV whereas first-chance fission dominates the 7 MeV case.

Near edge X-ray absorption mass spectrometry on coronene.

We have investigated the photoionization and photodissociation of free coronene cations C24H12 (+) upon soft X-ray photoabsorption in the carbon K-edge region by means of a time-of-flight mass spectrometry approach. Core excitation into an unoccupied molecular orbital (below threshold) and core ionization into the continuum both leave a C 1s vacancy, that is subsequently filled in an Auger-type process. The resulting coronene dications and trications are internally excited and cool down predominantly by means of hydrogen emission. Density functional theory was employed to determine the dissociation energies for subsequent neutral hydrogen loss. A statistical cascade model incorporating these dissociation energies agrees well with the experimentally observed dehydrogenation. For double ionization, i.e., formation of intermediate C24H12 (3+⋆)trications, the experimental data hint at loss of H(+) ions. This asymmetric fission channel is associated with hot intermediates, whereas colder intermediates predominantly decay via neutral H loss.

Experiments and Theoretical Data for Studying the Impact of Fission Yield Uncertainties on the Nuclear Fuel Cycle with TALYS/GEF and the Total Monte Carlo Method

We describe the research program of the nuclear reactions research group at Uppsala University concerning experimental and theoretical efforts to quantify and reduce nuclear data uncertainties relevant for the nuclear fuel cycle. We briefly describe the Total Monte Carlo (TMC) methodology and how it can be used to study fuel cycle and accident scenarios, and summarize our relevant experimental activities. Input from the latter is to be used to guide the nuclear models and constrain parameter space for TMC.The TMC method relies on the availability of good nuclear models. For this we use the TALYS code which is currently being extended to include the GEF model for the fission channel. We present results from TALYS-1.6 using different versions of GEF with both default and randomized input parameters and compare calculations with experimental data for 234U(n,f) in the fast energy range. These preliminary studies reveal some systematic differences between experimental data and calculations but give overall good and promising results.

True ternary fission of 252Cf(sf), the collinear decay into fragments of similar size

The ternary decay in 252Cf(sf, fff), with three cluster fragments of different masses (e.g.132Sn,52-48Ca,68-72Ni), has been observed by the FOBOS group in JINR. This work has established a new decay mode of heavy nuclei, the collinear cluster tripartition, (CCT). This "true ternary fission" of heavy nuclei has been predicted many times in theoretical works during the last decades. In the present report we discuss true ternary fission (FFF) into three nuclei of almost equal size (e.g. Z=98 → Zi = 32, 34, 32) and other fission modes in the same system. The possible fission channels for 252 Cf(sf) are predicted from potential-energy (PES) calculations. These PES's show pronounced minima for several ternary fragmentation decays, suggesting a variety of collinear ternary fission modes. The FFF-decays have very similar dynamical features as the previously observed collinear CCT-decays, the central fragment has very small kinetic energy. The data of the cited experiment allow the extraction of the yield for some FFF-decays, by using specific gates on the measured parameters.

Spontaneous fission withβ-parameterized quasimolecular shape

In the framework of a generalized liquid drop model (GLDM), a quasimolecular mechanism is introduced to describe the deformation of a nucleus in the procedure of nuclear fission or fusion. In order to more appropriately evaluate the shell correction on the fission or fusion path, the quasimolecular shape is described in terms of deformation parameters beta (i. e., so-called beta parameterized) by a transformation. For symmetric fission it is done analytically, whereas for asymmetric fission it is performed in a pure numerical way. Thereafter, for each quasimolecular shape, the shell correction can be calculated by the Strutinsky method where the single-particle energies are derived from a shell model in an axially deformed Woods-Saxon potential with the beta-parameterized quasimolecular shape. We then use this recipe to predict the half-lives of several spontaneous fission channels for some heavy nuclei, and the results are in agreement with the experimental data.

Excitation-energy dependence of fission in the mercury region

Background: Recent experiments on beta-delayed fission reported an asymmetric mass yield in the neutron-deficient nucleus 180Hg. Earlier experiments in the mass region A=190-200 close to the beta-stability line, using the (p,f) and (\alpha,f) reactions, observed a more symmetric distribution of fission fragments. While the beta-delayed fission of 180Hg can be associated with relatively low excitation energy, this is not the case for light-ion reactions, which result in warm compound nuclei. Purpose: To elucidate the roles of proton and neutron numbers and excitation energy in determining symmetric and asymmetric fission yields, we compute and analyze the isentropic potential energy surfaces of 174,180,198Hg and 196,210Po. Methods: We use the finite-temperature superfluid nuclear density functional theory, for excitation energies up to E*=30MeV and zero angular momentum. For our theoretical framework, we consider the Skyrme energy density functional SkM* and a density-dependent pairing interaction. Results: For 174,180Hg, we predict fission pathways consistent with asymmetric fission at low excitation energies, with the symmetric fission pathway opening very gradually as excitation energy is increased. For 198Hg and 196Po, we expect the nearly-symmetric fission channel to dominate. 210Po shows a preference for a slightly asymmetric pathway at low energies, and a preference for a symmetric pathway at high energies. Conclusions: Our self-consistent theory suggests that excitation energy weakly affects the fission pattern of the nuclei considered. The transition from the asymmetric fission in the proton-rich nuclei to a more symmetric fission in the heavier isotopes is governed by the shell structure of pre-scission configurations.

The r-Java 2.0 code: nuclear physics

[Aims:] We present r-Java 2.0, a nucleosynthesis code for open use that performs r-process calculations as well as a suite of other analysis tools. [Methods:] Equipped with a straightforward graphical user interface, r-Java 2.0 is capable of; simulating nuclear statistical equilibrium (NSE), calculating r-process abundances for a wide range of input parameters and astrophysical environments, computing the mass fragmentation from neutron-induced fission as well as the study of individual nucleosynthesis processes. [Results:] In this paper we discuss enhancements made to this version of r-Java, paramount of which is the ability to solve the full reaction network. The sophisticated fission methodology incorporated into r-Java 2.0 which includes three fission channels (beta-delayed, neutron-induced and spontaneous fission) as well as computation of the mass fragmentation is compared to the upper limit on mass fission approximation. The effects of including beta-delayed neutron emission on r-process yield is studied. The role of coulomb interactions in NSE abundances is shown to be significant, supporting previous findings. A comparative analysis was undertaken during the development of r-Java 2.0 whereby we reproduced the results found in literature from three other r-process codes. This code is capable of simulating the physical environment of; the high-entropy wind around a proto-neutron star, the ejecta from a neutron star merger or the relativistic ejecta from a quark nova. As well the users of r-Java 2.0 are given the freedom to define a custom environment. This software provides an even platform for comparison of different proposed r-process sites and is available for download from the website of the Quark-Nova Project: http://quarknova.ucalgary.ca/

Pyrolysis of ethanol: A shock-tube/TOF-MS and modeling study

The pyrolysis of ethanol was studied in a shock tube behind reflected shock waves in the temperature range 1300–1510K at pressures of about 1.1bar by using time-of-flight mass spectrometry for detection. For the first time, concentration–time profiles of C2H5OH, C2H4O, (C2H2+C2H4+C2H6), H2O, and CH4 were simultaneously recorded. It could be shown that under our experimental conditions, the major products are H2O and C2H4, which are mainly formed in the reaction C2H5OH→C2H4+H2O (R1). The rate coefficient of reaction (R1) could be determined for the first time from directly measured concentration–time profiles of H2O in a shock tube. The results can be expressed by the Arrhenius equation k1(T, P ∼1.1bar)=(2.5±1.0)×1010 exp (−23320K/T)s−1 with an estimated uncertainty of ±40%. The multiple-species concentration–time profiles from our experiments can be used for a most rigorous test of ethanol pyrolysis mechanisms. As an exemplary case, we employed the mechanism from Marinov (1999) and found a very good agreement between the experimental and simulation results. We also calculated rate coefficients for the most important decomposition steps of ethanol from statistical rate theory by solving a thermal multichannel master equation. Reaction (R1) as well as the bond fission channels C2H5OH→CH3+CH2OH (R2) and C2H5OH→C2H5+OH (R3) were considered. The specific rate coefficients were obtained from RRKM theory (R1) and the Statistical Adiabatic Channel Model (R2 and R3) with molecular and transition state data from quantum chemical calculations at CCSD(T)-F12/def2-QZVPP//MP2/cc-pVQZ level of theory. The rate coefficient obtained from the ab initio data were slightly fitted to recently published experimental values of k1, k2, and k3 and inserted into Marinov’s mechanism. A similar good representation of the concentration–time profiles as with the original mechanism was obtained.

True ternary fission, the collinear decay into fragments of similar size in the 252Cf(sf) and 235U(nth, f) reactions

The collinear cluster decay in 252Cf(sf, fff), with three cluster fragments of different masses (e.g. 132Sn, 52–48Ca, 68–72Ni), which has been observed by the FOBOS group in JINR, has established a new decay mode of heavy nuclei, the collinear cluster tripartition (CCT). The same type of ternary fission decay has been observed in the reaction 235U(nth, fff). This kind of “true ternary fission” of heavy nuclei has been predicted many times in theoretical works during the last decades. In the present note we discuss true ternary fission (TFFF) into three nuclei of almost equal size (e.g. Z=98→Zi=32, 34, 32) in the same systems. The possible fission channels are predicted from potential-energy (PES) calculations. These PES's show pronounced minima for several ternary fragmentation decays, e.g. for 252Cf(sf) and for 235U(nth, f). They suggest the existence of a variety of collinear ternary fission modes. The TFFF-decays chosen in this letter have very similar dynamical features as the previously observed collinear CCT-decays. The data obtained in the above mentioned experiments allow us to extract the yield for these TFFF-decays in both systems by using specific gates on the measured parameters. These yields are a few 1.0⋅10−6/(binary fission).

Spontaneous Fission Barriers Based on a Generalized Liquid Drop Model

The barrier against the spontaneous fission has been determined within the Generalized Liquid Drop Model (GLDM) including the mass and charge asymmetry, and the proximity energy. The shell correction of the spherical parent nucleus is calculated by using the Strutinsky method, and the empirical shape-dependent shell correction is employed during the deformation process. A quasi-molecular shape sequence has been defined to describe the whole process from one-body shape to two-body shape system, and a two-touching-ellipsoid is adopted when the superdeformed one-body system reaches the rupture point. On these bases the spontaneous fission barriers are systematically studied for nuclei from 230Th to 249Cm for different possible exiting channels with the different mass and charge asymmetries. The double, and triple bumps are found in the fission potential energy in this region, which roughly agree with the experimental results. It is found that at around Sn-like fragment the outer fission barriers are lower, while the partner of the Sn-like fragment is in the range near 108Ru where the ground-state mass is lowered by allowing axially symmetric shapes. The preferable fission channels are distinctly pronounced, which should be corresponding to the fragment mass distributions.

The Impact of Intermediate Structure on the Average Fission Cross Sections

This paper discusses two common approximations used to calculate average fission cross sections over the compound energy range: the disregard of the WII factor and the Porter-Thomas hypothesis made on the double barrier fission width distribution. By reference to a Monte Carlo-type calculation of formal R-matrix fission widths, this work estimates an overall error ranging from 12% to 20% on the fission cross section in the case of the 239Pu fissile isotope in the energy domain from 1 to 100 keV with very significant impact on the competing capture cross section. This work is part of a recent and very comprehensive formal R-matrix study over the Pu isotope series and is able to give some hints for significant accuracy improvements in the treatment of the fission channel.

Measurement of prompt neutron spectra from the239Pu(n,f) fission reaction for incident neutron energies from 1 to 200 MeV

Prompt fission neutron spectra in the neutron-induced fission of ${}^{239}$Pu have been measured for incident neutron energies from 1 to 200 MeV at the Los Alamos Neutron Science Center. Mean energies deduced from the prompt fission neutron spectra (PFNS) lead to the observation of the opening of the second chance fission at 7 MeV and to indications for the openings of fission channels of third and fourth chances. Moreover, the general trend of the measured PFNS is well reproduced by the different models. The comparison between data and models presents, however, two discrepancies. First, the prompt neutron mean energy seems constant for neutron energy, at least up to 7 MeV, whereas in the theoretical calculations it is continuously increasing. Second, data disagree with models on the shape of the high energy part of the PFNS, where our data suggest a softer spectrum than the predictions.

Elastic and inelastic scattering of neutrons on238U nucleus

Advanced modelling of neutron induced reactions on the 238 U nucleus is aimed at improving our knowledge of neutron scattering. Capture and fission channels are well constrained by available experimental data and neutron standard evaluation. A focus of this contribution is on elastic and inelastic scattering cross sections. The employed nuclear reaction model includes - a new rotational-vibrational dispersive optical model potential coupling the low-lying collective bands of vibrational character observed in even-even actinides; - the Engelbrecht-Weidenmuller transformation allowing for inclu- sion of compound-direct interference effects; - and a multi-humped fission barrier with absorption in the secondary well described within the optical model for fission. Impact of the advanced modelling on elastic and inelastic scattering cross sections including an- gular distributions and emission spectra is assessed both by comparison with selected microscopic experimental data and integral criticality benchmarks including measured reaction rates (e.g. JEMIMA, FLAPTOP and BIG TEN). Benchmark calculations pro- vided feedback to improve the reaction modelling. Improvement of existing libraries will be discussed.

Fragmentation dynamics of the ethyl bromide and ethyl iodide cations: a velocity-map imaging study

The photodissociation dynamics of ethyl bromide and ethyl iodide cations (C2H5Br(+) and C2H5I(+)) have been studied. Ethyl halide cations were formed through vacuum ultraviolet (VUV) photoionization of the respective neutral parent molecules at 118.2 nm, and were photolysed at a number of ultraviolet (UV) photolysis wavelengths, including 355 nm and wavelengths in the range from 236 to 266 nm. Time-of-flight mass spectra and velocity-map images have been acquired for all fragment ions and for ground (Br) and spin-orbit excited (Br*) bromine atom products, allowing multiple fragmentation pathways to be investigated. The experimental studies are complemented by spin-orbit resolved ab initio calculations of cuts through the potential energy surfaces (along the RC-Br/I stretch coordinate) for the ground and first few excited states of the respective cations. Analysis of the velocity-map images indicates that photoexcited C2H5Br(+) cations undergo prompt C-Br bond fission to form predominantly C2H5(+) + Br* products with a near-limiting 'parallel' recoil velocity distribution. The observed C2H3(+) + H2 + Br product channel is thought to arise via unimolecular decay of highly internally excited C2H5(+) products formed following radiationless transfer from the initial excited state populated by photon absorption. Broadly similar behaviour is observed in the case of C2H5I(+), along with an additional energetically accessible C-I bond fission channel to form C2H5 + I(+) products. HX (X = Br, I) elimination from the highly internally excited C2H5X(+) cation is deemed the most probable route to forming the C2H4(+) fragment ions observed from both cations. Finally, both ethyl halide cations also show evidence of a minor C-C bond fission process to form CH2X(+) + CH3 products.

One-neutron and noncompound-nucleus decay contributions in the12C+93Nb reaction at energies near and below the fusion barrier

The dynamical cluster-decay model (DCM), with effects of deformations and orientations of nuclei included in it, is used to study the decay of the hot and rotating compound nucleus ${}^{105}$Ag${}^{*}$ formed in a ${}^{12}\mathrm{C}+{}^{93}\mathrm{Nb}$ reaction at near and below barrier energies. The only parameter of the model is the neck-length parameter, which varies smoothly with the temperature of the compound nucleus, and its value remains within the range of validity ($\ensuremath{\sim}$2 fm) of the proximity potential. The emissions of both the observed light particles (${A}_{2}=1--4$) representing the evaporation residue (ER) and the (energetically favored) intermediate mass fragments (IMFs; $5\ensuremath{\le}{A}_{2}\ensuremath{\le}13$), together with the so far unobserved fission channel, are considered as the dynamical collective mass motions of preformed fragments or clusters through the barrier. A best fit to data is shown to be obtained only if a large noncompound-nucleus (nCN) contribution, calculated as a quasifission or capture process, is allowed in the DCM. Another interesting result is that the one-neutron contribution is large, rather the largest, when best fits are attempted only for the ER cross sections, but, in turn, it reduces to zero or becomes relatively small if the data on the total fusion cross section ${\ensuremath{\sigma}}_{\mathrm{fus}}$, i.e., ${\ensuremath{\sigma}}_{\mathrm{ER}}+{\ensuremath{\sigma}}_{\mathrm{IMFs}}$, are considered. Furthermore, the fusion-fission (ff) cross section ${\ensuremath{\sigma}}_{\mathrm{ff}}$, consisting of symmetric and near-symmetric fragments, at the considered three energies of the experiment, is predicted to be of the order of 10${}^{1}$ to 10${}^{3}$ mb, depending on the choice of neck-length parameter for fission region. Further experimental studies are called for both the nCN and ff channels in this reaction.

Variational RRKM theory calculation of thermal rate constant for carbon—hydrogen bond fission reaction of nitro benzene

The present work provides quantitative results for the rate of unimolecular carbon-hydrogen bond fission reaction of benzene and nitro benzene at elevated temperatures up to 2000 K. The potential energy surface for each C-H (in the ortho, meta, and para sites) bond fission reaction of nitro benzene was investigated by ab initio calculations. The geometry and vibrational frequencies of the species involved in this process were optimized at the MP2 level of theory, using the cc-pvdz basis set. Since C-H bond fission channel is barrier less reaction, we have used variational RRKM theory to predict rate constants. By means of calculated rate constant at the different temperatures, the activation energy and exponential factor were determined. The Arrhenius expression for C-H bond fission reaction of nitro benzene on the ortho, meta and para sites are k(T) = 2.1 × 1017exp(−56575.98/T), k(T) = 2.1 × 1017exp(−57587.45/T), and k(T) = 3.3 × 1016exp(−57594.79/T) respectively. The Arrhenius expression for C-H bond fission reaction of benzene is k(T) = 2 × 1018exp(−59343.48.18/T). The effect of NO2 group, location of hydrogen atoms on the substituted benzene ring, reaction degeneracy, benzene ring resonance and tunneling effect on the rate expression have been discussed.

High Temperature Shock Tube and Theoretical Studies on the Thermal Decomposition of Dimethyl Carbonate and Its Bimolecular Reactions with H and D-Atoms

The shock tube technique was used to study the high temperature thermal decomposition of dimethyl carbonate, CH3OC(O)OCH3 (DMC). The formation of H-atoms was measured behind reflected shock waves by using atomic resonance absorption spectrometry (ARAS). The experiments span a T-range of 1053-1157 K at pressures ∼0.5 atm. The H-atom profiles were simulated using a detailed chemical kinetic mechanism for DMC thermal decomposition. Simulations indicate that the formation of H-atoms is sensitive to the rate constants for the energetically lowest-lying bond fission channel, CH3OC(O)OCH3 → CH3 + CH3OC(O)O [A], where H-atoms form instantaneously at high temperatures from the sequence of radical β-scissions, CH3OC(O)O → CH3O + CO2 → H + CH2O + CO2. A master equation analysis was performed using CCSD(T)/cc-pv∞z//M06-2X/cc-pvtz energetics and molecular properties for all thermal decomposition processes in DMC. The theoretical predictions were found to be in good agreement with the present experimentally derived rate constants for the bond fission channel (A). The theoretically derived rate constants for this important bond-fission process in DMC can be represented by a modified Arrhenius expression at 0.5 atm over the T-range 1000-2000 K as, kA(T) = 6.85 × 10(98)T (-24.239) exp(-65250 K/T) s(-1). The H-atom temporal profiles at long times show only minor sensitivity to the abstraction reaction, H + CH3OC(O)OCH3 → H2 + CH3OC(O)OCH2 [B]. However, H + DMC is an important fuel destruction reaction at high temperatures. Consequently, measurements of D-atom profiles using D-ARAS allowed unambiguous rate constant measurements for the deuterated analog of reaction B, D + CH3OC(O)OCH3 → HD + CH3OC(O)OCH2 [C]. Reaction C is a surrogate for H + DMC since the theoretically predicted kinetic isotope effect at high temperatures (1000 - 2000K) is close to unity, kC ≈ 1.2 kB. TST calculations employing CCSD(T)/cc-pv∞z//M06-2X/cc-pvtz energetics and molecular properties for reactions B and C are in good agreement with the experimental rate constants. The theoretical rate constants for these bimolecular processes can be represented by modified Arrhenius expressions over the T-range 500-2000 K as, kB(T) = 1.45 × 10(-19)T(2.827) exp(-3398 K/T) cm(3) molecule(-1) s(-1) and kC(T) = 2.94 × 10(-19)T(2.729) exp(-3215 K/T) cm(3) molecule(-1) s(-1).