Fission Fragment Angular Distributions Research Articles

Fission fragment mass and angular distribution in6,7Li+235,238U reactions

Fission fragment (FF) angular distributions for 6,7 Li+235,238 U reactions and mass distributions for 6,7 Li+238 U reactions have been measured at energies near and above the Coulomb barrier. The angle integrated fission cross sections for 6 Li induced reactions at sub-barrier energies are found to be higher than 7 Li induced reactions possibly due to larger contribution of breakup induced fission in case of the former compared to the latter. The FF anisotropy for 6,7 Li+235 U was found to be smaller than 6,7 Li+238 U, manifesting the effect of target ground state spin. The statistical saddle point (SSP) model predictions were found to be consistent with the measured FF anisotropy for 6,7 Li+235 U, however they were under-estimated for 6,7 Li+238 U particularly at lower energies. Observation of larger FWHM of FF folding angle distribution and sharp increase in peak to valley ratio for FF mass distribution with the decrease in bombarding energy in 6,7 Li+238 U reactions confirms the presence of breakup induced fission.

Fission fragment angular distribution measurements for 16O +194Pt reaction at energies near the Coulomb barrier

Abstract Fission fragment angular distributions have been measured for 16O + 194Pt reaction forming the compound system 210Rn, in the laboratory energy range from 79 to 90 MeV. The measured fission fragment anisotropies as a function of E c . m . / V B are compared with the predictions of standard saddle point statistical model ( sspm ). Anisotropies calculated using the average excitation energy and angular momentum values could not reasonably fit the experimental data. Statistical model calculations were performed using the pace with modified fission barrier and level density parameters. Fission probability, evaporation residue cross section and neutron multiplicity were simultaneously used to fix the statistical parameters. Model calculations incorporating the chance nature of fission decay and scaled values of the rotating finite range model (RFRM) moment of inertia could reasonably fit the fragment angular anisotropies.

Measurement of prompt neutron polarization asymmetries in photofission of235,238U,239Pu, and232Th

A photofission experiment was performed on targets of ${}^{235}$U, ${}^{238}$U, ${}^{239}$Pu, and ${}^{232}$Th using nearly 100$%$ linearly polarized, high-intensity ($\ensuremath{\sim}$10${}^{7}$ $\ensuremath{\gamma}/$s), and nearly monoenergetic $\ensuremath{\gamma}$-ray beams having energies between 5.6 and 7.3 MeV at the High Intensity $\ensuremath{\gamma}$-ray Source (HI$\ensuremath{\gamma}$S). An array of 18 liquid scintillator detectors was used to measure prompt fission neutron polarization asymmetries. An asymmetry close to zero was found for ${}^{235}$U and ${}^{239}$Pu while a significant asymmetry ($\ensuremath{\sim}$0.5) was found for ${}^{238}$U and ${}^{232}$Th. A simplified model of near-threshold photofission has been developed in an attempt to explain the systematic difference between the even-even and even-odd targets. The results of a simulation, based on this model and using previous measurements of fission fragment angular distributions, are shown to accurately reproduce the essential features of the data.

Collective spectra along the fission barrier

Discrete and continuous spectra of fissioning nuclei at the humps of fission barriers (Bohr transition states) and in the intermediate wells (superdeformed and hyperdeformed states) play a key role in the calculation of fission cross sections. A theoretical evaluation of the collective parts of the spectra is possible within the framework of the dinuclear system model, which treats the wave function of the fissioning nucleus as a superposition of a mononucleus configuration and two–cluster configurations in a dynamical way, permitting exchange of upper–shell nucleons between clusters. The impact of theoretical spectra on neutron–induced fission cross sections and, in combination with an improved version of the scission–point model, on angular distribution of fission fragments is evaluated for plutonium isotopes of interest to nuclear energy applications.

A new approach to prompt fission neutron TOF data treatment

Abstract The prompt neutron emission in spontaneous fission of 252Cf has been investigated applying digital signal electronics along with associated digital signal processing algorithms. A new mathematical approach, applicable to single events, was developed for prompt fission neutron (PFN) time-offlight distribution unfolding. The main goal was to understand the reasons of the long existing discrepancy between theoretical calculations and the measurements of prompt fission neutron (PFN) emission dependence on the total kinetic energy (TKE) of the fission fragments (FF). Since the 252 Cf (sf) reaction is one of the main references for nuclear data the understanding of the PFN emission mechanism is very important both for nuclear fission theory and nuclear data. The experimental data were taken with a twin Frisch-grid ionization chamber and a NE213-equivalent neutron detector in an experimental setup similar to the well known work of C. Budtz-Jorgensen and H.-H. Knitter. About 2.5 × 10 5 coincidences between fission fragment (FF) and neutron detector response to prompt fission neutron detection have been registered ( ∼ 1.6 × 10 7 of total recorded fission events). Fission fragment kinetic energy, mass and angular distribution, neutron time-of-flight and pulse shape have been investigated using a 12-bit waveform digitizer. The signal waveforms have been analyzed using digital signal processing algorithms. The main goal of this work was a detailed description of the prompt fission neutron treatment.

Neutron induced fission of234U

The fission fragment properties of 234 U(n,f) were investigated as a function of incident neutron energy from 0.2 MeV up to 5 MeV. The fission fragment mass, angular distribution and kinetic energy were measured with a double Frisch-grid ionization chamber using both analogue and digital data acquisition techniques. The reaction 234 U(n,f) is relevant, since it involves the same compound nucleus as formed after neutron evaporation from highly excited 236 U*, the so-called second-chance fission of 235 U. Experimental data on fission fragment properties like fission fragment mass and total kinetic energy (TKE) as a function of incident neutron energy are rather scarce for this reaction. For the theoretical modelling of the reaction cross sections for Uranium isotopes this information is a crucial input parameter. In addition, 234 U is also an important isotope in the Thorium-based fuel cycle. The strong anisotropy of the angular distribution around the vibrational resonance at En = 0.77 MeV could be confirmed using the full angular range. Fluctuations in the fragment TKE have been observed in the threshold region around the strong vibrational resonance at En = 0.77 MeV. The present results are in contradiction with corresponding literature values. Changes in the mass yield around the vibrational resonance and at En = 5 MeV relative to En = 2 MeV show a different signature. The drop in mean TKE around 2.5 to 3 MeV points to pair breaking as also observed in 235,238 U(n,f). The measured two-dimensional mass yield and TKE distribution have been described in terms of fission modes. The yield of the standard 1 (S1) mode shows fluctuations in the threshold of the fission cross section due to the influence of the resonance and levels off at about 20% yield for higher incident neutron energies. The S2 mode shows the respective opposite behaviour. The mean TKE of both modes decreases with En . The decrease in mean TKE overrules the increase in S1 yield, so the mean TKE is dropping as a function of En above 2.5 MeV.

MEASUREMENT OF PHOTOFISSION CROSS-SECTION OF238UUSING MICROTRON FACILITY

Photofission cross-section of238U was measured using bremsstrahlung radiation energy 7.4–9.0 MeV with energy step of 0.4 MeV by employing Lexan polycarbonate film as solid state nuclear track detector (SSNTD). The photon intensity from the Microtron accelerator at a distance of 15 cm from the bremsstrahlung converter (tantalum target) facility was estimated to be 1010photons/sec using the code EGS-4. In this paper, details of the fission fragment angular distribution measurements of238U target using Lexan polycarbonate have been discussed. The photofission cross-section was calculated using the angular distribution of fission fragments and the results were compared with those obtained using the code EMPIRE-II and various barrier parameters of the RIPL-1, RIPL-2 libraries and with the new analytical fission barrier formula based on the Hugenholtz–Van Hove theorem. The present experimental measurements were in good agreement with the results obtained from the Empire-II code predictions for potential parameter taken from RIPL-1 and a newly developed analytical fission barrier formula.

Effective moment of inertia for several fission reaction systems induced by nucleons, light particles, and heavy ions

The effective moment of inertia of the compound nucleus is calculated for several fission reaction systems induced by nucleons, light particles, and heavy ions. The determination of this quantity for these systems is based upon the comparison between the experimental data of the fission fragment angular distributions as well as the prediction of the standard saddle-point statistical model. For the systems, the two cases, namely with and without neutron emission corrections, were considered. Afterward, our theoretical results are compared with the data obtained from the rotational liquid drop model as well as the Sierk model and satisfactory agreements were found. We also calculated the quadrupole deformation and mass asymmetry parameters of the fissioning nuclei at the saddle point by two methods, the first method is based on an ellipsoid shape assumption for the fissioning nucleus, and the other is based on dinuclear system model, since in heavy ion induced fission reactions, the compound nuclei which undergo fission can be considered as states of two touching clusters (dinuclear system). Afterward, the results of these two methods were compared and satisfactory agreements were found between them.

Heavy ion reactions around the Coulomb barrier

The angular distributions of fission fragments for the 32S+184W reaction near Coulomb barrier energies are measured. The experimental fission excitation function is obtained. The measured fission cross sections are decomposed into fusion-fission, quasi-fission and fast fission contributions by the dinuclear system (DNS) model. The hindrance to completing fusion both at small and large collision energies is explained. The fusion excitation functions of 32S+90,96Zr in an energy range from above to below the Coulomb barrier are measured and analyzed within a semi-classical model. The obvious effect of positive Q-value multi-neutron transfers on the sub-barrier fusion enhancement is observed in the 32S+96Zr system. In addition, the excitation functions of quasi-elastic scattering at a backward angle have been measured with high precision for the systems of 16O + 208Pb, 196Pt, 184W, and 154,152Sm at energies well below the Coulomb barrier. Considering the deformed coupling effects, the extracted diffuseness parameters are close to the values extracted from the systematic analysis of elastic and inelastic scattering data. The elastic scattering angular distribution of 17F+12C at 60 MeV is measured and calculated by using the continuum-discretized coupled-channels (CDCC) approach. It is found that the diffuseness parameter of the real part of core-target potential has to be increased by 20% to reproduce the experimental result, which corresponds to an increment of potential depth at the surface region. The breakup cross section and the coupling between breakup and elastic scattering are small.

Publisher’s Note: Fission fragment angular distributions in theBe9+Th232reaction [Phys. Rev. C83, 067601 (2011)

Publisher’s Note: Fission fragment angular distributions in the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi mathvariant="normal">Be</mml:mi><mml:mprescripts /><mml:none /><mml:mrow><mml:mn>9</mml:mn></mml:mrow></mml:mmultiscripts></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi mathvariant="normal">Th</mml:mi><mml:mprescripts /><mml:none /><mml:mrow><mml:mn>232</mml:mn></mml:mrow></mml:mmultiscripts></mml:math>reaction [Phys. Rev. C<b>83</b>, 067601 (2011)

Fission fragment angular distributions in theBe9+Th232reaction

Fission fragment angular distributions have been measured for a $^{9}\mathrm{Be}$ $+$ $^{232}\mathrm{Th}$ system at four different beam energies around the Coulomb barrier. The experimental results on fission fragment anisotropies have been compared with predictions of the standard statistical saddle-point model (SSPM) and the preequilibrium fission (PEQ) model including projectile ground-state spin. It is observed that both SSPM and the PEQ model fail to reproduce the experimental results, indicating that projectile breakup may affect the fission fragment anisotropies.

Measurement of photofission excitation of 237NP

Abstract The angular distribution of 237Np fission fragments was measured over the energy range 7.4 MeV–9.2 MeV in steps of 0.4 MeV of bremsstrahlung radiation. Lexan polycarbonate films (SSNTD) were used to detect the fission fragments. The photon intensity from the Microtron accelerator at a distance of 15 cm from the bremsstrahlung converter (tantalum target) facility was estimated to be 1010 photons s−1 using the code EGS-4. The photofission cross-section was calculated using the angular distribution of fission fragments and the results were compared with those obtained using the code EMPIRE-II and various barrier parameters of the RIPL-1, RIPL-2 libraries and with the new analytical fission barrier formula based on the Hugenholtz–Van Hove theorem. The results are in good agreement with the prediction of the RIPL-1 barrier parameter and analytical fission barrier formula.

Non-equilibrium processes in Heavy-ion induced Fission Reactions

Heavy ion induced reactions offer a wide range of possibilities in exploring the collective behavior of nuclei and nuclear processes. Recent years have seen a spurt in the experimental and theoretical studies to understand the mechanism of the fission process in heavy target projectile systems, arising from the interest to produce elements in the superheavy region. A number of new features are observed experimentally with respect to the kinetic energy, mass and angular distributions of fission fragments and their correlations, implying prevalence of non-equilibrium phenomena in the fission process in many target projectile systems. There are many theoretical attempts both in terms of static potential energy considerations and dynamics to understand the important degrees of freedom that govern the evolution of the nuclear system from initial interaction to the final fission stages. Depending on the degree and nature of equilibration, various processes such as fast-fission, quasi-fission and pre-equilibrium fission have been invoked that can compete with the fully equilibrated compound nuclear fission process. In what we call a C-T Fissility plot, we show the remarkable dependence of the onset of these non-equilibrium processes on the entrance channel parameters.

Fusion-fission and quasifission competition in the 32S+184W reaction

The angular distribution of fission fragments for the 32S+184W reaction at center-of-mass energies of 118.8, 123.1, 127.3, 131.5, 135.8, 141.1 and 144.4 MeV were measured. The experimental fission excitation function is obtained. The fragment angular anisotropy is found by extrapolating the fission angular distributions. The measured fission cross sections are decomposed into fusion-fission, quasifission and fast-fission contributions by the dinuclear system model. The total evaporation residue and fusion-fission excitation functions are calculated in the framework of the advanced statistical model. The hindrance to complete fusion at small collision energies increases due to the increase of quasifission events and it is explained by the elongated shape of the dinuclear system which is formed in collisions with small orientation angles to the beam direction. An increase of the hindrance to complete fusion at large beam energies is explained by the dependence of the quasifission and intrinsic fusion barriers of dinuclear system on its angular momentum: at large angular momentum the quasifission barrier decreases and the intrinsic fusion barrier increases. In this reaction the contributions of fusion-fission and quasifission fragments are comparable.

Experimental determination of photofission cross-sections of 232Th using electron accelerator

Photofission cross-section of 232Th was measured using Bremsstrahlung radiation energy 7.4–9.2 MeV with energy step of 0.3 MeV by employing Lexan polycarbonate film as Solid State Nuclear Track Detectors (SSNTD). The photon intensity from the Microtron accelerator facility was estimated to be 10 10 photons/s at a distance of 15 cm from the Bremsstrahlung converter using EGS-4 code ( Nelson et al., 1985). Photofission cross-sections were evaluated using fission fragment angular distribution measurements. The present experimental results were compared with EMPIRE-2.19 ( Herman et al., 2005) code prediction of RIPL-1 and RIPL-2 ( Mamdouh et al., 2001, 1998) and a new analytical formula ( Gupta and Saxena, 2005) for fission barrier.

Angular distribution of fission fragments of excited compound nuclei in multidimensional Langevin dynamics

Angular distributions of fission fragments were calculated within a multidimensional approach to the fission dynamics of excited nuclei, and the results of these calculations are presented. The evolution of the shape parameters of a fissile nucleus was described by the set of three-dimensional Langevin equations for collective coordinates introduced on the basis of the {c, h, a} parametrization. The evolution of the orientation degree of freedom (K mode, K being the projection of the total angular momentum on the symmetry axis of the nucleus under study) was described with the aid of the Langevin equation in an overdampedmode. The coupled Langevin equations for the shape and K-mode collective coordinates were integrated simultaneously. The friction parameter for the K mode was set to 0.077 (MeV × 10−21 s)−1/2, which is the estimate obtained previously for this quantity in calculating angular distributions of excited compound nuclei with allowance for the effects of the orientation degree of freedom. The developed model was used to analyze the anisotropy of angular distribution of fission fragments in 16O+208Pb, 16O+232Th, and 16O + 238U reactions over a broad interval of projectile-ion energies. The results of the calculations show that the developedmodel, usedwith the above value of the friction parameter for theK mode, leads to a rather good description of experimental data on the anisotropy of angular distributions of fission fragments. The effect of the dimensionality of the dynamicalmodel used to describe the evolution of the shape of a fissile nucleus on the results obtained by calculating the anisotropy of angular distributions is discussed.

Quasifission and Fusion-Fission Competition in32S +184W Reaction

The angular distribution of fission fragments for the 32 S+ 184 W reaction at center-of-mass energies of 118.8, 123.1, 127.3, 131.5, 135.8, 141.1 and 144.4 MeV were measured. The experimental fission excitation function is obtained. The fragment angular anisotropy is found by extrapolating the fission angular distributions. The measuredfission cross sections aredecomposed into fusion-fission, quasifission and fast-fission contributions by the dinuclear system model. Thetotal evaporation residue and fusion-fission excitation functions are calculated in the framework of the advanced statistical model.

DYNAMICAL STUDY OF ANISOTROPY IN ANGULAR DISTRIBUTION OF FISSION FRAGMENTS IN 16O+181Ta AND 16O+208Pb FUSION–FISSION REACTIONS

The three-dimensional Langevin equation (LE) and Transition State Model (TSM) are applied to calculate prescission neutron multiplicity and anisotropy of fission fragment angular distribution for 16O+181Ta and 16O+208Pb systems. Dynamical calculations were carried out considering one-body dissipation. Theoretical results of pre-scission neutron multiplicity and anisotropy have been compared with experimental data for given systems. Calculations show that the results of anisotropy based on dynamical approach are in better agreement with the experimental data as compared with the TSM (in saddle point).

Calculation of spin distribution for several fission reaction systems induced by nucleons and heavy ions

Compound nucleus spin distribution has been calculated for several fission reaction systems induced by nucleons and heavy ions. Determination of the spin distribution for these systems is based upon the comparison between the experimental data of the fission fragment angular distributions as well as the prediction of the standard saddle-point statistical model (SSPSM). For the systems, the two cases, namely with and without neutron emission corrections were considered. This method is used for the first time to determine compound nucleus spin distribution. Afterwards, our theoretical results have been compared with the data obtained from the coupled-channel technique as well as the Wong model, and satisfactory agreements were found. Furthermore, we have introduced a semiclassical approximation relation for the compound nucleus spin distribution of these systems.

DYNAMIC-STATISTICAL APPROACH TO THE DESCRIPTION OF THE INDUCED FISSION IN WIDE EXCITATION ENERGY RANGE

It is developed a new approach to analyze the induced fission of heavy nuclei, which takes into account the nuclear dissipation phenomenon and the shell structure of nuclei in determination of fission barriers. The approach is based on the combined dynamical and statistical simulations. In the framework of this approach the method of calculation the fission fragment angular distributions in a wide energy range is discussed. The ability of the method is demonstrated in the simultaneous description of the experimental data of the fission probabilities on the Am and Pu nuclei, yields of the spontaneous fission of isomers for the α + 238 U reaction at E α = (20 - 32) MeV and the d + 242,240 Pu reactions at E d = (20 - 30) MeV , the fission times for the α + 238 U reaction and angular distribution of fission fragments for the α + 238 U , 237 Np reactions at E α = (20 - 100) MeV . A further information on the value of the nuclear dissipation at low excitation energies is obtained. In addition, it is demonstrated that the influence of the shell effects on the deformation potential energy holds up to excitation energies of about (50 - 60) MeV.