Post-scission Neutron Multiplicities Research Articles

Fission as a relevant probe of the nuclear level density away from β-stability

Fission at moderate to high excitation energy is proposed as an innovative approach to study the evolution of the nuclear level density (NLD) away from β-stability. A four-dimensional dynamical Langevin model, implementing various prescriptions of the asymptotic level-density parameter a˜, is employed to demonstrate the relevance of considering typical fission observables, and namely the pre- and post-scission neutron multiplicities, to investigate the evolution of a˜ with N and Z. The richness of fission is shown to offer an alternative route, that is very complementary to the conventional fusion-evaporation method from which no consensus emerged yet. In particular, to investigate the isospin-dependence of the NLD, the present calculations lead us to propose a series of fusion-induced fission measurements based on Ca + Pb collisions which are readily available at current experimental facilities. This set of reactions can be further enhanced with today, as well as future, radioactive beams.

Inference on fission timescale from neutron multiplicity measurement in 18O + 184W

The pre-scission and post-scission neutron multiplicities are measured for the 18O + 184W reaction in the excitation energy range of 67.23–76.37 MeV. Langevin dynamical calculations are performed to infer the energy dependence of fission decay time in compliance with the measured neutron multiplicities. Different models for nuclear dissipation are employed for this purpose. Fission process is usually expected to be faster at a higher beam energy. However, we found an enhancement in the average fission time as the incident beam energy increases. It happens because a higher excitation energy helps more neutrons to evaporate that eventually stabilizes the system against fission. The competition between fission and neutron evaporation delicately depends on the available excitation energy and it is explained here with the help of the partial fission yields contributed by the different isotopes of the primary compound nucleus.

Probing entrance channel effects in fusion-fission dynamics through neutron multiplicity measurement of 208Rn

The pre- and post-scission neutron multiplicities are measured for the fission of 208Rn, populated with the reactions 30Si+178Hf and 48Ti+160Gd in the excitation energy range of 54.5 - 80.8 MeV. We found significant differences in the pre-scission neutron multiplicities of these two systems. Also, both the reactions exhibit an enhancement in the pre-scission multiplicity with respect to the existing data for the 16O+194Pt reaction populating 210Rn. The measured multiplicities are compared with the theoretical predictions obtained from the Langevin dynamical model for fission. Dynamical simulations are also performed to estimate the capture cross-sections. Contribution of the neutrons, emitted during the fusion process, is shown to be crucial in determining the neutron multiplicities for the present systems. Moreover, in case of the 48Ti+160Gd reaction, rapid quasi-fission dynamics prevents neutron emission from the thermalized target-projectile composite. The present measurement helps us to understand the relative importance of fusion and quasi-fission processes and their timescales depending on the entrance channel mass asymmetry.

Pre-scission neutron multiplicity in the S32+W184 reaction

The neutron energy spectra in coincidence with both the correlated fission fragments in $^{32}\mathrm{S}+^{184}\mathrm{W}$ reaction forming the compound nucleus $^{216}\mathrm{Th}$ at an excitation energy of 71.31 MeV have been measured at various angles with respect to one of the fission fragments. Pre- and post-scission neutron multiplicities of $2.68\ifmmode\pm\else\textpm\fi{}0.48$ and $3.3\ifmmode\pm\else\textpm\fi{}0.20,$ respectively, have been extracted by analyzing angular correlation of the neutron energy spectra using moving source fitting procedure. The excess pre-scission neutron multiplicity in comparison to statistical model joanne2 code predictions have been converted into total fission delay. A comparative study of fission delay has been carried out for the $^{16}\mathrm{O}+^{208}\mathrm{Pb}$ reaction forming the compound nucleus $^{224}\mathrm{Th}$ at similar excitation energy as in $^{32}\mathrm{S}+^{184}\mathrm{W}$ reaction. Both the reactions have been analyzed within the frame work of joanne2 code including fission delay, deformation dependent level densities, particle binding energies, and transmission coefficients. The observed fission delay for both the reactions can be quantitatively understood if different formation time predicted by dynamical model hicol code are considered.

Systematic study ofPo192,202,206,210compound nuclei using neutron multiplicity as a probe

In the present work we have measured pre- and post-scission neutron multiplicities $({M}_{\mathrm{pre}}$ and ${M}_{\mathrm{post}})$ from two compound nuclei, namely $^{192,202}\mathrm{Po}$ populated by $^{48}\mathrm{Ti}+^{144,154}\mathrm{Sm}$ systems at 72 MeV of excitation energy using the National Array of Neutron Detectors (NAND) facility at IUAC, New Delhi. Statistical model analysis has been performed for $^{48}\mathrm{Ti}+^{144,154}\mathrm{Sm}$ along with already existing data for $^{12}\mathrm{C}+^{194}\mathrm{Pt}$ and $^{18}\mathrm{O}+^{192}\mathrm{Os}$ covering compound nuclei of Po $(^{192,202,206,210}\mathrm{Po})$ with neutron number ${N}_{C}=108$, 118, 122, and 126 respectively. Variation of experimental ${M}_{\mathrm{pre}}$ with $N/Z$ of the compound nucleus does not show any special feature at neutron shell closure $({N}_{C}=126)$ at compound nucleus excitation energy around 72 MeV considered here. In particular, it is found that dissipation alone is unable to reproduce the experimental ${M}_{\mathrm{pre}}$ for $^{192}\mathrm{Po}$, and the role of entrance channel dynamics should be considered in future works.

Measurement of mass-gated neutron multiplicity for the Ti48+Pb208 reaction at 57.4 MeV excitation energy

The neutron emission in coincidence with the fragments have been investigated for the $^{48}\mathrm{Ti}+^{208}\mathrm{Pb}$ reaction populating the near superheavy compound nucleus $^{256}\mathrm{Rf}$ at an excitation energy of 57.4 MeV. The National Array of Neutron Detectors facility is used for a precise determination of the prescission (${M}_{n}^{\mathrm{pre}}$) and postscission (${M}_{n}^{\mathrm{post}}$) neutron multiplicities as a function of fission observables. A moving source fitting procedure has been adopted to deduce ${M}_{n}^{\mathrm{pre}}$ and ${M}_{n}^{\mathrm{post}}$. The variation of ${M}_{n}^{\mathrm{pre}}$ with the mass-split and total kinetic energy (TKE) of the fission fragments have been studied to understand the fission dynamics of $^{256}\mathrm{Rf}$. It is observed that ${M}_{n}^{\mathrm{pre}}$ increases from the value of 1.66 $\ifmmode\pm\else\textpm\fi{}$ 0.07 to 2.23 $\ifmmode\pm\else\textpm\fi{}$ 0.07 with transition from the asymmetric to the symmetric mass region. ${M}_{n}^{\mathrm{pre}}$ is also found to increase with the decrease in TKE, which is probably due to the neutron emission during the acceleration time of the fission fragments in this heavy system. The experimental results for neutron multiplicity have also been compared with the theoretical predictions from the statistical model calculations. From this comparison, the value of reduced dissipation strength for the $^{256}\mathrm{Rf}$ nucleus is found to be (13.0 $\ifmmode\pm\else\textpm\fi{}$ 1.0) $\ifmmode\times\else\texttimes\fi{} {10}^{21} {\mathrm{s}}^{\ensuremath{-}1}$ and a fission delay time of (${67.3}_{\ensuremath{-}3.9}^{+5.3}$) $\ifmmode\times\else\texttimes\fi{} {10}^{\ensuremath{-}21}$ s has also been estimated. For the spontaneous fission of $^{256}\mathrm{Rf}$, the extracted average neutron multiplicity ${M}_{n}^{sf}$ is found to be 4.4 $\ifmmode\pm\else\textpm\fi{}$ 1.0 which is in good agreement with the recently reported value for the $^{258,260}\mathrm{Rf}$ isotopes.

Dissipative effects in fission investigated with proton-on-lead reactions

The complete kinematic measurement of the two fission fragments permitted us to investigate dissipative effects at large deformations, between the saddle-point and the corresponding scission configurations. Up to now, this kind of study has only been performed with fusionfission reactions using a limited number of observables, such as the mass distribution of the fission fragments or the neutron multiplicities. However, the large angular momenta gained by the compound nucleus could affect the conclusions drawn from such experiments. In this work, the use of spallation reactions, where the fissioning systems are produced with low angular momentum, small deformations and high excitation energies, favors the study of dissipation, and allowed us to define new observables, such as postscission neutron multiplicities and the neutron excess of the final fission fragments as a function of the atomic number of the fissioning system. These new observables are used to investigate the dissipation at large deformations.

Probing fusion-fission dynamics inBi203

Background: Complete fusion between two massive nuclei after capture inside the potential barrier is inhibited by competing fission-like processes. The target-projectile composite system may reseparate after capture without proceeding towards formation of the compound nucleus (CN), which is equilibrated in all degrees of freedom. The nature of these non-CN fission (NCNF) processes and factors that affect them are not completely known yet.Purpose: The nuclear mass regions from where NCNF processes begin to manifest themselves are not clearly demarcated. This work aims to study the onset of NCNF, if any, in the mass region $\ensuremath{\sim}200$.Methods: Fission fragment (FF) mass and angular distribution (MAD) and pre-scission and post-scission neutron multiplicities were measured for the reaction $^{19}\text{F}+^{184}\text{W}$ at a laboratory energy $({E}_{\mathrm{lab}})$ range of 84--125 MeV. The measurements were carried out using two multiwire proportional counters (MWPC) to detect the FFs in coincidence and four neutron detectors to measure neutron time of flight (TOF). Statistical model (SM) calculation was performed.Results: No significant mass-angle correlation was observed in the MAD plots. Extracted mass ratio distributions were single-peaked and of Gaussian shape. Measured pre-scission neutron multiplicity values indicated dissipative nature of CN decay for this reaction.Conclusions: No clear signatures of NCNF were observed in the studied reaction, indicating that the target-projectile composite system predominantly proceeds towards formation of the CN after capture.

Effect of shell structure on neutron multiplicity of fissioning systems220,222,224Th nuclei

The pre- and post-scission neutron multiplicities have been extracted for the 220,222,224Th nuclei for the excitation energy range of 40 MeV to 64 MeV using the National Array of Neutron Detectors (NAND). The Th isotopes are populated from the fusion reaction of 16O+204,206,208Pb systems in order to investigate the dynamics of fusion-fission reactions using the neutron multiplicity as a probe. The theoretical calculations were performed using the Bohr-Wheeler fission width as well as the dissipative dynamical fission width from Kramers prescription. It is observed that the Bohr-Wheeler fission width underestimates the pre-scission yields to a large extent. A large amount of dissipation is required in the Kramers width to fit the observed pre-scission neutron multiplicities.

Neutron multiplicity measurements for19F+194,196,198Pt systems to investigate the effect of shell closure on nuclear dissipation

Pre- and post-scission neutron multiplicities are measured for the three isotopes of Fr (${}^{217}$Fr, ${}^{215}$Fr, and ${}^{213}$Fr) in the excitation energy range of 48--91.8 MeV. Out of these three isotopes, ${}^{213}$Fr has shell closure (${N}_{C}=126$) while the other two are non-closed-shell nuclei. Statistical model calculations using Kramers' fission width are performed to investigate shell effects on the dissipation strength which fit the experimental data. It is observed that shell correction to the binding energies of the evaporated particles strongly affects the fitted values of the dissipation strength. However, the best-fit dissipation strength is only weakly influenced by the inclusion of shell correction in fission barrier.

Fission properties of actinide nuclei from proton-induced fission at 26.5 and 62.9 MeV incident proton energies

Fission properties of proton-induced fission on {sup 232}Th, {sup 237}Np, {sup 238}U, {sup 239}Pu, and {sup 241}Am targets, measured at the Louvain-la-Neuve cyclotron facility at proton energies of 26.5 and 62.9 MeV, are compared with the predictions of the state-of-the-art nuclear reaction code talys. The code couples the multimodal random neck-rupture model with the pre-equilibrium exciton and statistical models to predict fission fragment mass yields, pre- and post-scission neutron multiplicities, and total fission cross sections in a consistent approach. The sensitivity of the calculations to the input parameters of the code and possible improvements are discussed in detail.

Measurement of neutron multiplicity from fission ofU228and nuclear dissipation

Pre- and post-scission neutron multiplicities are measured at different excitation energies of the compound nucleus $^{228}\mathrm{U}$ populated using the $^{19}\mathrm{F}+^{209}\mathrm{Bi}$ reaction. The measured yield of pre-scission and total neutrons are compared with the statistical model calculation for the decay of a compound nucleus. The statistical model calculations are performed using the Bohr-Wheeler transition state fission width as well as the dissipative dynamical fission width due to Kramers. Comparison between the measured and the calculated values shows that, while the Bohr-Wheeler fission width grossly underestimates the pre-scission neutron yield, a large amount of dissipation is required in the Kramers width to fit the experimental pre-scission multiplicities. Various factors contributing to the large excitation energy dependence of the fitted values of the dissipation coefficient are discussed.

Entrance channel effects in fission ofTl197

The pre- and post-scission neutron multiplicities are measured for $^{16}\mathrm{O}+^{181}\mathrm{Ta}$ and $^{19}\mathrm{F}+^{178}\mathrm{Hf}$ systems where the same compound nucleus $^{197}\mathrm{Tl}$ is formed at the same excitation energies (${E}^{*}=72,$ 76, and 81 MeV). The measured pre-scission neutron multiplicities are found to be different for the two reactions and this difference in neutron yield increases with the excitation energy of the compound nucleus. The experimental pre-scission neutron yield is compared with predictions from the statistical model of compound nuclear decay containing the strength of nuclear viscosity as a free parameter. The magnitude of nuclear viscosity required to fit the experimental yield is found to be different for the two reactions. Because the two systems under consideration lie on the two sides of the Businaro-Gallone point, this observation indicates that the entrance channel mass asymmetry plays an important role in determining the number of neutrons emitted prior to scission in fusion-fission reactions.

Particle multiplicities in the fission-like reactions of 340 MeV 28Si on 232Th

Pre-scission and post-scission multiplicities of neutrons and alpha particles have been simultaneously measured for the fission-like reactions of 340 MeV 28Si on 232Th. Dynamical model calculations using HICOL code predict that about 90% of the observed events are of quasi-fission type while the remaining 10% are from compound nucleus fission decay. Moving source fits were carried out to the observed neutron and alpha particle spectra, measured at different angles with respect to the fragment directions. The pre-scission and post-scission neutron multiplicities are deduced to be 8.7±2.0 and 9.4±2.0, respectively. The corresponding multiplicity values for alpha particles are found to be 0.22±0.08 and 0.1±0.03. From the measured post-scission neutron multiplicity, it is inferred that about 65±20 MeV of the initial excitation energy remains at scission. This may be compared to the value of 85±30 MeV estimated from PACE2 statistical model calculations, adjusted to reproduce the measured pre-scission neutron multiplicity. From a comparison of the Statistical Model predictions with the measured pre-scission neutron multiplicity, the fission delay is estimated to be of 5 +7 −3×10 −20 s which overlaps with the average duration of fission-like process from the contact to the scission point (2×10 −20 s) as determined from HICOL-based dynamical calculations. For the delay time deduced as above, the pre-scission alpha particle multiplicity calculated by the PACE2 code is about a factor two larger than the experimental one, demonstrating the difficulties in modelling the alpha particle emission from highly elongated shapes that characterize the fissioning system from the contact point to scission.

Reduction coefficient in surface-plus-window dissipation: Analysis of experimental data from fusion-fission reactions within a stochastic approach

A stochastic approach based on three-dimensional Langevin equations was applied to detailed study of fission dynamics in fusion-fission reactions. The reduction coefficient in surface-plus-window dissipation is deduced by analyzing the available experimental data on various observable quantities in fusion-fission reactions. Analysis of the results shows that not only characteristics of the mass-energy distribution of fission fragments but also the mass and kinetic-energy dependence of the prescission and postscission neutron multiplicities, the angular anisotropy, and fission probability can be reproduced using surface-plus-window dissipation with the reduction coefficient from a wall formula ks =0 .25-0.5 for com- pound nuclei lighter than 224 Th. The analysis performed reveals that the coordinate-independent reduction coefficient ks is not compatible with the simultaneous description of the main fission characteristics for fissioning systems heavier than 224 Th. c � 2003 MAIK "Nauka/Interperiodica".

Pre- and post-scission neutron multiplicities in $19F$ + $209Bi$ reaction at 108 MeV

The pre- and post-scission neutron multiplicities have been determined from the fragment-neutron angular-correlation measurements in 19F + 209Bi reaction at bombarding energy of 108 MeV. The average pre-scission neutron multiplicity ( νpre) is significantly larger than that calculated on the basis of the statistical model. The excess in νpre value is used to estimate the dynamical delay in the fusion-fission time scale for the 228U compound system. For a level density parameter a n = A/10 MeV-1 and a n = a f, the fusion-fission dynamical time was established to be 120 ± 10 × 10-21 s. We also observe that νpre is higher for asymmetric mass splits as compared to symmetric mass splits. The enhancement in the νpre for the asymmetric region could be due to strong variations in the fragment mass distribution at different stages in multiple chance fission at medium excitation energies.

Neutron and fragment yields in proton-induced fission of 238U at intermediate energies

The primary fission fragment mass and kinetic energy distributions, and neutron multiplicities as function of fragment mass have been measured in the proton-induced fission of 238U at energies E p=20, 35, 50 and 60 MeV using time-of-flight technique. Pre-scission and post-scission neutron multiplicities have been extracted from double differential distributions. The fragment mass dependence of the post-scission neutron multiplicities reveals the gross nuclear shell structure effect even at the higher proton energies we measured. The yields of neutron-rich fission products in the fission of 238U by 25 MeV protons were measured using an ion guide-based isotope separator technique. The results indicate enhancement for superasymmetric mass division at intermediate excitation energy of the fissioning nucleus. The experimental results have been analysed in the framework of a time-dependent statistical model with inclusion of nuclear friction effects in the fission process.

Temperature dependence of friction from pre- and post-scission neutron emission

The magnitude of nuclear friction has been investigated in recent years by measuring pre- and post-scission neutron multiplicities. These data have been analysed mostly in the frame of the statistical model by introducing ad-hoc delays to go from equilibrium to saddle and then to scission. We present here a more simple approach based on remaining excitation energies at saddle. This leads to analytical formulae which reproduce almost all the published data. The temperature at which one observes transition from pre-scission to post-scission emission is the only parameter in the calculation. Its values show no significant variation with the total mass nor with bombarding energy, supporting the concept of temperature dependent friction. The mean value averaged over all published data is 1.11±0.05 MeV.

Fission dynamics for capture reactions in [formula omitted] systems: New results in terms of thermal energy and neutron multiplicity correlated distributions

The neutron multidetector DéMoN has been used to investigate the symmetric splitting dynamics in the reactions 58,64Ni + 208Pb with excitation energies ranging from 65 to 186 MeV for the composite system. An analysis based on the new backtracing technique has been applied on the neutron data to determine the two-dimensional correlations between the parent composite system initial thermal energy ( E CN th ) and the total neutron multiplicity ( ν tot), and between pre- and post-scission neutron multiplicities ( ν pre and ν post, respectively). The ν pre distribution shape indicates the possible coexistence of fast-fission and fusion-fission for the system 58Ni + 208Pb ( E beam = 8.86 A MeV). The analysis of the neutron multiplicities in the framework of the combined dynamical statistical model (CDSM) gives a reduced friction coefficient β = 23 ± 12 25 × 10 21 s −1, above the one-body dissipation limit. The corresponding fission time is τ f = 40 ± 20 46 × 10 −21 s.

Thermal and dynamical properties of fusion-fission and fusion-evaporation processes in 20Ne + 159Tb and 20Ne + 169Tm reactions between 8 and 16 MeV/nucleon

We have undertaken a study of the thermal and the dynamical properties of fission and evaporation processes through the observation of neutron and light charged particle (LCP) emission (p, α) in collisions of 20 Ne + 159 Tb and 20 Ne + 169 Tm at 8, 10, 13 and 16 MeV/nucleon using the neutron multidetector DEMON. Both systems lead to the formation of compound nuclei (CN) with initial excitation energies ( E x * ) ranging between 100 and 250 MeV. These CN, formed through complete (CF) or incomplete (IF) fusion reactions, decay through either fission or evaporation residue (ER) formation. Our experimental goal in this study is to establish, as a function of E x * , i) σ ER /σ fission cross-section ratios, ii) pre- and post-scission neutron and LCP multiplicities with a clear cut assignment to CF and IF processes and iii) to try to consistently interpret these data in the framework of the recently modified statistical code GEMINI, including time-dependent fission width probabilities in order to establish the fission time scales.