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Binary fragmentation of the near super-heavy compound nucleus Rf has been studied through the reaction Ti + Pb at a bombarding energy well above the Coulomb barrier. For a better understanding of its reaction dynamics, the mass distribution, mass-energy distribution and mass-angle distribution of the fission fragments produced from Rf have been investigated thoroughly. The masses and kinetic energies of the fission fragments were reconstructed event-by-event from their measured velocities and emission angles. From the mass-energy analysis, a sizeable contribution from the asymmetric fission was observed on the edges of symmetric mass distribution. Evidence of asymmetric fission was also clued from the observed correlation between the masses and emission angles of the fission fragments. Contribution of the quasi-fission products has also been estimated by performing the theoretical dinuclear system calculations. [ABSTRACT FROM AUTHOR]

225Pa nuclei exhibit a fusion-fission mechanism below the fusion barrier energy. Mass angle distribution measurements indicate absence of noncompound events at this bombarding energy. The effect of asymmetric fission mode on mass distribution of fission fragments is visible at low excitation energy (31.8 MeV). [ABSTRACT FROM AUTHOR]

Two large area multistep position sensitive (two dimensional) multiwire proportional counters have been developed for experiments involving study of fission dynamics using general purpose scattering chamber facility at IUAC. Both detectors have an active area of 20×10 cm2 and provide position signals in horizontal (X) and vertical (Y) planes, timing signal for time of flight measurements and energy signal giving the differential energy loss in the active volume. The design features are optimized for the detection of low energy heavy ions at very low gas pressures. Special care was taken in setting up the readout electronics, constant fraction discriminators for position signals in particular, to get optimum position and timing resolutions along with high count rate handling capability of low energy heavy ions. A custom made charge sensitive preamplifier, having lower gain and shorter decay time, has been developed for extracting the differential energy loss signal. The position and time resolutions of the detectors were determined to be 1.1 mm full width at half maximum (FWHM) and 1.7 ns FWHM, respectively. The detector could handle heavy ion count rates exceeding 20 kHz without any breakdown. Time of flight signal in combination with differential energy loss signal gives a clean separation of fission fragments from projectile and target like particles. The timing and position signals of the detectors are used for fission coincidence measurements and subsequent extraction of their mass, angular, and total kinetic energy distributions. This article describes systematic study of these fission counters in terms of efficiency, time resolution, count rate handling capability, position resolution, and the readout electronics. The detector has been operated with both five electrode geometry and four electrode geometry, and a comparison has been made in their performances. [ABSTRACT FROM AUTHOR]

The fission dynamics has been studied for a near super heavy compound nucleus 260Rf populated through 28Si + 232Th reaction at an excitation energy of 85.7 MeV. Full momentum transfer binary events were separated from the transfer induced fission events. The contribution from transfer induced fission has been found to be 7 ± 2 %. Mas ratio distribution, mass-total kinetic energy (TKE), and mass angle correlation have been extracted for the full momentum transfer events using two body kinematics. The experimentally extracted width of mass distribution is higher than the mass width calculated theoretically using the saddle-point model, which indicates the presence of non-compound nuclear fission in the reaction under study. The mass-TKE distribution obtained for 260Rf nucleus matches with the theoretical predictions from the Viola systematics and GEneral description of Fission observables (GEF) model. The mass-angle distribution for the reaction under study indicates no significant correlation between the mass and emission angles of the fission fragments. [ABSTRACT FROM AUTHOR]

Fission fragment mass–angle and mass ratio distributions have been measured for the 30 Si + 180 Hf reaction in the beam energy range 128–148 MeV. Quasifission signature is observed in this reaction, forming the compound system 210 Rn. The results are compared with a very asymmetric reaction 16 O + 194 Pt, forming the same compound nucleus. Calculations assuming saddle point, scission point and DNS models have been performed to interpret the experimental results. The results strongly suggest the entrance channel dependence of quasifission in heavy ion collisions. [ABSTRACT FROM AUTHOR]

Background: Detailed studies on the effect of the breakup of weakly bound projectile on fission are scarce. Distinguishing the events of compound nuclear (CN) fission from the breakup or transfer induced fission to understand the properties of measured fission fragments is difficult but desirable. Purpose: To investigate the effect of projectile breakup and its breakup threshold energy on fission-fragment (FF) mass distributions and folding angle distributions for 6,7Li + 238U reactions and find out the differences in the properties of the fission events produced by complete fusion (CF) from the total fusion (TF). Methods: The FF mass and folding angle distributions have been measured at energies around the Coulomb barrier using gas detectors by time-of-flight technique. The results are compared with the ones involving tightly bound projectiles as well as predictions from systematics to bring out the effect of the breakup. Results: A sharp increase in the peak to valley (P: V) ratio of FF mass distribution with the decrease in bombarding energy for 6,7Li + 238U reactions is observed when all events are assumed to be CN fission. As the beam energy falls through the fusion barrier, the full width half maximum (FWHM) of the FF folding angle distribution is found to increase at sub-barrier energies, unlike the reactions involving tightly bound projectiles where a linear decrease in FWHM is expected. By selecting pure CN events from the scatter plot of the velocity components of the composite nuclei, the energy dependence of the deduced FWHM is found to be consistent with the ones involving tightly bound projectiles. Similarly, the P:V ratio obtained for the selected CN events is consistent with the theoretical calculations as well as the experimental data for the proton induced reaction forming similar CN. Conclusions: The presence of projectile breakup induced fission and a relatively low breakup threshold for 6Li compared to 7Li explains the observed differences in the energy dependence of the P:V ratio and the FWHM of FF folding angle distributions for CF and TF fission in the present reactions. [ABSTRACT FROM AUTHOR]

Pre- and post-scission neutron multiplicities are measured for the three isotopes of Fr (217Fr, 215Fr, and 213Fr) in the excitation energy range of 48-91.8 MeV. Out of these three isotopes, 213Fr has shell closure (Nc = 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 [ABSTRACT FROM AUTHOR]

Pre-scission neutron multiplicities from fission of the compound nuclei 210,212,214,216Rn have been measured in order to investigate the N/Z dependence of fission hindrance. The Rn isotopes are populated through the fusion of 16.18O + 194.198pt systems an(j are formed at four excitation energies in the range of 50-79 MeV. The experimental pre-scission neutron yield is compared with predictions from the statistical model of compound nuclear decay containing the strength of nuclear dissipation as a free parameter. The strength of nuclear dissipation obtained in the present analysis does not show any specific dependence on the N/Z values of the fissioning nuclei at large excitation energies. The dissipation strength at the lowest excitation energy, however, indicates a shell closure effect at N = 126 for the 212Rn isotope. [ABSTRACT FROM AUTHOR]

The article discusses the experimental developments in nuclear physics. It presents the study which investigated the shell effects on fission hindrance through the measurement of the precision neutron multiplicity excitation function. Based on the statistical model analysis of the experimental data, the strengths of nuclear dissipation for nuclei away from shell closure are very similar.

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 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. [Copyright &y& Elsevier]

The fission fragment mass angle correlations and mass ratio distributions have been investigated for the two systems 16O+197Au and 27Al+186W, leading to the same compound nucleus 213Fr around the Coulomb barrier energies. Systematic analysis of the variance of the mass distributions as a function of temperature and angular momentum suggests true compound nuclear fission for both the reactions, indicating the absence of nonequilibrium fission processes. [ABSTRACT FROM AUTHOR]

Abstract: Evaporation residue (ER) cross sections and gamma multiplicity distributions have been measured for 16O+ 184W and 19F+ 181Ta systems in the excitation energy range of 50–90 MeV, leading to the same compound nucleus 200Pb∗. Comparison of experimental results of both the systems shows that ER cross sections and moments of gamma multiplicity distribution of 16O+ 184W system are significantly higher than those of 19F+ 181Ta system at higher excitation energies. Present measurements directly shows the experimental signature of entrance channel effect even with the systems which are not very different with respect to their entrance channel mass asymmetry. It is further demonstrated that the reduction in the ER cross section and moments of spin distribution for 19F+ 181Ta system is mainly due to the suppression of fusion of higher l values. [Copyright &y& Elsevier]

Neutron multiplicity excitation function has been measured for the 30Si+197Au reaction populating the 227Np compound nucleus at excitation energies in the range 44.1-78.8 MeV using the National Array of Neutron Detector facility of Inter University Accelerator Centre, New Delhi. Measured pre-scission neutron multiplicity values are analyzed using a statistical model incorporating Krammer's fission width due to the dissipative drag in nuclear fission, shell corrections in fission barrier and level density, collective enhancement of level density, and K-orientation effect. The present work demonstrates that a strong fission hindrance is essential to reproduce the experimental pre-scission neutrons, whereas the temperature dependent dissipation coefficient as observed in a few recent measurements is not required to reproduce the experimental νpre data. No substantial effect of collective enhancement of nuclear level density and tilting away effect of compound nucleus spin on neutron emission prior to the scission configuration was observed unlike fission of preactinides. [ABSTRACT FROM AUTHOR]

The multinucleon transfer reaction or incomplete fusion reaction (ICF) is a powerful tool to study fission of exotic nuclei that cannot be formed by stable heavy-ion fusion reactions. In the present work, mass distributions of fission fragments (FFs) from fissioning nuclei 241,242,243,244Pu and 240,241Np populated in multinucleon transfer or ICF reactions on 6,7Li+238U systems have been studied. Among these, 244Pu,243Pu, and 241Np are formed by the capture of unstable nuclei 6He,5He, and t, respectively, by the target 238U. Identification of fissioning nuclei and determination of excitation energies have been performed by finding the details of the outgoing projectile-like fragments detected in coincidence with both the fission fragments on an event-by-event basis. The measurements of FF mass distributions and FF folding angle distributions of different ICF channels confirm that these channels are the prime factors for the modifications in the experimental ratio of asymmetric to symmetric fission yields and the width of folding angle distributions for inclusive fission reported earlier on the same reactions. Comparison among the ratio of asymmetric to symmetric fission yields from 241,242,243,244Pu and 240,241Np nuclei formed in the present reactions, available literature data, and the theoretical calculations using gef code shows that the shell correction for symmetric fission channels plays an important role in describing the experimental mass distribution. [ABSTRACT FROM AUTHOR]

Pre- and post-scission α-particle multiplicities have been measured for the reaction 16O+194Pt at 98.4 MeV forming 210Rn compound nucleus. α particles were measured at various angles in coincidence with the fission fragments. Moving source technique was used to extract the pre- and post-scission contributions to the particle multiplicity. Study of the fission mechanism using the different probes are helpful in understanding the detailed reaction dynamics. The neutron multiplicities for this reaction have been reported earlier. The multiplicities of neutrons and α particles were reproduced using standard statistical model code joanne2 by varying the transient (τtr) and saddle to scission (τssc) times. This code includes deformation dependent-particle transmission coefficients, binding energies and level densities. Fission time scales of the order of 50-65 ×10-21 s are required to reproduce the neutron and α-particle multiplicities. [ABSTRACT FROM AUTHOR]

Background: Complete fusion of two nuclei leading to formation of a heavy compound nucleus (CN) is known to be hindered by various fission-like processes, in which the composite system reseparates after capture of the target and the projectile inside the potential barrier. As a consequence of these non-CN fission (NCNF) processes, fusion probability (PCN) starts deviating from unity. Despite substantial progress in understanding, the onset and the experimental signatures of NCNF and the degree of its influence on fusion have not yet been unambiguously identified. Purpose: This work aims to investigate the presence of NCNF, if any, in pre-actinide nuclei by systematic study of fission angular anisotropies and fission cross sections (sfis) in a number of nuclear reactions carried out at and above the Coulomb barrier (VB). Method: Fission fragment angular distributions were measured for six 28Si-induced reactions involving isotopically enriched targets of 169Tm,176Yb,175Lu,180Hf,181Ta, and 182W leading to probable formation of CN in the pre-actinide region, at a laboratory energy (Elab) range of 129-146 MeV. Measurements were performed with large angular coverage (θlab=41∘-170∘) in which fission fragments (FFs) were detected by nine hybrid telescope (E−ΔE) detectors. Extracted fission angular anisotropies and σfis were compared with statistical model (SM) predictions. Results: Barring two reactions involving targets with large non-zero ground state spin (J), viz., 175Lu... and 181Ta..., experimental fission angular anisotropies were found to be higher in comparison with predictions of the statistical saddle point model (SSPM), at Ec.m. near VB. Comparison of present results with those from neighboring systems revealed that experimental anisotropies increasingly deviated from SSPM predictions as one moved from pre-actinide to actinide nuclei. For reactions involving targets with large nonzero J, this deviation was subdued. Comparison between measured sfis and predictions of SM indicated the presence of NCNF in at least four systems, when shell effects, both in the level density and the fission barrier, were included in the calculation. Conclusions: Systematic SM analysis of measured fission angular anisotropies and σfis confirmed the onset of NCNF in pre-actinide nuclei. Discrepancies between results about the degree of its influence on complete fusion, as deduced from various experimental probes, remain challenges to be solved. Complete measurement of all signatures of NCNF for many systems and preferably a dynamical description of the collisions between projectile and target nuclei are warranted for a deeper understanding. [ABSTRACT FROM AUTHOR]

Abstract: Fission fragment mass angle correlation and mass ratio distributions have been measured for 16O + 194Pt reaction forming the composite system 210Rn in the energy range 75 MeV to 102 MeV in laboratory frame. No mass angle correlation was observed and the mass ratio distributions were compared with theoretical calculations assuming compound nucleus formation. No evidence of quasi-fission process is observed in the reaction. [Copyright &y& Elsevier]