Experimental signature of entrance channel effect in heavy mass region via evaporation residue cross section and spin distribution measurements

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.     

The influence of the fission barrier on the mass distribution in reactions with symmetric fragmentation

The mass distributions of reaction products with symmetric fragmentation resulting from the bombardment of¹⁴⁴Sm,¹⁶⁵Ho,¹⁷⁵Lu,¹⁷⁶Hf,¹⁸¹Ta, and²⁰⁸Pb with⁴⁰Ar projectiles have been studied. The incident energies were chosen to populate nearly the same angular momentum range of the composite system while the nuclear temperature varies only weakly. The reaction products were identified by their energy and time of flight using a solid state detector placed at 60°. The width of the mass distribution was found to increase by nearly a factor of two as the liquid drop fission barrier of the different compound nuclei decreases from 11 to 2.2 MeV. For all systems, the survival of evaporation residues proves that compound fission contributes to the observed symmetric reaction component.     

Langevin study of neutron emission in the reactions 16O+181Ta and 19F+178Hf

The pre-scission neutrons measured in the reactions 16O+181Ta and 19F+178Hf are studied via a Langevin equation coupled with a statistical decay model.We find that because of the mass asymmetry of different entrance channels,the spin distributions of compound nuclei would be different,consequently,the measured neutrons in these two reactions would also different.This means that the entrance channel will affect the particle emission in the fission process of hot nuclei.     

Quasifission and difference in formation of evaporation residues in the O + W and F + Ta reactions

The role of the entrance channel has been studied to ascertain a cause of the observed difference between the evaporation residue cross sections normalized to the fusion cross sections in the ¹⁹F + ¹⁸¹Ta and ¹⁶O + ¹⁸⁴W reactions at high excitation energies. The theoretical analysis performed in the framework of the dinuclear system and advanced statistical models showed that the more intense yield of evaporation residues in the ¹⁶O + ¹⁸⁴W reaction in comparison with that in the ¹⁹F + ¹⁸¹Ta reaction was explained by the large capture and fusion cross sections in the former reaction, which is in agreement with the experimental data. The observed decrease in the evaporation residue cross section normalized to the fusion cross section in the ¹⁹F + ¹⁸¹Ta reaction, in comparison with one in the ¹⁶O + ¹⁸⁴W reaction at large excitation energies, is caused by the unintentional inclusion of the quasifission and fast fission contributions in the fissionlike fragment yields that were used in reconstructing the experimental fusion cross section in the normalizing procedure. The range of the angular momentum distribution for both systems was similar, but the partial cross sections are different, showing the presence of a difference in the hindrance to complete fusion in both reactions.     

Fission lifetimes of Th nuclei measured by crystal blocking

Crystal blocking lifetime measurements have been made for highly excited Th nuclei with neutron number well removed from the stability line. Thin W crystals were bombarded with ³²S ions in the energy range 170-180 MeV and the yield of fission fragments was measured for emission close to a 〈111〉 axis. The fission blocking dips are compared to the appropriately scaled ones for elastic scattering of the ³²S beam ions and no significant difference is seen between the dips. This implies that the fraction of nuclei fissioning with lifetimes longer than 10 as is less than 2%. Fission lifetimes are increased by viscosity in the nuclear mass flow and comparison with a statistical model calculation indicates that the viscosity parameter, η, must be lower than for Th and U nuclei near β-stability. The effect of the N = 126 magic number is discussed. Received: 2 October 2002 / Accepted: 16 January 2003 / Published online: 29 April 2003     

Measurement of fission anisotropy for 16O+181Ta

Anisotropies in fission fragment angular distributions measured for the system ¹⁶O + ¹⁸¹Ta over a range of bombarding energies from 83 MeV to 120 MeV have been analysed. It is shown that statistical transition state model (TSM) with pre-scission neutron correction described adequately the measured anisotropy data. Strong friction parameter is found to be necessary to estimate the pre-saddle to pre-scission neutron ratio.     

Ternary fission involving4He emission in the16O (144 MeV)+232Th and12C (108 MeV)+197Au reactions

The results of coincidence measurements of⁴He emission with fission fragments in reactions of¹²C(108 MeV) ions with a¹⁹⁷Au target and of¹⁶O(144 MeV) ions with a²³²Th target are presented. On the basis of a Monte Carlo kinematic simulation of nuclear reactions the experimental energy spectra and velocity distributions of alpha particles have been analyzed. A conclusion has been drawn that the main source of⁴ He emission is evaporation from the fissioning compound nucleus. Substantial part of alpha particles was emitted from fully accelerated fission fragments. Some of⁴He nuclei with an average energy of about 16 MeV (in the CM system) emitted mainly perpendicular to the fission axis were identified as being similar long-range alpha particles produced in ternary fission of heavy nuclei at a low excitation energy. The emission multiplicities of these particles are considerably higher than those observed at a low excitation energy. The experimental results are compared with the statistical model predictions.     

Variation of total kinetic energy and mass distributions in the fusion-fission reaction of 16O,19F + 209Bi at near barrier energies

Fission fragment mass and energy distributions and their correlations have been measured for the ¹⁶O and ¹⁹F + ²⁰⁹Bi reactions over a wide range of excitation energies ( E ^* = 30-50 MeV). It is observed that in the case of ¹⁶O + ²⁰⁹Bi reaction, the average total fragment kinetic energy, <TKE> is nearly independent of the bombarding energy. However, in the case of ¹⁹F + ²⁰⁹Bi reaction, the average total kinetic energy of the fission fragments shows a peaking behaviour near the barrier. The variation in <TKE> at near barrier energies in the ¹⁹F + ²⁰⁹Bi system seems to be correlated with corresponding strong variation in the variance of the fragment mass distribution. The present results may imply certain dynamical effects leading to compact scission configurations in the fission of ¹⁹F + ²⁰⁹Bi system at near barrier bombarding energies. Received: 9 April 2001 / Accepted: 26 May 2001     

ANGULAR DISTRIBUTIONS OF FISSION FRAGMENTS AND THE SHAPES OF THE SADDLE POINT OF COMPOUND NUCLEI

We have measured the angular distributions of fission fragments for a series of compound nuclei formed in the bombardment of ¹⁶⁹Tm, ¹⁷⁵Lu, ¹⁸¹Ta, ¹⁹⁷Au, ²⁰⁹Bi, ²³²Th and ²³⁸U by 78 MeV ¹²C ions and in the bombardment of natural lead by 96.6, 92.2, 88.1, 79.0 and 75.6 MeV ¹⁴N ions with mica track detectors. All the measured angular distributions can be fitted by the theoretical formula given by Halpern and Strutinski. The effective moments of inertia of the fissioning nuclei at the saddle point are derived from the angular distribution anisotropies. A comparison is made for the shapes of experimental saddle point with the theoretical predictions by the liquid drop model.     

MEASUREMENT OF FISSION FRAGMENT ANGULAR DISTRIBUTION IN HEAVY-IONS INDUCED REACTION

The angular distributions of fission fragments for a series of compound nuclei formed in the bombardment of ¹⁸¹Ta,¹⁹⁷Au Pb and ²⁰⁹Bi by ¹²C ions and in the bombardment of natural lead by ¹⁴N ions were measu ed with mica track detectors and gold surface barrier detectors.It is shown that all experimental data for the fission fragment angular distributions can be fitted by the statistical scission model of fission fragment angular distribution.     

Inclusive and exclusive measurements of high energy γ-rays in the 101 MeV <Superscript>19</Superscript>F +<Superscript>181</Superscript>Ta fusion reaction

The high energy γ-rays from the fusion-evaporation reaction 101 MeV ¹⁹F +¹⁸¹Ta have been measured in coincidence with different γ-ray fold windows or discrete γ-rays in final residual nuclei. The line-shape analysis of the high energy γ-ray spectra confirms the large deformation of the ²⁰⁰Pb nuclei at high angular momenta. Coincidences with discrete γ-transitions evidence a strong correlation between γ-rays in the Giant Dipole Resonance region (E_γ≥ 8 MeV) and final evaporation residues. Received: 14 September 1999 / Revised version: 17 December 1999     

FISSION EXCITATION FUNCTION IN INTERATIONS OF 12C IONS WITH VARIOUS TARGETS

Angular distributions of fission fragment and fission excitations was measured for a series of reactions ¹²C+¹⁶⁹Tm, ¹⁷⁵Lu, ¹⁸¹Ta, W, Re, Pt, ¹⁹⁷Au, Pb and ²⁰⁹Bi in the 57.0—73.0MeV energy region with mica detectors and gold surface barrier silicon detectors. The experimental values <Γ_f/Γ_n> we are abtained from fission excitation functions and compared to theoretical ones. The variation of the experimental values for the fission barrier height with the mass number A of the compound nuclei was studied and compared to the theoretical predictions by the liquid drop medel.     

Crystal blocking measurements of the time delay of fission induced by 32S, 48Ti, and 58Ni bombardment of W

The time delay in fission induced by bombardment of W with 180 MeV 32S, 240-255 MeV 48Ti, and 315-375 MeV 58Ni has been measured by observation of crystal blocking. There is a clear narrowing and a small increase in the minimum yield of the angular dips for fission compared with scaled dips for elastically scattered ions. This is interpreted as a fission delay of about 2 as, only weakly dependent on energy and atomic number. The delay is longer by 1 to 2 orders of magnitude than obtained from standard interpretations of measurements of prescission neutrons and giant-dipole-resonance gamma rays and from calculations of the nuclear dynamics in heavy-ion reactions.     

ANGULAR DISTRIBUTIONS OF FISSION FRAGMENTS IN INTERACTIONS OF 12C IONS WITH VARIOUS TARGETS

Angular distributions of fission fragments were measured for a series of compound nuclei formed in the bombardment of ¹⁶⁹Tm,¹⁷⁵Lu,¹⁸¹Ta,W,Re,Pt,¹⁹⁷Au,Pb and ²⁰⁹Bi by ¹²C ions with mica track detectors and gold surface barrier silicon detectors.All the measured angular distributions can be fitted satisfactorily by the theoretical formula based on saddle point model.The variation trend for K² with the excitation energy increasing was given at various ranges of the fission parameter Z²/A.     

Langevin study of neutron emission in the reactions ^16O＋^181Ta and ^19F＋^178Hf^＊

The pre-scission neutrons measured in the reactions 16O 181Ta and 19F 178Hf are studied via a Langevin equation coupled with a statistical decay model. We find that because of the mass asymmetry of different entrance channels, the spin distributions of compound nuclei would be different, consequently, the measured neutrons in these two reactions would also different. This means that the entrance channel will affect the particle emission in the fission process of hot nuclei.     

Evaporation residues from heavy (Z=90–93) compound nuclei at excitation energies of Ex ≃ (75–110) MeV

Evaporation residue cross sections have been measured in reactions⁴⁰Ar +¹⁴⁰Hf,¹⁸¹Ta,¹⁸²W,¹⁸⁴W,^natRe at bombarding energiesE/A=(5.5–6.3) MeV/u. The cross sections do not decrease with increasing nuclear temperature, as expected from statistical model calculations. Rather they are found to be roughly constant. This behavior is predominantly affected by an enhanced emission probability of charged particles. Influence of a ‘fission delay’, which is expected atE ^x>60 MeV from measurements of pre- and post-scission neutrons, on thexn- cross sections has been observed at excitation energiesE ^x>90 MeV. The possibility to produce transfermium isotopes in ‘hot fusion’ reactions has been investigated in an irradiation of²³²Th with⁴⁰Ar. Upper limits of 10 nb for evaporation residues were obtained.     

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.     

A possible mechanism in heavy ion induced reactions: “Fast fission process”

The influence of the orbital angular momentuml on the mass distribution of fission fragments is studied, both on previously available data on heavy ion induced fission and in new specifically planned experiments: systems⁴⁰Ar+¹⁶⁵Ho and²⁴Mg+¹⁸¹Ta at bombarding energies ranging from 180 up to 391 MeV and leading to the same fissioning nucleus²⁰⁵At with differentl distributions. Whenl values corresponding to a vanished fission barrier are reached, the mass distribution broadens. This suggest the existence of a specific process, “fast fission”, atl-values intermediate betweenl-values leading to compound nucleus formation and deep inelastic collisions, respectively. This process and its conditions of occurence are discussed; of special interest are the correlated differences between the limitations to the fusion cross-section and the fission mass distributions broadenings, respectively, for the Ar+Ho and Mg+Ta systems.     

Evaporation residue formation competing with the fission process in the197Au+16O,12C reactions and fission barriers at a specifiedJ window

Evaporation-residue excitation functions for¹⁶O and¹²C+¹⁹⁷Au reactions were measured by means of the activation technique. The competition between evaporation and fission of the compound nuclei was studied by comparing the observed evaporation-residue data with the published fission excitation functions. A newly devised analysis was applied in order to deduce a fission barrier height at a specified angular momentum and determine the relevant fissioning nucleus as well. We found the fission barriers to be 8.2 MeV for the²¹¹Fr nucleus at 16? and 8.2 MeV for the²⁰⁷At nucleus at 27?.