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.     

THE MEASUREMENT OF COMPLETE FUSION CROSS-SECTIONS FOR THE REACTIONS 12C+27Al, 12C+209Bi AND 14N+Pb

Mica track detectors were used for the measurement of evaporation residues and fission fragments of compound nuclei formed in ¹²C+²⁷Al, ¹²C+²⁰⁹Bi and ¹⁴N+Pb reactions. The complete fusion cross-sections and excitation functions were then obtained. By using the sharp cut-off model approximation, the values of the critical angular momenta were extracted from the complete fusion cross-sections. The results obtained were compared with the calculations based on current theories for critical angular momentum; they were found to agree within experimental uncertainties.     

EFFECTIVE MOMENT OF INERTIA OF MEDIUM MASS NUCLEI AT THE SADDLE POINT

The angular distributions of fission fragments are measured for systems of 120MeV ¹⁴N+¹¹⁸Sn with mica track detectors and gold-surfacebarrier silicon detectors. The measured angular distributions can be fitted by the standard theory of fission angular distributions on the basis of statistical model. The effective moments of inertia of fissioning nuclei at saddle point are derived from angular distribution anisotropies. The shapes of saddle point of fissioning nuclei in the region of Z²/A<30 are discussed.     

INFLUENCE OF THE NEUTRON NUMBER OF TARGET NUCLEUS ON CROSS SECTIONS OF PARTICLES EMISSION IN 12C+112,124Sn

The energy spectra and angular distributions of Li, Be, B particles emitted from the ¹²C+¹¹²Sn and ¹²C+¹²⁴Sn reactions at E_L=70.0MeV are measured. The angular distributions of the Li, Be, and B particles are found to be peaked around the grazing angle, without any forward peaked component.
The experimental results also showed that the cross section for emission is larger in ¹²C+¹¹²Sn reaction than that in ¹²C+¹²⁴Sn reaction, in contrast with the cross sections for Li, Be and B emissions. The indicates the effect of the neutron number of the target nucleus on the emission probability.     

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.     

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.     

Fission barrier of 209Bi measured by electron induced fission

For ²⁰⁹Bi an electrofission cross section below 10^?36 cm² was measured by the observation of correlated binary fission tracks in mica detectors. From the cross-section data between 28 and 41 MeV electron energy, a fission barrier of 25.5 ± 1.5 MeV was deduced. The method offers the possibility to measure fission barriers ar low angular momentum and for nuclei not accessible otherwise.     

Fission studies of gold induced by (1665 MeV) π- using a CR-39 detector

The fission cross section and fission probability of ¹⁹⁷Au, induced by (1665 MeV) π^-, have been studied using CR-39 track detectors. A 4π-geometry was used to count track statistics. A beam of negative pions of 1665 MeV was produced at AGS of Brookhaven National Laboratory, USA, and allowed to fall normally on the stack. Two detectors from the stack were scanned for fission fragment tracks after etching in 6N NaOH at 70℃. The statistics of fission fragment tracks in both detectors were obtained. It was found that there was a marked asymmetry of registered tracks with respect to the forward and backward hemispheres. This asymmetry could be partly accounted for on the basis of momentum transfer to the struck nucleus. On the basis of counting statistics fission cross section was measured, and fission probability was determined by dividing the fission cross section with the reaction cross section. The fission cross-section and fission probability were compared with the computed values using the cascade-exciton model code CEM95.     

A fast-fission component with small mass drift in the reaction84Kr+27Al atE lab=5.9 MeV/u

All reaction products in the range from target- and projectile-like to fission-like fragments were measured for the system⁸⁴Kr+²⁷Al at 5.9 MeV/u beam energy. They are assigned to the various reaction mechanisms on the basis of experimental signatures (energy dissipation, mass and angular distribution). The sum of the measured partial cross sections, including the evaporation residue yield obtained previously, agrees with the total reaction cross section derived from elastic scattering. A small fast-fission component was found, discernible from deep-inelastic reactions by its 1/sin θ angular distribution, and distinguished from compound-nucleus fission by an incomplete mass asymmetry relaxation.     

Investigation of the role of the projectile-target orientation angles on the evaporation residue production

The measured yield of evaporation residues in reactions with massive nuclei have been well reproduced by using the partial fusion and quasifission cross sections obtained in the dinuclear-system model. The influence of the orientation angles of the projectile- and target-nucleus symmetry axes relative to the beam direction on the production of the evaporation residues is investigated for the ⁴⁸Ca + ¹⁵⁴Sm reaction as a function of the beam energy. At the low beam energies only the orientation angles close to αP = 30° (projectile) and αP = 0°–15° (target) can contribute to the formation of evaporation residues. At large beam energies (about E _c.m. = 140–180 MeV) the collisions at all values of orientation angles αP and α _T of reactants can contribute to the evaporation residue cross section which ranges between 10–100 mb, while at E _c.m. > 185 MeV the evaporation residue cross section ranges between 0.1–1 mb because the fission barrier for the compound nucleus decreases by increasing its excitation energy and angular momentum.     

Subbarrier Complete Fusion-Fission Reactions of 16)O+232Th

We have succeeded in isolating the complete arising from complete fusion-fission in terms of the fragment folding angle technique,and measured complete fusion-fission cross sections and fragment angular distributions for the ¹⁶O+²³²Th system in the center-of-mass energies between 72.61 and 80.11 MeV.The observed fission excitation function is in quite good agreement with the expectation of the coupled-channels theory.However,the measured fragment angular distributions are more anisotropic than the predictions of both the saddle-point transition-state model and scission model.     

MEASUREMENT OF COMPLETE FUSION CROSS SECTION OF 12C+159Tb,12C+165Ho

Complete fusion cross sections have been measured for ¹²C+¹⁵⁹Tb and ¹²C+¹⁶⁵Ho reactions by using K-X rays of evaporation residues with Si(Li)spectrometer.The half-lives of evaporation residues and its yield distributions as a function of incident energy have also been obtained.The experimental values for the complete fusion cross section were compared with the theoretical ones.     

Fission of medium mass elements induced by 126 MeV 14N ions

Ni, Se, Mo, Ag, Ho and Au targets were bombarded by 126 MeV ¹⁴N ions in order to study compound nucleus fission. The large angular momentum brought in by the projectile implies much greater fission probabilities than those obtained when the same compound nuclei are created with light projectiles. We employed two surface-barrier detectors. The results obtained are fission cross sections, angular correlations and kinetic energy and mass distributions. Fissilities are found to decrease exponentially with the ratio Z²/A of the fissioning nucleus until Z²/A = 19 (molybdenum) and then to increase again for lower Z²/A. The calculations of fissilities performed with the fission barrier of Myers and Swiatecki, and including angular momentum effects in evaporation-fission competition, gives the same variation of fissilities as the experimental one, but the results obtained are lower than the experimental values. The total kinetic energies measured are higher than the predictions of the liquid drop model by around 10 MeV (Nix). The shapes of the mass distributions indicate that the value of x_BG (Businaro-Gallone point) is lower than 0.4 (silver).     

Measurement of Angular Cross Correlation of the Cross Section Fluctuation in Dissipative Collision 27Al+27Al

The excitation function was measured from θ_lab=10.4° to 57.4° in dissipative collision ²⁷Al+²⁷Al with incident energy ranging from 114 to 123 MeV in step of 200 keV. The Angular-cross Correlation Functions (ACFs) of cross section fluctuation within large angle region were obtained through experiment for the first time. It is found that the angular coherent width is at least 40°. It is interesting that the ACFs is not simple bell shape but shows strong asymmetry. The over-all behavior is that ACF varies more quickly for the backward angles. Itis also noticed that the cross section fluctuation in the forward angle region (θ_cm<70°) shows much stronger angular coherence than in the backward angle region (θ_cm>70°). This angular dependence may be attributed to the coherent rotation effect of intermediate dinuclear system.     

Evaporation residues from the fusion of 32S on 27Al at E = 142–227 MeV

The evaporation residues from the reaction ³²S on ²⁷Al were measured at 142, 153, 175, 187 and 227 MeV incident sulphur energies, using a time-of-flight spectrometer. The measured fusion-evaporation residue excitation function shows a maximum around E_lab = 170 MeV, which corresponds to a limiting angular momentum of 43 ħ. Several possible mechanisms were investigated as being responsible for this limitation. It was found that both fission and entrance channel effects could be the limiting factors for the fusion-evaporation cross section at high energies.     

Theoretical Analysis of Double Differential Cross Sections for n+ 238U Reaction

Based on the new measurements of total, nonelastic, elastic cross section and elastic scattering angular distributions for n+U reactions, a set of neutron optical model potential parameters is obtained in the region of incident neutron energy from 0.1 to 20 MeV. The cross sections, angular distributions, energy spectra and double differential cross sections are calculated and analyzed by optical model, nuclear fission theory, distorted wave Born approximation theory, coupled channel theory, the unified Hauser-Feshbach theory, as well as exciton model. The results indicate that our theoretical model can reasonably analysis n+ ²³⁸U reaction data with neutron energy lower than 20 MeV.     

Evaporation residue cross sections from the fusion of52Cr+110Pd

Evaporation residue cross sections for the fusion of⁵²Cr and¹¹⁰Pd were studied at four energies close to the barrier (up toE _CM ?E _B =26 MeV). The cross sections were analysed using a multiple-chance fission/evaporation calculation. Agreement with the data is obtained with a fission barrier reduced by 40 to 20%, depending on incident energy.     

A study of the differential cross section in subbarrier photofission of238U

A measurement of the angular distribution and yield of fission fragments from photofission of²³⁸U has been performed between 5.2 MeV and 6.4 MeV. Asγ-source the bremsstrahlung from a microtron has been used. For the detection of the fission fragments, solid state track detectors were used. The yield data were evaluated to approximate cross sections. The data were analyzed within the framework of the double hump barrier model.     

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.