Evidence of complete fusion in the sub-barrier 16O+238U reaction

Evaporation residue cross sections in the 16O+238U reaction were measured for the energy range from above- to extreme sub-barrier. The cross sections are reproduced by a statistical model calculation, for which partial cross sections are calculated by a coupled-channel model taking into account the prolate deformation of 238U. Complete fusion was observed in the collision of the projectile with the tips of the 238U target, in the same way as the side collision.     

Fission product energies and anisotropies following complete fusion of16O+238U

Radioactive recoil techniques have been developed for measuring angular distributions and range distributions of individual fission products following heavy ion induced fission. From these measurements, values can be extracted for the recoil velocity of the fissioning nucleus, the velocity imparted by fission, and the fission anisotropy. These techniques were applied to reactions of 101 MeV¹⁶O on²³⁸U, and confirmed that the reaction mechanism is essentially entirely complete fusion-fission. Accepting this, the data determine the kinetic energy release in forming the various products to a precision of 1%; while the overall magnitude of the energy is in good agreement with previous results, the data suggest a systematic correlation between kinetic energy and the position of a product on the nuclear charge dispersion curve, not previously reported, which is similar to but significantly larger in magnitude than the effect expected from simple Coulomb repulsion. Significant variations in anisotropy are also observed between products, which appear to be partially correlated with the variations in kinetic energy.     

Fission following complete or incomplete fusion of 135 MeV16O and238U

From measurements of the laboratory angular distribution and the projected range distribution for each of 26 individual radioactive fission products from the interaction of 135 MeV¹⁶O on²³⁸U, we have determined for each product quantities including the average velocity,ν, of the parent fissioning system and the anisotropy,ω, of the fission angular distribution. We have interpreted the results in terms of two competing mechanisms: complete fusion and a process involving transfer of anα-particle from projectile to target followed by fission of²⁴²Pu. The values ofν lie betweenν _cn, the complete fusion value, and 0.3ν _cn, and can be taken as a measure of the relative importance of the two processes competing in populating each observed product. These two contributions to each measured product yield can then be separated, and understood using a simple model of the fission process. The data suggest that approximately 24% of the overall fission yield is due to incomplete fusion processes. The anisotropy,ω, is strongly correlated with the value ofν, indicating anisotropies of 1.4 and 2.8 for the two individual processes.     

Measurement of evaporation residue cross-sections of the reaction 30Si + 238U at subbarrier energies

The reaction ³⁰Si + ²³⁸U → ²⁶⁸Sg^* was studied at beam energies close to the Coulomb barrier. At a center-of-mass energy of E _c.m. = 144.0MeV for reactions at half thickness of the target we measured three decay chains of ²⁶³Sg produced by evaporation of five neutrons. The cross-section was ( 67⁺⁶⁷ _-37) pb. At E _c.m. = 133.0MeV we measured three spontaneously fissioning nuclei which we assigned to the isotope ²⁶⁴Sg. The production cross-section was ( 10⁺¹⁰ _-6) pb and a half-life of ( 120⁺¹²⁶ _-44) ms was determined. This half-life is a factor of twenty shorter than theoretical predictions. At E _c.m. = 128.0MeV an upper cross-section limit of 15pb was measured. The cross-section data reveal a strong influence of the orientation of the deformed target nucleus on the production yield. Compared to excitation functions measured for the lighter system ¹⁶O + ²³⁸U → ²⁵⁴Fm^*, a reduction of the fusion probability was observed at low beam energies indicating increasing competition from quasifission processes.     

The reaction238U +238U at 7.42MeV/u

One-particle-inclusive measurements have been performed for the charge, kinetic energy and angular distributions of reaction products from²³⁸U +²³⁸U at 1 766MeV (7.42MeV/u) incident energy. The deep inelastic products exhibit features similar to those seen in reactions induced by medium heavy nuclei: increasing particle transfer is observed with increasing energy damping, the angular distributions are peaked near the grazing angle, they broaden significantly with increasing energy loss and/or charge transfer. The dominant reaction mechanism, however, is found to be sequential fission of one or both primary reaction products. The reconstructed primaryZ- andQ-value distributions show more particle transfer at a given energy loss than in other systems, i.e. the diffusion process seems to proceed colder in this system. This is confirmed by relatively large cross sections for surviving deep inelastic reaction products belowZ=92. A direct search forα-decay or fission of superheavy nuclei being produced in a deep inelastic reaction and being implanted in a surface barrier detector resulted in an upper cross section limit of 2 ×10^?32cm².     

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.     

Enhancement of the subbarrier fusion reaction due to neck formation

Subbarrier fusion reaction cross sections have been calculated based on the idea of the neck formation. Barrier heights for the no neck configuration and for the neck formed one are calculated by the Krappe-Nix-Sierk model and the transition between them are calculated by the perturbation formula. The relative motion between two nuclei is treated by the semiclassical model introducing the imaginary time. The formulation was applied to the fusion of identical nuclei and for six systems we examined we got a good reproduction of the data. Our calculation shows that transfer of about one tenth of the incident flux to the neck formed configuration is necessary and enough to explain the experimental data.     

Fission in238U+238U collisions below the Coulomb barrier

Integral fission cross sections in the system²³⁸U+²³⁸U were measured at beam energies below the interaction barrierV _C. Scattering angle dependent probabilities and integral cross sections for Coulomb fission were calculated. It is concluded that earlier observed discrepancies between measured and calculated angular distributions for the one-neutron transfer product²³⁹U cannot be explained by sequential fission. Multi-nucleon transfer induced fission is observed down to energies (0.90±0.02)×V_C.     

Fragment correlations in the reaction 9.03 MeV/u238U+natU

Kinematically almost complete correlation studies on heavy fragments in the reaction 9.03 MeV/u²³⁸U+^natU were performed using metaphosphate-glass detectors in a 2π-geometry technique. Absolute data for the total reaction cross-section and the partial cross-sections for the three-, four-, and five-body reaction channels were measured. The tracks of correlated particles were measured and converted into masses and energies. The reaction mechanism of each individual event was studied in terms of the separation velocities of correlated fragment pairs. Sequential fission following inelastic collision is found to account for all analysed events with more than two heavy nuclei in the exit channel. The masses, scattering angles, and energies of the first reaction step were reconstructed event by event.     

A scenario for the 220-MeV40Ar+238U reaction

Singles and coincidence charge distributions are combined to illustrate the mechanism for the 220-MeV⁴⁰Ar+²³⁸U reaction. It is found that the apparent peak in the coincidence fragment distribution corresponding toZ=82 (A=208) can be explained in terms of the fissionability of the target-like fragments produced in deep-inelastic collisions rather than as a manifestation of shell effects in compound-nucleus fission, as has been postulated for a similar system.     

Study on the formation of the composite system of 238U+238U

Strongly damped reactions of 238U+238U, at Ecm = 680-1880 MeV have been studied based on the improved quantum molecular dynamics model. We find that at a certain energy region the entrance channel potential is weakly repulsive and the dissipation is very strong after touching configuration, these two effects make the time delay of re-separation for colliding system. The single particle potential well of the transiently formed composite system has Coulomb barrier about 15-20 MeV high at the surface, which makes the excited unbound protons being still embedded in the potential well and moving in a common mono-single particle potential for a period of time and thus restrains from quick decay of the composite system.     

Search for production of superheavy elements via “fusion after instantaneous fission” in the reaction238U+208Pb

Superheavy elements (SHE) might be formed via a reaction mechanism called “fusion after instantaneous fission”, which is supposed to occur during the collision of a deformed very heavy nucleus with a spherical one. We bombarded natural uranium targets with lead ions and searched for alpha-emitting and spontaneously fissioning reaction products. Different techniques were used: a rotating wheel, a gas jet system and a radiochemical procedure. No SHE have been found. The upper cross section limits are between 10^?33 cm² and 10^?34 cm² within the half-life range from 1 ms to 1 year. In addition, the production cross sections of some Cm and Cf isotopes and of Am fission isomers were determined.     

Potential pockets in the238U+238U system and their possible consequences

Within the double-folding model the separation, shape, and orientation dependence of the interaction potential is studied for two heavy ions. An effective nucleon-nucleon interaction (M3Y) derived fromG-matrix elements and based upon the Reid soft-core potential is used. Deformed Fermi-type matter densities with static quadrupole and hexadecapole deformations were utilized. The model is applied to the²³⁸U+²³⁸U system and shows dramatic dependence on the deformations and orientations.     

Experimental determination of the total reaction cross section of the stellar nuclear reaction 16O + 16O

The total reaction cross section for ¹⁶O + ¹⁶O has been measured at six energies between E_c.m. = 6.8 and 11.9 MeV. Cross sections for the production of protons, alphas, neutrons, deuterons, ³¹S, ³⁰P, ¹²C(g.s.) + ²⁰Ne(g.s.) and the relative γ-yield were obtained with a variety of experimental methods. No ³H or ³He were found. All cross sections are normalized to ¹⁶O + ¹⁶O elastic scattering at θ_c.m. = 90°, which was measured separately with high precision between E_c.m. = 7.3 and 14.4 MeV. The elastic scattering and relative γ-yield of ¹²C + ¹²C were measured between E_c.m. = 3.9 and 7.5 MeV. The elastic scattering and neutron yield of ¹²C + ¹⁶O were measured between E_c.m. = 5.4 and 10.1 MeV.     

Coincidence measurements of light particles and evaporation residues in the 16O+27Al fusion reaction

Energy and angular distributions of protons and α-particles in coincidence with fusion residues produced in the reaction of 150 MeV ¹⁶O with ²⁷Al were measured. Experimental results are reproduced by a statistical model calculation except a forward component of α-particle yield. About 20% of the total α-particle yield is considered to originate from incomplete fusion process.     

Cross sections for nuclear reactions in collisions of238U+238U and238U +197Au near and below the coulomb barrier

Cross sections for nuclear reactions at beam energies near and below the spherical Coulomb barrier V _c were measured in the very heavy collision systems²³⁸U +²³⁸U and²³⁸U +¹⁹⁷Au. The most probable reaction channel with mass transfer is the one-neutron transfer. Its excitation function is understood in terms of Rutherford trajectories together with the quantal process of neutron tunnelling over large distances. In addition, the exchange of up to 15 nucleons is observed down to 0.90 V _c . The excitation functions for the multi-nucleon transfer products have much steeper slopes than that for one-neutron transfer, and are steeper for²³⁸U +¹⁹⁷Au than for²³⁸U +²³⁸U, suggesting that nuclear contact is established in the associated collisions. The angular distribution for one selected multi-nucleon transfer product,²²⁷Th, shows that its formation occurs in more central collisions within contact times shorter than about 10^?21 s. There is no evidence for very longlived di-nuclear systems in the these reactions.     

Incomplete fusion reactions in 16O+165Ho

Excitation functions for evaporation residues of the system ¹⁶O + ¹⁶⁵Ho have been measured up to 100 MeV. Recoil range distribution of long lived reaction products were measured at ¹⁶O beam energy of 100 MeV. Detailed Monte Carlo simulation of recoil range distributions of products were performed with the help of PACE2 code, in order to extract the contributions of incomplete fusion in the individual channels. The results clearly show the incomplete fusion contributions in the tantalum and thulium products. This is confirmed by the predictions of breakup fusion model of the incomplete fusion.     

Comparison of the fusion reactions12C+20Ne and16O+16O near the Coulomb barrier

The partial cross sections of heavy residual nuclei produced in the heavy ion fusion of¹²C+²⁰Ne have been measured atE _c.m.=6–15 MeV viaγ-ray spectroscopy with a Ge(Li) detector. Windowless and recirculating gas target systems have been used. The dominant residual nuclei are²⁴Mg,²⁷Al,²⁸Si,³⁰Si,³⁰P and³¹P, which arise from two- and three-body breakups in the exit channels. The observed excitation functions are smooth in their energy dependence and give no indications for the existence of pronounced resonance structures, in contrast to theoretical predictions. The Coulomb excitation of²⁰Ne served as an intrinsic calibration standard in the determination of absolute partial and total fusion cross sections. The same experimental set-up was also used in the reaction studies of¹⁶O+¹⁶O atE _c.m.=7–14 MeV, going through the same compound nucleus³²S at similar excitation energies. The observed energy dependence in the excitation functions is in good agreement with previous work. The total fusion cross section agrees fairly well with two sets of values reported previously, but deviates significantly from other reported absolute cross section values. The relative evaporation distributions of the residual nuclei are similar for both heavy ion reactions. However, the ratio of their total fusion cross sections deviates from model predictions and suggests that compound nucleus formation does depend on the microscopic structure of the colliding nuclei in the entrance channel. From the observed energy dependence of the above ratio, particularly at subcoulomb energies, geometrical effects in the entrance channel (due to deformed and spherical nuclei) appear to be weak. The astrophysical aspects of the data in the context of late stellar nucleosynthesis are discussed.