Further experimental evidence for fast fission

Fusion-fission products have been studied for three reactions: Ar + Au, Ar + Bi and Ar + U (5.25–7.5 MeV/u). By measuring symmetric fragmentation components (fission-like events), cross sections for fusion were deduced and compared with the predictions of static and dynamic models. With increasing projectile energy, the width of the mass distributions strongly increases for the two lighter systems. By contrast, for Ar + U it remains essentially constant at a very large value. These results clearly demonstrate that the large increase of the width of the mass distribution cannot be attributed simply to large values of the angular momentum. However, they can be explained by the occurence of a different dissipative process, fast fission, which can be expected if there is no barrier to fission. For the reaction Ar + Au, the total kinetic-energy distributions were also studied in detail. In this case fast fission occurs only at high incident energy. The average total kinetic energy (TKE) was found to be constant with increasing energy whereas the widths of the TKE distribution increase.     

Possible Mechanisms of Ternary Fission in the 197Au+197Au System at 15 A MeV

Ternary fission in ^197Au＋^197Au collisions at 15 A MeV is investigated by using the improved quantum molecular dynamical （ImQMD） model. The experimental mass distributions for each of the three fragments are reproduced for the first time without any freely adjusting parameters. The mechanisms of ternary fission in central and semicentral collisions are dynamically studied. In direct prolate ternary fission, two necks are found to be formed almost simultaneously and rupture sequentially in a very short time interval. Direct oblate ternary fission is a very rare fission event, in which three necks are formed and rupture simultaneously, forming three equally sized fragments along space-symmetric directions in the reaction plane. In sequential ternary fission a binary division is followed by another binary fission event after hundreds of fm/c.     

Fission of238U induced by136Xe for energies close to the Coulomb barrier

Excitation functions and angular distributions have been measured for fission of²³⁸U induced by¹³⁶Xe ions with bombarding energies between 4.5 and 5.9 MeV/N. Structures expected theoretically as characteristic for Coulomb fission have not been observed. The Q-value of ?(7.5±1.0) MeV measured for bombarding energies below 4.7MeV/N, however, appears to be compatible with inelastic scattering (e.g. Coulomb excitation) rather than subcoulomb transfer followed by fission. The total kinetic energy of deep inelastic events at 5.9 MeV/N is consistent with the current knowledge about mass diffusion, but also (for the highest excitation energies) with a fast fission process in the presence of the projectile.     

Determination of the disorder profile in an ion‐implanted silicon carbide single crystal by Raman spectroscopy

Silicon carbide is an interesting material for GEN IV fission reactor projects because of its excellent properties. However, these properties will be altered under extreme conditions such as irradiation because of accumulation of damage. Mechanisms playing a role in defect formation require further studies in the case of high energy heavy ion irradiations. In this work a silicon carbide single crystal slice has been implanted with 20 MeV Au ions and probed by using Raman spectrometry. The resulting Raman spectra recorded as a function of depth clearly show a damaged zone, in which the width is in agreement with the projected range of the incident ions (R_p) calculated by using SRIM code. In this area, three damaged zones have been brought to light because of the high spatial resolution of the Raman spectrometry technique. The existence of these zones is discussed with regard to the different energy loss regimes of the implanted ions such as the electronic and nuclear ones. Copyright © 2012 John Wiley & Sons, Ltd.     

The interaction of 605 MeV 63Cu with 197Au

Binary coincident fragments from the ⁶³Cu + ¹⁹⁷Au reaction at a copper energy of 605 MeV have been studied. Fragment energies were measured and fragment masses determined by a kinematic method. Three types of event are defined by suitable adjacent limits in the mass versus total kinetic energy event space. The angular distributions of cross section, average total kinetic energy and average mass have been determined for each event region. Total cross sections determined in the present experiment are compared to those found at lower bombarding energies. Further information on the sequential fission process has been obtained from measurements of yields of radioactive isotopes resulting from bombardment ofthin and thick targets of Au by 605 MeV Cu ions.     

Quasi-fission and other strongly damped collisions between 63Cu ions and 197Au nuclei

The interaction of ⁶³Cu ions with ¹⁹⁷Au nuclei have been studied experimentally at incident energies of 365 and 443 MeV (1.1 and 1.4 times the Coulomb barrier). Mass and kinetic energy distributions of reaction products have been measured at several angles. Near the grazing angle, a continuous transition was found from elastic events to partially damped (PD) events, and to fully damped events (quasi-fission, QF). Away from the grazing angle a clean separation between elastic and QF events was observed. Events that may be due to fission following fusion (CF) were also obtained. Results are discussed in terms of decomposition into PD, QF, and CF components. The QF kinetic energy is independent of the incident energy (implying full damping of the initial relative motion). It is lower than the Coulomb barrier and close to the kinetic energies from the fission of similar systems. The angular distribution is peaked somewhat forward of the grazing angle for low mass transfers. For large mass transfers the yield increases slowly with decreasing angle. At 443 MeV a large contribution from negative angles is present. σ_QF accounts for more than 65 % of the reaction cross section σ_R at 443 MeV and for more than 50 % at 365 MeV. The upper limit on CF is about 10 % of σ_R, and σ_PDis of the order of 25 % of σ_R.     

Production of heavy fragments in the fission mass region in C+U, Bi, Au and Ag reactions at 26.5 GeV

Formation of heavy fragments in the fission mass region in the interaction of 26.5 GeV with U, Bi, Au and Ag is studied using a sandwich configuration of the Makrofol polycarbonate track detector. Events in which at least one heavy fragment in the fission mass region is detected are analyzed. Fragments produced in these events are identified and an event-by-event model-free analysis is performed in order to separate different production mechanisms. We have identified the events produced in fission, deep spallation and fragmentation processes. The cross sections and experimental features are determined for these reaction mechanisms, and their variations as a function of the target mass have been investigated. The results show that in the interactions of with U the dominant process is fission, while in the interactions of with lighter target nuclei the dominant processes are deep spallation and fragmentation.     

重离子裂变反应中的能级密厦参数和裂变几率

本工作从Ｎｉｌｓｓｏｎ单粒子能级入手,计算了形变核１８６Ｏｓ、１８７Ｉｒ、１８９Ｏｓ和１９３Ａｕ作为激发函数的固有能级密度参数ａｉｎｔ和有效能级密度参数（包括核集体运动效应）ａｅｆｆ．由这样能量相关的能级密度参数计算的裂变几率Ｐｆ（Ｕ）与本工作的裂变几率符合很好． Based on the single-particle levels given by Nilsson,the intrinsic and effective(with collective effects)level density parameters as a function of theexcitation energy for the 186Os, 187Ir, 189Os and 193Au deformation nuclei have been calculatedin the range of the excitation energy up to 150MeV. The calculated fission probabilities Pf(U) are consistent satisfactorily with the experimental data when a nonadiabtic estimation ofthe collective effects was used to calculate the nuclear level density parameters.     

Fission of Pb in the He-induced reactions at 0.65–12.7 GeV

The fission processes from the interactions of 0.65, 1.74, 5.1, 8.8 and 12.7 GeV ⁴He with Pb have been analyzed using the polycarbonate track detector Makrofol in the form of a sandwich. The decay channels ending with one, two, or more (three, four) fragments were detected. We analyzed events in which only two heavy fragments (Z>20) are detected, irrespective of the existence of coincident intermediate mass fragments (8Z20). Using the correlation between the common observables which characterize fission events, we identified events originated from thermal (soft) fission and more violent processes. Cross sections, angular distributions and excitation energy have been determined for two fission mechanisms, and their variation as a function of the incident energy has been investigated.     

Separation of Coulomb fission from nuclear fission at medium energies

The cross sections for fission induced by one neutron transfer and by the electromagnetic field in the reaction 24.3 MeV/u ²³⁸U + ¹⁹⁷Au are calculated and compared to the experimental data. It turns up that the two calculated cross sections differ by five orders of magnitude at a distance of closest approach of 25 fm. It is shown that in the experiment in which one is able to select the events corresponding to a large distance of closest approach, a separation of Coulomb fission from nuclear fission events can be efficiently obtained.     

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.     

Energy dependence of fission probabilities induced by negative pions in Sn,Au and Bi

Fission probabilities induced by negative pions in Sn, Au and Bi at different energies using variety of nuclear track detectors have been studied. The target-detector assemblies in 4π-geometric configuration were irradiated at the AGS facility of Brookhaven National Laboratory, USA. After etching the exposed detectors at appropriate etching conditions the detectors have been scanned for the tracks of fission fragments produced as a result of interaction of pions with the target nuclei. Based on the track counts, the values of fission cross sections have been measured and fission probabilities have been calculated using the reaction cross-section calculated with the help of the cascade-exciton model code CEM95. The values of fission probability based on experimental fission cross-sections have been compared with the theoretically calculated values of fission probabilities obtained using the CEM95 code. Theoretical values of fission probability have been computed for incident pion energy up to 2500 MeV in Sn, Au and Bi for comparison to fission data for high energy negative pions at 500, 672, 1068, 1665 MeV and 2300 MeV energies for the same targets. The values of fission probability based on the experimental fission cross section and theoretically calculated values of fission probability have been compared. Reasonable agreement has been observed among the experimentally measured and theoretically computed values of fission probabilities. A saturation of probabilities has been observed for Au and Bi at higher energies, but for Sn an increase of probability with the increase of pion beam momentum up to 1665 MeV has been observed.     

Mass spectrometer for measuring the mass distributions of cluster ions in the range 105–107 amu

The mass analysis of heavy cluster ions in the range m/q=10⁵–10⁷ amu is investigated. A dynamic mass spectrometer is described for measurements of the mass distributions of charged particles generated in pulsed sources of polyatomic clusters in the given range. The main parameters and characteristics of the instrument are given. The distribution of cluster ions with respect to the ratio m/q is determined for a specific source of gold cluster ions formed in the inelastic sputtering of gold island films with fission fragments of ²⁵²Cf nuclei. Zh. Tekh. Fiz. 67, 107–112 (April 1997) The reported investigations have been performed under the auspices of International Science Foundation Grant No. R1S000, ISF Grant No. R1S300, ant the Government of the Russian Federation, 1994–1995.     

The collinear cluster tri-partition (CCT) of 252Cf (sf): New aspects from neutron gated data

Results of two different experiments for the study of fission of ²⁵²Cf (sf) events in coincidence with neutrons are reported. Two time-of-flight-energy (TOF-E detectors systems have been used. The fission fragment masses were obtained in a double arm coincidence set-up, where the missing mass in the binary decay is used to characterise ternary fission as a collinear cluster tri-partition (CCT). The ³He filled neutron counters have been arranged so as to detect principally neutrons emitted from an isotropic source in the laboratory frame. The fission events connected to the larger experimental neutron multiplicities show a wide range in the missing-mass spectrum, down to $ \alpha$ -particles, carbon and oxygen isotopes. These are linked with magic nuclei in the binary mass-mass correlations of the fission fragments. These neutron gated data are virtually free from background events from scattered binary fission fragments. The ungated spectra are compared to those of the previous data from our previous article (Eur. Phys. J. A. 45, 29 (2010)), the observed structures agree well with the manifestations of the collinear cluster tri-partition of ²⁵²Cf (sf) observed earlier. Several new families of the CCT modes are observed.     

A search for rare types of fission induced in silver and bromine nuclei

Ternary and quaternary fission produced in silver and bromine nuclei have been studied withK5 nuclear emulsion exposed to 1.8 GeV/cK ⁻ beams. The frequency of the ternary events is found to be ∼0.08 of that of the binary events produced in the same volume of the emulsion. The range ratio and range distribution of the fission fragments are studied and the angles between each pair of the fragments are determined. Ranges are found to vary from 5 to 40μ with a maximum number lying between 5 and 10μ. The angles between the fission fragments are found to form a broad distribution extending from 40° to 180°. A few of the events have also been analysed to give them a possible identity. A possible case of quaternary fission has also been reported. This paper was presented at the Symposium on 3rd High Energy Physics held at Bhubaneswar during November 1976.     

The binary decay of U,Th, Pb and Au induced by 380 MeVK − mesons

The binary decay of U, Th, Pb and Au induced by 380 MeVK ^? mesons has been studied by means of a polycarbonate plastic track detector makrofol. The range distributions of the produced fragments are discussed and some conclusions are derived on the ratio of the processes of fission and fragmentation. Also calculated mass and energy distributions of the fragments produced by the uranium fission are presented.     

Mass distribution of fission fragments in the19F+197Au reaction

Mass distributions of fission fragments have been measured for the reaction of¹⁹F on¹⁹⁷Au as a function of scattering angle. The mean fragment mass is dependent on the scattering angle, the asymmetry increasing with increasing projectile energy.     

Average spins of primary fission fragments

The average spins at which final fragments are formed after fission following fusion of ¹⁹F with ¹⁹⁷Au have been measured directly from spectroscopic studies of discrete γ-ray lines. The final fragment spins can be translated into average spins at which primary fragments are formed by adding changes due to statistical γ-ray emission and post-fission neutron emission. The average primary fragment spins show marked variations with fragment mass. Possible causes for the structure are discussed within the framework of the statistical scission model of heavy-ion induced fission.     

非全熔合反应裂变角分布

采用核固体径迹探测器测量了¹⁶O+¹⁵⁹Tb、¹⁶O+¹⁹⁷)Au和¹⁶O+²⁰⁹Bi反应碎片角分布,扣除中等质量碎片(IMF)的贡献,用过渡态统计模型(TSM)和单个自旋标准理论拟合实验的裂变角分布,提取裂变核的自旋. 讨论了它随质心系入射能量的变化.     

Mechanisms for emission of4He in the reactions of 334 MeV40Ar with238U

Emission of⁴He in the reaction 334 MeV⁴⁰Ar+²³⁸U has been studied by triple coincidence measurements that allow the separate identification of fusion fission and sequential fission. For the⁴He evaporative spectra from fusion fission the composite system is shown to be the predominant contributor; whereas, for sequential fission the dominant emission is from the fragments. This result demonstrates a correlation between evaporative emission probability and lifetime expectancy of the composite system. To account for the observed⁴He spectra two other mechanisms are necessary in addition to nuclear evaporation. At forward angles, the⁴He spectra from both fusion fission and sequential fission exhibit higher intensities and larger energies than those expected from purely evaporative processes. This forward-peaked component must be related to a very rapid or pre-thermalization stage of the reaction. At backward angles yet another component is observed for fusion fission. As it is sensitive to the fragment masses but does not carry the kinematic shift characteristic of their full acceleration, this component must originate near to the time of scission. The average⁴He energy for this component is approximately 17 MeV (c.m.), and its intensity is correlated with a plane perpendicular to the fission fragment separation axis. These signatures are similar to those for long range alpha particle emission in low energy fission. Alpha particles evaporated from the composite nuclei in fusion-fission reactions are shown to be preferentially associated with fission events which result in the more symmetric masses. This result is consistent with the notion that mass asymmetric fission is a faster process than symmetric fission. Such a correlation between mass asymmetry and lifetime is an essential part of the “fast fission” or “quasifission” idea, which has attracted much current attention.