Kinetic energy and angular distributions of instantaneous fission fragments in a classical model

The process of instantaneous fission in deep inelastic collisions is investigated in a classical model. Kinetic energies and angular distributions of the fragments are calculated for the proposed reaction Pb+U atE _cm ^inc =750 MeV; an experimental setup for the separation of the fragments originating from instantaneous fission from the fragments of thermal fission is explained. We also discuss fusion following instantaneous fission as a mechanism for the production of superheavy elements and arrive at rather promising estimates.     

Search for delayed fission products from superheavy nuclei in the reaction136Xe+208Pb

The production of superheavy elements in binary reactions of the type²⁰⁸Pb (¹³⁶Xe, X) Y was investigated atE _c.m.=470 MeV. The experiment was designed to search for delayed fission products from elements withZ between 108 and 116 and fission lifetimes ofΤ?10^?12 s. No fission events were observed the upper limit for the formation cross section being 1.2 Μbarn.     

Light particle accompanied quasifission in superheavy composite systems

The dynamics of the excited superheavy system with Z = 118 in the reaction ⁸⁶Kr + ²⁰⁸Pb at E _Kr = 460, 500, and 600 MeV has been investigated. The mass and kinetic energy of binary fragments were measured by the time-of-flight method. Double-differential distributions of protons and α particles were measured in coincidence with fragments. The proton spectra can be described considering only evaporation from fragments. Evidence of the neck fragmentation was obtained from analysis of double-differential α spectra. Properties of the α-particle neck fragmentation component are close to those known from the ternary fission of actinide nuclei, but the multiplicity is much larger than can be expected from extrapolation of the ternary fission data. The text was submitted by the authors in English.     

Total cross sections for fission of heavy nuclei by intermediate-energy protons

The results of measuring total cross sections for proton-induced fission of ^natPb, ²⁰⁹Bi, ²³²Th, ²³³U, ²³⁵U, ²³⁸U, ²³⁷Np, and ²³⁹Pu nuclei at proton energies from 200 to 1000 MeV are reported. The measurements were carried out in steps of ≈100 MeV. Complementary fission fragments were detected in coincidence by two parallel-plate avalanche gas counters placed in the beam line on both sides of the thin target close to it. Energy and isotope dependences of the fission cross sections are analyzed.     

Electromagnetic and nuclear fission of238U in the reaction of 100, 500, and 1000 A·MeV208Pb with238U

The folding- and azimuthal-angle and velocity distributions for the²³⁸U fission fragments have been measured in reactions with 100, 500, and 1000 A·MeV²⁰⁸Pb. These distributions were used to decompose the fission cross section into its electromagnetic and nuclear components. The fraction of electromagnetic fission was found to be 0.16±0.07, 0.48±0.08, and 0.60±0.04, respectively. The electromagnetic fission cross section as a function of the²⁰⁸Pb nucleus energy is compared with theoretical predictions. The measured fission cross section from nuclear reactions (≈1.5 b) is approximately constant between 100 and 1000 A·MeV.     

The study of the (α, α'f) reaction at 120 MeV on 232Th and 238U: (I). Fission probabilities and angular distributions in the region of the giant quadrupole resonances

The fission decay channel of ²³²Th and ²³⁸U has been investigated, using the (α, α'f) reaction at 120 MeV bombarding energy. The angular distributions of the fission fragments and the fission probabilities up to around 15 MeV excitation have been measured. No evidence for the fission decay of the giant quadrupole resonance has been found, although for ²³⁸U, a weakly excited structure is seen in the (α, α'f) spectrum at about 9.5 MeV excitation at backward angles with respect to the recoil axis. This effect is similar to what has been found in a (⁶Li, ⁶Li'f) experiment reported recently. The over-all feature of the fission probability for excitation energies above the fission barrier are well reproduced by statistical calculations.     

On search and identification of relatively short-lived superheavy nuclei (<Emphasis Type="Italic">Z</Emphasis> ≥ 110) by fossil track studies of meteoritic and lunar olivine crystals

The main goal of the present work is the search for and identification of relatively stable nuclei of superheavy elements (SHE) (Z>110) in galactic matter by fossil track study of nonconducting crystals from the surface of meteorites and rocks from the lunar regolith. Nuclei of SHE are thought to be the products of nucleosynthesis in explosive processes in our Galaxy (supernova r-process nucleosynthesis and, especially, neutron-star formation, etc.). When accelerated to relativistic energies in the Galaxy, they can produce extended trails of damage in nonconducting extraterrestrial crystals. The lifetime of such SHE in galactic cosmic rays will range from 10³ to 10⁷ yr to be registered in extraterrestrial crystals. To search for and to identify the superheavy nuclei in the galactic cosmic rays, it was proposed to use the ability of nonconducting extraterrestrial crystals such as olivines, pyroxenes, and feldspars to detect and to store for many millions of years the trails of damage produced by fast Z≥23 nuclei coming to rest in the crystalline lattice. The track lengths of fast Z≥23 nuclei are directly proportional to Z² of these nuclei. The nuclei of SHE produce, when coming to rest in a crystal volume, tracks that are a factor of 1.6–1.8 longer than the tracks due to cosmic-ray Th and U nuclei. To identify the tracks due to superheavy nuclei, calibrations of the same crystals were performed with accelerated Au, Pb, and U nuclei. For visualization of these tracks inside the crystal volume, proper controlled annealing and chemical etching procedures were developed. Since 1980, fossil tracks due to Th and U nuclei have observed and unambiguously identified (1988) by subsequent calibrations of the olivine crystals with accelerated U, Au, and Pb ions. The number of tracks of Th and U nuclei measured in olivine crystals totaled more than 1600, as compared with the prior 30 events. The other approach to identifying SHE in nature is to search for tracks in phosphate crystals from spontaneous fission of Z ≥ 110 nuclei; these produce two-prong and three-prong fission fragment tracks and differ significantly from the tracks from spontaneous fission of ²³⁸U and ²⁴⁴Pu nuclei. Extraterrestrial phosphate crystals of lunar and meteoritic origin will be investigated. Such SHE nuclei can survive in crystals of extraterrestrial rocks and produce spontaneous fission tracks, if the lifetime is more than 5×10⁷ yr.     

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.     

Reaction and fission cross-sections of 750 A·MeV238U ions on Pb,Cu and Al-targets

Charge-loss and fission cross-sections of²³⁸U at 750 A·MeV were measured on Al, Cu and Pb targets. The charge-loss rate was obtained by the attenuation method. Fission was selected by detecting the pair of highly ionizing fragments. Since the neutron-loss cross sections were measured in a parallel experiment for the same projectiles, all cross sections contributing to²³⁸U collisions on nuclei are available now as function of the target mass number and can be compared with current models.     

Mass distribution in the bremsstrahlung-induced fission of Th,U and Pu

The yields of various fission products in the 10 MeV bremsstrahlung-induced fission of ²³²Th, ²³⁸U and ²⁴⁰Pu were determined using a recoil catcher and off-line γ-ray spectrometric techniques. From the yield data, mass yield distributions were obtained using charge distribution corrections. The higher yields of fission products around mass numbers 133–135, 138–140, 143–145 and their complementary products in the neutron and bremsstrahlung-induced fission of ²³²Th, ²³⁸U and ²⁴⁰Pu were interpreted based on nuclear structure effects. From the mass yield distribution, the peak-to-valley (P/V$P/V$) ratio was also obtained for the above fissioning systems. The present data, along with data from the literature on different bremsstrahlung- and mono-energetic neutron-induced fissions of ²³²Th and ²³⁸U are interpreted to examine the influence of excitation energy on the peak to valley ratio. For the same compound nucleus ²⁴⁰Pu^?, the data in the 10–30 MeV bremsstrahlung-induced fission of ²⁴⁰Pu were compared with similar data of thermal to 14 MeV neutron-induced fission of ²³⁹Pu and the spontaneous fission of ²⁴⁰Pu to examine the role of excitation energy due to bremsstrahlung radiation and mono-energetic neutrons.     

Low-energy fission investigated in reactions of 750 AMeV238U-ions with Pb and Be targets

Charge distributions of fragments from low energy nuclear fission are investigated in reactions of highly fissile²³⁸U projectiles at relativistic energies (750 A·MeV) with a heavy (Pb) and a light (Be) target. The fully stripped fission fragments are separated by the Fragment Separator (FRS). Their high kinetic energies in the laboratory system allow the identification of all atomic numbers by using Multiple-Sampling Ionization Chambers (MUSIC). The elemental distributions of fragments observed at larger magnetic rigidities than the²³⁸U projectiles show asymmetric break-up and odd-even effects. They indicate a low energy fission process, induced mainly by dissociation in the electro-magnetic field for the U/Pb-system, or by peripheral nuclear interactions for the U/Be-system.     

Combined dynamical and statistical approach to describing induced fission of heavy nuclei

A combined dynamical and statistical approach to describing induced fission of heavy nuclei is proposed. This approach takes into account the nuclear-dissipation phenomenon and the double-humped structure of the fission barrier. A method that is intended for calculating the angular distribution of fission fragments and which is applicable over a broad range of excitation energies is discussed. The potential of the approach is demonstrated by addressing the problems of self-consistently describing experimental data on fission probabilities for plutonium and americium isotopes, the yields of shape isomers in the α+²³⁸U reaction at alpha-particle energies in the range E _α = 20–32 MeV and the d+^242,240Pu reactions at deuteron energies in the range E _d = 20–30 MeV, fission times in the α + ²³⁸U reaction at alpha-particle energies in the range E _α = 20–32 MeV, and angular distributions of fission fragments in the α + ²³⁸U, ²³⁷Np reactions at alpha-particle energies in the range E _α = 20–100 MeV.     

Fusion-fission dynamics in the superheavy nucleus production

The fusion-fission reaction mechanism leading to the massive nucleus formation is studied. We investigate the superheavy nucleus formation in heavy-ion induced reactions by analysing the evaporation residue (ER) production in order to study the fusion dynamics and the decay properties of nuclei close to the stability island at Z=114. We consider the ⁶¹Ni+²⁰⁸Pb, ⁴⁸Ca+²³⁸U and ⁴⁸Ca+²⁴⁴Pu reactions that lead to the Z=110, 112 and 114 superheavy elements respectively. By using the dinuclear system (DNS) concept of the two interacting nuclei we calculate the quasifission-fusion competition in the entrance channel and the fission-evaporation competition along the de-excitation cascade of the compound nucleus. The dynamics of the entrance channel allows us to determine the beam energy window which is favorable to the fusion, while the dynamic evolution of the compound nucleus on the shell correction to the fission barrier and the dissipative effects influence the fission-evaporation competition in order to obtain the residue nuclei from the superheavy nucleus formation. We also calculate the τ _n /τ_tot ratio at each step of the de-excitation cascade of the compound nucleus and we present a systematics of τ _n /τ_tot (at first step of the cascade) for many reactions that lead to nuclei with Z=102–114.     

The decay of 31 sec98Zr, 2.9 sec98Nb and 51 min98Nb

The- and-radiations of⁹⁸Zr and^{98m, g} Nb have been investigated employing scintillation and semiconductor spectrometers and coincidence techniques. Sources of⁹⁸Nb were produced by the⁹⁸Mo(n, p)⁹⁸Nb reaction, sources of⁹⁸Zr by fission of²³⁵U with thermal neutrons applying chemical separations. For⁹⁸Zr, a half-life of 30.7±0.4 sec and aQ -value of 2.3±0.2 MeV were obtained, for the⁹⁸Nb isomers, half-lives of 2.86±0.06 sec and 51.3±0.4 min, andQ -values of 4.8±0.2 MeV and 4.5±0.2 MeV, respectively. No-rays were observed in the decay of⁹⁸Zr. The decay of 2.9 sec⁹⁸Nb was found to involve 11-ray transitions. In the decay of 51 min⁹⁸Nb, 54-transitions were detected. Spin and parity of 1⁺ and 4^– were deduced for the isomeric states of⁹⁸Nb.     

An experimental study of mass asymmetry in subbarrier photofission of 238U

A measurement of symmetric and asymmetric photofission yields of²³⁸U in the energy region from 4.5 MeV to 6.5 MeV has been performed. As γ-source the bremsstrahlung from a microtron has been used. The fission yields are obtained from β-counting of chemically separated isotopes ¹¹¹Ag, ¹¹⁵Cd and ¹¹⁷Cd for symmetric fission and ¹³⁹Ba for asymmetric fission. Some structures in the yield curves at about 5.3 MeV and a pronounced maximum at about 6.0 MeV in the valley-to-peak ratio in the fission fragment yield distribution are observed. It is suggested that these effects are connected with the double-hump fission barrier concept and indicate differences between fission barriers for symmetric and asymmetric fission.     

Single-particle enhancement of heavy-element production

Fusion barriers are calculated in a macroscopic-microscopic model for several cold-fusion heavy-ion reactions leading to heavy and superheavy elements. The results obtained in such a picture are very different from those obtained in a purely macroscopic model. For reactions on ²⁰⁸Pb targets, shell effects in the entrance channel result in fusion-barrier energies at the touching point that are only a few MeV higher than the ground state for compound systems near Z = 110. The entrance-channel fragment-shell effects remain far inside the touching point, almost to configurations only slightly more elongated than the ground-state configuration, where the fusion barrier has risen to about 10 MeV above the ground-state energy. Calculated single-particle level diagrams show that few level crossings occur until the peak in the fusion barrier very close to the ground-state shape is reached, which indicates that dissipation is negligible until very late in the fusion process. Whereas the fission valley in a macroscopic picture is several tens of MeV lower in energy than is the fusion valley, we find in the macroscopic-microscopic picture that the fission valley is only about 5 MeV lower than the fusion valley for cold-fusion reactions leading to compound systems near Z = 110. These results show that no significant “extra-extrapush” energy is needed to bring the system inside the fission saddle point and that the typical reaction energies for maximum cross section in heavy-element synthesis correspond to only a few MeV above the maximum in the fusion barrier.     

Dynamical approach to calculating angular distributions of fission fragments

A dynamical approach is proposed for calculating the angular distributions of fission fragments. The relaxation time for the degree of freedom associated with the projection of the total angular momentum of the nuclear system onto the symmetry axis and the coefficient of damping of the fission mode are the basic parameters of this approach. Experimental data on the anisotropy of the angular distributions of fission fragments and on the multiplicities of prescission neutrons are analyzed within the proposed model for ¹⁶O+²⁰⁸Pb (E _lab=110–148 MeV), ¹⁶O+²³²Th (120–160 MeV), ¹⁶O+²⁴⁸Cm (110–148 MeV), and ¹⁶O+²³⁸U (96–148 MeV). The relaxation time and the damping coefficient are estimated at τ_K=(5–6)×10^?21 s and β=4×10²¹ s^?1, respectively.     

Search for spontaneous fission emitters in Atlantis II (Part II)

In this paper we describe an attempt to isolate superheavy elements from Atlantis II hot brine deposits. Volatile, sulfidic elements (Cd, Pb) were isolated and investigated for spontaneous fission events with a mica-plastic solid state track detector sandwich. During nearly two years exposure time no spontaneous fission activity was observed. The upper limit for the concentration of superheavy elements (following this chemistry) in the Atlantis II-material is found to be 1.4·10^?14g/g. This limit is more than one order of magnitude lower than the concentration of a spontaneous fission activity in material from the Cheleken hot brine, as observed by Flerov and coworkers.     

Cumulative fission yield measurements with 14.7 MeV neutrons on 238U

The fission yield data in the 14 MeV energy neutron induced fission of ²³⁸U play an important role in decay heat calculations and generation-IV reactor designs. In order to accurately measure fission product yields (FPYs) of ²³⁸U induced by 14 MeV neutrons, the cumulative yields of fission products ranging from ⁹²Sr to ¹⁴⁷Nd in the ²³⁸U(n, f) reaction with a 14.7 MeV neutron were determined using an off-line γ-ray spectrometric technique. The 14.7 MeV quasi-monoenergetic neutron beam was provided by the K-400 D-T neutron generator at China Academy of Engineering Physics (CAEP). Fission products were measured by a low background high purity germanium gamma spectrometer. The neutron flux was obtained from the ⁹³Nb (n, 2n)^92mNb reaction, and the mean neutron energy was calculated using the cross-section ratios for the ⁹⁰Zr(n, 2n)⁸⁹Zr and ⁹³Nb(n, 2n)^92mNb reactions. With a series of corrections, high precision cumulative yields of 20 fission products were obtained. Our FPYs for the ²³⁸U(n, f) reaction at 14.7 MeV were compared with the existing experimental nuclear reaction data and evaluated nuclear data, respectively. The results will be helpful in the design of a generation-IV reactor and the construction of evaluated fission yield databases.     

Fission and gamma-ray decay of the238U shape isomer

The decay of the²³⁸U superdeformed shape isomer has been reinvestigated by detecting for the first time simultaneously the fission and the gamma-back decay. An electrostatic deflection system has been used to transport the^238mU recoils, produced in a²³⁸U(d, pn) reaction with a pulsed beam of 18 MeV deuterons, in front of a detector set-up consisting of three ion-implanted solidstate detectors and a Ge(Li) gamma-detector. The gamma-back decay has been measured in coincidence with conversion electrons of the 2⁺ 0⁺ transition deexciting the first rotational state in²³⁸U. Two gamma-transitions of 2.513 MeV and 1.878 MeV have been observed with half-lives consistent with the result obtained for the decay by delayed fissionT _1/2=(298±18) ns.Dedicated to Prof. Dr. P. Kienle on the occasion of his 60th birthday