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.     

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.     

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.     

Light charged particle emission and fission in238U+238U collisions at 1,740 MeV

Inclusive⁴He and⁴H energy spectra and heavy fragment coincidence correlations have been measured for reactions of 7.31 MeV/u²³⁸U with²³⁸U and^?197Au targets. The H/He production cross sections are in the range 15–26 mb, and their emission spectra are very similar for the two systems. The observed strong kinematic shifts with angle are reproduced in shape and magnitude by Monte Carlo simulations of particle evaporation from projectile-like and target-like fragments, indicating competition between charged particle emission and sequential fission. No evidence is found for high energy charged particle emission associated with ultra-highZ composite systems. Heavy fragment measurements indicate an abundance of quasielastic and deeply inelastic reaction fragments, as well as sequential fission of target and projectile nuclei. For²³⁸U nuclei, the fission occurs predominantly in an asymmetric mode, reminiscent of fission at low excitation energy. For²³⁸+²³⁸U reactions in the vicinity of the grazing angle, the frequency of single sequential fission (with survival of the partner fragment) is twice as large as double sequential fission in which both the target and projectile undergo fission. In²³⁸U+¹⁹⁷Au reactions, the survival probability of the heavy fragments is even greater. The surprisingly high survival probabilities of high-Z fragments imply a preponderance of very soft collisions in these very-heavy-ion reactions, at least at energies not very far over the Coulomb barrier.     

Sub-Coulomb transfer and fission in collisions of129Xe,132Xe and136Xe with238U

Integral cross sections for fission and for one- and two-neutron transfer reactions in the system¹³²Xe+²³⁸U were measured radiochemically in the energy range 0.7≦E/E _Coul≦1. The excitation functions for fission and transfer are found to be essentially parallel below 0.85×E _Coul. Even at the lowest energies the transfer cross sections exceed the fission cross section by more than one order of magnitude. With the other projectiles¹²⁹Xe and¹³⁶Xe different transfer cross sections illustrating their sensitivity for the ground stateQ-values,Q _gg , are observed while the fission cross sections are the same as in the¹³²Xe +²³⁸U reaction. The fission data are interpreted in terms of a continuous transition between Coulomb fission and several transfer-induced fission processes.     

Search for new isotopes of element 112 by irradiation of 238U with 48Ca

The reaction ⁴⁸Ca+²³⁸U was investigated at the recoil separator VASSILISSA at the Flerov Laboratory of Nuclear Reactions, JINR Dubna, in attempts to synthesize new isotopes of element 112. The experiments were performed at two beam energies resulting in excitation energies of the compound nucleus of 33 and 39 MeV. The collected beam dose was 3.5 × 10¹⁸ and 2.2 × 10¹⁸, respectively. Two spontaneous fission events were observed at the lower beam energy, which tentatively were assigned to the new neutron rich isotope ²⁸³112 produced in the reaction ²³⁸U(⁴⁸Ca,3n)²⁸³112. The measured cross-section is (5.0^+6.3 _−3.2) pb and the half–life is (81⁺¹⁴⁷ ₋₃₂) s. No event was observed at the higher beam energy resulting in the upper cross–section limit of 7.3 pb. Received: 12 October 1998 / Revised version: 12 January 1999     

Possible observation of light neutron nuclei in the alpha-particle-induced fission of 238U

Searches for nuclear-stable multineutrons among products originating from the fission of ²³⁸U nuclei that is induced by 62-MeV alpha particles were performed by the activation method. The reaction involving the transfer of four neutrons and occurring on the isotope ⁸⁸Sr, ⁸⁸Sr( ^x n, (x ? 4) n)⁹²Sr??⁹²Y, was used to identify nuclear-stable multineutrons. A line at the energy of E = 1384 keV was found in the measured gamma-ray spectra of irradiated samples. This line, together with the measured time dependence of the decrease in its activity, is indicative of the formation of the beta-active nucleus ⁹²Sr. This result was reproduced in repeated measurements. It suggests the possible existence of nuclear-stable multineutrons ( ^x n) for x ?? 6. The differential cross section for the ^x n yield at an angle of 30° in the alpha-particle-induced fission of ²³⁸U was about 6 × 10^?2 mb/sr.     

Photofission of 238U at the energy of the giant dipole resonance

Photofission of ²³⁸ U by bremsstrahlung photons is studied at four energies of an electron accelerator: 19.5, 29.1, 48.3, and 67.7 MeV. The yields of the fission fragments after the emission of prompt neutrons are obtained using the gamma-ray spectroscopic technique. The mass distributions of photofission are obtained at different upper energies of the bremsstrahlung spectrum. The ratio the symmetric-fission mode to the asymmetric mode is obtained from the mass distribution. The symmetric mode becomes 3–4 times greater than the asymmetric as the excitation energy of the ²³⁸U nucleus increases from 12 to 16 MeV.     

Quasielastic transfer reactions in 238U + 197Au and 197Au + 197Au collisions near the Coulomb barrier

Differential cross sections as a function of cm angle were measured for 1n- and 2n-transfer reactions in ²³⁸U + ¹⁹⁷ Au and¹⁹⁷ Au + ¹⁹⁷ Au collisions in the energy range from 0.881 V_c to 1.093 V_c and 0.825 V_c to 0.964 V_c, respectively. For ¹⁹⁸Au and ¹⁹⁹Au from the ²³⁸U + ¹⁹⁷Au collisions, for reduced distances of closest approach d _o 1.55 fm, the angular distributions at all bombarding energies are well described by the semiclassical theory. Equivalently, the transfer probabilities show the expected exponential decrease with increasing d _o over many orders of magnitude. For all other transfer products from ²³⁸U + ¹⁹⁷Au collisions, and for all transfer products from ¹⁹⁷Au + ¹⁹⁷Au collisions, markedly reduced cross sections relative to the semiclassical theory are observed for central collisions at all bombarding energies, even for values of d _o that are well outside the region where absorption is known to set in. Only for the more peripheral collisions, one observes agreement of the angular distributions (transfer probabilities) with the semiclassical expectations. The deviations for central collisions are absent for reactions with positive Q _gg values and scale roughly with increasingly negative values of Q _gg, i.e. with increasing Q-value mismatch. Channel coupling is proposed as the relevant mechanism.     

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.     

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.     

Small-angle neutron scattering from Pb and 238U between 0.6 and 2.2 MeV

Differential cross sections for neutrons scattered from natural Pb and 99.9 % isotopically pure ²³⁸U have been measured at 0.5°, 1.0°, and 1.5°. A neutron energy continuum was produced by bombarding a thick natural lithium target with a 4 MeV, nanosecond-pulsed proton beam. Neutron energies were determined by time-of-flight techniques. Flight paths from the neutron source to the scatterer and from the scatterer to the detector were each about 5 m. For the 0.5° measurements an annular detector geometry with an angular resolution of ± 0.1° was developed to maximize detection solid angle. Data were averaged over 100 keV intervals from 0.6 to 2.2 MeV and were corrected for backgrounds, multiple scattering and inelastic scattering. Measured cross sections were compared to optical-model calculations which included electromagnetic interactions of neutrons with the nuclear Coulomb field. Inclusion of an induced neutron electric dipole moment interaction was not warranted by the data. The angular dependence of the cross section was fitted with a function $\text{A + B}\text{cot}{}^2\text{case1}\text{2}$θ at each energy. Mean values of B for ²³⁸U are in agreement with theoretical predictions. Values of B for Pb are apparently 15 % too low.     

Energy Dependence of the Most Stable Nuclei Production in Proton-Induced 238U and 232Th Fission

Using the available experimental data, production cross sections of the most stable nuclei have been calculated for the proton-induced fission of ²³⁸U and ²³²Th at 12, 20, 35, 50, 96 MeV in case of ²³⁸U and at 8, 9.3, 12, 19.55, 32.2, 44.7, 53 MeV for ²³²Th. The analysis has been carried out for the fission fragment mass ranges corresponding to N/Z ratios in the ranges 1.33 ± 0.09 and 1.32 ± 0.08 for ²³⁸U and ²³²Th respectively. Results have been compared with the ones generated indirectly by employing GEF nuclear reaction code, version 2017/1.1. From the production cross sections point of view, for the same energy, ²³²Th is found to be a better target than ²³⁸U for producing nuclei A around the symmetric mass peak, while ²³⁸U comes out to be a better one than ²³²Th for producing the fission fragments around the asymmetric peaks of the mass distribution.

     

238U photofission in the energy region of the giant dipole resonance

²³⁸U photofission is studied in the region of nuclear excitation energies that extends up to 20 MeV. Independent photofission-fragment yields and cumulative photofission-fragment yields after the emission of fast neutrons are measured by gamma-spectroscopy methods. The mass distributions of photofission fragments are obtained at the endpoint bremsstrahlung energies of 19.5, 29.1, 48.3, and 67.7 MeV. A comparative analysis of the behavior of the symmetric and asymmetric modes of photoninduced fission as a function of the average excitation energy of the fissioning nucleus is performed. The integrated yield of ²³⁸U photofission is calculated, and the evaluated cross sections for photofission and for photoneutron reactions are discussed.     

Heavy-nucleus multifragmentation induced by coherent bremsstrahlung photons with endpoint energy <Emphasis Type="Italic">E</Emphasis><Stack><Subscript>γ</Subscript><Superscript>max</Superscript></Stack>=4.1 GeV

New experimental data are presented that were obtained by studying the multifragmentation of ¹⁹⁷Au, ²⁰⁹Bi, ²³⁸U, and ²⁴³Am nuclei in a coherent bremsstrahlung-photon beam with an endpoint energy of 4.1 GeV from the Erevan Synchrotron. For the first time, four or more fragments are observed. For all the nuclei studied here, the respective production cross sections are about (1–3)×10^?3 of the total inelastic photon-nucleus cross section. The fragment yields are isotropic.     

Photofission of 238U in the giant-resonance region

Data on cross sections for the reaction ²³⁸U(γ,F) in the giant-resonance region were analyzed in connection with the preparation of new experiments aimed at studying ²³⁸U photofission—in particular, in beams of photons from in-flight positron annihilation on internal targets of positron storage rings. These data were taken from measurements also performed with annihilation photons but from positron beams external to the accelerators used. The procedures applied in such measurements and based both on processing the multiplicity of detected neutrons and on detecting fission fragments were also analyzed.     

Symmetric and asymmetric fission modes in proton-induced fission at 660 MeV of <Superscript>238</Superscript>U

Fission product cross sections of intermediate-energy fission of ²³⁸U were used in order to construct the charge and mass yield distributions. Enriched target of ²³⁸U was irradiated by proton beam with energy 660 MeV for several hours at the LNP Phasotron, Joint Institute for Nuclear Research (JINR), Dubna, Russia. The charge distribution of the fission fragments was analyzed for calculation of isobaric cross sections. The mass yield curves were expanded into symmetric and asymmetric components according multimodal fission approach. The fissility values of actinides were calculated at given proton energy. The obtained results have been compared to the same data for targets ²³⁷Np and ²⁴¹Am.     

The study of the (α, α′ f) reaction at 120 MeV on 232Th and 238U: (II). Fission barrier properties deduced from fission probabilities and angular distributions

The fission probabilities and angular distributions of the fission fragments for the (α, α'f) reaction on ²³²Th and ²³⁸U at a bombarding energy of 120 MeV have been measured from about 4 to 14 MeV excitation energy. Evidence for sub-barrier resonances has been found, the negativeparity ones occurring at the same excitation energy where photofission resonances have been observed. The data are analyzed with the two-humped barrier model. For ²³⁸U the data are reasonably well fitted with barriers similar to those known from the literature. For ²³²Th though, the outer barrier parameters are quite different: the height E_B = 6.6 MeV and the width (khω)_B = 1.2 MeV. Also for ²³²Th, introducing an additional mass symmetric and axially asymmetric outer barrier, as was previously found necessary for ²³⁸U, does not result in a good fit to the data at higher excitation energies.     

Large contribution of deep inelastic processes to reactions of40Ar and48Ca with238U

Differential recoil range distributions have been measured for heavy-reaction products ranging from Te(Z=52) to quasielastic transfer products near the charge and mass of the targets for the reactions of 276 MeV⁴⁸Ca+²³⁸U, 237MeV and 250 MeV⁴⁰Ar+²³⁸U, and 259 MeV⁴⁰Ar+¹⁹⁷Au. The measured recoil range distributions for the⁴⁰Ar+¹⁹⁷Au reaction agree with range distributions calculated from the known projectile-like fragment angular distributions for this reaction. The angular distributions of recoil products formed in the uranium target reactions are deduced and show that the products in the₇₅Re to₈₃Bi region have backward peaked angular distributions characteristic of deep inelastic reactions. The heavy product angular distributions smoothly vary from a (1/sinθ) shape to an exponential shaped backward peak as the atomic number of the product increases from 52 to 83. The trend in the deduced angular distributions for those elements for which recoil range distributions were determined in the⁴⁰Ar+¹⁹⁷Au reaction and the 250 MeV⁴⁰Ar+²³⁸U reaction is similar, suggesting that just as for the Ar+Au system the composite system for the uranium target reaction is also not fully equilibrated along the mass asymmetry coordinate. These conclusions show that the fraction of the total reaction cross section resulting in complete fusion must be re-evaluated for the⁴⁰Ar+²³⁸U reaction and similar heavy-target reactions.