Dynamical calculation of the mass fragmentation in the collision238U-238U

The fragmentation of the nuclear system²³⁸U-²³⁸U is studied by treating the fragmentation coordinate quantum mechanically. The time dependent Schrödinger equation, the Hamiltonian of which is calculated from the microscopic asymmetric two center shell model, is solved by the finite difference method. In order to study the fragmentation mechanism, model calculations have been carried out, by assuming the collective fragmentation potential as an oscillator potential which is adjusted to the realistic potential. Effects arising from the dynamical treatment of the fragmentation coordinate are found to be important in the collision of²³⁸U on²³⁸U. The dependence of the fragmentation on the incident energy is discussed.     

The interaction of 538 MeV 56Fe with 238U

Binary coincident fragments from the ⁵⁶Fe+²³⁸U reaction at 538 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 energy event space. The angular distributions of cross section, energy and mass have determined for each event region and particular attention paid to that part containing possible fissions following complete fusion. The total reaction cross section consists of about 50 % of binary fragmentation channels. The remaining channels correspond to a ternary process of heavy recoil fission.     

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.     

Photoinduced fission of the doubly even uranium isotopes 234U, 236U and 238U

A measurement of the angular distributions and yields of fission fragments in the photofission of ²³⁴U has been performed between 5.2 and 6.4 MeV. As γ-source, the bremsstrahlung from a microtron was used. For the detection of the fission fragments, solid-state track detectors were used. The present data for ²³⁴U have been analysed together with earlier obtained data for ²³⁶U and ²³⁸U. The values of the fission barrier parameters obtained are compared to results in theoretical macroscopic and microscopic fission potential energy calculations.     

Heavy-ion potentials for ellipsoidally deformed nuclei and application to the system 238U + 238U

The nuclear potential between deformed nuclei is calculated within the framework of the double-folding model. An analytical expression for the potential is obtained on condition that the two-body interaction is of gaussian type and the nuclear densities have ellipsoidal equidensity surfaces. The computed potential values are used for a least-square fit of the radial potentials in a multipole expansion of the heavy-ion potential. The method is applied for the calculation of the ²³⁸U + ²³⁸U potential.     

Comparison of non-compound neutron and proton emission in 16O induced reactions on 238U at 310 MeV

The emission of energetic neutrons and protons in coincidence with fission fragments was measured for the reactions ²³⁸U(¹⁶O, nf) and ²³⁸U(¹⁶O, pf). Larger cross sections were observed for the emission of high-energy protons than for the emission of high-energy neutrons. The differences in spectral shapes and the implications for the coalescence model are discussed.     

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 of 232Th, 238U and 235U induced by negative muons

The absolute yields of prompt and delayed fission induced by negative muons in ²³²Th, ²³⁸U and ²³⁵U have been measured. The delayed fission yields are much lower than could be predicted from Γ_n/Γ_f systematics for 15–20 MeV nuclear excitation. The systematics of prompt fission yields are compared with recently obtained photofission data. It is suggested that prompt fission can be used for investigating the channel structure of the fission barrier.     

Study of the giant dipole resonance of 232Th and 238U using elastic and raman photon scattering

Differential cross sections for elastic and Raman scattering of photons from ²³²Th and ²³⁸U targets were measured. Eight photon energies in the range 7.9–11.4 MeV were used and were obtained from thermal neutron capture in Fe, Cr, Cu and Ni. The angular distribution of the elastic and Raman scattered radiation from ²³²Th was measured. The results are compared with calculations of the simple rotator model and the dynamical collective model of the giant dipole resonance after incorporating the effect of Delbruck scattering.     

The influence of the nature of soil and plant and pollution on the 238U, 232Th, 222Rn and 220Rn concentrations in various natural honey samples using nuclear track detectors: Impact on the adult consumers

²³⁸U and ²³²Th concentrations as well as ²²²Rn and ²²⁰Rn α-activities per unit volume were measured in various natural honey samples collected from different regions in Morocco using CR-39 and LR-115 type II solid state nuclear track detectors (SSNTDs). These radionuclides were also measured in soils, plant flowers and nectar solutions corresponding to the honey samples studied. In addition, these radionuclides were measured in different imported honey samples. The measured ²³⁸U, ²³²Th, ²²²Rn and ²²⁰Rn concentrations ranged from (1.5±0.1) mBq kg⁻¹ to (10.6±0.6) mBq kg⁻¹, (1.1±0.1) mBq kg⁻¹ to (4.2±0.2) mBq kg⁻¹, (1.5±0.1) Bq kg⁻¹ to (10.6±0.6) Bq kg⁻¹ and (1.1±0.1) Bq kg⁻¹ to (4.2±0.2) Bq kg⁻¹ for the honey samples studied, respectively. Annual ²³⁸U, ²³²Th and ²²²Rn intakes by Moroccan adults from the consumption of honey were assessed. The influence of the nature of soil and plant on the ²³⁸U and ²³²Th contents of the studied honey samples was investigated. These measurements were completed by an investigation of the ²³⁸U and ²³²Th transfer between soils and plant flowers and that between plant flowers and honey, and also by the investigation of the influence of pollution due to different material dusts on ²³⁸U, ²³²Th and ²²²Rn in the honey samples studied. Committed equivalent doses due to the annual intake of ²³⁸U, ²³²Th and ²²²Rn were evaluated in the organs of adult members of the Moroccan rural population from the ingestion of the honey samples. The maximum total committed effective dose due to ²³⁸U, ²³²Th and ²²²Rn from the ingestion of natural honey by the Moroccan rural population was found to be equal to 0.64 μ Sv y⁻¹.     

Non-axiality effects in quadrupole hyperfine splitting in muonic 238U

The quadrupole hyperfine splitting in muonic atoms with nuclei having non-axial symmetry is calculated within the rigid asymmetric rotor model. The X-rays emitted in 4F-3D and 3D-2D transitions of muonic ²³⁸U were measured with high resolution and an analysis of the spectrum was performed to find the quadrupole deformation parameters β and γ.     

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.     

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.     

Evidence of complete fusion in the subbarrier 16O+238U reaction

Evaporation residue cross sections in the ¹⁶O+²³⁸U reaction were measured for the energy range from above-to extreme subbarrier. We used a He-gas-jet system to transport the fusion products, and the α decay of the evaporation residues was measured by using a rotating wheel system. The measured cross sections for ^248,249,250Fm 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. We conclude that complete fusion is the main process in the subbarrier energy region, and quasifission is not an important channel. The text was submitted by the authors in English. On leave from China Institute of Atomic Energy, Beijing, China.     

Search for coulomb fission induced by 84Kr ions on 238U

We have searched for Coulomb fission induced by ⁸⁴Kr ions on a ²³⁸U target at energies ranging from the interaction barrier down to 37 MeV below (408–458 MeV lab). No event attributed to Coulomb fission was detected; it was deduced that the cross section for this reaction is lower than 0.3 mb/sr near the interaction barrier. This value was compared to theoretical predictions. However, fission events originating from transfer reactions at the interaction barrier have been detected.     

Multimode approximation for 238U photofission at intermediate energies

The yields of products originating from ²³⁸U photofission are measured at the bremsstrahlung endpoint energies of 50 and 3500 MeV. Charge and mass distributions of fission fragments are obtained. Symmetric and asymmetric channels in ²³⁸U photofission are singled out on the basis of the model of multimode fission. This decomposition makes it possible to estimate the contributions of various fission components and to calculate the fissilities of ²³⁸U in the photon-energy regions under study.     

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.     

Emission of high-energy alpha particles in nuclear reactions of 48Ca and 56Fe ions on 181Ta and 238U targets

The results of experiments on measuring the energy spectra of alpha particles in reactions with heavy ions are presented. The measurements were performed using the high-resolution magnetic analyzer MAVR with beams of ⁴⁸Ca (280 MeV) and ⁵⁶Fe (320 and 400 MeV) on ¹⁸¹Ta and ²³⁸U targets at an angle of 0°. A strong dependence of the double differential cross sections for production of alpha particles on the atomic number of the target nucleus was observed, which indicates that fast alpha particles are mainly emitted from the target nucleus; this conclusion was also confirmed by calculations within the time-dependent Schr?dinger equation approach. An analysis of the obtained experimental data was carried out within the model of moving sources modified to consider the kinematic limits for two-body and three-body exit channels.     

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.     

Measurement of the total cross section for 235U and 238U photofission in the δ-resonance region

The total cross section for the photoemission of fission fragments σ(γ, F) has been measured for ²³⁵U and ²³⁸U using tagged bremsstrahlung photons in the energy range 120–460 MeV. The fission fragments were detected in parallel plate avalanche detectors. The results are compared with other information on the photonuclear absorption cross section.