Fragment angular distributions from fission of238U,209Bi and197Au by 140-MeV4He Ions

The fission fragment angular distributions from reactions of 140-MeV⁴He ions with²³⁸U,²⁰⁹Bi and¹⁹⁷Au have been studied. From the anisotropies in the angular distributions, values for? _eff, the effective moment of inertia at the fission saddle point, have been estimated and compared with results obtained at lower bombarding energies. The derivation of? _eff values has included corrections for the effects of incomplete fusion mechanisms on the orbital angular momentum distribution of the fissioning nuclei and for neutron evaporation prior to fission. The results are also compared with heavy-ion-induced fission data for systems of similar fissility. Also, examination of the forward-backward symmetry of the²³⁸U angular distribution substantiates other results which show that the fraction of fission reactions which follow complete fusion of the target and projectile is less than 0.5 for 140-MeV⁴He-ion bombardment of²³⁸U.     

Studies of4He emission in both fusion-like and inelastic reactions of 340-MeV40Ar+238U

The fission-fragment angular correlation technique has been used in conjunction with a position-sensitive avalanche detector to identify the fusion-like and inelastic collisions of 340-MeV⁴⁰Ar+²³⁸U. For each of these two reaction types, we have characterized the correlated⁴He emission by measuring three-fold coincidences between two fission fragments and the light charged particle. We find an abundance of both evaporation-like and direct⁴He emission, each of which is associated with both fusion-fission and sequential-fission processes. Several comparison tests of both the coincidence and singles data strongly indicate that very little⁴He emission is due to evaporation from accelerated fission fragments in this system. The evaporative⁴He emission appears instead to have a strong contribution originating from the composite system prior to fission. The forward angle⁴He emissions, characterized by prominent forward-peaking and relatively high energies, suggest an origin associated with the very early stages of the collision.     

Fission reactions of 469-MeV56Fe+238U: Detection of4He/1H emission from pre- and post-fission sources

The emission of⁴He and¹H has been measured in coincidence with fission for reactions of 469-MeV⁵⁶Fe+²³⁸U. By using a gas-ionization telescope in kinematic coincidence with a position-sensitive avalanche detector, the folding angle between two fission fragments was determined in order to distinguish fusion reactions from fission following smaller-momentum-transfer collisions. In both fusion fission and sequential fission reactions, the⁴He/¹H energy spectra are relatively narrow with relatively flat angular distributions at backward angles and become broader in energy with enhanced cross-sections at forward angles. The extent of forward peaking is significantly greater for peripheral collisions than for central collisions. The light-charged-particle multiplicities are quite similar for⁴He and¹H, being much larger for fusion fission than for sequential fission. Detailed comparisons of the spectral shapes with Monte Carlo simulations of reaction kinematics impose strong constraints on the participation of different emission sources. We find important contributions to the observed⁴He/¹H emission both from accelerated fragments (FE) and from the composite system prior to fission (CE). For⁴He emission, the multiplicity of CE is much larger for fusion fission than for sequential fission, possibly as a consequence of the higher spins and shorter reaction times associated with deeply inelastic and quasi-elastic processes. For¹H emission, a corresponding but somewhat smaller difference is observed for the CE multiplicities. An excess of⁴He/¹H particles, found at forward angles in both fusion and sequential fission processes, cannot be attributed to evaporative emission from any fragments and therefore must originate in pre-thermalization emission.     

Muon capture rates in233U,234U,235U,236U,238U,and237Np

The time spectra for muon induced fission of²³³U,²³⁴U,²³⁵U,²³⁶U,²³⁸U, and²³⁷Np were measured simultaneously, detecting both fragments in coincidence in a fast multi parallel plate avalanche detector. The observed mean lifetimes τ are 68.9 ± 0.3 ns, 70.6 ± 0.2 ns, 72.2 ±0.2 ns, 74.3 ± 0.3 ns, 77.0 ±0.4 ns, and 69.8 ±0.2 ns for the above isotopes, respectively. No second time component, tentatively ascribed to a possible fission isomer, was observed. The deduced total capture rates are compared with theoretical predictions.     

Reactions of40Ar with233U,235U,and238U at the barrier

Production cross sections for target-like transfer products in reactions of⁴⁰Ar with^233,235,238U at the barrier were determined using radiochemical techniques. The heaviest products detected are isotopes of californium (Z=98). In addition to the quasi-elastic component of the isotopic distributions observed in the vicinity of uranium, there are also relaxed contributions throughout the entire region. The peak positions of the isotope distributions of this component for fixed atomic number, after the transfer of > 3 charges, approach closely the minimum of the potential energy surfaces (PES). The experimental results thus indicate the cold formation of the reaction products. A search for long-lived heavy actinides produced, by fusion-evaporation, via isotopes of element 110 and their subsequent decay through α-decay chains, remained unsuccessful at a cross section limit of 21 pb.     

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.     

Electrofission of 234U, 236U and 238U: Angular distributions and E2 strength functions

The electrofission angular distributions for ²³⁴U in the energy range 5.5 to 25 MeV were measured and are analyzed together with those obtained previously for ²³⁶U and ²³⁸U. The competition between the K = 0 and K = 1 fission channels following E2 excitation is established, showing a dominance of the K = 0 channel for near-barrier fission. The E2 fission strength functions for ²³⁴U, ²³⁶U and ²³⁸U are deduced as well, and the E2 fission probabilities (at energies below the pairing gap) are estimated. A substantial concentration of E2 strength near the fission barrier is found, in good agreement with earlier photofission angular-distribution studies.     

Measurements of235U/238U isotopic ratio in the photoproduct UF5 by multiphoton ionization and time-of-flight mass spectrometry

A MultiPhoton-Ionization Time-Of-Flight Mass Spectrometry (MPI/TOFMS) apparatus was developed for real-time measurement of the uranium isotopic ratio in nascent UF₅ formed by the 266 nm photolysis of effusive UF₆ ( < 300 K, 1.3 × 10^–4 Pa). The UF₅ was selectively and efficiently multiphoton ionized by 532 nm radiation at appreciably low fluences ( < 10 J/cm²). The main ions observed, U⁺ and U²⁺, were subsequently analyzed with a TOFMS with mass resolution of 1190 to separate²³⁵U ⁿ⁺ and²³⁸U ⁿ⁺ completely. The isotopic ratio measurements showed good precision resulting from the excellent agreement which was observed between the isotopic ratios in UF₅ products and those in a parent UF₆ sample. These results suggested that the MPI/TOFMS method can be applied to the real-time analysis of separation factors in the molecular laser isotope separation of uranium by ionization of UF₅ following the infrared photodissociation of UF₆.     

Cross sections for 209Bi, 232Th, 235U, 238U, and 237Np fission induced by intermediate-energy protons and deuterons

The cross sections for ²⁰⁹Bi, ²³²Th, ²³⁵U, ²³⁸U, and ²³⁷Np fission induced by protons of energy in the range 1.0–3.7 GeV and by deuterons of energy 1.0 GeV were measured. The results are compared with data from other experiments, with available estimates, and with the predictions of theoretical models.     

Range and energy-loss of 16.34 Mev/u 238U in kapton polyamide

Abstract

The range and energy-loss of 16.34 MeV/u ²³⁸U in Kapton-polyamide plastic have been measured by the nuclear track technique. The experimental ranges are compared with two sets of theoretical values. It has been observed that ranges obtained from the computer code ‘DEDXT’ are found to be 5–8% lower as compared to measured ones whereas those from Northcliffe and Schilling are about 10–15% overestimated above 8 MeV/u.     

Symmetric and asymmetric yields in the photofission of232Th,235U,238U and239Pu

Measurements are presented for the mass and energy distributions of fragments from the photofission of²³²Th,²³⁵U,²³⁸U and²³⁹Pu. The experiments were performed for bremsstrahlung endpoint energies between 15 and 55 MeV, using silicon surface barrier detectors. The results are discussed with respect to the competition between the symmetric and asymmetric fission modes.     

Study by track detectors of the 580-MeV photofission of U,Au, Te,Ag, Se,Cu and Cr

The fission of U, Au, Te, Ag, Se, Cu and Cr induced by photons with a maximum energy of 580 MeV has been investigated using mica and glass track detectors in a sandwich configuration. Cross sections for U, Au, Te and Ag fission have been obtained which are in fair agreement with existing data for charged-particle induced fission, but are substantially lower than previous photofission results, obtained by other authors for Te and Ag targets.