W. Lang

Nuclear charge and mass yields for 235U(nth, f) as a function of the kinetic energy of the fission products☆

Abstract The mass spectrometer “Lohengrin” of the Institut Laue-Langevin in Grenoble was used to determine the yields of the light fission products as a function of A, Z, the kinetic energy E and the ionic charge state q. The nuclear charge and mass distributions summed over all ionic charge states are given for different kinetic energies between 83.6 MeV and 112.0 MeV. The proton pairing causes fine structures in both the mass and nuclear charge distributions at all kinetic energies. Additional fine structures due to shell effects are apparent in the yields at high kinetic energies. With the aid of Monte Carlo calculations the isobaric nuclear charge and energy distributions before neutron emission were determined. The mean nuclear charge Z of the fragments before neutron emission depends on the kinetic energy of the fragments. The pre-neutron emission variance σz2 of the isobaric Z-distributions is found to be independent of the total excitation energy Extot in the region 12 MeV ≦ Extot ≦ 37 MeV. This may be an indication for quantum-mechanical zero-point motion. The kinetic energy distributions for odd-Z elements are shifted toward lower kinetic energies by 0.4 MeV with respect to even-Z elements. The weak dependence of the proton odd-even effect on the kinetic energy of the fragments leads to the conclusion that the energy dissipation between saddle and scission is almost independent of the asymptotic kinetic energy. Furthermore it can be estimated that in about 25% of all fission events all protons remain in a paired state. The mean total excitation energy of the fragments shows no influence of the proton shell Z = 50 and the neutron shells N = 50 and N = 82.

Nuclide yields of light fission products from thermal-neutron induced fission of 233U at different kinetic energies

The yields of light fission products from thermal-neutron induced fission of 233U are measured as a function of their mass A, their nuclear charge Z, their kinetic energy E and their ionic charge state q at the recoil spectrometer Lohengrin of the Institut Laue-Langevin in Grenoble. The mass yields are determined by intercepting the fragments with an ionization chamber of high energy resolution positioned at the focal plane of the spectrometer. The nuclear charges and their yields are determined with the same ionization chamber by measuring the residual energy of fission products, selected monoenergetically by Lohengrin, behind a passive absorber made of parylene-C. The nuclear charge resolution enabled by this detector device is considerably improved to Z/dZ = 58. The nuclear charge and mass distributions summed over all ionic charge states are listed within the mass range 79 ⩽ A ⩽ 106 at 6 energies: E = 85.34, 90.41, 95.46, 100.50, 105.55 and 110.55 MeV. The energy-integrated nuclear charge and mass yields are also given. The isotonic and isotopic yields are shown. An odd-even effect in the yields is found for the protons as well as for the neutrons at all kinetic energies. The yield weighted total odd-even effect for the protons is found to be (22.1 ± 2.1)%, for the neutrons (5.4 ± 1.7)%. An odd-even effect for the protons in the mean kinetic energy is also observed. The displacement of the mean isobaric nuclear charges from the unchanged charge-density values and the variances of the isobaric nuclear-charge distributions reveal fine structures in their mass dependences.

Separation of isobaric elements by the energy-loss dispersion in carbon absorber foils

Abstract Fission fragments provided by the mass separator “Lohengrin” were slowed down in a homogeneous 1.2 mg/cm 2 thick carbon absorber. By measuring the residual energy with a time-of-flight method, a resolution of Z / ΔZ = 45 at Z = 40 was achieved.

Cold fragmentation in thermal-neutron-induced fission of 233U and 235U

Abstract The mass spectrometer Lohengrin of the Institut Laue-Langevin in Grenoble was used to measure fission-fragment mass yields in the mass range 80 ≤ A ≤ 107 for light-fission-fragment kinetic energies up to about 115 MeV for the reactions 233,235 U(n th , f). The kinetic energies corresponding to a common fixed yield level for each isobar reflect the influence of the proton pairing energy, but not of the neutron pairing energy. By using calculated Q -values for the different mass splits, mass distributions at fixed total excitation energy are deduced from the data. At a fixed total excitation energy of about 7 MeV, the yield increases from very asymmetric mass splits ( A L ≈ 80) to more symmetric mass splits ( A L ≈ 105) by more than two orders of magnitude. This strong dependence on the mass split seems to be correlated with the decreasing surface-to-surface distance of the unaccelerated fission fragments in this range of mass splits, as calculated under the assumption that the total Q -value is represented by the mutual Coulomb repulsion of the two fragments. The influence of the fission-fragment ground-state deformations on the yield in cold fragmentation could not be detected unambiguously.

212Th, a new isotope

The new isotope212Th has been found to decay withEα= (7.802 ±0.010) MeV andt1/2 =(30−10+20ms. The decay energies of the previously observed isotopes213Th and214Th could be confirmed. The measured half-lives are in agreement with calculated values.

The ionic charge distribution of fission products and the influence of internal conversion on highly preionized heavy ions

At the mass spectrometer LOHENGRIN of the Institut Laue-Langevin the ionic charge state distributions of235U(nth,f)-fission products separated according to their mass, nuclear charge and kinetic energy were determined. The mean values and the widths of the ionic charge state distributions are compared with semiempirical predictions. Deviations between the experimental data and the estimation of Nicolaev and Dmitriev are found. Furthermore, the influence of the internal conversion process on the ionic charge state distribution of highly ionized fission products was established.Internal conversion is observed mainly for odd-odd nuclei and nuclei with 59 neutrons. The Auger cascade following the internal conversion process shifts the ionic charge state distribution by about 3 charge units. The yield of conversion electrons per fragment was determined in the mass range 85≦A≦103.

Energy loss, energy straggling and angular straggling of heavy ions in carbon foils☆

Abstract The mass-separated fission product beam provided by the mass separator “Lohengrin” was used to investigate the energy loss, the energy loss straggling and the angular straggling of 235U(nth, f)-fission products in carbon foils. The energy loss of adjacent elements was measured by a time-of-flight technique. The dependence of the energy loss on the nuclear charge Z1 of the projectile deviates systematically from semiempirical predictions thus indicating Z1-oscillations. The fluctuations of the energy loss were found to be about twice as large as predicted for totally ionized projectiles. Empirical values for the counting losses under zero degree due to angular scattering are given.

A fast zero-time detector for time-of-flight measurements with heavy ions

Abstract Two variants of a zero-time detector for heavy ions using secondary electrons emitted by a thin carbon foil are described. The secondary electrons are accelerated in a parallel-plate geometry and detected by a surface-barrier detector. With the carbon foil inclined to the beam axis by 62°, a time resolution (fwhm) of 280 ps was obtained for 252 Cf-fission products. With the foil oriented perpendicular to the beam axis, the system is especially suited for slow particles with a large beam diameter. A time resolution of 370 ps was obtained at 8 mm beam diameter.

Pulse height defect of fission products in a silicon surface barrier detector

Abstract The pulse height defect of mass and energy separated 235 (n nt , φ) fission products awas measured and the change of the pulse height defect with the mass of the fission products was compared with the results of two previous calibration procedures.

Dependence of the evaporation residue production on the excitation energy of the compound nucleus in the reaction40Ar+164Dy

With the velocity filter SHIP at the heavy ion accelerator UNILAC the energy dependence of the evaporation residues production in the reaction40Ar+164Dy was investigated. The results show a much smaller shift of the (204Po, xn) excitation functions with respect to theoretical predictions than has been reported by other authors.