J.P. Theobald

Fission yields at different fission-product kinetic energies for thermal-neutron-induced fission of 239Pu☆

Abstract At the recoil spectrometer “Lohengrin” of the Institut Laue-Langevin in Grenoble, the yields of the light fission products from the thermal-neutron-induced fission of 239 Pu were measured as a function of A , Z , the kinetic energy E and the ionic charge states q . The nuclear charge and mass distributions summed over all ionic charge states were determined for different light fissionproduct kinetic energies between 93 and 112 MeV. The proton odd-even effect which was measured to be (11.6 ± 0.6)% causes considerable fine structure in the yields. The average kinetic energy of even- Z elements in the light fission-product group is 0.3 ± 0.1 MeV larger than for odd- Z elements. The neutron odd-even effect is (6.5 ± 0.7)%. The comparison with previously published data 1 ) for thermal-neutron-induced fission of 235 U reveals a correlation between the proton odd-even effect in the yield and in the kinetic energy of the elements. The dependence of the proton odd-even effect on the fragmentation is very similar for 235 U and 239 Pu when it is considered as a function of the nuclear charge of the heavy fission products. The isobaric variances σ z 2 . for thermal-neutron fission of 235 U and 239 Pu coincide at all kinetic energies if the influence of the proton odd-even effect is averaged out. This supports the hypothesis that the magnitude of σ z 2 is determined only by quantum-mechanical zero-point fluctuations. The influence of the spherical shells Z = 50 and N = 82 on the fragmentation is discussed.

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.

Fission mode fluctuations in the resonances of 235U(n,f)

Abstract Fission fragment mass- and total kinetic energy distributions were measured for single, isolated resonances and neutron energy bins covering the incident neutron energy range from 0.006 eV to 130 eV. The measurements were performed at the Geel Electron Linear Accelerator (GELINA) of the European Communities using a Frisch-gridded ionization chamber. Fluctuations of the fission fragment mass distributions as function of resonance energy were observed, which are correlated with fluctuations of the reaction Q-value and with the measured total kinetic energy 〈TKE〉 averaged over all fragments. In the resonance region the fluctuations in 〈TKE〉 from resonance to resonance are observed with amplitudes up to about 450 keV. The correlations between the mass-distribution fluctuations and other parameters like spin J, spin orientation quantum number K, angular distribution fluctuations and the fluctuations of the average number of neutrons emitted in fission, v , are evaluated and discussed. An interpretation of the v - fluctuations observed in other experiments is given in terms of the mass distribution fluctuations. The fluctuations of the mass-distribution parameters and of the total kinetic energy distributions as function of mass are viewed in the frame of the fission channel model of Bohr and Wheeler and of the recent multi-fission mode random neck-rupture model of Brosa, Grossmann and Muller.

Fission modes in the compound nucleus 238Np

Abstract Potential energy calculations in conjunction with the multi-modal random neck-rupture model of Brosa, Grosmann and Muller (BGM-model) indicate the existence of four fission modes in the compound nucleus 238Np. For the first time the splitting of the standard mode into three submodes could be demonstrated theoretically. The existence of a third standard mode in fission has already been proposed for heavier nuclei, where a nonnegligible yield of more asymmetric fission fragments is experimentally observed. The calculated fission fragment properties for 238Np are discussed and compared to experimental results.

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.

Cold fission of 233U(nth, f)☆

Abstract The recoil spectrometer Lohengrin of the Institute Laue-Langevin in Grenoble was used to measure the yields of light fission fragments separated according to their mass and nuclear charge for the mass-number range 76 ⩽ A L ⩽ 93 in the cold fission regime. Neutron emission being forbidden energetically, the correlated heavy fission fragments including their kinetic energies, as well as the total excitation energy TXE of both fragments were determined by conservation laws. Close to TXE = 0, the yields seem to be strongly influenced by the level density of the fragments. In order to obtain yields for the fission into the ground-states of the two fragments, the measured yields per excitation energy interval were divided by calculated level densities for the two fragment system and extrapolated to TXE = 0. These deduced ground-state yields are influenced by the Q values and by the potential energy of the fragments at the scission configuration. The observed trends in the ground-state yields as a function of nuclear charge and mass may qualitatively be reproduced by calculated penetrabilities through the scission barrier. In these calculations, no correspondence between the calculated penetrabilities and the ground-state yields is found if calculated fragment ground-state deformations are used. There is no difference in the influence of neutron and proton pairing on the ground-state yields.

Low-energy ternary fission

Abstract The actual situation of the research on ternary fission, mainly α-particle accompanied fission, is reviewed, with the accentuation on recent experimental and theoretical investigations. The results of multiparameter experiments are discussed in more detail, and the role of fragment deformation for a correct description of the ternary fission process is outlined.

A vacuum photodiode as light sensing element for gas scintillation counters

Abstract The authors investigated the possibility of replacing the conventional photomultiplier tube with a vacuum photodiode as light sensor for gas scintillation counters. This was successfully done for heavy ions in counters where a high light output was available. It is shown that the diode can also be used with a gas scintillation proportional counter measuring low-energy particles if a large light amplification factor exists. Test results with 238 U ions at 1.4 MeV/amu, 252 Cf fission fragments and 241 Am α-particles are presented.

Noble-Gas Scintillation Counters for the Detection of Energetic Heavy Ions

Different types of noble-gas scintillation counters are described, which were advantageously applied to the detection of energetic heavy ions: (a) a low-pressure gas scintillator, operated with Xe, Ar and Ar-N2 mixtures, (b) a photo-diode gas scintillator, (c) a gas scintillator with built-in bidimensional position-sensitive drift chamber, and (d) a solid-Xe scintillator. Details of design and performance are presented.

A Multiparameter Gas-Scintillation Counter for Heavy Charged Particles

Recent results with gas-scintillation detectors for heavy charged particles are summarized and a heavy-ion detector telescope is described, with a multiparameter gas scintillator as the main detector component.