Experimental beta-decay energies of short-lived light fission products

The beta-decay energy of 20 nuclides in the mass region 88≦A≦102 has been measured using the on-line mass separator LOHENGRIN. The experimental values obtained are compared with theoretical predictions of different mass formulae.     

Experimental beta-decay energies of very neutron-rich Cs isotopes

TheQ _β-values of the very neutron-rich cesium isotopes with mass number 142≤A≤146 have been measured with a plastic scintillation detector telescope at the fission product separator OSTIS. The mass excesses calculated from these decay energies agree well with the values obtained by direct mass determinations.     

Total kinetic energies and mass yield distributions of Cf fission fragments

The variation in the mass yield distribution of the spontaneous fission of ²⁵²Cf as a function of the total kinetic energy (TKE) of the two fragments is investigated. With increasing TKE the peaks of the mass yield distributions become narrower.     

The beta-decay energies of103In and104In

The β⁺ spectra of neutron-deficient nuclei with mass numbers 103 and 104 have been measured, using a semiconductor beta spectrometer. The nuclei were produced by on-line mass separation of reaction products formed by bombarding an enriched Mo target with Ne ions. Values for Q_EC have been deduced. The decay energies are in agreement with systematics. The decay of¹⁰⁴In shows remarkably strong feeding to levels above 5 MeV. In addition, Q_EC values for₁₀₃Cd,₁₀₃Ag,₁₀₄Ag and_104mAg have been determined.     

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.     

Total cross sections for fission of heavy nuclei by intermediate-energy protons

The results of measuring total cross sections for proton-induced fission of ^natPb, ²⁰⁹Bi, ²³²Th, ²³³U, ²³⁵U, ²³⁸U, ²³⁷Np, and ²³⁹Pu nuclei at proton energies from 200 to 1000 MeV are reported. The measurements were carried out in steps of ≈100 MeV. Complementary fission fragments were detected in coincidence by two parallel-plate avalanche gas counters placed in the beam line on both sides of the thin target close to it. Energy and isotope dependences of the fission cross sections are analyzed.     

Experimental Beta-decay energies of very neutron-rich fission products with 107≦A≦109

Theβ-decay of neutron-rich fission products with mass numbersA=107, 108 and 109 has been investigated at the mass separator LOHENGRIN of the Institute Laue-Lange-vin (ILL) in Grenoble by measuringβγ-coincidences with a large plastic scintillator telescope and a Ge(I)-detector. Theβ-decay energies of 8 nuclei were obtained from the evaluation of more than 40β-endpoint energies. For the nuclei¹⁰⁷ Mo,¹⁰⁷Tc,¹⁰⁸Tc,¹⁰⁹Tc and¹⁰⁹Ru, theβ-decay energies were determined for the first time; the experimental error in the decay energy of three daughter nuclei was considerably reduced. In addition, the two-neutron separation energies and the nuclear mass excesses were calculated from these experimentalQ _β-values and compared with different mass predictions.     

Separation of Coulomb fission from nuclear fission at medium energies

The cross sections for fission induced by one neutron transfer and by the electromagnetic field in the reaction 24.3 MeV/u ²³⁸U + ¹⁹⁷Au are calculated and compared to the experimental data. It turns up that the two calculated cross sections differ by five orders of magnitude at a distance of closest approach of 25 fm. It is shown that in the experiment in which one is able to select the events corresponding to a large distance of closest approach, a separation of Coulomb fission from nuclear fission events can be efficiently obtained.     

Recoil studies of fission products formed in the fission of U by protons of energies 25–85 MeV

The recoil properties of ten fission products with masses ranging from 72 to 136 formed in the fission of ²³⁸U with protons of energies 25–85 MeV have been determined radiochemically by the integral-range method. From the recoil properties of the products and the Monte Carlo cascade calculations the average kinetic energy, cascade deposition energy, and anisotropy parameter for each fission product has been calculated. The kinetic energy and the excitation energy of the primary fragments leading to the observed fission product, and the total kinetic energy and the total excitation energy of the primary fragment pair have also been calculated.

The results indicate that up to a bombarding energy of 40 MeV fission takes place predominantly by the compound nucleus mechanism, with an increasing contribution of the direct interaction mechanism as the bombarding energy increases. The kinetic energy deficit was found to decrease with increasing bombarding energy. The fission products formed from the symmetric mode of fission have a larger separation distance between the charge centres of their respective primary fragments than those for the asymmetric mode of fission.     

Heavy-ion induced fission at near-barrier energies

The study of heavy-ion induced fission fragment angular distributions continues to be a source of rich information as regards fission process in general and fission dynamics in particular. Considerable progress has been made towards understanding many features of the fission phenomenon. While some of the new sets of data measured in the last few years have confirmed the theoretical expectations, the others have provided surprises not quite anticipated. In the present review article the emphasis will be mainly on the recent experimental results of heavy-ion-induced fission fragment angular distributions at energies near the fusion barrier, their implications and new puzzles in this area which require not only suitable explanation but also additional measurements.     

Puzzling features of heavy-ion fission at sub-barrier energies

Elastic and inelastic scatterings cross-sections of¹²C+¹²C in the energy range 11–24 MeV/n have been analyzed within the coupled channels framework including appropriate symmetrization. The set obtained withr ₀=1.01 fm,r _I =0.86 fm for seven incident energies gave the best agreement with the data and could be, therefore, considered as really significant when compared with other sets. Coupled channels calculations have been made to fit the data and it has been found that they require a reduction of the imaginary potential. The total reaction cross-sectionσ _R (E) determined from this analysis is in good agreement with a direct and recent measurement of σ _R . Quadrupole deformation parameterβ ₂ has been extracted and compared with those obtained for proton and alpha scatterings. Some small energy dependence of β₂ over this energy range has been found, ¦β ₂¦ ～0.39–0.47. Finally, these calculations further confirm that β₂ depends on the probe used in the various scattering experiments. For composite projectile such asα or¹²C,β ₂ is clearly smaller than that one for nucleon as projectile.     

Puzzling features of heavy-ion fission at sub-barrier energies

The heavy-ion induced fission fragment angular distributions measured for systems with Th, U and Np as targets have revealed “anomalous” values of anisotropies at energiesE≤V _B (fusion barrier) and this feature is observed to be independent of the entrance channel mass-asymmetry. While this puzzling feature is exhibited by the deformed targets like Th, U and Np, most of the fission data measured for the spherical targets like Pb and Bi can be satisfactorily explained using the standard saddle point statistical model with moderate correction for pre-fission neutron emission. Plausible reasons for this anomalous behaviour are explored.     

Experimental beta-decay energies of several neutron-rich nuclides with mass number 131≦A≦146

With sources obtained from the on-line mass separator LOHENGRIN the beta-decay energies of¹³¹Sn,¹³⁴Sb,^135,136Te,^143,146Ba and¹⁴⁶La have been determined from measuredβ-endpoint energies. The experimental values are compared with the predictions of several mass formulae.     

The relationship between energies and spontaneous fission periods

This present study is an attempt to establish the relationship between energies and spontaneous fission periods of even—even nuclei. Spontaneous fission is a tunnel effect, and thus from general considerations one might expect that spontaneous fission periods, in analogy toα-decay [1, 2], would show a tendency to increase with increase in nuclear charge Z and with decrease in spontaneous fission energy E～f,? and would also depend on mass number A. In fact, for other conditions equal increase in Z leads to an increase in barrier expansion, while increase in E～f leads to a decrease in its permeability. Growth in A on the one hand increases the corrected fragment mass, thus hindering its passage through the barrier, while on the other hand it leads to increase in fragment dimensions, which reduces barrier height. By considering the influence of shell and subshell effects on E～f we may, to a certain degree, consider the influence of these effects on spontaneous fission period. For calculation of E～f the combined method was employed. Accurate experimental values of nuclear mass and transition energy [3] were used; in their absence formulas for nuclear transition energy base on the detailed experimental data of [4] were employed.     

Cold fission as heavy ion emission

The last version of the analytical superasymmetric fission model is applied to study cold fission processes. Strong shell effects are present either in one or both fission fragments. A smooth behaviour is observed when the proton or the neutron numbers are changed by four units. IncreasingZ andN, in the transuranium region, a sharp transition from asymmetry with a large peak-to-valley ratio to symmetry atZ=100 and/orN=164 is obtained. The transition toward asymmetry at higherZ andN is much smoother. The most probable cold fission light fragments from²³⁴U,²³⁶U,²³⁹Np and²⁴⁰Pu are¹⁰⁰Zr,¹⁰⁴Mo,¹⁰⁶Mo and¹⁰⁶Mo, respectively, in good agreement with experimental data. The unified treatment of alpha decay, heavy ion radioactivities and cold fission is illustrated for²³⁴U — the first nucleus in which all three groups have been already observed.     

Experimental beta-decay energies of very neutron-rich isobars with mass numbers A=101 and A=102

TheQ -values of¹⁰¹Rb,^101,102Sr and^101,102Y have been measured for the first time at the mass separator ISOLDE by means of-coincidence techniques with a plastic scintillation detector telescope and a large Ge(HP)-detector. For some of these nuclei, also new details of their decay schemes could be derived from the measured-decay properties. The experimental results are discussed, as far as the systematics of two-neutron separation energies derived from them is concerned. In addition, the nuclear masses deduced from theseQ -values are compared with the predictions of recent mass calculations.Dedicated to Prof. Dr. Peter Armbruster on the occasion of his 60th birthday     

The beta-decay of 8He

A many-level, many-channel R-matrix formula is used to fit the measured ⁸He β-decay data, including the half-life, branching ratios and delayed neutron and triton energy spectra. Satisfactory fits are found with contributions from four 1⁺ levels of ⁸Li, at 0.98, 3.08, 5.15 and 9.06 MeV. The 32 reduced width amplitudes are taken from a shell-model calculation, with only two requiring adjustment. Gamow-Teller matrix elements obtained from the fits are in reasonable agreement with shell-model 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(n _th,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.