Particle evaporation from hot nuclei withA ≦85 mainly prior to scission

We report on measurements of fusion-fission fragments and evaporation residues from the reaction⁵⁸Ni +²⁷Al at 835 MeV incident energy leading to compound nucleiA ≦ 85 with high excitation energies. In the case of fission the mass dependence of the velocity widths of separated fragments indicate that the evaporation of light particles predominantly occurs from the composite nucleus prior to scission. The fraction evaporated from the fully separated fragments after scission can be estimated to approximately 20%.     

Formation and decay of the compound nucleus studied in the reaction20Ne+27Al

Energy and velocity spectra, angular and mass distributions have been measured for evaporation residue products of the^{20, 22}Ne+²⁷Al system in the energy range ofE _L (²⁰Ne)=51 to 395 MeV and in the angular rangeθ=2° and 30°. Calculations with simple assumptions of the velocity and angular distributions of the evaporation residues are presented and compared to the data. The structure seen in the mass distributions, a competition between α-particle and nucleon evaporation in the deexcitation of the compound nucleus, is described well by calculations with the computer code CASCADE. The evaporation residues exhibit mass distributions varying systematically as a function of the excitation energy. The excitation function of the evaporation residue cross section is compared with theoretical models. At higher incident energies contributions of incomplete momentum transfer (incomplete fusion) are observed. A limitation for complete compound nucleus formation with following light particle evaporation is found.     

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.     

Beta-decay energies and nuclear masses of very neutron-rich Rb and Cs isotopes

Theβ-endpoint energies of very neutron-rich Rb and Cs isotopes with mass numbers 94≦A≦98 and 142≦A≦146, respectively, have been measured with a plastic scintillatorβ-telescope at the on-line mass separator OSTIS. From these,Q _β-values and mass excesses are calculated and compared with the results obtained in direct mass determinations.     

Fission accompanied by alpha particle emission in the12C(85MeV)+232Th reaction

Inclusive and coincident spectra of alpha particles and fission fragments were measured for the²³²Th+¹²C (85 MeV) reaction to study the influence of the excitation energy and the angular momentum on the fission of the compound nucleus and to separate different alpha particle emission mechanisms. At backward angles α emission can be accounted for by the evaporation processes. At forward angles the most important contribution is given by the break-up fusion process. Mass distributions for compound nuclei²⁴⁴Cm (E^*=58 MeV,ff coincidences), and²⁴⁰Pu (E^*=37 MeV,ff α coincidences) were obtained. In the case of²⁴⁰Pu mass distribution has a shape different from those obtained in light ion reactions at the same excitation energy, indicating the strong influence of the entrance channel. The dependence of the mass distribution shape on the α particle energy is also examined.     

Nuclear-charge polarization at scission in the 12 MeV proton-induced fission of <Superscript>232</Superscript>Th

The most probable charges of secondary fragments, produced after neutron evaporation from primary fragments, have been evaluated using fractional cumulative and mass yields in the 12MeV proton-induced fission of ²³²Th . The nuclear-charge polarization of primary fragments at scission has been obtained by correcting the most probable charge of secondary fragments for neutron evaporation. The fragment mass dependence of the nuclear-charge polarization at scission shows good agreement with that for thermal neutron-induced fission of ²³⁵U , indicating that the nuclear-charge polarization is nearly insensitive to mass and excitation energy of the fissioning nucleus for asymmetric fission in the actinide region.     

Study of the heavy products from84Kr+238U collisions at 522 MeV

Angular distributions and integral cross sections for heavy reaction products from⁸⁴Kr +²³⁸U collisions at 522 MeV were measured using catcher foil techniques and off-line X-ray identification of individual reaction product isotopes. There is no evidence for a fission-fusion mechanism being responsible for the formation of the “gold finger”. The data show that usual deep inelastic collisions account for the formation of all products with 70≦Z≦86. Compared to existing data atE≦605 MeV, the deduced mass distribution indicates reduced fission probabilities forZ≧80 fragments at 522 MeV. This is consistent with the expected dependence of fission probability on excitation energy and especially on angular momentum.     

Photoproduction of24Na from targets with 63≦A T ≦238

The mean cross sections for the photoproduction of²⁴Na in the energy range 400 to 1000 MeV have been measured for nine elements with mass number 79≦A _T ≦238. The exponential decrease with increasingA _T observed for lighter targets fades out and changes to a slight increase forA _T >100.     

Excitation energy dependence of fragment characteristics for the photofission of 232Th

Independent and cumulative product yields were measured for the photofission of ²³²Th with bremsstrahlung with endpoint energies 6.5, 7.0, 8.0, 11.0, 12.0, and 14.0 MeV, applying γ spectrometric techniques on catcherfoils and pneumatically transported ²³²Th-samples. The independent heavy fragment yields for the fission of the ²³²Th compound nucleus at excitation energies in the vicinity of the fission barrier were deduced. Postneutron mass, isobaric charge, isotopic mass distributions, isotonic and elemental yield distributions and proton odd-even effects were obtained from these independent yields. In the mass distributions a maximum yield is observed for mass splits with heavy fragments in the region of A = 142, corresponding with a high production of Ba(Z = 56) - isotopes. A slightly increased yield is also observed for mass splits with heavy mass in the vicinity of A = 134. The latter effect increases with increasing compound nucleus excitation energy. The similarity between the mass distributions of the N = 142 fissioning systems ²³²Th, ²³⁴U and ²³⁶Pu is striking. For low excitation energy the proton odd-even effect in the element distributions amounts to 30%, while on the other hand no sizeable neutron odd-even effect could be deduced from the isotonic distributions. The proton odd-even effects remain constant up to compound nucleus excitation energies of about 7.85 MeV. For higher compound nucleus excitation energies the proton odd-even effect drops rapidly. A possible explanation of these observations in terms of pair breaking at the outer barrier is proposed.     

Partial level density of n-quasiparticle excitations, radiative strength functions and new experimental information on the dynamics of nuclear structure change in the B n range

The most reliable at present values of the level density in the fixed spin window and the sums of radiative strength functions of cascade gamma transitions are obtained from analysis of intensities of two-step cascades excited upon thermal neutron capture for approximately 40 nuclei in the mass range 40 ≤ A ≤ 200. The maximal reliability of these data is provided by the experimental conditions—minimum possible propagation error coefficients and practically unique solution of the problem of determination of gamma decay parameters from measured spectra. The experimental data are approximated by the sum of partial level densities corresponding to excitation of n quasiparticles. Steplike structures in the level density at excitation energies smaller than 3–4 MeV are described with good accuracy as the superposition of two-quasiparticle (three-quasiparticle in odd A nuclei) and vibrational excitations with the coefficient of collective density enhancement K _coll ≈ 10?20. They correspond to excitation-energy-correlated maximum enhancement of the radiative strength functions of primary gamma transitions. The level density at larger excitation energies is well reproduced if the breakup of at least two more Cooper pairs of nucleons is taken into account. The increase in the number of excited quasiparticles in the nucleus corresponds to unconditional reduction of the radiative strength functions of primary gamma transitions of the compound state decay. However, the maximum possible value of partial widths of primary transitions increases regularly with decreasing energy. Some ambiguity in the results of approximation and divergence from existing theoretical ideas of the energy dependence of nucleon correlation functions in an excited nucleus point to the possibility of direct extraction from experiment of fundamentally new information on the structure of excited nuclear levels in the range of the neutron binding energy. These are, first of all, the parameters of dependence of nucleon correlation functions on the excitation energy of the nucleus.     

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

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 ^233,235U(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.     

Excitation-energy sharing in uranium induced dissipative collisions

Strongly damped collisions were studied in uranium induced reactions on¹¹⁰Pd and¹²⁴Sn target nuclei near the barrier. The excitation-energy splitting was deduced from binary reaction yields and those in which the heavy fragment undergoes sequential fission. For systems with 87≦Z≦95 for the heavy fragment, the excitation energy is concentrated in the lighter nucleus, at best shared equally. The results indicate a non-equilibrated energy dissipation, and support nucleon exchange based on average single-particle strength functions as the underlying dissipation mechanism at the barrier.     

Evolution of the giant dipole resonance properties with excitation energy

The studies of the evolution of the hot Giant Dipole Resonance (GDR) properties as a function of excitation energy are reviewed. The discussion will mainly focus on the A ∼ 100-120 mass region where a large amount of data concerning the width and the strength evolution with excitation energy are available. Models proposed to interpret the main features and trends of the experimental results will be presented and compared to the available data in order to extract a coherent scenario on the limits of the development of the collective motion in nuclei at high excitation energy. Experimental results on the GDR built in hot nuclei in the mass region A ∼ 60-70 will be also shown, allowing to investigate the mass dependence of the main GDR features. The comparison between limiting excitation energies for the collective motion and critical excitation energies extracted from caloric curve studies will suggest a possible link between the disappearance of collective motion and the liquid-gas phase transition.     

Exciton number dependence of the Griffin model two-body matrix element

The exciton number dependence of the residual two-body matrix element has been determined using published particle-hole pair creation rates obtained from semi-phenomenological calculations. The previous energy dependence of the mean squared matrix element has been replaced by a dependence on the average excitation energy per degree of freedom,E/n. ForE/n less than 7 MeV or greater than 15 MeV, the form of the energy dependence has been slightly modified. These changes result in a factor of three reduction in the predicted Griffin model composite nucleus equilibration times but in only minor changes in the calculated energy spectra for emitted particles.     

‘Slow’ residues observed in the reaction20Ne+208Pb at E/A=8.6, 11.4 and 15.0 MeV/u

Velocity distributions of heavy residuesA _Res>A _tar,Z _Res>Z _tar identified by means ofα spectroscopy, have been investigated at the velocity filter SHIP in reactions²⁰Ne+²⁰⁸Pb at projectile energies E/A=8.6, 11.4 and 15.0 MeV/u. Besides products from complete or nearly complete fusion, characterized by velocity distributions peaking atν/ν _CN?0.8–1.0, heavy residues with mean velocities of about half of the compound nucleus velocity were observed. The Z-distribution of this component was found to peak atZ=87. It is interpreted as residues from fusion of target nuclei with projectile fragments produced by nearly symmetric break-up. The experimental results were compared with predictions of theoretical models: cross sections for incomplete fusion were calculated using the sum-rule model of Wilcynski et al., while residue cross sections were calculated using the evaporation code HIVAP. A fair agreement between experimental and calculated mass distributions of heavy residues and transferred projectile fragments is achieved if an energy dissipation of ?23% (at E/A=8.6 MeV/u) and ?8% (at E/A=11.4 MeV/u) of the incident projectile energy is introduced. The observed peak of theZ-distribution atZ=87 is predominantly effected due to a higher fission probability of products withZ>87 during the deexcitation process and experimental limitations in the identification of products withZ≦86 by means of a spectroscopy, which cause a decrease of the observed production rates towards lowerZ.     

Nucleus excitation and deexcitation following at rest

The evolution of a nucleus after at rest is studied. The process is divided into two successive phases: a rapid phase, described by an intranuclear cascade model and a slow phase, akin to an evaporation process. The distribution of the residue masses after the second phase is calculated and compared to the recent radiochemical measurements for ⁹⁸Mo. The evolution of the number of pions, and the distribution of the number of fast ejected nucleons and of participants is studied. The latter two are fitted by negative binomials. The meaning of this observation is discussed. The mass dependence of several quantities, like the maximum excitation energy and the average mass loss is also investigated. Finally, the possibility of a multifragmentation is discussed.     

Gamma-spectroscopy within the island of high-spin isomers nearN=82

A NaI sum-spectrometer combined with Ge-counters has been used to characterize the members of the island of high spin isomers nearN=82. On the basis of half lives, totalγ-decay energies and discreteγ-lines, assignments of 22 isomers are given or confirmed. The isomers are localized to the region 82≦N≦86 andZ≦68, and the excitation energies vary from 3 MeV to 12.2 MeV. An empirical relation between spin and excitation energy is presented and on this basis isomeric spin values up to (33±2)? are deduced. The isomers are thought to be due to strong alignment of 2 to 8 shell-model particles in a spherical or possibly weakly oblate potential.     

Investigation of α-induced reactions on niobium and tantalum

Excitation functions of (α,xn yp) reactions on⁹³Nb withy≦6 andx≦10 have been measured in four different irradiations from 16 to 171 MeV bombarding energy by Ge(Li) spectroscopy of the residual activity of stacked foils. Similarly, previous measurements of¹⁸¹Ta(α,x n y p) reactions from 15 to 103 MeV were extended up to 170 MeVα-energy for residual nuclei withy≦4 andx≦12. Including isomeric states a total number of 37 residual activities could be identified in⁹³Nb+α. From the rather complexγ-decay spectra altogether 28 different excitation functions of sizable length could be established. For181Ta+α 10 different residual activities were analyzed so that together with the former data a set of 15 excitation functions is available. The whole set of excitation functions provides a large data basis for probing the validity of combined equilibrium and preequilibrium reaction models in a wide energy range.     

The barrier for compound-nucleus formation in the nearly symmetric systems86Kr+123Sb→209Fr and124Sn +94Zr→218Th

Evaporation residues from the fusion of the nearly symmetric systems⁸⁶Kr+¹²³Sb and¹²⁴Sn+⁹⁴Zr have been detected. The compound nucleus²¹⁸Th could be produced with an excitation energy as low as 21 MeV. The barriers for the formation of a compound nucleus were deduced from the evaporation residue cross sections. There is no evidence for an additional amount of energy needed to fuse these systems, if compared with the extrapolation of fusion barrier systematics or with results of fusion model calculations not including friction phenomena. One of the systems reaches a value ofZ ₁·Z ₂=2,000, about the highest value where evaporation residues have ever been observed.     

Nuclear structure effects in the mass region aroundA=100, derived from experimentalQ β -values

The endpoint energies ofβ-transitions in 15 neuron-rich nuclei with mass numbers 101≦A≦106 have been measured with a plastic scintillator telescope in coincidence with a large Ge(Li)-detector at the on-line mass separator LOHENGRIN. From theQ _β -values obtained in these experiments, nuclear structure effects in this region of the nuclear chart are deduced. Finally, nuclear masses derived from theseQ _β -values are compared with the predictions of different theoretical mass calculations.