K. H. Schmidt

Some Remarks on the Error Analysis in the Case of Poor Statistics

A prescription for the error analysis of experimental data in the case of stochastic background is formulated. Several relations are given which allow to establish the significance of mother-daughter relationships obtained from delayed coincidences. Both, the probability that a cascade is produced randomly and the probability that the parameters of an observed event chain are incompatible with known properties of a given species are formulated. The expressions given are applicable also in cases of poor statistics down even to single events.

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.

Fusion probability of symmetric heavy, nuclear systems determined from evaporation-residue cross sections

Abstract Cross sections were measured for the formation of evaporation residues in 48 Ca-, 86 Kr- and 124 Sn-induced reactions with Yb, Sb and Zr isotopes. For the nearly symmetric systems, the energy dependence of the fusion probability in central collisions was determined. The fusion probability at the expected fusion barrier as calculated from a one-dimensional heavy-ion potential was found to be reduced by several orders of magnitude. The fusion cross sections increase gradually, and only at energies appreciably above the expected fusion barrier the major part of the cross section of central collisions leads to fusion. The observed hindrance of the fusion process is compared with model predictions.

Cold fusion in symmetric 90Zr-induced reactions

Abstract Excitation functions for evaporation residues were measured for the reactions 90 Zr + 89 Y, 90 Zr, 92 Zr, 96 Zr, and 94 Mo. Deexcitation by γ-radiation only was found for the compound nuclei 179 Au, 180 Hg, 182 Hg and 184 Pb. Fusion probabilities as well as fusion-barrier distributions were deduced from the measured cross sections. There are strong nuclear structure effects in subbarrier fusion. For energies far below the fusion barrier the increase of the fusion probabilities with increasing energy is found to be much steeper than predicted by WKB calculations. As a by-product of this work new α-spectroscopic information was obtained for neutron-defi- cient isotopes between Ir and Pb.

Alpha decay properties of new protactinium isotopes

Abstract Evaporation residues produced in the reactions 40 Ar + 181 Ta and 40 Ar + 184 W were separated from the primary beam by the velocity filter SHIP and detected by a Δ E-E counter telescope. The technique of delayed coincidences was applied to individually identify the reaction products implanted into a Si surface barrier detector by their subsequent α-decays. The previously unknown nuclei 215 Pa and 218 Pa were identified by their known daughter decays. 215 Pa was found to decay with E α = 8.085±0.015MeV and t 1/2 = 14± 2 3 ms. For 218 Pa two α-lines were found at 9.535±0.015MeV and 9.614±0.020MeV with a corresponding half-life of 120 +40 -20 μs. The half-life of the 8.33 MeV α-decay of 217 Pa. was determined to be 4.9 +0.6 -0.4 ms. Furthermore a new isomer was found in 217 Pa which decays with E = 10.16±0.02MeV and t 1/2 = l.6 +1.0 -0.5 ms. The decay of this isomer is tentatively interpreted as being hindered by an angular momentum of about 10ħ of the emitted α-particle. The ground state α-decay energies of these very proton-rich isotopes agree well with systematics but deviate considerably from theoretical mass predictions.

Fusion near the threshold: A comparative study of the systems 40Ar + 112, 116, 122sn and 40Ar + 144, 148, 154Sm

Abstract Fusion excitation functions for the systems 40 Ar + 112, 116, 122 Sn and 40 Ar + 144, 148, 154 Sm have been determined, covering cross sections ranging from several hundred mb down to the μb level. The data show a pronounced correlation of the subbarrier behaviour with low-energy collective properties of the nuclei involved and are well reproduced by simplified coupled-channel calculations coupling fusion to inelastic channels. The possibilities of parameterizing the data in terms of a simple dynamic barrier-fluctuation phenomenon are discussed and result in the prediction of remarkably diffuse partial-wave distributions above the barrier. This is shown to be important for the analysis of deexcitation phenomena following fusion reactions.

Cross sections for evaporation residue production near theN=126 shell closure

In fusion reactions of40Ar with isotopes of Ho, Tm, Yb, Lu, Hf and Ta, cross sections for the production of proton-rich evaporation-residues near the 126 neutron shell were measured. This first comprehensive study of very fissile spherical residues reveals a surprisingly low stabilizing influence of the sphericalN=126 shell on the survival probability. The experimental results are compared with evaporation calculations. Conclusions for the production of superheavy nuclei are drawn.

Entropy driven excitation energy sorting in superfluid fission dynamics.

It is shown that the constant-temperature behavior of nuclei in the superfluid regime leads to an energy-sorting process if two nuclei are in thermal contact, as is the case in the fission process. This effect explains why an increase of the initial excitation energy leads an increase of the number of emitted neutrons from the heavy fission fragment, only. The observed essentially complete energy sorting may be seen as a new counterintuitive manifestation of quantum-mechanical properties of microscopic systems.

Distribution of Ir and Pt isotopes produced as fragments of 1 A GeV 197Au projectiles: a thermometer for peripheral nuclear collisions

Abstract Cross sections of iridium and platinum isotopes have been measured, which were produced as fragments from 197 Au projectiles interacting with aluminum target nuclei at 1 A GeV. The fragments were identified by a kinematic analysis ( ΔE − Bρ -time-of-flight) and by an ion-optical separation at the projectile-fragment separator FRS at GSI. Indications are found for high excitation energies and weak proton-neutron correlations in the peripheral nuclear-collision process.

Shell effects in the properties of the heaviest nuclei

Abstract Shell effects are extracted in the following experimental quantities: mass, alpha-decay energy and half-life, spontaneous-fission barrier and lifetime. The heaviest even-even nuclei known experimentally, with Z = 92–108 are considered. This extraction is based on the Yukawa-plus-exponential model, used for the description of the macroscopic part of nuclear mass. The effects are found to be large; e.g. they increase the spontaneous-fission half-life of some nuclei by about 15 orders of magnitude. The sensitivity of these effects to changing the macroscopic model is discussed.