C. C. Sahm

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.

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.

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.

Evidence for element 109 from one correlated decay sequence following the fusion of58Fe with209Bi

An experiment to synthesize element 109 is presented. Decay patterns characteristic of complete fusion products were searched for in an irradiation of209Bi targets with58Fe projectiles at specific incident energies of 4.95, 5.05, and 5.15 MeV/u. A total dose of 7 ×1017 particles was obtained. The experimental method involves in-flight separation of forward peaked reaction products with a static-field velocity filter, their passage through a time-of-flight device and their final implantation into position sensitive solid state detectors to measure their kinetic energy, approximate mass and their time and position of incidence. The subsequent decay of the narrowly localised reaction products by cascades of alpha particles and/or spontaneous fission is also registered in terms of the energies and times of all the emitted particles. One outstanding decay sequence that started with the emission of two alpha particles within subsequent time intervals of 5 ms and 22 ms and ended with spontaneous fission after 13 s was found at 5.15 MeV/u. The first alpha particle had a kinetic energy of (11.10±0.04) MeV. A detailed analysis of all the alternative interpretations of this observation, such as a purely random correlation of signals, the decay of a product from a transfer reaction or of any of the various energetically possible evaporation residues, shows that the isotope with mass 266 of element 109, i.e. the one neutron evaporation channel after complete fusion, is the statistically most significant assignment. The outlook for new element synthesis is also briefly discussed.

Fusability and fissionability in 86Kr-induced reactions near and below the fusion barrier

Abstract Evaporation-residue excitation functions for the reactions 86 Kr + 70,76 Ge, 92,100 Mo, 99,102,104 Ru have been measured using activation methods and the velocity filter SHIP. The data span the region from well below the fusion barrier up to, and beyond, the energy where limitation by fission competition takes place. The data are shown to be compatible with the concept of complete fusion followed by the statistical decay of the equilibrated compound nucleus. Information on both the fusion probability at and below the fusion threshold and the fissionability of the compound nuclei formed, is extracted. The model dependence of the extracted fission barriers is discussed in detail. In analogy to studies involving lighter projectiles, strong correlations between the low-energy nuclear-structure properties of the nuclei and the subbarrier fusion probability are found. A relative shift of the fusion barrier to higher energies, that increases with the number of valence neutrons in the target nuclei, is observed.

The new isotopes258105,257105,254Lr and253Lr

Evaporation residues from the heavy-ion fusion reaction50Ti on209Bi were investigated. They were separated from the projectile beam by the velocity filter SHIP and identified after implantation into an array of position-sensitive surface-barrier detectors by analyzing theirα-decay chains. Spontaneous fission was also observed.Four newα emitters,258105 (T1/2=4.4−0.6+0.9s),257105 (T1/2= 1.4−0.3+0.6s),254Lr (T1/2= 13−2+3s), and253Lr (T1/2=1.3−0.3+0.6s) could be identified. For the isotope257105 we obtained a spontaneous-fission branch of about 20%. A spontaneous-fission activity with a halflife comparable to that for theα decay of258105 was explained as fission of258104, formed by electron capture from258105.An excitation function for evaporation-residue production was measured for bombarding energies in the range ofECM=184.4 MeV toECM=196.6 MeV. Nearly all evaporation residues we observed, could be attributed to the 1n and 2n deexcitation channels. The maximum cross sections wereσ(1n)=c/2.9±0.3) nbarn, andσ(2n)=c/2.1±0.8) nbarn, respectively.We could measure the total kinetic energy of the fission fragments of258104 to be TKE=(220±15) MeV, a value that fits into empirical systematics based on aZ2/A1/3 dependence.

Gamma-Spectroscopic investigations in the radiative fusion reaction 90Zr + 90Zr

Abstract The γ rays emitted in the radiative fusion reaction 90 Zr+ 90 Zr→ 180 Hg+ xy were measured. No indication was found for a specific direct capture process associated with a high-energy γ ray. The shape of the observed γ spectrum is compatible with a statistical deexcitation of the compound nucleus.

Hindrance of fusion in central collisions of heavy, symmetric nuclear systems

The energy dependence of the fusion probabilities for central collisions of the systems124Sn +92,94,96Zr and86Kr +123Sb was determined from neutron-evaporation-residue cross sections. Near the barrier as calculated from one-dimensional barrier-penetration models, the fusion probability was found to be reduced by several orders of magnitude.

Fusion-fission and neutron-evaporation-residue cross-sections in 40Ar- and 50Ti-induced fusion reactions

Abstract For the reactions 40Ar+ 165Ho. 169Tm, 174Yb. 115Lu, 176–180Hf, 181Ta, 208Pb and 50Ti + 208Pb, 209Bi the cross sections for the fusion-fission process were determined by measuring energy and time-of-flight of the reaction products. In addition, the neutron-evaporation-residue cross sections were measured by using the velocity filter SHIP. A σ versus 1/E analysis of the fusion-fission cross sections is used to determine fusion barriers and fusion radii. The evaporation- residue cross sections are used to extract in an approximate way barriers for compound-nucleus formation. These barriers are found to agree with the fusion barriers determined from the fission cross sections. For all systems investigated the neutron-evaporation-residue cross sections reach their maximum close to the fusion barrier as calculated from the Bass potential.