W. Reisdorf

Alpha decay studies of very neutron deficient isotopes of Hf, Ta, W, and Re

Very neutron deficient isotopes in the Hf-Re region were produced by bombarding targets of107, 109Ag,nat, 108, 110Pd, and103Rh with58Ni accelerated by the linear accelerator UNILAC at GSI. After separation from the projectile beam by the velocity filter SHIP, the fusion products were implanted with their full recoil energy into a silicon surface barrier detector. The subsequent alpha decay of the implanted ions was measured with the same detector. In some of the experiments the evaporation residues were implanted into a position sensitive silicon surface barrier detector. With a newly developed position and time correlation technique parent daughter relationships, half lives, andα branching ratios of a large number of isotopes could be determined. In the investigated reactions the eleven new isotopes161–164Re,160W,157–161Ta, and156Hf could be identified. The measuredQα values, half lives, and a branching ratios are discussed. Two new high energyα. transitions of (7,408±10) keV and (7,804±15) keV were also found with half lives of 2.7 ms and 0.52 ms, respectively.A tentative mass and atomic number assignment givesA=153–156 andZ=70–72. The energies of the excited isomeric states are between 2 MeV and 3 MeV. From the systematics of alpha ground state transitions in even nuclei a hindrance of 105 can be deduced for both transitions, possibly indicating orbital angular momenta on the order of 10ħ for the alpha emitting states. The isomers are proposed to belong to shell model isomers beyond146Gd.

Central collisions of Au on Au at 150, 250 and 400 A·MeV

Abstract Collisions of Au on Au at incident energies of 150, 250 and 400 A ·MeV were studied with the FOPI-facility at GSI Darmstadt. Nuclear charge ( Z ⩽ 15) and velocity of the products were detected with full azimuthal acceptance at laboratory angles 1° ⩽ θ lab ⩽ 30°. Isotope separated light charged particles were measured with movable multiple telescopes in an angular range of 6–90°. Central collisions representing about 1% of the reaction cross section were selected by requiring high total transverse energy, but vanishing side flow. The velocity space distributions and yields of the emitted fragments are reported. the data are analysed in terms of a thermal model including radial flow. A comparison with predictions of the quantum molecular model is presented.

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.

A highly-segmented ΔE-time-of-flight wall as forward detector of the 4π-system for charged particles at the SIS/ESR accelerator

Abstract At the SIS/ESR accelerator facility at GSI in Darmstadt the 4π-detector system FOPI is under construction at present. It is designed for the investigation of central collisions of heavy ions in the energy range up to 2 A GeV. As phase I of this detector a forward wall has been built and used in various experiments. It comprizes a total number of 764 scintillators with an additional shell of 188 thin ΔE -detectors in front of it and covers the full azimuth of the polar angles from 1° to 30°. The velocity and the nuclear charge of the fragments are determined by a combined time-of-flight and ΔE measurement.

Proton radioactivity of151Lu

A (1231±3) keV proton activity has been observed in the fusion reaction58Ni +96Ru→154Hf*. The production cross section peaks at 50 MeV of excitation energy with a value of about 70 μb. No coincidences with annihilation radiation or withK X-rays could be observed. The activity is assigned to direct proton decay of the new isotope151Lu. The measured half life is (85±10) ms.

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.

Cold fusion reactions with 48Ca

Abstract Fusion reactions of 48 Ca projectiles with 180 Hf, 184 W, 197 Au, 208 Pb and 209 Bi targets were studied. Highest cross sections were found in the 208 Pb( 48 Ca,2n) 254 No channel with σ max 3.2 ± 0.3 μb. The results of the heavier systems are discussed in terms of the extra — push model. For the lighter systems missing cross section is observed if compared to estimates made by the evaporation code HIVAP.

Systematics of central heavy ion collisions in the 1A GeV regime

Using the large acceptance apparatus FOPI, we study central collisions in the reactions (energies in A GeV are given in parentheses): Ca-40 + Ca-40 (0.4, 0.6, 0.8, 1.0, 1.5, 1.93), Ni-58 + Ni-58 (0.15, 0.25, 0.4), Ru-96+Ru-96 (0.4, 1.0. 1.5), (96)zr+(96)zr 1.0, 1.5), Xe-129+CsI (0.15, 0.25, 0.4), Au-197 + Au-197 (0.09, 0.12, 0.15, 0.25, 0.4, 0.6, 0.8, 1.0, 1.2, 1.5). The observables include cluster multiplicities, longitudinal and transverse rapidity distributions and stopping, and radial flow. The data are compared to earlier data where possible and to transport model simulations