V. Metag

Fission Isomerism Induced by Helium Ions

Delayed fission events due to isomeric fission have been produced using He3 and He4 beams from the MP Tandem Van de Graaff accelerator. The isomeric nuclei which recoil from the target and subsequently fission in flight were detected by a square cone array of polycarbonate foil detectors mounted along the beam direction. From the distribution of fission tracks along the detector foils, half-lives in the nanosecond region were deduced.

Spin dependence and angular correlation of giant resonances studied in (HI, xn) reactions

Abstract The high energy component above 10 MeV in the γ decay following the 128 Te + 34 S compound nucleus reaction is studied by exploiting the new experimental possibilities provided by the NaI crystal ball spectrometer.

FISSION ISOMERS IN Cm AND Bk ISOTOPES.

A new extremely shortlived fission isomer presumably in245Bk has been detected following the bombardment of243Am with4He particles of 26 MeV. Proton bombardment of243Am gives some evidence for the excitation of two shortlived fission activities which are tentatively assigned to isomers in241,242Cm. When irradiating targets of237Np,241Am, and243Am with deuterons in the energy range from 12 to 20 MeV previously known isomers are observed. Excitation functions are given for the production of these fission isomers.

Evidence for a direct reaction mechanism in the production of fission isomers

Abstract Targets of 233, 235, 236, 238 U were bombarded with douterons and protons in the energy range from 11 to 20 MeV. Excitation functions for the production of fission isomers were obtained and are interpreted in terms of direct reaction processes. The most probable reactions are (d, pχγ) and (d, pnχγ) leading to isomers assigned to 236 U and 238 U with half-lives of 70 nsec and 110 nsec respectively.

Angular momentum splitting and spin dependence of the giant dipole resonance in hot nuclei

Abstract High energy γ-rays from the decay of the electric giant dipole resonance (GDR) in the reaction 130Te(34S,xn)164−xEr at Elab = 156 MeV have been measured as a function of the γ-ray multiplicity using the Darmstadt-Heidelberg crystal ball. From the analysis of the γ-angular distributions the γ-rays with Eγ ≥ 4 MeV were found to be consistent with pure dipole radiation. Moreover, in the region of the GDR pronounced anisotropies in the γ-angular distributions are observed, proving an angular momentum splitting of the giant resonance into the spin components with ΔI = 0 and ΔI = ± 1. A consistent description of both the angular distributions and energy spectra can only be obtained (i) by assuming the nuclei in the spin- and excitation energy region studied ( I ≥30 h , E ∗ ≈ 50 MeV ) to have an oblate deformation with the spin axis pointing in the direction of the symmetry axis (γ = -60°) and (ii) by allowing the centroid energy ER to decrease with increasing spin.

Correlation between fission isomer half-lives and liquid-drop model parameters

Abstract A semiempirical systematics of known fission isomer half-lives is proposed. For doubly odd and odd-mass nuclei there seems to be a pronounced correlation between the half-lives and the liquid-drop model fissility parameter with an additional modulation due to the variation of shell effects with neutron number. For even nuclei the observed isomeric half-lives are nearly constant irrespective of Z and A . A model based on a simplified double-humped fission barrier is discussed which may account for the general trend of fission isomeric half-lives of doubly odd and odd-mass nuclei. Parameters are given which reproduce the observed half-lives. Excitation energies and fission barrier heights are compared with the predictions of the model. Other explanations are considered for isomers in doubly even nuclei which do not follow the systematics.

Decay modes and lifetimes of levels in 39K below 7.3 MeV excitation

Abstract Levels in 39K have been populated using the (p, pγ) reaction and the decays of levels below 7.3 MeV have been established by a particle-gamma coincidence experiment. The mean lifetimes of 24 levels have been determined in a singles experiment using the Doppler shift attenuation method. Limits on the lifetimes of a further 10 levels in 39K have been obtained. Discussion of the properties of levels in 39K is given.

A secondary minimum in the potential energy surface of 40Ca

Abstract Strutinsky type calculations are extended to lighter nuclei. Secondary minima in the potential energy surface generally considered to account for the existence of fission isomers occur also in the lighter mass region. Results for 40 Ca are compared with experimental evidences.

Physics with 4π−γ-detectors

Abstract The broad range of applications of the Darmstadt-Heidelberg crystal ball is illustrated. First results are discussed which have been obtained in investigations of the nuclear structure of rapidly rotating nuclei, in studies of nuclear reaction mechanisms, and in experiments with radioactive sources.

Differences in angular momentum distributions for near-barrier fusion of 32S+100Mo and 36S+96Mo

Abstract Angular momentum distributions for the fusion reactions 32S+100Mo and 36S+96Mo, both leading to the compound nucleus 132Ce, have been measured for energies between 1.08 and 0.89 times the barrier. The system 32S+100Mo, characterized by a stronger enhancement of the sub-barrier fusion cross section compared to 36S+96Mo, exhibits broader angular momentum distributions at all measured bombarding energies. Simple coupled channel calculations indicate that target and projectile excitation alone cannot describe the strikingly different behavior of the two S+Mo systems.