Gamma decay of giant resonances excited by heavy ions

Experiments on /sup 208/Pb bombarded by /sup 17/O at 22 MeV/nucleon (ORNL) and 84 MeV nucleon (GANIL) are reviewed. Inelastically scattered projectiles were detected at forward angles in coincidence with gamma rays seen in NaI (ORNL) or in BaF/sub 2/ (GANIL). The /sup 17/O were identified by 6 Si telescopes covering THETA = 11. 5/degree/--14.5/degree/ (ORNL) or by the focal-plane detector system of the energy-loss spectrometer SPEG, set to accept THETA = 1. 5/degree/--5.0/degree/ (GANIL). The ..gamma..-ray data provide information on the multipole character of various parts of the giant resonance region, matrix elements between the GR region and low-lying states in /sup 208/Pb, and the relative contribution of direct and compound process to ..gamma../sub 0/ decay. At the higher energy the 9--15 MeV GR region is excited very strongly. The isovector giant dipole is dominant over most of the angles studied. Significant contributions from the isoscalar giant quadrupole and monopole resonances are also present. Decomposition of the GR into L = 1, 2, and 0 components was based on coincidences with the overwhelmingly dipole ..gamma../sub 0/ transitions. The magnitude (1.7 +- 0.2%) and energy distribution of the ..gamma../sub 0/ branch can be reproduced well by a parameter-free calculation. The ..gamma../sub 0/ decay of the isoscalar giant quadrupole resonance is more easily observed at the lower energy. The ..gamma../sub 0/ angular correlations confirm the presence of E2 radiation from states in the 9--11 MeV region. The B(E2) implies that the ratio of neutron to proton matrix elements is consistent with the expected value of N/Z. This conclusion is confirmed by evidence form Coulomb-nuclear interference in the singles data at 84 MeV/nucleon. Photon decays to excited states indicate that 4/sup +/ and/or 6/sup +/ strength is present around 9--10 MeV, and are consistent with a monopole contribution from 12.5--15.5 MeV. 20 refs., 14 figs

Heavy-ion reaction mechanisms studied with the spin spectrometer

Experimental data and statistical-model calculations for xn and ..cap alpha..xn products of the reaction /sup 20/Ne + /sup 146/Nd at 136 MeV are shown to be in generally good agreement, indicating that equilibrium processes are dominant. Preliminary results on the heavy-ion ejectiles from /sup 19/F + /sup 159/Tb are presented

Angular Momentum Effects in Subbarrier Fusion

Analyses of published experimental data leading to angular-momentum distributions for subbarrier fusion of {sup 64}Ni and {sup 100}Mo have been re-examined, especially in the low-{ell} region. Our previous results are substantially unchanged. 3 refs., 6 figs

Crystal Ball Studies of Giant Resonance Gamma Decay

We have carried out coincidence experiments to investigate the photon and neutron emission from the giant resonance region in /sup 208/Pb and /sup 90/Zr using the ORNL Spin Spectrometer, a 72-segment NaI detector system. States in /sup 208/Pb and /sup 90/Zr were excited by inelastic scattering of 380-MeV /sup 17/O. We have determined the total gamma-decay probability, the ground-state gamma branching ratio, and the branching ratios to a number of low-lying states as a function of excitation energy in /sup 208/Pb to approx. 15 MeV. Especially interesting observations include the absence of a significant branch from the giant quadrupole resonance to the 3/sup -/ state at 2.6 MeV, a strong branch from this resonance to a 3/sup -/ state at 4.9 MeV, and the dominance of decays to various 1/sup -/ states at 5 to 7 MeV from the region around 14 MeV of excitation (EO resonance). Comparable but less complete data were also obtained on /sup 90/Zr. 14 references

Photon decay of giant resonances

Measurements have been made of the photon decay of the giant multipole resonances in /sup 208/Pb. The giant resonances were excited by inelastic scattering of 380 MeV /sup 17/O projectile and the photons were detected in the ORNL Spin Spectrometer. The results show a quadrupole resonance ground state gamma branch of 20% while only less than or equal to 2% of the GQR decay proceeds through the 2.6 MeV, 3/sup -/ state. Nearly one half of the GQR decay through a 3/sup -/ state at 4.974 MeV. Photon decay from the dipole and monopole resonances and high spin resonances (4+,6+) are also observed

Photon decay of giant resonances

The total gamma-decay probability, the ground-state gamma branching ratio, and the branching ratios to a number of low-lying states as a function of excitation energy have been determined in /sup 208/Pb to approximately 15 MeV. The total yield of ground-state E2 gamma radiation in /sup 208/Pb can only be understood if decay of compound states is considered. Other observations in /sup 208/Pb include the absence of a significant branch from the giant quadrupole resonance (GQR) to the low-lying collective states at 2.6 MeV and 4.08 MeV, and a strong branch to a 3/sup -/ state at 4.97 MeV. 20 refs., 6 figs., 1 tab

MEASUREMENT OF CARBON ARC DENSITY

The density of carbon arc was investigated by elastic scattering of 28- Mev N ions from the ORNL 63-inch cyclotron. The apparatus and method are described, and a discussion of results is presented. If the excess of three counts above background was real and not due to noise, the probable density of the arc is 1.1 x 10/sup 14/ listing the probability of 3 coincidences as a functio

Entry states in subbarrier fusion

The cross section for fusion of heavy ions below the Coulomb barrier can be orders of magnitude larger than the predictions of models that are quite successful above the barrier. Recent studies of ..gamma..-ray multiplicity have shown that the average l of the partial waves participating in subbarrier fusion is much higher than expected. The discrepancies become larger as the mass asymmetry of the projectile and target decreases. We have used the Spin Spectrometer in coincidence with identified products from two reactions leading to the same compound nucleus, /sup 164/Yb, to study entry-state angular-momentum effects. The reactions were /sup 64/Ni + /sup 100/Mo and /sup 16/O + /sup 148/Sm; the conditions of bombardment are listed. The O + Sm energies were chosen to match two of the Ni + Mo compound-nucleus excitation energies. Exit channels were identified by known ..gamma..-ray lines from the residual nuclei observed in six Compton-suppressed Ge detectors which replaced a like number of pentagonal NaI units of the Spin Spectrometer. Recording of events was triggered by detection of a ''clean'' Ge pulse (i.e., no ..gamma.. ray detected in its surrounding Compton shield). 10 refs., 5 figs., 2 tabs

THE SPIN SPECTROMETER AT THE HOLIFIELD HEAVY-ION RESEARCH FACILITY AND SOME PLANNED EXPERIMENTS

The 4π multidetector γ-ray spectrometer at the Holifield Heavy-ion Research Facility (HH IRF) is described in some detail. The following important features of this spectrometer are discussed : (a) the geometric arrangement, (b) the actual performance of the individual detector elements, (c) the associated electronics and data acquisition system, and (d) the response of the system to input γ-cascades including the effect of crystal-to-crystal scattering and the response to neutrons. The first few experiments to be performed are briefly described