Scattering of alpha particles from 44Ti, the lightest unstable alpha-particle nucleus above A=40, has been measured at backward angles. The "anomalous" order-of-magnitude enhancement that is ...characteristic of 40Ca and other light alpha-particle nuclei is not observed. Instead, the backward yield is similar to that observed for other nuclei heavier than 40Ca, and is well described with average optical model parameters.
Photon energy spectra up to the kinematic limit have been measured in 190 MeV proton reactions with light and heavy nuclei to investigate the influence of the multiple-scattering process on the ...photon production. Relative to the predictions of models based on a quasifree production mechanism, a strong suppression of bremsstrahlung is observed in the low-energy region of the photon spectrum. We attribute this effect to the interference of photon amplitudes due to multiple scattering of nucleons in the nuclear medium.
The giant dipole resonance built on excited states was observed in very fissile nuclei in coincidence with evaporation residues. The reaction
48Ca
+
176Yb populated evaporation residues of mass
A
=
...213
–220 with a cross section of ∼200 μb at 259 MeV. The extracted giant dipole resonance parameters are in agreement with theoretical predictions for this mass region.
Existing data on the prescission neutron multiplicities have been analyzed with a new method combining time-dependent statistical cascade calculations with Feldmeier's model of the dynamics of ...nucleus-nucleus collisions. The analysis resulted in the determination of the time scale tausub ital f of fusion-fission reactions for different composite systems with excitation energies ranging from 80 to 240 MeV. The deduced values of tausub ital f range from 5times10sup minus20 to 10sup minus18 s. These time scales are approximately 10 times longer than those deduced for the same set of data with the static'' model used by Hinde and collaborators.
The dynamics of heavy-ion collisions is studied in an energy domain in the vicinity of the Fermi energy. The early history of the collision is analyzed from the theoretical and experimental point of ...view in which the message conveyed by bremsstrahlung photons and neutral pions is exploited. The Boltzmann-Uehling-Uhlenbeck model and the Dubna Cascade Model, both based on similar principles but each adopting different computation technics, are briefly described and their respective predictions are discussed. In particular the emission pattern of bremsstrahlung photons is discussed. The photon production has been measured in the systems
86Kr+
58Ni at 60
A MeV,
181Ta+
197Au at 40
A MeV and
208Pb+
197Au at 30
A MeV and energy spectra, angular distributions and two-photon correlations have been analyzed. We find that bremsstrahlung photons are emitted from two distinct sources that can be correlated with nuclear-matter density oscillations. The properties of photon emission are discussed in terms of collective properties of nuclear matter. The high energy tail of the photon spectrum is interpreted by
π
0 and Δ decay but predominantly by radiative capture of pions. The
π
0 absorption in the nuclear medium is further analyzed by examining their emission pattern.
The azimuthal angle distributions of neutral pions at midrapidity from Au+Au reactions at 1 GeV/nucleon incident energy were measured. An enhanced emission of pi exp 0 's perpendicular to the ...reaction plane was observed. The azimuthal asymmetry was found to be dependent on the pi exp 0 momentum. The pi exp 0 spectrum perpendicular to the reaction plane was harder than that in the reaction plane. The strength of the observed asymmetry appeared to be more pronounced for pi exp 0 than for charged particles and neutrons. A squeeze out effect for neutral pions was observed in Au+Au collisions at 1 GeV/nucleon. A determination of the reaction plane was achieved with a resolution of 27 deg .
The production of nuclear bremsstrahlung photons (E{sub {gamma}}>30 MeV) has been studied in inclusive and exclusive measurements in four heavy-ion reactions at 60A MeV . The measured photon spectra, ...angular distributions, and multiplicities indicate that a significant fraction of the hard photons is emitted in secondary nucleon-nucleon collisions from a thermally equilibrated system. The observation of the thermal component in multifragment {sup 36}Ar+{sup 197}Au reactions suggests that the breakup of the thermalized source produced in this system occurs on a rather long time scale.
Photons originating from coherent bremsstrahlung have been measured over a large dynamic range for the reaction of 200 MeV alpha particles with protons. At low photon energies the bremsstrahlung ...spectrum exhibits the classical behavior with an approximate 1/E(gamma) shape. At higher photon energies there is a pronounced contribution from capture into the unbound ground state and first excited state of 5Li. These results allow one, for the first time, to test theoretical models for a consistent description of bremsstrahlung and radiative capture in a complex system. Calculations predict both features qualitatively but fail to account for their relative importance.
We present some recent results from studies of light nuclei using exotic beams from ATLAS at Argonne National Laboratory. Light nuclei far from stability provide ideal testing grounds for modern ...theoretical methods, and may provide information about astrophysical environments. We focus on the nuclei 9Li and 7He, populated with the (d, p) and (d,3He) reactions.
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