Measurements of dielectron production in p + p and p + d collisions with beamkinetic energies from 1.04 to 4.88 GeV are presented. The differential cross section is presented as a function of invariant pair mass, transverse momentum, and rapidity. The shapes of the mass spectra and their evolution with beam energy provide information about the relative importance of the various dielectron production mechanisms in this energy regime. The p + d to p + p ratio of the dielectron yield is also presented as a function of invariant pair mass, transverse momentum, and rapidity. The shapes of the transverse momentum and rapidity spectra from the p + d and p + p systems are found to be similar to one another for each of the beam energies studied. The beam energy dependence of the integrated cross sections is also presented.
The disappearance of directed, collective nuclear motion ("flow") away from the interaction region of heavy-ion collisions has been observed using the Lawrence Berkeley Laboratory Streamer Chamber. We find that flow vanishes at a beam energy near 50 MeV/nucleon for the La+'s La system and near 60 MeV/nucleon for the Nb+ Nb system. The disappearance of flow may be understood as resulting from a balance between attractive and repulsive scattering strengths. Pull calculations with the Boltzmann-Uehling-Uhlenbeck model show that the disappearance of flow is sensitive to the assumed nuclear equation of state (EOS) and to the in-medium scattering cross section (o&z). Also, in the Nb+ Nb system, the purely attractive contribution to the reduced flow does not appear to be strongly sensitive to the EOS assumptions. PACS number(s): 25.70.Pq
An oxidosqualene cyclase from Arabidopsis thaliana makes arabidiol, a tricyclic triterpene reported with indeterminate side-chain stereochemistry. We established the full structure of arabidiol through chemical degradation, NOE experiments, and molecular modeling. By examining the mechanistic constraints that govern water addition in triterpene synthesis, we further show how the stereochemistry of hydroxylation can generally be deduced a priori, why deprotonation is more common than hydroxylation, and why cyclases that perform hydroxylation also generate olefinic byproducts.
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