Chemical structural effects on γ-ray spectra of positron annihilation in fluorobenzenes

Spectra of ?-ray Doppler shifts for positron annihilation in benzene and its fluoro-derivatives are simulated using low energy plane wave positron (LEPWP) approximation. The results are compared with available measurements. It is found that the Doppler shifts in these larger aromatic compounds are dominated by the contributions of the valence electrons and that the LEPWP model overestimates the measurements by approximately 30%, in agreement with previous findings in noble gases and small molecules. It is further revealed that the halogen atoms not only switch the sign of the charges on carbon atoms that they bond to, but that they also polarize other C-H bonds in the molecule leading to a redistribution of the molecular electrostatic potentials. As a result, it is likely that the halogen atoms contribute more significantly to the annihilation process. The present study also suggests that, while the Doppler shifts are sensitive to the number of valence electrons in the molecules, they are less sensitive to the chemical structures of isomers that have the same numbers and type of atoms and, hence, the same numbers of electrons. Further investigation of this effect is warranted. © EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2012.

Acceleration of ions up to 20MeV/nucleon in the ultrashort, high-intensity regime

The measurements reported here provide scaling laws for the ion acceleration process in the regime of ultrashort (50 fs), ultrahigh contrast (10) and ultrahigh intensity (> 10W/cm ), never investigated previously. The scaling of the accelerated ion energies was studied by varying a number of parameters such as target thickness (down to 10nm), target material (C and Al) and laser light polar- ization (circular and linear) at 35° and normal laser incidence. A twofold increase in proton energy and an order of magnitude enhancement in ion flux have been observed over the investigated thickness range at 35° angle of incidence. Further- more, at normal laser incidence, measured peak proton energies of about 20 MeV are observed almost independently of the target thickness over a wide range (50nm- 10 µm). 1. © 2012 by Società Italiana di Fisica.

Laser-driven acceleration of ions:State of the art, perspectives and applications

The acceleration of ions with high-power lasers has been a very active field of research during the past 10 years. This paper summarizes the main results obtained in the field, detailing the mechanisms of the acceleration process and the main observed beam characteristics. Perspectives for future development of the field and current and future applications are also discussed. © 2012 by Società Italiana di Fisica.