Utilising a high-resolution, trap-based positron beam, we have measured both elastic and inelastic scattering of positrons from water vapour. The measurements comprise differential elastic, total elastic, and total inelastic (not including positronium formation) absolute cross sections. The energy range investigated is from 1 eV to 60 eV. Comparison with theory is made with both R-Matrix and di...

The 1.25 A, 40 ns electron beam is accelerated in the 200 MeV electron linac. This beam is then focused onto a 3 rom diameter spot at the 7 rom thick (i. e., two radiation length) tungsten target. By the process of multiple nuclear-scattering the target generates the positron particles. These positrons, in general, can have a large diverging angle. The empirical formula for an angular dependenc...

A general method for calculating next-to-leading order cross sections in perturbative QCD is presented. The algorithm is worked out for calculating N -jet cross sections in hadron-hadron collisions. The generalization of the scheme to performing caclulations for other QCD process, such as electron-positron annihilation or in deep inelastic scattering is also straightforward. As an illustration ...

Monte Carlo simulation is a very powerful tool in understanding performances of positron tomographs as well as in assessing image reconstruction algorithms and their implementations. We present an object-oriented Monte Carlo simulator developed for 3D positron tomography. Results from phantom simulation studies including absorption and scattering of the photons in the field-of-view are presente...

An instrument has been designed and constructed to provide new insights into fundamental, low energy positron scattering processes. The design is based on the Surko trap system and produces a pulsed positron beam with an energy resolution of as good as 54 meV. The design and operation of the apparatus is explained, while the first experimental results from this apparatus have been demonstrated ...

We present the observation of an efficient mechanism for positron sticking to surfaces termed here Auger-mediated sticking. In this process the energy associated with the positrons transition from an unbound scattering state to a bound image potential state is coupled to a valence electron which can then have sufficient energy to leave the surface. Compelling evidence for this mechanism is foun...

The aim of this work is to identify the mechanisms responsible for very large rates and other peculiarities observed in low-energy positron annihilation on molecules. The two mechanisms considered are: (i) Direct annihilation of the incoming positron with one of the molecular electrons. This mechanism dominates for atoms and small molecules. I show that its contribution to the annihilation rate...

Absolute measurements are presented for the positron-impact cross sections for positronium formation, direct ionization, and total ionization of the diatomic molecules N2, CO, and O2, in the range of energies from threshold to 90 eV. Cross sections for the electronic excitation of the a 1 and a 1 state in N2 and the A 1 state in CO near threshold are also presented. The experiment uses a cold, ...

The coherent scattering of photon in the Coulomb field (the Delbrück scattering) is considered for the momentum transfer ∆ ≪ m in the frame of the quasiclassical operator method. In high-energy region this process occurs over rather long distance. The process amplitude is calculated taking into account the multiple scattering of particles of the intermediate electron-positron pair in a medium. ...

It is shown that induced Raman scattering of electromagnetic waves in the strongly magnetized electron-positron plasma of pulsar magnetosphere may be important for wave propagation and as an effective saturation mechanism for electromagnetic instabilities. The frequencies, at which strong Raman scattering occurs in the outer parts of magnetosphere, fall into the observed radio band. The typical...