Electro-produced slow positrons

During the last 6 years it has been demonstrated that electro-produced intense beams of slow positrons are possible. High energy electrons from an accelerator generate bremsstrahlung in a thick conversion target of high element number Z. The photons produce electronpositron pairs and a small fraction of the positrons may be moderated to thermal energies. A review is given of the existing slow positron beam lines using this technique. At accelerator energies of 100 MeV total conversion efficiencies of several slow positrons per 10⁶ primary electrons have been obtained, resulting in average intensities of several 10⁹ slow positrons per second or more than 10⁵ slow positrons in pulses having a duration of a few ns. A further increase in intensity by at least one order of magnitude seems possible at higher accelerator energies.     

Some fundamental physics experiments using atomic clocks and sensors

We present several experiments in fundamental physics that use atomic clocks and sensors together with high performance time/frequency transfer methods. Our account is far from being exhaustive and instead concentrates on a chosen subset of present and future experiments, whilst providing some theoretical background. We only give very brief overviews of the experiments and theories, but provide ample references for the interested reader.     

Of atomic collisions with positrons and positronium

Recent progress in the study of atomic collisions using positronic projectiles is outlined and advances which might ensue from the production of more intense positron beams are indicated. Material in this paper was contained in a lecture delivered at the Workshop on Intense Positron Beams in Europe held at UCL on the 18th and 19th September, 1995.     

Time bunching of slow positrons for annihilation lifetime and pulsed laser photon absorption experiments

Methods are described for gathering positrons from an extended region and causing them to impact upon a target surface within a small time interval. The most optimistic of these proposed schemes suggests that one should be able to produce ∼10⁻⁹s positron pulses with peak intensities of ∼10¹¹s⁻¹ starting with a ∼200 m Ci⁵⁸Co β⁺ source. These pulses should be useful for studying time-dependent interactions of positrons at surfaces.     

Elastic scattering of slow positrons on atoms

The results of calculations of the elastic scattering cross section of positrons on noble gas and alkali atoms are presented. The calculations are performed within the one-electron Hartree-Fock approximation with multielectron correlations in the so-called random phase approximation with exchange taken into account. Virtual positronium formation is taken into account and proved to be very important. Arguments are presented that the positron polarization potential is repulsive for alkali atoms. The results obtained are in a reasonable agreement with experiment and with some previously reported calculations.     

Production of slow positrons with the Giessen 65 MeV LINAC

The experimental arrangement to produce the slow positrons with the Giessen 65 MeV LINAC is described. Some results of obtained slow positron yields are shown. At the present a maximum intensity of 1.05×10⁸ slow positrons per second is measured at the detector place. Actually we are optimizing different parameters of our experimental facility.This paper is based upon a talk given by F. E. at the Intl. Symposium Production of Low-Energy Positrons with Accelerators and Applications (Giessen 1986)     

The positronium atom as a benchmark for Rydberg excitation experiments in atomic physics

Positronium is a hydrogen-like pure leptonic atom that has gained great attention in basic physics for its role in antimatter studies, in quantum electrodynamics tests and in material science. Positronium spectroscopy is also an interesting research field, especially in the again unexplored region of Rydberg states, where motional effects turns out of overwhelming importance in determining the level structure, at variance with the usual Rydberg atomic spectroscopy. In this paper we present a simple theory of optical excitation of positronium high-n levels in strong magnetic fields, and determine the conditions for obtaining saturation of the transitions. It is shown that positronium atom can be an atomic physics benchmark for laser excitation experiments on Rydberg states in magnetic environments.     

Nuclear physics parallels in atomic cluster physics

This paper considers some examples of physical phenomena, manifesting themselves in electron scattering on atomic clusters, which are analogous with those known from nuclear physics. It is demonstrated that the electron diffraction plays an important role in the formation of both elastic and inelastic electron scattering cross sections. The essential role of the multipole plasmon excitations in the formation of electron energy loss spectra on clusters is elucidated. The main emphasis in the paper is laid on electron scattering on fullerenes and metal clusters, however, results are applicable to some extent to other types of clusters as well.     

Chaos in atomic physics

The influence of quantum chaos on small atomic systems is reviewed. It now seems clear that chaos in the usually understood sense (e.g. exponential sensitivity to perturbations) is not found in isolated quantum systems. However, there are phenomena which appear only when the corresponding classical system is chaotic. The stability of the classical dynamics, in other words whether it is regular or chaotic, has a profound effect on the character of the corresponding quantum spectrum and wavefunctions.     

物理学中的示零实验

示零实验采用完全补偿法 ,在物理实验中具有独特的作用 .本文介绍了示零实验在提高测量精度、修正系统误差和为理论提供依据方面的特殊功能     

Generation of slow positrons on an electron LINAC

A target and converter set-up for production of slow positrons on linear electron accelerators is described. It makes use not only of the pair-production processes but also of a positron source created on-line by nuclear reactionThis paper is based upon a poster presented at the International Symposium Production of Low-Energy Positrons with Accelerators and Applications (Justus-Liebig-Universität Giessen, FRG, 25–27 September, 1986)     

Detection of helium implanted into nickel by slow positrons

Slow/monoenergetic positrons were used to detect helium atoms implanted in nickel. Doppler broadening profiles of positron annihilation radiation as a function of incident positron energy were found to be quite sensitive to the helium implantation. The detection of helium implanted with a concentration of several ppm was proved to be possible by this technique.     

The emission of slow positrons from magnesium oxide powder

From Monte Carlo analyses of measured time-of-flight spectra for slow positrons traveling down a tube which is first evacuated and then filled with low-density helium gas, it is shown that the positrons leave the MgO-coated source grid at angles close to the normal direction. This result is important for low-energy positron collision studies. An interpretation of the result based on the surface charge held by the oxide is proposed.     

Positronium interactions in atomic physics

The role of positronium in the investigation of fundamental aspects of atomic physics is discussed. The recently developed technique of timed positronium beam production and the characterisation of the beam constituents in the energy range 7–41 eV are outlined. Examples of positronium atoms as probes in atomic scattering interactions are given.     

Intense source of slow positrons from pulsed electron accelerators

A pulsed LINAC is used for pair production in a tantalum target of 2.5 radiation lengths in an energy range from 80 to 260 MeV. Several well-annealed tungsten vanes are placed immediately behind the target and thermalize a small fraction of the fast positrons. The slow positrons are extracted from the target region and magnetically guided over a distance of 17 m to the detector at the end of an S-shaped solenoid. Two Nal detectors with well-known detection efficiency are used to register the 511 keV annihilation-rays. To reduce pile-up effects 50 mm of Pb were placed in front of the detectors. At an average electron current of 1 A we could detect about 10⁷ slow positrons per second. The positron yield is proportional to the electron current, and shows an increase with the electron energy for our target. The positron energy distribution has a FWHM of 1.8 eV.