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[en] This chapter discusses the use of high-energy electron rays and positrons to study quasi atoms. The high-energy tail of the electron-ray spectrum contains detailed information about the high-energy component of the momentum distribution of the bound electrons in the quasi atom. It is demonstrated that, at least in principle, the most direct way to investigate the question of the binding energy of very high Z systems is the spectroscopy of positrons, since a preionized diving level may decay by spontaneous positron emission. Topics considered include the spectroscopy of high-energy electron rays (general considerations, physical quantities, experimental arrangements, test of the basic concept, electron-ray spectroscopy in the high-Z /SUB u/ region), and the spectroscopy of positrons (positron creation in strong electric fields, physical quantities in positron spectroscopy, evaluation of atomic positrons). All the positron experiments described were performed for scattering systems in which the collison partners followed nearly pure Rutherford trajectories

[en] High precision beta decay experiments with polarized neutrons, employing magnetic guiding fields for the decay electrons in combination with energy dispersive detectors, initiated detailed studies of the point spread function (PSF) for homogeneous magnetic fields. A PSF describes the radial probability distribution of mono-energetic electrons at the detector plane which were emitted from a point-like source. With regard to accuracy considerations for high-precision experiments unwanted singularities occur as function of the radial detector coordinate which have recently been discussed in detail by Dubbers (2015) [3]. In the present article mathematical inconsistencies in the approximations to calculate PSFs have been corrected. In addition, numerical orbit calculations have been performed for inhomogeneous magnetic fields which show that, on the one hand, generalizations on the basis of adiabaticity considerations must be handled with care but indicate, on the other hand, that non-adiabaticity would not prevent a proposed check of magnetic field configurations.

[en] Some new aspects of in beam spectroscopy of electrons and positrons after heavy ion collisions are discussed. The experimental set-up used is essentially an electron transport system with normal conducting solenoidal coils and a Si(Li)-detector as energy dispersive element. Spectroscopy of low energy conversion electrons emitted form recoil nuclei in flight after (HI,xn)-reactions is possible with a longitudinal semicylindrical baffle between target and Si(Li)-detector. Special features of this method are high transmission and strong suppression of the prompt delta-electron background. This method was applied for the investigation of a subnanosecond 12⁺ isomer in the ¹⁸⁸Pt-nucleus by electron-gamma, electron-electron coincidence measurements as well as lifetime-measurements based on the detection of conversion electrons. The first observation of positron creation in 1.1 GeV²⁰⁸Pb+²⁰⁸Pb collisions is reported. It is established that a dominant fraction of the positron yield is from none nuclear origins. (author)

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[en] This contribution emphasizes a pedestrian introduction to the physics of superheavy quasimolecules with united nuclear charges Zsup(u) = Z₁ + Z₂ >approx. 137. First, the author investigates the simple electron bound-state and continuum solutions of the Dirac equation in a Coulomb potential with charge number Z > 1/α. Then the question is discussed whether quasimolecules are formed in uranium-uranium collisions and whether the predicted phenomena of the preceding section (like positron production) will be observed. Then models for inner-shell ionization are discussed and experiments are proposed. Finally the positron experiments are investigated further. (Auth.)

[en] The measurements of positron spectra resulting from the bombardment of U and Cm targets with U beams of energies between 5.8 and 8.4 MeV/u is reviewed. The positron spectrum of the quasielastic ₂₃₈U + ₂₃₈U scattering system at 5.9 MeV/u and scattering angles thetasub(p)sup(lab) = 45₀+-10₀ exhibits a typical broad energy distribution in accord with the dynamic mechanism of positron production involving Rutherford trajectories. Under more restricted kinematical conditions narrow peak structures were observed in the ₂₃₈U + ₂₃₈U and the ₂₃₈U + ₂₄₈Cm systems. The possibility that these peak structures may signal the spontaneous positron production mechanism is discussed. In cases in which the colliding nuclei result in a fission reaction (e.g. at 7.5 and 8.4 MeV/u), the atomic positron spectra drop off more steeply at high energies than expected from calculations based on Rutherford scattering. This effect is in accord with a nuclear contact time of about u^-₂₁ s. (orig.)

[en] The positron production yields in close ion-atom collisions have been investigated for the U + Cm, U + U and U + Pb scattering systems at U beam energies between 4.7 and 5.9 MeV/u. Furtheron positron spectra were measured for the overcritical U+U and the undercritical U+Pb system at 5.9 MeV/u and a scattering angle thetasup(Ion)sub(Lab) = 45⁰ +- 10⁰. All positron yields, including earlier measurements, follow a simple scaling law. No characteristic deviations from this or other signatures indicating level diving for the overcritical U + Cm and U + U systems have been observed within the experimental errors of about 30%. (orig.)