Modern Storage Rings Research Articles

Photoionization of helium : A showcase of the continuous improvements in the production and in the use of synchrotron radiation for atomic physics

During the past twenty years, considerable progress has been made in the production and in the use of synchrotron radiation. Nowadays, the advent of ultra-high vacuum toroidal grating monochromators coupled to modern storage rings has made possible a new class of sophisticated experiments whose development has gone hand in hand with the continuous improvements in the technology of synchrotron radiation. The simple example of photoionization processes in helium is taken to illustrate these parallel developments. The present status of experiment is summarized, with particular emphasis on recent measurements of very weak photoionization processes leaving the positive He+ ion in the n = 2 and n = 3 excited states. Further improvements are still expected from the use of ondulators mounted on new storage rings, which will offer spectacular possibilities to the experimental investigations, in particular in atomic physics.

Die Änderung der Polarisation eines Strahles von Elektronen im homogenen Magnetfeld auf Grund der Wechselwirkung mit dem elektromagnetischen Strahlungsfeld

In order to get full information on the behaviour of the polarization of electrons moving in a uniform magnetic field, the corresponding density matrix is studied in the Furrypicture, and its rate of change caused by the interaction with the radiation field is calculated up to second order in the coupling constant e. In doing so one has to take account of matrix elements of higher, that is second order, because of their interference with forward scattering. We obtain the wellknown precession of the polarization vector against the direction of momentum owing to the anomalous magnetic moment, superimposed by a turning towards the opposite direction of the magnetic field, the degree of polarization tending to a constant limiting value about 0.92. This holds for an arbitrary initial polarization, in particular for originally unpolarized electrons. The polarization time decreases with increasing energy and magnetic field. In a modern storage ring this time is of the order of a few minutes and small compared to the storage time.