SPARC: Photon and X-ray Spectrometers

FOCAL – Precision X-Ray Spectroscopy for QED in the Strong-Field Domain

In a production run conducted over a period of three weeks at the gas jet of the ESR storage ring the 2p1/2,3/2→1s1/2 Lyman-α transitions of hydrogen-like Au78+ were measured in high resolution via spectroscopy of the the corresponding x-rays located near 63 keV. The aim of the experiment is to access the quantum-electrodynamic (QED) contributions to the 1s binding energy experimentally in order to provide an accurate comparison with the most advanced QED calculations.

The experiment was made possible by the development of the FOCAL FOcusing Compensated Asymmetric Laue x-ray crystal optics which overcomes both the limiting spectral resolving power of previously used germanium solid-state detectors and the prohibitively low detection efficiency of conventional crystal spectrometers. Still the event rate observed is very low amounting to only two events per hour in the Lyman-α1 line in each of the two spectrometers. The amazingly low background revealed in the measured spectra is due to a number of measures including active shielding, optimized 2D position-sensitive germanium strip detectors allowing energy gating and fast timing necessary for measuring the x rays in coincidence with particles undergoing charge loss and being detected in a high-performance gaseous particle detector downstream the gas jet. The crystal spectrometers were set up in twins for a Doppler self compensation leading to a cancellation of angular uncertainties. Last but not least the present measurement has become feasible because, since it’s inauguration, the ESR’s average ion number has dramatically increased.

See, also, this report (in German).