First use of an internal superconducting ‘holding magnet’ in an η-photoproduction experiment

A new concept of a small superconducting ‘holding magnet’, placed inside a polarization refrigerator, has been developed for frozen spin targets. The superconducting wire is wound on the inner cooling shield of the vertical dilution refrigerator of the Bonn Frozen Spin Target (BoFroST). The maximum field of the magnet is 0.38tesla. Based on this concept, the frozen spin target was used for a measurement of the target asymmetry in the η-photoproduction γp↑→ηp with the PHOENICS-detector at the Bonn electron stretcher accelerator ELSA.