Polarised targets for 4π detectors at MAMI

The spin, as a fundamental property of a particle, has been a main object of investigation in particle and nuclear physics research in recent decades. Consequently, the investigation of polarisation observables has become more and more important. Nowadays continuous wave (cw) beams with high duty cycles are standard technology. In parallel, the development of detectors with full angular acceptance, so called 4π detectors, has kept pace with the accelerator technology to allow for precise determination of the final state of the reaction under investigation. The availability of high speed data acquisition systems has enabled an increase in the luminosities by orders of magnitude. For complete spin investigation, one requires a polarised target in addition to polarised beams and recoil polarimeters. The advent of advanced beam and large acceptance detector technologies has driven the development of highly polarised, full angular acceptance targets. Highest nucleon polarization in solid-state target materials is obtained by a microwave pumping process, known as 'Dynamic Nucleon Polarization' (DNP). This process is applicable to any nucleus with spin and has already been used in different experiments with polarized proton and deuteron targets. The 'frozen-spin target' concept is a two stage process: in the initial step, a high degree of nucleon polarization is achieved through the DNP process. This requires placing the target material in a highly uniform magnetic field of typically 2.5 Tesla and applying 70 GHz microwaves. In a second step the microwaves are switched off