A new frozen-spin target for 4π particle detection

A new frozen-spin target has been developed, that allows the detection of emitted particles in an angular acceptance of almost 4 pi in the laboratory frame. The central part of this new target represents a He-3/He-4 dilution refrigerator that is installed horizontally along the beam axis. The refrigerator includes an internal superconducting holding coil to maintain the nucleon polarization in the frozen-spin mode longitudinally to the beam. The design of the dilution refrigerator and the use of an internal holding coil enabled for the first time the measurement of a spin-dependent total cross section in combination with a polarized solid state target. This new frozen-spin target was used successfully to measure the helicity asymmetry of the total photoabsorption cross-section at the Mainz accelerator facility MAMI. This experiment has been performed in order to verify for the first time the GDH sum rule. (C) 1999 Elsevier Science B.V. All rights reserved.