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Development of compact magnetostatic cavities for 3He spin filter cells

In order to preserve the 3He polarization in a spin filter cell used as a polarizer or analyser on a neutron beam line, the cell should be situated in a homogeneous magnetic guide field. Mu-metal magnetic shields are used to assure the stability of the guide field in the presence of moderate magnetic stray fields. Very compact (shoe box dimension) magnetostatic cavities made of mu-metal and permanent magnets have recently been constructed and optimized for rather large (13 cm long, 6 cm diameter) spin filter cells for use with hot neutrons. The relaxation time due to the magnetic gradients measured in the cells, situated in these compact boxes, is more than 500 h at 1 bar 3He pressure. Here we present the construction of such cavities and discuss the possibilities of further improvement in performance.