Three types of spin-filter cells are currently in use at the ILL : reblown GE180 aluminosilicate glass, GE224 quartz glass and Pyrex glass with single-crystal Si-windows. The cells are coated inside with Caesium to improve the relaxation time. We routinely obtain wall relaxation times of the order of 200 hours for standard size cells (5 cm diameter, 10 cm length). For cells with a higher ratio of the volume to the surface (e.g. 14 cm diameter, 12 cm length) the wall relaxation time becomes even longer, sometimes exceeding 500 hours, which is a world-record for a valve-sealed cell.
The material and shape of the NSF cell is adapted to the instrument. Glass cells can go to high pressure and/or cover large solid angles, while single-crystal silicon windows cells can be used to further minimise parasitic scattering.