Objectives of the Joint Research Activity
There are essentially two competing techniques for polarising 3He gas: Metastable Exchange Optical Pumping (MEOP) and Spin Exchange Optical Pumping with optically pumped Rb vapor (SEOP). For both techniques, circularly polarised light passes through cells aligned with an applied magnetic field. With the Rb technique, the 3He gas is polarised directly at the desired pressure. The simplicity and compactness of the required apparatus makes the method attractive but the maximum polarisation reached is presently limited to about 60% for large cells and 72% for small cells. On the other hand, metastable 3He pumping takes place in a discharge at low pressure with a follow-up polarisation preserving compression by a factor of a few 1000. The compression phase requires some technical effort which is rewarded by the shorter spin exchange time constant per 3He ground-state atom, and the better polarisation achieved (about 75%). Compared to SEOP, this technique requires a big apparatus, and up to now only a remote type of operation has been developed: NSF cells are detached from the filling station and carried to the instrument in a small magnetic holding field device. Because of the relaxation of the 3He polarisation, cells must be regularly replaced which is not possible on all instruments.
As regards the use of 3He neutron spin filters on neutron beamlines, a few simple devices necessary for maintaining the 3He polarisation are routinely used on selected instruments. The relaxation of the 3He polarization is due to three contributions:
- the wall relaxation scales with the surface-to-volume ratio and depends strongly on the quality of the inner surfaces of the 3He container,
- a magnetic field with very low relative gradient must be applied over the volume of the 3He neutron spin filter,
- the 3He pressure must be low enough for minimizing the dipolar relaxation.
3He containers produced from aluminosilicate glasses like Supremax and Corning 1720 are known to have good relaxation times. However, they contain boron leading to significant transmission losses for thermal and cold neutrons. Therefore, detachable cells are usually made from quartz glass and are cęsium coated. Typical relaxation times vary from 60 to 200 hours. The longer relaxation times are enough for carrying out real experiments if the cell is replaced with a fresh one every one or two days. Very few large containers have also been constructed. While they could not be used in combination with complex neutron polarisation manipulation devices, they have been tested successfully in low stray field environment at steady-state sources.
In order to render magnetic field relaxation negligible, the magnetic field applied over the whole volume of the NSF must have a relative gradient of the order of 10-4 cm-1 or better. Neutron spectrometers contain generally large quantities of magnetic material which is often magnetised. Moreover, many experiments are performed with a high magnetic field applied at the sample position and large inhomogeneous stray fields must be screened. To cope with these fields, new devices (magneto-static cavities) must be designed for carrying out experiments successfully. Most of the partners have already built such devices for simple cases: monchromatised beams, single detectors, mid-range magnetic fields and direct beam configurations.
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Supported by the EU through FP5 and FP6