COW before TYREX

A very first test of a ³He filter was performed at the ILL on D3 in 1990 [1] using the spin-exchange optical pumping (SEOP) to polarise the gas. It was promising but the relaxation time (the time during which the polarisation is kept) was far to short. Hence a collaboration was started with the goal of producing a cheaper and more efficient ³He polariser.

In 1996, a first filling station for polarising ³He gas, nicknamed COW, was built at the ILL which used the metastability-exchange optical pumping (MEOP) technique. This was a collaboration with Mainz University [2,3]. The COW facility provided 60% nuclear-spin polarisation in cells of up to 5 bars pressure (250 cm³) at a production rate of about 0.3 bar•l/h. On the 2rd photo one can see in the heads of 7 small compressed air valves (3 are vertical and in the background), a technology developped by the ILL for its cryostats. On the right, the liquid nitrogen trap used to clean up the ³He gas from impurities. On the left, the buffer where the gas is compressed to a few tenths of mbar.

References:
1- F Tasset, TE Chupp, JP Pique, A Steinhof, A. Rhompson, E Wasserman, M. Ziade (1992) Physica B: Condensed Matter, 180-181, 896-898. DOI: 10.1016/0921-4526(92)90502-J
2- F. Tasset (1995) Physica B 213&214, 935-938. DOI: 10.1016/0921-4526(95)00327-6
3- W. Heil, K.H. Andersen, D. Hofmann, H. Humblot, J. Kulda, E. Lelièvre-Berna, O. Schârpf, F. Tasset, (1997) Physica B, 241–243, 56-63. DOI: 10.1016/S0921-4526(97)00511-5

Movie description:
Files: mp4 (mp4 - 10.59 Mi) and webm (webm - 19.07 Mi)

See F. Tasset, assisted by H. Humblot and W. Heil, installing the spin filter on the D3 diffractometer. After several years of strong development of both the technique and the special bottle, this is a crucial moment.

The cell filled with polarised ³He arrives on D3. The transport container includes two magnetic coils which maintain the polarisation. H. Humblot, F. Tasset, W. Heil and E. Bourgeat-Lami prepare for the measurements.
F. Tasset transfers the cell into CRYOPAD, which provides a homogeneous magnetic field and also acts as a spin flipper.
The film ends with a brief view of the zero-field polarimeter developed at PNPI (Gatchina, Russia). This polarimeter is capable of measuring very small rotations of the neutron polarisation.

During this experiment, the neutron beam polarisation achieved by the filter was determined with high accuracy from spin dichroism measurements. It was verified that both no significant relaxation was induced by neutron capture, and the wavelength variation of the polarisation down to 0.33 Å was in good agreement with the 1/v dependency of the absorption cross-section.

This experiment also made it possible to measure the complementary effect of P-odd spin rotation near the same p-wave resonance of ¹³⁹La and confirmed the prediction of the theory [1].

Reference
1- W. Heil at al. (1999) Physica B: Condensed Matter, 267–268, 289-293. DOI: 10.1016/S0921-4526(99)00020-4