As mentioned above, the difficulty in using 3He neutron spin filters on beamlines resides in the ability to preserve the polarisation: the magnetic field applied on the whole volume of the NSF must have a relative gradient of the order of 10-4 cm-1 or better. Two types of magneto-static cavities have been constructed for hosting detachable 3He containers which are 6 cm diameter and 10 cm long. The first one, Decpol (see photo), is based on resistive solenoid coils and the second, Cryopol, on a superconducting cylinder, both being included in high permeability yokes and polar pieces. Cryopol has been developed by participants P1 and P2 and has been tested successfully in a stray magnetic field up to 350 Gauss (i.e. 6 Tesla applied on the sample).

No magneto-static cavity exists for very large 3He containers covering position sensitive detectors. Moreover, there is a need for working at much higher magnetic stray fields. Participants P1 and P2 have a long experience in developing such equipment, requiring perfect integration on a polarised neutron instrument in avoiding depolarisation of the beam. Their collaboration with the other participants will lead to the development of new magneto-static cavities and a better exploitation of the 3He neutron spin filters.

Here are the main objectives:

• Design study and construction of a transport unit made of permanent magnets and able to screen low fields,
  
• Design study, construction and neutron tests of a “single-k” magneto-static cavity hosting a 3He cell in the large stray field of a cryomagnet,
  
• Design study, construction and neutron tests of a “multiple-k” magneto-static cavity hosting a 3He cell in a low or large stray field environment (PSD detector).