Introduction

Since January 2004, CEA-MDN, FRM-II, FZJ, HMI, ILL and ISIS, have actively developed advanced modular devices with the aim of improving and widening the exploitation of spin filters. This work focusses on the following tasks: the production of polarised 3He gas using both the spin-exchange (SEOP) and metastability-exchange (MEOP) optical pumping techniques and the exploitation of the polarised gas on instruments with improved containers and diverse magnetostatic cavities necessary for the slow decay of the 3He polarisation.

Results and applications

Today, FZJ has a conventional SEOP station polarising 3He up to 62% and ISIS takes advantage of a new 50W Quintessence diode in an external cavity diode laser (ECDL) providing a spectrally narrowed laser light. Since the beginning of the JRA, the maximum 3He polarisation has increased from 32 to 70% and beam tests have been very successful on some instruments. ILL has also built an ECDL pumping cells up to 72% polarisation. FZJ has started working on the hybrid technique and the components for implementing spin filters on the instrument KWS-1 at FRM-II will soon be tested.

In order to improve the performance of its MEOP station, ILL has built a set of electronics and optics with which to stabilise automatically the wavelength of the light. ILL has also replaced the optics with ones offering better performance at high power and changed the shape of the electrodes applying the discharge and placed on the optical pumping cells. With all these modifications performed, the maximum polarisation has raised from 75 to 83% in static mode and the production rate now reaches 15 bar.l/day when filling NSF with 76% polarised gas. In the meantime, FRM-II has acquired a filling station built at Mainz showing almost comparable performance. Concerning the other facilities, HMI is finishing the construction of its own MEOP filling station and ISIS has recently signed a contract with ILL for the construction of a MEOP station.

At the beginning of the project, the production of valve-sealed cells for MEOP was not reliable, with lifetimes varying between 60 and 200 hours. After many investigations at all facilities and some fruitful discussions with colleagues from the USA, we have finally adopted a more reliable recipe leading to the production of containers with long wall relaxation times (200 to 450 hours) at HMI, FRM-II, FZJ, ILL and ISIS. HMI has also investigated the influence of external magnetic fields on the relaxation behaviour of polarised 3He in NSF cells. These investigations have shown that the relaxation drops considerably if the cell is magnetised with a field of only 200 G. This result is now taken into account when building new magnetostatic cavities.

ILL has constructed containers with single-crystal silicon windows so as to minimise the diffuse scattering of the neutrons and therefore the background seen in the detectors of SANS instruments and reflectometers (lifetimes > 240h). ILL has also performed finite-element calculations of banana-shaped cells and manufactured prototypes in electronic-grade quartz-glass (lifetime > 400h after Cs coating).

In order to render the field relaxation of the 3He polarisation negligible, the magnetic field applied on the whole volume of the NSF must have a relative gradient of the order of 1e-4/cm or better. To cope with the fields encountered on the instruments, new devices must be designed for bringing cells to the instruments and for carrying out experiments successfully. ILL has designed, constructed and tested two transport units (called ``magic boxes'') made of µ-metal and permanent magnets. They screen low environmental magnetic fields, protect the users from accidental explosion of the container, do not require the transport of a battery and feature field relaxation times of 500 and 120 hours respectively for the large and small versions. The smallest box can host a standard NSF cell and only weight 6 kg. Several copies of this ``magic box'' have been manufactured for the JRA partners.

An upgrade of this ``magic box'' has been developed at ILL and tested at both the ILL and on the instrument MIRA at FRM-II. This version holds a 3He spin flipper and can therefore be used on the instruments to analyse and flip the neutron polarisation. ILL has also calculated and built a set of coils able to produce a homogeneous magnetic field that can be rotated towards 3 perpendicular directions. A neutron test has been performed on IN3 at ILL. With the applied field rotating every 5 min toward the X, Y and Z directions, the relaxation time measured was exactly the one expected for a cell filled with 3 bar of polarised gas, i.e. about 100 hours. A second version featuring a wider beam access is in production.

At HMI, FRM-II and FZJ, a few magnetostatic cavities have been built with the aim to start using spin filters on beam lines. HMI is finishing the construction of a detector with polarisation analysis following ILL Decpol design for the three-axis spectrometers E1. FRM-II is building specific cavities for the three-axis and neutron resonance spin-echo spectrometers and FZJ is finalising the construction of a cavity to be installed on the SANS instrument KWS-1. An upgraded version of Cryopol is been assembled at ILL for the polarised hot-neutron beam facility D3 and the powder diffractometer D20. This new version will still take advantage of a Meissner screen but will be cooled down with a cryorefrigerator and will be able to host a MEOP detachable cell or a SEOP cell for on-line pumping.

The partners and observers have met regularly during this project. The description of these meetings and the minutes are available from the Meetings section. Up to now, the partners and observers have exchanged 17 reports, presented 6 talks and produced 20 publications.

Co-ordinator: Eddy Lelièvre-Berna (ILL)