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Polarised Helium-3 Station
Instrument Scientist
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| Instrument Layout of the Polarised Helium-3 Station |
During 2012, we will take delivery of a new state-of-the-art metastable-exchange optical-pumping helium-3 polarising system. The 3He station has been developed and constructed for us by the Institut Laue Langevin, in Grenoble, France.
It will enable polarisation-analysis experiments, for the first time, on five of our existing instruments (WOMBAT, QUOKKA, TAIPAN, PELICAN and SIKA), and will extend the present polarisation-analysis capability on PLATYPUS (which uses supermirrors) to off-specular diffuse scattering.
Our contract with the Institut Laue Langevin includes the magnetic-field environments and in-situ glass cells for all of these instruments.
The filling station itself consists of a set of glass cells containing low-pressure discharge-excited 3He gas, a laser system that pumps the the helium atoms into a spin-polarised state, all contained in a uniform magnetic field generated and maintained by a large set of Helmholtz coils.
The whole assembly will be located on top of the cold-guide bunker in the Neutron Guide Hall. As of January 2012, the platform for the station has been installed, along with underfloor radiation shielding, and the railings are currently being modified to bring them up to the right height in non-magnetic construction.
Some of the magnetic-field environments have already arrived, and been installed in the neutron beam instruments, and the 3He station itself has been fully tested in Grenoble.
The basic physical idea is that while the 3He is a good neutron absorber, and it is routinely used in neutron detectors, the absorption cross-section is extremely spin dependent: the absorption occurs almost exclusively when the neutron spin is antiparallel to that of the helium-3 nucleus, while there is very little interaction if the neutron and 3He spins are parallel. Therefore, if one can polarise a volume of helium-3 gas, it will be transparent to one neutron spin polarisation, and black to the the other polarisation.
The resultant device acts as a spin-dependent neutron filter (or valve), over a relatively wide range of neutron energies and independent of the neutron trajectory through the gas. The competing technologies of (a) supermirrors (as already implemented on PLATYPUS and the incident beams of QUOKKA and PELICAN), and (b) polarising Bragg crystal reflection (for instance from Cu2MnAl Heusler alloys) have the constraints of working best for cold neutrons and for beams of limited divergence in the case of supermirrors, and that crystal polarisers also monochromate the beam, whether or not one wants to.
The main disadvantage of polarised 3He, aside from its complication, is that it is very sensitive to magnetic fields, and cannot be used in close vicinity to high-field magnets.
References:
[1] Neutron Spin Filters Based on Polarized Helium 3 (NIST)
[2] 3He neutron spin filter (HZB-Berlin)