Helios
The optical pumping facility HELIOS provides the ability of a large-scale production of a dense spin-polarized 3He gas at FRM II. The use of polarized 3He gas is of significant importance in many research areas where polarized gas is considered as a subject or a tool in investigations. In particular, it makes a great impact on the instrumentation for neutron polarization and polarization analysis since polarized nuclei of helium-3 possess very high spin-dependent neutron absorption efficiency over a wide range of neutron energies. Neutron spin filters (NSF) based on a dense hyperpolarized 3He gas may compete in polarization efficiency with common devices such as magnetized single crystals or supermirrors. Although these other methods are rather simple in operation, their applications are strongly limited by the acceptable neutron energy and the allowed range of scattering angles. In contrast, broadband neutron spin filters can be built to a predetermined size and shape in such a way that they will meet just about all practical needs.
Neutron Spin Filter – a new tool for neutron instrumentation
Application of Neutron Spin Filters with polarized 3He gas is most favorable in experiments with requirements of:
- capability to measure with any divergent scattered beam,
- homogeneous analyzing efficiency,
- predictable analyzing efficiency and transmission,
- negligible small angle scattering from an analyzer,
- low gamma-ray background produced
For example, such applications as:
- small-angle neutron scattering with polarization analysis,
- off-specular reflectometry with polarization analysis,
- large solid angle polarization analysis
will profit from using NSF.
What can be provided for a neutron experiment
The gas polarization is a crucial parameter in many neutron experiments and, thus, the main requirement for the HELIOS is to be able to provide a highly polarized gas. Currently, the steady-state gas polarization in the optical pumping volume may reach 82%. The normal mode of operation is, however, optimized for the production of gas with 75% polarization at the high rate of 1 bar×liter/hour. NSF cells can be filled with polarized 3He gas up to 3 bar gas pressure (up to 5 bar by special request) and transported to a neutron instrument with the use of a special transporter. At the neutron instrument the NSF cell is placed in a special magnetostatic cavity that assures a long lifetime of a polarized gas.
Magnetostatic cavity
Magnetostatic cavity is an important unit used to hold the polarized 3He NSF in a homogeneous guiding magnetic field and to screen the polarized gas against environmental magnetic field at a neutron instrument. Generally, the relaxation rate of a gas polarization in a NSF cell may be represented as a sum:
Here T1 is the total relaxation time, Tcell – the relaxation time related to the cell (relaxation due to interactions with the cell walls and due to dipole-dipole interactions) and Tmag – the relaxation time related to the gradient of the magnetic field over the cell volume. The last term can be written as
where p is the gas pressure, B0 refers to the mean value of a holding magnetic field and B⊥ - to the component of the magnetic field normal to B0. It follows then that the field gradient less than 3x10-4 cm-1 is required to assure Tmag > 500 h in a 1 bar pressure cell. There are two types of a magnetostatic cavity developed and built by the Neutron Optics group. The first one is a compensated solenoid with the magnetic field gradient of about 1x10-4 cm-1 over the 10x10x10 cm volume. The solenoid is not shielded and can be used at the instruments where the volume shielded against environmental magnetic field is available. The second one is a magnetostatic cavity with permanent magnets used to provide a homogeneous field in a box made of steel and mu-metal plates . The measured gradient of the field in the central area of 10x10x10 cm volume is about 3x10-4 cm-1. The box provides a good shielding of the NSF cell against environmental magnetic fields as well.
Cells
The cells of different size and shape (see Fig.) are currently available for experiments at FRM-II. They are made of high purity quartz glass and show the transmission of about 90% for a wide range of neutron wavelength.
NSF cells at FRM II: for common use and instrument-oriented.
The lifetime of the polarized gas in the cells varies from 100 to 200 hours in a very homogeneous magnetic holding field. The cells may fit the neutron beam with the maximal size up to 10 cm.