During the last user cycle of 2025, the FLYNN team supported its highest number of experiments so far, providing 23 polarized 3He cells.
The polarized neutron group at ISIS runs the FLYNN polarized 3He filling station at ISIS, which provides 3He spin-filters to act as polarisation analysers for the LET, Larmor and Zoom instruments at ISIS.
Neutrons have a spin of ½ and are produced at ISIS with half of them pointing “up" and half “down". For some experiments, being able to use neutrons with their spins all pointing in one direction (polarized) gives an additional insight.
For example, a group of ISIS researchers recently used neutron polarisation analysis to discover that coherent scattering in QENS protein studies cannot be fully attributed to the buffer solution, as previously thought, showing the importance of using polarized neutrons.
To polarize a beam, we need to remove either the “up" or “down" spins, which we can do with supermirrors, or polarized 3He gas filters, such as those prepared using FLYNN. A similar polarization analyser is used between the sample and the detector to determine whether the scattered neutrons are spin-up or spin-down.
When 3He gas is polarized, it strongly absorbs neutrons with spins in one direction, leaving the others to pass through to the detectors. It can therefore be used as a spin analyser, but the process of polarizing 3He, and keeping it polarized, is not straightforward.
FLYNN is a facility at ISIS that uses high power lasers to polarize 3He inside purpose-built cells. Originally designed and delivered by the team of David Jullien of the ILL, FLYNN is now operated by Mark Devonport and Kirill Nemkovskiy, who are part of the ISIS sample environment team.
The 3He cells are designed in ISIS and built by a specialist glass manufacturer. Because of the instrument's detector geometry, the cell used on LET is the largest 3He cell in the world, holding 2500 cm3 of 3He at pressures up to 0.9 bar. Once the cells arrive, their inner surface needs to be coated with a thin layer of caesium in order to better maintain the 3He polarization, and their performance often gets better as this coating ages over a period of months.
After a certain time after filling with polarized 3He gas, the 3He polarization decays; after which its effectiveness reduces. At the moment, the team have increased this lifetime up to the point where they now only need to replace the 3He cells daily to maintain their effectiveness. Mark and Kirill are investigating ways to increase this lifetime further by designing new cells and treatment methods.
“We work closely with the team from the ILL, who are the experts in this field," says Kirill, adding: “we have also been helped along the way by industrial placement students who have contributed a lot to the project."
Within ISIS, the cells are primarily used for experiments on LET, but there is an increasing demand for use on Larmor and Zoom. The last cycle may have been their busiest yet, but things are only set to get busier.
“We are constantly challenged," says Mark, “but we enjoy it!"
