Speaker
Description
Polarising beams of ultracold neutrons (UCN) is crucial for many precision experiments, e.g., the measurement of the neutron electric dipole moment or the beta decay asymmetry. Modern polarisers usually rely on the longitudinal Stern-Gerlach effect, where strong (superconducting) magnets create fields of several tesla, effectively repelling the undesired spin state and only transmitting the desired high-field seekers. While this method is highly efficient, it usually consumes large spaces, requires constant cooling, and is very expensive.
We propose a novel polariser based on Halbach arrays, which are circular arrangements of small permanent magnets. These magnets are arranged in a way to create a sextupole field inside a neutron guide made from polyethylene. When stacking multiple of these arrays, the gradient field causes UCN with their spin antiparallel to the magnetic field to be deflected towards the surface of the guide, where they will be upscattered inside the polyethylene. In cotrast, UCN with their spin parallel to the magnetic field will be guided along the field minimum.
This approach is extremely cost-efficient, since such magnets are widely available and cheap. In addition, this polariser provides a small footprint, since it closely surrounds the guide.
Recently conducted transport simulations show that a $30\,$cm long arrangement could in theory polarise the entire PF2 spectrum with an efficiency of more than $0.9$.
We carried out a series of measurements with this novel polariser at the PF2-EDM platform as a proof-of-concept. Using previously characterised iron-foil analysers and an adiabatic spin flipper, we measured the transmission and polarising efficiency of the Halbach polariser.
While the overall transmission of approximately $0.13$ is very low due to the losses inside the polyethylene, we measured a spectrally-integrated polarising efficency of $0.9$, with flipping ratios exceeding ten for the slower fraction of the spectrum.