Study of spin-exchange optically pumped 3He cells with high polarisation and long lifetimes

We present a detailed investigation into 3He neutron spin filter cells polarised by spin exchange optical pumping (SEOP). We include measurements of the absolute 3He polarisation using neutron transmission and characterisation of both the X-factor and 3He relaxation times (T1) for a number of cells. For one cell we calculated a maximum 3He polarisation of 79% with a T1 of 633 h. The measured X-factor of this cell, X=0.17±0.01, is low. For all cells polarisations of >71% are observed. In addition we present 3He relaxation data for a new design of magneto-static cavity with a field of high homogeneity ΔB/B0∼3.5x10-4cm-1. This compact device provides a magnetic field in an orientation suitable for in situ optical pumping that minimises the field inhomogeneity contribution to the T1 to 930 h in a 1 bar cell, the longest reported on beam thus far. The results suggest that high 3He polarisation with long relaxation times can now be routinely obtained with SEOP, enabling time independent incident beam polarisation to be easily implemented across many different neutron scattering instruments.