World class 80% ³He polarization maintained constant with on-beam polarizer at JCNS

Only ten years ago, world leading polarized neutron scattering experiments using neutron spin filters (NSF) were being conducted using starting ³He polarizations of 55%. This polarization typically decayed with T₁ time constants of about 100 hours. Immense progress in the last 10 years has resulted in ³He polarizations of up to 80% being reported in NSFs by the world's leading labs. The experimental team, based at the JCNS in Garching, was lead by E. Babcock, S. Mattauch, S. Starringer, and A. Ioffe. They used an in-house made polarizer utilizing the workshops at the JCNS in Garching and the IFF in Juelich with a cell constructed by the glass workshop at the ZAT also in Juelich. This in-situ ³He polarizer obtained a saturation polarization of 80% which was maintained constant, via the on-beam optical pumping, with high stability, for over a day.

The in-situ ³He polarizer, based on the spin-exchange optical pumping method, reached this high level of ³He polarization in a recent test performed on the TREFF reflectometer. The major components are similar to one utilized in prior work with an in-situ SEOP polarizer developed during the sixth European framework program, or FP6 [1,2]. The system has been redesigned and re-optimized for use as an wide angle analyzer for a new magnetic reflectometer at the JCNS called MARIA . Eventually the system is designed to accept a cell of up to 15 cm in diameter. It incorporates two frequency narrowed diode array bar lasers [3], a "SEOP magic box" style magnetic cavity [1], and an integrated AFP ³He flipper [4] driven by an amplified RF pulse provided by a high speed DAQ card on a PC.

For this early test we have polarized a ³He cell, called J1, which was prepared in collaboration with the ISIS ³He team [5]. This cell was previously tested and known to achieve a high level of ³He polarization when polarized in the laboratory [6]. This cell has an inside diameter of 5.6 cm and contains 5.2 bar cm of ³He. We monitored the ³He polarization over time with an NMR free induction decay system and unpolarized neutron transmission. The transmission of unpolarized neutrons then gives us the absolute ³He polarization, knowing the ³He pressure length product of the cell and the neutron wavelength.
As can be seen in the graph, after final optimization, the ³He polarization of cell J1 climbed to a high level of 80%. Very good stability was also observed over the course of the two day measurement with the polarization exceeding 75% for the whole period. During this time the system was allowed to run without any adjustments to the lasers or optical pumping parameters. As a reference, the green and blue lines are added to demonstrate the polarization over time if the same ³He cell were polarized offline and allowed to undergo T₁ decay of polarization, assuming a 200 hour and 100 hour on-instrument T₁ respectively.

These initial test are very encouraging and further developments will focus on refinement of the device, polarizing larger diameter cells, interfacing the system with the MARIA instrumentation, and obtaining practical experience to best apply the system and techniques for routine daily use. We expect the system to be able to provide polarization analysis concurrent with the commissioning of MARIA in 2010.

[1] E. Babcock et al. PRA 80, 033414, (2009)
[2] S. Boag et al. Physica B 404, 2659, (2009)
[3] E.Babcock et. al. App. Optics 44, 3098 (2005)
[4] E. Babcock et al. Physica B 397, 172, (2007)
[5] www.isis.stfc.ac.uk/groups/excitations/excitations-4640.html
[6] S.R. Parnell et al. NIMA 397, 172, (2007)

For more information please contact
Earl Babcock