3He-based Neutron Spin Filters

My second, highly related, area of research is in polarizing neutron beams with polarized 3He gas. Polarized neutrons are necessary for nuclear physics experiments such as EmiT and NPDGamma. However, most research neutrons give their short, happy lives for neutron scattering and materials science, not for nuclear physics. Materials scientists use polarized neutron scattering to study magnetic materials such as the storage medium and read-head in your computer hard drive. NIST, Hamilton and Indiana received a grant from the DOE in 2000 to develop 3He-based polarizers specifically for use in neutron scattering instruments. This grant has been renewed until 2006. Materials science experiments tend to be much shorter in duration than the multi-year nuclear physics experiments. Students can see an entire run in a few intense days of beam time. I have brought students to runs at NIST and Argonne National Laboratory (ANL) to let students see how their Hamilton research fits into a larger experimental program.

3He-based neutron polarizers rely on the highly spin dependent absorption cross section for neutrons on 3He. When a beam of neutrons passes through the polarized 3He gas, the neutrons with spins in one direction are transmitted, but the neutrons with spins in the opposite direction are absorbed and filtered out. The result is a beam of neutrons all pointing in the same direction (i.e. polarized neutrons). 3He-based spin filters offer many advantages over supermirrors and other neutron polarization techniques. They are especially useful at pulsed neutron sources like the SNS because they can work over a broad energy range of neutrons including epithermal neutrons. For neutron scattering applications, spin filters have a wide angular acceptance and do not alter the divergence of the transmitted neutron beam. For nuclear physics applications, the resulting polarization can be accurately measured using only neutron transmission measurements. However, more development work is required to make this technology practical.

The difficulty in making 3He-based spin filters is in polarizing the 3He gas. I use spin-exchange optical pumping to polarize the 3He. A diode laser array optically pumps rubidium vapor which in turn polarizes the 3He nuclei. This is a very slow process, so the 3He gas must be contained in specially prepared glass cells that do not cause the 3He polarization to relax.

Spin-Exchange Infrastructure

My first few years at Hamilton were spent building infrastructure for 3He-based neutron polarizers. In order to make neutron polarizers, I need a vacuum system capable of making spin-exchange cells and an NMR system to test these cells. Due to demand from my collaborators, I concentrated on the NMR first. In the fall of 2000, Eileen Wildman, ’01, built a portable NMR system for her senior thesis. Eileen’s system was subsequently shipped to Indiana University. This system performed so well that Indiana asked for a second system which was built by Lubi Kutua, ’04 during the summer of 2002. Since then, Professor Brian Collett has completely redesigned the NMR system using a digital signal processor. The new system shows improved signal to noise and is much easier to use. At a recent conference, the European community showed a great deal of interest in Professor Collett’s NMR boxes, and we intend to get the design into the literature later this year. Over the summer of 2001, Andrew Magyar, ’03, built a much more stable, if less portable, system for the NPDGamma experiment at Los Alamos.

For his thesis project, Kris Gerardi, ’02, designed and built the vacuum system to make spin exchange cells. Kris produced a cell (named “Dilbert”) with a fantastic 300 hour spin lifetime. The factors that make good cells are not well understood, and I know of labs that have set up systems but have never produced good cells. Very few groups have produced cells as good as Dilbert. I intend to start regular cell production this year. There is a very high demand for the cells made at NIST, which has a similar system, and a second cell production facility at Hamilton would relieve some of this pressure while providing ideal student projects.

Compact Neutron Spin Filter

With this infrastructure in place, I have somewhat unique capabilities here at Hamilton. At a conference in March of 2003, it became clear that the Single Crystal Diffractometer (SCD) to be built for the SNS in 2009 would require a 3He-based polarizer. Within a year, I was able to produce a prototype polarizer for testing on the SCD at ANL. Due to severe space constraints, the SCD at ANL is a challenging place to insert a spin-exchange system. Using the summer work by M. Freddie Dias, ’06, and Andrew Yue, ’04, Jim Baker, ’04 produced a very compact 3He-based neutron polarizer for his senior thesis. Jim, Freddie, and I successfully tested the device at ANL in March. The SCD is being modified to reduce some of the space constraints, and a second run is planned for late 2004 or early 2005. In addition to producing a prototype for the SNS SCD, this test was also a way to break down the sociological barriers between the nuclear physicists who produce 3He polarizers and the materials science community who are legitimately concerned about the complexity and reliability of the new spin filter technology. Jim’s work is the first continuously running spin filter for use in neutron scattering.