SBIR Phase I: A High Pressure Polarized 3He Neutron Spin Filter

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This Small Business Innovation Research Phase I project is to develop an improved method for polarizing cold, thermal, and epithermal neutrons. It has long been realized that neutrons can be polarized by passing them through a dense, spin-polarized, gaseous 3He target. Because the thermal neutron-absorption cross section of polarized 3He is 10,000 times larger for the anti-parallel spin case then for the parallel case, a sufficiently thick polarized 3He target efficiently transmits neutrons that are in one spin state and strongly attenuates the others. This type of transmission polarizer has several advantages over currently available reflection polarizers (`supermirrors`). Unlike supermirror polarizers, the 3He filter can be applied to epithermal neutrons and can be used as a polarization analyzer in a number of standard neutron-scattering instruments, including Small Angle Neutron Scattering (SANS) spectrometers, which presently have substantial private sector applications. Two methods have previously been used to spin-polarize 3He. The first is spin exchange with an optically pumped alkali species (rubidium vapor) which is an extremely slow method, taking up to a day to achieve full polarization. The second is direct pumping of metastable 3He which has only been attempted at very low pressures (2 torr) and thus must have an added capability to pressurize the polarized gas. The physics of the direct method bears great similarity to that of excimer lasers with which Science Research Laboratory has a long history. This project will develop a high pressure ( 1 ATM) highly polarized (75%) 3He neutron-spin filter that will find application in polarized neutron scattering and other areas. The apparatus will employ a proprietary method for polarizing the 3He quickly and directly at high pressures, avoiding the problems faced by other methods, and making for a low cost, reliable device. The feasibility of this concept will be investigated in detail in the proposed Phase I program. Phase II will consist of a proof-of-principle demonstration of a neutron-spin filter. It will be followed by a privately funded Phase III program in which a commercial prototype instrument will be developed. If this research is carried over into Phase II and Phase III, the 3He Neutron Spin Filter will allow for efficient thermal and epithermal neutron polarization. Such a polarizer will find applications at thermal and epithermal neutron facilities in the USA, Canada, Europe, and Asia as an upgrade to established neutron-scattering instruments. Other applications exist as well in nuclear physics and in MR imaging.