Precision Neutron Scattering Length Measurements with Neutron Interferometry

Since its inception, single-crystal neutron interferometry has often been utilized for precise neutron scattering length, $b$, measurements. Scattering length data of light nuclei is particularly important in the study of few nucleon interactions as $b$ can be predicted by two + three nucleon interaction (NI) models. As such they provide a critical test of the accuracy 2+3 NI models. Nuclear effective field theories also make use of light nuclei $b$ in parameterizing mean-field behavior. The NIST neutron interferometer and optics facility has measured $b$ to less than 0.8\% relative uncertainty in polarized $^3$He and to less than 0.1\% relative uncertainty in H, D, and unpolarized $^3$He. A neutron interferometer consists of a perfect silicon crystal machined such that there are three separate blades on a common base. Neutrons are Bragg diffracted in the blades to produce two spatially separate (yet coherent) beam paths much like an optical Mach-Zehnder interferometer. A gas sample placed in one of the beam paths of the interferometer causes a phase difference between the two paths which is proportional to $b$. This talk will focus on the latest scattering length measurement for n-$^4$He which ran at NIST in Fall/Winter 2010 and is currently being analyzed.