Precision measurement of the coherent scattering length of gaseous helium-four using neutron interferometry
This dissertation details a measurement of the n-4He coherent scattering length to be b4He = [3.0982±0.00214 (stat)±0.00077 (sys)] fm utilizing a perfect silicon crystal neutron interferometer. This measurement provides over a factor of 10 improvement in precision and differs by 0.162 fm compared to the most commonly used value. Neutron interferometry provides a tool for precision scattering lengths measurements for a variety of isotopes. Examples include coherent scattering length measurements for 1H, 2H, 3He and the incoherent scattering length of 3He. Neutron scattering lengths of light nuclei provide useful tests of nuclear potential models and serve as inputs for nuclear effective field theories. A monolithic, perfect silicon neutron interferometer splits the wave function of a single neutron via Bragg diffraction into two coherent paths spatially separated to the extent of a few centimeters. A sample of 4He gas, contained within an aluminum cell, is introduced into one beam path which produces a phase shift directly proportional to b4He. Significant effort has been spent quantifying important systematic considerations that include thermal transfer from the gas cell to the interferometer crystal and deformation of the gas cell walls due to gas pressure which ranges from 7 bar to 13 bar which were calculated by an FEA simulation. Thermal transfer between the gas cell and interferometer crystal induces a change of the intrinsic interferometer phase which is dependent on sample position. This additional systematic phase has been named the shadow phase. A glycol cooling system was used to mitigate the shadow phase and a special measurement pattern was devised to account for possible shadow phase drift. This work was performed at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR).