Abstract – Michael G. HuberMay 16, 2019 3:43 pm
We have measured the n-4He coherent scattering length to be b(4He) = (3.0982 +/- 0.0023) 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 scattering data with light nuclei targets provide useful tests of nuclear potential models and can serve as inputs for nuclear effective field theories. Here, a sample of helium gas, contained within an aluminum cell, is introduced into one beam path of the interferometer which produces a phase shift directly proportional to b(4He). Important systematic considerations include; thermal transfer from the gas cell to the interferometer crystal and deformation of the gas cell wall due to gas pressure which ranges from 7 bar to 13 bar which was calculated by an FEA simulation. A gylcol cooling system was used mitigate thermal effects and a measurement pattern was devised to account for possible phase drifts due to time dependent thermal gradients. This work was performed at the National Institute of Standards and Technology (NIST) Center for Neutron Research (NCNR).