Neutron grating interferometry evaluation of test masses*

Newton’s gravitational constant, G, remains the least well-known of the fundamental constants. Multiple precision measurements are highly discrepant, differing up to 10-sigma. This suggests a lack of understanding of all measurement systematic effects. One possible systematic is mass density gradients which are difficult to measure in general. One solution is to employ optically transparent test masses whose mass gradients can be assessed by laser interferometry. A candidate test mass material is PbWO₄, with density 8.28 g cm^-3. Given the use of PbWO₄ as a scintillator, large quantities of single crystal are available. Another solution, and the focus of this presentation, is to employ penetrating radiation, such as neutrons. For example, whether in elemental form or in a chemical compound, Pb and Bi have negligible absorption cross sections, allowing slow neutrons to pass through several cm of material. Neutron grating interferometry, whose signal depends on the coherent rather than absorption cross section, is a sensitive method for measuring density gradients. We report on recently published measurements of PbWO₄ crystals using a neutron Talbot-Lau interferometer in which mass gradients < 5×10^-7 cm^-1 were measured. We also provide estimates on future measurement possibilities.