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Abstract The use of transparent test/source masses can benefit future measurements of Newton’s gravitational constant G . Such transparent test mass materials can enable nondestructive, quantitative internal density gradient measurements using optical interferometry and allow in-situ optical metrology methods to be realized for the critical distance measurements often needed in a G apparatus. To confirm the sensitivity of such optical interferometry measurements to internal density gradients it is desirable to conduct a check with a totally independent technique. We present an upper bound on possible internal density gradients in lead tungstate (PbWO 4 ) crystals using a Talbot-Lau neutron interferometer on the Cold Neutron Imaging Facility at NIST. We placed an upper bound on a fractional atomic density gradient in two PbWO 4 test crystals of 1 N d N d x < 0.5 × 10 − 6  cm −1 . This value is about two orders of magnitude smaller than required for G measurements. We discuss the implications of this result and of other nondestructive methods for characterization of internal density inhomogeneties which can be applied to test masses in G experiments. 

Abstract It is highly desirable for future measurements of Newton’s gravitational constant G to use test/source masses that allow nondestructive, quantitative internal density gradient measurements. High density optically transparent materials are ideally suited for this purpose since their density gradient can be measured with laser interferometry, and they allow in-situ optical metrology methods for the critical distance measurements often needed in a G apparatus. We present an upper bound on possible internal density gradients in lead tungstate (PbWO 4 ) crystals determined using a laser interferometer. We placed an upper bound on the fractional atomic density gradient in two PbWO 4 test crystals of 1 ρ d ρ d x < 2.1 × 1 0 − 8 cm −1 . This value is more than two orders of magnitude smaller than what is required for G measurements. They are also consistent with but more sensitive than a recently reported measurements of the same samples, using neutron interferometry. These results indicate that PbWO 4 crystals are well suited to be used as test masses in G experiments. Future measurements of internal density gradients of test masses used for measurements of G can now be conducted non-destructively for a wide range of possible test masses. 

The QCD axion is a particle postulated to exist since the 1970s to explain the strong-CP problem in particle physics. It could also account for all of the observed dark matter in the Universe. The axion resonant interaction detection experiment (ARIADNE) intends to detect the QCD axion by sensing the fictitious ‘magnetic field’ created by its coupling to spin. Short-range axion-mediated interactions can occur between a sample of laser-polarized³He nuclear spins and an unpolarized source-mass sprocket. The experiment must be sensitive to magnetic fields below the 10⁻¹⁹T level to achieve its design sensitivity, necessitating tight control of the experiment’s magnetic environment. We describe a method for controlling three aspects of that environment which would otherwise limit the experimental sensitivity. Firstly, a system of superconducting magnetic shielding is described to screen ordinary magnetic noise from the sample volume at the 10⁸level, which should be sufficient to reduce the contribution of Johnson noise in the sprocket-shaped source mass, expected to be at the 10⁻¹²T$/\sqrt{\mathrm{H}\mathrm{z}}$level, to below the threshold for signal detection. Secondly, a method for reducing magnetic field gradients within the sample up to 10²times is described, using a simple and cost-effective design geometry. Thirdly, a novel coil design is introduced which allows the generation of fields similar to those produced by Helmholtz coils in regions directly abutting superconducting boundaries. This method allows the nuclear Larmor frequency of the sample to be tuned to match the axion field modulation frequency set by the sprocket rotation. Finally, we experimentally investigate the magnetic shielding factor of sputtered thin-film superconducting niobium on quartz substrates for various geometries and film thicknesses relevant for the ARIADNE axion experiment using SQUID magnetometry. The methods may be generally useful for magnetic field control near superconducting boundaries in other experiments where similar considerations apply.