Neutron Phase Contrast Imaging of PbWO$_{4}$ Crystals for G Experiment Test Masses Using a Talbot-Lau Neutron Interferometer

TL;DR

This study uses a Talbot-Lau neutron interferometer to set an upper limit on internal density gradients in PbWO$_4$ crystals, supporting their suitability for precise gravitational measurements and nondestructive testing.

Contribution

It provides the first independent neutron-based measurement of internal density gradients in PbWO$_4$ crystals relevant for G experiments.

Findings

Upper bound on density gradient: <0.5×10^{-6} cm^{-1}

Result is two orders of magnitude below the threshold for G measurements

Demonstrates neutron interferometry as a nondestructive characterization method

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 (CNIF) at NIST. We placed an upper bound on a fractional atomic density gradient in two PbWO$_{4}$ test crystals of ${1 \over N}{dN \over dx}<0.5 \times 10^{-6}$ 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.