Imaging of PbWO4 Crystals for G Experiment Test Masses Using a Laser Interferometer

TL;DR

This study demonstrates that PbWO4 crystals have extremely low internal density gradients, making them suitable for use as test masses in precise gravitational constant measurements, with laser interferometry providing a sensitive, non-destructive measurement method.

Contribution

The paper provides the first upper bound on internal density gradients in PbWO4 crystals using laser interferometry, showing their suitability for G experiments and surpassing previous neutron interferometry sensitivity.

Findings

Density gradient < 2.1 x 10^{-8} cm^{-1} in PbWO4 crystals.

PbWO4 crystals are well suited for G measurement test masses.

Laser interferometry is effective for non-destructive internal density measurements.

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 ${1 \over \rho}{d\rho \over dx}<2.1 \times 10^{-8}$ 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.