Collaborative Research: New Source and Test Masses and their Metrology for G Experiments

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The research supported by this award focuses on conducting high precision measurements of G, Newton's gravitational constant. Despite a long history of experiments, there are serious inconsistencies in our current knowledge of the value of G. All previous measurements of G have used large masses made from high density metals, which have some inherent limitations on how well one can find tiny cavities inside them. These cavities, if present, cause small variations in the density of the masses and introduce errors in the measurement of G. The work in this project seeks to help mitigate this problem by developing high density transparent materials, such as lead tungstate, for use in experiments that measure G. These materials have much smaller variation in their density compared to metals and, being transparent, one can map the tiny cavities without cutting them open. These tiny cavities will be imaged by two different techniques that can be compared against each other. One method will use a laser beam to optically inspect the material and the second method will use a beam of neutrons to scan the material. In addition to experiments measuring G, this research will also benefit other scientific and technical areas. For example, since lead tungstate is used to build detectors for nuclear and high energy physics, this project could lead to improvements in the uniformity of lead tungstate and help improve these large detector systems. Additionally, the methods for the quantitative measurement of internal density variations could help establish absolute standards applicable to the very wide variety of materials. One of the most important impacts of this research will be education of young scientific researchers. The project offers an intellectually stimulating environment for the scientific and educational activities of undergraduates and graduate students in the respective universities. This challenging project will provide an excellent atmosphere for first-rate education in experimental physics.G is the only fundamental constant for which the uncertainty has risen over time as more and more measurements are made. This project has the potential to help resolve this rather puzzling situation by helping eliminate one of the known limitations of all previous measurements, the metrology of test and source masses. The group proposes to address these limitations by developing high density transparent materials, such as lead tungstate, for use as test and source masses in the next generation of experiments. This choice of new materials is motivated by the fact that the density variations in glass and single crystals are significantly smaller than in metals and they can be measured non destructively. Consequently, the group proposes to develop a laser interferometer for the non destructive measurement of the internal density gradients in source and test masses constructed from high density transparent materials, and to validate such measurements with neutron interferometry. The team will measure the density gradients in the test and source masses to better than 10 parts per million, thereby, reducing their contribution to the systematic uncertainty in the measurement of G to less than 0.5 parts per million.