Alvin J. Sanders

Ultra‐High Precision Phosphor Thermometry Near 1100 K

Studying the fluorescent emission of the dominant 611 nm line from Y2O3:Eu (6.8 %) in the temperature region around 1100 K shows that the decay time changes correspond to changes in temperature with precisions of less than 10 mK. The apparatus used includes fluorescent activation by a 337 nm nitrogen gas laser and gradual cooling of an alumina thermal mass heated in a controlled‐temperature furnace. Newton’s cooling law and autocorrelation methods are used in the data analysis. Since the measurement technique involves the temperature response of molecular lattices it can be expected, with improvements of signal analysis and other laboratory refinements, to provide metrological precision and accuracy in the important temperature region above the aluminum freezing point.

Thermal metrology for a space-based gravity experiment

A proposed space-based test of gravitational theory requires unique performance for thermometry and ranging instrumentation. The experiment involves a cylindrical test chamber in which two free-floating spherical test bodies are located. The test bodies are spheres which move relative to each other. The direction and rate of motion depend on the relative masses and orbit parameters mediated by the force of gravity. The experiment will determine Newtons gravitational constant, G; its time dependence, as well as investigate the equivalence principle, the inverse square law, and post- Einsteinian effects. The absolute value of the temperature at which the experiment is performed is not critical and may range anywhere from approximately 70 to 100 K. However, the experimental design calls for a temperature uniformity of approximately 0.001 K throughout the test volume. This is necessary in order to prevent radiation pressure gradients from perturbing the test masses. Consequently, a method is needed for verifying and establishing this test condition. The presentation is an assessment of the utility of phosphor-based thermometry for this application and a description of feasibility experiments. Phosphor thermometry is well suited for resolving minute temperature differences. The first tests in our lab have indicated the feasibility of achieving this desired temperature resolution.

Prospects for a Space-Based Determination of G with an Error Below 1 PPM

It is not clear that ground-based laboratory determinations of Newton’s gravitational constant G will eventually converge on a common value that has an accepted uncertainty appreciably below 100 ppm. For many years G has been one of the least well known of the fundamental physical constants, and the uncertainty in the CODATA value of G was raised recently from 128 ppm to 1500 ppm, in part because of the very puzzling results from the Physikalisch-Technische Bundesanstalt (PTB) in 1995. Intense efforts by a number of groups during the past 8 years to resolve this difficulty are now beginning to bear fruit, but unfortunately the basic situation is unchanged. New results from three groups doing precision measurements of G disagree by over 200 ppm, although the reported errors of the individual experiments are below 50 ppm. The seeming intractability of this situation suggests that a useful alternative may be a space-based determination of G. We present and discuss a projected error budget for one such proposed measurement of G using the SEE (Satellite Energy Exchange) observatory. A SEE mission is also foreseen as being capable of measuring the time variation of G (G-dot) to about 1 part in 1014 per year. A finding of a non-zero value of G-dot would have immediate and profound cosmological significance. The authors have previously shown how a measurement of the required accuracy might be accomplished, entailing a synergism between SEE and a geopotential mission similar to the current GRACE mission of NASA.