Michael Kurt Bantel

High precision measurement of torsion fiber internal friction at cryogenic temperatures

Cryogenic torsion pendulums being developed for research in gravitational physics serve well for exceptionally accurate measurements of nonlinear and anelastic properties of torsion fibers at low temperature (77 K and 4.2 K) at low frequency (|0.01 Hz) in a pre-plastic regime of high shear strain (0.0001-0.003). The measurements use a torsion pendulum suspended by a thin fiber (typically|20 mm324 cm), oscillating with an amplitude of many revolutions. In a few oscillation cycles, oscillation amplitude and frequency may be 8 determined to better than one part in 10 , and harmonic deviations from simple harmonic motion (at 77 K) with fractional accuracy better 29 than 10 , enabling very accurate determination of the decrement, modulus defect, and hysteresis loop of the system as a function of oscillation amplitude.

A measurement of G with a cryogenic torsion pendulum

A measurement of Newtons gravitational constant G has been made with a cryogenic torsion pendulum operating below 4 K in a dynamic mode in which G is determined from the change in torsional period when a field source mass is moved between two orientations. The source mass was a pair of copper rings that produced an extremely uniform gravitational field gradient, whereas the pendulum was a thin fused silica plate, a combination that minimized the measurements sensitivity to error in pendulum placement. The measurement was made using an as-drawn CuBe torsion fibre, a heat-treated CuBe fibre, and an as-drawn Al5056 fibre. The pendulum operated with a set of different large torsional amplitudes. The three fibres yielded high Q-values: 82 000, 120 000 and 164 000, minimizing experimental bias from fibre anelasticity. G-values found with the three fibres are, respectively: {6.67435(10),6.67408(15),6.67455(13)}×10−11 m3 kg−1 s−2, with corresponding uncertainties 14, 22 and 20 ppm. Relative to the CODATA2010 G-value, these are higher by 77, 37 and 107 ppm, respectively. The unweighted average of the three G-values, with the unweighted average of their uncertainties, is 6.67433(13)×10−11 m3 kg−1 s−2 (19 ppm).

Measurements of torsion fiber anelastic properties in preparation for a new measurement of G

We present results of measurements of anelastic properties of CuBe and Al5056 torsion fibers at low temperature, and discuss their implications for a planned measurement of G using a cryogenic torsion pendulum. It appears that anelastic behavior should not limit a G measurement at a level of a few ppm.