F. J. van Kann

Search for coincident excitation of the widely spaced resonant gravitational wave detectors EXPLORER, NAUTILUS and NIOBE

We report the search for coincidences among three resonant mass detectors: EXPLORER at CERN and NAUTILUS in Frascati of the Rome group and NIOBE in Perth of the UWA group. The three detectors have a sensitivity for short bursts of GW in the h ≈ 10−18 range, about one thousand times better in energy than Webers original detectors. The analysis is based on the comparison of candidate event lists recorded by the detectors in the period December 1994 through October 1996. The events have been obtained by applying a pulse detection filter to the raw data and using a predetermined threshold. Due to the different periods of data taking it was not possible to search for triple coincidences. We searched for coincidences between EXPLORER and NAUTILUS during the years 1995 and 1996 for a total time coverage of 1372 hours and between EXPLORER and NIOBE in 1995 for a coverage of 1362 hours. The results have been: a weak indication of a coincidence excess with respect to the accidental ones between EXPLORER and NAUTILUS and no coincidence excess between EXPLORER and NIOBE.

Laboratory tests of a mobile superconducting gravity gradiometer

Abstract We describe a gravity gradiometer designed to measure off-diagonal components of the earths gravity gradient tensor, and intended for airborne geophysical exploration. The instrument consists of an orthogonal pair of mass quadrupoles pivoted about a common vertical axis. Each quadrupole is a rectangular bar of niobium having square cross section and with integrally machined flexural micro-pivots. Superconducting niobium pancake coils form the basis of the SQUID based transducers and provide supplementary magnetic springs to enable matching of mechanical parameters.

Performance of a superconducting gravity gradiometer

Abstract We report the laboratory operation of an rf SQUID-based superconducting gravity gradiometer designed to measure from an aircraft, off-diagonal components of the earths gravity gradient tensor. A detailed numerical model of the multi-mode electromechanical system shows excellent agreement with the experimental observations. We have demonstrated a common mode rejection ratio to linear accelerations of 180dB, as well as 60dB of active thermal compensation below 0.01Hz. Short term measurements show the instrument to be SQUID noise limited at 0.5Eo¨/√Hz in a frequency band from 50mHz to 1Hz. After correcting for the remaining effects of thermal fluctuations and flux creep the noise below 50mHz has a 1/f characteristic which is also dominated by the SQUID.

The University of Western Australia's resonant-bar gravitational wave experiment

The cryogenic resonant-mass gravitational radiation antenna at the University of Western Australia (UWA) has obtained a noise temperature of <2 mK using a zero order predictor filter. This corresponds to aIms burst strain sensitivity of 7x 10-19 . The antenna has been in continuous operation since August 1993. The antenna operates at about 5 K and consists of a 1· 5 tonne niobium bar with a 710 Hz fundamental frequency, and a closely tuned secondary mass of 0·45 kg effective mass. The vibrational state of the secondary mass is continuously monitored by a 9·5 GHz superconducting parametric transducer. This paper presents the current design and operation of the detector. From a two-mode model we show how we calibrate, characterise and theoretically determine the sensitivity of our detector. Experimental results confirm the theory.

A prototype superconducting gravity gradiometer

We report the successful laboratory test of a single-axis gradiometer designed to measure a diagonal component of the earths gravitational gradient tensor. It Consists of a pair of accelerometers mounted with their sensitive axes vertical and in line. The difference in displacement of the accelerometers is proportional to the component of the tensor gradient and is sensed via the modulated inductance of a superconducting coil coupled by a superconducting transformer into an RF biased SHE SQUID with energy sensitivity 4 × 10-29J/Hz. Rejection of in-line common mode accelerations is achieved by trimming the natural resonant frequency of each accelerometer: the restoring force acting on an accelerometer test mass is partly magnetic and can be trimmed by adjusting the persistent currents in a pair of force coils. A common mode rejection ratio exceeding 95 dB has been achieved in the presence of linear accelerations \sim 10^{-3} ms-2, and a laboratory generated gradient of 30 Eo rms has been detected with a signal to noise ratio of about 100. The dependence of this signal on the distance between source and detector has the expected Newtonian form. Under quiet conditions the background noise level of the instrument is at present 3 Eo/ \sqrt{Hz} . ( 1 Eo = 10-9s-2.) This is close to the practical limit achievable for such a single axis configuration: a three axis instrument for geophysical application is under development.

Estimating volumes of intra-abdominal blood using electrical impedance imaging

Electrical Impedance Imaging is a non-invasive method of imaging which we intend to apply to the problem of detecting intraabdominal bleeding in Emergency Centre patients. Experiments performed using a cylindrical saline-gel phantom show that as little as 25 ml of free blood within the abdomen may be detected. We also find a strong linear correlation between the total resistivity change created by the anomaly and the volume of the equivalent blood anomaly; thus enabling us to easily quantify the volume of blood perfused into the abdomen over a period of time directly from a reconstructed image. The total resistivity change produced by an anomaly is also observed to be independent of its radial position within the phantom.

Rotating Michelson-Morley experiment based on a dual cavity cryogenic sapphire oscillator

Recent experiments based on cryogenic microwave oscillators have tested the isotropy of the speed of light (Michelson-Morley experiment) at sensitivities of the order of a part in 10/sup 15/, which is a similar sensitivity to other best tests. Further improvements of the accuracy in this type of experiment are not expected due to the already long data set and the systematic error limit. We have constructed a new rotating Michelson-Morley experiment consisting of two cylindrical cryogenic sapphire resonators. The temperature of the dual cavity is controlled at approximately 6 K where the beat frequency between the two oscillators is independent of temperature. By rotating the experiment, an improvement of several orders of magnitude in our sensitivity to light speed anisotropy is expected, as the relevant time variations will now be at a rotation frequency where the frequency stability of the cryogenic oscillators is at its best.

Low field flux creep in niobium superconducting wires

Abstract Using a SQUID magnetometer we have measured flux creep in superconducting persistent current loops made of niobium and niobium-titanium wire. The observed creep is 4 orders of magnitude less than that reported in niobium alloys above H c1 , but 2 to 4 orders of magnitude larger than reported for trapped flux in niobium tubes well below H c1 A novel ‘ soak ’ technique, which eliminates the need for a persistent current and precise temperature regulation, is described.

A cryostat for measuring low field flux creep

We describe a SQUID-based magnetometer for measuring low field flux creep (LFFC) over the temperature range of 2–7.5 K. Temperatures below 4.2 K are achieved by pumping on a reservoir of liquid helium; the upper temperature limit is imposed by the SQUID magnetometer. Valid LFFC measurements to 105 seconds in niobium at 4.2 K require temperature regulation of the sample to within 1 μK. The white noise floor of the thermal system has been measured at 3–4 μK rms/√Hz, and the increased low frequency noise limits the temperature regulation to somewhat better than 1 μK at 10−5 Hz. Preliminary measurements of LFFC made using this magnetometer are presented.

Novel large volume horizontal cryostat

Abstract This paper describes a simple liquid helium cryostat with a main vacuum tank constructed from industrial steel pipes and an evacuated horizontal experimental can with a volume of 200 dm3. The novel construction allows high efficiency cooling and simple helium storage and avoids the problem of thermal oscillations, which are experienced in many other horizontal cryostats. This paper describes the main features of the cryostat and some performance data.