C. Bradaschia

The VIRGO Project: A wide band antenna for gravitational wave detection

Abstract The status of advancement of the VIRGO Project is presented: the first-generation results from the Pisa seismic noise super attenuator give an upper limit to the noise transfer function of 2 × 10 −8 at 10 Hz. The upper limit to the absolute noise of the 400 kg test mass at 10 Hz has been measured to be 1.5 × 10 −13 m/√Hz. The scheme and the related problems of the VIRGO interferometer, which is supposed to work down to 10 Hz, are also presented. At the 3rd Pisa Meeting in 1986 we presented the idea of what could be a very efficient seismic noise reduction system able to give a sensitivity h ∼ 10 −25 at 10 Hz, in a 3 km interferometer for 1 year integration time. Now we have two new facts to present: the first is that the attenuation has been built, is working in Pisa, and shows remarkable characteristics. The second is the Italian-French interferometer VIRGO [1,2], a 3 km long antenna for low and high frequency (10–1000 Hz) gravitational wave (GW) detection. These two items will be presented in this article.

Seismic vibrations mechanical filters for the gravitational waves detector VIRGO

VIRGO is a detector of gravitational waves based on a laser interferometer with 3 km long arms, now under construction. In order to detect gravitational waves down to 10 Hz, the seismic noise has to be reduced by several orders of magnitude. New mechanical filters using cantilever springs as elastic elements have been developed for the VIRGO seismic isolation system. The spring total stiffness is reduced by more than one order of magnitude using magnetic antispring. A control system allows one to keep the spring length constant. The new filters provide an attenuation ≥40 dB each for frequencies above 10 Hz and, compared to the previously adopted solution (gas springs), are much less sensitive to temperature changes.

First results on the electronic cooling of the Pisa seismic noise super-attenuator for gravitational wave detection

Abstract The seismic noise excitation of the normal modes in the three-dimensional Pisa Super Attenuator, to be used in a long base interferometric gravitational antenna, produces large movements of the 400 kg test mass. In this paper it is shown, for the first time, that it is possible to damp, in a stable way, these normal modes using a six-dimensional damping system acting on the second suspended mass instead of the test mass itself, with the purpose not to reintroduce seismic noise. In this way the large displacement normal modes, including the verticals and the rotations, have been damped and the pendulum maximum displacement, originally varying from 15 to 30 μm according to the seismic noise intensity, has been reduced to less than 3.4 μm allowing locking to a fringe of the interferometer.

An improvement in the VIRGO Super Attenuator for interferometric detection of gravitational waves: The use of a magnetic antispring

We present a method of lowering below 2.5 Hz the vertical normal mode frequencies of the Pisa Super Attenuator by using permanent magnets which provide an antispring force. This method allows a more efficient suppression of the seismic noise decreasing the lower limit of the frequency region devoted to gravitational wave detection.

Sensitivity of a rigid small interferometer in the 10 Hz frequency region

Abstract In the framework of the development of the super attenuators for VIRGO, the French-Italian long arm interferometric antenna for gravitational wave detection, a small interferometer has been built in the Pisa laboratory, to investigate the sensitivity in the low frequency region. The noise measured at 10 Hz is below 10 −13 m/√Hz, about a factor of 10 less than previous measurements in Munich. Calculation shows that the noise between 10 and 100 Hz is compatible with seismic noise.

Operation of limited streamer tubes with the gas mixture Ar + CO2 + n-pentane

The active detectors of the ALEPH hadron calorimeter at LEP consist of plastic streamer tubes developed in Frascati. The standard gas mixture for the operation of such devices is argon-isobutane (3070). However, in underground experiments, for safety reasons, one has to reduce the hydrocarbon content. Therefore a study of the behaviour of streamer tubes operated with a Ar/CO2/n-pentane mixture has been performed. The influence of gas composition on efficiency, charge distribution and stability of operation has been investigated, and the results of these tests are presented.

A GeSi active target for the measurement of short lifetimes

Abstract A new GeSi active target is presently used in the NA1 experiment at CERN to study photoproduction of charmed particles and to measure their lifetimes. Some general comments on the active target technique are made.

Plane parallel mirrors Fabry-Perot cavity to improve Virgo superattenuators

Abstract The measurement of the finesse and the longitudinal locking of a 100 μm long plane mirrors Fabry-Perot cavity is described. a finesse of up to 5400 has been found. If attached to the Virgo antenna suspension (SA), a similar device is sensitive enough to detect the thermal displacement noise of the SA test mass, which is expected to limit the Virgo antenna sensitivity between 10 and 600 Hz. Reducing the Virgo noise floor is a challenge for future antenna development, as this region is the most promising one for coalescing binary stars and pulsar signals.

Virgo: Very wide band interferometric gravitational wave antenna

Abstract The sensitivity required to detect the small signals of gravitational waves in a wide spectrum is achievable in interferometers of long enough arms. Six interferometric gravitational wave antennas are planned all over the world. Virgo, one of such experiments, will be built in Italy by a French-Italian collaboration. The sensitivity goal of the Virgo antenna has been chosen at the level where one expects to observe at least few events per year. Virgo is designed for broad band detection (from 10Hz to 3 kHz), in order to get the best chance of detecting different kinds of sources, from pulsars to supernovae, alone or in coincidence with other detectors.

Vertical and horizontal transfer function measurements on a magnetic gas spring

A magnetic gas spring (MGS) has been developed to be the filter element of the super attenuator (SA), the device designed to support the optical components of the gravitational interferometric antenna VIRGO. Measurements have been performed on a MGS in order to study the resonance structure of the attenuation spectra in both the vertical and horizontal directions. Results guarantee an attenuation of at least 40 dB almost everywhere from 10 Hz to 1 kHz. By using experimental data the calculated performances of the SA turn out to be quite adequate for the VIRGO project requirements.