The Gravitational Wave Detector EXPLORER

%RE5 EXPLORER is a cryogenic resonant-mass gravitational wave (GW) detector. It is in operation at CERN since 1984 and it has been the first cryogenic GW antenna to perform continuous observations (since 1990).\\ \\EXPLORER is actually part of the international network of resonant-mass detectors which includes ALLEGRO at the Louisiana State University, AURIGA at the INFN Legnaro Laboratories, NAUTILUS at the INFN Frascati Laboratories and NIOBE at the University of Western Australia. The EXPLORER sensitivity, at present of the same order of the other antennas, is 10$^{-20}$ Hz$^{-1/2}$ over a bandwidth of 20 Hz and 6 10$^{-22}$ Hz$^{-1/2}$ with a bandwidth of about 0.5 Hz, corresponding to a sensitivity to short GW bursts of \textit{h} = 6 10$^{-19}$.\\ \\This sensitivity should allow the detection of the burst sources in our Galaxy and in the Local Group. No evidence of GW signals has been reported up to now.\\ \\The principle of operation is based on the assumption that any vibrational mode of a resonant body having a mass quadrupole moment, such as the fundamental mode of a cylindrical bar, can be excited by a GW with non zero energy spectral density at the mode eigenfrequency.\\ \\The EXPLORER detector consists of an Al5056 cylindrical bar with length L = 3 m, diameter of 0.6 m and mass of 2270 kg, resonating in its first longitudinal mode of vibration at a frequency of about 900 Hz.\\ \\GW signals due to astrophysical sources such as gravitational collapses or merger of neutron stars and/or black holes are predicted to exist in this frequency range. However, even the largest hypothetical signal, such as a supernova occurring in our Galaxy, would produce a strain $\Delta L/L$ in the bar of the order of the GW amplitude \textit{h} $\sim$ 10$^{-18}$. Detectability of such small effects requires extreme isolation from the ambient noise and reduction of thermal and electronic noises. \\ \\The cylindrical bar is suspended in vacuum inside a cryostat and is kept cold at a temperature \textit{T} = 2K by a superfluid helium bath surrounding the bar vacuum chamber. The isolation of the bar from external acoustic and seismic disturbances is provided by a system of low-pass mechanical filters in cascade, with a total attenuation of -210 dB around the antenna resonance frequency. \\ \\The vibrations of the bar are converted into electrical signals by a capacitive transducer resonating at the antenna frequency in order to improve the energy transfer from the bar to the electronics. The signals are applied to the input coil of a dc SQUID amplifier by means of a superconducting transformer, which provides the required impedance matching. The transducer and the bar form a system of two coupled oscillators with frequencies f$_{-}$ = 906 Hz and f$_{+}$ = 922 Hz. The output signal from the SQUID instrumentation is directly sent to the acquisition system, together with the timing information.\\ \\The detector is also equipped with auxiliary sensors (accelerometers, search coil, etc.) that monitor the environment of the laboratory and allow to veto any event, observed by the detector, that occurs in the presence of external disturbances. The detector duty cycle is about 70 \% due to cryogenic operations and maintenance.\\ \\Absolute calibrations of the EXPLORER detector have been performed using the sinusoidal gravitational near field generated by a rotating quadrupole. \\ \\In addition to the search for short bursts, the data collected by EXPLORER are now used to search for periodic waves over long time periods, for measuring the stochastic background of cosmological origin, and for studying possible correlations with other astrophysical phenomena, such as gamma ray bursts and neutrinos emitted by gravitational collapses.\\ \\\\ \\\\ \\\\ \\\\ \\\\ \\\\ \\