Elsevier

Astroparticle Physics

Volume 30, Issue 4, November 2008, Pages 200-208
Astroparticle Physics

Detection of high energy cosmic rays with the resonant gravitational wave detectors NAUTILUS and EXPLORER

https://doi.org/10.1016/j.astropartphys.2008.09.002Get rights and content

Abstract

The cryogenic resonant gravitational wave detectors NAUTILUS and EXPLORER, made of an aluminum alloy bar, can detect cosmic ray showers. At temperatures above 1 K, when the material is in the normal-conducting state, the measured signals are in good agreement with the expected values based on the cosmic rays data and on the thermo-acoustic model. When NAUTILUS was operated at the temperature of 0.14 K, in superconductive state, large signals produced by cosmic ray interactions, more energetic than expected, were recorded. The NAUTILUS data in this case are in agreement with the measurements done by a dedicated experiment on a particle beam. The biggest recorded event was in EXPLORER and excited the first longitudinal mode to a vibrational energy of ∼670 K, corresponding to ∼360 TeV absorbed in the bar. Cosmic rays can be an important background in future acoustic detectors of improved sensitivity. At present, they represent a useful tool to verify the gravitational wave antenna performance.

Introduction

Cosmic ray showers can excite mechanical vibrations in a metallic cylinder at its resonance frequencies and can provide an accidental background for experiments searching gravitational waves (gw): this possibility was suggested many years ago and a first search, ending with a null result, was carried out with room temperature Weber type resonant bar detectors [1].

More recently, the cryogenic resonant gw detector NAUTILUS has been equipped with shower detectors and the interaction of cosmic ray with the antenna has been studied in detail.

The first detection of cosmic ray signals in a gw detector took place in 1998 in NAUTILUS. During this run many events of very large amplitude were detected. This unexpected result suggested in 2002, the construction of a cosmic ray detector even for the EXPLORER detector.

In Section 2, we briefly recall the main features of the thermo-acoustic model (TAM), that successfully describes the interaction between a solid elastic resonator and a charged particle, or a beam of such particles; some of these features are extended to the regime of superconducting metal for the elastic resonator.

In Section 3, we describe the NAUTILUS and EXPLORER cosmic ray detectors and we specialize the TAM model to the interaction with cosmic rays computing the expected event rates.

In Section 4, we describe the results of coincidence measurements between the output of each gw antenna and its respective cosmic ray monitor, in different periods of data taking: for NAUTILUS during the year 1998, with the antenna in superconductive state, and then in the years from 2003 to 2006, while for EXPLORER in the period from 2003 to 2006. The data are interpreted with the help of some results obtained by the RAP experiment. Finally, some conclusions of this extended analysis are drawn: we show the good agreement of our data with the TAM predictions and the consistency of data taken by two detectors with various different experimental setups (temperature, bandwidth, readout and acquisition hardware and software). As a central result, the relevance of the conducting state of the antenna material on the strength of the interaction is proven.

We have then learned that cosmic rays can also provide a good calibration source for resonant gw detectors, as they very closely mimic the signal expected by short bursts of gravitational waves, i.e. the tidal excitation of the longitudinal modes.

Section snippets

The thermo-acoustic model and its experimental validation with particle beams

The interaction of energetic charged particles with a normal mode of an extended elastic cylinder has been extensively studied over the years, both on the theoretical and on the experimental aspect.

The first experiments aiming to detect mechanical oscillations in metallic targets due to impinging elementary particles were carried out by Beron and Hofstander as early as in 1969 [2], [3]. A few years later, Strini et al. [4] carried out an experiment with a small metallic cylinder and measured

The cosmic rays detectors of NAUTILUS and EXPLORER, and their expected rates

The gw detector NAUTILUS [17] is located in Frascati (Italy) National Laboratories of INFN, at about 200 m above sea level. It is equipped with a cosmic ray detection telescope made of seven layers of gas detectors (streamer tubes) for a total of 116 counters [18]. Three superimposed layers, each with an area of 36 m2, are located above the cryostat. Four superimposed layers are below the cryostat, each with area of 16.5 m2. The signal from each counter is digitized to measure the charge that is

NAUTILUS in 1998

The ultra-cryogenic resonant-mass gravitational wave (gw) detector NAUTILUS [19] operating since 1996 at the INFN Frascati Laboratory, consists of a 3 m 2300 kg Al 5056 alloy bar. The cryostat mainly consists of seven concentric layers: three steel vessels, two thin aluminum plus three thick copper thermal shields. During the run of 1998 it was cooled at 140 mK. The quantity that is observed (the “gw antenna output”) is the vibrational amplitude of its first longitudinal mode of oscillation. This

Conclusions

We have discussed the NAUTILUS and EXPLORER response to small signals (energy E a few mK) and to large signals (from energy E20mK up to events of E600K). Table 2 shows the main results obtained.

For small signals, we have found that the acoustic gw detector response well agrees with the predictions based on the thermo-acoustic model, once the corrections to this model, provided by a dedicate experiment [14], [15], [16], are applied.

The large signals are the kind of events that can represent a

Acknowledgements

We thank our technicians F. Campolungo, G. Federici, M. Iannarelli, R. Lenci, R. Simonetti, F. Tabacchioni, and E. Turri, for their help in building and running the detectors and for the cryogenic operations. This work is partially supported by the EU Project ILIAS (RII3-CT-2004-506222).

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