Cosmic Ray Veto Methods

Perhaps the most powerful noise elimination method is to find a coincidence between two antennas which are themselves triggering at a slow rate. Since there will be accidental coincidences, it is still important to address all possible techniques for reducing the noise contamination of individual data sets. To the extent that local cosmic radiation causes noise (remembering that it also may come as gamma-rays from the source to be detected and may thus retain characteristics of the source), a charged-particle detector monitoring cosmic ray bursts will be a useful noise veto. Additionally, Chiang et al. (1992) have noted that the lowest allowed moment of gravitational radiation is the quadrupole and the second longitudinal mode cannot be excited. Their simulation indicates a significant reduction in noise from cosmic ray muons may be achieved if a second longitudinal mode veto is also applied.

We have seen that energy deposition of a few hundred GeV in a gravitational antenna by a cosmic ray can be a significant source of noise at a rate of a few events per year. We have also seen that this deposition can be through the interaction of an air shower core with the detector or through the rare interaction of a very energetic muon. In the latter case, energy is deposited through the production and absorption of secondary particles and in the former case by the interactions and absorption of existing particles. In both cases, an antenna component of 10 to 100gcm thickness will not absorb all the incident energy and some will flow through. This remaining energy, or the incident energy for a core, can be detected and the detection can flag the experimenter to ignore any coincident signal from the gravitational antenna. Large area (a few square metres) cosmic ray detector are simple and cheap (approximately $10,000) to build and can readily respond to signals below 10MeV. Such a detector responding to an energy deposition of, say, 100MeV below an antenna could provide an efficient and cost-effective conservative veto. This level of 100MeV would result in a veto trigger typically at a rate of one event every few hours with a detector area of about one square metre.

Coccia et al. (1995) have described a cosmic ray veto system for the NAUTILUS gravitational wave detector. This detector uses a 2300kg aluminium bar and a few events per day are expected to result from cosmic ray effects. Their cosmic ray detector system consists of layers of limited streamer tubes and the trigger logic responds to high energy muons and hadrons as well as extensive air showers. Coincidences are taken between layers of tubes above and below the gravitational wave detector. ADC saturation sets an upper limit of about 1000 particles for any given tube.

We note that, if a future generation of antennas approaches the quantum limit, then a cosmic ray system will be unable to assume that substantial energy can leak through the detector housing. Any veto system will need to be installed within the detector itself.

A Proposed Veto Array for the Perth Antenna

We have provided a two scintillation detector coincidence system for use with the University of Western Australia niobium bar gravitational wave antenna to veto possible events in coincidence with cosmic ray showers. This has been available for some time. It has a primary particle threshold energy for shower detection of about 10TeV. We have now designed an upgraded system based on our Thebarton Array (Smith and Clay 1996) which will provide an air shower arrival trigger and will also provide an estimate of the number of particles crossing the antenna bar for each event. This requirement is rather different to conventional air shower measurements which concentrate on estimating the total particle content of the shower. Here, we wish to have an estimate of the particle density in the antenna. This is similar to measuring the particle density spectrum (also known as the burst spectrum) over the whole of the antenna.

The design is based on five scintillation detectors, each 400mm square. We are proposing to have the detectors at the corners and the centre of a square with 5m sides. Since we are interested in determining the density of particles up to quite high levels, we will set the individual detector thresholds at about six particles which will give an array primary energy threshold at about 500TeV for threefold coincidences. We anticipate an event rate of about one per hour. It will also be possible to trigger the system on a single large particle density in the detector below the gravitational antenna. If that detector has a threshold set at 15 particles, we again would expect a trigger rate of about one per hour with a primary energy threshold of about 200TeV (Ashton and Parvaresh 1975).

Most array triggers will include the central detector which will be placed below the antenna bar. As a result there will be a direct estimate of the particle number in the antenna with a range up to 10,000 particles. If that detector were to saturate, the remaining detectors would provide density information which could be used to estimate the density in an independent way to better than a factor of two. Those detectors would also serve as a check on the central density which will be rather dependent on the exact impact point of the air shower core.

© Copyright Astronomical Society of Australia 1997