Effect of Cosmic Ray Showers on Gravitational Detectors

There would appear to be two likely modes by which cosmic rays may induce noise signals in gravitational wave antennas. These are associated with the ways in which energy and momentum deposition can respectively affect the antenna. Energy deposition in the antenna may cause heating and a thermal expansion of the antenna components. Amaldi and Pizzella (1986) note that resonant bar antennas might aim for a sensitivity of the order of K and that rather simple arguments equate this temperature to a typical energy deposition of a few GeV due to cosmic radiation. Also, in an interferometer, the momentum of the incoming particle may be transferred to a mirror and cause it to move on its pendulum support. The relative importance of these two effects is detector dependent and either could be important. Remarkably, one way or another, a few hundred GeV of energy deposition seems to be capable of producing significant noise in presently proposed antennas and a few GeV may be important for antennas of the following generation.

Giazotto (1988) has considered the effect of momentum deposition on internal oscillations of the mirror and its surface in an interferometer and concluded that such an effect is dominated by the heating term (see his equation (7)). However, the effect of momentum deposition on pendulum oscillation of the mirror needs to be considered and an energy of the order of a thousand GeV (1TeV) would seem to be a threshold for such a noise effect to be important.

We can check the likely rate of potential cosmic ray noise events from the measured cosmic ray density spectrum (e.g. Ashton and Parvaresh 1975). This describes the rate at which various densities of particles (in particles per square metre) are detected. The density spectrum is closely related to the cosmic ray energy spectrum and has a similar power-law shape. If we assume that near the core each particle carries about 100MeV (e.g. Dawson 1995), we are interested in the frequency of bursts containing 10000 particles or more in order to have 1TeV (eV) passing into the detector. This is expected roughly once in 5000 hours or a few times per year.

Thermal Effects

When a cosmic ray event deposits energy in a bar, the energy is degraded to heat and causes the bar to expand. Grassi Strini et al.\ (1980) derived the amplitude of the fundamental mode of the bar under such heating and this is close to the magnitude of thermal expansion of the bar but, naturally, depends on the way that the excitation occurs, particularly the position of the heating site. For a system such as a resonant bar, the effect can be large compared with the sensitivity required to detect a distant source.

Momentum Effects

Giazotto has examined in some detail the effect on an interferometer of the deposition of momentum by cosmic rays. We can examine such a process in a simple way by considering the effect of an impulsive deposition of momentum (E/c) where E is the cosmic ray energy deposited. (There will be a geometrical factor to determine the horizontal component of this momentum for a horizontal interferometer). The resulting change in speed of the mirror (mass M) will be (E/c)/M and the displacement in time dt will be this speed times dt. The displacement expressed as a fraction of the interferometer arm can then be expressed as a measurability limit, h. As Giazotto finds, using typical interferometer parameters, this limit for h is about where E is the energy deposition in GeV and dt is taken to be of the order of the pendulum oscillation time. Thus, a deposition of about 1TeV would again give a threshold signal.

© Copyright Astronomical Society of Australia 1997