Cosmic Ray Induced Noise in Gravitational Wave Detectors
R. W. Clay, A. G. K. Smith, J. L. Reid, PASA, 14 (2), in press.
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.
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.
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