Cosmic Rays at Ground Level

To a reasonable approximation for most absorbing materials, the interaction properties of cosmic rays can be adequately described by the absorber thickness measured in grams per square centimetre. Primary cosmic rays interact with the atmosphere (of vertical absorbing depth about 1000gcm) with a mean free path of below about 100gcm. In effect then, all ground level cosmic rays are secondaries resulting either from the first interaction or from a cascade of secondaries (an extensive air shower, or EAS). The rate of cosmic ray secondary shower cores passing through one square metre of horizontal detector is about one per year at a primary energy of eV and is correspondingly higher (given the steep power-law energy spectrum) at lower energies.

The secondary particles which result from the interactions of primary particles with the atmosphere are conventionally divided into three groups: the hadrons, the muons and the electromagnetic component (electrons, positrons and photons). The origins of these components can be seen by considering the first interaction of a primary cosmic ray with a nucleus in the atmosphere. The mean free path for this interaction is below 100gcm (or within the top 10% of the atmosphere) and the interaction results in the production of secondary hadrons (mainly pions) and the degradation of the energy of the primary particle (typically by 50%). Each following interaction of the primary particle adds to the hadron number and degrades the primary energy so that the central region of the EAS develops a core of nuclear active particles. This core is well collimated and narrow since the hadrons have high longitudinal momentum and have relatively low momentum in the centre of mass.

Pions resulting from nuclear interactions in the core are responsible for the other two shower components. The charged pions may interact or may decay. The decays produce muons (and neutrinos) which are generally assumed to be unlikely to interact further in the atmosphere except by progressive ionisation. It is possible that some high energy muons may have catastrophic energy loss in a gravitational detector thus producing a mechanism for inducing noise.

As a result of the high altitude of many of the hadronic processes which generate them and their angle of production, muons spread in a geometrical way through the atmosphere and some can be found over a kilometre from the core, where they dominate the charged particle density. Also, for the lower energy primary particles, the central hadronic core may be absorbed before ground level is reached leaving only remnant muons which then constitute a background of `unaccompanied' muons. These are numerically the dominant form of sea-level cosmic radiation. Singly, they do not present a serious noise source to current gravity wave detectors. However, it is possible that their arrival time distribution might contain a significant component at the resonant frequency of the antenna.

Neutral pions quickly decay to gamma-rays which initiate cascades of electrons and gamma-rays fed by bremsstrahlung and pair production until the average energy of the cascade particles drops below the critical energy in air of a few tens of MeV. These cascades consist of many relatively short paths and the cascades reach a form of equilibrium with a characteristic spread from the core (the Moliére radius) of about 80m at sea-level.

The three shower components then have quite different lateral scales. These are: a few metres for the central hadronic core, about 100m for the electromagnetic component and several hundred metres for the muons. The electromagnetic component dominates numerically when integrated over the whole shower but the highest energy particles are found near the core which has the greatest particle density per square metre. This central core, or individual particles within it, presents the greatest potential for inducing noise in gravitational systems.

© Copyright Astronomical Society of Australia 1997