In the last decade of the nineteenth century, scientists began investigating ionising radiation. The most common instrument used was an electroscope, a device invented by Jean Nollet in 1748. The electroscope consists of an insulated container in which are suspended two very light conducting leaves joined at a common point to a rod that is led vertically out of the container to a metal disc.

Electroscope When the electroscope is charged, the leaves fly apart as shown in the diagram opposite. When a radioactive substance is brought near the charged electroscope, the conductors slowly collapse together, the rate of collapse being proportional to the ionising radiation level. The early experimenters discovered that even when all radioactive materials were removed, the conductors would still collapse.

Even when the best efforts to insulate the electroscope from the environment were made, this inevitable discharge would occur. It was thought that natural radioactive materials in the ground might be responsible. To test this hypothesis, Victor Hess flew himself and an electroscope in a balloon in 1912.

As expected, the radiation level appeared to decrease with distance from the ground - at least up to about 1 km in altitude. However, above this point, the radiation increased, until at the maximum height reached (5 km), the radiation level was almost three times the ground level radiation. It appeared that some ionising radiation must be coming from space. This then was the discovery of cosmic rays.

Most cosmic rays (which are really particles) come from well outside the solar system and are termed Galactic Cosmic Radiation (GCR). This radiation contains the highest energy particles known - some way in excess of the energy that can be produced in the world's largest particle accelerators. A smaller and much more variable component comes from the Sun and is termed Solar Cosmic Radiation (SCR).

The Australian Antarctic Division maintains large neutron monitor cosmic ray detectors at Mawson (Antarctica) and Kingswood (Hobart). This data is now available on the IPS web site.

Neutron Monitor
Technician works on Neutron Monitor (Australian Antarctic Division)

Cosmic ray data from a neutron monitor near Moscow

ASAAustralian Space Academy