Today, most people who know about the existence of cosmic rays take it for granted that the earth is constantly being bombarded by particles coming from outer space since all of existence and the time to come. This is a story of the discovery of cosmic rays and human’s knowledge about it.
The journey starts from the “father” of cosmic rays studies, Victor Hess. He was a graduate in physics and during his work as an assistant in the institute of Radium research at the Austrian Academy of Sciences in the early 1900’s, he was astounded by the fact that there are residue charges trapped inside a sealed electroscopes no matter how good is the quality of the instrument. Scientist of his day proposed that it was caused by terrestrial ionising radiation – radioactivity from rock minerals. Hence the ionisation measured by the electroscopes should reduce if brought up higher into the atmosphere.
Oddly, previous experiments seems to indicate that the ionisation level is actually increasing with altitude, for example in 1910, Theodore Wulf measured the ionisation levels at the bottom and on top of Eiffel tower in Paris and it is found that the ionising radiation levels detected at higher level is much higher compared to the ground which is contradicting to the proposed terrestrial ionising radiation theory. Many other scientists at that time tried other methods to send instruments to record the ionisation levels in higher altitude using balloons but the data retrieved are inconclusive due to instrument defects under the conditions of high altitude.
Now speculating that the radiation is actually coming from the sky instead of the ground, Hess improvised the experiment by designing instruments that can withstand the pressure and temperature conditions of high altitude. He also determined that terrestrial radiation will no longer produce ionisation effect in altitudes higher than 500 meters.
Hess then mounts his instrument on balloons and sent to the skies ten times in the course of three years during (1911-1913). He found that at the height of several kilometres into the sky, the ionisation level is a few times higher than the surface of earth. Hence he concluded that “a radiation of very high penetrating power enters our atmosphere from above.”
Another conclusive data comes from one of Hess’s experiment on 12 April 1912, during a near-total eclipse of the sun. The ionising level did not reduce under the eclipse means that the radiation could not be from the sun itself; it has to come from further out in space. Hess’s experiments are later confirmed by Robert Milikan in 1925 who coined the term “cosmic rays”. And for this discovery, Hess shared the Nobel Prize in physics with the discoverer of the Positron, Carl D. Anderson in 1936.
For a period of time, cosmic rays are referred to as rays because it was believed to be a part of the electromagnetic spectrum, but during the 1930’s it was discovered that the cosmic rays must be composed of charged particles because they are affected by earth’s magnetic field.