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Mémoire online Cosmic Ray Dosimetry, tutoriel & rapport PDF.
Chapter 1: Cosmic rays
Cosmic rays (CRs) are particles pervading the Universe, much more energetic than the ones created in our best accelerators. In this chapter, I briefly review the history of their discovery, the main results of their observation, their origin and acceleration mechanisms, and finally the detection methods at different energies.
History
The story of cosmic rays began around 1900 with some people interested by measuring the remnant conductivity of air. They found that their electroscopes which measured the amount of ionization discharged even if they were protected from any visible radiation. They assumed that the radioactivity of certain ores present in the Earth’s crust was responsible for this effect. This assumption was apparently confirmed in 1910 by Wulf when he made precise measurements on the top of the Eiffel Tower. He observed that discharging rate was decreas-ing more slowly with altitude. But during 1911-1912 the Austrian physicist Victor Hess made a series of high altitude balloon flights and found that above 1.5 km the ionization rate was stronger. Hess was convinced that the explanation was an extraterrestrial origin of “penetrating radiation”. Hess was awarded the Nobel Prize in 1936 for the discovery of cosmic rays.
Before trying to identify the actual sources of this penetrating radiation, the big issue was to know its nature. In 1925 Millikan thought that these radiations were γ-photons with greater penetrating power than those observed in natural radioactivity. They were coined the term “cosmic-rays”. However, in 1927 Clay demonstrated that cosmic rays intensity depends on the geomagnetic latitude. This was a clear indication of the charged particle nature of cosmic rays, since photons would not have been influenced by the Earth’s magnetic field.
In February 1938, Kolhörster measured coincident signals in Geiger Muller counters set as 75 m apart. He concluded that the counters were hit by secondary particles or showers generated by cosmic rays in the atmosphere. In the late 1930s Pierre Auger undertook in-vestigations of cosmic radiation at the Jungfraujoch (Switzerland) at 3500 m above see level [1]. Similar to Kolhörster, Auger concluded that the registered particles are secondaries generated from a single primary cosmic ray when it penetrates the Earth’s atmosphere. Subse-quent experiments showed that the coincidences continued to be observed even at a distance of 200 m. This led Auger in 1939 to conclude that the energy spectrum of cosmic rays can ex-tend up to 1015 eV.
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