Cosmic ray physics not only is an interesting and challenging field of research, but can also serve to introduce interested high school students to many concepts of modern physics. Within the Nikhef Institute for Subatomic Physics, the Hisparc project has been developed for outreach experiments on cosmic ray air showers, that originate from the collision of high energy particles with nitrogen and oxygen molecules in the earth's atmosphere. Next to the Hisparc project, in recent years an experimental set-up has been developed for introductory experiments on muons, also suitable for outreach in particle physics. In the MuonLab project, measurements can be performed on muons from cosmic rays. Because the lifetime of the muon is relatively long, muons from cosmic rays at energies > 1 GeV reach the surface of the earth with a rate of approximately 160 muons per m²/sec and can be detected directly. In the MuonLab project the lifetime and the velocity of the muons can be measured. Also, the original experiment of Rossi showing the creation of secondary particles in materials surrounding the detectors can be performed.

The discovery of cosmic rays dates back to 1910 when Theodor Wulf took his hand-crafted electrometer from Maastricht to the Eiffel tower in Paris. His experiment led him to the conjecture that the ionizing radiation, responsible for the electrometer’s discharge, does not come out of the earth itself, but out of the sky! In 1912,Victor Hess ascended in a balloon up to the altitude of 5 kilometers, and found that his electroscope discharged more rapidly as he ascended. He had to attribute this to radiation entering the atmosphere from above. Ever since, people have climbed mountains, gone up in hot air balloons or sent experiments up in spacecrafts to study these ‘space invaders’. This discovery of cosmic rays marked the start of elementary particle physics. Today, we know that cosmic rays are particles with high energy, consisting of bare nuclei, about 90 % hydrogen, 9% helium and 1 % heavier ions, originating in outer space and travelling at nearly the speed of light. When these high-energy particles impinge on our atmosphere, they collide with air molecules and produce a cascade of secondary particles that shower down through the atmosphere. The majority of the secondary particles decays very rapidly and does not reach the surface of the earth. One of the particles that can be detected on earth is the muon. Muons travel at almost the velocity of light and reach the surface of the earth with an average intensity of about 160 muons per m²/sec.

Traditionally in high-schools, cosmic rays, notably muons, are made visible with relatively simple devices such as a cloud chamber or a spark chamber. In our MuonLab experiment the setup of the instrument allows high-school students to measure the mean lifetime and the velocity of muons. The students are confronted with the paradox that a particle with a very short lifetime manages to reach the surface of the earth while being created at a height of many kilometers. In this way a phenomenon outside the scope of classical Newtonian physics gives the student the opportunity to not only enter the world of quantum mechanics but also to apply Einstein’s theory of special relativity to find the solution for this apparent paradox.

Today, almost 100 years after their discovery, we still have not identified the mysterious sources in our universe responsible for cosmic rays. Several huge experiments as the Pierre Auger Observatory in Argentina, the Ice Cube neutrino telescope deep in the Antarctic ice and the ANTARES neutrino telescope at the bottom of the Mediterranean Sea try to resolve this mystery by trying to detect the highest energy of the incoming cosmic rays.