National Institute for Subatomic Physics

What are cosmic rays?

Every second, millions of cosmic rays bombard the atmosphere, causing a shower of subatomic particles. These cosmic particles can be divided into low-energetic particles and high-energetic particles. We already know where low-energetic particles come from (sun, solar flares).

However, the origin and the nature of the high-energetic variety is still unclear. We can’t imagine these particles gaining their enormous energy in our own Milky Way. Furthermore, these particles are not captured by the Galactic magnetic fields, therefore we can state that these particles have to have an extragalactic origin. The way in which nature accelerates the particles, the place where this happens, and the type of particle that is accelerated are subjects of international research.

What does the research entail?

Throughout the centuries, people have climbed mountains and the Eiffel tower, or have taken flight with hot-air balloons full of equipment to study cosmic radiation. 

Together with their scientific colleagues, Nikhef researchers have now developed and built a detector in Argentina with which particles can be studied that have an energy between 1017 eV and 1021 eV, the highest studied energy until now. The Argentinean set-up consists of different types of detectors. The entire particle detector consists of 1600 water basins, divided over 3000 square kilometers, with which the cosmic radiation from different directions is detected. When a shower of particles comes through the atmosphere, fluorescent light is emitted. This light is captured by 27 very sensitive telescopes that have been placed on the border of the observatory. At the observatory, new techniques are also developed, such as the measurement of radio waves that originate from the interaction of the particle shower with the atmosphere and the earth magnetic field.

How do the researchers know where the radiation originates from?

By very carefully measuring the characteristics of the particle shower, the researchers determine where the radiation comes from and which energy it has. They also determine what the mass is of the original cosmic radiation (does it resemble hydrogen or is it more like iron?). By comparing the arrival directions of the high-energetic radiation with the position of known objects in the sky (black holes, galaxies, supernovas) they try to determine what the sources are of this radiation.

What is Nikhef’s contribution to the project?

Nikhef is interested in finding out the nature of cosmic radiation. What is heading towards Earth and what happens when it collides with our atmosphere. Thus, we mainly want to better understand the particle shower. The detection of the fluorescent light is an ideal measurement for this, but unfortunately, this detector only works when it’s completely dark, without moonlight, when there are no clouds. The detection of radio waves of the particle shower doesn’t have these limitations. Nikhef is therefore very active in developing this new technique. Together with their colleagues, the Nikhef physicists develop an entirely new type of detection station. This way, they hope to study in detail the formation of radio waves. They hope to eventually find out more about the development of the particle shower in the atmosphere, and with that the composition of cosmic radiation.

Why do we want to know this and what are possible applications?

We want to know what the origin is of cosmic radiation and how these particles are able to accelerate from their most likely extragalactic location and be detected on Earth. Apart from that, the collision energies in the upper layers of the atmosphere are much higher than we can achieve with an accelerator on Earth. All kinds of exotic particles that may exist but haven’t been observed in the laboratory, can exist in the particle avalanche. If we find a way to accurately measure the characteristics of the particle avalanche in such a way that we can indicate at which energy we are likely to encounter new physics, then this would mean a lot to increasing our understanding of the fundamental building blocks of matter.

Website of the international collaboration

Pierre Auger Observatory