1912 – 2012: a century of studying cosmic rays

One year ago, the Alpha Magnetic Spectrometer was docked to the International Space Station. This state-of-the-art tool for studying cosmic rays has revolutionised methods of detecting cosmic radiation, which was discovered barely a century ago.


Victor Francis Hess (in the basket), back from his balloon flight in August 1912. Source: American Physical Society.

Exactly one hundred years ago, the Austrian-American physicist Victor Francis Hess discovered cosmic rays. The researcher observed the phenomenon while on board a balloon; he found that at an altitude of 1,000 to 5,000 metres, the wires of his Wulf electrometer (a tool used to measure radiation) showed an increase in electrical charge. Hess had just proven the existence of ionising radiation coming from outside the Earth’s atmosphere. Twenty years or so later, the invention of the Geiger-Müller counter enabled physicists to study the properties of the rays more precisely.

One century later, cosmic rays and the question of their origin - which has never had a satisfying answer - arouse as much interest as ever. The only thing to have changed is the technology used to detect them. Today, this radiation can even be studied from space using the Alpha Magnetic Spectrometer (AMS-02) – the dark matter and antimatter detector assembled at CERN. Installed on the International Space Station (ISS) in 2011 by the crew of the STS-134 mission (find more information about the astronauts' visit in this week's DG Word), AMS-02 has already recorded around 18 billion events in which cosmic rays have interacted with the machine’s numerous sensors. For the first time, electrons with an energy greater than 1 TeV and positrons with an energy greater than 200 GeV have been recorded before they enter the atmosphere.

AMS-02 has an ambitious scientific programme to perform from its base on the ISS. By collecting precious data over several years, researchers will be able to study the specturm of cosmic rays with unprecedented sensitivity. They also hope to learn what has happened to primordial antimatter and what the invisible mass of the Universe is made of.

Increase of ionization with altitude as measured by Victor Francis Hess using two electroscopes on 7 August 1912. Source: P. Carlson, A. De Angelis, Eur. Phys. J. H 35, 309 (2011). Image of a typical electroscope, as used during Victor Francis Hess' era. When the device is charged, the gold sheets inside the chamber move apart. As the ionization of the gas inside the device leads to it being discharged, the sheets move towards each other. The frequency of this occurring is related to the amount of ionization.


Cosmic rays, a shared passion

Numerous experiments around the world are devoted to studying phenomena linked to cosmic rays. Some are conducted in underground laboratories (which means they can naturally filter certain types of cosmic particles and target those being sought after), while others are above ground in observatories using very large telescopes to detect particles such as very high-energy gamma or cosmic rays and neutrinos - the HESS-II telescope, the largest gamma ray telescope ever constructed, has just been brought into operation. It is equipped with a mirror with a 28-meter diameter! Lastly, some experiments rely on satellite observatories to detect these same particles.

Several experiments at CERN are dedicated to cosmic rays. First and foremost, LHCf (Large Hadron Collider forward), a detector that uses forward particles in the LHC to simulate cosmic rays under "laboratory" conditions. One of the major LHC experiments, ALICE, whose main purpose is to study a new state of matter, is also partly devoted to detecting muons produced by cosmic ray interactions with the Earth's atmosphere. The CLOUD (Cosmics Leaving Outdoor Droplets) experiment uses a cloud chamber to explore the possible link between cosmic rays and the formation of clouds.

Not forgetting ASPERA (AStroParticle European Research Area), a network of European funding agencies for astroparticle physics, which has partnered with CERN to support several projects dedicated to cosmic-ray studies at European level.


A laboratory for the study of cosmic rays, just around the corner from CERN

In 1943, the Centre national de la recherche scientifique (French National Centre for Scientific Research, CNRS) created the high-altitude laboratory Les Cosmiques. Situated more than 3,600 metres above sea level on the Col du Midi (Mont Blanc), this laboratory was dedicated to the study of cosmic rays and their applications in nuclear physics. The facility remained open until 1955.

On 23 July this year, to celebrate the 100th anniversary of the discovery of cosmic rays, the astronauts of the STS-134 mission laid a commemorative plaque awarded to Les Cosmiques laboratory by the European Physical Society (EPS), granting it the status of ‘EPS Historic Site’.

Want to know more? Don't miss the special edition of the CERN Courier (Volume 52, Number 6, July-August 2012) celebrating the 100th anniversary of the discovery of cosmic rays.

by Anaïs Schaeffer