The Highest Energy Cosmic Rays come from outside our Galaxy, and just what are Cosmic rays anyway?

Even after more than a hundred years of study the origin and to a lesser extent the nature of Cosmic Rays is still something of a mystery. It was in 1912 that Victor Hess used a balloon to sent three electrometers, an early device for measuring radiation, to an altitude of 5300 meters. His discovery that the intensity of radiation increased as you ascended into the atmosphere stunned scientists. For his discovery Hess would be awarded the Nobel Prize in Physics in 1936.

It was quickly realized that the radiation being detected by Hess and others was actually the secondary products of collisions taking place in the upper reaches of our atmosphere between atoms of gas and some very powerful sources of energy coming from outer space. At first scientists believed that the primary component of the radiation was some form of X-ray or Gamma Ray, hence the name Cosmic Rays. It wasn’t until 1927 that physicist Jacob Clay was able to demonstrate that the source of Cosmic Rays was affected by the Earth’s magnetic field and therefore had to consist of charged particles.

In the years that followed physicists slowly learned that most (~90%) of cosmic ray showers are produced when a proton, with velocity nearly that of light slams into an atom in the air, shattering the atom and producing a spray of particles. In very energetic events the secondary particles produced by the initial collision may still have enough energy to strike and shatter further atoms leading to a cascade of sub-atomic particles. The diagram below illustrates such a cascade.

Primary cosmic ray. Development of an extensive air. shower in the Earth’s. atmosphere. Mostly muons, electrons and photons at Earth’s surface. (Credit: Pierre Auger)

Now I said that 90% of the primary particles are simple protons but about 9% have been found to be the nuclei of Helium atoms (two protons and two neutrons). The last 1% is composed of the nuclei of all the known atoms up to and including Uranium. In many ways the primary Cosmic Rays look just like the nuclei of the elements that the Sun is made of, accelerated to nearly the speed of light.

That resemblance to the composition of Stars gives us a clue as to where the Cosmic Rays get their energy. Our best model for the generation of Cosmic Rays uses the powerful explosions known as Supernova to boost some atoms to incredible energies. However calculations show that even Supernova are not powerful enough to produce the most energetic Cosmic Rays. Over the last 30 years astrophysicists have added black holes to the list of possible Cosmic Ray factories but not even black holes can account for some of the most energetic Cosmic rays that have been observed. Where these Ultra High Energy Cosmic Rays (UHECR) come from is still a hot question in astrophysics. The Cosmic Ray spectrum, that is the number of incident particles as a function of energy, is shown in the diagram below.

Cosmic Ray Flux. Number of Particles vs. Energy (Credit: Sven Lafebre)

It should be mentioned at this point that some of the Cosmic Ray particles that have been observed are millions of times more energetic than the particles accelerated in the Large Hadron Collider (LHC) at CERN. Now the LHC is the most powerful particle accelerator humanity has ever built, accelerating protons to an energy of 13 Trillion electron volts. That amount of energy would be about in the middle of the diagram above. Therefore all of the Cosmic Rays on the right hand side of the diagram are more powerful than anything humanity has ever produced. It’s easy to understand why physicists are so curious about where their energy comes from.

The Ultra High Energy Cosmic Rays (UHECR) are being studied by the Pierre Auger experiment, a vast array of detectors spread out over 3,000 square kilometers of the grasslands of Argentina. Since the more powerful the initial Cosmic Ray particle the larger the cascade it produces at Earth’s surface the Pierre Auger experiment must be physically large in order to capture the largest, most energetic cascades.

Recently the scientists at Pierre Auger have published a paper in which they announce that the very highest energy Cosmic Rays, those more than a million times the energy of the LHC, come from outside our Galaxy. This result comes from the study of 30,000 such particles. This is only one more clue in our attempts to unravel the mystery of Cosmic Rays but we have already learned much in the last century. If you’d like to learn more about Cosmic Rays or the Pierre Auger experiment click on the link below to be taken to the Pierre Auger website.