Tag: UHECR

The Ice Cube Experiment. High Energy Physics at the Bottom of the World.

Physicists who study the way the Universe works at its simplest, most fundamental level do so by examining the collisions between elementary particles like the electron, quarks and neutrinos. The higher the amount of energy in those collisions the more we can learn about their behavior, the more we learn about the rules by which the universe is built. This is why physicists need to build such powerful particle accelerators such as the Large Hadron Collider at CERN.

However every day the Earth is struck by particles coming in from outer space with billions of times as much energy as the most powerful ever produced by human science. These particles are called the Ultra High Energy Cosmic Rays (UHECR) and physicists would love to be able to use their enormous energy in their experiments. The problem is that we never know exactly where on earth the most powerful of these particles are going to strike so how do we study them? How do we get them inside our instruments so that we can study them? Obviously in order to catch these UHECR you need a really big detector, and a lot of patience.

Enter the Ice Cube Experiment down in Antarctica. This experiment uses a cubic kilometer of the ice on that frozen continent as a detector for UHECR collisions. (That’s a block of ice one-kilometer long by one-kilometer wide by one-kilometer deep) The image below shows the experiments control facility sitting on the Antarctic ice.

The Ice Cube Experiment’s above ground (ice) Control Center (Credit: Scientific American)

The Ice Cube Experiment was constructed by drilling eighty-six, one-and a half kilometer deep holes into the ice and inserting long strings of ultra sensitive light detectors (Called Digital Optical Modules or DOMs) into the holes. (There are 5,160 of these DOMs total) Deep within the ice the only light that will be possible for the DOMS to detect will be that which is given off by the UHECR as they collide with atoms in the ice. The image below shows the overall layout of the Ice Cube Detector.

The Layout of the Ice Cube Experiment (Credit: Ice Cube Collaboration)

The light given off by these fast moving particles is called Cherenkov radiation, which is best known as that eerie greenish-blue glow around a nuclear reactor. Just what Cherenkov radiation is requires a little bit of explanation.

Cherenkov radiation coming from a Nuclear Pile (Credit: Reed Z)

We all know that nothing can travel faster than the speed in a vacuum. However the speed of light in transparent materials, like air or water or glass or ice is lower than the speed of light in a vacuum. So what happens when a sub-atomic particle like a proton is traveling through ice faster than the speed of light in ice? Well, what happens is the particle emits energy in the form of Cherenkov radiation until its velocity is below the speed of light in ice and it is this Cherenkov radiation that the DOMs of Ice Cube detect. The image below shows one of the DOMs.

One of Ice Cube’s Digital Optical Modules (DOMs) (Credit: The Ice Cube Collaboration)

The primary type of elementary particle that Ice Cube is designed to study are neutrinos and yes I know I talk about neutrinos all of the time (See posts of 30 July 2017, 2 December 2017 and 6 June 2018). Over the last twenty years however we have learned so much about our Universe by studying neutrinos and we have the possibility of learning more about Supernova, Gamma Ray Bursts, Blazars and even the Big Bang itself by studying the high-energy neutrinos given off by those events.

In fact according to two recent papers from the teams of scientists running Ice Cube a neutrino event that was detected by Ice Cube on 22 September of 2017 has been traced back to it original source, a Blazar designated as TXS 0506+056. Objects like Blazars have been observed in the past with optical and radio telescope along with X-ray and gamma ray telescopes and now the Ice Cube Detector as well. The image below shows graphically what a detection by Ice Cube looks like.

The Highest Energy Event yet measured by the Ice Cube Experiment (Credit: The Ice Cube Collaboration)

As you might guess there are already plans to expand Ice Cube. Called Ice Cube Gen-2 the design calls for a detector that could be as large as four kilometers in diameter. Such a large detector could record hundreds of events every day that are millions of times as powerful as those produced at CERN. Ice Cube Gen-2 will benefit from the knowledge gained in the construction of the current Ice Cube in an effort to reduce cost.

It’s as simple as this, the more different ways we look at the Universe the more we learn about the Universe. The Ice Cube experiment in Antarctica is a new way of looking and I think that we’re going to learn a lot. If you’d like to learn more about the Ice Cube Experiment click on the link below to be taken to the experiment’s website.

http://icecube.wisc.edu/

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.

https://www.auger.org/