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High Energy


The muons that reach our detector originally have energies of greater that 3 GeV. To illustrate how large an energy of one GeV is, we give several examples using the electron volt (eV) energy unit. (The G in GeV stands for giga = billion = 1,000,000,000 = 109 An eV is a unit of energy convenient when discussing atoms. Atomic energy processes, which underlie chemistry, typically involve energies of 1 eV, and visible light photons given off by atoms carries approximately an eV of energy. Formally, and something we can immediately forget, an eV is the amount of energy gained by an electron when it is accelerated through a one volt electrical potential.

A penny with mass = 2.5 gm gains 1.5x1016 eV (otherwise known as 2.5x10-3 Joules if you have taken a physics course and are familiar with other energy units) when you drop it vertically 10cm. That energy is so large because the penny contains so many atoms (2.4x1022 atoms). We are more interested in the energy carried by each individual particle. Each atom in the penny would have gained only 6.4x10-7 eV due to its fall to Earth.

Carrying less than one eV of energy per particle are radio waves with approximately 10-8 eV and microwaves with 10-4 eV. Other particles carry more than an eV of energy. X-rays have 1000 eV (keV) and gamma rays that come from nuclear physics have 1,000,000 eV (MeV). Our muons have at least 3,000,000,000 eV (3GeV) of energy. They are not even that rare. You have 10 muons created by cosmic rays from outer space, going straight through your body every second.

energy scale

Note: 1 eV = 1.6x10-19 Joules

No natural process in our solar system can create energies above a GeV. We know that the cosmic rays come from outside of our solar system, because not even the hottest source in the solar system, the core of our Sun, which is powered by nuclear physics, can make particles with high energies above several MeV (sources of cosmic rays). It is postulated that one source of cosmic rays come is exploding stars (supernovae). So, the cosmic rays that we observe on Earth are really tiny pieces of distant stars that have exploded long ago. Humans can produce higher energies. The highest energy particles produced on Earth, have 1000 times the energy of our muons (1012 eV = TeV), and are produced at the Tevatron accelerator at the Fermilab National Accelerator in Batavia, Illinois.

Cosmic ray flux versus
energy

Cosmic rays with even higher energies exist, but the rate of cosmic rays decreases quickly with increasing energy. The highest energies ever observed are approximately 1020 eV. Although these very high energy particles were seen by astronomers in an early version of the Fly's Eye experiment, their source is not yet understood. They are the focus of a new laboratory (Pierre Auger Observatory) in Argentina, whose director emeritus talked to us about our cosmic ray experiments at our first QuarkNet summer workshop.