Frequently Asked Questions about Cosmic Rays

What are cosmic rays?

These are very energetic charged particles that continually bombard the earth. These particles are usually protons, but can also be larger nuclei. When such a particle strikes the earth's atmosphere, it creates a shower of lower energy secondary particles, and these are observed to reach the ground. In fact, about a hundred of these secondary particles pass through our bodies every second. Exposure to cosmic rays is even greater at high altitudes.
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Where do cosmic rays come from?

We still don't know. Part of the problem is that unlike light, which travels directly from a star to us, cosmic rays are charged particles, and so they are influenced by magnetic fields which extend throughout space. The magnetic fields cause the lower energy cosmic rays to swerve along complicated trajectories, and in most cases we can't determine their point of origin.
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How and when were cosmic rays discovered?

In 1912 a scientist named Viktor Hess carried an instrument called an ionization chamber in a balloon to high altitudes. An ionization chamber is a device that records the passage of charged particles. As Hess made his ascent in the balloon, the ionization fell off a little, up to an altitude of 2000 meters. The interpretation is that some of this ionization is due to the natural radioactivity of the earth, and its influence decreases with altitude. Above 2000 meters, however, the ionization slowly increased, and the increase became even more rapid as his balloon reached its maximum altitude of 5350 meters. Hess correctly guessed that this increase was due to radiation entering the atmosphere from space. On one occasion he rode the balloon during a solar eclipse, and found no decrease in ionization. From this he concluded that the radiation was coming from somewhere other than the sun. We now know that much of this cosmic radiation originates far outside the solar system.
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How much energy do cosmic rays have?

They have a very broad range of energies. The weakest ones have an energy of about 1 billion electron volts (1 GeV), which is about the minimum energy needed for a particle to get from beyond the solar system through the magnetized solar wind. The highest energy cosmic ray ever recorded had an energy of about 1020 eV, or about one hundred billion GeV. In contrast, the highest energy man-made particles, produced by very expensive machines called accelerators, have energies of about 1000 GeV.
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How many cosmic rays strike the ground each second?

The flux of cosmic rays falls off rapidly as the cosmic ray energy increases. For 1 GeV particles, the rate is about 10,000 per square meter per second. At 1000 GeV (or 1012 eV), the rate is only 1 particle per square meter per second. The rate starts to decrease even more rapidly around 1016 eV (this is the so-called "knee" of the cosmic ray spectrum). At these energies, there are only a few particles per square meter per year. The highest energy particles, above 1019 eV, arrive only at a rate of about one particle per square kilometer per year. The "knee" is itself quite interesting, because we don't yet understand why the spectrum experiences an abrupt change in slope at that point. There is also an "ankle" in the spectrum around 1019 eV, where the rate is found to be somewhat higher than expected.
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How do we observe cosmic rays?

The technology of recording cosmic ray showers has evolved since the early days. At first, they were studied using instruments such as ionization chambers, Geiger counters, and cloud chambers. These instruments recorded a signal when an energetic charged particle passed through them. In the late 1920s, the French scientist Pierre Auger discovered the phenomenon of extensive air showers using these techniques. What he found was that very energetic cosmic rays were capable of producing showers of secondary particles which spread over a large area up to hundreds of meters. These methods only see particles that reach the ground, but do not tell us about how a cosmic ray shower develops in the atmosphere. A new technique was developed in the 1980s based on the phenomenon of atmospheric fluorescence. When a charged particle passes close to molecules in the atmosphere, it transfers some energy to the molecules, in effect "shaking up" the electrons inside. The molecules respond by emitting light as their electrons return to their normal arrangement, and this light is known as fluorescence. Nitrogen molecules, which make up most of the air, make blue fluorescent light. This light can be seen by sensitive instruments called photomultipliers. Even so, the light is so faint that it can only be seen on moonless nights without clouds. This technique has been successfully used by the HiRes Fly's Eye experiment in Utah and the Pierre Auger Observatory in Argentina. Another technique, useful for measuring cosmic rays that reach the ground, uses a phenomenon called the Cerenkov effect. In transparent materials, the speed of light is less than its value in vacuum (300,000 kilometers/second). In water, for example, light travels at 70% of its speed in vacuum. When a high energy charged particle, such as a cosmic ray, passes through the water at speeds greater than this, it creates a shock front of light that spreads out in a cone around the particle. Photomultiplier tubes placed in the water see the Cerenkov light. An array of these detectors was used in an experiment in Haverah Park, England, for more than 20 years until 1991. Tanks of water using photomultipliers to see Cerenkov light are currently in use at Pierre Auger Observatory.
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How far do cosmic rays travel before they reach the earth?

Measurements from spacecraft, which can directly detect the primary cosmic ray, rather than the secondary shower particles that we observe on the ground, show us that the majority of particles are protons (hydrogen nuclei). In low energy cosmic rays, one also founds some amount of heavier nuclei. The abundances, or proportions, of the various nuclei are about what we would expect given our knowledge of the composition of the universe. However, we find that there are many more light nuclei (particularly lithium, beryllium and boron) than one expects. A possible answer is that as a cosmic ray travels through interstellar space, it can occasionally strike an atom found in a tenuous gas cloud. The target atom, which may be made of carbon or other heavy element, breaks up on impact with the cosmic ray in a process called spallation, creating secondary particles of light nuclei. If we know the density of atoms in the interstellar medium, then it's possible to estimate how far the initial cosmic rays traveled, based on the abundances of light nuclei that we see. For the low energy cosmic rays, this turns out be around a few million light years.
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Are low energy cosmic rays produced inside the Milky Way?

We think this may be true. Results from the Compton Gamma Ray Observatory satellite tell us information about the distribution of gamma rays (very high energy photons) in the sky. We expect that gamma rays are produced when cosmic rays interact with the diffuse gas in our galaxy, the Milky Way. The satellite data show that the intensity of these gamma rays falls off with increasing distance from the galactic center. This would happen if lower energy cosmic rays were produced in the central bulge of the galaxy.
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How does the composition of cosmic rays change with energy?

As discussed above, low energy cosmic rays consist of mainly protons and light nuclei. Measurements taken in a high altitude balloon, the Japanese-American Cooperative Emulsion Experiment (JACEE), show that as the cosmic ray energy increases, the proportion of heavier nuclei also increases. This suggests that as the energy reaches the "knee" of the spectrum, around 1015 eV, heavy nuclei become the dominant component. It is very difficult, however, for a satellite or balloon experiment like JACEE to study particles at these high energies. This is because the flux of particles at these energies is very low, and the detector area that can be carried aboard a satellite is so small. A better alternative is to use a ground based detector to sample the energies from extensive air showers and infer the particle energies indirectly. The situation is complicated because there is a lot of fluctuation in the way a shower develops, but in general, a heavy nucleus will start to shower higher up in the atmosphere than a light nucleus.
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What can we learn from the arrival direction of cosmic rays?

Because cosmic rays are deflected by magnetic fields, we expect to see them arriving from all directions. We do, and in fact the deviation from directional uniformity, or anisotropy, is less than 1%. Because the earth, along with the solar system, is moving through the galaxy at 200 kilometers/second, we expect a small anisotropy due to this motion. This is called the Compton-Getting effect: we should see slightly more cosmic rays in the direction we're moving. As yet, we have not yet observed this effect even though some experiments should be sensitive enough.
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Is there a maximum energy for cosmic rays?

There is a predicted cutoff energy which was calculated by Kenneth Greisen in the United States and G.T. Zatsepin and V.A. Kuzmin in the Soviet Union in 1965. It is called the GZK cutoff after the three scientists who discovered it. Space is filled with microwave radiation, called the cosmic microwave background, which is leftover radiation from the Big Bang. While a microwave photon doesn't have much energy, a sufficiently energetic cosmic ray would see the photon's wavelength to be compressed due to the Doppler effect. From the cosmic ray's perspective, the microwave photon would appear to be a gamma ray. Collisions between protons (cosmic rays) and gamma rays have been studied in accelerators, and these collisions often result in the production of particles called pions, which cause the proton to lose energy. A collision in space between a cosmic ray proton and a microwave photon would result in the same production of pions. With each collision, the proton would lose roughly 20% of its energy. This only happens for cosmic rays that have at least 6 x 1019 eV of energy, and this is the predicted GZK cutoff. So if cosmic rays were given an initial energy greater than that, they would lose energy in repeated collisions with the cosmic microwave background until their energy fell below this cutoff. However, if the source of the cosmic ray is close enough, then it will not have made very many collisions with microwave photons, and its energy could be greater than the GZK cutoff. This distance is about 150 million light years.
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What is the highest energy ever seen in a cosmic ray?

There are several events worth mentioning. In the 1960's, a ground array of 19 detectors spread over 8 square kilometers was built at Volcano Ranch, New Mexico, by a team led by John Linsley. In 1963, his team reported an observation of a cosmic ray with an energy greater than 1020 eV. Since then, several large detector arrays have been built to search for very high energy csomic rays. One such detector, called the Fly's Eye in the Utah desert, observed a cosmic ray shower in 1991 that at it's maximum contained 200 billion particles in the shower. The energy of the primary particle was 3 x 1020 eV, the highest energy cosmic ray ever observed. While the composition of the primary particle isn't known with certain, the best guess is that it was a moderate mass nucleus (something like oxygen).
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