Frequently Asked Questions


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.

Where Do Cosmic Rays Come From?

The answer depends largely on the energy of the particle, but the short answer is that 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 paths, and in most cases we can't determine their point of origin. Low to medium energy cosmic rays, up to energies of about 1018 eV, are thought to originate within the Milky Way galaxy. Higher energy cosmic rays may have an extragalactic origin. More information on this topic is found here.

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 chamber recorded fewer particles, up to an altitude of 2,000 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 2,000 meters, however, he recorded more particles, and the increase in particles became even more rapid as his balloon reached its maximum altitude of 5,350 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.

How Much Energy Do Cosmic Rays Have?

They have a very broad range of energies. The weakest ones have an energy of about 1,000,000,000 electron volts , which is about the minimum energy needed for a particle to get from beyond the solar system through the magnetized solar wind. (It is much easier to work with very large numbers by writing them in scientific notation. For example, we can write 1,000,000,000 as 1 x 109.) The highest energy cosmic rays ever recorded had energies of about 1 x 1020 eV. In contrast, the highest energy man-made particles, produced by very expensive machines called accelerators, have energies of about 1 x 1012 eV.

How Many Cosmic Rays Strike The Earth's Atmosphere Each Second?

The rate of cosmic rays reaching us 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, where 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.

How Do We Observe Cosmic Rays?

The technology of recording cosmic ray showers has improved over the years. 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 detect 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 detected by sensitive instruments called photomultipliers. Even so, the light is so faint that it can only be observed on moonless nights without clouds. This technique has been successfully used by the Fly's Eye experiment in Utah, and will also be used by future experiments including HiRes and the Pierre Auger Observatory.

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 detect 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 also being used by the Pierre Auger Observatory.

Are Low Energy Cosmic Rays Produced Inside Our Galaxy, The Milky Way?

We think this may be true. We know that very low energy cosmic rays are produced by the sun. We believe, however, that the vast majority of cosmic rays come from outside the solar system. 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. Exploding stars, called supernovae, may be responsible for producing many of the cosmic rays within our galaxy.

Where Are Very High Energy Cosmic Rays Produced?

Nobody knows for sure. We believe that cosmic rays with energies up to about 1017 or 1018 eV are trapped within our Milky Way galaxy by magnetic fields. Cosmic rays with higher energies would escape from the galaxy and wander through the vast distances of intergalactic space. Very high energy cosmic rays produced by other galaxies would be able to travel to us as well.

How Does The Composition Of Cosmic Rays Change With Energy?

When we ask about composition, we are asking about what cosmic rays are made of. Are they protons, electrons, or something else? 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 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 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. Observations suggest that at the highest energies, there are very few heavier nuclei, and the cosmic rays are mostly protons. This question is still a subject of considerable research.

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.

Cosmic rays with the highest energies are deflected the least by magnetic fields. Based on current understanding of magnetic fields in galaxies, these ultra-high energy rays may only be deflected by less than 3 degrees. Thus, it may be possible to correlate the arrival directions with known astronomical objects, such as nearby galaxies. Establishing this correlation would usher in the era of cosmic ray astronomy.

Is There A Maximum Energy For Cosmic Rays?

There is a predicted maximum energy 6 x 1019 eV, which was calculated by Kenneth Greisen in the United States and G.T. Zatsepin and V.A. Kuz'min in the Soviet Union in 1965. It is called the GZK cutoff after the three scientists who discovered it. Space isof 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 sufficently 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. This is expected to happen by the time a cosmic ray has traveled about 150 million light years. To summarize, we expect to see very few cosmic rays above the GZK cutoff energy, unless there are "nearby" sources.

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, and built 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).

What Does It Mean For A Cosmic Ray To Have An Energy 1020 eV?

Physicists have learned that one way of understanding nature is by classifying it by sizes. Everyone sees small things and big things, but scientists quantify these notions. Here's an example: A mouse is about 10 cm long. The Sun is about 10,000 million mice across. Now imagine if one day you ran across a mouse that was the size of the Sun. You might say: Cheese, I wonder how it got that big!

Physicists are asking the same question when it comes to cosmic rays. On Earth we know what "everyday" fundamental particles, such as protons, are like. (We quantify them by measuring their mass-energy.) But when we study radiation from space, we find a few of them with as much more energy as the everyday ones on Earth, as the size of a mouse when compared to the size of the Sun. The curiosity of the situation is that cosmic rays and particles we find on Earth are both the same type of object. How is it that nature can produce such a variety?

Why Should Society Support Cosmic Ray Research?

Indeed, why should society support basic research at all? Our colleagues at Fermilab deal with this question a lot; they have created a web page summarizing the benefits of supporting basic science: Why Support Science?

Ref: Cosmic Bullets, written by Roger Clay and Bruce Dawson.

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