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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
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.
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 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, the ionization slowly increased, and the increase 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
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.
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
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.
How do we observe cosmic
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 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 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 will
also be used in Auger.
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
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.
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
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.
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.
Kuz'min 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 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. 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.
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).
How will water in the Auger
detector tanks be kept from freezing?
Winter temperatures at the Colorado site will require
that the water tanks be insulated. Conventional foam insulation (about
20 centimeters thick) is good enough to protect the water from freezing
temperatures. This insulation is what gives the detectors their
Why should society support
cosmic ray research and the Pierre Auger Project?
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.
(The source for many of these questions and answers is Cosmic
Bullets, written by Roger Clay and Bruce Dawson.)