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Cosmic Rays

Cosmic-ray Composition

Because cosmic rays don't point back to their sources, we must use indirect methods to determine their sources and the way they have traveled (or "propagated") through the Galaxy. The chemical composition of the cosmic rays provide a surprisingly rich source of such knowledge. The chemical composition of the solar system has been determined from a combination of spectroscopy on the Sun, studies of the solar wind, and by chemical analysis of meteorites, which are presumed to have a purer sample of the early solar system than terrestrial rocks. The composition of cosmic rays is important because cosmic rays are a direct sample of matter from outside the solar system and contain elements that are much too rare to be seen in spectroscopic lines from other stars. Cosmic rays provide important information on the chemical evolution of the universe.

If we look at the elemental composition measured for cosmic rays and compare it to our best understanding of the composition of the solar system, we quickly see some large differences.

Solar and GCR Composition

Solar and GCR Composition

Cosmic-ray Secondaries

In the figure above, we take the abundance of silicon as a "standard candle" or reference point, and compare the relative abundances (relative to silicon) of the elements in the solar system and in galactic cosmic rays. Silicon is used as the reference because it is a common intermediate-weight element that is easy to measure. We see that there is less hydrogen and helium in the cosmic rays than in the solar system, we think because hydrogen and helium are harder to accelerate to high energies than heavier elements. We also see that some light elements (lithium, beryllium, and boron) that are rare in the solar system (and in the rest of the universe) are quite common in cosmic rays. We also see more cosmic ray elements between silicon and iron than in the solar system.

The accepted reason for all the observed cosmic ray lithium, beryllium, and boron is that these are pieces of heavier cosmic ray elements, especially carbon and oxygen, that have had high speed collisions with the very tenuous gas in interstellar space. Likewise, the elements between silicon and iron have been supplemented by fragments of heavy cosmic rays such as iron and nickel. These fragments are known as secondary cosmic rays or just secondaries.

From the number of secondaries observed at Earth, and with knowledge of the probability of these collisions (which can be measured in particle accelerators here on Earth), it is possible to calculate the amount of matter that the cosmic rays have traveled through. More matter would breakup more primary cosmic rays. If the cosmic rays have stayed in the Galaxy, the amount of matter that they have passed through divided by the average density of interstellar space (about one atom per cubic centimeter) gives the "age" of cosmic rays. With this method, we determine an average cosmic ray age of about two million years. But this turns out to be a bit wrong.

Radioactive Clocks

Another way to obtain the age of cosmic rays is to use radioactive isotopes as clocks in a way very similar to the way carbon-14 is used by archaeologists. There are several isotopes, beryllium-10, aluminum-26, chlorine-36, etc., which are almost entirely secondaries. After they are created, they begin to decay, and the fraction that reach us at Earth gives the age of the cosmic rays. With this method, the average age of cosmic rays comes out to approximately ten million years. The reason the two million year age from the previous paragraph is wrong is that cosmic rays actually don't just hang out in the one-atom-per-cubic-centimeter Galaxy proper (also known as the "galactic disk"). Cosmic rays spend a large fraction of their time in the low density galactic halo, bouncing back and forth through the galactic disk many times.
EGRET Gamma Ray All Sky Survey

EGRET Gamma Ray All Sky Survey

As the cosmic rays interact with interstellar gas, they can produce gamma rays, which can be seen in the EGRET gamma ray image of the Milky Way galaxy shown above.

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Nicholas White (Director), within the Exploration of the Universe Division (EUD) at NASA's Goddard Space Flight Center.

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