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The Question
(Submitted July 18, 1997)
I want to know more about the gamma-rays that do make
it through the atmosphere. Are these the same as the 'cosmic rays' that
are constantly bombarding earth and have a bearing on mutation? Do any
gamma-rays get through, what happens to them, are they measured down
here? Would the quality of our atmosphere have any bearing on how many
rays get through? I know that rays do not cause the ozone hole, but
does the hole in the ozone let more rays in? Do our increased levels of
CO2 in the atmosphere cause changes in the 'cosmic ray' barrier?
The Answer
Very few gamma-rays make it through the atmosphere. The atmosphere is as
thick to gamma-rays as a twelve-foot thick plate of aluminum. Gamma-rays
are very very unlikely to go through that much material. However, they can
strike the material and produce 'secondary' particles which are more
penetrating, and can go through the material.
Most of the cosmic rays which reach the Earth's surface are 'secondary
cosmic rays', produced by gamma-rays or (much more commonly) 'primary
cosmic rays' hitting the top of Earth's atmosphere. These primary cosmic
rays are high energy particles (such are protons and the nuclei from iron
atoms) moving at very close to the speed of light. These primary cosmic
rays have a hard time even getting to the top of our atmosphere--the
Earth's magnetic field deflects most of them away. If Earth didn't have a
magnetic field, there would be many more primary cosmic rays hitting the
atmosphere, and many more secondary cosmic rays hitting us.
There is a page in Imagine the Universe! about observations of the light
produced when cosmic rays and gamma-rays hit the top of the atmosphere. It
is at:
http://imagine.gsfc.nasa.gov/docs/science/how_l2/cerenkov.html
The cosmic rays are not very sensitive to the quality of the air (the
chemical composition--how the nitrogen, oxygen, carbon and other elements
in the air are joined together to make ozone, smog and other chemicals).
They are more affected by the quantity of the air, because most
interactions depend only on the nuclei of the atoms, and not on entire
molecules. Three O2 molecules and two O3 (ozone) molecules look exactly
the same to a cosmic ray. A carbon atom looks only slightly different from
an oxygen or nitrogen atom, so the increased CO2 level has almost no
effect. Nothing we do is likely to significantly change the number of
cosmic rays hitting Earth.
David Palmer
for Ask a High-Energy Astronomer
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