High Energy Atmospheric Absorption
What happens to the energy of a gamma ray or X-ray
when it is absorbed by the atmosphere?
The energy of any absorbed electromagnetic wave (gamma, X-ray, ultraviolet,
even visible light) is transferred to the particles that the waves hit. The
waves are made of little pieces called photons. Each photon can either hit
something or pass through. The particles can be whole molecules, individual
atoms, or sometimes just electrons.
This energy can be held in these particles, causing the atmosphere to warm>up.
The energy can be released in another direction, never reaching th
Earth. The energy can be released as several lower energy photons, making>it
Different particles are better at absorbing different energies. A great
deal of visible light gets through because the atmosphere is not really good
at absorbing visible light. The sky looks blue because the sky can absorb
blue light, but cannot hold it. The result is scattering. Blue light from
the sun can go off to one side, scatter in the atmosphere, and bounce back
to your eyes. Red light travels in a straight line, so you only see suc
colors at or near the sun. In certain situations, the atmosphere scatters>red
light. At these times the sky looks red.
Dr. Ken Mellendorf
Illinois Central College
In short, a trail of gas-ionization, which eventually subsides into heat.
"Eventually" here means from microseconds to minutes.
Incident Gamma rays and X-rays occasionally kick electrons
along their path. (Compton effect)
Then they continue on, minus the amount of energy expended.
Thus a gamma ray will gradually be demoted to an X-ray, then a UV photon,
and then it can finally be absorbed whole in one encounter.
Some of those kicked electrons are from inner shells of atoms
(this costs >100eV, very roughly) ; they leave behind an empty orbital
underneath a number of the atom's other electrons.
One of those overlying electrons falls into the vacancy,
releasing the energy difference as:
1) a lower-energy X-ray (X-ray fluorescence),
2) a freed higher-orbital electron, moving fast in any random direction,
with energy equivalent to such an X-ray (Auger electrons).
Eventually all these primary and secondary radiations put their energy
into knocking loose (ionizing) outer electrons (100eV).
When these vacancies are eventually re-filled,
the energy is released as heat, or as UV photons which are soon
absorbed by gas molecules and become heat.
A small percentage (order of 0.01%) of the energy is more directly
converted to momentum of nuclei, and that is heat. After all,
these encounters slow down and/or deflect the high-energy photon,
which involves a change in momentum.
Equal and opposite reaction, you know.
Ejected electrons impart noticeable recoil to the source atom, too.
If the atoms are 10,000 times heavier than the ejected electrons,
then nuclear momentum gets 1/10,000'th of the energy of the ejection.
An extremely small fraction of gamma rays succeed in causing nuclear
excitations or reactions. A nucleus is a very tiny target,
both in cross-section area and in frequency-bandwidth accepted.
X-rays cannot excite nuclei; they do not have enough energy
to cause the minimum excitation a nucleus knows how to do.
That pretty much defines the informal boundary
between X-rays and gamma rays.
Both being highly ionizing radiation, they initiate a cascade of
ions, photons and electronically excited molecules/atoms as they
enter the upper atmosphere. "Cosmic rays", which are not really
"rays" but very high speed nuclei also initiate a cascade of
particles and radiation.
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Update: June 2012