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Name: Robert
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Question:
What happens to the energy of a gamma ray or X-ray when it is absorbed by the atmosphere?



Replies:
Robert,

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 less dangerous.

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
Physics Instructor
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.

Jim Swenson


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.

Vince Calder



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