I would like to know how gamma rays from cosmic rays are blocked by the earth's atmosphere. I know that the sun's gamma radiation don't usually reach the earth so I wouldn't be worrying much about it. I have did some research and I only see that ozone will block uv radiation but not gamma radiation. So I am wondering how is gamma radiation being blocked by the atmosphere? What molecules in the atmosphere specifically absorb gamma radiation? Thank you!
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6$\begingroup$ Possible duplicate of How Earth protected from the gamma rays generated by Sun? $\endgroup$– engineerJan 29, 2018 at 12:43
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$\begingroup$ Why say that you know that the Sun's gamma radiation doesn't usually reach Earth, then award Best Answer to an answer which gives that explanation? $\endgroup$– sammy gerbilJan 30, 2018 at 3:35
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$\begingroup$ It seems to me te proposed dupe is more about the sun's emissions of gamma rays while this question is about what it is in the atmosphere that stops gamma rays from hitting us. $\endgroup$– Kyle KanosJan 30, 2018 at 11:01
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2 Answers
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Cosmic gamma rays are largely absorbed in the atmosphere by three ionizing interactions with atmospheric molecules: photoelectric effect, Compton scattering and pair production. Therefore, in order to observe cosmic gamma rays for astronomical purposes, gamma ray observatories (like the Compton Gamma Ray Observatory) have to be placed in space.
answered Jan 29, 2018 at 18:00
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$\begingroup$ Good grief, you couldn't find list of the dozen $\gamma$-ray or cosmic ray telescopes made in the last decade? You had to go early-90s with CGRO? $\endgroup$ Jan 29, 2018 at 22:37
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$\begingroup$ @KyleKanos - That's probably an effect of age, that you remember the old stuff best. It stopped working in 2000 I think. It would be interesting, of course, if you could name a couple still operational gamma observatories. $\endgroup$ Jan 29, 2018 at 22:42
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$\begingroup$ possibly, but at this point it's been dead twice as long as it was even alive (17 vs 9 years) that it's hard to fathom it being referenced these days (especially with the massive success of Integral and Fermi). $\endgroup$ Jan 29, 2018 at 22:45
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$\begingroup$ @Kyle Kanos - I was not aware that there are so many in orbit. I think that it is very good that they are many continuing this important work. $\endgroup$ Jan 29, 2018 at 22:49
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1$\begingroup$ This should be the accepted answer because it actually answers the question. To make this answer even better, you could write that gamma rays are absorbed in the upper reaches of the atmosphere, the thermosphere. This absorption is why the thermosphere is so named. Temperature in the thermosphere increases with increasing altitude due to this absorption. $\endgroup$ Jan 29, 2018 at 23:35
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Gammas lose energy predominantly via Compton scattering, $\gamma +e\to \gamma +e$, during which they impart much of their energy to the electron and change direction. Low-energy gammas, which you might better call x-rays, tend to induce bound-free transitions, $\gamma +atom\to e+ion$.
