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Questions about radiation

  1. Oct 23, 2013 #1
    1) Why can hitting a heavy metal target with high-speed electrons produce X-rays?

    2) Alpha particles have a larger mass and lower speed. Thus, they are more likely to knock electrons out of air molecules. Why? Is it about momentum? I thought particles with high speed can knock electrons out of air molecules easily.

    3) Can we completely block gamma radiation? My teacher said that we can't. A piece of lead of 25mm can only reduce the strength by 2 times. If you add one more lead, then the strength will be reduced by 4 times. This goes on and on and then eventually the strength is still not zero. Is he incorrect? I think we can come up the number of leads that are used to block gamma radiation. (Am I right?)

    4) I found on the Internet that "lead is good for shielding against X-rays, gamma rays because lead has lots of electrons the rays can interact with". Then what about those elements that have high atomic numbers? Are they really good at shielding?

    Thank you so much for your help.
    Last edited: Oct 23, 2013
  2. jcsd
  3. Oct 23, 2013 #2


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    Note that x-rays are just higher frequency (energy) photons; knocking an electron around (or off from) an atom requires energy, and when the electron "falls" back into place a photon is created which equals the energy difference between the states "filled" and "missing" for that particular electron orbital. These electronic transitions are occurring all of the time and the light we see from objects is partly due to those interactions.

    1) If the energy of the incoming electron is great enough it can eject an electron from one of the "deepest" orbitals. When this happens another electron will "fall" from an upper orbital to fill that vacancy. The emitted photon here is called a "characteristic x-ray"; if you do this with aluminum you will get a characteristic x-ray of about 1.4 keV - here is a great reference: http://xdb.lbl.gov/Section1/Table_1-2.pdf

    The greater the atomic number of the element, the more tightly bound are the inner orbital electrons, which thus requires more energy to eject, and the resulting characteristic x-ray will have higher energy. Lead (Pb) is about 75 keV.

    2) Alpha particles are charged particles (++) and so interact strongly with electrons. As they pass the air molecules an electron may be pulled off, and the alpha particle slows down a bit (loses kinetic energy). The ejected electrons will in turn eject other electrons from air molecules. Eventually the alpha particle slows enough (sheds enough energy) so that it can capture first one, then a second electron. At that point it is a neutral helium (He) atom, and though it may still be moving, has lost its strong interaction with the air. Neutral particles may still knock off an electron, but now it needs a direct collision; the interactions are much weaker.

    3) Gamma rays are photons emitted by nuclear processes (x-rays come from electronic transitions), and have energy measured in MeV, much more than x-rays (keV). Thus it takes longer to bleed off the energy. For a given density of material and distance traveled you will lose a percentage of the incoming gamma rays of a specific energy. This is usually presented as a "50% reduction" or something equivalent. So you can calculate the thickness required to reduce the gamma ray flux from a given source to a safe level (below background), or more, but there is no thickness which would guarantee a zero flux. So your teacher is correct.

    Note: when the petawatt laser in the lab across the hall from mine was upgraded so that it could accelerate electrons and protons to hundreds of MeV the experimental chamber was lined with lead bricks. All of the neighboring offices (and people) were then issued film badges so that any leakage could be monitored. In this case about 30 cm of lead blocks was sufficient to put us back at natural background levels.

    4) Elements with high atomic numbers can make good radiation shields; lead (Pb) at atomic number 82 is the usual choice because it is at the end of a radioactive decay chain. It is also cheap and easily available. Depleted uranium (U-238) is used for some special applications: http://en.wikipedia.org/wiki/Depleted_uranium#Shielding_in_industrial_radiography_cameras
  4. Oct 24, 2013 #3
    Thank you so much for your detailed explanations!!!

    But can particles with high speed knock electrons out of air molecules easily?

    "At that point it is a neutral helium (He) atom, and though it may still be moving, has lost its strong interaction with the air."
    Is it because alpha particles has a short range in air?
  5. Oct 24, 2013 #4


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    Alpha particles have a short range in air _because_ they interact strongly; this bleeds off energy (reduces the speed). And once they slow down enough they acquire an "upgrade" from He++ (ion) to He (neutral atom).

    At that point they cease to interact strongly because they are electrically neutral. But they don't collect the required electrons until the speed has been bled off ... so they are no longer fast.

    Neutral particles may knock off electrons, but it will be by indirect means - such as neutron-proton scattering in the nucleus. See http://en.wikipedia.org/wiki/Ionizing_radiation#Neutrons
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