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General Nuclear questions

  1. Mar 13, 2006 #1
    Why does gamma rays wavelength affect how it interacts with matter and how does it affect it? (gamma goes through most materials but lower frequency energy like light penetrates only alittle and forms lots of heat right away)

    Can energy such as xrays or gamma rays induce nuclear decay?

    Why do like charges repel and opposites charges attract?
     
  2. jcsd
  3. Mar 13, 2006 #2

    Astronuc

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    Gamma rays are photons that originate in the nucleus or in a subatomic or particle interaction, e.g. electron-positron annihilation. X-rays originate from the electrons, K and L electrons, of the atom. The energy of a photon is proportional to the frequency, E = h [itex]\nu[/itex], where h = Planck's constant and [itex]\nu[/itex] is the frequency. The frequency is just c/[itex]\lambda[/itex], where [itex]\lambda[/itex] is the wavelength.

    Photons like X-rays and gamma rays interact primarily with atomic electrons in the photoelectric effect and Compton scattering, but if energy high enough, pair production may occur if a gamma ray interacts with a nucleus. Pair production has a threshold of 1.022 MeV which is twice the rest mass of an electron/positron. X-rays are limited by the differences in energy levels of the atomic electrons.

    Photoneutron reactions are possible for certain radionuclides and generally require gamma energies slightly above the threshold for pair-production.

    Other photo disintegration modes would require much greater gamma energies. For example - http://www.gwu.edu/~ndl/research6kk.htm

    Some abstracts on photodisintegration (or use Google) - http://www.citebase.org/cgi-bin/citations?id=oai:arXiv.org:astro-ph/0507121

    That is the way Nature is, otherwise we would not be around to wonder why like charges repel and opposites attract. :biggrin:
     
  4. Mar 14, 2006 #3
    so the only reason gamma and xrays penetrate further is because of their higher energies?

    can gamma rays be reflected? Why about the material of a mirror makes light reflect?
     
  5. Mar 14, 2006 #4

    ZapperZ

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    Er.. be very careful here. Just because a range of light has a higher energy, does not automatically mean that it will penetrate further than those with lower energies. For example, UV light has higher energy than visible light. Yet, UV light cannot even pass through a simple ordinary glass that is transparent to our visible light.

    The reason why certain energy of photons can penetrate further than others has a lot to do with the properties of the "absorber" also. So this aspect of it should not be ignored.

    Zz.
     
  6. Mar 15, 2006 #5
    what are these properties of the absorber? and what causes these properties?
     
  7. Mar 15, 2006 #6

    Astronuc

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    The primary property of interest is electron density, which is a combination of atomic density and the number of electrons (Z) of the atoms comprising the absorber.

    The atomic density, N, is given by [itex]\rho[/itex]*NA/A, where [itex]\rho[/itex] is the mass density (g/cm3 or kg/m3), NA is Avogadro's number (atoms/mole or atoms/kg-mole) and A is the atomic mass (g/g-mole or kg/kg-mole). Make sure units are consistent.

    ZapperZ makes some good points, particularly the one about the properties of the absorber.

    The other consideration is that the more energy a photon has, the more energy there is to distribute in the absorber. X-rays are relatively low energy 10 eV - ~150 keV, whereas gamma rays can be up to several MeV. A high energy photon will scatter, usually with a small angle (see Compton effect - http://hyperphysics.phy-astr.gsu.edu/Hbase/quantum/comptint.html#c1) with a subsequent photon of lower energy, which scatters to another photon of lower energy and so on until the final photon is completely absorbed.

    The other property of the absorber is the probability of interaction with a photon of given energy, and this is related to the electron density, which is related to the atomic density. The probability of interaction is described in terms of a cross-section.
     
    Last edited: Mar 15, 2006
  8. Mar 16, 2006 #7

    ZapperZ

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    The phonon modes of the absorber, the atomic composition of the absorber, etc. You may want to read the FAQ in the General Physics forum on the apparent slowdown of light speed in a dispersive medium.

    Zz.
     
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