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Gamma spectrum Na-22: Peak origins

  1. Feb 17, 2010 #1
    Hey,

    For my first post here, I had a question about the gamma spectrum of Na-22.

    I measured the gamma spectrum of Na-22 with a Ge-spectrometer. Using Genie-2000 software I saw two peaks in the spectrum, 1274 keV (peaksurface 3981±1,74%) and 511 keV (peaksurface 25494±0,68%). (Horizontal axis is divided into channels, each representing an energy band. The vertical axis represents the number of counts)

    As an assignment, I have to explain the origins of these peaks and why there is so much difference in peaksurface.

    The peak at 511 keV is electron-positron annihilation, I guess. And if I interpret this source* correctly, the peak at 1274 keV is the result of both electron capture (which the assignment wants me to ignore) and beta+ decay?

    Another question I had was why the peak at 511 keV is so much higher (a lot more surface)? It is true that electron-positron annihilation yields two photons, but they travel in opposite directions so, in generl, the chance the Ge-spectrometer measures both is quite small. (The sample lies on top of the spectrometer)

    Thanks in advance for the help.

    *=http://community.middlebury.edu/~PHManual/gamma.html
     
  2. jcsd
  3. Feb 17, 2010 #2
    Look at this plot of attenuation coefficients for photons in germanium:

    http://physics.nist.gov/PhysRefData/XrayMassCoef/ElemTab/z32.html

    Make sure you are measuring peak areas and not just the counts in the highest channel. In particular look at these attenuation coefficients in germanium.

    0.50 MeV 8.212E-02 cm2 per gram
    1.25 MeV 5.101E-02 cm2 per gram

    Bob S
     
  4. Feb 18, 2010 #3
    I hadn't considered this effect before, thanks for bringing it to my attention. I think I can finish my assignment with this.

    Thanks again!
     
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