Co60, Gamma Spec, Different Counts in Peaks

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Discussion Overview

The discussion revolves around the observed differences in peak counts for gamma emissions from Co60 in a spectrum, specifically focusing on the 1.1 MeV and 1.3 MeV peaks. Participants explore potential reasons for the discrepancies in counts, considering factors related to detection efficiency, background noise, and the nature of photon interactions.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants note that the two peaks at 1.1 MeV and 1.3 MeV should theoretically appear with similar frequencies, yet the counts differ significantly, with the 1.1 MeV peak being counted about 10% more often.
  • One participant suggests that the type of detector used, a NaI(Tl) scintillation detector, may have varying detection efficiency at different energy levels, potentially explaining the count discrepancies.
  • Another participant raises the importance of background noise and fitting the spectrum to accurately assess peak heights and total counts, indicating that resolution differences could affect the observed counts.
  • Some argue that the decreasing slope of efficiency with energy and the QED cross section's behavior may contribute to the observed differences in counts.
  • There is a suggestion that the photoelectric effect is not the only interaction occurring, with some proposing that successive Compton scattering could also play a role in the counts observed.
  • A later reply emphasizes that the efficiency curve of the detector would provide valuable insights into the observed discrepancies.

Areas of Agreement / Disagreement

Participants express various hypotheses regarding the reasons for the differing counts, indicating that there is no consensus on a single explanation. Multiple competing views remain regarding the contributions of detector efficiency, background noise, and photon interaction types.

Contextual Notes

Limitations include the lack of specific efficiency curves for the detector in question and the potential influence of background noise on the observed counts. The discussion also highlights the need for further analysis to clarify the contributions of different factors.

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To explain my uncertainty more.

For each Gamma with 1.1MeV (appearing>99%) theire will be a Gamma with 1.3MeV(appearing>99%) but in the Spectrum the 1.1MeV Gamma ist counted about 10% often.

How could that be explained?
 
What kind of detector produced the spectrum? The detector might be less efficient at the higher energy.
 
Thank You

A NaI(Tl) Scintillation Detector.

They have a Detection Efficiency and its depending on the Energy and more.
 
Where does that number 10% come from?
The background is clearly different for the two energies so this has to be subtracted. Did you make a fit to the spectrum? The resolution can be different, so peak height and total number of events in the peak don't have to be proportional.

I would expect the lower efficiency as main reason. Maybe pair production outside the detector can be an issue as well.
 
I think that it is due to the decreasing slope of efficiency vs energy as was stated.
Also, the QED cross section drops with energy so if efficiency was flat I would expect a (small) decrease in counts as energy increases.
 
jtbell said:
What kind of detector produced the spectrum? The detector might be less efficient at the higher energy.

RotBlau said:
A NaI(Tl) Scintillation Detector.

They have a Detection Efficiency and its depending on the Energy and more.

The two peaks at the far right end of the spectrum are due to the photoelectric effect. The interaction cross-section σ for the photoelectric effect decreases rapidly with increasing photon energy. See here for example:

http://www.upscale.utoronto.ca/GeneralInterest/DBailey/SubAtomic/Lectures/LectF05/Lect05.htm

Scroll down near the bottom of the page to the section "Photon interactions" and see the graph of σpe.
 
It is not necessarily *only* photoelectric effect that takes place there.
It could be successive compton scattering with all the energy absorbed within the detector, taken that the detector is thick enough.
A 10% effect is an order of magnitude difference in counts and thus in the cross section. This argument is not supported by the plot you pointed us to if you look at the range 1.17 - 1.3 MeV!
Thus one may conclude that the effect is mainly detector efficiency and the decreasing cross section is a smaller additional effect.

If RotBlau had the efficiency curve of the detector in question, it would be most enlightening.
Take this for example from the web, although I don't know how relevant it might be
http://radware.phy.ornl.gov/esclev/esclev_fig2.gif
 

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