Co60, Gamma Spec, Different Counts in Peaks

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In summary: This plot of detector efficiency vs photon energy has a slope of approximately 1.3 MeV/keV. Assuming that the plot is linear and the slope is constant, the 10% effect would correspond to a count decrease of around 10-15% for photons energies over 1.3 MeV.In summary, the Spectrum of Co60 shows two peaks at 1.1 and 1.3 MeV. The higher energy peak is due to the photoelectric effect, and the lower energy peak is due to successive compton scattering.
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  • #2
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?
 
  • #3
What kind of detector produced the spectrum? The detector might be less efficient at the higher energy.
 
  • #4
Thank You

A NaI(Tl) Scintillation Detector.

They have a Detection Efficiency and its depending on the Energy and more.
 
  • #5
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.
 
  • #6
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.
 
  • #7
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.
 
  • #8
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
 

What is Co60?

Co60, or cobalt-60, is a radioactive isotope of cobalt that emits gamma rays. It is commonly used in medical and industrial applications, such as cancer treatment and sterilization.

What is Gamma Spec?

Gamma Spec, short for gamma spectroscopy, is a technique used to identify and measure the amount of gamma radiation emitted by a sample. It involves using a gamma spectrometer to detect and analyze the energy levels of gamma rays.

What are counts in peaks?

Counts in peaks refer to the number of gamma rays detected at a specific energy level, or peak, in a gamma spectrum. This information can be used to determine the presence and quantity of certain radioactive isotopes in a sample.

Why do different peaks have different counts?

The number of counts in a peak is influenced by several factors, including the energy and intensity of the gamma rays emitted by the isotope, the detector's efficiency, and the sample's radiation concentration. Therefore, different peaks may have different counts due to variations in these factors.

How is Co60 detected and measured using gamma spectroscopy?

To detect and measure Co60 using gamma spectroscopy, a sample is placed in a gamma spectrometer and bombarded with gamma rays. The spectrometer then identifies and measures the energy levels of the gamma rays emitted by the sample, allowing for the identification and quantification of Co60. The number of counts in the Co60 peak can also be used to determine the amount of Co60 present in the sample.

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