The Compton edge, don't let it fool you!

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The Compton edge, don't let it fool you!

Post by Sesselmann » 09 Nov 2019, 09:47

While many of you will have a good understanding of this subject, some might find it rather confusing, so I thought it might be a good idea top open a discussion on the Compton edge.

As always Wikipedia has a nice entry on the subject here: , but in any case I want to post the simple version for those who find the maths a bit scary.

Basically Compton scattering occurs when a gamma ray collides with an electron inside the scintillation crystal, some of the gamma ray energy is absorbed and released as a lower energy photon, which of course is the intended purpose, and when all the energy of the gamma ray is completely scattered inside the crystal, the light output is amplified by a PMT which outputs an electric impulse proportional to the gamma ray.

Now if we had an infinitely large crystal, that would always be the case, but for practical reasons we don't, and our crystals are typically cylinders of diameter 1" to 3" with one end surface coupled to a photo detector. What happens in reality is some of the photons generated near the surface of the crystal escape, and consequently the electrical impulse from the gamma ray interaction misses out on a bit of energy.

The energy of the escaped photon depends on the angle at which the gamma ray interacted with the electron, which is why we see a Compton continuum (that flat area to the left on a typical Cs137 spectrum), however there is a maximum scattering angle and it is referred to as the Compton Edge

The Compton edge is defined on Wikipedia as;

\[ E_{compton} = E_γ (1-\frac{1}{1+ \frac{2E}{m_e C^2} }) \]

I think it looks simpler like this.

\[ E_{compton} = E_γ (1-\frac{1}{1+ \frac{2E_γ}{E_e} }) \]

Here is a classic example from one of my 3" NaI(Tl) detectors, believe it or not, but this is a background spectrum in my brick house, no source anywhere near my detector. The tall peak with ROI in yellow is at 2614 keV and I assume it comes from from Thorium in the bricks. The smaller peak at around 2378, immediately to the left of the 2614 peak I suspect is the Compton edge.

Lets see if it makes sense...

\[ E_{compton} = 2614 keV (1-\frac{1}{1+ \frac{2*2614 keV}{511keV} }) = 2381.25 keV\]

and yes it matches the observed peak perfectly, so my message here is to be aware of the Compton edge and understand how to identify it, otherwise we might suspect it to be a gamma peak and go looking in the wrong place.

3" NaI(Tl) Background spectrum (Aussie brick house)
bg-nai-3inch.png (31.46 KiB) Viewed 2977 times

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Re: The Compton edge, don't let it fool you!

Post by Cosmic » 09 Nov 2019, 21:34

Hi Steven,

thank you for this interesting post!

I'd like to add that with Interspec (viewtopic.php?f=7&t=504) you don't need to do the calculation by hand, you can simply show "Feature Markers" like the compton edge and move it around through the spectrum so you can visually check whether a "peak" is real or corresponds to a "feature".
feature_markers.PNG (66.23 KiB) Viewed 2963 times
Part of "my" background

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Re: The Compton edge, don't let it fool you!

Post by Go-Figure » 24 Nov 2019, 22:32

Thank you for this Steven,
Yeah, when we try to identify a peak which doesn't quite fit with the overal picture (not belonging to any of the detected decay chains) we should always keep in mind that it could be the Compton Edge, otherwise it's very easy to be fooled by it.
I think this will help the beginners and work as a useful reminder for the more experienced ones.

I guess your spectrum is the "energy per bin" view.


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Re: The Compton edge, don't let it fool you!

Post by Sparky » 01 Jan 2020, 01:34

I found an article/demonstration by David Prutchi also very useful for understanding Compton interactions. He goes into more detail about the lower end of the Compton plateau. The Compton Peak (or "Compton minimum" ) is caused by 180 degree back-scattered photons in or near the source. It is sort of a compliment to the Compton edge, you see the minimum scattered photon's energy instead of the maximum recoil electron's energy. The sum of the two values will equal the energy of the original, incident photon. You can see this feature in Steven's spectrum around 230 keV. The Calculator linked below shows the relationship quite well.

Prutchi's demonstration is very simple to perform. You just intentionally enhance the backscatter peak by placing a target (of an appropriate Z value) behind the source. I used the copper disk from the bottom of my shield. My version of his demonstration using a TG30 (Cs-137) spark tube is shown below. The calculated edge and backscatter peak energies are marked. ... rutchi.pdf ... cattering/
Copper disk backscatter.JPG
Michael Loughlin

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Re: The Compton edge, don't let it fool you!

Post by iRad » 01 Jan 2020, 03:40

Nice post Mike, thanks for the useful links.
Hmmm... "His Girl Friday"; a nice classic comedy I haven't seen in a very long time...
Cheers, Tom Hall / IRAD INC / Stuart, FL USA
Please check out my eBay Store:

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Re: The Compton edge, don't let it fool you!

Post by Sparky » 02 Jan 2020, 07:46

Hi Tom,

You're a pretty good detective to figure out that DVD. The case was just the right thickness to center the probe and the spark-tube on the copper disk.
That movie is still worth a watch.
Michael Loughlin

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