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

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

Post by Steven 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 74 times
Steven Sesselmann | Sydney | Australia | | | |

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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 60 times
Part of "my" background

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