Positron-Electron Annihilation Peak Disappearance

Hello all,

I have been trying to debug a missing positron-electron annihilation peak in my simulations for some time now. My simulation consists of a germanium cylinder in a world volume of Galactic material, and a sodium-22 point source decaying in the center of the world volume.

To debug this, I decided to modify example B3a, which studies positron emission. In this simulation, the positron’s annihilation gammas are detected, and the expected peak at 511 keV appears in the spectra. This simulation uses a Lutetium-based material for the detectors, and it is placed in a world volume of air. The positron emitters decay inside of a patient made of what I think is brain matter.

I made slight adjustments to this simulation to see if I could reconstruct my simulation’s geometry in this example. I changed one of the provided macro files to decay sodium-22, reduced the number of detectors to one singular germanium detector, and changed the other volume’s materials to Galactic.

What I had found was that when I only had the one germanium detector, and the world and patient volumes’ materials were changed to Galactic, the 511 keV peak disappeared. I then decided to place a “dead layer” on the germanium detector, and once I did this, the missing peak reappeared.

I believe that once the positron annihilates with an electron, the resulting gammas do not interact with the volume it annihilated in. I have seen this in previous debugging simulations where I had varied the detector size, and the gammas would not deposit any energy on those detectors. I also saw that this height peaked at a dead layer thickness of 500 micrometers, and then decreased with length. I guess that the decrease could be the result of the gammas escaping before the enter the old detector.

Also, when I added the dead layer in my simulation, I noticed the 511 keV peak height vary with the thickness of the layer. I expected this, as changing the thickness should change the number of positrons that annihilate in the dead layer instead of the original detector.

My edited exampleB3a simulation can be found on my GitHub at this link:

Within the directory called “My_rootFiles,” I have a README file that details the changes I made to the B3a example and the generated ROOT files from the simulation after each change.

Would anyone know why the gammas from the positrons do not interact with the volume they originated from? I still lack the Geant4 knowledge to determine why.

Thank you!

Geant4 Version: 11.1.0
Operating System: Ubuntu 20.04