Questions about Geant4-Garfield++ interface example

:slight_smile: ear experts:
I’m currently working on simulations related to beam-induced backgrounds for future lepton colliders, with a focus on estimating their impact on gaseous tracker detectors, specifically Time Projection Chambers (TPCs). Our full detector simulation is based on Geant4.

Preliminary findings indicate that a significant number of low-energy gamma rays (~1 MeV) will enter the TPC gas region. Among these, a fraction will interact with the TPC gas (e.g. T2K gas) through processes such as Compton scattering, photoelectric effect, and electron-positron pair production. These interactions result in secondary electrons that ionize the gas, leading to a substantial number of hits and ions, which can adversely affect the TPC’s performance.

From the Geant4 simulation data, we observe that with each bunch crossing, approximately 7.4 e+3 low-energy gamma rays traverse our TPC, depositing around 16 MeV of energy within the gas region. The attached figure illustrates a typical event showcasing this process.

background ~e^{+} annihilation \rightarrow \gamma(0.511MeV) \rightarrow Compton~ e^{-}

I am in the process of cross-checking my simulation results using the Geant4-Garfield++ interface example. I just changed the geometry (T2K gas region, |z|<2.9 m, 0.6 m<radius<1.8 m) of this example and shoot 7.5e+3 \gamma with 0.511 MeV at Z=1m. The physics list is PAI by default.
Snipaste_2024-12-19_00-30-05

The total energy deposit is only 0.14 MeV, much smaller than Geant4.

So, my question are:

  • Is this example appropriate for simulating low-energy gamma rays (~1MeV)?
  • Are there any additional considerations that I might have overlooked in my simulation code?

Thank you in advance for any insights you can provide.:slight_smile:

Hi,

Adding @hschindl in the loop.

D

Dear @Lan-sx

I think you need to dig a bit in the code to make sure you understand what happens inside, and make sure that simulates it in the way you would like it to work. From your screenshot with RDataFrame I cannot grasp anything useful. Can you tell us whether you launched for each of them a photon with heed, and what is the amount of photons that interacted?

In case you didn’t please have a thourough look at the NIM A paper describing the GEANT-Garfield interface [1]. I am not an expert on the topic, but I can see there that you have different options available for the simulation of a ~MeV photon, which one did you configure to use?

Your photons do not only interact in the gas, but also (and most of all) in all material surrounding the gas volume. If you have 7.4e+3 gammas traversing your TPC, is it than enough to simulate 7.5e+3 originating from outside? From your event display I would think that many photons are also going outside your TPC volume towards the interaction point / beampipe.

Kind regards
Piet

[1] Redirecting

Thanks for your suggestions,

  • Upon reviewing the log information, it’s confirmed that the gammas emitted by the particle gun indeed enter the TPC gas region and are subsequently simulated by fTrackHeed within the GarfieldPhysics::DoIt() function. The figure illustrates an event where a 0.511 MeV gamma interacts with the gas, resulting in the generation of electrons (note that most gammas simply pass through the gas).
  • It’s possible that the event display images are somewhat deceptive. In our Geant4 simulation, the TPC is enveloped by the Ecal on the outside, and the majority of low-energy gammas (averaging 7.4e+3 per bunch crossing will cross TPC) originate from materials within the TPC, such as LumiCal. This is why I shoot 7.5e+3 gammas from a position of Z=1.1 m, which corresponds to the location of LumiCal. However, I have not constructed any sub detectors within the TPC itself. If I account for the energy loss of gammas due to interactions with these internal materials, the gammas’ energy will decrease before they reach the TPC. Consequently, this could lead to an increase in energy deposition, as lower energy gammas (<100keV) are more likely to deposit their energy.

I plan to delve deeper into this example, but I’m also concerned about whether this example can accurately simulate a gamma particle that penetrates out of the TPC and then re-enters after undergoing Compton scattering with the gas.

I truly appreciate your insights and the heads-up on this matter. Thanks for your help.

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