Simulating silicon pixels detector in water phantom

Dear experts,

Following up on my previous inquiry regarding simulating an HVCMOS silicon detector in Allpix² here, I’ve successfully completed the initial stage of my simulation and get reasonable results agree with measurements.

As the next step in my research, I am considering placing the detector within a water phantom to simulate the interaction of particles with the detector under different conditions. Before proceeding, I wanted to ask whether it’s feasible to incorporate a water phantom into the simulation setup using Allpix².

If anyone has experience or insight into incorporating a water phantom into Allpix² simulations or if there are alternative approaches to achieve similar goals, I would greatly appreciate your guidance and advice.

Thank you for your assistance.

Best regards,

Hi @Fajer ,

Nice to hear you have it running with reasonable results!

The easiest way to perform a simulation with a detector in water would be to set world_material = "WATER" in [GeometryBuilderGeant4].

Another way would be to change in the model configuration, and add support material with material = WATER around the sensor, if you want more control of the shape of the water.
An example of this would be to add

thickness = 20mm
size = 30mm 20mm
location = "sensor"
material = "WATER"

to the model file, which will add 20 mm of water in front of the sensor. More info about placing passive material in this way can be found at Detector Models | Allpix Squared

Kind regards,


Many thanks @hwennlof for your help.
Sorry for the late response, I was occupied in the lab, confirming a couple of parameters for the simulation study. Unfortunately, this caused me to respond later than intended.
Regarding your suggestion, I attempted to implement it. However, I encountered an unexpected issue. It appears that the beam is being refracted upon hitting the phantom, resulting in the majority of events failing to reach the detector. This behavior has left me puzzled, and I am unsure how to proceed.

I have included the main files for your reference. If you have any further advice or insights on how to address this issue, please let me know. Your assistance would be greatly appreciated.

Once again, thank you for your support. Your expertise is invaluable, and I am thankful for your help.

Best regards,

hv.conf (734 Bytes)
main_file.conf (1.6 KB)
pos.conf (243 Bytes)

Hi @Fajer ,

This looks interesting. I don’t have your latest geometry file, but running with one I do have, it seems that your carbon ions just have too low an energy to penetrate very far in the water. Do you know how far you expect carbon ions of this energy (2.4 GeV) to penetrate?

Here’s an event display of a single event with your current settings
I added a detector at -325 mm here, just to get the display to show nicely in an easy way (i.e. behind the beam source at -300 mm).

If I up the energy to 240 GeV instead (just to have something extreme), I instead get this:

Now, this is very messy due to interactions with the water, but the main point is that your ion beam now reaches your sensor at -125 mm.
So, I think the issue is just that your ions get absorbed too quickly. Remember that the whole simulated setup is water now, so all the white areas in the attached screenshots are water. Your ions have to traverse 175 mm of water to hit the detector.

Kind regards,

Dear Håkan,

Thank you so much for your insightful response. Your explanation opened my eyes to the issue I’ve been facing, and your event displays are beneficial for visualizing the problem.

I tried testing it with different distances between the source and the detector, and it gave reasonable results!

I appreciate your effort in providing these valuable insights. They have certainly helped me understand the dynamics of the simulation better. Apologies for the delay in my response.

Best regards,

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