How to understand the waveforms when energy deposit in specific positions

[zql2021]

Hi Felix, Happy New Year!

Recently, while using SSD to simulate waveforms, I’ve encountered some results that I can’t understand and would like your advice.
My detector is a high-purity germanium detector with single-sided multi-strip electrodes.The crystal diameter is 30mm, height is 10mm, the bottom is a lithium-diffused anode with a height of 3mm and thickness of approximately 0.5mm.The shape of the detector is shown below.

You can find the detector details in detector.yaml. detector.zip
The strip electrodes are numbered from left to right as 1, 2, 3...7, and the x-coordinates of the strips are shown below.

My question is:
①When energy deposits at position = (7.9, 0, 3) #mm, which is under Strip7, the simulated waveforms indicates Strip6 as the main collect electrode, but Strip7 also collect some energy.

begin
    T=Float32
    position = (4.9,0,3)  # mm
    starting_positions = [CartesianPoint{T}(position[1] / 1000.0, position[2] / 1000.0, position[3] / 1000.0)]  # convert mm to m
    energy_depos = T[100] * u"keV"
    evt= Event(starting_positions, energy_depos)
    simulate!(evt, sim, max_nsteps = 100000, Δt = 0.1u"ns", verbose = true,diffusion=true,self_repulsion=true)
    plot(
        plot( evt.waveforms[3] ,lw = 1.5, xlims = (0, 200), xlabel = "Time", ylabel = "Charge", 
    unitformat = :slash, legend = false, tickfontsize = 12, guidefontsize = 14,title="Strip 1"),
        plot( evt.waveforms[4] ,lw = 1.5, xlims = (0, 200), xlabel = "Time", ylabel = "Charge",
    unitformat = :slash, legend = false, tickfontsize = 12, guidefontsize = 14,title="Strip 2"),
        plot( evt.waveforms[5] ,lw = 1.5, xlims = (0, 200), xlabel = "Time", ylabel = "Charge",
    unitformat = :slash, legend = false, tickfontsize = 12, guidefontsize = 14,title="Strip 3"),
        plot( evt.waveforms[6] ,lw = 1.5, xlims = (0, 200), xlabel = "Time", ylabel = "Charge",
    unitformat = :slash, legend = false, tickfontsize = 12, guidefontsize = 14,title="Strip 4"),
        plot( evt.waveforms[7] ,lw = 1.5, xlims = (0, 200), xlabel = "Time", ylabel = "Charge",
    unitformat = :slash, legend = false, tickfontsize = 12, guidefontsize = 14,title="Strip 5"),
        plot( evt.waveforms[8] ,lw = 1.5, xlims = (0, 200), xlabel = "Time", ylabel = "Charge",
    unitformat = :slash, legend = false, tickfontsize = 12, guidefontsize = 14,title="Strip 6"),
        plot( evt.waveforms[9] ,lw = 1.5, xlims = (0, 200), xlabel = "Time", ylabel = "Charge",
    unitformat = :slash, legend = false, tickfontsize = 12, guidefontsize = 14,title="Strip 7"),    
    size = (1400, 600), layout = (2, 4)
    )
end

②When the position changes (only the Z-coordinate) to (7.9, 0, 5) #mm, the waveforms shows Strip7 as the main collect
electrode, but Strip6 also collect some energy..

③Similarly, when energy deposits at position = (4.9, 0, 3) mm, the simulated waveform shows Strip6 as the main collecting electrode, but Strip5 also unexpectedly collects some energy.

I have checked detector.yaml, as well as the electric field and potentials, and haven’t found any issues.
I can’t understand the waveforms described above and I suspect that detector.yaml might have issues. Could you please take a look and provide some suggestions on interpreting these waveforms?

[fhagemann]

Happy new year.

I had a look at your config file, but it looks good to me.
I rewrote the config file, mainly removing phi: from: 0; to: 360 (you don't need this if its 360°), reducing r: from: 0; to: 11 to just r: 11 (if from is 0, you don't need to specify it) and replacing your strip geometries by intersection rather than difference, which makes it easier to cross-check the detector strip position: detector.zip

I do not think it is an issue with the config file but with the simulation settings.
I assume you use something like

simulate!(sim)

to calculate the potentials and fields. I can reproduce your plot when using this for (7.9, 0, 5):

You might wanna increase the refinement for the electric potential calculation, for example using:

# increase refinement of electric potential and field calculation
simulate!(sim, min_tick_distance = 0.001u"mm")
calculate_electric_potential!(sim, refinement_limits = [0.2,0.1,0.05,0.02], min_tick_distance = 0.001u"mm")
calculate_electric_field!(sim)

Then, the plot looks just fine for (7.9, 0, 0.5):

[fhagemann]

Maybe some more explanation why to increase the min_tick_distance to 0.001u"mm":

The minimum tick distance in the electric potential calculation is set to 1e-5 (in units of meters, so 10µm). (check the docstring of simulate! or the code here).
This can result in the fact, that when painting the contact onto the grid, it might end up being some tens of µm wider than the original geometry. This can cause the charge drift to end before the charges reach the contact).

With default simulate!

With optimized simulate!

The left-over charge on the non-collecting strips are because the weighting potential value at that position is not 0 (because it is not "on" the contact), resulting in a non-zero induced charge on these contacts.

[zql2021]

Thank you for your patient reply. I was indeed using the default min_tick_distance parameter of 1e-5, after optimizing to 1 μm, the issue was indeed resolved. Thanks again for your help!