Photoelectric emission from SS Grids

[kvjmistry]

There is a small probability of photoelectric effect of VUV photons on the SS mesh. This can lead to additional light in the EL region due to additional liberated electrons. NEXUS implements this process as an optional physics process, however, it does not input the PE emission probability.

The following paper from LUX reports a quantum efficiency of 4e-4 on SS304: https://journals.aps.org/prd/abstract/10.1103/PhysRevD.102.092004

Other measurements from Gonzalo report some other values: https://next.ific.uv.es/cgi-bin/DocDB/private/ShowDocument?docid=1361

Other interesting reports of the quantum efficiency of TPB are in: https://arxiv.org/abs/1411.4524 This could also be implemented for future studies.

This is worth adding as a configurable option in the code with all these reported values.

[gonzaponte]

LUX's results are more trustworthy, but they are in LXe, not in HPGXe, and therefore

1. the backscatter probability will probably be quite different.
2. the electron affinity of LXe can decrease the work function

These two effects go in different directions. 1. works towards a lower (effective) QE while 2. favors a higher QE. That said, my data were taken at low pressure, so it's not a straightforward comparison either. The best would be to measure it directly. Perhaps we can do it during the LPR.

Because we don't know the exact value, I left it as an external parameter in the NEW simulation via the /Geometry/NextNew/photoe_prob command. This would have to be implemented for Next100 (if it isn't already) and for stainless steel (if a realistic mesh is used instead of the FakeGrid).

I don't fully understand from your comment if you want to hardcode the values in the simulation or if you didn't know that this was already an external parameter.

[kvjmistry]

These are very good points!

Yes if it's possible to measure it with the LPR we can investigate!

[gonzaponte]

I don't fully understand from your comment if you want to hardcode the values in the simulation or if you didn't know that this was already an external parameter.

Sorry, this was meant to be a question. Which one is it? :)

[kvjmistry]

I was thinking we have some kind of example config file with example values we can use. I also didn't know there was already a way to configure it as an external :D

[gonzaponte]

That sounds like a good option. I thought we had one already, but I must have had it in another branch and never made it to the PR.

[gonzaponte]

This would have to be implemented for Next100 (if it isn't already)

This was done in #240 using the FakeGrid.

and for stainless steel (if a realistic mesh is used instead of the FakeGrid).

And this is what might be missing. This can be go in a small PR after #187 is merged.

[paolafer]

This issue seems more complicated than expected. In order to trigger the current process, the photon needs to enter the steel volume, which at the moment is not the case since it is either reflected or dies in the optical surface around the grid. If we make the surface of dielectric_dielectric type and the steel dielectric, two unwanted effects occur: 1. many photons are absorbed in the boundary, therefore do not enter steel and don't have the chance of undergoing the photolectric process, 2. when a photon is reflected, the process is activated because the post-step volume is the grid (and the process is activated by the post-step volume). As far as I know, there is no way of triggering a process in the boundary, else than the boundary processes already defined in Geant4 (I posted a question in the forum, and I received no answer so far).