Hi G4 experts…
I am not a G4 expert so… sorry if my question is trivial…Some of my collegues ask me if it is possible to simulate an experimental setup that use optical photons source in a very narrow bandwith to irradiate and deposit energy in well plate for cell.
In fact i would like to use G4 to optimise the optical flux in the setup
any advices will be appreciate
Optical photons don’t deposit energy. You can run a simulation where you generate “gammas” (i.e., G4Gamma) with any energy you like. For example, you could illuminate a chip with 532 nm green laser light by setting your gamma energy to 1.95 eV.
Sorry… wanting to simplify my explanation, I actually indicated that photons deposit energy…
in fact photons at 660 nm excite molecules in water which, when de-excited, produce radicals which can kill cells…
before to start such a simulation I try to identify the difficulties.
It seems that the major difficulty is to correctly parameterize the properties of the media: reflection, absorption of photons at a given wavelength…
is there data in G4 on these parameters for the most common materials water, air PMMA well plates
In Geant4 10-series, optical properties must be specified entirely by the user application. My understanding is that in G4 11, optical properties of “common” or “well-known” materials are provided automatically, rather than requiring every user to chase them down. I haven’t found documentation for this, so I’m hesitant to claim that it’s true.
Welcome to Geant4!
I can also only speak for G4-10 as I haven’t used G4-11 myself yet. But I make the same observations as Mike.
I recommend to start with the Absorption and Rayleigh documentation, the Boundary Processes and the optical example OpNovice2.
In G4-10 optical photons only deposit energy at surfaces (if EFFICIENCY is set), n̶o̶t̶ i̶n̶ t̶h̶e̶ b̶u̶l̶k̶ o̶f̶ a̶ m̶a̶t̶e̶r̶i̶a̶l̶. T̶h̶i̶s̶ i̶s̶ s̶o̶m̶e̶t̶h̶i̶n̶g̶ y̶o̶u̶ w̶o̶u̶l̶d̶ n̶e̶e̶d̶ t̶o̶ c̶o̶d̶e̶ y̶o̶u̶r̶s̶e̶l̶f̶, b̶u̶t̶ t̶h̶a̶t̶’s̶ a̶ s̶o̶l̶v̶a̶b̶l̶e̶ p̶r̶o̶b̶l̶e̶m̶.
I think G4 provides the refractive index (RINDEX) for some materials, but I’m not sure which ones and whether they are wavelength dependent. But you can define all optical properties. I think there was a difference between RINDEX and REALRINDEX, but I don’t quite remember what is was.
You can get the refractive index from https://refractiveindex.info, e.g. air and PMMA. With the refractive index, you can calculate the absorption length (G4 doesn’t do this for you). If you define REALRINDEX (or RINDEX), G4 uses Fresnel’s formulas to calculate the reflectivity. If you have a reflectivity measurement, you can also define REFLECTIVITY, but this will only be dependent on the wavelength, not on the incident angle of the photon.
If you got all this, you can shoot optical photons of a desired wavelength through your setup and get the deposited energy. What G4 is not capable of is to excite molecules in water with optical photons (G4OpticalPhoton). Only gammas can do this (G4Gamma) and these are not the same particle type in G4 and do not share the same processes.
According to the code, absorption of optical photons in the bulk (by G4OpAbsorption) deposits energy. If Birgit and Mike find something different, please let me know.
The material properties provided in Geant4 are limited to a few refractive indices. We’re always happy to add more if the source permits it (manufacturers’ websites may not). The properties are documented here:
Thank you, Daren! The phrase that I’ve read more than once is “optical physics processes don’t conserve energy.” Which makes sense. If G4OpticalPhoton does carry along an energy, and can produce a step’s EnergyDeposit(), that’s useful to know.
Thank you, Daren for clarifying this. I have crossed out my wrong statement above. You are completely right, optical photons do deposit energy in the bulk by G4OpAbsorption. For surfaces, one needs to set EFFIENCY and the optical photons absorbed at the surface will deposit energy according to the probability defined in EFFIENCY.