# Diffusion coefficient

Dear FLUKA expert,
At the card “OPT-PROP”,if I let the type to blank,WHAT(3) is called diffusion coefficient,but I studied some literature and found this parameter’s unit is m^2/s or cm^2/s,could you tell me what it really mean?

Dear Xiong,

my understanding is that the definition of the diffusion coefficient is simmilar to the one of the attenuation coefficient - meaning that when a photon beam passes 1 meter of material with a diff. coeff of 1 m^-1, only 1/e of initial photons would pass without being diffused.

However, I am not 100% sure with this - I will ask another expert for confirmation.

Vojtěch

Dear Xiong,

I concur with Vojtěch’s assessment.

Optical photons in transport can essentially do one of three things (see manual for more details):

• Be absorbed (and optionally produce wavelength-shifted secondary optical photons),
• Be scattered (a la Thomson, i.e. with a differential cross section that goes like 1+\cos^2\theta, where \theta is the scattering angle),
• Be reflected/refracted at a material boundary.

For bulk processes (the first two) the code needs to know how often to sample each, so it asks from the user an inverse mean free path in units of \text{cm}^{-1}.

For the absorption part, in FLUKA this quantity is called the “absorption coefficient”, and for the scattering (or diffusion) part, the “diffusion coefficient”. Note that both have units of \text{cm}^{-1}. Thus, in the present context the diffusion coefficient is indeed not what is ordinarily meant when writing down the diffusion equation, where it has dimensions of L^2T^{-1}, as you noted.

With kind regards,

Cesc

Dear cesc,
I understand it,thank you for your help.