Can't set absorption coefficient for inert particle
 

Hi, I'm using Ansys fluent 12.1 and I'm modelling a UV-reactor for water disinfection. I have been able to release particles to the reactor, but I can't set the absorption coefficient for the particles.
I have tried to follow the instructions at the UsersGuide. It says:

ANSYS FLUENT will include the effect of particles on the absorption coefficient if you have turned on the Particle Radiation Interaction option in the Discrete Phase Model dialog box.

I have done that but I can't still set the absorption coefficient for particles. After I have checked the Particle Radiation Interaction at the Discrete Phase Model dialog box. I can set particle emissivity and scattering coefficient but not absorption coefficient.
So how I do that?
Thanks for helping!

Did change the material property in Material>particle>particle emissivity? It should be non-zero value so the impact on absorption coefficient can be added.