X-Ray diffraction pattern through a grating

I want to simulate X-Ray diffraction pattern through a grating. Basically same thing you see in the attached image, but instead of a light source I want an X-Ray source. I used Helmholtz equation, I dnt see anywhere there where I can set the wavelength. How can I replace the light source with a X-Ray source? I am new in COMSOL, I will appreciate your help.