Thanks juancarlosgzrz for your reply but actually I couldn't reach to my wanted result.
I used radial_gradient.osl too but I may missing something, and I can't get them to work as realistic fade on edge of atmosphere.
Actually I can reach my wanted effects in simple Corona MTL like assigning a FallOff map or GradientRamp to Opacity slot and making edges go fade but when using CoronaVolumeMaterial, it behaves as sharp edge shrink instead of fade.
I uploaded 3DsMax scene file with materials I used on slate material editor. I hope someone can help me.
Thanks.