unwanted lighting intensity steps/rings

I’m sure this has been answered somewhere, but I’ve searched and searched and can’t find it…

I’m working on a ray-traced shadow-only (alpha) rendering that results in various fields of grey and black depending on the interference patterns of various light sources. My problem: in areas that should be of fairly consistent tone, I’m getting noticeable steps or rings of light intensity (particularly noticeable in the blacks of the shadows) – based, I think (and I’m waxing here) on distance from the light sources. I don’t mind the gradient, I just want it to be smooth.

In a normal rendering, I would muck up the shadow-sensitive surface (with a noisy texture mapped to the surface normal) to break up these rings a bit and force more continuous results. However, in shadow-only, the texture is ignored.

Any suggestions on smoothing things out? Is it a lighting setting, or something having to do with the illuminated material?

(new file. new plane, scale 10. cube–>“only cast” [links and pipeline]. plane–>“only sha” [shaders]. sky to white for contrast. zoom in and preview.)