How can I get the previous frame from a scene capture?

I am trying to implement a postprocess effect that is essentially temporal ‘image subtraction’ - subtracting the current image from the previous to track changed pixels.

I have a scene capture component linked to a render target, but when I associate this to texture parameters, it’s continuously rendering to all textures it is linked to even when I try to introduce delays. For instance, Is there any way of getting the previous frame, by buffering it somehow?