Is it impossible to read Rendertarget2d through Grid2DCollection?

I posted yesterday and then did some deep test after that.
I created a user parameter(RenderTarget2D,R16F,width-256 height-1,filled color(2000,0,0,1),and binded it to a Grid2DCollection
and tried to read the grid as below

and captured the attributes of the particles:
I found the results(position) are always zero. I just googled and can not find any READ sample about GRID2DCollection with RenderTarget2D,Is it impossible to read like this? Or in Niagara, the render target should only be the result buffer?