Reduce Overdraw on Particles using RWTexture2D

Hey, I just had the idea to use a RWTexture2D within the pixel shader of my particles to try to reduce overdraw/fillrate. I have created a R32_Float texture with UAV and bound it together with the RenderTarget. In my pixel shader I just add a contstant value to pixel of the current fragment while I am checking for a maximum at the beginning. However it does not work. It seams that the texture is not getting written. What I am doing wrong? Or is it not possible to read/write at the same time in PixelShader? Thx, Thomas