Unity ParticleSystem collision with wall. Coords on wall.

So basically i would like to know if there is any way, of knowing which part of wall is hit by a particle. I managed to get the particle world coords, which doesnt transfer to my wall coords very well.

The thing i want to achieve is leaving decals on a wall(white decal 4x4 to 16x16, modified color with code), specifically oriented on a face hit by a projectile, while not hitting the performance, by checking each x,y,z for collision.

Another thing is, is there a way to generate sphere with given radius, that will select all pixels on walls texture by checking points contained by sphere AND wall? That way i could hit the corner and get pixels changed on two or even three walls.

And the last thing. Is there a possibility of making decals or pixels illuminate?

Thank you.