Violence

Hello, what I want to achieve is a matte that only displays objects within a certain distance to the camera. Let's say I create a random particle stream where the particles are far away from the camera on one point and on another point they are a little bit more far away. Now I want to create a matte for only the front particles (the nearest to the camera) that I can use in compositing later. Could be called some kind of a depth buffer or so. I have not found any answer anywhere. Thanks