Niagara Emitter Particles

I have set up particles on a rectangle grid and after 0.5 secs they are all attracted to a point force. This all works except that it takes longer for the outer edges to reach the point than the middle ones. As a result the middle ones start to bounce back. Is there a way to kill particles when/as they reach the attraction point.
image
As can be seen a sphere appears to grow in the middle, this is what I would like to avoid.
Thx.