Sadly with longform expressions like that I am clueless, hehe, so I wouln't be the one to ask about that
Regarding your collision event, however, have you checked the event editor at the bottom of the dialog is a checkmark to "Kill" the particles rather than split or emit others?
Just making sure, as mine kept defaulting to split, so I had to check it twice before it took correctly
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS
izzylong.com