CHICAGOUniversity of Chicago particle physicist Matthew Sharp drove halfway home before he was struck with the fear that he'd left the Argonne Tandem Linac Accelerator System running Tuesday night. 'I think I powered it down after smashing those 9-GeV electrons into 3.1-GeV positrons, but I don't specifically remember flipping the switch,' Sharp said. 'Not only does a nine million volt electrostatic tandem Van de Graaff injector accelerator cost a lot to run; it's also a pretty serious fire hazard.' Sharp almost turned his car around, but didn't, because the past three times he's gone back to check on the accelerator, he's found it off. 