Former banker Paul Douglas Peters broke into a suburban Sydney home in August 2011 and put the fake bomb around 18-year-old Madeleine Pulver's neck, the Australian Broadcasting Corp. reported Tuesday.
After leaving an extortion note, Peters fled to the United States where he was arrested by police in Kentucky and later extradited to Australia.
It took police 10 hours to determine the bomb locked around Pulver's neck was a fake. They were so baffled by the device they had to call British military experts for help.
District Court Judge Peter Zahra said Peters, who reportedly had two personalities, identified with a character in a novel he was writing and could not remember anything that happened at the Pulver home.
Despite his mental problems, the judge ruled he engaged in a "deliberate act of extortion" and sentenced him to prison.