Guns and masks and getaway cars are not required for a stiff sentence for bank robbery. Prosecutors told a U.S. District judge that on July 10, Scott F. Backus, 46, of Dorchester, passed a note demanding money to Citizens Bank employee in downtown Boston. When the employee didn't immediately respond, Backus backed out and walked to a second Citizen's Bank and repeated the process. Again he got cold feet and walked out, but this time he ran smack into federal agents who had just left the first bank to answer the call at the second bank. The investigators knew his face from a surveillance video. Backus pled guilty and was sentenced to 100 months in prison and three years probation for his crimes.