A 76-year-old Boston man received a nine-year prison sentence on Thursday for plotting to murder a Worcester prosecutor, authorities said. Pernell Powell was sentenced in federal court in Worcester for his conviction on a charge of using interstate commerce in the commission of a murder-for-hire, US Attorney Carmen M. Ortiz’s office said. Powell tried to hire his cellmate in 2011 at the Worcester County House of Correction to kill a prosecutor who he thought was working his pending drunken driving case, authorities said. The plot was not carried out.Travis Andersen can be reached at email@example.com. Follow him on Twitter @TAGlobe.