PA31 WITH K90 EXPERIENCED CONFLICT WITH C402 ON DEP FROM ACK.

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THE RTE OF FLT IS AS IN THE LOA WITH ACK ATCT. THIS CLRNC MEANS (FLT IDENT) IS CLRED TO THE HYANNIS ARPT; VIA RADAR VECTORS DIRECT HYA. UPON ESTABLISHING COM WITH CAPE APCH; YOU MAY EXPECT; 'VECTORS TO JOIN THE MARCONI (LFV) 200 DEG RADIAL.' WE DEPARTED NANTUCKET RWY 24 FOR HYA; IFR. ON THE RWY HDG; CLBING TURN TO 270 DEGS; CONTACT CAPE DEP. CAPE DEP ISSUED A R TURN TO 030 DEGS AND INTERCEPT TO LFV 200 DEG RADIAL; CLB TO 3000 FT. APPROX 5 MINS INTO THE FLT; CAPE APCH ISSUED A TFC ALERT! 'TFC AT 12 O'CLOCK POS; YOUR ALT; TFC ALERT; IMMEDIATE TURN TO 090 DEGS!' MAKING AN IMMEDIATE TURN; I OBSERVED THE TFC OFF MY R WING; APPROX 100-150 FT VERTICALLY AND 300-350 FT HORIZONTALLY IN THE OPPOSITE DIRECTION. WAY TOO CLOSE FOR IFR OP UNDER RADAR CTL. THE TFC WAS A FREIGHT OP ACFT. THE INITIAL CONVERSATION WITH THE FREIGHTER ACFT AND APCH WAS; APCH: 'ARE YOU ON THE RADIAL?' ACFT: 'NO! SHOWING TO THE L PAUSE; NO TO THE R OF COURSE; CORRECTING.' APCH NOW THINKING THAT ACFT WOULD INTERCEPT THE RADIAL FROM THE R SIDE; GIVING THE ACFT A CHANGE IN SECTOR FREQS. THE PLT WAS CONFUSED ON WHICH WAY TO TURN TO INTERCEPT THE RADIAL; AND THE SIT GOT WORSE WHEN THE OTHER ACFT CONTINUED TO TURN 180 DEGS. TURNING BACK AT ME WHILE I WAS STARTING THE TURN AND NOW TURNING INTO THE ACFT AND ATC DID NOT HAVE A GOOD PICTURE ON WHAT WAS GOING TO OCCUR; AND IT'S A GOOD CHANCE THAT OPERATIONAL POS TRAINING WAS IN PROGRESS AT THE TRACON PROVIDING THE RADAR SVC AT THAT TIME.