C172 PLT WAS TOLD BY ZBW TO MAINTAIN VISUAL SEPARATION FROM A B737 WHICH RESULTED IN A CONFLICT.

Narrative

C172 ON FLT FROM LEB TO 9B1 DIRECT (PILOTAGE; DR; LORAN; AND PORTABLE GPS; COURSE 170 DEGS MAGNETIC); CONTACTED ZBW UPON DEP AND ESTABLISHED WITH THEM A SQUAWK (MODE C GARMIN 330) CODE; ALTIMETER SETTING; RTE (LEB-9B1 DIRECT); ENRTE ALT (7500 FT); ACFT TYPE AND RA'S REQUEST. ZBW RPTED RADAR CONTACT. APPROX 35 NM FROM MHT BOSTON HANDED ME OFF TO MANCHESTER APCH. PLT COMPLETED RADIO CONTACT WITH MANCHESTER APCH; AND RECEIVED AN UPDATED CURRENT ALTIMETER. AT APPROX 270 DEG RADIAL 18 NM FROM MHT PLT HEARD ACR CLRED (FROM 9000 FT?) TO 5000 FT. APCH CONTACTED ME BY CALL SIGN AND ADVISED OF TFC AT 2 O'CLOCK POS DSNDING TO 5000 FT WITH MENTION OF 'PASSING BELOW YOU.' BOTH ACFT RPTED TFC IN SIGHT. APCH REQUESTED ME TO TAKE RESPONSIBILITY FOR VISUAL SEPARATION; I ACCEPTED RESPONSIBILITY. APCH INFORMED ACR I WAS ACCEPTING RESPONSIBILITY FOR VISUAL SEPARATION. WITHOUT FURTHER COM BTWN EITHER PLT AND APCH; I CLBED 200 FT AND 7700 FT BEFORE ACR PASSED BENEATH AND IMMEDIATELY RPTED PASSING 7000 FT TO APCH. SOON AFTER; ACR CONTACTED APCH TO DETERMINE IDENT OF PASSING ACFT. FBO AT 9B1 RECEIVED AND PASSED ON TO ME INSTRUCTION TO CONTACT THE SUPVR AT MANCHESTER APCH. I PLACED THAT CALL IMMEDIATELY; AND CONVERSATION WITH THE SUPVR INCLUDED DISCLOSURE THAT ACR HAD TCASII MONITORING THE SIT; THAT MANCHESTER ATC RADAR HAD BEEN UNABLE TO RESOLVE THE 2 ACFT (A RADAR SHADOWING PROB?); AND PLT ACKNOWLEDGED HE WAS CLR ABOUT ACCEPTING VISUAL SEPARATION RESPONSIBILITY AND FELT THAT SEPARATION HAD BEEN MAINTAINED.