NMAC. CONFLICT WITH VFR ACFT ON DSCNT. UNDER ATC CTL AND COM.

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THE INCIDENT FLT WAS FROM PHL TO LNS AND RETURN. WE HAD SLOWED TO 190 KIAS. TURB WAS CONTINUOUS LIGHT CHOP. AS WE CROSSED 10 MI NW OF MXE VORTAC AND WERE OUTBOUND ON THE 295 DEG RADIAL; WE WERE ADVISED BY PHL DEP TO CHANGE TO MDT APCH. AFTER CHKING IN WITH MDT; WE WERE INFORMED OF TFC AT '1 O'CLOCK 5500 FT; DSNDING.' WE INTENSIFIED OUR SCAN AND CHKED OUR 1 O'CLOCK. NEGATIVE CONTACT. APPROX 1 MIN LATER WE WERE INFORMED TFC NOW AS 12-1 O'CLOCK 4500 FT DSNDING 2 MI SEBOUND. SINCE W WERE IN A NW DIRECTION WE WERE CLOSING IN PRETTY FAST ON OUR TFC. MDT APCH CLRED US TO 3000 FT AND ASKED US TO EXPEDITE OUR DSCNT. STILL NO CONTACT. AS WERE PASSING THROUGH 3600 FT I SAW A CESSNA (152 OR 172 TYPE) PASS UNDERNEATH US. HAD WE BEEN 100-150 FT LOWER WHEN PASSING; A COLLISION WOULD HAVE BEEN UNAVOIDABLE. MY FO WAS THE PF; BUT I IMMEDIATELY GRABBED THE YOKE AND STOPPED THE DSCNT SINCE I SAW TFC FIRST. TIME FRAME FROM THE TIME I SAW TFC UNTIL HE WAS GONE WAS 2 SECONDS TOPS. I ASKED MDT APCH IF THE TFC PASSING UNDER US WAS THE TFC WE WERE SUPPOSED TO LOOK FOR SINCE OUR TFC REPORTEDLY WAS HIGHER THAN US. HE CONFIRMED THAT INDEED THAT WAS OUR TFC AND HE SEEMED SURPRISED WHEN I SAID THAT THE TFC PASSED UNDER INSTEAD OF OVER US. HE ALSO VERIFIED THE CESSNA'S ALT OF BEING 3500 FT. I TOLD THE CTLR THAT I HAD NO CHOICE BUT TO FILE A RPT ON THIS NEAR HIT. REMAINDER OF FLT PROCEEDED WITHOUT INCIDENT AND ARR AND LNDG IN LNS WAS UNEVENTFUL. I DON'T KNOW WHERE THE SYS FAILED BUT IT WAS EITHER THE CESSNA'S MODE C WHO WAS GIVING THE CTLR FALSE READINGS OR HE WAS DSNDING AT A RATE THAT WAS EXCESSIVE AND THE CTLRS EQUIP COULDN'T KEEP UP. INCIDENT OCCURRED IN CLASS E AIRSPACE. HAD WE HAD A TCASII ONBOARD WE MIGHT HAVE BEEN MORE ENLIGHTENED OF THE SIT. IF SEPARATION REQUIREMENTS BTWN IFR AND VFR TFC IN VMC WAS STRICTER THIS INCIDENT WOULD NEVER HAVE HAPPENED. THE BLAME WAS NO ONES; REALLY. I AM JUST GLAD THAT WE DIDN'T HAVE A COLLISION THIS TIME.