NCT CTLR EXPERIENCED NMAC WITH SMF DEP AND UNKNOWN CESSNA.

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I AM AN AIR TFC CTLR AT NORTHERN CALIFORNIA TRACON. ACR X DEPARTED SMF ON A SID AND ABOUT 6 MI AFTER TKOF; I VECTORED HIM 090 DEGS TO GO AROUND ACR Y AT 5500 FT. I ISSUED ACR X TFC '2 O'CLOCK POS; 5 MI 5500 FT NBOUND; ACR Y.' THE PLT LOOKED FOR THE TFC; BUT INSTEAD SAW A CESSNA AND HAD TO PUSH THE PLANE DOWN TO AVOID COLLISION. NO TARGETS WERE OBSERVED BTWN THE ACR X AND THE ACR Y ON RADAR AT THE TIME I ISSUED TFC. UPON REPLAY OF THE RADAR DATA; A DIAMOND SYMBOL; USED FOR PRIMARY TARGETS ON THE ARTS DISPLAY WAS OBSERVED TO BE FIRST DISPLAYED 5 MI S OF THE NMAC. NO SLASH REPRESENTING A PRIMARY TARGET SKIN PAINT RETURN OR XPONDER RETURN WAS DISPLAYED. THERE WAS NO HISTORY OF THIS TARGET BEING DISPLAYED PRIOR TO THIS POINT; WHICH IS UNUSUAL; SINCE; IN MY EXPERIENCE; PRIMARY TARGETS OF THIS SORT SHOW UP ON THE RADAR QUITE WELL. IN ADDITION; DIAMOND SYMBOLS OF THIS SORT; WITHOUT ACCOMPANYING PRIMARY OR BEACON SLASHES; REGULARLY APPEAR ON THE RADAR DISPLAY FOR A FEW SECONDS AND THEN DISAPPEAR. OUR RADAR IS ON A DIGITIZED DISPLAY; WHICH DECIDES WHICH TARGETS ARE STRONG ENOUGH TO BE DISPLAYED. ON EARLIER TYPES OF RADAR; ALL RETURNS WERE DISPLAYED FROM BIRDS; TO CLUTTER AND ACTUAL ACFT AND THE CTLR WAS ABLE TO MAKE HIS OWN INTERP OF THE VALIDITY OF THE RETURN. THE MOST IMPORTANT FACTOR PREVENTING ME FROM VIEWING THE CESSNA TARGET WAS THE FACT THAT THE DIAMOND SYMBOL OF THE CESSNA WAS CONCURRENT WITH THE DATA BLOCKS AND TARGETS OF 2 OTHER ACFT CIRCLING IN THE AREA AT LOWER ALTS. AFTER THE NMAC; WE WERE ABLE TO TRACK A TARGET THAT APPEARED 3 MI BEHIND ACR X UNTIL WE LOST RADAR CONTACT WITH IT APPROX 35 MI N OF SMF. AT NO TIME DID IT DISPLAY A XPONDER REPLY. MY OPINION OF THE CAUSE? A BETTER RADAR RETURN ON THE CESSNA CERTAINLY WOULD HAVE HELPED; BUT THE HVY CONCENTRATION OF TFC MENTIONED ABOVE AND SEVERAL OTHER ACFT IN THE IMMEDIATE AREA CLUTTERED THE RADAR PRESENTATION SO MUCH THAT JUST SEEING ALL THE DATA BLOCKS; LET ALONE STRAY TFC; BECOMES DIFFICULT. EVEN MORE IMPORTANT; AS A CTLR AND PLT; I FEEL THAT FLYING THE RTE AND ALT THE CESSNA TOOK OVER A MAJOR AIR TFC AREA WAS NOT CONSISTENT WITH GOOD PLANNING AND PROCS UNLESS THE PLT USED FLT FOLLOWING FROM ATC. I KNOW LOTS OF PLTS THINK ATC CAN SEE THEM AND WE WILL TAKE OUR PLANES AROUND THEM BUT IF THEIR XPONDER IS NOT WORKING PROPERLY WE WON'T SEE THEM AND THEY WON'T KNOW IT. EDUCATION FOR THE PLTS COULD HELP. PERHAPS THIS COULD BE ADDRESSED AT THE FAA'S PLT BRIEFINGS OR BI-ANNUAL REVIEWS.