Medium Large Transport; Low Wing; 2 Turbojet Eng
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 0.5
Human-Machine Interface; Communication Breakdown; Situational Awareness
ATC Issue All Types; Conflict NMAC
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action; Flight Crew FLC complied w / Automation / Advisory
Aircraft; Airspace Structure
Aircraft X was on the RNAV arrival at 5;000 feet. The Controller in Charge (CIC) pointed out a primary target when traffic was about 7 miles away from Aircraft X. I issued traffic to Aircraft X with the primary target's position and that type and altitude were unknown. When traffic was about 3 miles away I updated Aircraft X on the position of the traffic and Aircraft X reported they were looking. When traffic was about a half mile from the primary target Aircraft X responded that they were making a hard left bank and climbing; leveling at about 5;300 feet. After the traffic passed Aircraft X descended back to 5;000 feet and returned on the arrival. Aircraft X reported when they got the traffic in sight they appeared to be 100 feet below them. Aircraft X continued onto and landed at without further incident. We were able to track the primary target and it appeared they landed at another airport. The primary target was extremely close to the go Class Bravo airspace if not actually inside of. The primary target was definitely within the Mode C veil without an operating transponder.Inexperienced and unfamiliar pilots from across the country [fly] into our airspace transitioning to and from VFR [during this time]. Every year we are dodging and weaving from these aircraft; many times who are violating airspace; not squawking altitude; or not using a Mode C transponder at all. For one week a year it is a free for all with these VFR aircraft skirting controlled airspace with no idea what they are doing. They regularly come dangerously close to IFR aircraft without the slightest concern. Something needs to change with the procedures with inexperienced/incompetent pilots.
C90 TRACON Controller reported they observed a non-transponder (primary) target in conflict with an arriving IFR aircraft.