Date: 201605
Local time of day: 0601-1200
Location: P80.TRACON, OR
Flight conditions: VMC
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E P80
ATC / advisory: TRACON P80
Second aircraft: Small Aircraft; Low Wing; 1 Eng; Fixed Gear
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Second reporter function: Approach
Anomaly: Conflict NMAC
Detector: Person Flight Crew
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: On descent into PDX; descending via the TMBRS 2 RNAV Arrival; around the VANTZ point; at approximately 11;600 ft and 250 knots; I noticed motion to my right in the F/O's (First Officer) forward windscreen. Looking to the right; I momentarily saw a light aircraft pass within several hundred feet to the right-side of our aircraft; and very close to co-altitude. The traffic was past us before we had time to react. I reached for the yoke; but the conflict was over before I even had time to maneuver. We did not receive a TA; RA; or even a TCAS target; nor did we receive any warning from ATC. The traffic appeared to be a Piper Cherokee or similar type aircraft.We reported the near miss to Portland Approach and asked if they had the traffic on radar. They said no but they had previously seen a primary only target.We continued uneventfully to PDX and reported the incident to Dispatch and ATC Management upon landing. We conferred with the F/As (Flight Attendants) to see if any passengers had reported seeing the aircraft pass by us; but no comments were made; nor questions asked by the passengers.We were VFR and looking outside to acquire the airport. The traffic approached from a head on aspect and apparently did not have a transponder on. We did not see the traffic until it was on right in front of us. The conflict happened very quickly with no TCAS or ATC warning. It was very close; way too close.A heads up from ATC that there had been a primary only target on radar ahead of us would have been helpful. But; it would have been even better for the uncontrolled GA aircraft to have a transponder on so we received a TCAS alert and target.


Second narrative: [B737] checked in descended via the [TMBRS] arrival and was told to expect RWY 28L. After acknowledging the read back; I continued to scan my radar and do other duties. As I looked down at [the B737]; I noticed a very faint blue primary target which at first appeared to be a history trail. My radar is not adjusted that bright for that. Immediately after; [the B737] called and reported an small aircraft passed adjacent to his aircraft at the same altitude. I called Seattle sector controller and asked if he was painting a target and he replied no at first. I do not recall seeing traffic; but the small aircraft was there.


Synopsis: B737 Captain and a P80 Approach Controller reported a NMAC with a light aircraft in the vicinity of PDX. Light aircraft reportedly did not have an operating transponder and ATC was unaware of their position and altitude.