Date: 201908
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Light: Daylight
Aircraft: B777 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Airspace: Class B ZZZ
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Troubleshooting; Situational Awareness
Second reporter function: Flight Attendant (On Duty)
Anomaly: Deviation / Discrepancy - Procedural Hazardous Material Violation
Detector: Person Other Person
When detected: Pre-flight; Routine Inspection
Result: Flight Crew Became Reoriented; General Maintenance Action; General Work Refused
Contributing factors: Human Factors; Procedure
Primary problem: Ambiguous

Narrative: I received word from [Gate Agent] that boarding had been stopped. While in the course of random outbound security checks by Customs officers; they had received an alert on their PRDs (Personal Radiation Detectors) near our aircraft and needed to individually scan passengers and potentially the aircraft for excessive radiation. Passengers both on the jet bridge and in First Class were scanned by PRDs and a Customs dog. It was then determined that the excessive radiation was being emitted from the forward cargo area. Customs officers initially said it was at a LEVEL 7. However; upon further review from an upgraded Professional Radiation Detector and Customs Radiation Expert from [the terminal]; they determined it was off scale high in excess of a LEVEL 10 and were unable to extrapolate the precise reading. The phrase;' potential bomb making materials' was used several times by the Customs agents. Both in lieu of this information; and with the support of the [Chief Pilot]; and a lack of information from some departments at [the company] as to the exact nature of what we were shipping; I elected to remove the dangerous goods.As a follow up to this incident; I spoke with both our DG (Dangerous Goods) Hot-line and a Customs agent [a few days later]. The Hot-line was unable to delineate what exactly we were shipping except to merely read back what was already on the manifest. And a Customs Agent indicated that they have a scientist on call 24/7 to precisely interpret the results of the professional Radiation Meter. He indicated that the DG materials may have been legitimate; but the packaging was not.


Second narrative: Part of the way through the boarding process I saw security officers and a security dog on the jet bridge. They boarded and walked around with meters. They said all the passengers had to be seated. They focused on the area around 2L and questioned passengers in that area. They had some of the passengers from that area deplane. After some time I looked at the Flight Info app on my [device] and it said that 2400 lb of pallets covered with dry ice were removed from cargo. An officer returned to the First Class area and walked around with his reader and said the levels had become normal. They eventually completed boarding. During the layover I woke up with a headache and took Advil. Later that morning I took Advil again and the headache was relieved. After landing at ZZZ my flying partner showed me a photo of the officer's reader. It shows urem/h of 429; cps of 13672; and N-cps of 0.0. It says 'Alarm'; 'High Gamma Does Rate Alarm' with a red background. After landing at ZZZ the Captain spoke with someone in the Customs area about what the cargo was and he said the cargo was medical liquid.


Synopsis: B777 Captain and cabin crew reported Customs Officers conducted aircraft and passenger PRD (Personal Radiation Detector) scan due to excessive radiation levels from the aircraft. Captain elected to remove Hazmat cargo due to the lack of information from company regarding onboard Dangerous Goods cargo.