Customs has already been using gamma-ray detectors for a few years now... did you catch that first word? Customs.
Ports Operators do not do cargo inspection.
Ports Operators (generally) do not even know who the shippers or consignees or the commodities (exception: haz-mat, or to give high-value customers (wal*mart) better service,etc)
The US Gov't Agencies (Customs & Border Protection, USDA, and FDA) have the monopoly on national border security. These functions were never and are not delegated to the Port Operator. The P.O. only manages the physical footprint of the facility in order to facilitate the rapid loading/unloading of cargo from a vessel , and the truck gate.
It doesn't matter who runs the port, US Customs , USDA, FDA are the ones who provide the security.