A portal monitoring system has a cosmic ray charged particle tracker with
a plurality of drift cells. The drift cells, which can be for example
aluminum drift tubes, can be arranged at least above and below a volume
to be scanned to thereby track incoming and outgoing charged particles,
such as cosmic ray muons, whilst also detecting gamma rays. The system
can selectively detect devices or materials, such as iron, lead, gold
and/or tungsten, occupying the volume from multiple scattering of the
charged particles passing through the volume and can also detect any
radioactive sources occupying the volume from gamma rays emitted
therefrom. If necessary, the drift tubes can be sealed to eliminate the
need for a gas handling system. The system can be employed to inspect
occupied vehicles at border crossings for nuclear threat objects.