As pointed out in the passage, the United States ----.

One of the most pressing international priorities is to
control the dissemination of nuclear materials that
could be used in attacks by terrorists or rogue states.
Nuclear materials contain unstable isotopes, which
emit x-rays and gamma rays. The characteristic
energies of these photons provide a fingerprint
revealing which radioactive isotopes are present.
Unfortunately, some isotopes that occur in benign
applications emit gamma rays with energies that are
very similar to those emitted by materials used in
weapons, which leads to ambiguous identifications
and false alarms. This problem has been worrying
the United States, which is installing thousands of
radiation portal monitors to detect the gamma rays
emitted by nuclear materials carried by vehicles
crossing the Canadian and Mexican borders. One of
the worst fears of the authorities is that terrorists
might smuggle highly-enriched uranium into the
country to build a crude Hiroshima-style atomic
bomb.