There's some controversy over whether radiation sensors require warrants when used by police searches. Volokh suggests
that the standard should be based on precedents about heat sensors
pointed at homes (warrants are required), combined with those that
indicate serious crimes don't get exceptions for being serious
crimes. I agree with that principle, but I think radiation is
distinguishable; among other things it poses an independent health
hazard in high enough levels.
While, say, directing X-rays through a building to a receiver on the
other side would clearly be an unconstitutional search, merely checking
for excessive radiation levels outside a building is quite reasonable
and should be admissible in court.
The precedents for heat sensors are also correct; heat sensors are
substantially more invasive (because many normal and legal activities
generate detectable heat), and the level of heat at which a public
health hazard exists is generally quite visible to the naked eye in the
form of smoke and flame.
Once you start stepping on to (public spaces of) private property in
order to conduct such a search, however, things rapidly become more
complex. I would still be OK with people carrying detectors that
can register hazardous-to-health levels of radiation, chemical agents,
etc, so long as they remain in areas truly open to the public and don't
need to make false representations in order to gain entry.
I say this because, as a private citizen, I might well want to carry a
radiation detector along with me as an early warning of radiation
hazard, if I could do so cheaply, and if there was a perceived risk --
consider, for example, the laboratory facilities at many