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A comment on The NYPD Is Using Mobile X-Ray Vans to Spy on Unknown Targets. Parent comment

Re:The NYPD Is Using Mobile X-Ray Vans to Spy on Unknown Tar... by VLM on Tuesday October 20, 2015 @04:59PM (#10421780)
Its mostly the fault of one isotope of potassium and many hiker types eat bananas specifically to boost their blood K levels because otherwise they get really annoying muscle cramps. Its a moderately strong beta emitter and the range in water (close to human body) is about 2 cm or so. There are probably no parts of your innards more than 2 cm from the nearest bodily fluid (blood or whatever).

Humans are surprisingly radioactive BTW. We each have about a hundred grams of potassium happily radiating away. An interesting technological challenge for a startup might be using purely passive radiation monitoring to detect humans. For security sensing or something. Maybe in a 100 years automatic supermarket style doors will sense humans radioactively rather than current microwave doppler and infrared systems. Maybe.

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Re:The NYPD Is Using Mobile X-Ray Vans to Spy on Unknown Tar... by Florin_Andrei on Tuesday October 20, 2015 @07:04PM (#10422472)
> An interesting technological challenge for a startup might be using purely passive radiation monitoring to detect humans.

Beta is super-easily stopped. A piece of cardboard or tinfoil reduces it dramatically, or stops it completely, depending on energy per particle, total flux, etc.

I've a Luminox watch, the dial is visible in darkness all the time, you don't have to "recharge" it. It works by having small amounts of tritium decay within luminescent tubes. The radiation produced is beta - but the levels of radiation outside the watch are essentially zero. The body of the watch is enough to contain it.

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Re:The NYPD Is Using Mobile X-Ray Vans to Spy on Unknown Tar... by onnoonno on Wednesday October 21, 2015 @07:06AM (#10424679)
Again, and as others pointed out in other contexts here, it depends on the energy. Tritium has 20keV betas AFAIR. Some elements produce multi-MeV beta decay. That is not stopped by thin cardboard.
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