[Eric A. Taylor (OP)]

A few weeks ago the TV news reported that Radiation from Japan had been detected here in Portland, Oregon. Large grains of salt being considered, they showed a graphic which showed the levels detected compared to levels of concern. I don't remember the units used but the level of concern was 25 and the level detected was .0000008 which is very far below even the natural background.

My question here is how do you measure something that is so far below the background. I haven't done the math but it's got to about like trying to hear a whisper (30 dB) next to an F-16 in full afterburner (well over 160-180 dB loud enough to cause instant hearing loss anyway).

[graham.d]

Usually the way to extract data buried in large amounts of random noise is to measure for long enough. The random noise will eventually reduce by averaging and gradually you can start to resolve the (non-random) signal. When both the signal and the noise are random patterns (like looking for a tiny increase in radiation in a background) I think it is much harder. If the background radiation is constant when averaged over long periods of time then taking readings for long enough would show any change in this average value, but at the levels you describe, I would be skeptical at the validity of the results.

[Pikaia]

I don't know the complete answer, but radiation monitors are well above the ground and measure radiation in the air, which I presume would normally be far less than at ground level.

http://www.kgw.com/news/local/Residents--118446824.html

[JMLCarter]

They put sample (the grain of salt or seawater) and detector in a big lead box to get the readings - at least that is one method.

[Soul Surfer]

All radioactive decay is very characteristic and given the right sort of equipment the type and energy of the radiation can be detected with great precision this allows precisely the sort of atom that emitted the radiation to be identified so for example individual atoms of a short lived fission product can be identified as they decay.  There is no other natural source of such short lived isotopes so individual atoms getting to the UK in the air from Japan can be identified.  As there are an incredible number of atoms in even a small quantity of material the test is just about a sensitive as a test could possibly be.

Remember the story that if you managed to distribute a glass of water through all the water on the earth any glass of water would contain one hundred of the atoms in the original glass.

[Geezer]

They put sample (the grain of salt or seawater) and detector in a big lead box to get the readings - at least that is one method.

I suspect the grains of salt in question were more metaphorical than physical.

[Eric A. Taylor (OP)]

They put sample (the grain of salt or seawater) and detector in a big lead box to get the readings - at least that is one method.

I suspect the grains of salt in question were more metaphorical than physical.

Indeed. The news media typically gets things wrong. My father worked for 30 years at an oil refinery where he witnessed a great deal of accidents that were reported on the news. The media's score on accurate reporting was a big whopping 0.

[Eric A. Taylor (OP)]

All radioactive decay is very characteristic and given the right sort of equipment the type and energy of the radiation can be detected with great precision this allows precisely the sort of atom that emitted the radiation to be identified so for example individual atoms of a short lived fission product can be identified as they decay.  There is no other natural source of such short lived isotopes so individual atoms getting to the UK in the air from Japan can be identified.  As there are an incredible number of atoms in even a small quantity of material the test is just about a sensitive as a test could possibly be.

Remember the story that if you managed to distribute a glass of water through all the water on the earth any glass of water would contain one hundred of the atoms in the original glass.

I think I read in a Tom Clancy book that after a nuclear bomb was detonated they were able to tell where the bomb came from by testing the radio active signature of the fuel. But then Clancy had Jack Ryan referred to, in England, as "Sir Jack" (he was knighted for saving members of the Royal Family from a terrorist attack). This is improper as only British subjects are allowed to use the title "sir", though non-British knights are still able to use the letters after their name.

Even Bob Hope (KBE, KCSG, KSS) who was born in England but became a US citizen could not properly be referred to as "Sir Robbert" or "Sir Leslie" his real first name.

[Bored chemist]

Eric, since you pitch the question in terms of decibels, perhaps the best answer I can give is an analogy. Imagine that you are worried about exposure to sound.
There's a jet fighter taking off in the background and somebody starts whistling at 1 KHz.
You wouldn't hear them. But imagine you got a spectrum analyser and just looked for the 1 KHz signal. If the bandwidth of the source and analyser were narrow enough then you could spot the whistle.
The energy distribution of radiation is similar in a way. Also the material they are looking for emit characteristic radiation that is unusual compared to most background.

I forgot about the other trick.
Imagine simply taking the lone whistler away from the jet. That makes him much easier to hear.
The fission products that people have been measuring are typically isotopes of caesium and iodine.
You can use chemical techniques to isolate just the iodine from a sample. Then you can look at the radioactivity of that fraction. This removes most of the other material so most of the background radiation is removed too.