Sunday, March 13, 2011

Radiation Exposure of Japanese Residents

I have been seeing 'numbers' floating around about specific cases of contamination for a few of the citizens tested outside the nuclear evacuation zone. One, in particular I will quote here:
One person was measured to have an exposure of 18,000 counts per minute (cpm); another had a measurement of between 30,000 and 36,000 cpm; while a third evacuee had an exposure of 40,000 cpm. A fourth person initially gave a reading of over 100,000 cpm, but a second measurement taken after the person had removed their shoes was just under 40,000 cpm. Another five people were said to have "very small counts".
Numbers such as 18,000 and 40,000 counts per minute seem huge right? Well, unfortunately these numbers are completely meaningless. The only thing these numbers can be used for is to compare your 'counts per minute' to mine. This is a relative quantity and is dependent on the detector used, the voltage the detector is set at, the proximity with the person being evaluated, the normal background levels, the discriminator window or minimum threshold,.....oye and more!

First, you would need to tell me the type of detector used. Is it meant for photons, neutrons or charged particles?! Most likely, a portable counter is being used and that most likely means a geiger-muller counter. The only other that would make any sense to me is a proportional counter. Either way, if one unit of radiation enters into these detectors it causes either a full or partial dielectric breakdown (just like when you rub your socks on the carpet and touch a doorknob). This means that typically many many thousands of electrons move to create a signal that is detectable usually in the millivolt range. By design, neither of these detectors are EVER used for absolute counting.

I've conducted undergraduate labs with the same equipment, the same sources, the same detector settings, the same geometry and still get significantly different counts. This process is random and therefore almost always follows Poisson Statistics.

Any kind of exposure should be reported in Sevierts or rem. Different types of radiation interact with materials (including you) differently. These units take into account the difference from various types and energies of radiation (e.g. alpha, beta, gamma, neutron).

According to the EPA I get around 300 millirem per year. Now I'm a bit on the low side compared to most Americans which is around 620 millirem year. As I mentioned earlier this depends on if you smoke. If you smoke a pack a day, crank that up to about 8000 millirem per year! That's 160% the NRC regulatory limit for a trained radiation worker in one year!

Remember, just because we can detect radiation does not mean it is of any consequence. Hah, if you eat a banana a day you receive 3.6 mrem per year extra! If you like to lay out and get a tan, yeah that's right, you're getting a higher dose! What did you think sun burn was?! It's photons depositing their kinetic energy into your skin silly, i.e. ionizing radiation.

Without more information, I give no credit whatsoever to any claims of dangerous exposure.