Fukushima Fukushima radiation detection and measurement

[Bob S]

The best calibrated radiation dose rate (rad dose) instruments in my opinion are air or other gas ionization chambers. The best instruments for isotope identification are sodium iodide (with photomultiplier) or solid state diode detectors with pulse height analyzers (PHA) to identify gamma energies. The most common calibration isotope is probably Cs¹³⁷ with the 662 KeV gamma peak. This can be used for both dose rate and PHA calibration. Neutrons (especially pulsed neutrons) require special equipment.

There are special federal background level radiation air and fallout measuring stations (along the west coast of USA) that are constantly measuring the radiation background. The operators of this equipment are probably radiation physicists.

The dose limits (rems and Sieverts) are fixed, and set in 10 CFR 20 (Federal Code of Regulations). I think the radiation worker is 5 rads (rems) max per year, and 500 millirads max per year for general public (excluding medical and dental x-rays). Natural background is 100 to 300 millirads per year.

The failure of radiation monitoring equipment, if any, in Japan was due to operator error.

I believe that all qualified operators of radiation monitoring equipment should know the difference between Roentgens and rads, and be able to derive the numerical relationship.

Bob S

 

[vanesch]

Here is a back-of-the-envelope calculation of the radiation level required to cause immediate skin burns.

The specific heat of tissue is about 4 joules per gram-degree C. So it would require about 80 joules/gram to raise the skin temperature 20 deg. C (like spilling boiling water on skin).

Because the definition of a Sievert is 1 joule of energy deposition per kilogram, we have

20 deg C temp rise = 80 joules per gram = 80,000 joules per kilogram = 80,000 Sieverts.

This sounds like a lot. For comparison, I know (from personal experience) that I could not feel 42 doses of 1.8 Sieverts (per session) of focused gamma radiation for prostate cancer treatment last year.

Just a (very late) comment on this: the skin burns by radiation don't come from THERMAL heating of the tissues, which only play a role, as you calculate, at crazily high doses where thermal heating is actually the last of your worries. The "burning" actually means tissue destruction by the ionising effect of the radiation, which results in major chemical damage to the cells to a point where they are actually destroyed (their proteines are disrupted, and the membrane is broken). You can even have an effect at lower doses, where the cell's biological function has been destroyed, and will soon die off as it has no correct selfsustaining metabolism anymore.

 

[Bob S]

Just a (very late) comment on this: the skin burns by radiation don't come from THERMAL heating of the tissues, which only play a role, as you calculate, at crazily high doses where thermal heating is actually the last of your worries. The "burning" actually means tissue destruction by the ionising effect of the radiation, which results in major chemical damage to the cells to a point where they are actually destroyed (their proteines are disrupted, and the membrane is broken). You can even have an effect at lower doses, where the cell's biological function has been destroyed, and will soon die off as it has no correct selfsustaining metabolism anymore.

I understand and completely agree with your comments. Please note that I said immediate skin burns, like boiling water or hot torch applied to the skin. I was using this calculation to show that the thermal heating of my 42 prostate radiation treatments of 1.8 Sieverts each was orders of magnitude below the detectable thermal level. In fact, post irradiation blood tests can detect whole body doses below 500 milliSieverts. Whole body doses of 3 to 4 Sieverts is 50% mortality.

Bob S