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Question about a calculation of dose / exempt sources



Hopefully someone here can help me with my question. I'm living in switzerland and i'm working with radiation at a hobbyist level (gamma spectroscopy etc). Recently I requested a list of "exempt levels" from the EDI (basically equal to the U.S. NRC).

I noticed something really strange in the list what i don't understand. I send a question back, but did not get a reply (its understandable that a federal agency dont want to answer hobbyist questions  :) ). Maybe here someone can help me with this.

The exempt quanties list contains all isotopes and the activity in bequerel that do not require anykind of permit.

The limit is set as following:

Licensing limit for daily handling. The values for the licensing limits are derived from Column 4 since inhalation is the main risk when radionuclides are handled in the laboratory. The inhalation of an activity LA on a single occasion yields a committed effective dose of 5 mSv. In some cases, the value derived for LA is lower than the value for LE, which is not consistent. In such cases, the LA value has been replaced by the LE value [5].

So i picked two isotopes for example:

Cs-137 - Exempt Activity:  700'000 Bq = 18.9 uCi
Po-210 - Exempt Activity:  2000 Bq = 54 nCi

I calculated the yearly dose for the Cs-137 dose for 182 uSv / yr
and for the Po-210 dose for 24uSv / yr

If i would inhale the source, it would have to stay for the following time inside my body to get the 5mSv:  Cs-137 for ~27 yrs, and the Po-210 for ~208 yrs !! But Po-210 has only a half-life of 138days

Does anybody know where my calculation is wrong?? Or if its correct, why is the is the activity of Po-210 limited to such a few quanties?

Interesting is the fact that all alpha / fission / electron capture isotopes are limited to very few quanties.

I know that about 20 years ago, there were smoke detectors with Am-241 in use, these contained aournd 20uCi of Am-241, according to the current exempt list: Am-241 200 Bq = 5nCi! so the posession of a old smoke detector is literally illegal here! Interesting

Po-210 is an Alpha emitter, which does 20x more damage when inside your body than gamma.  Co-60 is a gamma emitter.  When assigning dose, and calculating limits, ONE of the things they consider is the type of radiation.

I haven't checked your math, but assuming you were correct, the Po-210 would give you a dose of 480 uSv/yr.

Additionally, they also take into account other limits, such as the Toxicity.  The limit of Pu, for example, is based more heavily on the toxicity than the radioactivity.

Also, the shorter half live you mentioned, also means that MORE energy is emitted when it decays, which means it does more damage.  The candle that burns brighter, burns hotter, and burns out faster.

So, Po-210 is more hazardous, and has a lower Bq limit, because of other hazards, not just the radiation aspect.

Po-210 is very poisonous (toxic).  It's much less a hazard in a sealed source.

Hi Rennhack,

Thanks for your answer, i was afraid that it has to do with alpha emssion. I looked all alpha emitting isotpes in my list up, and they are all way below 1uCi. So i won't get an alpha source for experimentation. To the regulation it won't matter if the isotope comes in the raw form or in a sealed source, only the containing activitiy is relevant :(


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