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TOPIC: I Hate Becquerels!

I Hate Becquerels! 27 Oct 2011 15:13 #69


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This week we’ve heard that the reactors at Fukushima released more radionuclides than was previously reported – eleventy hundred billion gazillions of becquerels!

I hate becquerels because they always are big numbers and frequently are enormous numbers. I also hate them because they are not easily translated into health risk. And I hate them because they have a name that replaced a clear description of what they are.

Radiation-measuring organizations like them, and they are used for regulations because they are easy to measure. One becquerel is one count per second. Click. Click. Click. Easily read on the meter.

Each count is one radionuclide atom breaking down. If you have a pile of radioactive atoms, they don’t all break down at once. It’s probabilistic: you can measure that half of them will break down in a certain amount of time, called the half-life.

So becquerels are an indication of how much radionuclide is there. But that’s not a simple calculation: it involves the half-life and moles and gram-atoms and atomic weight. So it’s not immediately obvious from the number of becquerels how much stuff is there.

Becquerels count atoms. It takes lots of atoms – eleventy hundred billion gazillion – to make up a bit of matter. Let’s get more precise. Chemists everywhere just celebrated Mole Day: 6.02 am on 10/23. That’s a nerd joke. :huh: A mole is 6.02 x 1023 molecules; if you’re talking about atoms, that number is called a gram-atom. So why would chemists come up with such a weird number, one followed by 23 zeroes? It’s as bad as the numbers you see for becquerels.

Chemical calculations are simpler if they are based on numbers of atoms or molecules. But mass is more easily measured. The mole connects numbers of atoms or molecules to their mass.

The atomic masses of the elements can be found on a periodic chart. When atoms are combined into molecules, the atomic masses are added to give the molecular mass. So water, H2O, is made up of two hydrogens, atomic mass 1, and one oxygen, atomic mass 16, for a molecular mass of 18. The molecular mass expressed in grams is a mole, and, it turns out, a mole has 6.02 x 1023 molecules.

Eighteen grams of water (density one gram per cubic centimeter) is about a tablespoon. Cesium radionuclides are one of the big concerns from Fukushima; cesium chloride (CsCl) is a crystalline solid, like table salt. The atomic mass of cesium is 55, that of chlorine 17, for a molecular mass of 72. The density of CsCl is 3.99 grams per cubic centimeter, so a mole of CsCl has a volume of 18 cubic centimeters, just about like water. Molar volumes are not all the same, though; this is a coincidence.

That tablespoon of water or CsCl contains 6.02 x 1023 molecules. So if some of them are unstable radionuclides, it wouldn’t be too surprising to see thousands (103) or millions (106) of becquerels coming from them.

Do you find those numbers as useless in news articles as I do?
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Re: I Hate Becquerels! 03 Nov 2011 21:58 #92


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Specific activity, which you can look up in a table, can greatly simplify some of these calculations.

So, becquerel means count per second. Why is a hertz a cycle per second? Why is a newton a kilogram-meter per second? We have some superfluous units. It's no big deal.

If you want convenience, then you need to express levels of activity in terms of something that is more relevant, say the activity of the tritium in one of those old exit signs. The SI units are designed to be consistent and easy to scale. They work very well if you are using a digital calculator or a computer. They are not necessarily the most intuitive units available.

Imperial units work much better on a human scale. Often they are easier to understand at this scale -- e.g., a foot is roughly the size of a foot, a cup is roughly the size of a cup -- and they work much better when calculating with fractions. The liquid measure units are entirely based on fractions. A foot can be divided by 2, 3, 4, and 6 to yield an integer number of inches. A mile can be divided by 2, 3, 4, 5, 6, 8, and 10 to yield an integer number of feet. Try that with a kilometer.
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Re: I Hate Becquerels! 15 Nov 2011 17:24 #119


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Hi Brian - Thanks for your comment.

I think you're not quite getting my objections, though.

One of the things I'd like to know when I read the stories about radiation is what the danger actually is. Becquerels are almost always big numbers, so that looks scary. Or I've seen a few stories lately in which the radionuclide loading is measured in fractions of a becquerel per cubic meter. Because one becquerel is a tiny measurement, those big numbers aren't as dangerous as they seem. And fractions of a becquerel per almost aren't there! But that's hardly ever explained.

And, even when there's that kind of context, becquerels tell you how much material is there and its emissions, but not how that translates into health risk. I enumerated the chain from becquerel to cancer risk - nothing easily understandable there!

What danger does the activity of tritiium in exit signs present? Presumably you're arguing that it's minimal or nonexistent. The activity of potassium-40 in bananas has also been mentioned, perhaps more understandable because we all eat bananas.

If the news stories said "counts per second" instead of becquerels, that would be a little easier to understand, but it still wouldn't give a sense of the danger involved. But people might get more of a sense that something real is involved. I have a personal thing about what might be called commemorative names, as opposed to descriptive names. It's nice to honor Henri Becquerel or Heinrich Hertz, or Isaac Newton, but I think it's nicer to make obvious what you're talking about.

It's not convenience I want, but comprehensibility.
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Re: I Hate Becquerels! 07 Dec 2011 16:41 #178


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heres a story about dry milk in japan:

"Tests conducted on Dec. 3 and 4 found Cesium-134 at levels as high as 15.2 becquerels per kilogram, while cesium-137 reached 16.5 Bq/kg, according to Meiji. A becquerel is a measure of radioactivity. The maximum permissible level for milk and dairy products for infants is 200 Bq/kg, the company said."

www.bloomberg.com/news/2011-12-06/radioa...all-shares-fall.html
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Re: I Hate Becquerels! 07 Dec 2011 18:06 #179


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Eh ... a typical beer contains over 14 Bq/kg of radioactive material in it.
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Re: I Hate Becquerels! 07 Dec 2011 19:10 #180


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dorkmo - you've got the relevant information:
The maximum permissible level for milk and dairy products for infants is 200 Bq/kg

But the problem I noted persists: the media don't understand what becquerels are, and they seem to think that any radioactivity must be bad. Brian's right: pretty much everything we eat is radioactive. :ohmy:
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