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Posts Tagged ‘radioactivity’

I’ll start with a limerick:

The Becquerel has me morose;

These units I can’t diagnose.

The rads and the Grays

Don’t measure decays—

But what of equivalent dose?

becquerel_postcard

1 Becquerel = 1 decay/s

There are at least seven units of radioactivity floating around out there, measuring at least three different kinds of things; a veritable zoo of scientific terms. Unfortunately, most people don’t know a rad from a Gray from a Becquerel. Here, then, is my attempt to sort out the confusion.

You’re welcome.

First, let me just say that most people (to my dismay) equate the terms “radioactivity” and “radiation”. There’s some disagreement on the meanings of these terms; I find myself in the conservative camp on this issue. To me, “radioactivity” refers to junk flying out of an unstable nucleus: alpha particles, gamma rays, and the like. “Radiation”, on the other hand, refers exclusively to electromagnetic radiation (anything from long-wavelength radio waves to ultra-short-wavelength gamma rays). By my fuddy-duddy standards, “radiation” is just light; it may or may not be biologically dangerous. Radiation is just one of the possible kinds of radioactivity.

Unfortunately, through the inevitable process of “language creep” (the same process by which the original four “collie” birds became four “calling” birds in the Twelve Days of Christmas, because people are just ignorant) the term “radiation” has come to encompass any ionizing junk from a nucleus.  So some people now call alpha particles and beta particles “particle radiation” to distinguish them from gamma rays, which is “electromagnetic radiation”. This usage bothers me, but I’ll get over it. Just note that I won’t use this terminology here.

So: unstable nuclei exist. They occasionally spit out things—a phenomenon I call radioactivity. These things can often knock electrons free from atoms (i.e. they can ionize atoms). Such ionization events can be detected by a Geiger-Müller tube (among other devices).

Activity. The first way to measure radioactivity is to measure these ionization events in a given amount of time, which in turn tells you how often decays are occurring. So we measure R, the “activity” of a nuclear sample. The metric system unit of activity is the Becquerel (Bq), which is defined to be one decay/second. (Note that 1 Bq is essentially equivalent to 1 Hz = 1 s–1.)

Unfortunately, the Becquerel is a small unit—if we’re talking about radioisotopes used in medicine, for example, we might have to speak of billions of Becquerels. So there’s another unit of activity: the Curie (Ci). One Curie is defined to be the activity of 1 gram of 226Ra. If you want to convert, 1 Ci = 3.7 x 1010 Bq.

There is a problem with measuring activity: it doesn’t really tell you how dangerous a particular sample is. Not all radioactivity particles are the same. Getting hit with millions of weak particles might be preferable to being hit by only a few high velocity ones. One bullet is more dangerous than 500 rapidly-fired marshmallows.

Absorbed dose. To get a feel for the dangerousness of a sample, we talk about absorbed dose: a measure of energy absorbed per kilogram of target material. In metric units, the applicable unit is the Gray (Gy): 1 Gy = 1 Joule/kg. Other people use the rad, with the conversion 1 rad = 1 erg/g = 0.01 Gy. Use of the rad is discouraged by the international scientific community but is still common in (surprise surprise!) the United States.

There’s still a problem. Suppose I’m exposed to 1 Gy of radioactivity (meaning that I expect to absorb a joule of energy per kilogram of my mass). It matters whether I’m absorbing beta particles (say) or alpha particles, because the damage done by alpha particles is worse, pound-for-pound. That is, different kinds of radioactivity are more or less dangerous, depending on the predilection of the given particle(s) for causing genetic damage and possibly causing cancer. This leads us to introduce…

Equivalent dose. Equivalent dose is basically just absorbed dose, times a “fudge factor” that depends upon the kind of radioactivity involved. The unit we use is the Sievert (Sv) = 1 J/kg (weighted). X-rays, gamma rays, and beta particles are all in a sense “equally” dangerous and have a weight factor of 1. So for those kinds of particles, 1 Gy → 1 Sv. Alpha particles, though, are around 20 times as “dangerous”, so if we’re dealing with alpha particles then 1 Gy → 20 Sv.

Of course Americans are contrary when it comes to units, and so the rem is still in common use; 1 rem = 100 erg/g (weighted) = 0.01 Sv. If you’re a science writer, you’d be best served by eliminating rad’s and rem’s altogether; why perpetuate archaic units? You don’t use furlongs/fortnight to measure speed, do you?

I can’t help but mention a seventh unit of radioactivity: the Banana Equivalent Dose, or BED; 1 BED = 0.1 μSv, and so represents an equivalent dose. It (roughly) equates to the amount of radioactive exposure you would get if you ate a banana. (Bananas are naturally radioactive, as they contain significant amount of radioactive potassium, 40K.) This kind of unit helps people put the hobgoblin of “radioactivity” into perspective. “Oh my God! The nuclear plant let off some radioactive steam! Am I doomed?” “Well, your exposure was about 10 BED’s. So basically eat 10 bananas for the same effect.” (There are some issues with the BED as a unit; see this for more information.)

tumblr_ndk1altiWI1r3kmkso3_400

In summary:

Unit                                                     Symbol            Note

Activity

Becquerel: one decay/s                     Bq                   Same as 1 Hz

Curie: activity of 1g of 226Ra              Ci                    Not SI unit, 1 Ci = 3.7×1010 Bq

Absorbed dose

Gray: 1 J/kg                                         Gy

rad: 100 erg/g                                     rad                  Not SI unit; 1 rad = 0.01 Gy

Equivalent dose

Sievert: 1 J/kg                                     Sv

rem: 100 erg/g rem                             rem                 Not SI unit; 1 rem = 0.01 Sv

Banana equivalent dose                      BED                Not SI unit; 1 BED = 0.1 mSv

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blinky

Blinky, a denizen of Springfield

From a recent blog post, high on hyperbole and low on science:

[Fukushima: At the Very Least, Your Days of Eating Pacific Ocean Fish Are Over  We’re writing this post becasue [sic] we feel that it is extremely important for everyone to be aware of this crisis, and it’s not being sufficiently reported on. In our entire month in the US, we did not see this in the mainstream media and hardly anyone knew about it…In a nutshell, Japan’s nuclear watchdog has now declared the leak of radioactive water from Fukushima a “state of emergency.” Each day, 300 tons of radioactive water seeps into the ocean…]

Ok.  Time out.  We need a reality check.  First of all, what’s “radioactive water”?  If I take a Geiger-Müller tube and wave it over any seawater, I will detect radioactivity.  I will also detect radioactivity in any banana (from 40K), and in my dog Banjo (from 14C).

Banjo

Banjo

In fact, just going outside in the open air I will detect radioactivity; in fact, over the course of a day I will absorb something like 10 μSv.  Why?  Because radioactivity is everywhere.  Radioactive isotopes aren’t like some nefarious pixie dust that is sprinkled here and there, giving us rare cancers and making us sprout third eyes.  Radioactive isotopes are everywhere.  So when you say the water is “radioactive” you’d better define what that means.

I assume that the author is just parroting what he or she read in some other blog, and doesn’t even know what “radioactive water” means (much less understand the difference between units such as Bequerels, Sieverts, Curies, Grays, rads, rems, and BED’s).  But let’s try to understand: what might “radioactive water” actually mean?  Some frustrated Google searching yielded little real science, until I stumbled upon a National Geographic article.  “…radiation levels in its groundwater observation hole on the east side of the turbine buildings had reached 310 becquerels per liter for cesium-134 and 650 becquerels per liter for cesium-137.”  Finally something we can work with!  I don’t know if this contaminated groundwater is leaking directly into the ocean, but let’s assume that it is…we might as well give in to some of the hyperbole.

650 Bq.  Is that a lot?  Well, it’s hard to say; Bq is a unit of activity (meaning decays per second) but it doesn’t tell you how much energy a person would be exposed to (for that, use the Gray; 1 Gy = 1 Joule/kg).  It also doesn’t take into account the type of radioactivity in question and how such radioactivity affects biological tissue (for that, use the Sievert, which includes a human-centric biological fudge factor).  But, as a physicist I have the right to make an educated guess, and (conservatively) say that drinking a liter of the water contaminated with 137Cs represents an exposure approximately equivalent to the EPA’s recommended limit on exposure for one year; that is, 1 mSv (one millisievert).

(It’s funny: the EPA recommends no more than 1 mSv per year, but the average person is exposed to 4 mSv per year, mostly from the air around us.  I suppose they mean to warn us not to expose ourselves to more than 1 mSv/yr beyond the 4 mSv/yr we normally get…but I digress.)

Don’t get me wrong; it’s quite a bit of radioactivity all at once, like getting 50 chest x-rays all at the same time.  But it’s still only half the dose you’d get if you got a single head CT scan.

And to get that dose, you’d have to drink the water.

But still.  The water’s “radioactive”, right?  And 300 tons of the stuff are being pumped into the Pacific every single day!

Here’s another good opportunity to practice unit conversion.  Go get a pencil.  I’ll wait.  Ready?  300 tons of water is 272,155 kg.  So that water has a volume of 272.155 m3, which is 272,155 liters.  OK so far?

272,155 x 1 mSv = 272 Sv.  A fatal dose is around 8 Sv, so this is a lot.  But you’d have to drink all 300 tons of water.

And the Pacific ocean is kinda big: maybe 6.4 x 1020 kg, or a volume of 6.4 x 1020 liters.  Imagine: each day, 300 tons of “radioactive water” enter the Pacific; but this water gets diluted (surely, it has mixed thoroughly before reaching California?)  272,155 kg / 6.4 x 1020 kg is a very, very, very small number: 4.25 x 10-16 .

This means that your initial dangerous level of radioactivity, 272 Sv, is diluted by a factor of 4.25 x 10-16, giving you 1.16 x 10-13 Sv, per day.  That is, if you drank 300 tons of water on the California coast.  If you’re numerically challenged, here’s a hint as to what this means.  1.16 x 10-13 is basically zero.  Go ahead.  Drink those 300 tons.

The hyperbolic blog continues:

[The contamination has made it’s [sic] way to the USA. A Stanford University study…]

Wait…what study?  Citation please!  Otherwise you’re just making stuff up!

[…just showed that every bluefin tuna tested in the waters off California has shown to be contaminated with radiation that originated in Fukushima. Every single one. Our FDA assures us that our food supply is safe. They LIE. Don’t trust the government testing. They are covering up the magnitude of this situation. The only safe level is zero.]

This is breathtaking.  For one thing, even if I take a Geiger counter and detect radioactivity from a fish—which I don’t doubt in the slightest; fish contain 14C, after all—how could I possibly know that the individual radioactivity events “came” from Japan?  And that final sentence…“The only safe level is zero.”  Wow.  Just, wow.  Doesn’t the author know that a “zero level of radioactivity” does not exist on this Earth?  Should we give up breathing, and eating bananas, and having basements, and walking out into the open air?

Imagine Frankenstein’s monster saying “Fire bad!”  Now imagine sciencephobes saying “Radioactivity bad!”  It amounts to the same thing.  People don’t like what they don’t understand.  And there are too many science illiterates in the world.

Here’s one more gem:

[In the wake of Fukushima, The White House has given final approval for dramatically raising permissible radioactive levels in drinking water and soil. The EPA says the new levels are within the “safe” range, but they keep moving those safe levels higher as things unfold. In soil, the PAGs allow long-term public exposure to radiation in amounts as high as 2,000 millirems. Welcome to the new normal.]

2000 millrems = 20 mSv, which is equivalent to getting 3 chest CT scans, but spread out over a whole year (which is a good thing).  This is still less than the maximum permitted yearly dose for radiation workers, which is 50 mSv.

Here’s a summary of what I’m saying.  Fukushima was a disaster, sure.  But no one in America should worry in the slightest.  You get way, way, way more radiation exposure from the person you’re sleeping next to, than you do from some water in the western Pacific.

Don’t blame the blog author.  I mean, the blog is called “Sprinter Life”.  Enough said.

[For help with radioactivity units, see this excellent graphic.]

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