The eight units of radioactivity

I’m reading an interesting-yet-frustrating book about Chernobyl called “Wormwood Forest: A Natural History of Chernobyl” by Mary Mycio. It is interesting simply because I didn’t know much about what happened at Chernobyl immediately after the disaster and in the two decades since.
The frustrating thing is that the narrative style is disjointed and rambling. Fascinating tidbits are sprinkled amongst sort of boring day-to-day info about her travels there. Also frustrating is her random use of units.
The units aren’t really her fault — just as with temperature and length measurements, there are “common” and “system international” (si, or metric) units for radioactivity, too. But to further confuse the issue, radioactivity is divided into four different categories : Radioactivity, Absorbed Dose, Dose Equivalent, and Exposure.

  Radioactivity Absorbed Dose Dose Equivalent Exposure
Common Units curie (Ci) rad rem roentgen (R)
SI Units becquerel (Bq) gray (Gy) sievert (Sv) coulomb/kilogram (C/kg)

The first unit, Radiation, is simply a measure of how many atoms spontaneously disintegrate in a unit of time. As you would expect from SI units, the becquerel is exceptionally straightforward at one disintegration per second. The curie is 37 billion becquerels. Why? That is how much radiation is in one gram of the element radium.
The rad and grey are both a measure of how much radiation is absorbed by the tissue of a person. Again, SI rules the day with a simple definition of a grey as one Joule of energy absorbed per kilogram. The older rad unit is 0.01 grey. If you’re good on SI prefixes, you’d realize that a rad is a centigrey (not to be confused with the old name for the Celcius temperature unit, centigrade).
Next up : Dose Equivalent. These units attempt to assess how much damage the radiation would do to living tissue based on the type of radiation. The effectiveness of radiation in damaging tissue is known as its relative biological effectiveness (r.b.e.). Alpha particles have a higher ability to damage tissue than X-Rays or gamma rays, so it may have an r.b.e. of up to 20 whereas the r.b.e for x-ray and gamma radiation is 1. The seivert is the SI unit for dose equivalent, and it equal to the greys times the r.b.e. The rem is the common unit for dose equivalent, and it is equal to the rads times the r.b.e. So 1 seivert is 100 rems.
So while this is confusing, imaging how frustrating it is to understand Mary Mycio’s book when she bandies about becquerels and curies in the same paragraph. It is like saying, “It was so hot in the desert today — it got up to 40 degrees C, but it was a mild 70 degrees F here in Minneapolis.” Not so intuitive.

Tune your glasses with salt?

My friend, Chris, mentioned an interesting scientific observation. We were stting on his front porch for the bimonthly knitting club. I was working on oboe reeds, and everyone else was mostly playing with the dog, Chloe. Chris had bacon and peanut-butter flavored bubbles that he would blow into the air and the dog would chase them. He related a scientific observation he had made earlier.
Chris had been mixing up some salt water in a glass. The spoon occasionally hits the side of the glass, and it makes a ringing sound. At first the pitch of the rining dropped. Then, as the salt dissolved, the pitch of the ringing sound went higher. He said he could tell when it had all dissolved because the pitch stopped going up.
Why does this happen?
The pitch that you hear is a function of resonance inside the glass. Let’s hypothesize that the pitch is due to sound waves going back and forth through the water. The speed of sound in tap water at room temperature is about 1500 meters per second. If we assume that the glass Chris used was about 0.05 meters across, we would expect to have a resonance frequence of about 30000 Hertz.
HUMMM, That is above the normal range of human hearing. I guess that the sound produced by hitting a glass with a spoon is not simply a function of the resonance of the glass. Indeed, if it were, you would expect that an empty glass (one filled with air, in which sound travels at 340.29 meters per second) would have a much lower pitch. In fact, an empty glass has a higher pitch when struck with a spoon.
I believe that what we’re observing is the same phenomena that allows a piano or a string instrument to produce lower pitches on strings of the same (or similar) length to those that produce pitches in higher registers. The low strings are heavier! The relationship shows that frequency (i.e. pitch) is inversely proportional to the density (weight) of the string.
The same rule applies to the ringing of the glass. The water in the glass impedes the motion of the glass in the same way that extra weight impedes the motion of the string. The more water the more weight. Thus the more water weight, the lower the pitch.
I believe the observation that the pitch changes as the salt dissolves is untrue has nothing to do with the salt. While experimenting with salt, water, and a glass just now, I observed that by far the largest change in pitch is due to the shape of the water in the glass when it rings. Stirring the water causes the water to rise up along the sides of the glass, effectively raising the mass of more of the it. This causes the pitch to drop. As you stop stirring as vigorously, the water settles down and the pitch rises. I could not detect any difference in pitch between a glass filled with tap water, and one filled to the same level with salt water.