Dating and Disaster

Homeless people are usually up early – like at the break of dawn. This is nice for me because it gives me a chance to run all 5 dogs at the dogpark peacefully (the normal limit is 2 to avoid pack behavior or ganging up). The downside is that there are somewhere between 3 and 4 hours that have to be filled before public buildings like the library or museums open.

So, I had invested a buck in some sewing needles, and another buck in some thread. and I got to work repairing my pants – 2 of them were unwearable they were torn so badly. Car repair is not kind to pants – especially when you have to climb in and out from under the hood of a truck – things tend to get caught.

Ripped Not-so-favorite jeans (there was a hole in one pocket)

Ripped Favorite Sweats

Repaired Jeans and Sweats

I fixed everything as the dogs peacefully enjoyed the sun:

I finished just as the Bradbury Science Museum opened. Although I had already visited once, I wanted to see their exhibit on radiation in light of the nuclear disaster that had happened in Japan. I also thought that it was important to review how carbon-14 dating works, since so much of the dating of archeological evidence relies on this.

They had a seismograph at the museum, and showed the measurement of Japan’s earthquake.

According to this site, the maximum level of radiation leaked so far from the nuclear plant that has failed in Japan is 0.809 microsieverts per hour (1/10 of a dental x-ray).

So, they had a radiation detector there, and the first thing I did was check to see how radioactive I was. Thankfully, not very…

I then proceeded to check the 3 sources that they had at the museum.
An alpha source: Polonium-210

A beta source: Strontium-90

A gamma source: Cobalt-60

measuring each of them at the source, and at a distance of 2 feet to watch the decay.
Alpha: 0 distance: 0.044 mRem 4″: 0.039 8″:0.034 12″: 0.030 16″: 0.021
Beta: 0 distance: 4.590 mRem 4″: 0.068 8″:0.050 12″: 0.039 16″: 0.025
Gamma: 0 distance:2.305 mRem 4″: 0.062 8″:0.051 12″: 0.042 16″: 0.034

I plotted everything out using this plotting software. In retrospect, I should have taken more measurements between 0 and 4 inches. The graph looks linear there (only because there are no middle points), but it should in fact be exponential.
Ok. I went back for a 3rd visit and got more data between 0 and 4 inches. The alpha source was not very active, so I didn’t really bother with these measurements. Because Geiger detectors normally work by the detection of ionization events in a sealed gas chamber with a window, they are usually sensitive to gamma radiation (photons) and beta radiation (electrons). They don’t detect alpha particles very well, although many alpha radiation sources also emit gamma radiation. Note that I used a different unit to report the values with (counts per minute, vs. mrem).

(Note: these values were corrected during a 2nd visit with a tape measure, and obviously replaced sources.)

The larger the radioactive particle emitted, the less penetrating the radiation. Alpha particles are small nuclei, beta particles are electrons, and gamma particles are photons. So the most penetrating radiation is the gamma particle. For more about atomic structure, visit this cool site that shows how particles work.

They had a nice little chart showing the relative exposure levels from different sources. Cigarette smoking is apparently correlated with a relatively level of exposure.

This site suggests that all 3 types are being emitted from the Japanese plant. This site explains the problem they are trying to avoid.

Carbon-14 is found in our bodies, and it is a weak beta emitter with a half-life of 5730 years. Carbon-14 is created in the atmosphere by the bombardment of nitrogen with cosmic rays where it forms carbon dioxide molecules with a carbon-14 atom. These molecules are evenly spread throughout the atmosphere, and are taken up by plants in photosynthesis. Since all living organisms ultimately derive their carbon from this, there is a constant ratio of carbon-14 in living matter (1 part per trillion parts normal carbon). For the rate equation for decay, see this article.

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