Still - not anywhere close to what I want - but better'n nuttin at all.
Nature's Radioactive Material
Andrew Lange
March 19, 2011
Submitted as coursework for Physics 241, Stanford University, Winter 2011
http://large.stanford.edu/courses/2011/ph241/lange1/
"Natural" Radioactive Isotopes
We know of three hundred and thirty nine naturally occurring isotopes. [1] The majority of them are theoretically unstable, but decay slowly enough that their decay processes are undetectable. A primordial isotope is one that has existed in it's present state on Earth for the lifetime of the earth. Of these, the decay processes of Uranium and Thorium contribute the most to the background radiation spectrum (by a substantial margin). [2]
Isotope Concentrations in Everyday Life
Three isotopes dominate the background ionizing radiation spectrum. Potassium-40 has the highest concentrations because it is ubiquitous in biological materials, while Uranium and Thorium are found primarily in rocks and soil (particularly igneous rocks like granite [3]). In fact, most of the radiation to which we are exposed comes from the accumulated Potassium in our bodies and concentrations of these three isotopes in the soil.
In 1950, E.C. Anderson and W.H. Langham conducted an experiment that showed that the amount of potassium in the body spiked around age 10 for women and age 20 for men. This data indicates that the average concentration for both sexes over all ages is approximately 2.0 g/kg. [4]. Potassium emits about 31.2 beta rays per gram per second and 3.6 gamma rays per gram per second. [5] Therefore a person weighing 180 lbs (82 kg) is exposed to 5117 beta rays per second and 590 gamma rays per second from the potassium in their bodies. Because soil is a major source of gamma radiation, there has been an international effort to survey the gamma ray spectra of soil in various parts of the world. The UN Scientific Committee released a report in 2000 which gives the average activity of Uranium, Thorium, and Potassium in Soil samples. These are 40 Bq/kg, 40 Bq/kg, and 370 Bq/kg respectively. [6] If we want to consider the amount of radiation from the soil to which we're exposed, we have to first halve the total activity because half of the radiation from a flat layer of soil radiates downward. This gives us 225 Bq/kg. Next, we assume that only radiation from soil within an area of 1 square ft of our feet reaches us. If we only consider the first half-foot of soil this corresponds to about 50 lbs(25kg). Therefore, we are exposed to about 5625 Bq or 5625 gamma rays per second from average soil sources. From our bodies and the soil at our feet alone (excluding cosmic rays which contribute significantly less) the average background level consists of about 6215 gamma rays per second and 5117 beta rays per second. To put this in perspective, 1 curie is the number of decays per second of on gram of radium and is equal to 37 billion. This stuff was played with daily by the early explorers of nuclear energy.
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