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Scientific
Notation Scientific notation is used to express numbers that are very small or very large. A very small number will be expressed with a negative exponent, e.g., 1.2 x 10-6. To convert this number to the more commonly used form, the decimal point must be moved left by the number of places equal to the exponent (in this case, six). Thus the number 1.2 x 10-6 is equal to 0.0000012. A large number will be expressed with a positive exponent, e.g. 1.2 x 106. To convert this number, the decimal point must be moved right by the number of places equal to the exponent. For example, the number 1.2 x 106 is equal to 1,200,000. Concentrations
of Radioactivity The amount of radioactivity in a substance of interest is described by its concentration. The concentration is the amount of radioactivity per unit volume or weight of that substance. Air, milk, and atmospheric moisture samples are expressed as activity per milliliter (mL). Concentrations in surface water, drinking water, and precipitation samples are expressed as activity per liter (L). Radioactivity in foodstuff and soil are expressed as activity per gram (g). Exposure, as measured by environmental dosimeters, is expressed in units of milliroentgens (mR). This is sometimes expressed in terms of dose as millirem (mrem) or microseiverts (µSv). Gross
versus Specific Analyses Some analyses are designed to detect specific radionuclides (specific analyses) while other analyses are designed to measure radiation of a particular type that can be from a large number of sources (gross analyses). Gamma-emitting radionuclides are determined by a specific analytical technique called gamma spectroscopy, for example. This specific analysis can show whether the radiation was produced from a natural source, or from a human made source. Analyses for specific alpha and beta emitting radionuclides, on the other hand, require more difficult and expensive radiochemical analyses. Low cost, but very sensitive, gross measurements are often substituted for the more expensive specific analyses as a screening procedure. The gross analyses are generally made first to determine the total amount of radioactivity that is present. The more expensive specific analyses of beta and alpha-emitting radionuclides are only made if the gross measurements are above background levels. When gross beta or gross alpha measurements are made, they measure all beta-activity or all alpha activity from all sources, including those that occur naturally and those that are manmade. There is no distinction between which beta-emitting or alpha-emitting radionuclides are present, just how much beta or alpha activity is present. Gross measurements are used as a method to screen samples for relative levels of radioactivity. Detecting
Radioactivity All
measurements have uncertainties. Uncertainty
associated with measurements of radioactivity arises from many sources
including: variations in
detection equipment and the number of particles/energy that actually
strike the detector, analysis procedures, natural background radiation,
the random nature of radioactive decay and variances in the distribution
of the targeted compound in the media being analyzed.
The level of uncertainty from many of these sources is reported
with each radioactive analysis presented here.
If the number of radioactive disintegrations from one sample is
counted multiple times, each for the same duration, that number will
vary around an average value. Background
radiation makes this true even for a sample that has no radioactivity.
If a sample containing no radioactivity was analyzed multiple
times, the net result should vary around an average of zero after
correction for background radiation (Figure 4).
Therefore, samples with radioactivity levels very close to zero
will have negative values approximately 50% of the time. In order to avoid censoring data, these negative values,
rather than “not detectable” or “zero,” are reported for
radionuclides of interest. This
provides more information than merely truncating to the detection limits
for results near background activities and allows for statistical
analyses and measures of trends in the data. Figure 4. Expected frequency distribution for a sample with no radioactivity. If a sample containing no radioactivity was analyzed multiple times, a distribution of net values with an average of zero would result. Samples with radioactivity levels very close to zero are expected to have net results that are negative values approximately 50% of the time after background is subtracted. |
