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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.



Radiation in Our World

Radiation has always been a part of the natural environment in the form of cosmic radiation, cosmogenic radionuclides [carbon-14 (14C), Beryllium-7 (7Be), and tritium (3H)], and naturally occurring radionuclides, such as potassium-40 (40K), and the thorium, uranium, and actinium series radionuclides which have very long half lives. Additionally, human-made radionuclides were distributed throughout the world beginning in the early 1940s. Atmospheric testing of nuclear weapons from 1945 through 1980 and nuclear power plant accidents, such as the Chernobyl accident in the former Soviet Union during 1986, have resulted in fallout of detectable radionuclides around the world. This natural and manmade global fallout radioactivity is referred to as background radiation. MORE

Radiation Exposure and Dose

The primary concern regarding radioactivity is the amount of energy deposited by particles or gamma radiation to the surrounding environment. It is possible that the energy from radiation may damage living tissue. When radiation interacts with the atoms of a given substance, it can alter the number of electrons associated with those atoms (usually removing orbital electrons). This is called ionization. MORE