|
| Home | Crystal | jmol | jPOWD | Chem | X Ray | Dana | Strunz | Properties | A to Z | Images | Share | News | Help | About |
Radioactivity in Minerals
Content
DefinitionRadioactivity in minerals are caused by the inclusion of naturally-occurring radioactive elements in the mineral's composition. The degree of radioactivity is dependent on the concentration and isotope present in the mineral. For the most part, minerals that contain potassium (K), uranium (U), and thorium (Th) are radioactive. This table lists all of the naturally-occurring radioactive isotopes. Mineral radioactivity is due to alpha, beta, and gamma radiation from the unstable isotopes in the composition. Alpha decay is due to the ejection of a helium nucleus (2 protons and 2 neutrons) from the parent isotope. This alpha particle is accompanied by gamma radiation and a daughter isotope which is two protons and two neutrons lighter than the parent isotope. Beta decay is due to the ejection of an electron from a neutron in the parent nucleus. This particle is accompanied by gamma radiation and a daughter isotope which is one proton heavier and one neutron lighter than the parent isotope. Electron Capture (EC) decay is very rare and is the result of the nucleus capturing one of the atom's orbital electrons. This decay is accompanied by gamma radiation and a daughter isotope which is one neutron heavier and one proton lighter than the parent isotope. Of the three main types of radioactive decay, gamma radiation causes the most damage because it has a greater effect on biological materials and is neutralized only by heavy shielding. The next most damaging type of radiation is beta particles which is absorbed by a few feet of air. The least damaging is alpha particles which has a range of 6 inches or less in air.
Caveats, Disclaimers, and WarningsRadioactive minerals emit alpha, beta and gamma radiation. The amount of that radiation is best measured with Geiger counters or scintillation counters. Warning: If you collect or possess radioactive minerals, you really should have a means of measuring radiation to determine if the extraordinary precautions are needed to store the mineral. If you don't have such equipment, inexpensive track-etch (Radon) detectors are a good way to determine if there is a danger. Get three detectors, place one in your mineral cabinet, the next in the room which houses the collection and the third as a blank to check on the laboratory that processes the detectors. The EPA limit on indoor radon exposure is 4 pCi/liter of air. Precautions for storing radioactive minerals are as follows:
The calculation of radioactivity in minerals is dependent on certain assumptions which affect the true activity of a mineral. The assumptions used in calculating mineral radioactivity are:
Calculation of ActivityActivity of a given amount of radioactive material found in nature is calculated as the decay constant l (related to the half-life T) multiplied by the number of radioactive nuclei. One kilogram of naturally-occurring element with X percentage of radioactive isotope with half-life T[sec] has activity R[Bq/kg]: where N = 6.023´1023 the Avogadro number times 1,000 gm/Isotope Mass times the % Abundance, ln 2 = 0.693, and T is the half-life in seconds. The unit of activity is Becquerel (1 Bq = 1 decay/sec) or Curie (1 Ci = 3.7´1010 Bq). kN is therefore, the activity of 1 kg of element with x percent of radioisotope in counts/sec. These values are multiplied by the chemical composition for each element in the empirical formula of the mineral to determine the theoretical radioactivity of that mineral. note: The origin of the Curie unit was based on the activity of 1 gram of 226Ra.
Gamma rays from all this activity pass through the mineral and experience successive Compton-scattering collisions with the mineral's atoms. Each collision reduces the the energy of the gamma ray until it is absorbed by the photo-electric effect. The rate of absorption varies with the electron density (re) of the mineral. Higher Z atoms (high electron density) have a greater adsorption factor than low Z atoms (low electron density). The end result from all this self-adsorption is the apparent radioactivity observed for the mineral specimen is much less than the absolute activities calculated by the rate equation. Electron density of the mineral is calculated as follows:
Photo-electric absorption effect is calculated from the following relationship:
Gamma Ray (GR) response for the mineral is estimated by taking the bulk activity calculation and correcting for absorption and density by comparing the gamma ray measurements derived from the geophysical log response (borehole geometry) from the American Petroleum Institute test facility in Houston, Texas:
Radiation Dose CalculationsDefinitions of Radiation Dose Measurements (see Natural Sources of Ionizing Radiation).
Radiation Dose Calculation is based on the GRapi response. Since the GRapi response is scaled to the borehole geometry where the detector is centered in a borehole with a 3 inch standoff, the estimate is based on the GRapi response of common clay minerals. The government estimated exposure to terrestrial sources is 30 mRem/year. If you hold a 1 kg sample, 1/2 the ratioactivity is absorbed in your hand and 1/2 radiates away in the other direction. Therefore using 200 GRapi as background of 15 mrem/year:
Tables of Radioactive Elements
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
