Radioactivity in Minerals

Content

1. Definition of Radioactive Minerals
2. Table of Naturally Occurring Radioactive Isotopes
3. Caveats, Disclaimers, and Warnings
1. Precautions for storing radioactive minerals
2. Caveats on the Calculation of radioactivity in minerals
4. Calculation of Radioactive Activity
1. Electron Density
2. Photo-Electric Effect
3. Gamma Ray (API Units)
5. Radiation Dose Estimation
6. Tables of  Radioactive Isotope Activities
   1. Table 1A (²³⁸U) Uranium Decay Series 
   2. Table 1B (²³⁵U) Actinium Decay Series
   3. Table 2 (²³²Th) Thorium Decay Series
   4. Table 3 All other Significant Isotopes

Definition

Radioactivity 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 Warnings

Radioactive 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:

1. Handle the specimen as little as possible. A good strategy is to place the specimen in a transparent container which is sealed. Wash your hands after handling the specimen.
2. Do not smoke, eat, or sleep near the specimens.
3. Limit the specimen size since exposure is related to the amount of mineral present. 
4. Secure the specimen from children or curious adults.
5. Alpha and beta radiation is absorbed by a coupe of feet of air or by a glass cabinet window. Gamma radiation is not.
6. The main environmental exposure hazard from radioactive minerals comes from the buildup of Radon gas emitted by Uranium- and Thorium- bearing minerals. Museums who display large radioactive specimens provide vents to the outside. A good strategy for personal collectors is to place a cheap aquarium pump inside a closed mineral cabinet. Route the output of the pump to the outside.

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:

1. Uranium (²³⁸U & ²³⁵U) and Thorium (²³²Th) decay produces a series of daughter isotopes which are themselves radioactive. The calculation assumes full secular-equilibrium with the daughter isotopes. Branching decay modes are ignored.
2. The isotopic composition is estimated from the natural abundance of the radioactive isotope in the element.  It is assumed that no isotopic fractionation of the element has occurred in the specimen. 
3. Minerals which contain Rare Earth Elements (REE) are generally more radioactive because thorium is commonly substituted for one or more of these REE. If thorium is not accounted for in the empirical formula, then thorium is added to the total activity by using the relationship % Th = S % REE x 0.05.
   This is a general relationship and is probably too high for carbonate and phosphate REE minerals and too low for silicate REE minerals.
4. Radioactive accessory minerals such as zircon may contribute to the radioactivity of a mineral which is otherwise non-radioactive by calculation. Many granites or other igneous rocks contain some radioactivity because of minor, but highly radioactive, accessory minerals.

Calculation of Activity

Activity 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 (r_e) 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:

r_e = mineral density  (S Atomic number / Molecular Weight)
where r_e is the electron density in grams/cc.

Photo-electric absorption effect is calculated from the following relationship:

Pe = (Z^3.6/10)*0.0025 where:
Z = Atomic number of the atom and Pe has the units of barns/electron.

Pe_min = S A_i Z_i P_i / S A_i Z_i where:
A_i = atomic number atom (i) in the mineral, Z_i = atomic number  atom (i), P_i = Pe value of atom (I).

U = Pe_min * r_e where:
U is the photoelectric effect in barns/cc

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:

GRapi = Bq/21 x 2 (S Z/mole wt.) where:
GRapi is gamma ray (in api units), 210 is the empirical proportionality factor from Bq to GRapi units, and Z = mean atomic number of the mineral.

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