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Concepts of Radioactivity

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Concepts of Radioactivity APPENDIX B: CONCEPTS OF RADIOACTIVITY APPENDIX B Concepts of Radioactivity This section introduces some of the basic concepts of radioactivity. It is designed to provide the general reader with an overall understanding of the radiological sections of this report. A discussion of the analyses used to qualitatively quantify radioactive material, the common sources of radioactivity in the environment, and how they contribute to an individuals radiation dose are provided. Some gen- eral statistical concepts are also presented, along with a discussion of radionuclides of environmental interest at BNL. RADIOACTIVITY more penetrating than alpha or beta radiation, ca- The atom is the basic constituent of all mat- pable of passing through dense materials such as ter and is one of the smallest units into which concrete. X-rays are similar to gamma radiation. matter can be divided. Each atom is composed of a tiny central core of particles, or nucleus, sur- NOMENCLATURE rounded by a cloud of negatively charged par- Throughout this report, radioactive elements ticles called electrons. Most atoms in the physical (also called radionuclides) are referred to by a world are stable, meaning that they are not radio- name followed by a number, e.g., cesium-137. active. However, some atoms possess excess en- The number following the name of the element is ergy, which causes them to be physically un- called the mass of the element and is equal to the stable. In order to become stable, an atom rids total number of particles contained in the nucleus itself of this extra energy by casting it off in the of the atom. Another way to specify the identity form of charged particles or electromagnetic of cesium-137 is by writing it as Cs-137, where waves, known as radiation. Cs is the chemical symbol for cesium as it ap- pears in the standard Periodic Table of the Ele- COMMON TYPES OF RADIATION ments. This type of abbreviation is used in the The three most important types of ionizing text and many of the data tables in this report. radiation are described below: Alpha - An alpha particle is identical in makeup to SOURCES OF RADIATION the nucleus of a helium atom, consisting of two Radioactivity and radiation are part of the neutrons and two protons. Alpha particles have a earths natural environment. Human beings are positive charge and have little or no penetrating exposed to radiation from a variety of common power in matter. They are easily stopped by mate- sources, the most significant of which are listed rials such as paper and have a range in air of only below. an inch or so. Naturally occurring radioactive ele- Cosmic - Cosmic radiation primarily consists of ments such as uranium and radon daughters emit charged particles that originate in space, beyond alpha radiation. the Earths atmosphere. This includes radiation Beta - Beta radiation is composed of particles that from the sun and secondary radiation generated are identical to electrons. As a result, beta particles by the entry of charged particles into the Earths have a negative charge. Beta radiation is slightly atmosphere at high speeds and energies. Radio- more penetrating than alpha but may be stopped active elements such as hydrogen-3 (tritium), be- by materials such as aluminum foil and Lucite ryllium-7, carbon-14, and sodium-22 are pro- panels. They have a range in air of several feet. duced in the atmosphere by cosmic radiation. Naturally occurring radioactive elements such as The average dose from cosmic radiation to a per- potassium- 40 (K-40) emit beta radiation. son living in the United States is about 26 mrem Gamma - Gamma radiation is a form of electro- per year. magnetic radiation, like radio waves or visible Terrestrial - Terrestrial radiation is released by light, but with a much shorter wavelength. It is radioactive elements present in the soil since the B-1 2002 SITE ENVIRONMENTAL REPORT APPENDIX B: CONCEPTS OF RADIOACTIVITY thorium decay series), and tobacco products (containing polonium-210 and lead-210). The average dose from consumer products to a per- Radon, son living in the United States is 10 mrem per year (excluding tobacco contributions). 200 Medical, 39 Manmade DOSE UNITS Nuclear Medicine, 14 The amount of energy that radiation deposits Consumer in body tissues or organs, when corrected for hu- Internal, Products, 10 man risk factors, is referred to as dose equivalent 40 or, more generally, as dose. Radiation doses are Figure B-1. Typical Annual Radiation measured in units of rem. Since the rem is a Cosmic, Doses from Natural and Man-Made fairly large unit, it is convenient to express most 26 Terrestrial, Sources (mrem). 28 Sources: NCRP Report No. 93 (NCRP 1987). doses in terms of millirem (1,000 mrem = 1 rem). To give a sense of the size and importance formation of the Earth about five billion years ago. of a 1 mrem dose, Figure B-1 indicates the num- Common radioactive elements contributing to ter- ber of mrem received by an individual in one restrial exposure include isotopes of potassium, year from natural and background sources. thorium, actinium, and uranium. The average dose These values represent typical values for resi- from terrestrial radiation to a person living in the dents of the United States. Note that the alternate United States is about 28 mrem per year. unit of dose measurement commonly used inter- Internal - Internal exposure occurs when radionu- nationally and increasingly in the United States is clides are ingested, inhaled, or absorbed through the sievert, abbreviated Sv. One Sv is equivalent the skin. Radioactive material may be incorporated to 100 rem. Likewise, 1 millisievert (mSv) is into food through the uptake of terrestrial radionu- equivalent to 100 mrem. clides by plant roots. Human ingestion of radionu- The unit used to express the quantity of radio- clides can occur when contaminated plant matter active material in a sample is the curie (Ci). This is or animals that consume contaminated plant mat- a measure of the rate at which radioactive atoms ter are eaten. Most exposure to inhaled radioactive are transformed to stable atoms. Since the curie is material results from breathing the decay products a relatively large unit (3.7 ´ 1010)for measuring of naturally occurring radon gas. The average dose environmental samples, the picocurie (pCi) is of- from eating foods to a person living in the United ten used. This unit is equal to one trillionth of a Ci, States is about 40 mrem per year; the average or 0.037 decays per second. The alternate unit for dose from radon product inhalation is about 200 quantifying radioactivity is the becquerel, abbrevi- mrem per year. ated Bq. One becquerel is equal to 1 decay per Medical - Millions of people every year undergo second. Additional units of measure and their con- medical procedures that utilize radiation. Such version factors can be found on the inside of the procedures include chest and dental x-rays, back cover of this report. mammography, thallium heart stress tests, and tumor irradiation therapies. The average dose TYPES OF RADIOLOGICAL ANALYSIS from nuclear medicine and x-ray examination The quality of environmental air, water, and procedures in the United States is about 14 and soil with respect to radioactive material can be 39 mrem per year, respectively. assessed using several types of analysis. The Anthropogenic - Sources of anthropogenic (man- most common analyses are described below. made) radiation include consumer products such Gross alpha - Alpha particles are emitted in a as static eliminators (containing polonium-210), range of different energies. An analysis that mea- smoke detectors (containing americium-241), sures all alpha particles simultaneously, without cardiac pacemakers (containing plutonium-238), regard to their particular energy, is known as a fertilizers (containing isotopes of the uranium and gross alpha activity measurement. This type of 2002 SITE ENVIRONMENTAL REPORT B-2 APPENDIX B: CONCEPTS OF RADIOACTIVITY measurement is valuable as a screening tool to nant radiation in a sample. To obtain a true mea- indicate the magnitude of alpha-emitting radionu- sure of the contaminant level in a sample, the clides that may be present in a sample. natural (or background) radiation level must be Gross beta - This is the same concept as de- subtracted from the total amount of radioactivity scribed above, except that it applies to the mea- measured by an instrument. Due to the random- surement of beta particle activity. ness of radioactive emissions and the very low Tritium - Due to the nature of the radiation, a concentrations of some contaminants, it is pos- low energy beta particle, emitted from the tritium sible to obtain a background measurement that is atom, it is detected and quantified by liquid scin- larger than the actual contaminant measurement. tillation counting method. (More information on When the larger background measurement is tritium is presented in the next section.) subtracted from the smaller contaminant, a nega- Strontium-90 - Due to the nature of the radiation tive result is generated. The negative results are emitted by strontium-90, a special analysis is re- reported because they are essential when con- quired. Samples are chemically processed to ducting statistical evaluations of data. separate and collect any strontium atoms that may be present. The collected atoms are then RADIONUCLIDES OF ENVIRONMENTAL INTEREST analyzed separately. (More information on stron- Several types of radionuclides are found in tium-90 is presented in the next section.) the environment at BNL due to historical opera- Gamma - This analysis technique identifies spe- tions and include the following: cific radionuclides. It measures the particular en- Strontium-90 - Strontium-90 is a beta-emitting ergy of a radionuclides gamma radiation emis- radionuclide with a half-life of 28 years (i.e., sions.
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