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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 individual’s dose are provided. Some gen- eral statistical concepts are also presented, along with a discussion of of environmental interest at BNL.

RADIOACTIVITY more penetrating than alpha or beta radiation, ca- The 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 , or nucleus, sur- NOMENCLATURE rounded by a cloud of negatively charged par- Throughout this report, radioactive elements ticles called . Most 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 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 is identical in makeup to SOURCES OF RADIATION the nucleus of a atom, consisting of two Radioactivity and radiation are part of the and two . Alpha particles have a earth’s 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 and daughters emit charged particles that originate in space, beyond alpha radiation. the Earth’s atmosphere. This includes radiation Beta - Beta radiation is composed of particles that from the and secondary radiation generated are identical to electrons. As a result, beta particles by the entry of charged particles into the Earth’s have a negative charge. Beta radiation is slightly atmosphere at high speeds and . Radio- more penetrating than alpha but may be stopped active elements such as hydrogen-3 (), be- by materials such as aluminum foil and Lucite™ ryllium-7, -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 , 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

decay series), and tobacco products (containing -210 and -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 of potassium, year from natural and background sources. thorium, , 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 , 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 -241), sures all alpha particles simultaneously, without cardiac pacemakers (containing -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 activity. ness of radioactive emissions and the very low Tritium - Due to the 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 ’s gamma radiation emis- radionuclide with a half-life of 28 years (i.e., sions. The energy of these emissions is unique after 28 years only one half of the activity from for each , acting as a “fingerprint” to iden- the original remains). It is found in the environ- tify a specific nuclide. ment principally as a result of fallout from aboveground nuclear weapons testing. (Fallout STATISTICS refers to the deposition of radionuclides on soils Two important statistical aspects of measur- and water bodies as a result of being dispersed ing radioactivity are uncertainty in results and high into the Earth’s atmosphere during nuclear negative values. explosions.) Strontium-90 released in the 1950s Uncertainty - Because the emission of radiation and early 1960s is still present in the environ- from an atom is a random process, a sample ment today due to its long half-life. Additionally, counted several times will yield a slightly differ- nations that were not signatories of the Nuclear ent result each time; a single measurement is, Test Ban Treaty of 1963 have conducted tests therefore, not definitive. To account for this phe- that have contributed to the global strontium-90 nomenon, the concept of uncertainty is applied to inventory. This radionuclide was also released radiological data. In this report analysis results as a result of the 1986 Chernobyl accident in are displayed with x ± y format, where x is the the former Soviet Union. result and ± y is the 95 percent confidence inter- The data in this environmental report are val of the result. That is, there is a 95 percent reported by method of analysis. Because stron- probability that the true value of x lies between x tium-90 requires a unique method of analysis, it + y and x - y. Conversely, there is a 5 percent is reported as a separate parameter in the data probability that the true value of x lies outside of tables. The level of sensitivity for detecting this range. strontium-90 using state-of-the-art analysis Negative values - There is always a small methods is quite low (less than 1 pCi/L), which amount of natural radiation in the environment. makes it possible to detect strontium-90 at lev- The instruments used in the laboratory to mea- els that are indicative of the environmental sure radioactivity in BNL site environmental me- sources described above. dia are sensitive enough to measure the natural, Tritium - Among the radioactive materials that or background, radiation along with any contami- are used or produced at BNL, tritium has re-

B-3 2002 SITE ENVIRONMENTAL REPORT APPENDIX B: CONCEPTS OF RADIOACTIVITY ceived the most public attention. Tritium exists Environmental tritium is found in two in nature and is formed when cosmic radiation forms: (1) gaseous elemental tritium and (2) tri- from space interacts with the gaseous tiated water (or water vapor), in which at least in the Earth’s upper atmosphere. Approximately one of the hydrogen atoms in the H O water 2 4 million Ci (1.5E+5 TBq) per year are pro- molecule has been replaced by a tritium atom duced in the atmosphere in this way, with the (hence, its shorthand notation HTO). All tritium total global quantity being about 70 million Ci released from BNL sources is in the form of (2.6E+6 TBq) at any given time (NCRP 1979). HTO. As a result of the 1950s and early 1960s Cesium-137 - Cesium-137 is a man-made, fis- aboveground weapons testing program, the glo- sion-produced radionuclide with a half-life of 30 bal atmospheric tritium inventory was increased years. It is found in the environment as a result by a factor of about 200. Other human activities of past aboveground nuclear weapons testing such as consumer product manufacturing and and can be observed in the upper levels of envi- nuclear power reactor operations have also re- ronmental soils at very low concentrations, usu- leased tritium into the environment. Commer- ally less than 1 pCi/g (0.04 Bq/g). It is a beta- cially, tritium is used in products such as self- emitting radionuclide, but can be detected by illuminating exit signs and wrist watches (exit by the gamma emissions signs may each contain as much as 25 Ci [925 of its decay product, barium-137m. GBq] of tritium). It also has many uses in medi- cal and biological research as a labeling agent in SCIENTIFIC NOTATION chemical compounds and is frequently used in Since many of the numbers used in mea- universities and other research settings. surement and quantification in this report are Of the sources mentioned above, the most either very large or very small, many zeroes significant contributor to tritium in the environ- would be required to express their value. Be- ment has been aboveground nuclear weapons cause this is inconvenient, scientific notation is testing. In the early 1960s, the average tritium used as a kind of numerical shorthand. Scien- concentration in surface streams in the United tific notation is based on the principle of repre- States reached a value of 4,000 pCi/L (148 senting numbers in multiples of ten. For ex- kBq/L) (NCRP 1979). Approximately the same ample, the number one million could be written concentration was measurable in precipitation. as 1,000,000. Alternatively, this number could Today, the level of tritium in surface waters in be written in scientific notation as 1 x 106. That New York State is below 200 pCi/L (7.4 kBq/L) is, “one times ten raised to the sixth power.” (NYSDOH 1993), less than the detection limit Since even this shorthand can be cumbersome, of most analytical laboratories. it can be reduced even further by using the Tritium has a half-life of 12.3 years. When capital E to stand for 10x, or “ten raised to an atom of tritium decays, it releases a beta par- the power of some value x.” Using this notation, ticle, causing transformation of the tritium atom 1,000,000 would be represented as 1E+06. Sci- into stable (nonradioactive) helium. This beta entific notation is also used to represent very radiation has a very low energy when compared small numbers like 0.0001, which can be writ- to the emissions of most other radioactive ele- ten as 1 x 10-4 or 1E-04. This notation is used in ments. The body’s outer layer of dead skin cells the text and some tables in this report. easily stops tritium beta radiation and therefore, only when taken into the body, can tritium PREFIXES cause an exposure. Tritum may be taken into Another method of representing very large the body by inhalation, ingestion, or absorption or very small numbers without the use of many through the skin. Because of its low energy ra- zeroes is to use prefixes to represent multiples diation and short residence time in the body, the of ten. For example, the prefix “milli” means health threat posed by tritium is very small for that the value being represented is one thou- most credible exposures. sandth of a whole unit, so that one-milligram is

2002 SITE ENVIRONMENTAL REPORT B-4 APPENDIX B: CONCEPTS OF RADIOACTIVITY equal to one thousandth of a gram. See table REFERENCES inside back cover. NCRP. 1979. Tritium in the Environment. NCRP Report No. 62. National Council on and Measurements. Bethesda, MD. DEFINITION OF RADIOLOGICAL TERMS NCRP. 1987. Ionizing of the Population of the Radiological terms are used throughout this United States. NCRP Report No. 93. National Council on report where radiation and radioactive material Radiation Protection and Measurements. Bethesda, MD. are discussed. The definitions of commonly used NYSDOH. 1993. Environmental Radiation in New York State. radiological terms are found in Appendix A. Bureau of Environmental Radiation Protection, New York State Department of Health, Albany, NY.

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