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Understanding Radiation

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Understanding Radiation APPENDIX B Understanding Radiation This section introduces the general reader to some basic concepts of radioactivity and an understanding of the radiation emitted as radioactive materials decay to a stable state. To better comprehend the radiological information in the Site Environmental Report (SER), it is important to remember that not all radiations are the same and that different kinds of radiation affect living beings differently. This appendix includes discussions on the common sources of radioactivity in the environment, types of radiation, the analyses used to quantify radioactive material, and how radiation sources contribute to radiation dose. Some general statistical concepts are also presented, along with a discussion of radionuclides that are of environmental interest at BNL. The discussion begins with some definitions and background information on scientific notation and numerical prefixes used when measuring dose and radioactivity. The definitions of commonly used radiological terms are found in the Technical Topics section of the glossary, Appendix A, and are indicated in boldface type here only when the definition in the glossary provides additional details. RADIOACTIVITY AND RADIATION ionizing or not, may pose health risks. In the All substances are composed of atoms that SER, radiation refers to ionizing radiation. are made of subatomic particles: protons, neu- Radioactive elements (or radionuclides) are trons, and electrons. The protons and neutrons referred to by name followed by a number, such are tightly bound together in the positively as cesium-37. The number indicates the mass charged nucleus (plural: nuclei) at the center of of that element and the total number of neutrons the atom. The nucleus is surrounded by a cloud and protons contained in the nucleus of the atom. of negatively charged electrons. Most nuclei Another way to specify cesium-37 is Cs-37, are stable because the forces holding the pro- where Cs is the chemical symbol for cesium in tons and neutrons together are strong enough to the Periodic Table of the Elements. This type of overcome the electrical energy that tries to push abbreviation is used throughout the SER. them apart. When the number of neutrons in the nucleus exceeds a threshold, then the nucleus SCIENTIFIC NOTATION becomes unstable and will spontaneously “de- Most numbers used for measurement and cay,” or emit excess energy (“nuclear” energy) quantification in the SER are either very large or in the form of charged particles or electromag- very small, and many zeroes would be required netic waves. Radiation is the excess energy to express their value. To avoid this, scientific released by unstable atoms. Radioactivity and notation is used, with numbers represented in radioactive refer to the unstable nuclear prop- multiples of 0. For example, the number two erty of a substance (e.g., radioactive uranium). million five hundred thousand (two and a half When a charged particle or electromagnetic million, or 2,500,000) is written in scientific wave is detected by radiation-sensing equip- notation as 2.5 x 06, which represents “2.5 ment, this is referred to as a radiation event. multiplied by 0 raised to the power of 6.” Radiation that has enough energy to remove Since even “2.5 x 06” can be cumbersome, the electrons from atoms within material (a pro- capital letter E is substituted for the phrase “0 cess called ionization) is classified as ionizing raised to the power of …” Using this format, radiation. Radiation that does not have enough 2,500,000 is represented as 2.5E+06. The “+06” energy to remove electrons is called nonionizing refers to the number of places the decimal point radiation. Examples of nonionizing radiation was moved to the left to create the shorter ver- include most visible light, infrared light, micro- sion. Scientific notation is also used to represent waves, and radio waves. All radiation, whether numbers smaller than zero, in which case a B- 2008 SITE ENVIRONMENTAL REPORT APPENDIX B: UNDERSTANDING RADIATION of cosmic radiation from outer space, radiation from radioactive elements in soil and rocks, and radiation from radon and its decay products in Radon, air. Some people use the term background when 200 Medical, 39 referring to all non-occupational sources com- Manmade monly present. Other people use natural to refer Nuclear only to cosmic and terrestrial sources, and back- Medicine, 14 ground to refer to common man-made sources Consumer such as medical procedures, consumer products, Internal, Products, 10 40 and radioactivity present in the atmosphere from former nuclear testing. In the SER, the term Cosmic, natural background is used to refer to radiation 26 Terrestrial, 28 from cosmic and terrestrial radiation. Cosmic – Cosmic radiation primarily consists of Figure B-1. Typical Annual Radiation Doses from Natural and Man- Made Sources (mrem). Source: NCRP Report No. 93 (NCRP 1987) charged particles that originate in space, beyond the earth’s atmosphere. This includes ionizing minus sign follows the E rather than a plus. For radiation from the sun, and secondary radia- example, 0.00025 can be written as 2.5 x 0-4 tion generated by the entry of charged particles or 2.5E-04. Here, “-04” indicates the number of into the earth’s atmosphere at high speeds and places the decimal point was moved to the right. energies. Radioactive elements such as hydro- gen-3 (tritium), beryllium-7, carbon-4, and NUMERICAL PREFIXES sodium-22 are produced in the atmosphere by Another method of representing very large cosmic radiation. Exposure to cosmic radiation or small numbers without using many zeroes is increases with altitude, because at higher eleva- to use prefixes to represent multiples of ten. For tions the atmosphere and the earth’s magnetic example, the prefixmilli (abbreviated m) means field provide less shielding. Therefore, people that the value being represented is one-thou- who live in the mountains are exposed to more sandth of a whole unit; 3 mg (milligrams) is 3 cosmic radiation than people who live at sea thousandths of a gram or E-03. See Appendix C level. The average dose from cosmic radiation for additional common prefixes, includingpico to a person living in the United States is ap- (p), which means trillionth or E-2, giga (G), proximately 26 mrem per year. (For an expla- which means billion or E+09, and tera (T), nation of dose, see effective dose equivalent in which means trillion, E+2. Appendix A. The units rem and sieverts also are explained in Appendix A.) SOURCES OF IONIZING RADIATION Terrestrial – Terrestrial radiation is released Radiation is energy that has both natural by radioactive elements that have been pres- and manmade sources. Some radiation is essen- ent in the soil since the formation of the earth. tial to life, such as heat and light from the sun. Common radioactive elements that contribute to Exposure to high-energy (ionizing) radiation terrestrial exposure include isotopes of potas- has to be managed, as it can pose serious health sium, thorium, actinium, and uranium. The risks at large doses. Living things are exposed average dose from terrestrial radiation to a per- to radiation from natural background sources: son living in the United States is approximately the atmosphere, soil, water, food, and even our 28 mrem per year, but may vary considerably own bodies. Humans are exposed to ionizing depending on the local geology. radiation from a variety of common sources, the Internal – Internal exposure occurs when ra- most significant of which follow. dionuclides are ingested, inhaled, or absorbed Background Radiation – Radiation that occurs through the skin. Radioactive material may be naturally in the environment is also called back- incorporated into food through the uptake of ter- ground activity. Background radiation consists restrial radionuclides by plant roots. People can 2008 SITE ENVIRONMENTAL REPORT B-2 APPENDIX B: UNDERSTANDING RADIATION ingest radionuclides when they eat contaminat- Therefore, beta particles have a negative charge. ed plant matter or meat from animals that have Beta radiation is slightly more penetrating than consumed contaminated plants. The average alpha radiation, but most beta radiation can be dose from food for a person living in the United stopped by materials such as aluminum foil and States is about 40 mrem per year. A larger expo- plexiglass panels. Beta radiation has a range in sure, for most people, comes from breathing the air of several feet. Naturally occurring radioac- decay products of naturally occurring radon gas. tive elements such as potassium-40 emit beta The average dose from breathing air with radon radiation. Some beta particles present a hazard byproducts is about 200 mrem per year, but to the skin and eyes. that amount varies depending on geographical Gamma Radiation – Gamma radiation is a form location. An Environmental Protection Agency of electromagnetic radiation, like radio waves (EPA) map shows that BNL is located in one of or visible light, but with a much shorter wave- the regions with the lowest potential radon risk. length. Gamma rays are emitted from a radioac- Medical – Every year in the United States, mil- tive nucleus along with alpha or beta particles. lions of people undergo medical procedures Gamma radiation is more penetrating than alpha that use ionizing radiation. Such procedures or beta radiation, capable of passing through include chest and dental x-rays, mammography, dense materials such as concrete. Gamma radia- thallium heart stress tests, and
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