A NOTE ON RADIOACTIVE MATERIALS AND THEIR MEASUREMENTS Elements are placed in the periodic table of elements according to atomic number or proton number often referred to as Z. Their atomic weights are also usually indicated and can be fractional if the element in nature contains more than one isotope. Isotopes of an element all have the same atomic number, Z, but have different numbers of neu- trons. Some of these isotopes, even in naturally occurring elements, may be radioactive, and are usually given the atomic weight of the longest known isotope. Radioactive decay occurs when an unstable atom loses energy by emitting particles and gamma radiation or by spontaneously fission- ing. (Spontaneous fission is one of the natural modes of radioactive decay in which fission occurs without the addition of external energy or particles.) The activity of a radioactive sample is simply the rate of radioactive decay, expressed as the number of decays per unit time. Traditionally, the curie (Ci) was used to quantify the decay rate and was defined as the radioactivity measured for one gram of pure radium, or ~3.7 × 1010 decays per second (d/s), but the value changed as measurement techniques improved. To avoid this problem of a “chang- ing” constant, the Curie is now defined as exactly 3.7 × 1010 d/s. The becquerel (Bq), equal to one d/s, is the official SI unit (International System of Units). The roentgen R( ) is commonly used as a unit of radiation exposure to X-ray or gamma radiation, and can be roughly defined as “that quan- tity of X-ray or gamma radiation needed to produce one electrostatic unit of ionic charge in one cm3 of air under standard temperature and pressure (STP) “normal conditions”. However, the dose from ”normal” background radiation in Maryland (sea level) is ~16 mR per year while in Denver (the mile-high city), it is ~75 mR. Other local sources of radiation must still be added as applicable. The rad is a unit of absorbed dose. The official SI unit of absorbed dose is the gray (Gy) and is equal to 100 rad. The sievert (Sv) is the SI unit of dose equivalent of radiation. It attempts to reflect the biological rather than just the physical effects of radiation. The sievert is intended to replace the older roentgen equiva- lent man (rem); 1 Sv ~ 100 rem. 594 a note on radioactive materials According to the International Union of Pure and Applied Chemistry (which has the final word on international nomenclature), in recogni- tion of the fact that these units of measurements are derived from the proper names of persons, the first letter of the symbol is uppercase (Bq), but the unit spelled out in English begins with a lower case letter (becquerel) unless it appears at the beginning of the sentence or another place where capitalization would be proper for any word. This volume adopts this convention. The following elements are the most commonly mentioned radioac- tive materials in this volume: Am Americium; Z = 95: A synthetic element first produced and identi- fied in 1944. All of the isotopes of Am are radioactive. Probably most relevant for this study is Am-241 (half-life = 433 years). Cs Cesium: Z = 55: Cesium is a stable element (atomic weight 132.91) found in nature. A large number of radioactive isotopes of Cs have been produced, many of which are produced in fission of uranium and plutonium. Among these is the radioactive isotope Cs-137 (half-life = 30 years), a high yield fission product which is also widely used as a gamma-ray standard. Eu Europium, Z = 63: A stable element (atomic weight = 151.96) found in nature. A large number of radioactive isotopes have been produced in fission of U and Pu. Pu Plutonium, Z = 94: A synthetic radioactive element first produced and identified in 1941 at Berkeley as the isotope 238. Best known isotope is the fissionable isotopeP u-239, which is, used in nuclear weapons. Traces of the longest-lived isotope Pu-244 (80 million yrs.) were found in nature in 1971. Ra Radium, Z = 88: An element with only radioactive isotopes, Ra- 226 has the longest half-life, 1600 years. It is found in nature as a result of its continual production from the decay of naturally occurring very long-lived isotopes of thorium and uranium. Sr Strontium, Z = 38: A stable element (atomic weight = 87.62) found in nature. A large number of radioactive isotopes have been produced. Of particular interest is Sr 90 (half-life 29 yrs) produced in high yield in fission of U and Pu. Tc Technetium, Z = 43: No stable isotopes are known. Longest known isotope is Tc-99 (0.2 million yrs.), produced as a fission product..