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Units and Constants

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Units and Constants Appendix A Units and Constants Energy 1 electron volt (eV) = 1.602 x 10- 12 erg 1 kiloelectron volt (keV) = 1.602 x 10-9 erg 1 million electron volts (Me V) = 1.602 X 10-6 erg 1 joule (J) = 10 7 ergs 1 watt (W) = 107 ergs/s = 1 J/s 1 rad = 1 x 10-2 J/kg = 100 ergs/g 1 gray (Gy) = 100 rad = 1 J/kg 1 sievert (Sv) = 100 rem = 1 J/kg 1 horsepower (HP) = 746W 1 calorie (cal) = 4.184 J Charge 1 electronic charge = 4.8 X 10-10 electrostatic unit = 1.6 x 10-19 C 1 coulomb (C) = 6.28 X 10 18 charges 1 ampere (A) = 1 Cis Mass and Energy 1 atomic mass unit (amu) = 1.66 X 10-24 g = 1/12 the atomic weight of 12C = 931 MeV 1 electron rest mass =0.511 MeV 1 proton rest mass = 938.78 MeV 1 neutron rest mass = 939.07 MeV 1 pound = 453.6 g Length 1 micrometer, or micron Cum) = 10-6 m = 104 A 186 Appendix A. Units and Constants 187 1 nanometer (nm) = 10-9 m 1 angstrom (A) = 10-8 cm 1 fermi (F) = 10-13 cm 1 inch = 2.54cm Activity 1 curie (Ci) = 3.7 X 10 10 disintegrations per second (dps) = 2.22 x 1012 disintegrations per minute (dpm) 1 millicurie (mCi) = 3.7 x 107 dps = 2.22 X 109 dpm 1 microcurie {J.lCi) = 3.7 x 104 dps = 2.22 X 106 dpm 1 becquerel (Bq) = 1 dps = 2.703 X 10- 11 Ci 1 kilobecquerel (kBq) = 103 dps = 2.703 X 10-8 Ci 1 megabecquerel (MBq) = 106 dps = 2.703 X 10-5 Ci 1 gigabecquerel (GBq) = 109 dps = 2.703 X 10-2 Ci 1 terabecquerel (TBq) = 10 12 dps = 27.03 Ci Constants A vogadro's number = 6.02 X 1023 atoms/g· atom = 6.02 x 1023 molecules/g· mole Planck's constant (h) = 6.625 x 10-27 erg· s/cycle Velocity oflight = 3 x 10 10 cm/sec IT = 3.1416 e = 2.7183 Appendix B Terms Used in Text Absorption. A process by which the total energy of a radiation is removed by an absorber through which it passes. Accelerator. A machine to accelerate charged particles linearly or in circular paths by means of an electromagnetic field. The accelerated particles such as!Y. particles, protons, deuterons, and heavy ions possess high energies and can cause nuclear reactions in target atoms by irradiation. Accuracy. A term used to indicate how close a measurement of a quantity is to its true value. Annihilation radiation. y-Radiations of 511 keV energy emitted at 180 0 after a [3+ particle is annihilated by combining with an electron in matter. Atomic mass unit (amu). By definition, one twelfth of the mass of l~C, equal to 1.66 x 10-24 g or 931 MeV. Atomic number (Z). The number of protons in the nucleus of an atom. Attenuation. A process by which the intensity of radiation is reduced by absorption and/or scattering during its passage through matter. Attenuation coefficient. The fraction of y-ray energy attenuated (absorbed plus scattered) per unit length of an absorber (linear attenuation co­ efficient, J1.) or per gram of an absorber (mass attenuation coefficient, J1.m)· Auger electron. An electron ejected from an energy shell, instead of a charac­ teristic x-ray, carrying the energy equal to that of the x-ray. Average life (r). See Mean life. Avogadro's number. The number of molecules in 1 g' mole of any substance or the number of atoms in 1 g' atom of any element. It is equal to 6.02 x 10 23• Binding energy. The energy to bind two entities together. In a nucleus, it is the energy needed to separate a nucleon completely from other nucleons in the nucleus. In a chemical bond, it is the energy necessary to separate two binding partners an infinite distance. Biological half-life (7;,). The time by which one half of an administered dose of a substance is eliminated by biological processes such as urinary and fecal excretions. 188 Appendix B. Terms Used in Text 189 Bremsstrahlung. y-Ray photons produced by deceleration of charged parti­ cles near the nucleus of an absorber atom. Carrier. A stable element that is added in detectable quantities to a radio­ nuclide of the same element, usually to facilitate chemical processing of the radionuclide. Carrier-free. A term used to indicate the absence of any stable atoms in a radionuclide sample. Collimator. A device to confine a beam of radiation within a specific field of view. Collimators may be converging, pinhole, diverging, and parallel-hole types. Collimator efficiency. The number of photons passing through the collimator for each unit of activity present in a source. Collimator resolution. A component of spatial resolution of an imaging sys­ tem contributed by the collimator. It is also called geometric resolution. Committed dose equivalent (H T •SO )' The dose equivalent to organs or tissues of reference (T) that will be received from an intake of radioactive material by an individual during the 50-year period following intake. Compton scattering. In this process, a (-ray transfers only a partial amount of energy to an outer orbital electron of an absorber, and the photon itself is deflected with less energy. Conversion electron (e-). See Internal conversion. Critical organ. See Organ, critical. Cross section (a). The probability of occurrence of a nuclear reaction or the formation of a radionuclide in a nuclear reaction. It is expressed in a unit termed barn; 1 barn = 10- 24 cm 2 • Curie (Ci). A unit of activity. A curie is defined as 3.7 x 10 10 disintegrations per second. Dead time. The period of time that a counter remains insensitive to count the next after an event. Decay constant ().). The fraction of atoms of a radioactive element decaying per unit time. It is expressed as ). = 0.693/t 1/2 , where t1/2 is the half-life of the radionuclide. Veep-dose equivalent (Hd)' Dose equivalent at a tissue depth of 1 cm (1000 mg/cm2 ) resulting from external whole-body exposure. Dose. The energy of radiation absorbed by any matter. Also, a general term for the amount of a radiopharmaceutical administered in microcuries or millicuries. Dosimeter. An instrument to measure the cumulative dose of radiation re­ ceived during a period of radiation exposure. Dosimetry. The calculation or measurement of radiation absorbed doses. Effective half-life CT.). Time required for an initial administered dose to be reduced to one half as a result of both physical decay and biological elimi­ nation of a radionuclide. It is given by T" = (Tp x 1/,)/(Tp + 1/,), where T" is the effective half-life, and Tp and 1/, are the physical and biological half­ lives, respectively. 190 Appendix B. Terms Used in Text Electron (e-). A negatively charged particle rotating around the atomic nu­ cleus. It has a charge of 4.8 x 10-10 electrostatic units and a mass of 9.1 x 10-28 g, equivalent to 0.511 MeV, or equal to 1/1836 of the mass of a proton. Electron capture (EC). A mode of decay of a proton-rich radionuclide in which an orbital electron is captured by the nucleus, accompanied by emis­ sion of a neutrino and characteristic x-rays. Electron volt (eV). The kinetic energy gained by an electron when accelerated through a potential difference of 1 V. Energy resolution. Capability of a detecting system to separate two y-ray peaks of different energies. It is given by the full width at half maximum (FWHM) of a given photopeak. Erg. The unit of energy or work done by a force of 1 dyne through a distance of 1 cm. Fission (f). A nuclear process by which a nucleus divides into two nearly equal smaller nuclei, along with the emission of two to three neutrons. Free radical. A highly reactive chemical species that has one or more un­ paired electrons. Generator, radionuclide. A device in which a short-lived daughter is separated chemically and periodically from a long-lived parent adsorbed on adsor­ bent material. For example, 99mTc is separated from 99Mo from the Moly generator with saline. Gray (Gy). The unit of absorbed radiation dose in SI units. One gray is equal to 100 rad. Half-life (t 1/ 2 ). A unique characteristic of a radionuclide, defined by the time during which an initial activity of a radionuclide is reduced to one half. It is related to the decay constant ), by t 1/2 = 0.693/),. Half-value layer (HVL). The thickness of an absorbing material required to reduce the intensity or exposure of a radiation beam to one half of the initial value when placed in the path of the beam. Internal conversion. An alternative mode to y-ray decay in which nuclear excitation energy is transferred to an orbital electron, which is then ejected from the orbit. Ion. An atom or group of atoms with a positive charge (cation) or a negative charge (anion). Intrinsic efficiency. The number ofradiations detected divided by the number of radiations striking the detector. Intrinsic resolution. A component of the spatial resolution of an imaging system that is contributed by the detector and associated electronics and depends on the photon energy, detector thickness, and the number of PM tubes. Isobars. Nuclides having the same mass number, that is, the same total num­ ber of neutrons and protons. Examples are ~~Fe and ~~Co.
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