Nuclide A species of atom characterized b> its mass number (number of neutrons and protons m nucleus), atomic number (number of electrons m the neutral atom), and energy state of the nucleus, provided that the mean life in that state is long enough to be observable

Neution An elementary particle, electricallv neutral, whose mass is approximate!) equal to that of a hydrogen atom which with a half-life of about 1 1 7 min, decays, m the free state, into a proton and an electron

thermal muttons Neutrons essentially in thermal cqui librium with the medium in which they exist

last millions Neutrons of kinetic energy greater than some specified value In reactor physics the value is frequently chosen to be 0 1 MeV

Beta particle An electron, of either positive charge ((?) oi negative charge ((3 ), which has been emitted bv an atomic nucleus or neutron in the process of a transfor­mation

Radioai live dc < ay A spontaneous nuelear transformation m which the nucleus emits particles or gamma radia­tion, or undergoes spontaneous fission, or in which the atom emits x-radiation or Auger electrons following orbital electron capture or internal conversion

Decay constant (or disintegration constant) for a radio­active nuclide (radionuclide), the probability per unit time for the spontaneous radioactive decay of a nucleus It is given by

. 1 dN

N dt

m which N is the number of nuclei of concern existing at time t

( tine The special unit of activity (nuclear disintegration rate) One Curie equals 3.7 XIO10 disintegrations per second, exactly “Curie” is abbreviated as Ci

Half life (lad/oac live half life) for a single radioactive decay process, the time required for the activity (dN/dt or AN) to decrease to half its value by that process I he half-life is related to the dccav constant T^ = (logt 2)/A ~ 0 6931 5/Л

( mss section measure of the probability of a specified interaction between an incident radiation and a target particle or svstem of particles It is the reaction rate per target particle for a specified process divided by the particle-flux densitv of the incident radiation (шию scopic ci oss section) In reactor physics the term is sometimes applied to a specified group of taiget particles, eg, those pci unit volume (mac I osc opic cioss section) or per unit mass, or those in a specified bodv |of< Unless otherwise qualified the term “cross section ’ means “micioscopic cross section ”]

Ifuioscopic cioss section 1 he cross section per unit volume of a given material for a specified process It has the dimension of recipiocal length, for a pure nuclide, it is the product of the microscopic cross section and the number of target nuclei per unit volume, for a mixture of nuclides, it is the sum of such products М/с і osc opn cioss section 1 he cross section per target nucleus, atom, or molecule It has the dimension of area and tnav be visuali/cd as the area normal to the direction of an incident particle which has to be attributed to the target particle to account geometri­cally for the interaction with the incident particle Microscopic cross sections are often expressed in bains, where 1 barn = 10 24 cm2

Panicle Jlu density At a given point in space, the number of particles or photons incident per unit time on a small sphere centered at that point divided by the cross-sectional area of that sphere It is identical with the product of the particle density and the average particle speed The term is commonly called flux (Xe utrou flu density Particle-flux density for neutrons Also commonly called neution flu Often denoted by nv or 0

Particle finance At a given point in space, the number of particles or photons incident during a given time interval on a small sphere centered at that point divided by the cross-sectional area of that sphere. It is identical with the time integral of the particle-flux density Often denoted by nvt

Particle density At a given point in space, the number of particles or photons per unit volume in a small sphere centered at that point

Ionizing radiation. Any electromagnetic or particulate radiation capable of producing ions, directly or indi­rectly, by interaction with matter Indirectly ionizing particles Uncharged particles or pho­tons which can liberate directly ionizing particles or tan initiate a nuclear transformation Directly ionizing particles. Charged particles having suffi­cient kinetic energy to produce ionization by collision.

Exposure. A measure of the ionization produced in air by x or gamma radiation. It is the sum of the electrical charges on all of the ions of one sign produced in air when all electrons liberated by photons in a volume element of air are completely stopped in the air, divided by the mass of the air in the volume element. The special unit of exposure is the roentgen.

Roentgen. The special unit of exposure. One roentgen = 1 R = 2.58 X 104 coulomb per kilogram of air.

Dose. A general term denoting the quantity of radiation or energy absorbed in a specified mass. For special purposes, its meaning should be appropriately stated, e. g., absorbed dose.

Absorbed dose. The energy imparted to matter in a volume element by ionizing radiation divided by the mass of irradiated material in that volume element. The special unit of absorbed dose is the rad. (Absorbed dose is often called dose.)

Rad. The special unit of absorbed dose. One rad equals 100 ergs/gram.

Dose equivalent (radiation protection). The product of absorbed dose, quality factor, dose distribution factor, and other modifying factors necessary to express on a common scale, for all ionizing radiations, the irradiation incurred by exposed persons. The special unit of dose equivalent is the rem.

Rem. The dose equivalent in rems is numerically equal to the absorbed dose in rads multiplied by the quality factor, the distribution factor, and any other necessary modifying factors.

Quality factor (radiation protection). A linear-energy — transfer-dependent factor by which absorbed doses are to be multiplied to obtain the dose equivalent. (Note: The term “RBE” should be used only in the field of radiobiology.)

Linear energy transfer (LET). The average energy locally imparted to a medium by a charged particle of specified energy per unit distance traversed. Noles: (1) The term “locally imparted” may refer either to a maximum distance from the track or to a maximum value of dis­crete energy loss by the particle beyond which losses are no longer considered as local. In either case, the limits chosen should be specified. (2) The concept of LET is different from that of stopping power. The former refers to energy imparted within a limited volume, the latter to loss of energy from the particle regardless of where this energy is absorbed.]

Dose distribution factor (radiation protection). A factor used in computing dose equivalent to account for the nonuniform distribution of internally deposited radio­nuclides.

Maximum permissible dose equivalent (MPD) (radiation protection). The largest dose equivalent received within a specified period which is permitted by a regulatory agency or other authoritative group on the assumption

that receipt of such dose equivalent creates no appre­ciable somatic or genetic injury. Different levels of MPD may be set for different groups within a population. (By popular usage, “maximum permissible dose” is an accepted synonym.)

Kerma (kinetic energy released in material). The ratio of the sum of the initial kinetic energies of all the charged particles liberated by indirectly ionizing particles in a volume element to the mass of the matter in the volume element.