R
racemate See racemic mixture.
racemic mixture (racemate) A
mixture of equal quantities of the dand l-forms of an optically active
compound. Racemic mixtures are denoted by the preÜx dl- (e.g. dl-lactic
acid). A racemic mixture shows no
*optical activity.
racemization A chemical reaction
in which an optically active compound is converted into a *racemic
mixture.
rad See radiation units.
radial distribution function The
average number of atoms found at a
distance r from a central atom. In the
case of a crystal, the radial distribution function is a series of regular
spikes, corresponding to the atomic
positions in the lattice. Calculating
the radial distribution function is of
fundamental importance in the
theory of liquids.
radiation 1. Energy travelling in
the form of electromagnetic waves or
photons. 2. A stream of particles, especially alpha- or beta-particles from
a radioactive source or neutrons
from a nuclear reactor.
radiationless decay Decay of an
atom or molecule from an excited
state to a lower energy state without
the emission of electromagnetic radiation. A common example of a radiationless process is the *Auger effect,
in which an electron rather than a
photon is emitted as a result of
decay.
radiation units Units of measurement used to express the activity of a
radionuclide and the dose of ionizing
radiation. The units curie, roentgen,
rad, and rem are not coherent with
SI units but their temporary use with
SI units has been approved while the
derived SI units becquerel, gray, and
sievert become familiar.
The becquerel (Bq), the SI unit of
activity, is the activity of a radionuclide decaying at a rate, on average,
of one spontaneous nuclear transition per second. Thus 1 Bq = 1 s
–1
.
The former unit, the curie (Ci), is
equal to 3.7 × 10
10 Bq. The curie was
originally chosen to approximate the
activity of 1 gram of radium–226.
The gray (Gy), the SI unit of absorbed dose, is the absorbed dose
when the energy per unit mass imparted to matter by ionizing radiation is 1 joule per kilogram. The
former unit, the rad (rd), is equal to
10
–2 Gy.
The sievert (Sv), the SI unit of dose
equivalent, is the dose equivalent
when the absorbed dose of ionizing
radiation multiplied by the stipulated
dimensionless factors is 1 J kg
–1
. As
different types of radiation cause different effects in biological tissue a
weighted absorbed dose, called the
dose equivalent, is used in which the
absorbed dose is modiÜed by multiplying it by dimensionless factors
stipulated by the International Commission on Radiological Protection.
The former unit of dose equivalent,
the rem (originally an acronym for
roentgen equivalent man), is equal to
10
–2 Sv.
In SI units, exposure to ionizing radiation is expressed in coulombs per
kilogram, the quantity of X- or
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racemate See racemic mixture.
racemic mixture (racemate) A
mixture of equal quantities of the dand l-forms of an optically active
compound. Racemic mixtures are denoted by the preÜx dl- (e.g. dl-lactic
acid). A racemic mixture shows no
*optical activity.
racemization A chemical reaction
in which an optically active compound is converted into a *racemic
mixture.
rad See radiation units.
radial distribution function The
average number of atoms found at a
distance r from a central atom. In the
case of a crystal, the radial distribution function is a series of regular
spikes, corresponding to the atomic
positions in the lattice. Calculating
the radial distribution function is of
fundamental importance in the
theory of liquids.
radiation 1. Energy travelling in
the form of electromagnetic waves or
photons. 2. A stream of particles, especially alpha- or beta-particles from
a radioactive source or neutrons
from a nuclear reactor.
radiationless decay Decay of an
atom or molecule from an excited
state to a lower energy state without
the emission of electromagnetic radiation. A common example of a radiationless process is the *Auger effect,
in which an electron rather than a
photon is emitted as a result of
decay.
radiation units Units of measurement used to express the activity of a
radionuclide and the dose of ionizing
radiation. The units curie, roentgen,
rad, and rem are not coherent with
SI units but their temporary use with
SI units has been approved while the
derived SI units becquerel, gray, and
sievert become familiar.
The becquerel (Bq), the SI unit of
activity, is the activity of a radionuclide decaying at a rate, on average,
of one spontaneous nuclear transition per second. Thus 1 Bq = 1 s
–1
.
The former unit, the curie (Ci), is
equal to 3.7 × 10
10 Bq. The curie was
originally chosen to approximate the
activity of 1 gram of radium–226.
The gray (Gy), the SI unit of absorbed dose, is the absorbed dose
when the energy per unit mass imparted to matter by ionizing radiation is 1 joule per kilogram. The
former unit, the rad (rd), is equal to
10
–2 Gy.
The sievert (Sv), the SI unit of dose
equivalent, is the dose equivalent
when the absorbed dose of ionizing
radiation multiplied by the stipulated
dimensionless factors is 1 J kg
–1
. As
different types of radiation cause different effects in biological tissue a
weighted absorbed dose, called the
dose equivalent, is used in which the
absorbed dose is modiÜed by multiplying it by dimensionless factors
stipulated by the International Commission on Radiological Protection.
The former unit of dose equivalent,
the rem (originally an acronym for
roentgen equivalent man), is equal to
10
–2 Sv.
In SI units, exposure to ionizing radiation is expressed in coulombs per
kilogram, the quantity of X- or
www.AzShimi.ir www.AzShimi.com