R

racemateSee racemic mixture.

racemic mixture (racemate)A
mixture of equal quantities of the d-
and l-forms of an optically active
compound. Racemic mixtures are de-
noted 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 com-
pound is converted into a *racemic
mixture.

radSee radiation units.

radial distribution functionThe
average number of atoms found at a
distance r from a central atom. In the
case of a crystal, the radial distribu-
tion 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.

radiation1.Energy travelling in
the form of electromagnetic waves or
photons. 2.A stream of particles, es-
pecially 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 radi-
ation. A common example of a radia-
tionless process is the *Auger effect,
in which an electron rather than a
photon is emitted as a result of
decay.

radiation units Units of measure-
ment 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 radionu-

clide decaying at a rate, on average,

of one spontaneous nuclear transi-

tion per second. Thus 1 Bq = 1 s–1.

The former unit, the curie (Ci), is

equal to 3.7 × 1010 Bq. The curie was

originally chosen to approximate the

activity of 1 gram of radium–226.

The gray (Gy), the SI unit of ab-

sorbed dose, is the absorbed dose

when the energy per unit mass im-

parted to matter by ionizing radia-

tion is 1 joule per kilogram. The

former unit, the rad (rd), is equal to

10 –2Gy.

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 dif-

ferent effects in biological tissue a

weighted absorbed dose, called the

dose equivalent, is used in which the

absorbed dose is modiÜed by multi-

plying it by dimensionless factors

stipulated by the International Com-

mission on Radiological Protection.

The former unit of dose equivalent,

the rem (originally an acronym for

roentgen equivalent man), is equal to

10 –2Sv.

In SI units, exposure to ionizing ra-

diation is expressed in coulombs per

kilogram, the quantity of X- or