【医】 β[射]线

Beta rays \Be"ta rays\ (Physics)
a form of ionizing radiation emitted by radioactive
substances (such as radium), more penetrating than alpha
rays, and consisting of negatively charged electrons. The
electrons are the same kind of particle as those of cathode
rays, but have much higher velocities (about 35,000 to
180,000 miles per second). They are readily deflected by a
magnetic or electric field.
[Webster 1913 Suppl.]

Electron \E*lec"tron\, n. [NL., fr. Gr. 'h`lektron. See
{Electric}.]
1. Amber; also, the alloy of gold and silver, called
{electrum}. [archaic]

2. (Physics & Chem.) one of the fundamental subatomic
particles, having a negative charge and about one
thousandth the mass of a hydrogen atom. The electron
carries (or is) a natural unit of negative electricity,
equal to 3.4 x 10^{-10} electrostatic units, and is
classed by physicists as a {lepton}. Its mass is
practically constant at the lesser speeds, but increases
due to relativistic effects as the velocity approaches
that of light. Electrons are all of one kind, so far as is
known. Thus far, no structure has been detected within an
electron, and it is probably one of the ultimate composite
constituents of all matter. An atom or group of atoms from
which an electron has been detached has a positive charge
and is called a {cation}. Electrons are projected from the
cathode of vacuum tubes (including television picture
tubes) as {cathode rays} and from radioactive substances
as the {beta rays}. Previously also referred to as
{corpuscle}, an obsolete term. The motion of electrons
through metallic conductors is observed as an electric
current. A particle identical to the electron in mass and
most other respects, but having a positive instead of a
negative charge, is called a {positron}, or {antielectron}
[Webster 1913 Suppl. +PJC]