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Cs133

Cs-133, or cesium-133, is the only stable isotope of the element cesium. It has 55 protons and 78 neutrons, giving a mass number of 133. This isotope does not undergo radioactive decay, making it the sole naturally occurring and long-lived form of cesium.

In atomic terms, a neutral Cs-133 atom has the electron configuration [Xe] 6s1. The nuclear spin of

Naturally, Cs-133 accounts for essentially all natural cesium; other cesium isotopes are radioactive and present only

Applications of Cs-133 are dominated by metrology. Because of its stable nature and well-defined hyperfine transition,

Safety-wise, Cs-133 is not radioactive, but cesium metal is highly reactive with water and air and must

Cs-133
is
I
=
7/2+.
The
ground
state
of
the
atom
is
a
6S1/2
state,
and
the
interaction
between
the
nuclear
spin
and
the
electron
angular
momentum
splits
this
level
into
two
hyperfine
components,
with
total
angular
momentum
F
=
4
and
F
=
3.
The
transition
between
these
two
hyperfine
levels
is
precisely
defined
and
is
the
basis
of
the
modern
definition
of
the
second:
exactly
9,192,631,770
periods
of
the
radiation
corresponding
to
the
F
=
4
to
F
=
3
transition
in
Cs-133.
in
trace
amounts.
Physical
properties
of
cesium
as
an
element
are
notable:
it
is
a
soft,
highly
reactive
alkali
metal
with
a
melting
point
of
about
28.5°C
and
a
boiling
point
around
671°C,
and
a
density
near
1.93
g/cm3
at
room
temperature.
Cs-133
serves
as
the
primary
standard
for
the
SI
second.
Atomic
clocks
based
on
this
transition,
including
fountain
clocks,
underpin
timekeeping,
global
positioning
systems,
and
high-precision
measurements
in
physics.
be
handled
with
appropriate
precautions;
other
radioactive
cesium
isotopes
require
radiological
safety
measures.