DETECTORS
45
•
controls
the
acceptance
angle
-
this
has
obvious
applications
in
defining
the
angular resolution
for
angle resolved XPS,
and it is
also
important
for
small area
and
imaging
XPS
because
the
angular accep-
tance
will determine
the
spatial resolution
as
well
as the
transmission.
In
the
past, these transfer lenses were exclusively electrostatic
but
there
are
some modern instruments which
are
fitted
with
a
magnetic
immersion lens.
This
is a
large electromagnet
the
current through
which
is
scanned
as a
function
of the
kinetic energy
of the
electrons
being
analysed. When using this type
of
lens,
the
specimen
is
situated
within
the
magnetic
field. A
major advantage
of
this type
of
lens
is
that
it can
collect electrons from
a
very wide range
of
emission angles;
typically,
a
collection angle
of 90° can be
achieved compared with about
25°,
usual
for a
purely electrostatic system. This
can
significantly
im-
prove
the
sensitivity
of the
instrument. Magnetic lenses
can
provide
better
spatial
resolution
than
electrostatic
lenses
of the
same focal length
because
their aberration
coefficients
are
lower.
Magnetic lenses cannot
be
used
for AES
because
the
magnetic
field
would deflect
the
primary electron beam
and
affect
the
spot
size
at the
sample surface.
The
extent
of
these
effects
would depend upon
the
mag-
netic
field
strength, which
is
varied with
the
kinetic energy being analysed.
2.6
Detectors
In
most
electron
spectrometers
it is
necessary
to
count
the
individual
electrons arriving
at the
detector.
To
achieve this, electron multipliers
are
used. Although there
are
many types
of
electron multiplier, only
two
types
are
commonly used
in
electron spectrometers: channel electron
multipliers
(channeltrons)
and
channel
plates.
2.6.1
Channel
electron
multipliers
These consist
of a
spiral-shaped glass tube with
a
conical collector
at
one
end and a
metal
anode
at the
other.
The
internal walls
of the
detector
are
coated with
a
material which, when struck
by an
electron