9—6.
ABSORPTION OF
201
_
unit cross section by each atom… For ordinary light rays, carbon
is transparent when in the form of diamond and opaque when in
the form of graphite. For X-rays, however, both p.a and M… are the
' .
same for the two forms of carbon.
.
_
If the X—ray beam contains more than one wave—length, the
softer rays will be absorbed by the rst portions of the material,
leaving the harder components to penetrate to greater dépths.
This is true for the emulsion of a photographic plate as well as
for
.
intermediate
absorbing screens. The qective aôsorpz‘ian .
coeïcimt, as computed from equation 9—1, is, therefore, greater
‘
for the rays which have just entered the absorber than for tho'se
which have penetrated to a greater depth. For a satisfactory
study of the absorption of different materials, it is necessary to'
use a monochromatic or one wave-length beam of X—ra'ys. Such a
monochromatic beam maybe obtained by the use of an X—ray
spectrometer, just as a monochromatic beam of light
is obtained
by the use of a spectroscope. As a crude but simpler method, one
may use a lter consistingbf a sheet of copper
Of such.thickness as
to reduce the original intensity of the beam to ten per cent or less
of its unltered value.
For the less penetrating rays from the '
lower voltage tubes, a copper
lter one-half millimeter.
in thicknessis commonly used and for the higher
tubes this
is
increased to one or two millimeters. It “is customary-to place
-
beyond these lters, a one millimetér sheet of aluminùm,in order '
‘
to reduce the eHects of the rays scattered by the copper.
9—7.
Processes—The term “absorption”
is customarily used in two senses. In the rst, a constant fraction
of
the energy in the parallel beam,
centimeter of path, 18 trans—
ferred to the absorbing material, ejecting electrons from its atoms,
giving them kinetic energy
and also, perhaps, adding potential
energy to électrons which remain in the atoms. In the second
prOcess,
“
absorption
”
is the result of scattering
or dev1at1‘on
of
part of the x-rays
their direct path, With conseqùent
tion of the” intensity along the parallel beam.
The former,
photo-electric or true absorption, may
be expressed by means of an
_
absorption coeicient 'r and the latter,
process, by
_‘
_
means of a coefcient 0, such that
—
'_
_
_
-
p=7+0‘
(9—4)
ABSORPTION OF
201
_
unit cross section by each atom… For ordinary light rays, carbon
is transparent when in the form of diamond and opaque when in
the form of graphite. For X-rays, however, both p.a and M… are the
' .
same for the two forms of carbon.
.
_
If the X—ray beam contains more than one wave—length, the
softer rays will be absorbed by the rst portions of the material,
leaving the harder components to penetrate to greater dépths.
This is true for the emulsion of a photographic plate as well as
for
.
intermediate
absorbing screens. The qective aôsorpz‘ian .
coeïcimt, as computed from equation 9—1, is, therefore, greater
‘
for the rays which have just entered the absorber than for tho'se
which have penetrated to a greater depth. For a satisfactory
study of the absorption of different materials, it is necessary to'
use a monochromatic or one wave-length beam of X—ra'ys. Such a
monochromatic beam maybe obtained by the use of an X—ray
spectrometer, just as a monochromatic beam of light
is obtained
by the use of a spectroscope. As a crude but simpler method, one
may use a lter consistingbf a sheet of copper
Of such.thickness as
to reduce the original intensity of the beam to ten per cent or less
of its unltered value.
For the less penetrating rays from the '
lower voltage tubes, a copper
lter one-half millimeter.
in thicknessis commonly used and for the higher
tubes this
is
increased to one or two millimeters. It “is customary-to place
-
beyond these lters, a one millimetér sheet of aluminùm,in order '
‘
to reduce the eHects of the rays scattered by the copper.
9—7.
Processes—The term “absorption”
is customarily used in two senses. In the rst, a constant fraction
of
the energy in the parallel beam,
centimeter of path, 18 trans—
ferred to the absorbing material, ejecting electrons from its atoms,
giving them kinetic energy
and also, perhaps, adding potential
energy to électrons which remain in the atoms. In the second
prOcess,
“
absorption
”
is the result of scattering
or dev1at1‘on
of
part of the x-rays
their direct path, With conseqùent
tion of the” intensity along the parallel beam.
The former,
photo-electric or true absorption, may
be expressed by means of an
_
absorption coeicient 'r and the latter,
process, by
_‘
_
means of a coefcient 0, such that
—
'_
_
_
-
p=7+0‘
(9—4)