202
9.
X—RAYS
_
,
X-rays whose wave-lengths are greater than approximately
ten angstroms are readily absorbed, even by a fraction of a millimeter of air and can only be studied with apparatus placed in an
evacuated chamber.
Shorter and shorter x-rays are increasingly
penetrating. In the wave-length range from, roughly 3 to 0.3
_
angstroms, the photoelectric type of absorption greatly exceeds
that due to scattering. Very approximately, one may then_say
” = 7' = €7\3
(9—5)
where 6 is a constant (=c’Z4, c' is another constant, Z is the atomic
_
number of the absorbing element) and )\ is the wave—length of the
x-rays.
This is known as Owen’s law.
Typical values are as
'
follows: between 0.4 and 2.3 angstroms, for a certain specimen
of mica, pt =
and for cellophane p, = 3.52À2’66.
A graph of the mass absorption coefcients of argon at different
wave—lengths is given in gure 9—8. Here, it is seen that the gas
'
_
becomes
more
and
more
..
,…
K
'
transparent
as
the
wave—
Ë ,…
'
length decreases (in approxi_
%
1200
!
_
mate agreement with
Oweh’s
1000
:
law) unt11 a crit1cal wave—
Ê
800
:
length, marked K in the
“°
!
gure, is reached. For silver,
;_ ::: a.… ;
this occurs at a wave—length
£
of 0.484 angstrom and for
'
?
3wÀvÊ—rÊmrf
?
a
9
A”
bromin_e
at _0:918 angstrom.
Flo. 9—8.
Mass absorption coefcients of
.
thls
crltzca.l absorp l1071
argon for different wave-length x-rays.
lim” the materlal SUddenlY
_
becomes
cô‘mparatively.
opaque to the x-rays.
In other Words, just below K, large amounts
_
of energy_are extracted from the beam by the atoms. -It can be
,
shown that this energy is used to raise electrons from the innerm03t
or K shell inside the atoms to higher energy levels. Upon the sûb— '
_
sequent return of these atomic electrons to their normal energy
-
levels, a part of the absorbed energy is re-radiated in the form of
—
uorescent x-‘mys whose wave—length is characteristic of the atoms:
,
of the absorbing material and independent of that of the incident
-
x-rays.
The shortest uorescent wave-length which can be excited
9.
X—RAYS
_
,
X-rays whose wave-lengths are greater than approximately
ten angstroms are readily absorbed, even by a fraction of a millimeter of air and can only be studied with apparatus placed in an
evacuated chamber.
Shorter and shorter x-rays are increasingly
penetrating. In the wave-length range from, roughly 3 to 0.3
_
angstroms, the photoelectric type of absorption greatly exceeds
that due to scattering. Very approximately, one may then_say
” = 7' = €7\3
(9—5)
where 6 is a constant (=c’Z4, c' is another constant, Z is the atomic
_
number of the absorbing element) and )\ is the wave—length of the
x-rays.
This is known as Owen’s law.
Typical values are as
'
follows: between 0.4 and 2.3 angstroms, for a certain specimen
of mica, pt =
and for cellophane p, = 3.52À2’66.
A graph of the mass absorption coefcients of argon at different
wave—lengths is given in gure 9—8. Here, it is seen that the gas
'
_
becomes
more
and
more
..
,…
K
'
transparent
as
the
wave—
Ë ,…
'
length decreases (in approxi_
%
1200
!
_
mate agreement with
Oweh’s
1000
:
law) unt11 a crit1cal wave—
Ê
800
:
length, marked K in the
“°
!
gure, is reached. For silver,
;_ ::: a.… ;
this occurs at a wave—length
£
of 0.484 angstrom and for
'
?
3wÀvÊ—rÊmrf
?
a
9
A”
bromin_e
at _0:918 angstrom.
Flo. 9—8.
Mass absorption coefcients of
.
thls
crltzca.l absorp l1071
argon for different wave-length x-rays.
lim” the materlal SUddenlY
_
becomes
cô‘mparatively.
opaque to the x-rays.
In other Words, just below K, large amounts
_
of energy_are extracted from the beam by the atoms. -It can be
,
shown that this energy is used to raise electrons from the innerm03t
or K shell inside the atoms to higher energy levels. Upon the sûb— '
_
sequent return of these atomic electrons to their normal energy
-
levels, a part of the absorbed energy is re-radiated in the form of
—
uorescent x-‘mys whose wave—length is characteristic of the atoms:
,
of the absorbing material and independent of that of the incident
-
x-rays.
The shortest uorescent wave-length which can be excited