Elements of Modern Physics
158
The main contribution to the attenuation of x-rays is the photoelectric
absorption of the photons with the emission of electrons. As the frequency is
lowered beyond v K , the photon energy is not sufficient to eject a K-shell electron.
The closure of this channel lowers the absorption coefficient µ and gives rise to
the K-absorption edge. When the energy is lower than v LI , the electrons in the
L I shell also cannot be ejected, giving rise to the L I absorption edge. Thus, there
are three absorption edges corresponding to the L I , L II and L III subshells, five
absorption edges corresponding to the M shell (see Fig. 5.8), etc. The frequencies
of these absorption edges are given by
hv = | E |
(5.55)
where E is the energy of the K shell, the L I shell, etc. Comparison of this
expression with that in Eq. (5.48) for characteristic emission frequencies shows
that the frequency of the absorption edge is always greater than the corresponding
characteristic emission frequencies (this may be observed in Fig. 5.10). The
reason for this is that emission lines correspond to transition between two shells
(or subshells). On the other hand, the frequency of the absorption edge,
corresponds to a transition between the lower shell or subshell and the continum
states. In comparison with Eq. (5.51), the frequency of the absorption edges is
given by
hv ≈ |E 0 |
2
2
(
)
f
f
Z
n
− σ
(5.56)
which is greater than the corresponding characteristic emission frequency in
Eq. (5.51), and is equal to the limit of the characteristic emission frequency for
n i → ∞.
Auger Effect
So far, it has been assumed that the vacancy in an inner shell, say the K shell, is
filled by an electron from an outer shell, along with the emission of a photon. It
is however found experimentally, that the fluorescence yield w defined as:
w =
p
e
n
n
(5.57)
n p being the number of K photons and n e being number of K electrons knocked
out (i.e. number of K-shell vacancies), is smaller than 1, ranging from a value of
0.1 for light elements to about 0.95 for uranium.
An explanation of the above observation was found by Auger (1925) who
noted that the ejection of the K-shell electron is often accompanied by the ejection
of another electron. This is due to the fact that the electron which undergoes a
transition to the vacant K shell may knock out another electron usually from the
same shell, i.e. its initial shell. This is known as an Auger transition and the
emitted electron is known as an Auger electron. It should be emphasized that
158
The main contribution to the attenuation of x-rays is the photoelectric
absorption of the photons with the emission of electrons. As the frequency is
lowered beyond v K , the photon energy is not sufficient to eject a K-shell electron.
The closure of this channel lowers the absorption coefficient µ and gives rise to
the K-absorption edge. When the energy is lower than v LI , the electrons in the
L I shell also cannot be ejected, giving rise to the L I absorption edge. Thus, there
are three absorption edges corresponding to the L I , L II and L III subshells, five
absorption edges corresponding to the M shell (see Fig. 5.8), etc. The frequencies
of these absorption edges are given by
hv = | E |
(5.55)
where E is the energy of the K shell, the L I shell, etc. Comparison of this
expression with that in Eq. (5.48) for characteristic emission frequencies shows
that the frequency of the absorption edge is always greater than the corresponding
characteristic emission frequencies (this may be observed in Fig. 5.10). The
reason for this is that emission lines correspond to transition between two shells
(or subshells). On the other hand, the frequency of the absorption edge,
corresponds to a transition between the lower shell or subshell and the continum
states. In comparison with Eq. (5.51), the frequency of the absorption edges is
given by
hv ≈ |E 0 |
2
2
(
)
f
f
Z
n
− σ
(5.56)
which is greater than the corresponding characteristic emission frequency in
Eq. (5.51), and is equal to the limit of the characteristic emission frequency for
n i → ∞.
Auger Effect
So far, it has been assumed that the vacancy in an inner shell, say the K shell, is
filled by an electron from an outer shell, along with the emission of a photon. It
is however found experimentally, that the fluorescence yield w defined as:
w =
p
e
n
n
(5.57)
n p being the number of K photons and n e being number of K electrons knocked
out (i.e. number of K-shell vacancies), is smaller than 1, ranging from a value of
0.1 for light elements to about 0.95 for uranium.
An explanation of the above observation was found by Auger (1925) who
noted that the ejection of the K-shell electron is often accompanied by the ejection
of another electron. This is due to the fact that the electron which undergoes a
transition to the vacant K shell may knock out another electron usually from the
same shell, i.e. its initial shell. This is known as an Auger transition and the
emitted electron is known as an Auger electron. It should be emphasized that