Biomedical Engineering Reference
In-Depth Information
Fig. 2.9
Typical continuous X-ray spectra from tube operating
at three different peak voltages with the same current.
this curve intersects the abscissa. The X-ray energies are commonly referred to in
terms of their peak voltages in kilovolts, denoted by kVp.
If the tube voltage is sufficient, electrons striking the target can eject electrons
from the target atoms. (The K-shell binding energy is
E
K
= 69.525
keV for tung-
sten.) Discrete X rays are then also produced. These are emitted when electrons
from higher shells fill the inner-shell vacancies. The photon energies are charac-
teristic of the element of which the target is made, just as the optical spectra are
in the visible range. Characteristic X rays appear superimposed on the continuous
spectrum, as illustrated for tungsten in Fig. 2.10. They are designated
K
α
,
K
β
,and
so forth, when the K-shell vacancy is filled by an electron from the L shell, M shell,
and so on. (In addition, when L-shell vacancies are filled, characteristic L
α
,L
β
,and
so forth, X rays are emitted. These have low energy and are usually absorbed in the
tube housing.)
Because the electron energies in the other shells are not degenerate, the K X rays
have a fine structure, not shown in Fig. 2.10. The L shell, for example, consists
of three subshells, in which for tungsten the electron binding energies in keV are
E
LI
=
12.098,
E
LII
=
11.541,and
E
LIII
=
10.204. The transition LIII → KgivesaK
α
1
photon with energy
E
K
-
E
LIII
=
59.321
keV; the transition LII
→
KgivesaK
α
2
photon with energy 57.984 keV. The optical transition LI
→
Kis
quantum mechanically forbidden and does not occur.
The first systematic study of characteristic X rays was carried out in 1913 by
the young British physicist, H. G. J. Moseley, working in Rutherford's laboratory.
69.525 - 10.204
=
