Biomedical Engineering Reference
In-Depth Information
Fig. 2.10
Spectrum showing characteristic K
α
and K
β
discrete
X rays in addition to the continuous X rays. Characteristic K X
rays are present only when the tube operating voltage is high
enough to give the incident electrons sufficient energy to eject
an electron from the K shell in the target atoms. Potential
difference across the tube in volts is then practically
≥
K-shell
binding energy in eV.
The diffraction of X rays by crystals had been discovered by von Laue in 1912,
and Moseley used this process to compare characteristic X-ray wavelengths. He
found that the square root of the frequencies of corresponding lines (e.g., K
α
1
)
in the characteristic X-ray spectra increases by an almost constant amount from
element to element in the periodic system. Alpha-particle scattering indicated that
the number of charge units on the nucleus is about half the atomic weight. Moseley
concluded that the number of positive nuclear charges and the number of electrons
both increase by one from element to element. Starting with
Z
=
1
for hydrogen,
thenumberofchargeunits
Z
determines the atomic number of an element, which
gives its place in the periodic system.
The linear relationship between
√
ν and
Z
would be predicted if the electrons
in many-electron atoms occupied orbits like those predicted by Bohr's theory for
single-electron systems. As seen from Eq. (2.13), the frequencies of the photons
for a given transition i
→
f in different elements are proportional to
Z
2
.
