X-rays
Henri G.J. Moseley
1887 – 1915
X-Ray Production
• While an electron is in the vicinity of a nucleus, it feels its electromagnetic
field
• Acceleration (v2/R) = Electromagnetic Radiation
• Bremsstrahlung (“Braking radiation”) effect Emission of Photons (Xrays: λ=0.01 to 1nm)
• Not a continuous process, since a photon can only be created with an
energy E = hν
X-rays
• When Bohr’s model appears
(1913), people know about
X-rays (Roentgen, 1895):
– Bombardment of e- into
materials:
• Bremstrahlung (Continuum)
• Peaks (which are different for
each element)
Shells
• Historically:
– States were given letters:
•
•
•
•
•
n=1
n=2
n=3
n=4
etc…
K shell,
L shell,
M shell,
N shell,
– Transitions ∆n between shells:
• ∆n=1
• ∆n=2
• etc…
α,
β,
Many electrons atom
• Bohr’s model works for Hydrogen
• What about the other elements ?
– Z: Atomic number Number of
electrons
•
•
•
•
Z=1 Hydrogen
Z=2 Helium
Z=6 Carbon
Z=92 Uranium
…
-1.51 Z2 eV
-3.40 Z2 eV
– Approximation: we ignore the mutual
repulsion of the electrons.
• The coulomb force between one electron
and the nucleus is: F = (Ze2)/(4πε0r2)
• Applying the same treatment than
before: En = Z2 E1/n2
10.2 Z2 eV
-13.6 Z2 eV
Characteristic X-ray Wavelengths
• 1913: H.G.J.Moseley 1887-1915
– Catalogs the X-ray emission (K) of various
elements
– Finds an empirical formula:
– Volunteered to fight in World war I, killed in
action in 1915 (he was 27 years old)
2
(Z-1) ? Why -1 ?
• Transitions measured (K) correspond to
the transition of the electrons deep in the
atom.
– Does not depend on Z Always the closest
– So why (Z-1) ?
– At the time, nobody knows, but there can be 2
electrons in the inner orbit Charge
screening effect
Moseley Plot
• X-ray energy depends on Z, so one
can determine Z by measuring the
energy of the X-rays
• Some elements were missing !
– Z=43
– Z=61
– Z=75
Technecium
Promethium (1940) !
Rhenium
• Element composition: X-ray
fluorescence
– Non-destructive