Nuclear Spectroscopy

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12
X-Ray Energy (keV)
10
8
6
4
2
X XXXXXXXXXXXXXXXXXXXXXX
0
0 200 400 600 800 1000 1200 1400
(Z - 1) 2
Figure 7.3 The data demonstrate the systematic change in K
x-ray energy with atomic number, much as Moseley discovered
just before World War I.
The decay of a nucleus to an excited state of another
nucleus, such as the beta decay of 137 Cs to the 661.6 keV
excited state of 137 Ba, frequently results in the emission
of a 661.6 keV gamma as the 137m Ba decays to its ground
state (see Figure 2.1). A competing process is the
internal transition (IT), whereby the decay of 137m Ba to
its ground state transfers 661.6 keV to a K-shell electron,
allowing the electron to escape the atom with a
kinetic energy of 661.6 keV less its binding energy
L
K
nucleus
A = Z + N
Z protons
N neutrons
Before
Auger
Figure 7.4 The internal transition process is depicted above.
The decay of an excited nuclear state results in the same
nucleus with an atomic K-shell vacancy. The vacancy is
filled by an Auger transition (or K x-ray emission).
L
K
nucleus
A = Z + N
Z protons
N neutrons
After
Figure 7.5 The electron capture process is depicted above as
the capture of a K-shell electron by the nucleus, converting one
proton into a neutron.,with the emission of a neutrino. The
emission of a K x ray (or an Auger electron) follows the EC.
(which is the same as the K-shell ionization energy).
The resulting vacancy in the K shell is filled by an L-
shell or M-shell electron and K x rays from barium are
observed from a 137 Cs radioactive source. This IT
process is depicted in Figure 7.4.
Capture of a K-shell electron (EC) by the nucleus
results in one more neutron, one less proton, the
emission of an electron neutrino, and a vacancy in the
K shell. The overlap of the K-shell electron’s
wavefunction with the nucleus results in a nonzero
probability for the electron to be captured by the
nucleus. As an example, the nucleus 65 Zn undergoes
both positron decay and electron capture to the ground
state of 65 Cu, and only electron capture to its 1115 keV
excited state (see Appendix D). Copper K x rays are
observed from a 65 Zn source, along with 1115 keV
gammas. The electron capture process is depicted in
Figure 7.5.
OBJECTIVE
Observe the K x rays after electron capture and after
internal conversion. K x rays from lead are usually
observed in your spectra. Some gammas are intercepted
by the lead shielding around your detector and
interact with the electrons of the lead by the photoelectric
effect or Compton effect, resulting in an
electron vacancy in one of the atomic shells. If this is
a K-shell electron, then a lead K x ray will usually be
emitted. See Figure 7.6.
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