PENELOPE 2003 - OECD Nuclear Energy Agency

2.6. Atomic relaxation 65
2.6 Atomic relaxation
Atoms are primarily ionized by photon interactions and by electron or positron impact.
There is a fundamental difference between the ionizing effects of photons and of charged
particles. A photon is only able to directly ionize a few atoms. In the case of photoabsorption,
when the photon energy is larger than the K-shell binding energy, about
80% of photoabsorptions occur in the K shell, i.e. the resulting ion with a vacancy in
the K shell is highly excited. Incoherent scattering is not as highly preferential, but
still the probability that an inner shell is ionized is nearly proportional to the number
of electrons in the shell. Conversely, fast electrons and positrons (and other charged
particles) ionize many atoms along their paths; the ionizations occur preferentially in
the less tightly bound atomic shells, or the conduction band in the case of metals (see
section 3.2), so that most of the produced ions are only weakly excited.
Excited ions with a vacancy in an inner shell relax to their ground state through a
sequence of radiative and non-radiative transitions. In a radiative transition, the vacancy
is filled by an electron from an outer shell and an x ray with characteristic energy is
emitted. In a non-radiative transition, the vacancy is filled by an outer electron and
the excess energy is released through emission of an electron from a shell that is farther
out (Auger effect). Each non-radiative transition generates an additional vacancy that,
in turn, migrates “outwards”. The production of vacancies in inner shells and their
subsequent relaxation must be simulated in detail, since the energetic x rays and/or
electrons emitted during the process may transport energy quite a distance from the
excited ion.
1.000
0.100
0.010
Auger
L3
M3
L2
M2
N3
N2
0.001
0 10 20 30 40 50 60 70 80 90
Z
Figure 2.11: Relative probabilities for radiative and non-radiative (Auger) transitions that
fill a vacancy in the K-shell of atoms.