6 Chapter 1. Properties and Sources of Radiation
Electron capture:^8136 Kr+e →^8135 Br + γ + νPositron decay:^4019 K →^4018 Ar + e+ + νThe electron captured in the first of these reactions actually transforms a proton
intoaneutron,thatis
p+e→n+ν.
That is why the daughter nuclide has one proton less and one neutron more than
the parent nucleus. Electron capture transforms the nuclide into a different element.
In a similar fashion, the positron emission is the result of transformation of a proton
intoaneutron,thatis
p→n+e++ν.
This implies that in case of positron emission the daughter nuclide has one proton
less and one neutron more than the parent nuclide. This is also apparent from the
positron emission example of potassium-40 given earlier. It is interesting to note
that in this reaction the mass of the proton is less than the combined mass of the
neutron, positron, and the neutrino^4. This means that the reaction is possible only
when enough energy is available to the proton. That is why there is a threshold
energy of 1.022MeVneeded for positron emission. Below this energy the nuclide
can decay by electron capture, though.
Note that the electron capture reaction above shows that a photon is also emitted
during the process. This photon can be an x-ray or aγ-ray photon. The x-ray
photon is emitted as one of the electrons in the higher orbitals fills the gap left
by the electron captured by the nucleus. Since in most cases a K-shell electron is
captured by the nucleus, the orbital is quickly filled in by another electron from one
of the higher energy states. The difference in the energy is released in the form of
an x-ray photon. It can also happen that the nucleus being in an excited state after
capturing an electron emits one or moreγ-rays. The electron capture reaction then
should actually be written as a two step process, that is
81
36 Kr+e →81
35 Br∗+ν
81
35 Br∗ → 81
35 Br+γ
In general, the subsequentγ-decay is not specific to electron capture. It can
occur in a nucleus that has already undergone any other type of decay that has left
it in an excited state. It is a natural way by which the nuclei regain their stability.
Sometimes it takes a number ofγ-decays for a nucleus to eventually reach a stable
state.
Althoughγ-emission is the most common mode of de-excitation after a decay, it
is not the only one. Another possible process is the so calledinternal conversion.In
this process, the excess energy is transferred to an orbital electron. If the supplied
energy is greater than the binding energy of this electron, the electron gets expelled
from the orbital with a kinetic energy equal to the difference of the atom’s excess
energy and its binding energy.
(^4) As of the time of writing this book, the neutrino mass is still unknown. However it has been confirmed
that the mass is very small, probably 100,000 times less than the mass of an electron. We will also learn
later that there are actually threeor moretypes of neutrinos.