2.3 Interconversion of Nucleons Within the Nucleus
17
n → p + β
−
+ μ
2.3.2 Conversion of Proton to Neutron
Like the conversion of an excess neutron to a proton, a nucleus with an excess proton
(i.e., excess than needed for its stability) can approach stability by converting a proton
into a neutron by capturing one of the nearest orbital electrons. This type of decay
is known as electron capture (E.C.). Unlike β-decay, the excess mass (i.e., energy
converted from the excess mass) left even after converting the proton by capturing the
orbital electron is converted into a neutrino (μ). This type of decay can be expressed
as follows:
p + e
−
→ n + μ.
If for some reasons, electron capture is not possible, the proton can be converted
into a neutron by the emission of a positron (β
+ ); this type of decay is known as
positron decay. In this type of decay also the neutrino is ejected for the same reason
as expressed earlier. This decay can be expressed as follows:
p → n + β
+
+ μ.
A positron is a positively charged electron designated by β
+ . The question is:
which of these two types of decays will an isotope having an excess proton prefer?
This can be found out by considering mass excess in each decay process. In decay
by positron emission, the atomic number of the atom is decreased by one unit, and
one of the orbital electrons becomes superfluous:
A K Z
→
A Y Z−1
+ β
+
+ e + decay enery
(Z electrons)
(Z − 1, electrons)
If atomic masses are used for the calculation of positron decay energy, the mass
of this superfluous electron must be considered. Thus,
(
A X Z −
A Y Z−1 ) − (β
+
+ e
−
) = positron decay energy
This calculation suggests that for positron emission to occur spontaneously, the
difference in mass between parent and daughter atoms must be greater than the
combined mass of the positron and electron. That is,
(
A X Z −
A Y Z−1 ) > (β
+
+ e
−
).
This may be illustrated by considering an example of Sodium-22 decay.
17
n → p + β
−
+ μ
2.3.2 Conversion of Proton to Neutron
Like the conversion of an excess neutron to a proton, a nucleus with an excess proton
(i.e., excess than needed for its stability) can approach stability by converting a proton
into a neutron by capturing one of the nearest orbital electrons. This type of decay
is known as electron capture (E.C.). Unlike β-decay, the excess mass (i.e., energy
converted from the excess mass) left even after converting the proton by capturing the
orbital electron is converted into a neutrino (μ). This type of decay can be expressed
as follows:
p + e
−
→ n + μ.
If for some reasons, electron capture is not possible, the proton can be converted
into a neutron by the emission of a positron (β
+ ); this type of decay is known as
positron decay. In this type of decay also the neutrino is ejected for the same reason
as expressed earlier. This decay can be expressed as follows:
p → n + β
+
+ μ.
A positron is a positively charged electron designated by β
+ . The question is:
which of these two types of decays will an isotope having an excess proton prefer?
This can be found out by considering mass excess in each decay process. In decay
by positron emission, the atomic number of the atom is decreased by one unit, and
one of the orbital electrons becomes superfluous:
A K Z
→
A Y Z−1
+ β
+
+ e + decay enery
(Z electrons)
(Z − 1, electrons)
If atomic masses are used for the calculation of positron decay energy, the mass
of this superfluous electron must be considered. Thus,
(
A X Z −
A Y Z−1 ) − (β
+
+ e
−
) = positron decay energy
This calculation suggests that for positron emission to occur spontaneously, the
difference in mass between parent and daughter atoms must be greater than the
combined mass of the positron and electron. That is,
(
A X Z −
A Y Z−1 ) > (β
+
+ e
−
).
This may be illustrated by considering an example of Sodium-22 decay.