an isomeric state. This is true for all b−-,b+-, or electron capture decays that
are followed by g-ray emission.
Some examples of b−-decay follow:
42(^99) Mo →99m
43 Tc +b
−+
131
53 I →
131
54 Xe +b−+
67
29 Cu →
67
30 Zn +b−+
90
38 Sr →
90
39 Y +b−+
It should be noted that in b−-decay, the atomic number of the daughter
nuclide is increased by 1 and the mass number remains the same.
Positron (b+)-Decay
When a radionuclide is proton rich—that is, the N/Zratio is low relative to
that of the nearest stable nuclide—it can decay by positron (b+) emission
accompanied by the emission of a neutrino (), which is an opposite entity
of the antineutrino. Positron emission takes place only when the energy dif-
ference (transition energy) between the parent and daughter nuclides is
Positron (b+)-Decay 17Fig. 2.5. Decay scheme of^99 Mo. Approximately 87% of the total^99 Mo ultimately
decays to 99mTc, and the remaining 13% decays to^99 Tc. A 2-keV transition occurs
from the 142-keV level to the 140-keV level. All the 2-keV g-rays are internally con-
verted. (The energy levels are not shown in scale.)