greater than 1.02 MeV. In b+-decay, essentially a proton is converted to a
neutron plus a positron, thus, decreasing the atomic number Zof the daugh-
ter nuclide by 1. Thus,
p→n+b++
The requirement of 1.02 MeV for b+-decay arises from the fact that one
electron mass has to be added to a proton to produce a neutron and one
positron is created. Since each electron or positron mass is equal to
0.511 MeV, one electron and one positron are equal to 1.02 MeV, which
is required as a minimum for b+-decay.
Some examples of b+-decay follow:
18
9 F →
18
8 O +b
++
68
31 Ga →
68
30 Zn +b++
13
7 N →
13
6 C +b++
15
8 O →
15
7 N +b++
The energetic b+-particle loses energy while passing through matter. The
range of positrons is short in matter. When it loses almost all of its energy,
it combines with an atomic electron of the medium and is annihilated, giving
rise to two photons of 511 keV emitted in opposite directions. These
photons are called annihilation radiations.
The decay scheme of^68 Ga is presented in Figure 2.6. Note that the b+-
decay is represented by a two-step right-to-left arrow.
18 2. Radioactive Decay
Fig. 2.6. Decay scheme of^68 Ga. The positrons are annihilated in medium to give
rise to two 511-keV g-rays emitted in opposite directions.