bei48482_FM

Nuclear Structure 399

nucleus of an electron from its innermost shell. The electron is absorbed by a nuclear

proton which is thereby transformed into a neutron:

Electron capture peSn^0

Figure 11.9 shows how alpha and beta decays enable stability to be achieved. Ra-

dioactivity is considered in more detail in Chap. 12, where we will find that another

particle, the neutrino, is also involved in beta decay and electron capture.

11.4 BINDING ENERGY

The missing energy that keeps a nucleus together

The hydrogen isotope deuterium,^21 H, has a neutron as well as a proton in its nucleus.

Thus we would expect the mass of the deuterium atom to be equal to that of an ordinary

1

1 H atom plus the mass of a neutron:

Mass of^11 H atom 1.007825 u

mass of neutron 1.008665 u

Expected mass of^21 H atom 2.016490 u

However, the measured mass of the^21 H atom is only 2.014102 u, which is 0.002388 u

lessthan the combined masses of a^11 H atom and a neutron (Fig. 11.10).

What comes to mind is that the “missing” mass might correspond to energy given

off when a^21 H nucleus is formed from a free proton and neutron. The energy equiva-

lent of the missing mass is

E(0.002388 u)(931.49 MeV/u)2.224 MeV

To test this interpretation of the missing mass, we can perform experiments to see how

much energy is needed to break apart a deuterium nucleus into a separate neutron and

Alpha decay

Neutron number (

N

)

Proton number (Z)

N decreases by 2

Z decreases by 2

Positive beta decay

or electron capture

Z decreases by 1

N decreases by 1Z increases by 1 N increases by 1

Stability curve

Negative beta decay

Alpha decay

Figure 11.9Alpha and beta decays permit an unstable nucleus to reach a stable configuration.

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