bei48482_FM

Nuclear Transformations 421

Gamma

decay

Alpha

decay

Beta

decay

Electron

capture

Positron

emission

Emission of positron

by proton in nucleus

changes the proton to a neutron

Capture of electron

by proton in nucleus

changes the proton to a neutron

Emission of electron

by neutron in nucleus

changes the neutron to a proton

Emission of alpha particle

reduces size of nucleus

Emission of gamma ray

reduces energy of nucleus

Proton (charge = +e)

Neutron (charge = 0)

Electron (charge = –e)

Positron (charge = +e)

= +

+ =

= +

Original nucleus Decay event

Final

nucleus

Reason for

instability

Nucleus has

excess energy

Nucleus too

large

Nucleus has too

many neutrons

relative to number

of protons

Nucleus has too

many protons

relative to number

of neutrons

Nucleus has too

many protons

relative to number

of neutrons

Figure 12.3Five kinds of radioactive decay.

Example 12.1

The helium isotope^62 He is unstable. What kind of decay would you expect it to undergo?

Solution

The most stable helium nucleus is^42 He, all of whose neutrons and protons are in the lowest

possible energy levels (see Sec. 11.3). Since^62 He has four neutrons whereas^42 He has only two,

the instability of^62 He must be due to an excess of neutrons. This suggests that^62 He undergoes

negative beta decay to become the lithium isotope^63 Li whose neutron/proton ratio is more

consistent with stability:

6

2 HeS

6

3 Lie



This is, in fact, the manner in which^62 He decays.

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