bei48482_FM

Positions were discovered in 1932 and two years later were found to be

spontaneously emitted by certain nuclei. The properties of the positron are identical

with those of the electron except that it carries a charge ofeinstead ofe. Positron

emission corresponds to the conversion of a nuclear proton into a neutron, a positron,

and a neutrino:

Positron emission p→ne (12.16)

Whereas a neutron outside a nucleus undergoes negative beta decay into a proton (half-

life10 min 16 s) because its mass is greater than that of the proton, the lighter

proton cannot be transformed into a neutron except within a nucleus. Positron emis-

sion leads to a daughter nucleus of lower atomic number Zwhile leaving the mass

number Aunchanged.

Closely connected with positron emission is electron capture. In electron capture

a nucleus absorbs one of its inner atomic electrons, with the result that a nuclear

proton becomes a neutron and a neutrino is emitted:

Electron capture pe→n (12.17)

Usually the absorbed electron comes from the Kshell, and an x-ray photon is emitted

when one of the atom’s outer electrons falls into the resulting vacant state. The wave-

length of the photon will be one of those characteristic of the daughter element, not

of the original one, and the process can be recognized on this basis.

Electron capture is competitive with positron emission since both processes lead to

the same nuclear transformation. Electron capture occurs more often than positron

emission in heavy nuclides because the electrons in such nuclides are relatively close

to the nucleus, which promotes their interaction with it. Since nearly all the unstable

nuclei found in nature are of high Z, positron emission was not discovered until several

decades after electron emission had been established.

Inverse Beta Decay

By comparing Eqs. (12.16) and (12.17) we see that electron capture by a nuclear proton

is equivalent to a proton’s emission of a positron. Similarly the absorption of an

Nuclear Transformations 439

The Weak Interaction

T

he nuclear interaction that holds nucleons together to form nuclei cannot account for beta

decay. Another short-range fundamental interaction turns out to be responsible: the weak

interaction.Insofar as the structure of matter is concerned, the role of the weak interaction

seems to be confined to causing beta decays in nuclei whose neutron/proton ratios are not ap-

propriate for stability. This interaction also affects elementary particles that are not part of a nu-

cleus and can lead to their transformation into other particles. The name “weak interaction” arose

because the other short-range force affecting nucleons is extremely strong, as the high binding

energies of nuclei attest. The gravitational interaction is weaker than the weak interaction at

distances where the latter is a factor.

Thus four fundamental interactions are apparently sufficient to govern the structure and be-

havior of the entire physical universe, from atoms to galaxies of stars. In order of increasing

strength they are gravitational, weak nuclear, electromagnetic, and strong nuclear. These inter-

actions and how they are related to one another and to the origin and evolution of the universe

will be discussed in Chap. 13.

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