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Nuclear Transformations 437

0

Energy equivalent

of mass lost by

decaying nucleus

0.2 0.4 0.6 0.8 1.0 1.2

Electron energy, MeV

Relative number of electrons

1.17

Figure 12.10Energy spectrum of electrons from the beta decay of^21083 Bi.

conservation principles of energy, linear momentum, and angular momentum are all

apparently violated in beta decay.

1 The electron energies observed in the beta decay of a particular nuclide are found

to vary continuouslyfrom 0 to a maximum value KEmaxcharacteristic of the nuclide.

Figure 12.10 shows the energy spectrum of the electrons emitted in the beta decay of

210

83 Bi; here KEmax1.17 MeV. The maximum energy

Emaxmc^2 KEmax

carried off by the decay electron is equal to the energy equivalent of the mass difference

between the parent and daughter nuclei. Only seldom, however, is an emitted electron

found with an energy of KEmax.

2 When the directions of the emitted electrons and of the recoiling nuclei are observed,

they are almost never exactly opposite as required for linear momentum to be conserved.

3 The spins of the neutron, proton, and electron are all ^12 . If beta decay involves just

a neutron becoming a proton and an electron, spin (and hence angular momentum)

is not conserved.

In 1930 Pauli proposed a “desperate remedy”: if an uncharged particle of small or

zero rest mass and spin ^12 is emitted in beta decay together with the electron, the above

discrepancies would not occur. This particle, later called the neutrino (“little neutral

one”) by Fermi, would carry off an energy equal to the difference between KEmaxand

the actual KE of the electron (the recoiling nucleus carries away negligible KE). The

neutrino’s linear momentum also exactly balances those of the electron and the recoiling

daughter nucleus.

Subsequently it was found that twokinds of neutrinos are involved in beta decay, the

neutrino itself (symbol ) and the antineutrino(symbol ). The distinction between

them is discussed in Chap. 13. In ordinary beta decay it is an antineutrino that is emitted:

Beta decay n→pe (12.15)

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