Beta Decay

Another type of radioactive decay is b decay. b decay can involve the emission of an electron (b-), a positron (b+), or electron capture. When a nucleus b decays, the Z of the nucleus changes by 1, while A does not change. The number of nucleons and the charge are conserved. We can again write this decay in terms of a general equation:

b- decay: X(Z,N) -> X'(Z+1,N-1) + e- + n'

b+ decay: X(Z,N) -> X'(Z-1,N+1) + e+ + n

In b decay, the number of leptons must be conserved also. Thus when a lepton is emitted, an anti-lepton must also be emitted. This anti-lepton emitted together with the electron, e-, or positron, e+, is called a neutrino, n, or anti-neutrino, n'.

One example of b decay is the decay of the neutron,

n Æ p + e- + n'

where n' is the anti-electron neutrino. The neutrino has no mass or charge but has energy and momentum.

Another example is the decay of 14C,

14C Æ 14N + e- + n'

This decay involves three particles, and the particles can take a variety of energies so that the electron will not be observed at a single energy but rather will take on a distribution of energies varying from almost zero to an energy corresponding to the case where the neutrino is emitted with almost zero energy. For 14C, the maximum energy is given by

(14.003242 u - 14.003074 u)·931.5 MeV = 0.156 MeV

(Here we have neglected the recoil energy that the nucleus receives. The recoil is the 14N nucleus is very small because the electron is so light.)

The case for b+ or positron decay is somewhat different. Since the positron is an antiparticle, it must be created in combination with a normal electron. This means that the maximum energy is given by

Mass(initial nucleus) - Mass(final nucleus) - 2·Mass(electron)



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