A22 Beta decay

Beta decay is the spontaneous transformation of an unstable nucleus into a new nucleus with charge differing by AZ = ±1, because of the emission of an electron (fl-decay) or a positron (fl+ decay) or the capture of an electron (e-capture).

In the first case (fl- decay) one of the neutrons of the nucleus becomes a proton, after emitting an electron. The mass number A does not change, while the new nucleus has an atomic number higher by 1.

Tritium (3H, often symbolized by a T), AT = 3 ZT = 1, fl- decays into 3He, AHe = 3 ZHe = 2, meaning that one of the two neutrons of the tritium nucleus emits an electron and becomes a proton; the mass number does not change, i.e., AT = AHe, while the positive charge of the new nucleus increases by 1,

The energy condition is that the mass (energy) of the parent nucleus is higher than the sum of the masses (energies) of the daughter nucleus and the electron, and is expressed by [Mukhin, 1987]:

In the fl + decay the unstable nucleus emits a positron (i.e., a positive electron). The fl + decay can be treated as the transformation of a proton into a neutron, because also in this case the parent nucleus and the daughter nucleus have the same mass number A, while the atomic number of the daughter Z is lower by 1. The proton mass is lower than the neutron mass (energy). The transformation of the proton into a neutron is possible since the proton is bonded to a nucleus and the excess energy to become a neutron is supplied by the nucleus itself. The energy condition can be expressed in analogy with the fl- case [Mukhin, 1987].

C11, AC = 11 ZC = 6, decays fl + into B11, AB = 11 ZB = 5, and the missing charge of boron-11 is that of the positron emitted.

The third type of beta decay is the electron capture: it consists in the capture of an electron by a nucleus from its own electron shell. For heavy nuclei with the K shell close to the nucleus, this phenomenon (also defined K-capture) is quite common; captures from L shell (L-capture), M shell (M-capture), etc. have also been observed. After the capture, the nucleus has the same mass number A, but its atomic number Z decreases by 1: the electron captured and one of the protons of the nucleus become a neutron in the daughter nucleus.

For instance, Be7, ABe = 7, ZBe = 4, after capturing an electron from its K shell, becomes Li7, ALi = 7 ZLi = 3, the mass number does not change ABe = ALi = 7, while the atomic number Z of the lithium is lower by 1. The mass (energy) condition is that the sum of the masses (energies) of the captured electron and the parent nucleus is higher than the mass (energy) of the daughter nucleus [Mukhin, 1987].

Because of the vacancy created in the electron shell, there is the transition of one of the shell electrons to that vacancy, accompanied by the emission of X-rays.

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