Page 78 - Radiochemistry and nuclear chemistry

Unstable Nuclei and Radioactive Decay 67

4.4.4. Double beta decay

The rather unusual (and very slow)/3-3--decay mode is energetically possible for several
even-even nuclei, and kinetically possible to observe in those cases where the separating
odd-odd nucleus of higher energy prevents normal 3--decay, see Fig. 3.6 V. It has recently
been observed for some such cases, e.g.

82
382Se--~ 36K1 " + 2~ + 2P
The half-life of about 1.7 • 1020 y can be observed because small amounts of the noble
gas Kr can be physically isolated from large amounts of S2Se (9 % natural abundance), and
then measured. Studies of 33-decay are of importance for evaluation of neutrino properties.

4.4.5. B--decay

This process can be written symbolically as follows:

kx-, z+'/x + _?a + (4.24)

However, if we take the electrons into account, the neutral parent atom has Z orbital
electrons, while the daughter atom, with a nuclear charge of Z + 1, must capture an
electron from the surroundings, in order to become neutral:

z+ X + + e- -, z+ X (4.25)

Moreover, since the negatron emitted provides an electron to the surroundings, the total
electron balance remains constant. As a result, in the calculation of the decay energy it is
not necessary to include the mass of the emitted 3-particle as the use of the mass of the
neutral daughter atom includes the extra electron mass. The equation for calculating the
Q-value in negatron decay is thus:

Q#- = -931.5 (Mz+ l - M z) (4.26)

As an example we can take the decay of a free neutron in a vacuum; it transforms
spontaneously with a half-life of 10.6 rain to a proton.

+ _l~ -
The Q-value for this reaction is

Qo- = -931.5 (1.007 825 - 1.008 665) = 0.782 MeV