Radon and thoron decay products as small ions

9 Radon and thoron decay products as small ions 

Although alpha decay carries away positive charge, electrons are stripped from the parent atom by its recoil, and decay products are formed as positive ions. Before discussing their properties, it is convenient to summarise briefly the formation, neutralisation and attachment to condensation nuclei of ordinary small ions in air, as described for example by Chalmers (1967). 

Air is ionised by radiation from natural activity in the air and on the ground, and by cosmic rays. Production of one ion pair requires 32.5 eV if ionisation is by fast electrons, 35.6 eV if by alpha rays. The total energy dissipated in air per decay of 222Rn depends on the equilibrium ratio of the decay products. Taking the mean of the results in Table 1.7, it is 10.3 MeV, and the rate of production of ion pairs is approximately 

In the free atmosphere, the rate of production of small ions is in balance with the rate of neutralisation by recombination and the rate of attachment to condensation nuclei. Condensation nuclei, otherwise called Aitken nuclei, are submicrometric particles mainly produced by combustion processes. If slight differences in the rates of attachment of positive and negative ions are ignored, 

In filtered air, N is zero, n = (qla) and the mean life before neutralisation is (aq)K Using (1.14), and taking a = 1.6 x 10-12 m3 s 1, the mean life before neutralisation by ions formed by 222Rn and daughters, is about 1400 Xos. This provides a test of whether the decay products 

---