Cluster radioactive decay

The thus obtained fission half lives are depicted in the lower part of Figure 8.11. Their distribution as a function of the fragment mass A2 resembles quite well the asymmetric mass distribution. Cluster radioactive decays correspond to the broad peaks around A2 = 20, 30 (200, 210). The confrontation of the calculated fission half lives with experiments is depicted in Figure 8.12. One notices "nearly quantitative" agreement over 20 orders of magnitude, which is—for an ab-initio calculation—remarkable ... 

There are four modes of radioactive decay that are common and that are exhibited by the decay of naturally occurring radionucHdes. These four are a-decay, j3 -decay, electron capture and j3 -decay, and isomeric or y-decay. In the first three of these, the atom is changed from one chemical element to another in the fourth, the atom is unchanged. In addition, there are three modes of decay that occur almost exclusively in synthetic radionucHdes. These are spontaneous fission, delayed-proton emission, and delayed-neutron emission. Lasdy, there are two exotic, and very long-Hved, decay modes. These are cluster emission and double P-decay. In all of these processes, the energy, spin and parity, nucleon number, and lepton number are conserved. Methods of measuring the associated radiations are discussed in Reference 2 specific methods for y-rays are discussed in Reference 1. 

Table 10. Exotic Radioactive Decay and Emissions of Clusters...

Presence of these interstices provides to the fluorite stmcture extremely specific features. In UO2 particularly, it allows for placement of some radioactive decay products, these sites are responsible for existence of hyperstoichiometric UO2+X phase, where the extra oxygen ions fill the empty interstitial sites in the fluorite lattice etc. First case is extremely important in radiation damaged UO2. Second one is cmcial in oxidation of pure UO2 in atmospheric conditions. Diffusion of atmospheric oxygen into the bulk of crystal brings excess oxygens into empty interstices. These become filled more or less randomly only at low x, at higher concentration of extra anions they form different types of clusters, including so-called 2 2 2 Willis dimers Willis), tetra- and pentameric defects clusters of cuboctahedral symmetry Allen and Tempest). Last defects appear due to interaction of extra anions with intrinsic crystal FP defects (anion Frenkel pairs, i.e. anion vacancies and anion interstitials). 

Half-lives for proton emission, ot decay, cluster radioactivity, and cold fission processes have been calculated in the framework of an effective liquid drop model by Duarte et al. (2002). The comparison with experimental data shows that the model is very efficient to describe these different decay processes in a unified theoretical framework. 

In the following, exotic nuclear decay modes of heavy nuclei, cluster radioactivities, delayed fission, and spontaneous fission (SF) together with the recent progress on deformation paths toward fission are briefly introduced. 

A summary of cluster radioactivities experimental Q-values, half-lives, Ti/2, and half-lives relative to those of the a decay in terms of the logarithm of T-i/ ITa... 

A model has been developed to calculate the size distributions of the short lived decay products of radon in the indoor environment. In addition to the classical processes like attachment, plate out and ventilation, clustering of condensable species around the radioactive ions, and the neutralization of these ions by recombination and charge transfer are also taken into account. Some examples are presented showing that the latter processes may affect considerably the appearance and amount of the so called unattached fraction, as well as the equilibrium factor. 

By now this process has been verified experimentally by research groups in Oxford, Moscow, Berkeley, Milan and other places. Accordingly, one has to revise what is learned in school there are not only 3 types of radioactivity a-, j8-, y-radioactivity), but many more. Atomic nuclei can also decay through spontaneous cluster emission (that is the spitting out" of smaller nuclei like carbon, oxygen,. ..). Figure 8.10 depicts some nice examples of these processes. 

For all issues relevant to the chemistry and physics of atoms, molecules, clusters, and solids only electromagnetic and — to a negligible extent — weak interactions, which are responsible for the radioactive /5-decay and the nonconservation of parity, contribute. The internal structure of hadrons, i.e., protons and neutrons built up by quarks governed by strong interactions and also gravitational forces, do not play any role and are therefore not covered by this presentation. 

When a radioactive nuclide decays, electrons are stripped from the parent atom by its recoil and decay products are formed as positive ions. These ions can attract liquid and even solid material, thus forming clusters of atoms or particles in the submicron region ranging from 0.001 to 0.01 pm. Air is permanently ionised by radiation from the natural radioactivity of air and by cosmic radiation which consists mostly of positively charged particles, 85% protons, 10% alpha particles with a smaller percentage of positively charged stripped nuclei of heavier elements, such as Fe, Co and Ni, etc. Production of an ion pair requires 35.6 eV if ionisation is by alpha particles and 32.5 eV if by fast electrons. 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 are mostly the Aitken nuclei, which are submicrometre particles in the range 0.005 to 0.01 pm. 

Most of the newly formed decay product clusters are positively charged and have a high mobility. The mobility is characterised by the diffusion coefficient that chiefly controls the formation of the radioactive aerosols by attachment and the deposition on surfaces. The measured values for the diffusion coefficient, D, range from 0.03 to 0.085 cm s depending on... 

Besides cluster formation, the radon decay products attach to the existing aerosol particles within 1-100 s, forming the radioactive aerosols of the radon decay products. Results of the activity size distribution measurements carried out at different places in outdoor air, dwellings and workplaces are presented in Table 5.2. In general, the activity size distribution of the radon... 

---