Rare decay modes

Nuclear reactions producing exotic nuclei at the limits of stability are usually very non-specific. For the fast and efficient removal of typically several tens of interfering elements with several hundreds of isotopes from the nuclides selected for study mainly mass separation [Han 79, Rav 79] and rapid chemical procedures [Her 82] are applied. The use of conventional mass separators is limited to elements for which suitable ion sources are available. There exists a number of elements, such as niobium, the noble metals etc., which create problems in mass separation due to restrictions in the diffusion-, evaporation- or ionization process. Such limitations do not exist for chemical methods. Although rapid off-line chemical methods are still valuable for some applications, continuously operated chemical procedures have been advanced recently since they deliver a steady source of activity needed for measurements with low counting efficiencies and for studies of rare decay modes. The present paper presents several examples for such techniques and reports briefly actual applications of these methods for the study of exotic nuclei. 

Table II Experimental and theoretical values of rare decay modes of Singlet (X<.) and Triplet (Xy) positronium... 

Proton Decay and Other Rare Decay Modes... 

The mode of radioactive decay is dependent upon the particular nuclide involved. We have seen in Ch. 1 that radioactive decay can be characterized by a-, jS-, and y-radiation. Alpha-decay is the emission of helium nuclei. Beta-decay is the creation and emission of either electrons or positrons, or the process of electron capture. Gamma-decay is the emission of electromagnetic radiation where the transition occurs between energy levels of the same nucleus. An additional mode of radioactive decay is that of internal conversion in which a nucleus loses its energy by interaction of the nuclear field with that of the orbital electrons, causing ionization of an electron instead of y-ray emission. A mode of radioactive decay which is observed only in the heaviest nuclei is that of spontaneous fission in which the nucleus dissociates spontaneously into two roughly equal parts. This fission is accompanied by the emission of electromagnetic radiation and of neutrons. In the last decade also some unusual decay modes have been observed for nuclides very far from the stability line, namely neutron emission and proton emission. A few very rare decay modes like C-emission have also been observed. 

New forms of radioactivity were reported. Proton emission from ground states, predicted as the simplest decay mode of proton-rich nuclei and long searched for, was observed in 1982 for Lu (81 ms) produced by a heavy-ion reaction (Hofinann et al. 1982). Unusual large nuclear radii were found for some very light nuclei (Tanihata et al. 1985) and later attributed to neutron haloes, e.g., for Li (8.5 ms) to a halo of two neutrons around a i core. Even a new kind of natural radioactivity was discovered in 1984 (Rose and Jones 1984) emission of nuclei fi-om Ra (lid) leading to Discoveries of other rare decay modes involving the emission of a variety of fragments from very heavy nuclei soon followed. 

In addition to the study of CP-violating asymmetries, the collection of a large sample of taggcxl events with cleanly separated vertic cs will allow PEP-II to extend the study of the mechanism of B weak decays beyond what is ac hievalde in the symmetric situation. The separation of tlie two B decay vertices leads to significant background reduction in the reconstruction of rare decay modes. 

Finally the high Ps rate could be used to improve on the search for rare or forbidden decays of ground state Ps. Table II summarizes the present values of such decay modes and theoretical predictions. 

The various decay modes are listed in Table 5.1. Unstable, radioactive nuclei may be transformed by emission of nucleons (a decay and, very rarely, emission of protons or neutrons) or by emission of electrons or positrons and decay, respectively). Alternatively to the emission of a positron, the unstable nucleus may capture an electron of the electron shell of the atom (symbol e). 

Among the superheavy nuclei spontaneous fission is a common decay mode. At the same time, the cluster (e.g., C-, Ne-,...) decay of heavy nuclei is rather rare (see in Sect. 2.3.6). 

Class B clusters are also weakly bound, but their constituent molecular units have large density of states with low vibrational frequencies which approach those of the van der Waals modes. Thus, these clusters are much more likely to decay via rapid IVR followed by a statistical dissociation. At least this is their behavior on the electronically excited potential surface. Studies in the infra-red have not been carried out. Among the well studied systems are p-difluorobenzene—X (Tiller et al., 1989 Hye-Keun et al., 1988 Butz et al., 1986) and stilbene—X dimers, where A is a rare gas (Semmes et al., 1990 Khundkar et al., 1983 DeHaan et al., 1989). 

In fact, the NEET is a fundamental but rare mode of decay of an excited atomic state in which the energy of atomic excitation is transferred to the nucleus via a virtual photon. This process is naturally possible if within the electron shell there exists an electronic transition close in energy and coinciding in type with nuclear one. In fact, the resonance condition between the energy of nuclear transition wn and the energy of the atomic transition coa should be fulfilled. Obviously, the NEET process corresponds to time-reversed bound-state internal conversion. Correspondingly, the NEEC process is the time-reversed process of internal conversion. Here, a free electron is captured into a bound atomic shell with the simultaneous excitation of the nucleus. 

For stationary applications a PEM fuel cell is required to operate for 40,000 hours or longer. The major MEA failure modes that lead to a much shorter lifetime are the membrane breach and the electrode decay. The former will result in reactant crossover that causes a sudden and catastrophic failure. lonomer chemical structure and end-group modification, reinforcement of the membrane, incorporation of additives into the membrane, and use of a protective subgasket or full-gasket are effective methods to slow down the membrane breach process. The electrode decay causes gradual loss of the fuel cell performance and rarely causes catastrophic failures. The performance decay lowers the fuel cell efficiency and when the efficiency becomes lower than a predetermined value, the fuel cell reaches its end of life. In... 

Some half-lives of isomeric states can be very long, for example, lOmgj decays by alpha emission with a half-life of 3.0 X 10 year. Alpha decay is, however, a rare mode of decay from a metastable state gamma-ray emission is much more likely. A gamma transition from an isomeric state is called an isomeric transition (IT). On the Karlsruhe Nuclide Chart, these are shown as white sections within a square that is coloured (if the ground state is radioactive) or black (if the ground state is stable). 

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