Particle*like nuclei

Radionuclides that do not emit beta particles likely emit alpha particles. An alpha particle is, in effect, a helium atom (two protons and two neutrons) ejected from an unstable nucleus. An alpha particle can only travel a few inches in air and cannot penetrate the outer layers of dead skin cells. Therefore, alpha particles are not external hazards and produce tissue damage only if alpha-emitting radionuclides are ingested, inhaled, or injected. 

In three dimensions a spatial wave group moves around an harmonic ellipsoid and remains compact, in contrast to the dispersive wave packets of classical optics. The distinction is ascribed to the fact that the quantum wave packet is built up from discrete harmonic components, rather than a continuum of waves. The wave mechanics of a hydrogen electron is conjectured to produce wave packets of the same kind. At small quantum numbers the wave spreads around the nucleus and becomes more particle-like, at high quantum numbers, as it approaches the ionization limit where the electron is ejected from the atom. 

If one has differentiated information about elementary particles like protons, neutrons and electrons which connect to questions of chemical bonding, misconceptions can arise which mix bent water molecules or the 11-protons nucleus of a sodium atom with macroscopic characteristics of matter (see also Sect. 4.3). Since electrons are not ordinary basic particles of matter in the sense of atoms, ions and molecules, but are recognized more as charged clouds, orbital or through the particle-wave duality, the mixing of macroscopic and sub-microscopic characteristic properties should be avoided more carefully. 

The current information on size, structure and chemistry of diamond nuclei is primarily speculative, with a small number of conclusive results. It has been proposed that diamond nuclei may be multiple twinned particles, likely containing some of the structures related to the boat-boat conformer of bicyclodecane (10 carbon atoms) or boat-chair-chair-boat tetracyclo octadecane (18 carbon atoms) within higher molecular weight compounds formed by the partial hydrogenation of graphitic or polyaromatic hydrocarbons. The diameter of a critical nucleus of diamond is presumably around 3 nm. 

In this last section we mention a few cases, where properties other than the energy of a system are considered, which are influenced in particular by the change from the point-like nucleus case (PNC) to the finite nucleus case (FNC) for the nuclear model. Firstly, we consider the electron-nuclear contact term (Darwin term), and turn then to higher quantum electrodynamic effects. In both cases the nuclear charge density distribution p r) is involved. The next item, parity non-conservation due to neutral weak interaction between electrons and nuclei, involves the nuclear proton and neutron density distributions, i.e., the particle density ditributions n r) and n (r). Finally, higher nuclear electric multipole moments, which involve the charge density distribution p r) again, are mentioned briefly. 

Since that time, hundreds of radioisotopes have been produced in the laboratory. In order for charged particles like a-particles to penetrate a positively charged nucleus, or the electron cloud surrounding the nucleus, they must be moving at extremely high velocities. Such velocities are achieved in particle accelerators such as the cyclotron. All the elements above atomic number 92 have been created in this manner. Most radioisotopes used in medicine have been produced using neutrons as subatomic projectiles. 

There are two types of nuclear reactions. The first is the radioactive decay of bonds within the nucleus that emit radiation when broken. The second is the billiard ball type of reaction where the nucleus, or a nuclear particle (like a proton), is struck by another nucleus or nuclear particle. It is easy to remember these with the following element decay radiation (see Figure 11.3). 

Moreover, Dalton s model of the atom, which represented it as a tiny, indivisible particle, like a minute billiard ball, did not predict the existence of subatomic charged particles. These were observed in later experiments that led to the discovery of electrons and the atomic nucleus. Let s examine some of these experiments and the more complex atomic model that emerged fi om them. 

This Bohr model was eventually found to have some significant limitations because of its inability to explain several phenomena involving electrons. A resolution was reached with a wave-mechanical model, in which the electron is considered to exhibit both wavelike and particle-like characteristics. With this model, an electron is no longer treated as a particle moving in a discrete orbital rather, position is considered to be the probability of an electron s being at various locations around the nucleus. In other words, position is described by a probability distribution or electron cloud. Figure 2.3 compares Bohr and... 

Nuclei suitable for fusion must come near each other, where near means something like the nuclear radius of 10" cm. For positively charged nuclei to make such a close approach it requires large head-on velocities, and therefore multimillion-degree Celsius temperature. In contrast, fission can occur at normal temperatures, either spontaneously or triggered by a particle, particularly an uncharged neutron, coming near a fissionable nucleus. 

There are three common ways by which nuclei can approach the region of stability (1) loss of alpha particles (a-decay) (2) loss of beta particles (/3-decay) (3) capture of an orbital electron. We have already encountered the first type of radioactivity, a-decay, in equation (/0). Emission of a helium nucleus, or alpha particle, is a common form of radioactivity among nuclei with charge greater than 82, since it provides a mechanism by which these nuclei can be converted to new nuclei of lower charge and mass which lie in the belt of stability. The actinides, in particular, are very likely to decay in this way. 

An electron in an atom is like a particle in a box, in the sense that it is confined within the atom by the pull of the nucleus. We can therefore expect the electron s wavefunctions to obey certain boundary conditions, like the constraints we encountered when fitting a wave between the walls of a container. As we saw for a particle in a box, these constraints result in the quantization of energy and the existence of discrete energy levels. Even at this early stage, we can expect the electron to be confined to certain energies, just as spectroscopy requires. 

We can use Fig. 17.13 to predict the type of disintegration that a radioactive nuclide is likely to undergo. Nuclei that lie above the band of stability are neutron rich they have a high proportion of neutrons. These nuclei tend to decay in such a way that the final n/p ratio is closer to that found in the band of stability. For example, a l4C nucleus can reach a more stable state by ejecting a (3 particle, which reduces the n/p ratio as a result of the conversion of a neutron into a proton (Fig. 17.15) ... 

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