Charged particles, nuclei

Spin I> 0 nuclei possess a magnetic dipole or dipole moment, n, which arises from a spinning, charged particle. Nuclei that have a nonzero spin will also have a magnetic moment, and the direction of that magnetic moment is collinear with the angular momentum vector associated with the nucleus. This can be expressed as... 

However, the interaction potential between two charged particles, nucleus-electron or electron-electron, is not just the Coulomb interaction, since in the relativistic description a retarded, velocity-dependent interaction must be considered. The full and general derivation of these interaction potentials is involved and approximate relativistic corrections to the Coulomb interaction are used in general. The frequency-dependent correction to the electron-electron Coulomb interaction, the Breit operator... 

The atomic scattering factor for electrons is somewhat more complicated. It is again a Fourier transfonn of a density of scattering matter, but, because the electron is a charged particle, it interacts with the nucleus as well as with the electron cloud. Thus p(r) in equation (B1.8.2h) is replaced by (p(r), the electrostatic potential of an electron situated at radius r from the nucleus. Under a range of conditions the electron scattering factor, y (0, can be represented in temis... 

All the techniques discussed here involve the atomic nucleus. Three use neutrons, generated either in nuclear reactors or very high energy proton ajccelerators (spallation sources), as the probe beam. They are Neutron Diffraction, Neutron Reflectivity, NR, and Neutron Activation Analysis, NAA. The fourth. Nuclear Reaction Analysis, NRA, uses charged particles from an ion accelerator to produce nuclear reactions. The nature and energy of the resulting products identify the atoms present. Since NRA is performed in RBS apparatus, it could have been included in Chapter 9. We include it here instead because nuclear reactions are involved. 

Fermi had been fascinated by the discovery of the neutron by James Chadwick in 1932. He gradually switched his research interests to the use of neutrons to produce new types of nuclear reactions, in the hope of discovering new chemical elements or new isotopes of known elements. He had seen at once that the uncharged neutron would not be repelled by the positively-charged atomic nucleus. For that reason the uncharged neutron could penetrate much closer to a nucleus without the need for high-energy particle accelerators. lie discovered that slow neutrons could... 

In Chapter 2 you were introduced to atoms and in Chapter 6 they were described in more detail. You were told that the atom contains charged particles, that it has a nucleus made up of neutrons and protons, and that the nucleus is surrounded by electrons. The atom is incredibly small but the nucleus is even smaller. But also you were told that every theory (including the atomic theory) should be thought about and criticized—the evidence upon which it is based should be examined and understood. It is one thing to ask Do we believe in atoms and quite another to ask Why do we believe in atoms In this chapter we shall try to answer this last, harder question. 

You have been told that the atomic nucleus bears a positive charge and is surrounded by a number of negatively charged particles called electrons. Also, the nucleus is supposed to contain most of the mass of the atom and to be made of protons and neutrons, each of which has nearly two thousand times the mass of the electron. How do we know that atoms are built this way How do we know that there is such a particle as an electron Again, weight relations associated with chemical reactions provide key evidence. 

Nuclei that have too many protons relative to their number of neutrons correct this situation in either of two ways. They either capture one of their Is electrons or they emit a positron (a positively charged particle with the same mass as an electron). Either process effectively changes a proton to a neutron within the nucleus. 

When Rutherford allowed the radiation to pass between two electrically charged electrodes, he found that one type was attracted to the negatively charged electrode. He proposed that the radiation attracted to the negative electrode consists of positively charged particles, which he called a particles. From the charge and mass of the particles, he was able to identify them as helium atoms that had lost their two electrons. Once Rutherford had identified the atomic nucleus (in 1908, Section B), he realized that an a particle must be a helium nucleus, He2+. An a particle is denoted or simply a. We can think of it as a tightly bound cluster of two protons and two neutrons (Fig. 17.5). 

FIGURE 17.17 When a positively charged particle approaches a nucleus, it is repelled strongly. However, a very fast particle can reach the nucleus before the repulsion turns it aside, and a nuclear reaction may result. 

Neutrons readily induce nuclear reactions, but they always produce nuclides on the high neutron-proton side of the belt of stability. Protons must be added to the nucleus to produce an unstable nuclide with a low neutron-proton ratio. Because protons have positive charges, this means that the bombarding particle must have a positive charge. Nuclear reactions with positively charged particles require projectile particles that possess enough kinetic energy to overcome the electrical repulsion between two positive particles. 

Electron A negatively charged particle found outside the nucleus of an atom. Free electrons are called beta particles. 

Alpha Particle—A positively charged particle ejected spontaneously from the nuclei of some radioactive elements. It is identical to a helium nucleus, i.e., 2 neutrons and two protons, with a mass number of 4 and an electrostatic charge of +2. 

Bremsstrahlung—X rays that are produced when a charged particle accelerates (speeds up, slows down, or changes direction) in the strong field of a nucleus. 

Decay, Radioactive—Transformation of the nucleus of an unstable nuclide by spontaneous emission of charged particles and/or photons (see Disintegration). 

Low-LET—Energy transfer characteristic of light charged particles such as electrons produced by x and gamma rays where the distance between ionizing events is large on the scale of a cellular nucleus. 

Ans. (a) The only positively charged particles, the protons, arc located there, (b) The protons and neutrons, both massive compared to the electrons, arc located there, (c) The nucleus is positively charged and they are negatively charged, (d) The nucleus is tiny compared to the atom as a whole, and it contains most of the mass of the atom. Thus, the remainder of the atom contains little mass, and may be thought of as mostly empty space. 

FIGURE 3.1 Stopping power of water for various charged particles over a wide span of energy 1 electron, 2 (positive) muon, 3 proton, 4 carbon nucleus, and 5 fission (light) fragment. See text for details. Reproduced from Mozumder (1969), by permission of John Wiley Sons, Inc. ... 

Electron An electron is a negatively charged particle with a diameter of ICE12 cm. Every atom consists of one nucleus and one or more electrons. Cathode rays and negations are elections. 

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