Nucleonics

Mn is the mass of the nucleon, jis Planck s constant divided by 2ti, m. is the mass of the electron. This expression omits some temis such as those involving relativistic interactions, but captures the essential features for most condensed matter phases. 

Comphcated theoretical calculations, based on filled shell (magic number) and other nuclear stabiUty considerations, have led to extrapolations to the far transuranium region (2,26,27). These suggest the existence of closed nucleon shells at Z = 114 (proton number) and N = 184 (neutron number) that exhibit great resistance to decay by spontaneous fission, the main cause of instabiUty for the heaviest elements. Eadier considerations had suggested a closed shell at Z = 126, by analogy to the known shell at = 126, but this is not now considered to be important. 

Properties of Particles. From the research of the early part of the twentieth century, the existence of several types of particles was firmly estabhshed, and the properties were deterrnined. The particles that are involved in the decay of radioisotopes are given in Table 4. An additional type of conservation is that in all atomic and nuclear decays, the number of nucleons, ie, protons and neutrons, is conserved and the number of leptons, ie, electrons and neutrinos, is also conserved. 

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. 

Cluster emission is an exotic decay that has some commonalities with a-decay. In a-decay, two protons and two neutrons that are moving in separate orbits within the nucleus come together and leak out of the nucleus as a single particle. Cluster emission occurs when other groups of nucleons form a single particle and leak out. Several of the observed decays are shown in Table 10. The emitted clusters include C, Ne, Mg, and Si. The... 

E. Bleuler and G. J. Goldsmith, Experimental Nucleonics, Holt, Reinhart and Wiaston, Inc., New York, 1952. 

Note The mass of a nucleon (proton or neutron) is 1835 times heavier than an electron and moves much slower than an electron since. m,vf= m v) = constant. Where m is the mass and v. the velocity of an electron or a proton. The bulk of the arc is therefore caused by electrons rather than protons. 

Several further comparisons attest to the universal nature of the statistical fragmentation theory. In Fig. 8.28 the consequences of a nuclear fragmentation event brought about by the 70 MeV per nucleon collision of a carbon nucleus with a silver nucleus is shown (Greiner and Stocker, 1985). In this... 

For a molecular system, 4 is a function of the positions f the electrons and thenu 1 within the molecule, which we will designate as r and R, respectively These h are a shorthand for the set of component vectors describing the position of each particle. We ll use subscripted versions of theig to denote the vector correspondin to a particular electron or nucleus r, andR/. Note that electrons are treated individually, while each nucleus is treated as an aggregate the component nucleon.s are not treated individually. 

Since the radioactive half-lives of the known transuranium elements and their resistance to spontaneous fission decrease with increase in atomic number, the outlook for the synthesis of further elements might appear increasingly bleak. However, theoretical calculations of nuclear stabilities, based on the concept of closed nucleon shells (p. 13) suggest the existence of an island of stability around Z= 114 and N= 184. Attention has therefore been directed towards the synthesis of element 114 (a congenor of Pb in Group 14 and adjacent superheavy elements, by bombardment of heavy nuclides with a wide range of heavy ions, but so far without success. 

R. Hofstadter (Stanford) pioneering studies of electron scattering in atomic nuclei and discoveries concerning the structure of the nucleons. 

Nuclear fission is a process in which a heavy nucleus—usually one with a nucleon number of two hundred or more—separates into two nuclei. Usually the division liberates neutrons and electromagnetic radiation and releases a substantial amount of energy. The discoveiyi of nuclear fission is credited to Otto I lahn and Fritz Strassman. In the process of bombarding uranium with neutrons in the late 1930s, they detected several nuclear products of significantly smaller mass than uranium, one of which was identified as Ba. The theorectical underpinnings that exist to this day for nuclear fission were proposed by Lise Meitner and Otto Frisch. Shortly after Hahn and Strassman s discovery. 

In the model of nuclear structure you were given in Chapter 6, the nucleus was pictured as being built up of protons and neutrons. These two kinds of particles are given the general name nucleon. The mass number of a nucleus is equal to the number of nucleons present. The superscripts in our equation are mass numbers ... 

We may rephrase this in the form of a rule The total number of nucleons is unchanged during nuclear reactions. 

According to the model of Chapter 6, how many nucleons would be present in a uranium nucleus of mass number 235 How many protons are pictured as being present How many neutrons ... 

We can use this idea of the relation of mass to energy in several ways. The mass of a 3iU nucleus is less than the sum of the masses of the 92 protons and 143 neutrons postulated to lie in it. The diirercnce in mass represents the binding energy which holds the nucleons together in... 

By comparing these two answers we can see that the repulsive force between two protons in the nucleus is about ten billion times as great as the repulsive force between two protons bound together in a hydrogen molecule. In order to overcome these enormous intranuclear coulomb repulsions and hold the nucleus together there must exist some very strong attractive forces between the nucleons. The nature of these forces is not understood and remains a very important problem in physics. 

Notice that both the electric charge and the total number of nuclear particles (nucleons) are conserved in the nuclear decomposition. Careful study of the rate of this nuclear decay shows that in a given period of time a constant fraction of the nuclei present will undergo decomposition. This observation allows us to characterize or describe the rate of nuclear decay in a very simple manner. We simply specify the length of time it takes for a fixed fraction of the nuclei initially present to decay. Normally we pick the time for... 

Since there is a decrease of 0.03035 gram/mole number of nucleons in the nucleus. This calcula-of helium formed in this reaction, an equivalent tion provides us with the binding energy per... 

Brueckner, K. A., Phys. Rev. 103, 1121, "Relation between nucleon density and nuclear potential."... 

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