Nucleon numbers

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. 

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. 

Another, better, name for the mass number is nucleon number. 

The antinucleon distribution functions follow by changing the sign of the effective chemical potential [11], Since we are interested in the low density region the contribution of the antinucleons can be neglected. Then, in mean-field approximation the chemical potential is related to the nucleon number density according to... 

An element is specified by the number of protons in its nucleus. This equals the atomic number of the respective element, and thus determines its place within the periodic table of the elements. The atomic number is given as a subscript preceding the elemental symbol, e.g., gC in case of carbon. Atoms with nuclei of the same atomic number differing in the number of neutrons are termed isotopes. One isotope differs from another isotope of the same element in that it possesses a different number of neutrons, i.e., by the mass number or nucleon number. The mass number m is the sum of the total number of protons and neutrons in an atom, molecule, or ion. [1] The mass number of an isotope is given as superscript preceding the elemental symbol, e.g.,... 

The number of nucleons is equal to the sum of the number of protons (Z = atomic number) and number of neutrons (N) in the nucleus and is defined as the mass number (A = nucleon number)... 

Neutrons and protons have a similar mass. Electrons possess very litde mass. So the mass of any atom depends on the number of protons and neutrons in its nucleus. The total number of protons and neutrons found in the nucleus of an atom is called the nucleon number (or mass number) and is given the symbol A. 

Hence, in the example shown in Figure 3.3 the helium atom has a nucleon number of 4, since it has two protons and two neutrons in its nucleus. If we consider the metallic element lithium, it has three protons and four neutrons in its nucleus. It therefore has a nucleon number of 7. 

The proton number and nucleon number of an element are usually written in the following shorthand way ... 

The number of neutrons present can be calculated by rearranging the relationship between the proton number, nucleon number and number of neutrons to give ... 

Element Symbol Proton number Number of electrons Number of protons Number of neutrons Nucleon number... 

Not all of the atoms in a sample of chlorine, for example, will be identical. Some atoms of the same element can contain different numbers of neutrons and so have different nucleon numbers. Atoms of the same element which have the same proton number but different neutron numbers are called isotopes. The two isotopes of chlorine are shown in Figure 3.4 (p. 36). 

Isotopes Atoms of the same element which possess different numbers of neutrons. They differ in nucleon number (mass number). 

Nucleon number (mass number) Symbol A. The total number of protons and neutrons found in the nucleus of an atom. 

Element Proton number Nucleon number Electronic structure... 

These results demonstrate that a difference of only one neutron causes a considerable change of the nature of the nuclei at A 100 and that the study of the isotopes with odd nucleon numbers can provide insight into the details of the shape transition. The transition in the Y isotopes seems to be even more rapid than in the Sr and Zr chains where the N = 60 isotones still have coexisting shapes and where the shell-model character of the N 58 isotones at high excitation energies has not yet been tested.Further investigations are, however, needed in order to confirm in detail the proposed interpretation of the level schemes of the Y isotopes and to see whether similarly rapid structure changes occur in the Rb and Nb isotopes at N 60. 

Element abundance data were useful not only in astrophysics and cosmology but also in the attempts to understand the structure of the atomic nucleus. [74] As mentioned, this line of reasoning was adopted by Harkins as early as 1917, of course based on a highly inadequate picture of the nucleus. It was only after 1932, with the discovery of the neutron as a nuclear component, that it was realized that not only is the atomic mass number related to isotopic abundance, but so are the proton and neutron numbers individually. Cosmochemical data played an important part in the development of the shell model, first proposed by Walter Elsasser and Kurt Guggenheimer in 1933-34 but only turned into a precise quantitative theory in the late 1940s. [75] Guggenheimer, a physical chemist, used isotopic abundance data as evidence of closed nuclear shells with nucleon numbers 50 and 82. 

For a given nucleus, having nuclear charge number (atomic number) Z representing its number of nuclear protons and nucleon number (mass number) A representing its total number of nucleons (neutrons + protons), the mass excess of atom (Z, A) is defined by... 

The term nuclide implies an atom of specified atomic number (proton number) and mass number (nucleon number). Nuclides having the same atomic number but different mass numbers are called isotopic nuclides or isotopes. Nuclides having the same mass number but different atomic numbers are called isobaric nuclides or isobars. 

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