Neutrons number combinations

If we now bring together two nucleons, we find a rather important and interesting fact, only one combination produces a stable (bound) nucleus. One proton and one neutron will combine to form a deuteron, or one hydrogen atom plus one neutron will form a deuterium atom with its atomic electron. Both of the other combinations, two protons that can be labeled 2He and two neutrons, are unbound and come apart almost as rapidly as the constituents come together. It is easy to see that the diproton, or 2He, is more unstable than the dineutron due to the Coulomb repulsion between the two positively charged protons. Thus, we find a preference for equal numbers of neutrons and protons even in the smallest nucleus. 

When the more than 3600 known nuclides are plotted on the neutron/proton grid in Figure 22.3, they fall in a curved band sometimes called the "band of nuclear stability." Even within the band, only 264 of the nuclides are stable indefinitely. The remainder decay spontaneously, although their rates of decay vary enormously. On either side of the band is a "sea of instability" representing the large number of unstable neutron/proton combinations that have never been seen. Particularly interesting is the "island of stability" predicted to exist for a few superheavy nuclides near 114 protons and 184 neutrons. The first members of this group—287114,288114,289114, and 292116—were prepared in 1999 and do indeed seem to be unusually stable. Isotope 289114, for example, has a half-life of 30.4 seconds. 

Electron capture where a proton is changed to a neutron after combining with the captured extranuclear electron (from the K shell) the atomic number is decreased by one unit. 

Shortly thereafter, the strong nuclear force ensured that large numbers of protons and neutrons rapidly combined to form deuterium nuclei (p + n), then helium (2p -t- 2n). The process of element building had begun. During this small niche of cosmic history, from about... 

A number of investigations have been focused on alkali haloaluminates, that is mixtures (MX)y(AlXj)i y where M is an alkali metal and X is a halogen (Cl or Br). Blander et al. [4] have used neutron diffraction combined with quantum chemical calculations to investigate the salts formed from KBr and KQ where y = 0.25 and 0.33. They showed that for both bromide and chloride salts, [AI2X7] was the dominant species present, as expected, in fiiU agreement with other spectroscopic techniques such as Raman and infrared [5]. However, uneiqiectedly, in the case of... 

Tin has an atomic number of 50—a magic number. The neutron number is 68 in the nuclide Sn. This is an even-even combination, and we should expect the nucleus to be stable. Moreover, Figure 25-7 shows that this nuclide is within the belt of stability. Sn is a stable nuclide. 

Isotopes of an element are formed by the protons in its nucleus combining with various numbers of neutrons. Most natural isotopes are not radioactive, and the approximate pattern of peaks they give in a mass spectrum can be used to identify the presence of many elements. The ratio of abundances of isotopes for any one element, when measured accurately, can be used for a variety of analytical purposes, such as dating geological samples or gaining insights into chemical reaction mechanisms. 

This is a technique developed during World War II for simulating stochastic physical processes, specifically, neutron transport in atomic bomb design. Its name comes from its resemblance to gambling. Each of the random variables in a relationship is represented by a distribution (Section 2.5). A random number generator picks a number from the distribution with a probability proportional to the pdf. After physical weighting the random numbers for each of the stochastic variables, the relationship is calculated to find the value of the independent variable (top event if a fault tree) for this particular combination of dependent variables (e.g.. components). 

Any combination of reaction products consistent with these conseiwation principles is possible. For example, in the neutron-induced nuclear fission of it is possible to produce Xe, Sr, two neutrons, and 185 MeV of energy. The most likely reaction products are close in atomic number to xenon pCe) and strontium (Sr), but the possibilities number in the hundreds. 

Instead of producing new kinds of substances by combination of atoms, the element uranium has combined with a neutron and as a result has split into two other elements—barium and krypton—plus three more neutrons. Atoms of a given element are characterized by their atomic number, the number of units of positive charge on the nucleus. For one element to change into another element the nucleus must be altered. In our example the uranium nucleus, as a result of reacting with a neutron, splits or fissions into two other nuclei and releases, in addition, neutrons. ... 

Nuclei with even numbers of both protons and neutrons are more stable than those with any other combination. Conversely, nuclei with odd numbers ot both protons and neutrons are the least stable (Fig. 17.12). Nuclei are more likely to be stable if they are built from certain numbers of either kind of nucleons. These numbers—namely, 2, 8, 20, 50, 82,114, 126, and 184—are called magic numbers. For example, there are ten stable isotopes of tin (Z = 50), the most of any element, but... 

The Structural Basis of the Magic Numbers.—Elsasser10 in 1933 pointed out that certain numbers of neutrons or protons in an atomic nucleus confer increased stability on it. These numbers, called magic numbers, played an important part in the development of the shell model 4 s it was found possible to associate them with configurations involving a spin-orbit subsubshell, but not with any reasonable combination of shells and subshells alone. The shell-model level sequence in its usual form,11 however, leads to many numbers at which subsubshells are completed, and provides no explanation of the selection of a few of them (6 of 25 in the range 0-170) as magic numbers. 

Our picture of atomic architecture is now compiete. Three kinds of particles—electrons, protons, and neutrons-combine in various numbers to make the different atoms of aii the eiements of the periodic table. Table 2-1 summarizes the characteristics of these three atomic buiiding biocks. 

Although nuclides with mass numbers around 60 are the most stable, the balance of electrical repulsion and strong nuclear attraction makes many combinations of protons and neutrons stable for indefinite times. Nevertheless, many other combinations decompose spontaneously. For example, all hydrogen nuclides with j4 > 2 are so... 

Alpha (a.) decay. As we shall see later, the alpha particle, which is a helium nucleus, is a stable particle. For some unstable heavy nuclei, the emission of this particle occurs. Because the a particle contains a magic number of both protons and neutrons (2), there is a tendency for this particular combination of particles to be the one emitted rather than some other combination such as s3Li. In alpha decay, the mass number decreases by 4 units, the number of protons decreases by 2, and the number of neutrons decreases by 2. An example of alpha decay is the following ... 

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