Why nuclei decay

Today, the DOE Office of Science supports nuclear decay experiments to answer fundamental questions like: Why is there more matter than anti- matter? What is dark matter? And why do neutrinos have such a tiny mass?

Radioactive isotopes are also critical to modern society, with uses in medicine, chemistry, energy, environmental sciences, material sciences, manufacturing, and national security. A goal of the DOE Isotope Program is to support the research and development of methods and technologies to produce isotopes that are in short supply in the United States and reduce our dependence on foreign supplies. Some radioactive nuclides decay in a single step to a stable nucleus.

For example, 60 Co is unstable and decays directly to 60 Ni, which is stable. Others, such as U, decay to another unstable nuclide, resulting in a decay series in which each subsequent nuclide decays until a stable nuclide is finally produced. The decay series that starts from U is of particular interest, since it produces the radioactive isotopes Ra and Po, which the Curies first discovered see Figure 1.

Radon gas is also produced Rn in the series , an increasingly recognized naturally occurring hazard. Since radon is a noble gas, it emanates from materials, such as soil, containing even trace amounts of U and can be inhaled.

The decay of radon and its daughters produces internal damage. The U decay series ends with Pb, a stable isotope of lead. Figure 1. The decay series produced by U, the most common uranium isotope. Nuclides are graphed in the same manner as in the chart of nuclides.

The type of decay for each member of the series is shown, as well as the half-lives. Note that some nuclides decay by more than one mode. You can see why radium and polonium are found in uranium ore. A stable isotope of lead is the end product of the series. Beta decay is a little more subtle, as we shall see. In alpha decay , a 4 He nucleus simply breaks away from the parent nucleus, leaving a daughter with two fewer protons and two fewer neutrons than the parent see Figure 2.

The decay equations for these two nuclides are. Figure 2. Alpha decay is the separation of a 4He nucleus from the parent. The daughter nucleus has two fewer protons and two fewer neutrons than the parent. Alpha decay occurs spontaneously only if the daughter and 4He nucleus have less total mass than the parent. Then since four nucleons have broken away from the original , its atomic mass would be Linear and angular momentum are conserved, too.

Although conserved angular momentum is not of great consequence in this type of decay, conservation of linear momentum has interesting consequences. If the nucleus is at rest when it decays, its momentum is zero.

In that case, the fragments must fly in opposite directions with equal-magnitude momenta so that total momentum remains zero. Total mass—energy is also conserved: the energy produced in the decay comes from conversion of a fraction of the original mass. This is easily done using masses given in Appendix A.

The energy carried away by the recoil of the U nucleus is much smaller in order to conserve momentum. This decay is spontaneous and releases energy, because the products have less mass than the parent nucleus. The question of why the products have less mass will be discussed in Binding Energy. Note that the masses given in Appendix A are atomic masses of neutral atoms, including their electrons. In this case, there are 94 electrons before and after the decay. There are actually three types of beta decay.

It turns out that alpha decay is confined largely to relatively heavy nuclides, typically with mass numbers in the range of and greater. For typical alpha particle energies between 5 and 6 MeV, the recoiling ion will have a kinetic energy on the order of keV , eV. The result has some important implications. For example, when workers are dealing with highly radioactive solutions or even solid materials with high concentrations of alpha emitters, it has been observed that the radioactive material manages to leave open containers and migrate to various other locations in the area, seemingly under its own power.

This is a result of alpha decay events occurring close to the surface of the solution or the solid and the relatively large recoil kinetic energy of the residual ion being distributed among thousands of atoms in its immediate vicinity. In this instance radioactive aggregates collect on the filter surface as contaminated air is drawn through the filter.

When an alpha decay event occurs in the aggregate, the recoil energy sometimes tears free a smaller aggregate, which gets entrained in the moving airstream and gets transported deeper into the filter.

Eventually, some radioactivity may penetrate the filter as a consequence of such sequential events occurring. Regardless of where the residual ion of an alpha decay event ends up it will achieve electrical neutrality and reside as a foreigner among its neighboring atoms.

The neutrons of this larger nucleus act to insulate the protons from the effects of each other. The ratio of neutrons to protons, or N:Z ratio, is the primary factor that determines whether or not an atomic nucleus is stable.

There are also what are called magic numbers, which are numbers of nucleons either protons or neutrons that are especially stable. If both the number of protons and neutrons have these values, the situation is termed double magic numbers. You can think of this as being the nucleus equivalent to the octet rule governing electron shell stability.

The magic numbers are slightly different for protons and neutrons:. To further complicate stability, there are more stable isotopes with even-to-even Z:N isotopes than even-to-odd 53 isotopes , than odd-to-even 50 than odd-to-odd values 4. One final note: Whether any one nucleus undergoes decay or not is a completely random event. The half-life of an isotope is the best prediction for a sufficiently large sample of the elements. It can't be used to make any sort of prediction on the behavior of one nucleus or a few nuclei.

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