The 235 U 235 U nucleus can be left in an excited state to later emit photons ( γ γ rays). The energy carried away by the recoil of the 235 U 235 U nucleus is much smaller in order to conserve momentum. Most of this energy becomes kinetic energy of the α α particle (or 4 He 4 He nucleus), which moves away at high speed. The energy released in this α α decay is in the MeV MeV range, about 10 6 10 6 times as great as typical chemical reaction energies, consistent with many previous discussions. As discussed in Atomic Physics, the general relationship is Total mass–energy is also conserved: the energy produced in the decay comes from conversion of a fraction of the original mass. This results in the α α particle carrying away most of the energy, as a bullet from a heavy rifle carries away most of the energy of the powder burned to shoot it. In that case, the fragments must fly in opposite directions with equal-magnitude momenta so that total momentum remains zero. If the nucleus is at rest when it decays, its momentum is zero. Although conserved angular momentum is not of great consequence in this type of decay, conservation of linear momentum has interesting consequences. Linear and angular momentum are conserved, too. You can see from the equation Z A X N → Z − 2 A − 4 Y N − 2 + 2 4 He 2 Z A X N → Z − 2 A − 4 Y N − 2 + 2 4 He 2 that total charge is conserved. It is instructive to examine conservation laws related to α α decay. Then since four nucleons have broken away from the original 239, its atomic mass would be 235. So if you were told that 239 Pu 239 Pu α α decays and were asked to write the complete decay equation, you would first look up which element has two fewer protons (an atomic number two lower) and find that this is uranium. Where Y is the nuclide that has two fewer protons than X, such as Th having two fewer than U. The decay equations for these two nuclides are Another nuclide that undergoes α α decay is 239 Pu 239 Pu. One example of α α decay is shown in Figure 31.14 for 238 U 238 U. In alpha decay, a 4 He 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 31.15). No γ γ decays are shown in the figure, because they do not produce a daughter that differs from the parent. Beta decay is a little more subtle, as we shall see. The daughters of β β decay have one less neutron and one more proton than their parent. This seems reasonable, since we know that α α decay is the emission of a 4 He 4 He nucleus, which has two protons and two neutrons. Note that the daughters of α α decay shown in Figure 31.14 always have two fewer protons and two fewer neutrons than the parent. A stable isotope of lead is the end product of the series. You can see why radium and polonium are found in uranium ore. Note that some nuclides decay by more than one mode. The type of decay for each member of the series is shown, as well as the half-lives. Nuclides are graphed in the same manner as in the chart of nuclides. The 238 U 238 U decay series ends with 206 Pb 206 Pb, a stable isotope of lead.įigure 31.14 The decay series produced by 238 U 238 U, the most common uranium isotope. The decay of radon and its daughters produces internal damage. Since radon is a noble gas, it emanates from materials, such as soil, containing even trace amounts of 238 U 238 U and can be inhaled. Radon gas is also produced ( 222 Rn 222 Rn in the series), an increasingly recognized naturally occurring hazard. The decay series that starts from 238 U 238 U is of particular interest, since it produces the radioactive isotopes 226 Ra 226 Ra and 210 Po 210 Po, which the Curies first discovered (see Figure 31.14). Others, such as 238 U 238 U, decay to another unstable nuclide, resulting in a decay series in which each subsequent nuclide decays until a stable nuclide is finally produced. For example, 60 Co 60 Co is unstable and decays directly to 60 Ni 60 Ni, which is stable. Some radioactive nuclides decay in a single step to a stable nucleus. We call the original nuclide the parent and its decay products the daughters. Unstable nuclides decay (that is, they are radioactive), eventually producing a stable nuclide after many decays. Some nuclides are stable, apparently living forever. In this section, we explore the major modes of nuclear decay and, like those who first explored them, we will discover evidence of previously unknown particles and conservation laws. Nuclear decay gave the first indication of the connection between mass and energy, and it revealed the existence of two of the four basic forces in nature. Nuclear decay has provided an amazing window into the realm of the very small. Calculate the energy emitted during nuclear decay.By the end of this section, you will be able to:
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