1. Radioactivity – types of decays presentation for April 28, 2008 by Dr. Brian Davies, WIU Physics Dept.

2. Atomic and nuclear structure Atomic model – nucleus and electrons. Nucleus – contains protons and neutrons. Protons have charge Q p = +e, where e is defined as the magnitude of the electron charge. e = 1.6 x 10 -19 Coulombs = 1.6 x 10 -19 C Electrons have charge Q e = -e Neutrons have charge Q n = 0 (zero, exactly) The nucleons (protons and neutrons) are bound together by the strong nuclear force in a small nucleus which has a size of about 10 -15 m.

3. Nuclear notation Z = atomic number or proton number, is the number of protons in the nucleus. N = neutron number, is the number of neutrons in the nucleus. A = Z + N = mass number, is the number of nucleons in the nucleus. In general, the notation is Z X N For example, 6 C 6 has atomic mass 12.000 A 12

4. Nuclear isotopes Each element has a distinct proton number Z. The neutron number may vary for different isotopes of the element. The mass number A = Z + N will also vary for different isotopes. For example, 6 C 6 and 6 C 8 are two isotopes of carbon. These are also written C-12 and C-14. The neutron number is usually omitted, but can easily be calculated, since N = A – Z. Also, the proton number is often omitted: 14 C 1412

5. Radioactivity – unstable nuclides Each element may have several different isotopes, and some of these may be unstable. Radioactive decay occurs when a nucleus of an unstable isotope decays into product particles. Three common types of radiation are observed: Alpha particles are doubly-charged He nuclei. Beta particles are electrons (or positrons). Gamma rays are high-energy photons.

6. Alpha particles Alpha particles are doubly-charged He nuclei. The particle is a bare nucleus with 2 protons and 2 neutrons, and a charge of +2e They are identical to the nucleus of helium atom, and once the alpha slows down, it will capture two electrons from surrounding material and become a neutral He atom. The nuclear notation for the alpha is  or 2 He Most of the helium in rocks is due to alpha decay of heavy elements in the Earth. 4

7. Alpha decay Alpha particles are emitted in alpha decay. The parent nucleus is usually a heavy element. For example, polonium-214 decays by alpha decay to lead-210 and an alpha particle: 84 Po  82 Pb + 2 He Notice that this nuclear equation is balanced in both the proton number (84 = 82 + 2) and the nucleon number (214 = 210 + 4). 2142104

8. Conservation laws in nuclear decay The balance of the proton and mass numbers is due to conservation laws for nuclear decay: Conservation of nucleons (the mass numbers). Conservation of charge (the proton numbers). Conservation of mass-energy is also observed. The energy of the alpha particle from the decay of 214 Po is about 7.7 MeV. This energy is due to a small difference between the mass of 214 Po and the sum of the masses of 210 Pb and 4 He. The difference in mass between parent and products is converted to energy by E = mc 2.

9. Beta decay Beta particles are emitted in beta decay. The parent nucleus is usually an isotope with an excessive number of neutrons. For example, carbon-14 decays by beta decay to nitrogen-14 and a beta particle: 6 C  7 N + -1 e (beta with Q = -e) This equation is balanced in the charge number (6 = 7 + (-1)) and nucleon number (14 = 14 + 0). Z increases by one, and N decreases by one. 14 0

10. Beta decay of an isolated neutron. Beta decay can be thought of as the decay of a neutron in the nuclide into a proton and electron. An isolated neutron will also decay by beta decay to a proton and a beta particle: 0 n  1 p + -1 e (neutron decay) Notice that this nuclear equation is balanced in both the charge number (0 = 1 + (-1)) and the nucleon number (1 = 1 + 0). 110

11. Two types of beta decay:  + and  - Beta decay can also produce a positron, the anti-particle of the electron. An example of positive beta decay is oxygen-15: 8 O  7 N + +1 e (like a proton decay!) This process occurs for some nuclides which have more protons than neutrons. However, the proton by itself is stable (we believe). 15 0

12. Electron capture (EC) Another type of decay is electron capture, where an electron of the atom is captured by the nucleus and a proton is converted into a neutron. An example of electron capture is beryllium-7: 4 Be + -1 e  3 Li (and an X-ray) This process has the same resulting daughter nucleus, except that no beta particle is emitted. 770

13. Gamma decay Gamma decay occurs when a nucleus emits a gamma particle, a high-energy photon. This only occurs when a nucleus has extra energy, perhaps because it was just created in a previous nuclear decay. An example of gamma emission is barium-137m, which is a nucleus of Ba which has just been created in a beta decay of Cs-137: 55 Cs  56 Ba* + -1 e (a beta decay) 56 Ba*  56 Ba +  (a gamma decay) 137 0

14. Energy levels in gamma decay Barium-137m is an example of an isomer, which is a nucleus with excess energy. In some ways this is similar to a neutral atom in an excited state, which can make a transition to a lower energy level by emitting a photon. (Fluorescence is one example of this.) The nuclei have much larger energies than atoms, MeV instead of eV. 56 Ba*  56 Ba +  (661.66 keV) This isomer lives long enough to be separated from it’s parent (Cs-137) and studied in the lab. 137

15. Co-60: beta-gamma-gamma decay Cobalt-60 is an example of a nuclide that decays by beta decay to an intermediate excited state. The intermediate state then decays by emitting a gamma rays. Hence both beta and gamma rays are observed from this nuclide. The energy level scheme for this is not too complicated, and is shown in an on-line database of nuclide data. (link to Korean database)(link to Korean database) Co-60 decays with emission of a beta of energy less than 318 keV and two gamma photons of energies 1173 keV and 1332 keV (link to Lund)(link to Lund)