2. Radioactive Decay The nuclei of some chemical elements are unstable against the strong nuclear force holding them together, resulting in a spontaneous change of characteristic or identity of the element. This usually happens for elements above 92 There are 3 methods of decay

3. Decay Mechanisims  - decay  - decay  - decay A Helium nucleus seems to emerge from the unstable element An electron and neutrino emerge from the unstable element A photon emerges from the unstable element

4.  - decay Helium nucleus emerges from the unstable element. 92 238 U  90 234 Th + 2 4 He + energy electric repulsion becomes greater than the nuclear attraction/ contrast between short and long range forces. Masses do not balance!

5.  - decay An electron and a “neutrino” emerge from the unstable nucleus. 6 14 C  7 14 N + - 1 0 e + energy Weak force - a “down quark” in a neutron changes into an “up quark” changing it into a proton. Masses do not balance!

6.  - decay Photon emerges from the unstable element The element retains it’s identity 13 27 Al*  13 27 Al +  + energy nucleus is not changed but has an excess of energy - particles are agitated and farther away from each other. Masses don’t balance!

7. Einstein - mass IS energy E =  mc 2  m is the mass difference between the parent nuclei and the daughters. The equation gives the energy released. Mass is converted into energy!

8. Decay Process Rate of decay is proportional to the number of nuclei present. –  N/  t = N –N = N 0 e t This is the same for all nuclei, only the is different - it is related to the “probability” of decay for that nuclei!