2. Nuclear Decay Notes

3. Stability Curve Atomic number Z Neutron number N Stable nuclei Z = N 20406080100 40 100 140 20 60 80 120 Nuclear particles are held together by a nuclear strong force. A stable nucleus remains forever, but as the ratio of N/Z gets larger, the atoms decay. Elements with Z > 82 are all unstable. Page #866

4. Radioactivity As the heavier atoms become more unstable, particles and photons are emitted from the nucleus and it is said to be radioactive. All elements with Z > 82 are radioactive. Examples are: Alpha particles    particles (electrons) Gamma rays     Page #861

5. The Alpha Particle An alpha particle  is the nucleus of a helium atom ( 4 2 He) consisting of two protons and two neutrons tightly bound. Charge = +2 Mass = 4.001506 u Mass = 4.001506 u Relatively low speeds (  0.1c ) Not very penetrating Not very penetrating

6. The Beta-minus Particle A beta-minus particle    is simply an electron ( 0 -1 e) that has been expelled from the nucleus. Charge = e - = -1 - High speeds (near c) - Mass = 0.00055 u Mass = 0.00055 u - Very penetrating Very penetrating-

7. The Gamma Photon A gamma ray  has very high electromagnetic radiation carrying energy away from the nucleus. Charge = Zero (0)  Mass = zero (0)  Speed = c (3 x 10 8 m/s)  Most penetrating radiation 

8. Radioactive Decay As discussed, when the ratio of N/Z gets very large, the nucleus becomes unstable and often particles and/or photons are emitted. Alpha decay results in the loss of two protons and two neutrons from the nucleus. X is parent atom and Y is daughter atom The energy is carried away primarily by the K.E. of the alpha particle.

9. Example : Write the reaction that occurs when radium-226 decays by alpha emission. From tables, we find Z and A for nuclides. The daughter atom: Z = 86, A = 222 Radium-226 decays into radon-222.

10. Beta Decay Beta   decay results when a neutron decays into a proton and an electron. Thus, the Z-number increases by one. X is parent atom and Y is daughter atom The energy is carried away primarily by the K.E. of the electron. -

11. Example : Write the reaction that occurs when Carbon-14 decays by Beta emission. 14 6 C 14 7 N + 0 -1 β + energy