Unit 1: Radioactivity and nuclear physics

1. Unit 1: Radioactivity and nuclear physics

2. Traditional atomic model • Not correct but it is a good enough approximation. • Nucleus contains 2 types of nucleons: Protons and Neutrons. • Orbiting electrons. • Atomic number is number of protons. • Mass number is number of nucleons. • Elements have the same atomic number but can have different mass numbers = isotopes.

3. Chemical symbols for isotopes

4. Properties of subatomic particles

8. Hydrogen Isotopes

13. Radioactive isotopes • Every element has some unstable isotopes called radioisotopes. • These are rare. The most common isotope is usually stable (eg. C-12 not C-13 or C-14). • Radioisotopes become stable by decaying into a stable isotope. • They decay by emitting radiation.

14. Alpha decay • The nucleus ejects an α particle (2 protons and 2 neutrons) to become smaller and more stable.

15. Uranium-238: 92 protons, 146 neutrons. • Emits an α particle: 2 protons, 2 neutrons. • Thorium-234: 90 protons, 144 neutrons. • Th-234 is the daughter nucleus of the alpha decay

16. Beta decay • A neutron can decay into a proton and an electron (after about 10 minutes). • The electron (β particle) is ejected from the nucleus at close to the speed of light. • The atomic number increases by 1. The mass number stays the same.

17. Thorium-234: 90 protons, 144 neutrons • One neutron decays into a proton and an electron • Protactinium-234: 91 protons, 143 neutrons

18. Gamma decay • After an alpha or beta decay the nucleus can be momentarily “excited”. • The excited nucleus emits a high energy gamma ray (which is a photon). • If a nucleus is in an excited state we put an asterisk next to the symbol.

19. Protactinium-234*: 91 protons, 143 neutrons • Emits a γ particle • Protactinium-234: 91 protons, 143 neutrons

20. Decay of Uranium-238 • http://en.wikipedia.org/wiki/Uranium-238 • http://en.wikipedia.org/wiki/Decay_chain

21. Penetration power

22. Smoke detectors • Americium-241 emits α particles. • These charged particles allow a current to flow between two electrodes. If smoke blocks the α particles then the current is reduced and an alarm sounds.

23. Ionising power • Radiation can ionise atoms by removing one or more electron from it’s orbit. • α particles interact strongly with atoms because of their slow speed and +2 charge. This slows them down quickly (low penetration power). • β particles also with atoms but are moving faster and only interact with the outside electrons. They take longer to slow down and have higher penetration power than α particles. • γ particles interact with atoms very weakly because they have no charge and most of an atom is empty space. This means they pass through most materials.

24. Properties of α,β,γ particles