Radioactivity W Richards The Weald School

Structure of the atom A hundred years ago people thought that the atom looked like a “plum pudding” – a sphere of positive charge with negatively charged electrons spread through it… I did an experiment (with my colleagues Geiger and Marsden) that proved this idea was wrong. I called it the “Scattering Experiment” Ernest Rutherford, British scientist:

The Rutherford Scattering Experiment Alpha particles (positive charge, part of helium atom) Thin gold foil Most particles passed through, 1/8000 were deflected by more than 90 0 Conclusion – atom is made up of a small, positively charged nucleus surrounded by electrons orbiting in a “cloud”.

The structure of the atom ELECTRON – negative, mass nearly nothing PROTON – positive, same mass as neutron (“1”) NEUTRON – neutral, same mass as proton (“1”) Atoms are roughly m in diameter, while the nucleus is – m

The structure of the atom ParticleRelative MassRelative Charge Proton1u (1.7x kg)+1.6x C Neutron1u (1.7x kg)0 Electron0-1.6x C MASS NUMBER (A) = number of protons + number of neutrons SYMBOL No. of neutrons N = A - Z PROTON NUMBER (Z) = number of protons (obviously)

Isotopes An isotope is an atom with a different number of neutrons: Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more. Notice that the mass number is different. How many neutrons does each isotope have? A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating.

Quarks P e- Low energy scattering We can investigate the structure of protons by bombarding them with electrons: High energy scattering Elastic collision. Electrons and protons behave as expected. P e- Inelastic collision. Energy is “absorbed” by the proton and increases its internal energy. This is Deep Inelastic Scattering and suggests that the proton is made of smaller particles called quarks.

Introduction to Radioactivity Some substances are classed as “radioactive” – this means that they are unstable and continuously give out radiation: Radiation The nucleus is more stable after emitting some radiation – this is called “radioactive decay”.

Ionisation Radiation is dangerous because it “ionises” atoms – in other words, it turns them into ions by giving them enough energy to “knock off” electrons: Alpha radiation is the most ionising (although short range). Ionisation causes cells in living tissue to mutate, usually causing cancer.

The Geiger-Muller Tube Metallic case (cathode) Mixture of argon and halogen Central anode Mica end window

Types of radiation 1) Alpha (  ) – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of a ______ atom) 2) Beta (  ) – an atom decays into a new atom by changing a neutron into a _______ and electron. The fast moving, high energy electron is called a _____ particle. 3) Gamma – after  or  decay surplus ______ is sometimes emitted. This is called gamma radiation and has a very high ______ with short wavelength. The atom is not changed. Unstable nucleus New nucleus Alpha particle Beta particle Gamma radiation Words – frequency, proton, energy, neutrons, helium, beta

Changes in Mass and Proton Number Alpha decay: Am Np α C 11 6 B β Sr Y β 0 + Beta - decay: Beta + decay: “positron”

Blocking Radiation Each type of radiation can be blocked by different materials:    Sheet of paper (or 6cm of air will do) Few mm of aluminium Few cm of lead

Summary PropertyAlphaBeta -Beta +Gamma Charge Rest mass Penetration What is it? Ionising ability

Deflection by Magnetic Fields Alpha and beta particles have a charge: protons, 2 neutrons, therefore charge = +2 1 electron, therefore charge = -1 Because of this charge, they will be deflected by electric and magnetic fields:

Background Radiation Radon gas Food Cosmic rays Gamma rays Medical Nuclear power 13% are man-made

Nuclear fission Uranium nucleus Unstable nucleus New nuclei (e.g. barium and krypton) More neutrons Neutron

Chain reactions Each fission reaction releases neutrons that are used in further reactions.

Radioactive Decay Radioactivity is a random process. The number of radioisotopes that will decay clearly depends on the number of radioisotopes present at that point in time: Activity (in Bq) = λ N λ = “The decay constant” and has units of s -1. It is constant for a particular radioisotope.

Half Life The decay of radioisotopes can be used to measure the material’s age. The HALF-LIFE of an atom is the time taken for HALF of the radioisotopes in a sample to decay… At start there are 16 radioisotopes After 1 half life half have decayed (that’s 8) After 3 half lives another 2 have decayed (14 altogether) After 2 half lives another half have decayed (12 altogether) = radioisotope= new atom formed

A radioactive decay graph Time Count 1 half life

Half Life To calculate half life there are a few methods: 1) Read from a graph 2) Calculate using an equation t ½ = ln2 λ

Half Life questions 100s 1)The graph shows the activity of a radioisotope. Determine the half life and decay constant. 2)If there are 10 6 atoms present right now calculate how many will decay over the next second. 3)What percentage of a sample of radioactive material will exist after 200 years if the half life is 50 years? 4)Uranium decays into lead. The half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample.