Chapter 5: Radioactivity Form 5 Physics Next > The study of matter Chapter 5: Radioactivity 1

Physics: Chapter 5 Objectives: (what you will learn) 1) understanding nucleus of atom 2) radioactive decay 3) uses of radioisotopes 4) nuclear energy 5) management of radioactive substances < Back Next > 2

Nucleus of atom An atom consists of a central nucleus where most of the mass of the atom is concentrated. Orbiting around nucleus are electrons. n + – + proton n neutron – electron A helium atom 4 He 2 < Back Next > The nucleus is composed of protons that are positively charged, and neutrons that are neutral. Nucleons = protons + neutrons 3

Nucleus of atom 4 proton number (Z) = the number of protons in nucleus nucleon number (A) = the number of nucleons (protons & neutrons) in nucleus nuclide = a nucleus species with a certain proton number & certain nucleon number 4 He 2 represents nucleus with proton number Z & nucleon A A X Z represents nuclide with 2 protons & 4 nucleons The number of neutrons is 4 – 2 = 2 < Back Next > Isotopes = nuclides with same proton number, different nucleon numbers Isotopes of an element have the same chemical properties but different physical properties, such as mass. 4 1H 1 2H 3H Example isotopes: (tritium) (deuterium) (hydrogen)

Nucleus of atom Rutherford’s alpha-particle (α-particle) scattering experiment Rutherford bombarded gold foil with α-particles. Most α-particles go through gold foil undeflected as the nucleus is very tiny (occupies a small fraction of the volume of atom). Some α-particles are slightly deflected, others are deflected through large angles. The positive α-particles are repelled by a massive, positively charged nucleus. < Back Next > α-particle source α-particle deflected Gold foil Fluorescent screen Telescope vacuum 5

Radioactive decay Radioactivity = spontaneous disintegration of unstable nuclei accompanied by emission of energetic particles or radiations (photons). Spontaneous disintegration = emissions of the particles or photons are not planned in advance Radioactive decay is random because it is not possible to predict which nuclei the number of nuclei that would decay at a particular instant < Back Next > Radioactive decay is not affected by physical conditions such as temperature and pressure, chemical composition The particles emitted in radioactive decays are α-particles and β-particles, and the radiation emitted is gamma-ray (γ-ray). Apparatus used to detect radioactive emissions include cloud chamber and Geiger-Muller tube (GM tube). 6

Radioactive decay The tracks of radioactive emissions can be observed in a cloud chamber. α-particles’ tracks: thick because of their high ionizing power straight because of the comparative large mass all of same length because they are emitted with the same speed < Back Next > β-particles’ tracks: thin because of their weak ionizing power wavy because of the comparative small mass long because of its relative long range in air γ-rays’ tracks: identical to β-particles’ tracks but are short the tracks are those of electrons produced from ionisation of air 7

Radioactive decay 8 Geiger-Muller tube When connected to a counter, it will count the number of β-particles or γ-ray photons that enters it. When connected to a ratemeter, it will give the number of particles per seconds that enter the GM-tube. The GM-tube is unable to detect α-particles which cannot penetrate the window of the tube. < Back Next > doctronics 8

Radioactive decay Changes to structure of nucleus during radioactive decay. Alpha-decay A X Z A-4 Y Z-2 + 4 He 2 (α-particle) Proton number decreases by 2. Nucleon number decreases by 4. < Back Next > Beta-decay A X Z A Y Z+1 + 0 e -1 (β-particle) Gamma ray No changes in the proton number and nucleon number. Proton number increases by 1. Nucleon number unchanged. 9

Radioactive decay 10 N0 ½N0 ¼N0 T½ A0 ½A0 ¼A0 T½ The half-life, T½ of a radioisotope is the time taken for half of the number of nuclei in a sample to decay. N0 ½N0 ¼N0 T½ < Back Next > It is also the time taken for the rate of decay of a sample to become half. A0 ½A0 ¼A0 T½ 10

Radioactive decay 11 Radioisotope = an isotope that is radioactive Uses in medicine (a) γ-rays from cobalt-60: - radiotherapy to destroy cancerous cells - sterilization to destroy bacteria or germs (b) Radioactive tracers: - iodine-131 to evaluate function of thyroid gland - sodium-24 to estimate volume of blood in patient < Back Next > Uses in agriculture (a) Radioactive tracers used in plant nutrient research. (b) γ-rays used to sterilize insects, destroy pests/bacteria in food/fruits. Uses in archaeology (a) Carbon-14 dating: Proportion of C-14 to C-12 in living organism is the same as that of the atmosphere. When an organism dies, its proportion decreases. Its age is estimated by measuring its proportion in sample. 11

Radioactive decay 12 Uses in industry (a) Gauge control GM-tube connected to ratemeter measures thickness of paper by its constant count rate. (b) Leak tracer Sodium-24 used as tracer to locate damaged underground pipes. GM-tube is used to detect high count rate from leaks in the pipe. (c) Quality control γ-rays (Cobalt-60) used to detect flaws in joints between pipes carrying natural gas. (d) Smoke detector Americium-241 emits α-particles which ionizes air particles, allowing current to flow between charged plates. Smoke particles which reduces current flow by deflecting α-particles can then be detected. < Back Next > 12

Nuclear Energy The unit of mass used for measuring the mass of atoms, Atomic mass unit (a.m.u.), u = (mass of an atom of carbon-12) 1 u = 1.66 x 10-27 kg 1 12 < Back Next > Einstein’s energy-mass relation The energy equivalent E of mass m is given by Energy, E = mc2 where c = 3.0 x 108 m s-1 Nuclear fission = splitting of a nucleus into two nuclei Slow neutrons are used to split the nucleus. 13

Management 14 2 negative effects of radioactive materials Somatic damage: near-term death of cells of sensitive organs such as eyes. Genetic damage: long-term effect; mutation of cells in subsequent generations < Back Next > α-particles: Quite harmless outside body due to short range and weak penetration power. Inside body, they are the most damaging due to their strong ionizing power. β-particles: Harmful both outside and inside body due to stronger penetration power, but moderate ionizing power. 14 γ-rays: Harmful outside body due to strong penetration power.

Summary 15 What you have learned: understanding nucleus of atom 2. radioactive decay < Back 3. uses of radioisotopes 4. nuclear energy management of radioactive substances 15 Thank You