Atom Rutherford Next Slide Rutherford’s scattering experiment Photo Atomic model Diagram Rutherford’s scattering experiment Introduction 1

Atom Basic terms and definitions Atomic number (Z) : no. of protons in nucleus Next Slide Mass number (A) : total no. of protons and neutrons in nucleus Symbol of a nucleus with Z and A : Nuclide : a kind of atom with a particular A and Z Radionuclide : nuclide which is radioactive Introduction 2

Atom Basic terms and definitions Isotopes : nuclides with same value of Z Next Slide Radioactivity : emission of radiation by unstable nuclei Radioactive decay : the process of emission of radiation by unstable nuclei Radioactive decay, parent nucleus, daughter nucleus and decay products Diagram Radioactive Decay 1

Atom Radioactive decay Alpha emission Next Slide Beta emission Gamma emission Radioactive decay series Diagram Radioactive Decay 2

Atom Random decay Random decay : assumptions Next Slide Activity of the source : number of disintegrations per second of the source Time required for half of the radionuclides in the source to undergo radioactive decay Half-life of a radionuclide : Diagram Radioactive Decay 3

Atom Applications Radiotherapy : killing of cancer cells Next Slide Sterilisation : killing of bacteria and viruses Thickness gauge Tracers Explanations Diagram Tracing and monitoring flow systems Diagram Carbon dating Diagram Smoke detector Photo Uses of radioisotopes

Atom Nuclear fission and fusion Nuclear fission : Uranium-235 Next Slide Controlled and uncontrolled chain reactions Nuclear debate Nuclear fusion : Hydrogen-2 & Hydrogen-3 Diagram Fission and Fusion

END of Atom

Radioactivity Click Back to Rutherford Back to Introduction 1

Radioactivity Next Slide A piece of thin gold foil is placed in front of a radium source as shown in the following figure. A zinc sulphide screen is used to detect the path of alpha particles passing through the foil. movable detector movable detector gold foil  source Introduction 1

Radioactivity Click Back to Most of the particles pass through the gold foil without any deflection. Some particles are deflected and few (1 in 8000) is reflected backwards. movable detector movable detector gold foil  source Back to Introduction 1

Radioactivity Next Slide Atomic model to explain the results : i. Nucleus is a small point at the centre containing most of the mass of an atom. ii. There are two kinds of particles (protons & neutrons) in the nucleus. Each has a mass 1800 times that of an electron. iii. A strong force, which is called nuclear force, holds the protons and neutrons together in the nucleus. iv. Proton carries a +ve charge of the same magnitude as that of an electron. Introduction 1

Radioactivity Next Slide v. Electrons orbit around the nucleus at fixed energy levels which are called electronic shells. They constitute the “skin” of an atom. Most of an atom is empty space. vi. The no. of electrons and protons are the same to form a neutral atom. Introduction 1

Radioactivity Next Slide Atomic model of a helium atom : electron nucleus with 2 protons and 2 neutrons Introduction 1

Radioactivity Click Back to Scattering of  particles gold atoms alpha particles Back to Introduction 1

Radioactivity Click Back to Back to A radionuclide is shown below : Z and A are always conserved on both sides of the equation. 92 p 146n 90p 144n 2p 2n Uranium-238 Thorium-234  particle Parent nucleus Daughter nucleus Decay products Radioactive Decay 1

Radioactivity Click Back to Back to Alpha emission is shown below : 92 p 146n 90p 144n 2p 2n Uranium-238 Thorium-234  particle Radioactive Decay 1

Radioactivity Click Back to Back to Beta emission is shown below : 90 p 144n 91p 143n Thorium-234 Protactinium-234  particle When an electron is emitted, one neutron is changed to proton. The mass of an electron is very small compared with an proton or neutron, its mass no. is considered as zero. Radioactive Decay 1

Radioactivity Click Back to Back to Gamma emission is shown below : Sometimes a nuclide may contain more energy than usual (e.g. after emitting  or  particle). We say that it is in an excited state. The extra energy may be emitted in the form of EM waves (  ray). 90 p 144n Thorium-234 (excited state)  ray 90 p 144n Thorium-234 (normal) Radioactive Decay 1

Radioactivity Next Slide If the decay process repeats again and again for each daughter nuclides until a final stable nuclide is produced, then we have a decay series. The decay series could be shown graphically. Radioactive Decay 1

Radioactivity Click Back to Back to Z (atomic no.) N (neutron no.)  decay  decay Radioactive Decay 1

Radioactivity Click Back to Back to Radioactive decay is a random uncontrolled process. No definite answers to questions like : i. Which nucleus will undergo disintegration next? ii. When will a specific nucleus decay? iii. Where does the emitted particle go? Radioactive Decay 3

Radioactivity Next Slide The decay of a sample of radium-226 to radon-222 is illustrated. The half-life of radium-226 is 1620 year. Originally, we have 80 million radium-226. radium-226 radon million40 million20 million10 million5 million 0 million40 million60 million70 million75 million time0 year1620 year3420 year4860 year6840 year half-life Radioactive Decay 3

Radioactivity Next Slide A graph of no. of undecayed nuclei vs. time is shown below : Since activity is directly proportional to no. of radionuclides, the graph of activity vs time also has the same half-life: time no. of undecayed nuclei time activity Radioactive Decay 3

Radioactivity Next Slide Therefore, we can measure the activity of a source and hence deduce the half-life of the source. However, under normal situations, we measure the activity as well as the background radiation as shown : activity time Background radiation Radioactive Decay 3

Radioactivity Click Back to We subtract the background radiation from the activity to get the actual activity and hence deduce the half-life. time activity Back to Radioactive Decay 3

Radioactivity Click Back to We insert a small amount of weak radionuclide into a system. Then we can use a GM tube to detect the radiation as well as the flowing process of the system, e.g. bloodstream or water pipe. Back to Uses of radioisotopes

Radioactivity Click Back to Paper produced by a factory passes through a strontium-90 (beta source) and G.M. tube as shown. Back to Since beta radiation is partly absorbed by the paper, the reading detected by the G.M. tube could be used to monitor the thickness of the paper produced. source Uses of radioisotopes

Radioactivity Click Back to Gamma source may be used to detect any leakage in the water pipe as shown. If there is any leakage, the radiation may be detected by the G.M. tube. Back to radiation detected by G.M. tube Uses of radioisotopes

Radioactivity Click Back to A smoke detector is shown below : Back to Uses of radioisotopes

Radioactivity Click Back to The ratio of carbon-14 (radionuclide : half-life : 5600 years) to carbon-12 in atmosphere is a constant. Back to This ratio takes on the same value in animal and plant’s bodies due to respiration. However, for dead animals or plants, the ratio changes as carbon-14 undergoes radioactive decay. By measuring the difference between this ratio in a dead body and the normal value, we can deduce the time of death of the animal or plant. Uses of radioisotopes

Radioactivity Next Slide Uranium-235, which constitutes about 0.7% of natural uranium, can undergo a fission when bombarded by a slow neutron as shown in the following equation. The total mass of the product is smaller than the parent nuclides. The lost mass has been turned into energy. The neutrons produced would trigger other uranium-235 to undergo the same reaction. The process repeats again and again and we have a chain reaction. Fission and Fusion

Radioactivity Click Back to If the mass of the uranium is larger than a certain limit, the chain reaction takes place very quickly. Back to Fission and Fusion

Radioactivity Click Back to Uncontrolled chain reaction : atomic bomb Back to Controlled chain reaction : nuclear reactor to generate electricity Fission and Fusion

Radioactivity Click Back to If two light nuclei are joined to form a heavy nucleus, fusion occurs. Back to The fusion of two isotopes hydrogen-2 and hydrogen-3 is shown below : It is again a chain reaction. Now we cannot use controlled fusion to gain energy. An uncontrolled fusion is actually an hydrogen bomb. Fission and Fusion