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

Basic terms and definitions Introduction 2 Atom Next Slide Basic terms and definitions Atomic number (Z) : no. of protons in nucleus 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

Basic terms and definitions Radioactive Decay 1 Atom Next Slide Basic terms and definitions Isotopes : nuclides with same value of Z 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 Radioactive Decay 2 Atom Alpha emission Next Slide Radioactive decay Alpha emission Diagram Beta emission Diagram Gamma emission Diagram Radioactive decay series Diagram

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

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

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

END of Atom

Back to Radioactivity Introduction 1 Click Back to Rutherford

Radioactivity Introduction 1 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 gold foil  source

Back to Radioactivity Introduction 1 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 gold foil  source

Radioactivity Introduction 1 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.

Radioactivity Introduction 1 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.

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

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

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

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

Back to Radioactivity Radioactive Decay 1 Click 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.

Back to Radioactivity Radioactive Decay 1 Click Back to Gamma emission is shown below : 90 p 144n Thorium-234 (excited state)  ray (normal) 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).

Radioactivity Radioactive Decay 1 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.

Back to Radioactivity Radioactive Decay 1 Click Back to Z (atomic no.) N (neutron no.) 92 88 84 80 76 96 145 143 141 139 137  decay  decay

Back to Radioactivity Radioactive Decay 3 Click 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?

Radioactivity Radioactive Decay 3 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-222 80 million 40 million 20 million 10 million 5 million 0 million 60 million 70 million 75 million time 0 year 1620 year 3420 year 4860 year 6840 year half-life

Radioactivity Radioactive Decay 3 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 activity

Radioactivity Radioactive Decay 3 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

Back to Radioactivity Radioactive Decay 3 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 Radioactivity Uses of radioisotopes 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 Radioactivity Uses of radioisotopes Click Back to Paper produced by a factory passes through a strontium-90 (beta source) and G.M. tube as shown. 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

Back to Radioactivity Uses of radioisotopes 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. radiation detected by G.M. tube

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

Back to Radioactivity Uses of radioisotopes Click Back to The ratio of carbon-14 (radionuclide : half-life : 5600 years) to carbon-12 in atmosphere is a constant. 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.

Radioactivity Fission and Fusion 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.

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

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

Back to Radioactivity Fission and Fusion Click Back to If two light nuclei are joined to form a heavy nucleus, fusion occurs. 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.