Radioactivity (ONLINE CLASS) Lecturer Note PDF

# RADIOACTIVITY Radioactivity is the disintegration of atomic nuclei with spontaneous emission of particles and or radiation such as alpha α, beta β, and gamma γ. The branch of chemistry that deals with the study of radioactivity and radioactive substances is known as Nuclear Chemistry. Nuclear ch...

# RADIOACTIVITY Radioactivity is the disintegration of atomic nuclei with spontaneous emission of particles and or radiation such as alpha α, beta β, and gamma γ. The branch of chemistry that deals with the study of radioactivity and radioactive substances is known as Nuclear Chemistry. Nuclear chemistry deals with the structures and behaviour of atomic nuclei; (i.e, the tiny, massive centre of atoms). Atomic nucleus can be viewed as an aggregate of protons and neutrons (nucleons). A nuclide is represented as $^A_ZX$, where X is the symbol of the element corresponding to the nuclide, A is the nucleon number or mass number and Z is the atomic number. Radioactivity relates to the ratio of the number of neutron to protons in a nucleus. Generally, the ratio of neutrons to protons in stable (non radioactive) nuclides ranges from a minimum of 1:1 to a maximum of 1.6:1. A nuclide that has too low or too high a neutron-to-proton ratio than this is likely to be radioactive. Furthermore, all nuclei with 83 protons and above (i.e atomic number ≥ 83) are radioactive no matter how many neutrons are present. ## History Radioactivity was first observed and investigated in 1896 by Henri Becquerel, who found that uranium salt emitted radiation which could penetrate through opaque material, affect (blacken) photography plate, cause gases to ionize and make certain substance to fluoresce. Investigations by scientists in the early 1900, particularly by Pierre and Marie Curie, led to the discovery that elements including Uranium, Radium, Thorium and Polonium disintegrated spontaneously and emitting radiation. Element of these types are said to be radioactive elements. - Radioactive elements emit radiation continuously and spontaneously. Temperature and Pressure have no effect on the rate at which this radiation is emitted, whereas, such factors particularly temperature, influence ordinary chemical reaction. - Radioactivity is always associated with a release of energy. The energy is known as nuclear energy. The energies involved in nuclear processes are more than a million times greater than the energies released in ordinary chemical reactions because nuclear forces are correspondingly stronger than chemical bonds. # TYPES OF RADIOACTIVE DECAY Rutherford investigated the penetrating power of the radiation of the radio actions from the radioactive substance by subjecting them to electric and magnetic field, as a result, he deduced that there were three main types of radiation which are: 1. Alpha particles (α) 2. Beta particles (β), which is of two types: - Electron (β-) (e-) - Positron ( β+) (e+) 3. Gamma particles (γ) The following are the types of radioactive decay: 1. Alpha particles decay 2. Beta particles decay 3. Gamma particles decay 4. Electron capture (EC) ## 1. Alpha particle decay (emission): Alpha particles are fast moving Helium nuclei with a nuclei charge of +2. The symbol is $^4_2$He. i.e Nuclei with mass number of 4 and atomic number of 2. The emission of an alpha particle by an unstable nuclei results in the decrease of mass number by 4 units and atomic number by 2 units. - In general, emission of (n) alpha particles by a radioactive substance (N) with mass number M, with atomic number A is: $ ^M_AN \implies ^{M-4n}_{A-2n}N^* + n^4_2He$ Where N* is another nuclei with different composition to nuclei N. the parent nucleus. - For example: - Emission of Alpha particle by Radium- 226 and Plutonium- 240; - $ ^{226}_{88}Ra \implies ^{222}_{86}Rn + ^4_2He$ - $ ^{240}_{94}Pu \implies ^{236}_{92}U + ^4_2He $ ### Properties of Alpha particles 1. They are of high velocity 2. They have little penetrating power and can be stopped by a paper 3. They usually ionize gases (air) and resemble x-rays e.g. Air produces flashes on fluorescence screen when alpha particle is passed through it. 4. A few of alpha particles are deflected by thin metal foil 5. They are attracted to the negative plates and south poles of electromagnetic field, hence they are positively charged. 6. They could blacken a photographic plate. ## 2. Beta particle decay: The radiations involved in beta particle decay are of two types. - **i. Electron ( β-) (e-)**: The electrons are formed by decay of a neutron in the nucleus to form a proton and electron. $^1_0n \implies ^1_1P + ^0_{-1}e^-$ - **ii. Positron (β+) (e+)**: The positron is formed by the decay of a proton in the nucleus to form a neutron. $^1_1P \implies ^1_0n + ^0_{+1}e^-$ ### i. Electron (Beta negative particles) emission: The emission of Beta-particle result in no change of mass number but an increase by 1 unit in the atomic number brings about a change. - Generally, emission of n $^0_{-1}e^-$ by radioactive substance N with mass number M and atomic number A can be represented by the nuclei equation: $ ^M_AN \implies ^{M}_{A+n}N^* + n ^0_{-1}e^-$ - Where N* is another nuclei with different composition to nuclei N, the parent nucleus. - For example, Emission of β- by Thorium- 234 and Lead-214. - $ ^{234}_{90}Th \implies ^{234}_{91}Pa + ^0_{-1}e^-$ - $ ^{214}_{82}Pb \implies ^{214}_{83}Bi + ^0_{-1}e^-$ ### ii. Positron (Beta positive particles) emission: The emission of β+ particles result in no change in mass number but a decrease by 1 unit in the atomic number. - Generally, emission of n $^0_{+1}β$ particle by radioactive substance N with mass number M and atomic number A can be represented by the nuclei equation: $ ^M_AN \implies ^{M}_{A-1}N^* + n ^0_{+1}e^-$ - Where N* is another nuclei with different composition to nuclei N, the parent nucleus. - For example, Emission of B+ particles by radioactive element $ ^{38}_{19}K$ and $ ^{95}_{43}Tc$ - $ ^{38}_{19}K \implies ^{38}_{18}Ar + ^0_{+1}e^-$ - $ ^{95}_{43}Tc \implies ^{95}_{42}Mo + ^0_{+1}e^-$ ### Properties of Beta particles 1. They are of high velocity which is higher than alpha particle. 2. Have more penetrating power that alpha particle. 3. They ionize gases to a lesser extent than alpha particle. 4. A beam of beta particle is diverge by a metal foil and is repelled by electrons in the metal atoms. 5. They are attracted to the positive and north poles of electromagnetic plate, hence they are negatively charged. ## 3. Gamma particle (emission): These are high energy electromagnetic radiation. The rays are comparable to x-rays, although they have shorter wavelength. The symbol is γ. They are formed by a re-adjustment of the nucleus after Alpha or Beta emission. They have no charge. The emission of a gamma particle results in no change of mass and atomic numbers. - For example: The emission of gamma particle by Cobalt-60 - $ ^{60}_{27}Co \implies ^{60}_{27}Co + γ$ - They are also emitted in a process called electron capture. - For example; $ ^{37}_{18}Ar + ^0_{-1}e^- \implies ^{37}_{17}Cl + γ$ ### Properties of gamma particles 1. They have velocity of light 2. They are not deflected by a magnetic field 3. They are extremely penetrating with a range of (15-20 cm) in length. 4. They have the ability to eject high speed electrons from matter. 5. They have little ionizing power. 6. They are not attracted in a magnetic or electric field as they are uncharged. ## 5. Electron Capture: This is a decay process whereby an unstable nucleus captures or picks up the electron from an inner orbital of an atom. In effect, a proton is changed to neutron as in positron emission. i.e $^1_1P + ^0_{-1}e^- \implies ^1_0n $ - An example is given by $ ^{40}_{19}K$ which can decay by electron capture - $ ^{40}_{19}K + ^0_{-1}e^- \implies ^{40}_{18}Ar$ - The decay by electron captures result in no change in mass number but a decrease by 1 unit in the atomic number. # NUCLEAR FISSION AND NUCLEAR FUSSION ## NUCLEAR FISSION: This is the splitting of a heavy isotope by a neutron into light nuclei with the emission of two or more fast moving neutrons and high energy radiation. The neutrons produced react with more atoms of the heavy isotope setting up a chain reaction. Only a few kinds of isotopes undergo fission. These are the fissile isotopes. The most important fissile isotopes are Uranium-235 and Plutonium-239. The three possible fissions of Uranium-235 used in nuclear power station can be represented by the following nuclear equations: - $ ^{235}_{92}U + ^1_0n \implies ^{142}_{54}Xe + ^{90}_{38}Sr + 4^1_0n + energy.$ - $ ^{235}_{92}U + ^1_0n \implies ^{148}_{56}Ba + ^{85}_{36}Kr + 3^1_0n + energy.$ - $ ^{235}_{92}U + ^1_0n \implies ^{144}_{55}Xe + ^{90}_{37}Rb + 2^1_0n + energy.$ - Therefore, fission produces a mixture of fission products. - Plutonium-239 undergoes atomic fission as follows: - $ ^{239}_{94}Pu + ^4_2He \implies ^{242}_{96}Cm + ^1_0n$ - $ ^{242}_{96}Cm + ^4_2He \implies ^{244}_{98}Cf + 2^1_0n$ - If heavier bombarding molecules are used; - $ ^{242}_{96}Cm + ^{12}_6C \implies ^{254}_{102}No + 4^1_0n$ - $ ^{250}_{98}Cf + ^{11}_5B \implies ^{257}_{103}Lw + 4^1_0n$ ### Characteristics of Nuclear Fission. The two important characteristics of fission are: 1. Nuclear fission always give neutrons as products, usually two or more neutrons are formed for every one neutron used to initiate a nuclear fission. 2. When fission occurs, more stable product nuclei are formed from the less stable parent nucleus. This change from less stable to more stable nuclei produces a large amount of energy. This energy represents nuclear energy that accompanies fission. ## NUCLEAR FUSSION: This is another type of transmutation process that can produce energy. At high temperatures, it is possible for some lighter nuclei to fuse to form heavier nuclei. Fusion is the joining together of two or more small atomic nuclei to form heavier nuclei with evolution of large amount of energy. The energy available from a single fusion far exceeds that from a single fission. For example, these processes occur in the hydrogen fusion bombs: 1. $ ^3_1H + ^2_1H \implies ^4_2He + ^1_0n$ 2. $ ^2_1H + ^2_1H \implies ^4_2He + γ + energy$ 3. $ ^1_1H + ^1_1H + ^1_1H + ^1_1H \implies ^4_2He + 2^0_{+1}β + energy.$ The neutron formed in (i) above interacts with Lithium-6 to give more Trithium. - $ ^6_3Li + ^1_0n \implies ^4_2He + ^3_1H$ The fusion process can produce larger amounts of energy for a given amount of materials than the fission process. # Calculations of Energy from Nuclear Processes. Consider a typical fission of Uranium-235 and the masses of nuclei and particles involved. - $ ^{236}_{92}U + ^1_0n \implies ^{139}_{54}Xe + ^{94}_{38}Sr + 3^1_0n + energy.$ - RAM: 235.0439 1.0087 138.9178 93.9154 3(1.0087) - Total: 236.0526 235.8593 In nuclear fission, loss of mass occurs as it changes to energy. This is the basis of nuclear of nuclear energy. From the above equation; - Loss in mass = 236.0526-235.8593=0.1933g. The relation between mass and energy is given by the Albert Einstein equation: $E = mc^2$ Where E is the energy in joules, m is the mass in Kg and c is the speed of light in ms-¹. (c = 3 x 10^8 ms-¹). Therefore for the equation above; E = (0.1933 Kg x 10^-3) (3 x 10^8 ms-1)^2 = 1.74 x10^13 J.