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www.physics.me.uk Radioactive Decay Mechanisms Many nuclei are stable. Many are radioactive or unstable, and will tend to decay by emitting a particle, transforming the atom (made up of protons, neutrons and electrons1) into an atom of another element, or into a lower energy state. During radioactive decay, principles of conservation apply. conservation of energy conservation of momentum (linear and angular) conservation of charge conservation of nucleon number In this document we consider four methods of radioactive decay; alpha, beta minus, beta plus and electron capture. These may be accompanied by another emission called gamma radiation. Note a nucleon is a general term that includes protons and neutrons. Belt of Stability If we plot the number of neutrons against the number of protons for all stable atoms we get the following graph that is often referred to as the belt of stability. Belt of stability. Neutron versus protons for stable isotopes 140 120 Z, number of neutrons 100 80 60 40 20 0 0 10 20 30 40 50 A, Number of protons 1 Atomic Structure and the Periodic Table 60 70 80 90 www.physics.me.uk Note that 1. there are no stable nuclei with an atomic number of 84 or greater. 2. for smaller atoms the ratio neutrons:protons is 1:1 3. for larger atoms the ratio of neutrons:protons approaches about 1.5:1 Since the strong nuclear force weakens significantly with distance, as the atom gets bigger. If the number protons to neutrons remained in the ratio of 1:1 for larger atoms then the repulsive force would be much stronger than the ‘strong force’. To compensate, more neutrons are required to ‘pad out’ the nucleus to ensure its stability. One the number of protons in the nucleus is above 83, no amount of extra neutrons can make the nucleus stable. Alpha Decay Alpha decay is a Coulomb or electrostatic repulsion effect, which becomes more and more disruptive as the size of the nuclei increase; overpowering the strong nuclear force. In alpha decay, the nucleus emits an alpha particle; an alpha particle is essentially a helium nucleus; a group of two protons and two neutrons. A helium nucleus is very stable. An example of an alpha decay involves uranium-238: or The process of transforming one element to another is known as transmutation. As a result of alpha decay the mass number of the nuclide decreases by four units and the atomic number reduces by two units (ie element shifts two places down the periodic table. Note that elements with a high mass number tend to give off alpha radiation; they need to significantly reduce their size to ever become stable. Here are two further examples Exercise: Write down the alpha decay equations for the following nuclides: , , , . Go to answers Exercise: Write down the alpha decay equations for the following nuclides: , , , . Go to answers , , www.physics.me.uk Beta Decay A beta particle is either an electron or a positron, a positively-charged particle that is the anti-matter equivalent of the electron. Hence there are two distinct types of beta decay, the former is termed beta minus (β-) decay and the latter is termed beta plus (β+) decay. Beta minus decay In beta minus decay a neutron converts into a proton and emits an electron. Hence the atomic number increases by one and the mass number remains unchanged (ie the element moves up one in the periodic table). For example Where can be written and represents an antineutrino (symbol ν (Greek letter ‘nu’) represents a neutrino). Hence beta minus decay tends to occur when the neutron to proton ratio is too high, that is that the isotope is above the stability belt. Another example of beta minus decay is the decay of tritium A further example of beta minus decay is the decay of into into Exercise: Write down the beta minus decay equations for the following nuclides: , , . Go to answers , Exercise: Write down the beta minus decay equations for the following nuclides: , , , . Go to answers , Beta plus or positron decay Beta plus particles or positrons are the antimatter equivalent of beta minus particles. Positron particles have the same mass as an electron, but it has a +1 charge. Beta plus decay is the mirror image of beta decay in which a proton is converted into a neutron and emits a positron. The atomic number goes down by one and mass number remains unchanged. Beta plus decay tends to happen when the neutron to proton ratio is too low. www.physics.me.uk Here is an example of beta plus decay where is a neutrino. Exercise: Write down the beta plus decay equations for the following nuclides: , , . Go to answers Exercise: Write down the beta plus decay equations for the following nuclides: , , , . Go to answers , , Electron Capture Electron capture is different to the other forms of decay in that it involves something entering the nucleus, rather than being emitted from the nucleus. In electron capture an electron from the lowest energy level falls into the nucleus and the other electrons reorganise themselves into the normal shell structure. The negative electron combines with one of the protons to become a neutron and another particle called a neutrino (atomic number zero and mass number zero) is emitted. The atomic number is reduced buy one and the mass number remains unchanged. Here is an example of electron capture Where e represents the electron, which can be written , to show that it has the effect of reducing the atomic number by one, but leaving the mass number unchanged. Exercise: Write down the electron capture equations for the following nuclides: , , . Go to answers , Exercise: Write down the electron capture equations for the following nuclides: , , , . Go to answers , Gamma radiation Gamma radiation is often a by-product of alpha or beta radiation. Gamma radiation allows the nucleus to remove surplus energy by the nucleus changes from a higher-level energy state to a lower level. Similar to the energy levels for electrons in the atom, the nucleus also has energy levels. The shells within the nucleus are less well understood than the electron shells, but the energies involved with changes in the nuclear shells are typically 1000000x greater than the energies involved with changes in the electron shells.