General Radiation Physics Lecture 1 Radiation Physics PDF

General & Radiation Physics DARRION WALKER (PHD) LECTURE 1 Radioactivity Radioactivity is the act of emitting radiation spontaneously. This is as the nucleus of an atom is unstable. The nucleus of an atom becomes unstable when it either has excess nucleons or excess energy. The nucleus...

General & Radiation Physics DARRION WALKER (PHD) LECTURE 1 Radioactivity Radioactivity is the act of emitting radiation spontaneously. This is as the nucleus of an atom is unstable. The nucleus of an atom becomes unstable when it either has excess nucleons or excess energy. The nucleus of the atom therefore then tries to become stable by emitting either energy or nucleons. Radioactivity The Atom, smallest unit into which matter can be divided without the release of electrically charged particles. It also is the smallest unit of matter that has the characteristic properties of a chemical element. The atom is referred to as the basic building block of chemistry. Radioactivity Radioactivity A given Nucleus has the following particles: Total number of ‘nucleons’ = atomic mass number, with symbol “A” Proton number = atomic number, with symbol “Z” Neutron number with symbol N is therefore = A ­- Z Radioactivity Notation: a specific nucleus or ‘nuclide’ can be specified as: Radioactivity The total mass of a stable nucleus is always less than the sum of the masses of its separate pieces; the protons and neutrons. Where has the mass gone? Energy, as radiation or kinetic energy, is released during formation of a nucleus by ‘fusion’ of smaller nuclei, giving a net mass difference. This difference between the total mass of separate nucleons and the mass of the final nucleus is then the total binding energy of that nucleus. Radioactivity The force that binds the nucleons together is called the strong nuclear force. This is a very strong, but very short­range, force. It is essentially zero if the nucleons are more than about 10­-15 m apart, which roughly corresponds to the size of a nucleus. The Coulomb force is long-range; this is why extra neutrons are needed for stability of high­“Z” nuclei. Unstable nuclei decay; some decays are governed by another force, called the weak nuclear force. Radioactivity There are 3 types of Radioactive decay: These are: Alpha Decay (helium nuclei) Beta Decay (electrons) Gamma Decay (electromagnetic radiation) Radioactivity Regular Alpha and Beta rays are bent in opposite directions in a magnetic field, thus have opposite charge; while gamma rays are not bent at all. Radioactivity Alpha Decay: In alpha decay the nucleus emits a 4He nucleus (an alpha particle). Alpha decay occurs most often in massive nuclei that have too large a proton to neutron ratio. An alpha particle, with its two protons and two neutrons, is a very stable configuration of particles. Nuclei, which are more massive than lead, frequently decay by this method. Radioactivity Consider the example of 210Po decaying by the emission of an alpha particle. The reaction can be written 210Po → 206Pb + 4He. This polonium nucleus has 84 protons and 126 neutrons. The ratio of protons to neutrons is Z/N = 84/126, or 0.667. A 206Pb nucleus has 82 protons and 124 neutrons, which gives a ratio of 82/124, or 0.661. This small change in the Z/N ratio is enough to put the nucleus into a more stable state. In alpha decay, the atomic number changes, so the original (or parent) atoms and the decay-product (or daughter) atoms are different elements and therefore have different chemical properties Radioactivity In the alpha decay of a nucleus, the change in binding energy appears as the kinetic energy of the alpha particle and the daughter nucleus. Because this energy must be shared between these two particles, and because the alpha particle and daughter nucleus must have equal and opposite momenta, the emitted alpha particle and recoiling nucleus will each have a well-defined energy after the decay. Because of its smaller mass, most of the kinetic energy goes to the alpha particle. Radioactivity Alpha decay occurs when the strong nuclear force cannot hold a large nucleus together. The mass of the parent nucleus is greater than the sum of the masses of the daughter nucleus and the alpha particle; this difference is called the disintegration energy. All Alpha decay processes can be written as: Radioactivity Beta Decay: Beta decay occurs when a nucleus emits an electron/positron. Beta particles are electrons or positrons (electrons with positive electric charge, or antielectrons). Beta decay occurs when, in a nucleus with too many protons or too many neutrons, one of the protons or neutrons is transformed into the other. Radioactivity In beta minus decay a neutron decays into a proton, an electron, and an antineutrino: n → p + e- + ṽ. In beta plus decay, , a proton decays into a neutron, a positron, and a neutrino: p → n + e+ + v. Radioactivity These particular reactions take place because conservation laws are obeyed. Electric charge conservation requires that if an electrically neutral neutron becomes a positively charged proton, an electrically negative particle (in this case, an electron) must also be produced. Similarly, conservation of lepton number requires that if a neutron (lepton number = 0) decays into a proton (lepton number = 0) and an electron (lepton number = 1), a particle with a lepton number of -1 (in this case an antineutrino) must also be produced. The leptons emitted in beta decay did not exist in the nucleus before the decay (they are created at the instant of the decay). Radioactivity Beta Plus & Beta Minus: The atom phosphorous 32 (3215P) is an unstable isotope of phosphorous 31 (3115P). Its nucleus contains 15 protons and 17 neutrons ie, it has excess neutrons. It regains stability by transforming the excess neutron into a proton to form the stable isotope sulphur 32 (3216S). This is a Beta Minus decay. Carbon 14 undergo this as well. On the other hand: 1910Ne which has 10 protons and 9 neutrons undergo Beta Plus decay. Radioactivity Electron Capture: Atoms with excess protons can achieve stability through an alternative process to that of B+ decay. This occurs when an orbital electron is pulled into the nucleus and combines with a proton to form a neutron. We say that the electron has been ‘captured’. The only particle emitted by the nucleus in this case is a neutrino. Typically, the nucleus is left with an excess of energy ie, it is said to be in an ‘excited’ state and it releases this energy by emitting gamma radiation. Radioactivity Electron Capture: Radioactivity Gamma Decay: In gamma decay, a nucleus changes from a higher energy state to a lower energy state through the emission of electromagnetic radiation (photons). The number of protons (and neutrons) in the nucleus does not change in this process, so the parent and daughter atoms are the same chemical element. Radioactivity Gamma Decay: Radioactivity Three Types of Radioactive Decay: Radioactivity Radioactivity