Radioactivity & Radioactive Decay PDF

?What is Radioactivity or Radioactive Decay  Radioactivity (or radioactive decay) is a process by which the nucleus of an unstable atom loses energy by emitting radiation.  These unstable nuclei decay and emit radiations of three main types, called alpha, beta and gamma radiation.  The fa...

?What is Radioactivity or Radioactive Decay  Radioactivity (or radioactive decay) is a process by which the nucleus of an unstable atom loses energy by emitting radiation.  These unstable nuclei decay and emit radiations of three main types, called alpha, beta and gamma radiation.  The fact that some elements are naturally radioactive was first realized by Henri Becquerel in 1896. Types of Radioactive Decays  Atoms found in nature are either stable or unstable.  An atom is unstable (radioactive) if the nucleus has an excess of internal energy.  Instability of an atom's nucleus may result from an excess of either neutrons or protons.  A radioactive atom will attempt to reach stability by ejecting nucleons (protons or neutrons), as well as other particles, or by releasing energy in other forms.  There are four main decay types: 1- Alpha Decay 2- Beta Decay (β decay) [a. Negative Beta decay (β͞ ) b. Positive Beta decay ( β+ ) (positron decay) ] 3- Gamma Decay (γ-Decay) 4- Electron Capture (EC) 1- Alpha Decay An alpha particle is a highly energetic helium nucleus that is emitted from the nucleus of an unstable atom when the neutron-to-proton ratio is too low. It is a positively charged, massive particle consisting of an assembly of two protons and two neutrons. Since atomic numbers and mass numbers are conserved in alpha transitions, it follows that the result of alpha emission is a daughter whose atomic number is two less than that of the parent and whose atomic mass number is four less than that of the parent.  2- Beta Decay a. Negative Beta decay (β͞ ) Negative beta (or β͞ decay) is a process which a neutron transforms in to a proton, electron ( ejected as a β͞ particle ) and an anti-neutrino. The β͞ decay process take place when the nuclei have extra neutron: b. Positive Beta decay ( β+ ) It takes place when the nuclei have extra protons. At this decay, a neutron, a positron (emitted as β+ particle), and a neutrino is created from a proton conversion. Consequently, the nuclide charge decreases by one, as illustrated in the example: 3- Gamma Decay (γ-Decay)  Gamma-rays possesses energies that usually variety from 0.1MeV up to 10MeV.  γ-ray are just a photons produced from the nucleus having no mass and no charge so they are unlike beta or alpha particles. The two decay processes (Alpha and/or beta) may leave the produced nucleus in an unstable state , leading the exited nucleus to de-excite by emitting one or more of the characteristic γ -rays via γ- decay process. 4. Electron Capture (EC) The nucleus in the EC process captures one of the closest orbital atomic electron. The captured electron combines with one of the nucleuses proton and forms a neutron and emits a neutrino at a specifi c energy. It is somehow same as to β+ decay process.. Below an example illustrate the EC process: The Radioactive Decay law A radioactive nuclei in a radioactive material decay in a random manner (i.e. the chance of a nucleus to decay is independent of time). It was clear three years after the detection of radioactivity that the decay of a pure radioactive material decreases in time following an exponential law called the Radioactive Decay law. The law calculates the number of radioactive nuclei remains after a period of time t. Number of radioactive nuclei dN, decaying in dt is related to the number of nuclei N as : where λ is the decay constant, and represent the probability of an atom to decay per unit time. Solving the differential equation (2.1), gives the exponential law of radioactive decay shown below: where is the initial nuclei number present at t = 0.  The rate at which decays take place in a matter represent the activity (A), and it can be achieved by differentiating eq. (2.2). where , represent the original activity at t = 0 Activity is measured by the Curie (Ci) and the Becquerel (Bq), where 1 Ci = 3.7 x 1010 Bq Bq = 1 disintegration per second.  The half-life Time ( )  The half-life time : represents the time required for any given radionuclide to decrease to one-half of its original quantity represented by the symbol represent the half-life time, and λ is the decay constant.  The Figure illustrates the exponentially decays of the activity with time plotted for  After one half-life  After two half-lives …… and  After (n) half- lives  also……. Average or Mean-Life (τ) In radioactivity, mean life is defined as the average lifetime of all nuclei of a specific unstable atomic species. Mean life is also considered as the sum of lifetimes of all the particular unstable nuclei in a given sample divided by the total number of present unstable nuclei. Mean life of a specific element of unstable nucleus is 1.443 times more than its half – life (time interval needed for the decay of half of the unstable nuclei). Lead-209 decays to bismuth-209 whose mean life is 4.69 hours and a half- life is around 3.25 hours. τ