Introduction to Radioactivity

Natural Radioactivity:- Pioneer:- Radioactivity was first discovered by a scientist Henry Becquerel in 1896. Definition:- The natural emission of radiations from unstable nuclei is called radioactivity. Explanation:- Henry Becquerel discovered in 1896 that certain elements whose atomic number is greater than 83 (Z>83) are unstable. They are disintegrating themselves and emitting three types of radiations a, ß, and ? continuously. Such elements are called radioactive elements. the process of emitting radiations is known as radioactivity. These radioactive elements are radium (Z = 88), uranium (Z = 92) , polonium (Z=84) etc. The radioactivity is a natural process and it can not be expedited (increasing speed) by the increase of temperature, pressure etc. the heavy nuclei whose atomic number is greater than Z = 83 have strong Coulomb repulsive interaction. This reduces the net attractive force and so the nucleons are closely bound in a nucleus. These nuclei then may emit a, ß, and ? rays naturally. a is doubly positive helium nucleus (He++), ß is either an electron -1e0 or positron (+1e0) and ? radiations are high energy photons. Experiments and investigations reveal that a particles are positively charge helium nuclei and deflected by magnetic field. Alpha particles show small penetration in matter, produce great ionization and produce scintillations on fluorescent screen. The beta particles are electron, are deflected by electric and magnetic fields, show much greater penetration than alpha particles and produce less ionization. The gamma particles are highly penetrating electromagnetic radiations are not deflected by magnetic or electric fields and produced little ionization. Experiment:- The nature of these radiations can be studies from the simple experiments. Radioactive substance such as radium is placed in the center of a block of lead. The radiations pass through a hole in the lead block and enter a chamber in which a magnetic field is applied perpendicular to the plane of the paper (in to paper). The radiations are deflected due to magnetic field and form separate images on the photographic plate as shown in the fig. those radiations which are deflected towards left are called alpha rays having positive charge. Those radiations which hare deflected towards right are called beta rays having negative charge. Radiations which are not deflected by the magnetic field have no charge and are called gamma rays. Transmutation of elements:- With the emission of alpha, beta and gamma rays the atomic number of the original element called parent element is changed and we say that the elements decays to form a new element called daughter element. This phenomenon is known as transmutation of elements. Alpha emission:- As alpha particle is H++ (2X4), so the emission of alpha particle reduces the atomic number by 2 and mass number by 4, i.e. ZXA ? Z-2YA-4 + a + Q Q is the integration energy. Q is always positive, as the process is spontaneous. The decay product ZXA-4 is called daughter nucleus of the parent nucleus ZXA. the alpha particle is often written as 2He4. The daughter nucleus may also remain unstable and undergo further disintegration till it attains stability. Examples:- Following are examples of a decay: 92U238 ? 90Th234 + 2He4 + Q 88Ra226 ? 86Rn222 + 2He4 + Q Beta emission:- As beta particles are either electron (-1e0) or positron (+1e0), so its emission can either increase or decrease the atomic number by 1, i.e. ZXA ß+ particle emission Z+1Y A + Antineutrino + Q Or ZXA ß- particle emission Z-1W A + neutrino + Q The beta particle emission can be explained by the disintegration of neutron as follows. on1 ? -1eo + +1P1 + Antineutrino + Q +1P1 ? +1eo + on1 + neutrino + Q Examples:- An example of ß- particle emission is the decay of Thorium in to Protactinium. 90Th234 ? 91Pa234 + -1ß0 + antineutrino + Q1 The prototype of beta decay is the decay of neutron itself. The neutron in free space is unstable decaying to proton and electron with a half life of 12 minutes. 0n1 ? 1H1 + -1ßo + neutrino As an example of a positron emitter is carbon 11, which decays by the reaction. 6C11 ? 5ß11 + +1ß0 + neutrino Gamma Emission:- As ? radiation is energy, therefore its emission does not effect the atomic or mass number of the nuclei. ? radiations are emitted from excited nuclei which are left excited due to the emission of a and ß particles or some other excitation source. The process of gamma emission can be shown as, ZX*A ? ZXA + ? Where ZX*A shows the excited state of the nuclei and ZXA shows its normal state. Radioactive Decay:- Half Life:- Definition:- The time taken by the atoms or radioactive materials to decay to half of their original number is called the half life of the material. Explanation:- Radioactivity is a spontaneous emission of radiations but it does not mean that all the atoms disintegrate at the same instant. No body can predict which atom will disintegrate at what instant. It is a random process. To quantify this property, a special physical property is introduced called half life. Half life is a nuclear property and is not affected by external parameters like pressure, temperature etc. To explain it further, let us consider that a radioactive element has No number of atoms at a certain instant. If the half life of the element is 4 hours, then after 4 hours, the number of radioactive atoms left will be N0/ 2. After 8 hours, i.e. two half-lives, the number of atoms of original element left = No/2 x ½ = No/4. This means that the number of atoms decayed = N0 – No/4 = 3No/4. Similarly the number of atom left undecayed after 12 hours = No/4 x ½ = No/8 and so on. To represent the variation of undecayed atoms as a function of time, a graphical method is more suitable as shown in the figure. From the graph, it is very easy to find the half life of the material. Half life is a nuclear characteristics and with the help of this property, we can distinguish between different radioactive element. It means that by measuring the half life of a certain radioactive element, we can identify it. For example radium ha a half life or about 1600 years, radon has a half life of about four days and polonium has a half life of 3 minutes. Measurement of Half Life:- The half life of a radioactive material is determined by measuring the activity of a given sample over a period of time. A count rate meter such as a Geiger counter is used to measure the activity of a given sample. The activity versus time curve gives the decay curve shown in the fig. the half liafe of the sample can be determined from the decay curve. For this we have to note activity at any time t1, and then time t2, where the activity has reduced to half. The difference of both the time i.e. (t2 – t1) gives the half life of the given radioactive sample. A graph showing how the count rate decreases as time goes by will have a curve like the one below. For any particular radioisotope the count rate and time will be different but the shape of the curve will be the same. The easiest way to measure the half-life from the graph is to 1. First we read the original count rate at zero days. On our graph the reading is 1600 counts. 2. Then we go down to half the original count rate (800 counts) and we draw a horizontal line to the curve. Then we draw a vertical line down from the curve. We can read off the half-life where the line crosses the time axis. On our graph the half-life is 20 days. Radioactive Decay Law:- Activity of a radioactive sample is defined as: “The rate at which the radioactive nuclei disintegrate i.e. ?N/?t”. Where ?N is a disintegrated nuclei in time ? t. From experiments it was found that rate of disintegration is directly proportional to the number of initial radioactive nuclei (No) i.e. ?N/?t 8 No ?N/?t = - ? N ………….. 1 The negative sign shows the rate of decrease of radioactivity and ? is the decay constant.