 Dear students, the last few lectures we have discussed the different aspects of nuclear chemistry namely the radioactive decay, the different types of decays like alpha, beta and gamma and also the structure of nuclei, the different models like liquid drop model, shell model and how the model can explain the different properties of nuclei. Now, we will start discussing the different types of radiations, how they interact with matter and also that will help us in deciding how to make a detector for these radiations. So, in the coming few lectures, I will be discussing upon interaction of radiation with matter and radiation detection and measurements. Let us first discuss what type of radiations that we are trying to discuss, we will call them ionizing radiations. So, the ionizing radiations means that radiations which cause ionization when they pass through the medium and you can then really understand that these radiations have energies have then the ionization energy of different materials with which they pass. These radiations are in fact, invisible to the human eye and so you require detectors, if you want to detect these radiations there are suitable detectors for this purpose. Before that, let us discuss, we can try to categorize them depending upon the type of interactions that and that are go. So, we will discuss the interactions based on the types of different types of radiations. So, you can see here that we have the different types of radiations like heavy charged particles once which are of nuclear material like nuclei, protons, alpha particles, fish and fragments. So, these heavy ions which involve a nuclear matter nucleus, we will call them heavy charged particle at CP. Then we have the light charged particles like electrons and positrons, even the beta particles will come in that category even OZ electrons will come in that category. So, these are charged particles but light, they are not nuclear particles, they are the extra nuclear particles. Then we have the electromagnetic radiations gamma rays x rays and then we lastly we have the neutral particles like neutrons. These are the four classifications in which we will discuss them one by one. The examples of heavy charged particles are the alpha particles, the protons or even heavy ions. You can have in a accelerator you can produce carbon ions, lithium ions, oxygen ions. So, how to detect them also is interesting. Fish and fragments, when the fragments are emitted from the fissioning nuclei, they are heavily charged particles and therefore their interactions are similar to heavy charged particles and the sources of these HCPs like alpha particles are emitted by actinides like thorium, uranium, plutonium and so on. Protons are emitted, protons are in fact emitted in nuclear reactions or we can have an accelerator producing high energy protons and fish and fragments are emitted in spontaneous fission or induced fission. So, many times we need to detect the fish and fragments. For example, as 210 polonium is a source of alpha particles, even plutonium 239 or radium 226 are there are sources of alpha particles and if you have a spontaneous fissioning, I stopped like californium 252 that is a source of fish and fragments. Among the beta particles we have beta particles, beta plus, beta minus, we can have electrons from an accelerator. So, tritium, carbon 14, phosphorus 32, these are all beta emitters, they emit beta minus and in internal conversion or electron capture types of decay, you can have emission of OZ electrons. They are relatively low in energy, but they are having energy more than the emission energy of different materials. In the electromagnetic radiation category, we have gamma rays and x-rays. We have already discussed how gamma rays are emitted in host, beta or alpha decay like cobalt-60 or helium-137 and we will also discuss today a type of radiation called Brimstallen which is emitted when the beta particles or high energy electrons interact with the any material. The neutrons are available in the reactors in plenty. They are also emitted from the neutron sources like californium 252 or americium beryllium or antimony beryllium type of sources. So, we will discuss all these types of additions and their interactions in these coming lectures. Why we need to study this subject of interaction radiation matter? We know that radioactivity is there in environment all along our body contains radioactivity in terms of carbon 14, potassium 14 and nature contains uranium, thorium and earth crust. So, we are in amidst low levels of radiation and we need to know how much it is. Then we will be handling radioactive samples like actinides or you can have radioactive isotopes for your experiments or you will be working in a radioactive area. So, you need to know what type of radiation that you are going to have in the area, what is the level of that radiation. So, for all these since the radiations are invisible, we need to have detectors suitable detectors when we go to a radioactive area or when we want to use count radiations. Then to make the radiation detectors, then we need to know how these radiations interact with the matter. Depending upon that, we can discuss how to make a detector for a particular type of radiation. So, before we go to the actual interaction of different radiations with matter, I thought I will just give you a feel of how much they can travel in any medium. So, this slide just gives you kind of distances, these different radiations can travel in a material. For example, alpha particles. So, alpha particle anything but W charge the helium atom H e 2 plus and it can be stopped even by a thin sheet of paper. So, maybe few hundreds of microns. I suppose you have you are wearing surgical gloves, they are enough to stop the alpha, it will not reach your hands. The beta particles of energy one or two a levy can be stopped even by one or two millimeters of aluminum metal foil, aluminum metal. So, you can see they are having higher, they can travel more distances. So, their ranges are higher. The gamma rays are waves electromagnetic radiations. So, they can travel much more distance and you require a high jet material as you will discuss later on. Almost a two inch thick lead brick is required to stop the gamma ray. And the neutrons, neutrons are neutral particles, but certain materials have very high neutron absorption cross sections like paraffin wax. In fact, paraffin wax is a low jet material, it can stop the neutrons very fast and then if you have it contains some neutron poisons like boron, cadmium, gadolinium like that. So, they can be used to shield ourselves from the neutrons. So, typically paraffin wax, borator water are used as neutron shield. So, that gives you an even shield. So, this one should be two centimeters two inches thick shield for neutrons. So, now let us come to the interaction of recharge particles with matter. So, as I mentioned the heavy charge particles are particles associated with the nuclei like protons, alpha particles, heavy ions, H i means heavy ions and the fission fragments. So, the fission fragments actually know they were like for example, a californium 252 fissons, you will have fission fragments of masses 100 to 150. So, and then their charges could be 20 plus. So, heavily charged ions, so their interactions are also included when we discuss. But for the sake of simplicity, I will be taking example of protons and alpha, the same relations hold for heavy ions and fission fragments. The typical energies of this radiation is involved like in alpha decay as I mentioned earlier. The energy of alpha particles are in the range of 4 to 8 MeV. So, 1 MeV equal to 1.602 into 10 to the power minus 13 joules, 1 electron mole 10 to the power minus 19 joules and so you can calculate. So, what is the energy? So, this normally joules will be whatever the bond energies you say kilo calories per mole, kilo joules per mole that is per mole. When we are talking about MeV, 1 MeV or per decay, one single alpha particle will have energy of 4 MeV or 8 MeV. Similarly, the protons of energy to MeV to even 10 MeV can be produced in nuclear accelerators and in nuclear reactions protons will have energies 1 to 2 MeV. Fission fragments have energies of hundreds of MeV and the heavy ions can be of energy of tens or 2 to few tens of MeVs. So, we are talking about energies which are in the range of millions of electron volts which is much above the energy required to ionize a particular material. And so, because of their charge, they interact with the material basically by interaction with the atomic electrons. Now, in the material the major volume is occupied by electrons as you know by this time that the nuclei occupy a very small volume in an atom. For example, this is an atom, the nucleus occupy very small volume and the rest is all electrons are occupied. So, when the heavy charge particle is coming and interacting with the material, it will be seen mostly the electrons. So, the nucleus, the nucleus offers you know it is with electrons which the heavy charge particle is interacting and it is like in a collision now. So, it is a collision between a heavy ion and an electron. So, you can see the heavy ion is very, very heavy, massive particle compared to electron, mass of proton is of the order of 2000 times that of electron. So, it will just give a its momentum, significant part of its momentum electron and electron will be knocked out. So, that will be calling as a ionization. So, the ionization you can see here alpha particle when it is interacting with an atom A, then A will be ionized. For example, it is argon atom will become argon plus, helium atom, helium plus, oxygen plus and an electron is ejected out from the atom. But in addition to ionization many a times you know even though the energy of the alpha particle or charge particle is much higher than the energy of electrons, the atoms may remain in the excited state instead of ionization. So, ionization and excitation are these two modes of interaction of the heavy charge particle with any matter. Now, you may ask why nuclear reaction cannot take place. So, as I discussed earlier the nuclear reaction requires a certain coulomb barrier to be crossed. So, first of all the probability of interacting the nucleus is very small because the nucleus occupying very small volume in the material. And secondly, even if it is interacting with an electron with the nucleus, then the energy of the heavy charge particle should be sufficiently high to cross the coulomb barrier of this nucleus to induce any nuclear reaction. So, predominantly like the energy that we are talking about for alpha protons or QMEB, we will resolve that it is mostly electron, electronic interaction, interaction of electrons that is coulombic interaction. So, ionization and excitation are the two modes of interaction, we will call it electronic stop. Now, just to give you a field for the energies and the velocities of ions, why you know how when the alpha particle comes out of a nucleus, it remains such doubly charged helium atom just to compare the velocity of alpha and the electrons velocity in atoms. So, for a hydrogen atom the velocity of electron is 0.1810 power 8 centimeter per second and see the alpha particle of the compatible velocity is for a alpha particle of 0.08 MeV. So, you can imagine that if you have a 5 MeV alpha particles, it will have much higher velocity than the velocity of electrons orbiting around the nucleus. So, what happens when the alpha particle comes out of a nucleus is velocity is much, much higher than the velocity of electrons in its orbitals and therefore, alpha particle remains as a helium plus only when the velocity of alpha particle becomes comparable or less than the velocity of electrons in its orbits, it will start picking up the electrons and become a helium atom. So, that is why the alpha particle will keep on removing knocking out electrons on the atoms unless or until its velocity becomes less than the velocity of electrons in the orbits. So, at velocity of alpha more than the velocity of electrons in the atomic orbitals, we call it electronic stopping means interaction with electrons by ionization connection. Then as the particle slows down and assumes energy of the rotational velocity of electrons, then even then it is stopping by electronic mode, but it may start picking up electrons particularly in the case. And when the velocity of alpha particle has become comparable to the velocity of electrons, the balance of electrons, then now the charge particle will pick up electrons and become neutral and now the neutral atom, neutral like helium atom, when it is now interacting with the material, it will be colliding with the atoms. So, that we call as a nuclear stopping. The nuclear stopping happens at the end of the, when it is about to get completely stopped in the medium. Now, let us see in more details the interaction, the energy lost by the heavy charge particle when it is in a single collision. So, just to see the kinematics of a heavy charge particle of mass capital m velocity v and initial energy E0 colliding with an electron of mass small m. And the electron goes at theta angle with respect to the initial beam direction and the energy of electron is E2 velocity V2. So, we can set up the kinematics equation of conservation of energy in elastic collision. So, initial energy of charge particle is nothing but the energy, some of the energies of remaining inner particle and the electron that is limited. You can set up the equation for conservation of linear momentum. So, initial momentum mv is a final momentum of two particles, the residual particle alpha and the electron. So, you can convert this momentum into energy root 2 me. So, for the initial alpha, the remaining alpha and the residual alpha. So, alpha particle is losing some energy. So, that is why we call remaining alpha and for theta equal to 0, we will not bring in the theta dependence, but we can set up the equation for theta also. So, what we need to do is solve these two equations for E2. What is E2? Energy of electron. So, what is the energy given by alpha to electron that is what we are discussing here. And if you multiply this equation 2 by multiply square this equation 2, this one and multiply the first equation with 2 m and solve for E2, you get the equation E2 is the energy of the electron. In fact, this is the maximum energy that electron will gain because at theta equal to 0, electron energy is maximum, that becomes 4 mm E0 upon m plus m square. And so, now for the heavy charge particle, the mass of the ion is very small, very high compared to this of the electron. And so, you can actually neglect this small m compared to capital M. And so, this becomes energy of electron 4 small m upon capital M E0, where E0 is the energy of the ion. So, you can just see the values for 4 MVV alpha part of proton, EE energy of electron will be energy of proton that is E0, 4 m upon m. So, energy mass of an electron and mass of proton, electron upon proton 1 by 2000, 4 by 2500. So, EP upon 500 is the energy of electron. For a 4 MVV, we can calculate that electron will be acquiring 8 kV energy. So, proton of 4 MVV loses 8 kV energy in one collision. In the case of 4 MVV alpha particle, it will lose 2 kV in one collision. So, you can see a very small fraction of energy is lost by the alpha particle or any other charge particle in a single collision. So, essentially, it loses its small energy and goes undeflected in the same direction. That is why you say the heavy charge particle moves undeflected. So, it is a project track. So, this is what is the track of alpha particle. Alpha particle moves in a straight line. And along this path, it is removing the electrons from the medium. And these electrons or the pink ones this arrows, these electrons are having energies of 2 kV and they will cause for the ionization in the medium, they are called the delta rays. So, the important point is that this heavy charge particles move in a straight line, which we call as the track. And the electrons that are emitted during this passage of this heavy charge particle through the material, these electrons can further cause ionization which we will call secondary ionization or delta rays. In fact, this principle of track is used in track edge detectors. That means, if you take a dielectric material, then the path of the alpha particle, it will create a channel damage in the material which does not get annealed. And you can edge it with some acid or alkali. And you can see the path of the alpha particle in a microscope. So, tracks, each track you can count in a microscope, that is one of the principle of track detectors. Let us go into little more details of the interaction of heavy charge particles. So, we call a term term called stopping power. The stopping power of the medium, how fast it can stop the particle or the linear energy transfer. That means, in one unit distance path length, how much is the energy transferred by alpha to the medium. So, we define the linear stopping power as s equal to dE by dx, energy lost per unit thickness. Hence, since it is a loss, it is negative. So, minus dE by dx is the stopping power and it has the units of MeV per centimeter. So, you have the initial energy E0 passing through a small thickness of the adsorber material delta x, outgoing energy E. So, E0 minus E we can say delta E upon delta x is called the stopping power. In this particular subject, actually we are more comfortable with the mass stopping power because the stopping power depends upon the density. So, if you divide the stopping power by rho, the density we call it mass stopping power. So, m by s by rho we call it the mass stopping power. So, if you this MeV upon centimeter you divide by density, that means gram per centimeter cube, then it will be left with MeV upon gram per centimeter square or milligram per centimeter square. So, very simple to put in a simple way, if you want to find out the thickness of a thin file in you know how do you either use a variable or you say micrometer, much simpler than that is you take the area of the file and multiply it by the density. So, area into thickness into density, the volume density that is the mass. So, you take the mass divided by the area we will call it the thickness. So, mass upon area is rho dx, you can see here mass upon area is rho dx, mass upon area is milligram per centimeter square. So, thickness of a file take the weight mass milligram and area in centimeter square you can find out thickness which is multiplied by the density. Thickness with density is mass thickness you can see. So, way in fact we also call as the specific ionization that means how many ion pairs are formed during unit length in a medium we will call it stopping power upon W. So, W is the energy required to produce one ion pair. So, in unit thickness delta E is the energy lost that if the energy required to produce one ion pair is W, D upon W upon dx is the specific ionization per unit thickness how many ion pairs are produced that is also an important quantity for when you are stopping a ion in a medium. So, the W is the energy required to produce one ion pair. Many times now when the ion is passing through a medium which is a compound. So, we do not have an individual atom, but we have several atoms. So, the atom fractions are weighted together to find the stopping power of a compound for the every ion in a. So, we can see that the W1, W2, W3 are the atom fractions and S1, S2, S3 are the stopping powers in those different atoms. So, you can have a scaling law to find out the stopping power of a compound. So, now just to give you a value a feel of the values the W value energy required to produce a one ion pair these are the W values in different media. Actually speaking the W value should be equal to the ionization potential or ionization energy of a medium. So, for Genon ionized potential 12.1, but actually W value is higher, Ilium 25.4 actual value is 43, Ammonia 10.8, 39, Germanium 1 and actually so you can see that W values are much higher than the ionization potential. Why it is so? Because every time the chart particle interacts it may not lead to ionization, it can lead to excitation also. So, that is why the average value of W much higher than the ionization potential. Now, this stopping powers actually hence Bithay in 1930s, in 30s they derived the formula, we will not go into details of the stopping power formula, but the stopping power essentially depends, the formula is 4 pi e power 4 z square upon M0 v square and z into a term depending upon the velocity of the ion. So, the small z here is the charge of the ion, e is the energy so that e will not come into picture here, but you can later on convert into energy of the ion, the v is the velocity of the ion, M0 is the mass of the electron and nz actually the electron density. So, number of atoms per cc into the atomic number that becomes the electron density, number of electrons per cc in the medium and M0 is the mass of electron. Now, let us not bother about the, you see this is actually this term is the relativistic term for high net electrons, we have to consider the relativistic term. So, try to simplify this for the non relativistic low velocity particles. So, stopping power becomes nz, this is the absorber property and this is the ion property, z square by v square, velocity z is the ion of charge of the ion, e velocity of the ion. So, there are two terms, one term depends upon the particle, one term depends upon the medium. So, you can see here the stopping power minus d e by dx is proportional to z square upon v square into nz. Now, you can just do some jugglery, multiply by the mass of the ion. So, you have Mz square upon mv square into nz and mv square is nothing but e, half mv square, it is kinetic energy. So, you can say Mz square upon e nz. So, this is the Mz square by e dependence of the ion and this is the electron density of the medium. So, you can see here from the stopping power, the stopping power decreases with increasing energy of the ion. So, because it is in the denominator. So, as the energy of the ion increases the stopping power decreases, because it is moving very fast, the stopping power increases with the nz electron density, higher the electron density of the medium, higher is the stopping power and the ratio for two ions, stopping power ratio is nothing but M1 z1 square upon M2 z2 square for a given energy. Just to translate for protons and neutrons, you can see the stopping power of proton to alpha will be 1 by 16, because it will be 4 into 2 square for 16. So, 1 upon 16. So, you can see proton will have a much smaller stopping power than the alpha part. And this concept is very well utilized in the identification of particles by a set of two detectors. So, this is one detector, very thin detector, this is large detector, thick detector, let us say silicon. So, if you see the relationship between d e by dx e to e, d e into e. Suppose the detector determines that e delta e energy lost and this one detector total energy, the plot of delta e versus e will be giving you different ions, protons, alpha, carbon, so on like hyperbola and you can resolve them to find out the different ions. So, they are called effect telescopes to detect the different ions. Now, another important property of this heavy charge particle is the Bragg's card. Bragg's, you know, Bragg actually showed that the path, how that, how along the distance traveled by the ion, how the stopping power changes. So, just now we saw that the stopping power changes with n z square by e. And so, as the energy of the ion decreases along the path, there is a traveling in the medium, energy is decreasing, so stopping power is going up 1 by e. And then gradually, when the ion is about to pick up, when it starts picking up electrons, the jet becomes low because the ionic charge will reduce and the stopping power decreases. Finally, it will pick up electron, become neutral atom and only get stopped. Now, this is for the single ion, but when we have the beam of ions, then different, since it is a statistical process, every time it will not produce the same number of ion pairs. So, for a beam, there will be a straggling. So, the stopping power will not be that narrow, but it will have a wide distribution. So, at the low energy end of the track, the ion picks up electrons and stops. So, this Bragg's card actually very much used in fact in the, even now in the concept treatment, the Bragg curve is being used for the ions. And lastly, the range of the ion, how much distance the ion travels in the material, it can be calculated by the integration of the 1 by e stopping power over d e. So, from initially you have e0 and stop with e with 0. So, 1 by d e by d x into d e, if you integrate, then you get the range. And simply you can do experiment to initial intensity i0 and then small thickness, if it is travelling, it will pass through it. So, if you measure the i and i0, then the plot of i by i0 versus the distance travelled gives you what is called as the transmission curve. And this transmission curve for alpha particles will be like this, or for that matter for heavy, heavy ion, heavy chart particles. So, it does not get lost. You see here, all the heavy ions will pass through certain distance and then when they start picking up electrons, the intensity will go down. In a very narrow zone, all the ions will stop. So, when the intensity will become half, we call it main range. And if we extrapolate this decreasing term, this plot, we call it extrapolated range. So, why this decreases come because of the stochastic nature of the electron stopping power. So, you take a derivative of this poly part, then you get a Gaussian and the width of that is called the range tagline. So, the range is a very important property of the heavy ion, heavy chart particles. And there are now semi empirical relationship of r range equal to a is the power b, where b is close to 2 and a depends upon particle time. So, we will discuss now about other ions like electrons and gamma rays later on. For the moment, I will stop here. Thank you very much.