Lecture 3: Interaction of Radiation with Matter – Part 2

Added: 2026-05-06

[00:00:14]Good morning. Welcome you all to our course radiography and imaging technology lecture number three. In last class we have discussed the interaction of radiation with matter. The part one where we have discussed the interaction the term radiations and the term matter.

[00:00:34]We also discuss the various phenomenons attenuation phenomenons of photons. In this class we are going to learn uh or we are going to discuss about bound or f electron. We'll discuss various types of photon interactions. The uh photon interactions are coherent scattering, photoelectric absorptions, compton scattering and pair productions. We also cover the interaction of charge particle and interactions of neutrons. Although these two part two uh interactions may not be useful for your course radiography and imaging technology, still for the sake of the completeness we are going to cover these two points.

[00:01:17]Now as I have already discussed in last class that we have categorized the ionizing radiations and radiations and where classified as ionizing radiations and non-ionizing radiations. First we will going to discuss the electromagnetic radiations interactions with the matter where electromagnetic radiations are the x-rays and gamma rays. Now first bound and free electron.

[00:01:44]Bound and free electron is a relative concept which I am going to discuss because by this concept we can understand easily the photon interactions with the atoms and atoms electrons with the photons. So bound electron when the incident photon energy the incident photon energy is much much higher or is comparable with the binding energy of the orbital electron. This is the binding energy of the orbital electron.

[00:02:20]They are rotate around the nucleus with a with a binding energy that is electro electrical because nucleus is positively charged and electron is negatively charged. So they attract and this is the cause of this binding energy. But in the bound electron when the incident photon energy is comparable with the binding energy of the electron then we can call this electron as bound electron. Now if you consider that incident photon energy is much much higher than the binding energy of the electron then we can consider this the same electron as free electron means the binding energy uh and the incident photon energy comparison uh is the main concepts of b bound and free electrons. When the incident photon energy is comparable with the binding energy of the electron then we consider the electron as bound electron and if the incident photon energy is much much higher than the binding energy of the electron then we consider this is the same we can consider the same electron as free electron. Now there are the incident photons and these incident photons can interact the atomic orbital electrons and also can interact the nucleus because we have two type kind of particles or two kind of exist matter are exist in the atomic structures. One is electrons and another is nucleus. Now for by the elect as we have categorized the electron orbital electron as bound electron and free electron. So the bound electron interactions are uh coherent scattering and photoelectric absorptions and the free electron is inter interaction is the or of the photon with the orbital electrons is compton scattering and photon interactions with the nucleus. One is pair productions and another is photo disintegrations. As here we are going to discuss only five type of interactions though there are 12 types of photo interactions with the atoms. So but they are uh not our uh requirement and uh they are beyond our scope. So we just disc we will discuss only five type of interactions out of which three are interactions of photons with orbital electrons and two are interactions of photons with the nucleus. The orbital electrons interactions are coherent scattering photoelectric absorptions they are the interactions of bound electron and comton scattering is the interaction of photon with the free electrons. And similarly in the interaction photon interactions with the nucleus they are two type two type here I have included one is pair production and another is photo disintegration one by one. First is coherent scattering. In coherent scattering it is a classical scattering phenomenon. It is a classical scattering phenomenon of atom of the photons with the and it is a interactions of bound electron. So this is a bound electron means the incident photon energy is comparable and the photon energy here the photon energy this is photon energy lambda with wavelength lambda and it is completely absorbed by this uh elect the energy is completely transferred to the electron and after this energy absorptions there is a orbital electron there will be a oscillation there will be oscillation means there is a one type of vibration or oscillations And because of this oscillations the electron reradiate the same ph wavelength of photon energy in different directions in different directions and the photon energy at the same frequency or same wavelength and so there is no change in the energy but only there is a scattering although this scattering is very small angle scattering and this is uh the event where the atom atom is high uh the material is with high atomic number element uh the material is high atomic number and the interaction of low photon energy. So this is a classical scattering with bound electrons classical scattering of the photon with the bound electron. Photon energy absorbed completely and sets the orbital electron in oscillations and this oscillating electron reiate the photon energy the photon energy at the same frequency and there will be no change of uh energy only change is the directions mean scattering with a small angle uh scattering. So this is happen in for low photon energy and high atomic materials.

[00:07:22]Now next interaction is photoelectric absorptions. This is also uh an interactions of photons with the bound electron. This is also a similar also a interactions. Here the energy is little bit higher than the energy with the uh coherent scattering. Here also photon energy is completely absorbed and this photon energy gives the energy to the orbital electron and orbit and it removes the electron from the orbits they are known as photoeleron. So this is the photoeleron means incident photon is comparable with a binding comparable uh energy or binding energy of electron and this photon energy is completely absorbed. It transfer energy to the photo electron and photoeleron removes from its orbital and known as photo electrons removed from the orbits and known as photoeleron. And there is a uh kinetic energies of the photo electron will be h new is the energy of the incident photon and binding uh minus the binding energy of that particular electron. So after removal there will be a vacancy in the cell orbital there is a vacancy in the or orbit and this vacancy is filled by outer orbital electron to make this atoms stable. So this is the vacancy. This vacancy will be filled by the orbital electron from either from this cell or from this cell. And these emissions and they cause the uh because this is a higher energy because it comes to the down to the lower energy. So there will be an emission of X-rays and these emissions are known as characteristic X-rays. So probability of the photoelectric absorptions is directly proportional this uh absorptions this phenomenon happening probability is directly proportion inversely proportional to the e to the power 3 and directly proportional to the Jed to the power 3 where e is the incident photon energy and zed is the atomic number of material means the when energy is row there is a probability is high of this photoelectric absorptions and Jed to the power3 means when Jed is higher then interaction is also uh uh increased by manifold. So this is a typical uh interactions of photons and uh this interaction is very important in radiography imaging technology because where uh these uh interactions is dependent on the Jed to the power 3 means this interaction can cause the differential attinuations means we have our body there will be uh soft tissues There will be lung, there are bones. So we need to image all these internal structures. We can use this photoelectric absorptions common phenomenon in the diagnostic radiology because of this interaction is directly proportional to the uh to the Jed to the power three. So soft tissue bones and air and air cavities or any other type of tissues they have differences in atomic number. So we can imag uh the image resolution will be very high. So this is the uh common interaction phenomenon of diagnostic radiology. Now coming to the comton scattering this is a interaction this is an interaction with the free electron because here incident energy is very high in comparison with the binding energy of the electron. So we can consider this electron as free electron the photon energy absorbed partially. Here photon energy is absorbed partially and these remove the orbital electron from the orbits. They are known as cometon electron. So they are known as cometon electron. The photon scattered with reduced energy because it is already gives some energy to the electron for removal and for its kinetic energy. So there will be less energy of this photon and this photon is scattered uh through an angle. So this is the scattered photon. Now probability of the component scattering is uh decreases with the increase of the photon energy means when photon energy increases the the component scattering probability will be decreases and this is independent of Jed.

[00:12:23]These are the phenomenon of medium energy photons. There are the phenomenon of medium energy photons. And we use this uh Jed independence means these interactions are almost similar in bone and in soft tissue or in lung tissue though their atomic number is different but they the phenomenon or the radiation absorption uh will be the same or radiation interaction will be the same.

[00:12:51]So this is the uh so that's why we use this in uh radiation oncology where we can treat the target without any shielding of bones or any body tissues uh high atomic number body tissues. So compton scattering is the interactions of photons with free electron and photon energy is absorbed partially. We remove and this partial absorptions of the photon remove the electron from the orbits and these are known as compton electron and photon scattered with the reduced energy. Probability of this comturn scattering is decreases with the increasing of photon energy and independent of atomic number Jed.

[00:13:39]Because of this in atomic number independence, we can use these component scattering or medium energy uh photons for the treatment of uh cancer patients.

[00:13:50]As in the previous slide I have discussed that this is the photoelectric absorptions is the low energy photons where these are uh in dependent on J to the power 3 and because of this Jed dependence in highly depend dependence because it is in the power of J to the power 3 we can use this as differential for differential attinuation means the bone will attenuate higher than the soft tissue and uh soft tissue will attenuate this beam higher higher than the lung tissue. So there will be a differences in attenuations which can be projected in the our flame. So we can see what is the problem in this uh the structure can be visualized with high resolution. These all these coherent scattering photoelectric absorptions and compton scattering are the interactions of photons with the uh orbital electrons. Now the interaction of photons with the nucleus and here the uh interaction is known as pair production.

[00:15:00]Here the interaction is known as pair productions. It is the interactions of photons with nuclear field. It's the interactions of photons with nuclear field. Now photon energy is absorbed completely. Here the photon energy is absorbed completely and it creates an a electron and posetron pair. Electron is the particle and posetron is the antiparticles. Means by using this photon energy which is completely absorbed in the nuclear field it creates two uh particle this is the energy to mass conversion. This is the energy to mass conversion means photon energy is converted into the mass that is two particle one is uh electron that is particle and posetron that is antiparticle. The rest mass of the electron we know that we have discussed in the first class that is approximately 0.51 mega electron volt. So we need to create two particles means 1 0.51 + 0.51 at least energy is required to create these two particles pair uh one is particle and another is antiparticle and the threshold energy is 1.0 02 MB because 0.51 + 0.51 is 1.02 MV is required at least required for the creations of this pair. Now the rest of energy if this incident photon energy is higher than 1.02 MV the rest of energy H new minus 1.02 02 me is distributed equally as the kinetic energy of this pair particle means 1.02 02 is used uh is spent to create two particle and the rest of energy which is uh from of the photons that is completely absorbed or completely disappear uh in the nuclear field they are act this energy is distributed equally with the as the kinetic energy of the both particle. So what happened next? This is very interesting. The unstable position posetron later combined because antiparticle is very unstable and when they will get one particle means one orbital one electron atomic electron later is combined with the electron and it creates again the two photons again create two photons and this particle and antiparticle annihilate. So this process is known as anihilation process. The first is pair product production where energy is converted to the particle means energy is converted to the mass where two particles are one is particle and another is antiparticle. And these antiparticles posetron they are very unstable and they promptly almost fast recombine. They combine with the electron or any other electron and they form two photon again particle to uh two photon means mass to energy conversion.

[00:18:28]So they are annihilate two particles annihilate. So these part this process is known as annihilation process and this annihilation process is used in for the pet imaging. Now this pair production is uh a very uh this pair production is very common for high energy photon beams. So we have interactions of low energy photon beams.

[00:18:54]We have interactions with medium energy photon beam. We have interactions with high energy photon beams. One is low energy photon beam. They are coherent scattering. They are photoelectric absorptions. Medium energy scatter interactions. photon interactions. They are count scattering and high energy photon beam interactions with the nucleus. They are high energy photon uh beam interactions. And the probability of this pair production photo pair production uh is proportional to Jed to the power 2 and it increases with the increase of the incident photon energy. So it increases with the increase of the photon energy.

[00:19:36]Now the last and fifth one this is also the interaction or direct collision of the photon with nucleus. Earlier one the impair production that is not a collision that is a interaction of nuclear field with the photon where photon is absorbed completely but here this is the interaction with the nucleus and this is direct bombardment and where photon energy is absorbed completely and nucleus enters an excited state and immediately decays into subatomic particles means nucleus enters in higher high energy state or in excited state and it decays uh by emitting subatomic particles. So this incoming gamma rays are effectively knocks one or more nucleons. Here the photon is sufficiently energized so that it can knock out uh one or more nucleons mean protons or neutrons or proton neutron combinations. And these reactions are called gamma in reactions where uh a neutron is ejected and gamma proton reactions where one proton is knocked out from the nucleus and gamma alpha means we know that 2 h e4 this is the alpha particles and this is uh gamma alpha gamma alpha particle also can be removed from this from the nucleus. So this is a phenomenon of high energy photon beam and uh this uh is the uh this product of these uh interactions they are unstable radioisotope. So in to summarize all these interactions of photons with the atomic atoms and atomic nucleus orbital electrons and atomic nucleus. First we categorize the uh energy in low energy medium energy and very high energy. One is very low energy we have the coherent scattering. In the little bit higher or but it's low energy low energy photon beam we have the uh photoelectric absorptions. In medium energy we have interactions with the uh bound sorry free electron they are compton scattering and in very high energy means little bit higher we have the photon pair productions and uh in very high energy we have the photo disintegrations where photons can sufficiently energized to knock out one or more nucleons from the nucleus. Now interaction of charged particle as I have already mentioned that that these may not be uh very useful for your uh radiography course but for the sake of compliments I'm going to discuss here the interaction of charged particles as we have discussed that charged particles are two type one is light charged particle and another is heavy charged particles. So interaction of charged particles they can interact with the orbital electron. So these are excitation or ionization. Excitation means the electron moves from lower energy state to higher energy state and ionization is the knocking out of electron from the orbits of the atoms.

[00:23:03]So one is excitations and another is ionization. So these are two types of interactions w with the orbital electrons and interactions with the nucleus they may be the radiative loss and or they may be the brim star radiative loss means or bmstrong they are same. So the charged particle if it is interact with the orbital electron they are excitations means they move to the upper higher energy state and ionization means they removed from the atomic uh orbits. So this is ionization. But if it is interact with the nuclear uh field then we have a there will because it is a charge. So there may be uh and this is also char this is charged particle and nucleus also in charge form. So there in electrical interactions cause the breaking of the radiation and this is this will be very important that breaking of the interactions. uh we have the bream starlong uh interactions. So this is the uh interactions of charged particle with the uh char interaction of charged particle with the atoms. Now this is interaction of heavy charged particles because first we'll discuss the this is the common uh interactions we have mentioned here but in for heavy charged particle the rate of loss is directly proportional to the charge to the power squared and also directly pro indirect inversely proportional to the velocity squared. So the as particle slows down the rate of energy loss increases. Means when it enter materials it is fast there will be less interactions and there will be less energy loss. But when it slows down because it is inversely proportional to the velocity. When velocity is less it will interact more. So when the at the dose deposition in the lower depth is less. So here we will have the low dose depositions at the entry level but most of the energy deposited at the end of the range when because a charged particle they there they have the special range in photon energy we do not have any in photon we do not have any range but in charge particle we have range means it will travel only a particular definite distance. So it is similar to any bullets from any guns any type of guns. So they have range and here also in charge particle they have range. So in the end when velocity is closely equal to the zero it will it will deposit all the energy and we cause a uh peak that is known as brags peak and uh here we have the le scattering which is useful for particle therapy of the cancer patients and uh this brags peak is exploited different ways. So this is the heavy charged particle interactions with the nuclear with the atoms. Now light charged particles their interactions are also similar to those heavy heavy charged particles because of low mass the electron suffers multiple scattering and we will not this brags peaks will not is not visible in the low light charge particles as the example is electron and heavy charged particle as we have mentioned that this example is the pions protons and uh the other heavy charged particle ules. Now coming to the interaction of the neutrons. These interaction of neutrons there are two types. First one is recoiling of nucleus and nuclear disintegration.

[00:27:02]We can have recoiling of nucleus or we can have nuclear disintegration. This is recoiling means it is similar to the billyard ball collisions means one striker and one ball is need to hit. So they are similar to the billiard ball collision and energy distributed between the two colliding particles. So energy transfer is very efficient when the both particles mass are same. So hydrogenous materials are efficient for neutron shielding means hydrogen has one proton and neutron is the similar mass with that proton. So hydrogenous materials or low jade materials are more efficient for neutron shielding in comparison with the photons. They needs the high materials and uh the nuclear disintegration is one type of integration where the energy is very high and emission of second secondary heavy charged particles where the emissions of secondary heavy charged particles because it is the directly knock out a multiple number of nucleons from the nucleus.

[00:28:12]So in neutrons as it is a neutral charge so there will be no interactions of electron its interaction only having they have only interaction with the nucleus because orbital electron is very low mass and electron is also a electrically uncharged particles. So there will be no interaction of neutrons with the orbital electrons. This is all for the today's class. Thank you.