Features of Nuclear Radiation & Interaction (PDF)

Features of nuclear radiation and its interaction with absorbing material. Detection of radiation. The text under the slides was written by János Szöllősi 2019 This instructional material was prepared for the biophys...

Features of nuclear radiation and its interaction with absorbing material. Detection of radiation. The text under the slides was written by János Szöllősi 2019 This instructional material was prepared for the biophysics lectures held by the Department of Biophysics and Cell Biology Faculty of Medicine University of Debrecen Hungary https://biophys.med.unideb.hu 2  =  This slide summarizes the main aims and topic of the lecture.  In nature radioactive isotopes emit only α, β- (electron), and γ radiations. Artificially (in cyclotrons, nuclear power plants) radioactive isotopes can be generated which can emit β+ particles (positrons), protons or neutrons as well.  High ionization density means smaller penetration depths since in this case the energy of the radioactive particle is lost during a shorter distance.  The α and β particles are directly ionizing radiations since 90-95% of the ionization is done by the particles themselves, while the γ photon is indirectly ionizing particle since 5% of the ionization is done by the photon itself, 95% of the ionization is secondary ionization.  Detection of the nuclear radiations relates to the facts that the nuclear radiations can ionize or excite atoms during the interaction with matter. For example ionization is utilized in the ionization chambers, excitation in the scintillation detectors.  The beneficial effect of nuclear radiation during evolution was that it increased the mutation rate, thereby helped the evolution.  Knowing the harmful effects of nuclear radiations medical doctors can make a decision when and how a given nuclear radiation can be utilized in diagnosis or therapy. 3  ALL NUCLEAR RADIATIONS ARE EJECTED FROM THE NUCLEUS OF THE ATOM.  The α particle is made of two protons and two neutrons. The energy of α particle is discrete, α particles ejected from the same type of atom have the same energy.  γ photons are electromagnetic radiations and have discrete energies similar to α particles.  During β- radiation a neutron is converted to proton and an electron is ejected from the nucleus along with an anti-neutrino. The energy spectrum of the β- radiation is continuous.  During β+ radiation a proton is converted to neutron and a positron is ejected from the nucleus along with a neutrino. The energy spectrum of the β + radiation is continuous. The positron is the anti-matter equivalent of the electron. 4  This is an example of α decay. During this process 226Ra (radium) is converted to 222Rn (radon) and the atomic number decreases by 2 and the mass number by 4.  The energy of the α particle is very specific for this decay. In another α decay, e.g. from the 212Po the ejected α particle has 8.785 MeV kinetic energy. The energy of the ejected α particle is determined by the specific shell structure of the nucleus the α particle is ejected from.  After determining the precise kinetic energy of an α particle we can identify what kind of nucleus of the α particle was originated from. 5  Slow moving particles ionize more efficiently because they have more time to interact!  The electrostatic interaction is described by the Coulomb equation written in the left yellow box. The strongest interaction is when the moving charged particle (red circle) is at the closest “b” distance from the electron (blue).  The electrostatic force (green arrow) is changing along the path of the moving particle, so the total change of energy of the particle can be obtained by summing the small F· s changes for each position with the help of integration.  The result of the integration is given by the equation in the right blue box. Here E is the total energy transferred from the moving particle to the electron,  is the proportionality sign, z is the charge of the moving particle (in the case of the α particle +2), M is the mass of the moving particle, b is the closest distance, E is the kinetic energy of the moving particle. 6  According to the equation (blue box) the α particle has high ionization density because of the big mass M, (allowing a small speed at the same kinetic energy) and the 2 positive charges.  The path of the α particle is straight since the α particle has 8000 times more mass than the electron. Occasionally when the α particle hits a nucleus the path can be diverted. If this head-on collision happens with a small nucleus, the nucleus can be stripped of the electron shell, and the obtained high speed nucleus can ionize on its own, that will be a δ ray. A δ ray can also be caused by high energy electrons as well, liberated by the α particle.  The α particle can ionize or excite molecules of the matter. Along its path (red line), the α particle causes primary or direct ionizations, while along the δ ray secondary or indirect ionization can be observed.  The right figure in the slide shows that the ionization density (ionpair/cm) is the highest just before the α particle loses its ionizing capability, and this is called the Bragg peak. The reason is that as the α particle loses its energy it will slow down, it will have more time to interact, so it will slow down even more, it will have even more time to interact, and this vicious circle continues. This is a good example for 7  positive feedback. In the next lecture we will see that this Bragg peak formation is the base of proton therapy for tumors.  The number of α particles does not change along the penetration, so the intensity of α radiation is constant. When an α particle loses all of its energy it will not be able to ionize anymore, it will capture two scattered electrons and will become a helium (noble gas) atom. 8  The energy change during β- decay is also discrete as it is suggested by the shell structure of the nucleus; however this discrete energy is randomly distributed between the β- particle and the anti-neutrino. Therefore the energy spectrum of the β- particle becomes continuous.  Neutrinos and anti-neutrinos are neutral particles, almost massless (mass of a neutrino is one millionths less than that of an electron).  We are practically transparent to the neutrinos and anti-neutrinos, because these particles interact with matter very-very weakly. Therefore they are difficult to detect. 9  The intensity (J) of β- radiation decreases exponentially (equation in the yellow box with red margins) because the β- particles do not have the same energy to start with and the interactions with the matter are heterogeneous.  Here the δ ray is evoked by high energy electrons and causes secondary, indirect ionizations. When a β- particle flies in the very vicinity of the nucleus of an atom, the concentrated positive charges in the nucleus divert the path of the β- particle (centripetal acceleration), and at the same time β- particle can be slowed down significantly and the kinetic energy lost is emitted as electromagnetic radiation (breaking radiation) which is an X ray. Diversion of the path could be very large, e.g. in the figure at the wavy black arrow showing the braking radiation this diversion is around 330 degree.  During excitation the fast moving β- particle gives only a small amount of energy to the electrons orbiting on the outer shells of the atom, therefore the electron goes up to a higher shell having higher energy level. The transferred energy is not enough to completely remove the electron from the atom, so that the atom will not be ionized only excited. 10  The penetration depth of α particles is a few centimeter in the air, and a few meters for β- radiation.  The ionization density (linear energy transfer, LET) of α radiation is much higher than that of the β- radiation.  If an α radiating isotope is on the skin, it is not so dangerous since the penetration depth is very small, only a few micrometers. However, if this isotope is already in the body (swallowing, injection, etc) that cause a lot of damage because of the high LET value of the α radiation. 11  If an α and a β- particle have the same energy to start with, they will ionize the same number of molecules in the air, the only difference will be the penetration depth. The α particle can ionize the same number of nitrogen or oxygen molecules in a much shorter distance than the β- particle because it has a much higher LET value.  Please note that nuclear radiations are very dangerous because the emanated α, β- and γ particles have so high energy that a single particle is able to destroy (ionize) more than a hundred thousand biomolecules in living organisms. (Owed to the large size of some of these, several damages are made to the same molecule, and molecules can be relatively far apart, so in practice this number can be somewhat smaller, but still large. More detail follows in the next lecture.) 12  In nature we cannot find isotopes which emit only γ radiation, γ emission always accompanies an α or a β- decay. However, not all α or β- radiations are accompanied by γ radiation.  Unstable radioactive nuclei have surplus energy and therefore they emit α or β - particles to get rid of the surplus energy. However, in a few occasions the emanated α or β- particles do not take all the surplus energy so in the leftover nucleus one or two nucleons (protons or neutrons) are at a higher energy level in the nuclear shell. When these nucleons jump down to a shell with lower energy, the energy difference between the two shells is emitted as electromagnetic radiation (a discrete energy package), and this is the γ radiation.  Hard X ray can have more energy than the γ radiation. In the overlapping spectral region we can tell the X ray and γ photons apart only if we know the history, i.e. the origin of the photons (the fact that the γ photons have only discrete energies can also help in identification). 13  The primary, direct ionization of γ radiation is responsible only for 5% of the ions generated, 95% of the ions are created by the secondary, indirect ionization, by high energy electrons set in motion by primary interaction. This is why X and γ ray are indirectly ionizing radiations.  The primary interactions are photoeffect, Compton effect, and pair production just like in X ray absorption.  The intensity (J) of γ radiation is described by an exponential equation shown in the yellow box and depicted on the next slide. J0 is the original intensity of the γ radiation, µ is the attenuation coefficient, x is the thickness of the absorbing material.  The photoeffect and Compton effect have been discussed in detail at the X ray absorption. 14  The intensity (J) of γ radiation is described by exponential equation shown in the red box. J0 is the original intensity of the γ radiation, µ is the attenuation coefficient, x is the thickness of the absorbing material.  The graph shows the exponential attenuation of γ radiation passing through the matter. Both the x and y axes uses linear scale. So this is a linear-linear plot. 15  This is a revision, see previous lecture on X ray absorption.  During photoeffect electron from the inner shells (K and L shells) are ejected.  The total energy of the γ photon is used up, to cover the ionization energy and the kinetic energy of the ejected electron. The γ photon ceases to exist.  The empty space in the K (or L) shell will not stay empty for long, electrons from the higher energy level shells (M, N, or O) will jump in to fill the empty space. The energy difference between the two shells will be emitted as characteristic X ray, similar to the X ray tube. 16  This is a revision, see previous lecture on X ray absorption.  During Compton effect electrons from the outer shells are ejected.  Only part of the energy of the γ photon is used up, to cover the ionization energy and the kinetic energy of the ejected electron. The scattered γ photon will have less energy (lower frequency, i.e. longer wavelength).  The scattered γ photon can participate in new Compton effects or photoeffects. 17  During this process the conservation laws should be fulfilled, conservation of charge (no net charge is created), conservation of energy (energy can be converted to mass), and conservation of momentum.  The pair production can be observed only if the energy of the γ photon is more than 1.02 MeV. During pair production the γ photon ceases to exist.  The energy of the γ photon should cover the creation of the masses of the electron and positron pair (1.02 MeV) and the kinetic energy of the newly created particles.  The momentum of the high energy γ photon (h/λ) is so large that the newly created particles, the electron and positron, cannot take all the momentum of the γ photon. The conservation of momentum can be fulfilled in the presence of a heavy nucleus (having big mass, M), the nucleus will be kicked with small velocity (v) and the slowly moving nucleus will take over the surplus momentum (Mv).  The positron and the electron pair IS NOT KICKED OUT FROM THE NUCLEUS of the heavy atom. There are no changes in the mass of the heavy nucleus, only the velocity of the nucleus will increase a bit. THE POSITRON AND ELECTRON PAIR IS CREATED FROM THE ENERGY OF THE γ PHOTON!!  The positron which is the anti-matter of the electron, is not stable in this matter world. Annihilation will happen when the positron meets an electron, but only after the positron loses all of its kinetic energy. That takes some time and path length so the annihilation does not happen in the immediate vicinity of the heavy nucleus where the pair production happened.  During annihilation two γ photons are created going into opposite directions. 18  The mass attenuation coefficient (µm) is defined as µm= µ/ρm, where µ is the attenuation coefficient (its unit is 1/cm) and ρm is the density of the matter. Therefore the unit of µm is cm2/g.  The dependence of the mass attenuation coefficient on the photon energy in the case of the lead (e.g. radiation shielding) and of water (e.g. biological tissues.)  In case of water the Compton scattering is the major contributor to the mass attenuation coefficient. The photoeffect takes place only in case of low energy photons, since the energy of the K shell electrons are not low enough in water.  Note that pair production does not start exactly at 1.02 MeV energy level, since the photon should have some extra energy to cover the kinetic energies of the newly created particles (electron and positron) and the slowly moving heavy nucleus, in addition to covering the mass of the electron and positron. 19  The α and β particles are directly ionizing particles since 90-95% of the ionization is done by the particles themselves, while the γ photon is indirectly ionizing particle since 5% of the ionization is done by the photon itself (primary ionization), 95% of the ionization is secondary, indirect ionization. 20  The linear energy transfer (LET) or stopping power (s) is the ratio of the energy lost and the path length: s = ΔE / Δx or nEionpair / l.  For α radiation WR = 20 means that the α radiation has 20 times more biological effect than the γ radiation if the same amount of energy is absorbed but only when the α emitting isotope is inside the body.  The neutron radiation is dangerous since it can interact with the nucleus of the atoms in a human body and can generate radioactive isotopes which can cause damage in the body.  The biological effects of β particles are similar to those of the X ray and γ photons. 21  We learn the working principle of the ionization chamber because it is used in biophysical practicals and analysis techniques relevant to radiation safety.  The body of the ionization chamber serves as cathode and a thin wire in the axis of the cylinder electrically isolated from the outer cylinder acts as anode. Incoming radiation ionizes the filling gas (e.g. argon) in the chamber and the ions are separated by the electric field of the electrodes. The accumulated charges induce current impulses in the outer circuit that produce a voltage signal on the resistance R.  At low voltages (A) ions created by the nuclear radiation move with a relatively low velocity toward their corresponding electrodes therefore the possibility for recombination is high. Therefore a small proportion of the ions is detected. The efficiency of detection, however improves with increasing voltage.  In the region B the efficiency of detection is close to 100%. In this region the highly ionizing α particle produces higher current pulses than the β particles therefore the α and β particles can be told apart.  In the C (proportional) region the charged particles produced by the primary ionization accelerate so much that they are able to generate secondary ionizations through collisions and linear charge amplification occurs in the filling gas. Devices 22  operated in this mode can be used for distinquishing particles from various radioactive source atoms based on their energy.  In the Geiger-Müller (D) region the charge amplification following the primary ionization results in complete discharge of the tube, i.e. all the gas molecules present are ionized, the maximum current pulse is produced.  In the E region, self-maintaining discharge can happen. We try to avoid this region. 23  We learn the working principle of scintillation detectors because they are used in medical diagnosis. (In addition we will use one of them in biophysical practice as well.)  The X ray and γ photons cannot be detected efficiently in an ionization chamber. Instead scintillation crystals can be utilized for more efficient detection.  In suitable crystals e.g. NaI crystal spiked with thallium, the absorption of γ photons can produce light flashes (scintillations) which knock electrons out of the cathode of the photomultiplier attached to the scintillation crystal.  In the photomultiplier tubes these photoelectrons are multiplied with the help of dynodes to obtain measurable current at the end. 24  In scintillation crystals γ photon generate high energy electrons by photoeffect or Compton effect. Along its path the electron loses energy by freeing other electrons and also by exciting electrons in the crystal. The probability of excitation increases with decreasing kinetic energy.  The excited electrons lose their excess energy with great efficiency through luminescence in the scintillation crystal. The net result is that a high energy γ photon produces a great number of visible range photons (blue light photons). Since the process occurs on a very short timescale, we perceive it as a light flash (scintillation). The number of visible photons is proportional to the energy conveyed by X ray or γ photons.  The produced visible photons are converted into electrical signal by a photomultiplier. The photomultiplier tube has vacuum inside so that motion of free electrons is not hindered by collisions with air molecules. Here the blue photons eject electrons from the photocathode through photoelectric effect. A large voltage in applied between the cathode and anode, so that on the electrodes positioned between them (called dynodes), the voltage increases towards the anode.  The setup of intermediate electrodes ensures that the electrons are initially accelerated towards the first dynode. Upon reaching this electrode the electrons will 25  have 100 eV energy so they will be able to release new electrons from this dynode. In this way one electron is able to produce several other electrons (electron multiplication). (The name dynode comes from the feature of the electrode since it can serve as anode when receiving electrons and cathode when ejecting electrons.)  The electrons newly ejected from the first dynode will be accelerated towards the second dynode and the electron multiplication process is repeated. Eventually, when reaching the anode the multitude of electrons (10 billion times of the original number) produces substantial electric current. Since the light pulse is very short so is the electric pulse.  The amplitude of the electric pulse is proportional to the number of photoelectrons produced in the photoelectric effect. 26  The film dosimeters are evaluated in central laboratories. Blackening of the film is proportional to the absorbed dose of the radiation. Different absorbents placed on certain sections of the film enable the separation of individual components of the radiation.  In thermoluminescent dosimeter electrons participating in the forbidden transitions have very low probability to return the ground state. Therefore these electrons remain in the metastable state for a relatively long time.  The exposure and the detection can be separated in time and space. Very small amounts the detector components (i.e. microcrystalline dust) are sufficient for making measurements.  During readout the detector crystals are heated. After reaching sufficiently high temperatures, essentially all trapped electron enter an excited state from where a transition back to the ground state is permitted. A certain (known) portion of the electrons relaxing to the ground state by emitting photons. These photons can be measured by a photomultiplier tube and the detected signal is proportional to the absorbed dose. 27 28