Modes Of Radioactive Decay PDF

Chapter Two Modes of Radioactive Decay Prepared by Dr. Mohamed Hasabelnaby Lecturer of physics, School of Allied Health Sciences, Badr University. 2.1 General Concepts Radioactive decay is a process in which an unstable nucleus trans...

Chapter Two Modes of Radioactive Decay Prepared by Dr. Mohamed Hasabelnaby Lecturer of physics, School of Allied Health Sciences, Badr University. 2.1 General Concepts Radioactive decay is a process in which an unstable nucleus transforms into a more stable one by emitting particles, photons, or both, releasing energy in the process. Atomic electrons may become involved in some types of radioactive decay, but it is basically a nuclear process caused by nuclear instability. It is common terminology to call an unstable radioactive nucleus the parent and the more stable product nucleus the daughter. In many cases, the daughter also is radioactive and undergoes further radioactive decay. Radioactive decay is spontaneous in that the exact moment at which a given nucleus will decay cannot be predicted, nor is it affected to any significant extent by events occurring outside the nucleus. Radioactive decay results in the conversion of mass into energy. If all the products of a particular decay event were gathered together and weighed, they would be found to weigh less than the original radioactive atom. Usually, the energy arises from the conversion of nuclear mass, but in some decay modes, electron mass is converted into energy as well. The total mass-energy conversion amount is called the transition energy, sometimes designated Q , most of this energy is imparted as kinetic energy to emitted particles or converted to photons, with a small (usually insignificant) portion given as kinetic energy to the recoiling nucleus. Thus radioactive decay results not only in the transformation of one nuclear species into another but also in the transformation of mass into energy.  Each radioactive nuclide has a set of characteristic properties. These properties include: the mode of radioactive decay, the transition energy, and the average lifetime of a nucleus of the radionuclide before it undergoes radioactive decay. Because these basic properties are characteristic of the nuclide, it is common to refer to a 131 radioactive species, such as I , as a radionuclide. The term radioisotope also is used but, strictly speaking, should be used only when specifically identifying a member of an isotopic family as radioactive; for example, 131I is a radioisotope of iodine. 2.2 Chemistry and radioactivity Radioactive decay is a process involving primarily the nucleus, whereas chemical reactions involve primarily the outermost orbital electrons of the atom. Thus the fact that an atom has a radioactive nucleus does not affect its chemical behavior and, conversely, the chemical state of an atom does not affect its radioactive characteristics. 131 For example, an atom of the radionuclide I exhibits the same chemical behavior as an 127 131 atom of I, the naturally occurring stable nuclide, and I has the same radioactive characteristics whether it exists as iodide ion I. 2.3 Radioactive decay Radioactive decay occurs when an unstable atomic nucleus loses energy by emitting energy in the form of emitted particles or electromagnetic waves, called radiation. There are many types of emitted particles and radiation that radioisotopes produce when they decay. The types are: alpha particles, beta particles, and gamma ray. 2.3.1 Beta Decay The beta decay is a radioactive decay in which a proton in a nucleus is converted into a neutron (or vice-versa). In the process the nucleus emits a beta particle (an electron or a positron) and massless particle, is called the neutrino. There is two types of beta decay β + and β −. (i) DECAY BY β − particle − Radioactive decay by β emission is a process in which; essentially, a neutron in the nucleus is transformed into a proton and an electron. Schematically, the process is: The electron (e−) and the neutrino (ν) are ejected from the nucleus and carry away the energy released in the process as kinetic energy. The electron is called β − particle. The neutrino is a “particle” having no mass or electrical charge. It undergoes virtually no interactions with matter and therefore is essentially undetectable. Its only practical consequence is that it carries away some of the energy released in the decay process. The parent radionuclide (X) and daughter product (Y) represent different chemical − elements because atomic number increases by one. Thus β decay results in a transmutation of elements. Mass number A does not change because the total number of nucleons in the nucleus does not change. This is therefore an isobaric decay mode, that is, the parent and daughter are isobars.  Radioactive decay processes often are represented by a decay scheme diagram. The figure below shows such a diagram for 14C, a radionuclide that decays by β − emission. The line 14 representing C (the parent) is drawn above and to the left of the line representing 14N (the daughter). Decay is “to the right” because atomic number increases by one (reading Z values from left to right). The vertical distance between the lines is proportional to the total amount of energy released, that is, the transition energy for the decay process (Q = 0.156 MeV for 14C ). Fig (2.1): Decay scheme diagram for 14C, a Q = 0.156 MeV β− emitter.  − − The energy released in β decay is shared between the β particle and the neutrino. This sharing of energy is more or less random from one decay to the next. Beta particles present special detection and measurement problems for nuclear medicine applications. These arise from the fact that they can penetrate only relatively small thicknesses of solid materials. For example, the thickness is at most only a few millimeters in soft tissues. Therefore it is difficult to detect − β particles originating from inside the body with a detector that is located outside the body. − − It should be noted that β particles are energetic electrons in ‘appearance,’ but β particles differ from electrons in that they originate from inside the nucleus. Electrons are in orbit outside the nucleus and have no energy in their normal condition. − (ii) Decay by (𝛃 , γ) − In some cases, decay by β emission results in a daughter nucleus that is in an excited or metastable state rather than in the ground state. If an excited state is formed, the daughter nucleus decays to a more stable nuclear arrangement by the emission of a γ ray. This sequential decay − process is called (β , γ) decay. In standard nuclear notation, it may be represented as: − − An example of (β , γ) decay is the radionuclide 133Xe, which decays by β emission to 133 one of three different excited states of Cs. Figure below is a decay scheme for this radionuclide. If it is to another excited state, additional γ rays may be emitted before the ground − state is finally reached. Thus in (β , γ) decay more than one γ ray may be emitted before the daughter nucleus reaches the ground state (e.g., β2 followed by γ1 and γ2 in 133Xe decay).  Fig (2.2): Decay scheme diagram for 133Xe, a (β−, γ) emitter. More than one γ ray may be emitted per disintegrating nucleus. The heavy line (for β3) indicates most-probable decay mode. The number of nuclei decaying through the different excited states is determined by 133 probability values that are characteristic of the particular radionuclide. For example, in Xe decay, 99.3% of the decay events are by β3 decay to the 0.081-MeV excited state, followed by emission of the 0.081-MeV γ ray. Only a very small number of the other β particles and γ rays of other energies are emitted. + (iii) Decay by 𝛃 (positron) + In radioactive decay by β (positron emission), a proton in the nucleus is transformed into a neutron and a positively charged electron. The positively charged electron or positron + (β ) and a neutrino are ejected from the nucleus. Schematically, the process is: + In standard notation, β decay is represented as: It is another isobaric decay mode, with a transmutation of elements. A positron is the antiparticle of an ordinary electron. After ejection from the nucleus, it loses its kinetic energy in collisions with atoms of the surrounding matter and comes to rest, usually within a few millimeters of the site of its origin in body tissues. The positron then combines with the negative electron in an annihilation reaction, in which their masses are converted into energy. The mass-energy equivalent of each particle is 0.511 MeV. This energy appears in the form of two 0.511-MeV annihilation photons, which leave the site of the annihilation event in nearly exact opposite directions (180 degrees apart) as shown in figure (2.3). Fig (2.3): Schematic representation of mutual annihilation reaction between a positron (β+) and an ordinary electron. Pair of 0.511-MeV annihilation photons is emitted “back-to-back” at 180 degrees to each other + + In β decay, a positron is ejected from the nucleus, and because β decay reduces the atomic number by one, the daughter atom also has an excess electron that it releases to reach its + ground state. Thus two particles are emitted from the atom during β decay, and because the rest-mass energy of an electron or a positron is 511 keV, total transition energy of 1.022 MeV is − required. Note that no such requirement is present for β decay, because the daughter atom must take up an electron from the environment to become neutral, thereby compensating for the − + electron released during β decay. In β decay, the excess transition energy above 1.022 MeV is shared between the positron (kinetic energy) and the neutrino. + Figure (2.4) show a decay scheme for 15 O, β emitter of medical interest. Decay is “to the left” because atomic number decreases by one. The vertical line represents the minimum + transition energy requirement for β decay (1.022 MeV). The remaining energy (1.7 MeV) is Eβ + max. With some radionuclides, β emission may leave the daughter nucleus in an excited state, and thus additional γ rays may also be emitted [(β+, γ) decay].  Fig (2.4): Decay scheme diagram for15O, a β+ emitter. 2.3.2 Isomeric transition The daughter nucleus of a radioactive parent may be formed in a “long-lived” metastable or isomeric state, as opposed to an excited state. The decay of the metastable or isomeric state by the emission of a γ ray is called an isomeric transition. Except for their average lifetimes, there are no differences in decay by γ emission of metastable or excited states. 2.3.3 Internal conversion An alternative to γ-ray emission is internal conversion. This can occur for any excited state, but is especially common for metastable states. In this process, the nucleus decays by transferring energy to an orbital electron, which is ejected instead of the γ ray. It is as if the γ ray were “internally absorbed” by collision with an orbital electron. The ejected electron is called a conversion electron. These electrons usually originate from one of the inner shells (K or L), provided that the γ-ray energy is sufficient to overcome the binding energy of that shell. The energy excess above the binding energy is imparted to the conversion electron as kinetic energy. The orbital vacancy created by internal conversion subsequently is filled by an outer shell electron, accompanied by emission of characteristic x rays.  Fig (2.5): Schematic representation of internal conversion involving a K-shell electron. An unstable nucleus transfers its energy to the electron rather than emitting a γ ray. Kinetic energy of conversion electron is γ ray energy minus electron-binding energy (Eγ − KB). − Internal conversion, like β decay, results in the emission of electrons. The important − differences are that in β decay the electron originates from the nucleus, whereas in internal conversion it originates from an electron orbit. 2.3.4 Electron capture (EC) decay Electron capture (EC) decay looks like, and in fact is sometimes called, “inverse β− decay.” An orbital electron is “captured” by the nucleus and combines with a proton to form a neutron: The neutrino is emitted from the nucleus and carries away some of the transition energy. The remaining energy appears in the form of characteristic X rays, which are emitted by the daughter product when the resulting orbital electron vacancy is filled. Usually, the electron is captured from orbits that are closest to the nucleus, that is, the K and L shells. The notation EC (K) is used to indicate capture of a K-shell electron, EC (L) an L-shell electron, and so forth. EC decay may be represented as: Note that like β− decay it is an isobaric decay mode leading to a transmutation of elements. The characteristic x rays emitted by the daughter product after EC may be suitable for external measurement if they are sufficiently energetic to penetrate a few centimeters of body tissues. EC decay results frequently in a daughter nucleus that is in an excited or metastable state. Thus γ rays (or conversion electrons) may also be emitted. This is called (EC, γ) decay. Figure (2-6) shows a decay scheme for 125I, Note that EC decay is “to the left” because EC decreases the atomic number by one. Fig. (2.6): Decay scheme diagram for 125I, an (EC, γ) emitter. 2.3.5 Competitive β+ and EC decay Positron emission and EC have the same effect on the parent nucleus. Both are isobaric decay modes that decrease atomic number by one. They are alternative means for reaching the same endpoint (see Equations of both). Among the radioactive nuclides, one finds that β+ decay occurs more frequently among lighter elements, whereas EC is more frequent among heavier elements, because in heavy elements orbital electrons tend to be closer to the nucleus and are more easily captured. There also are radionuclides that can decay by either mode. An example is 18F, the decay scheme for which is shown in Figure (2.7). For this radionuclide, 3% of the nuclei decay by EC and 97% decay by β+ emission.  Fig. (2.7): Decay scheme diagram for 18F, which decays by both electron capture and β+ emission competitively. 2.3.6 Decay by α emission In decay by α-particle emission, the nucleus ejects α particle, which consists of two neutrons and two protons (essentially a 42𝐻𝑒 nucleus). In standard notation this is represented as: The particle is emitted with kinetic energy usually between 4 and 8 MeV. Although quite energetic, α particle has very short ranges in solid materials, for example, approximately 0.03 mm in body tissues. Thus they present very difficult detection and measurement problems. Decay by α-particle emission results in a transmutation of elements, but it is not isobaric. Atomic mass is decreased by 4; therefore this process is common among very heavy elements that must lose mass to achieve nuclear stability. Heavy, naturally occurring radionuclides such as 238U and its daughter products undergo a series of decays involving α-particle and β−-particle emission to transform into lighter, more stable nuclides. 2.3.7 Gamma ray Gamma rays are high-energy electromagnetic radiation emitted in the deexcitation of the atomic nucleus. Electromagnetic radiation includes such diverse phenomena as radio, television, microwaves, infrared radiation, light, ultraviolet radiation, X rays, and gamma rays. These radiations all propagate through vacuum with the speed of light. They can be described as wave phenomena involving electric and magnetic field oscillations analogous to mechanical oscillations such as water waves or sound. Gamma photons are the most energetic photons in the electromagnetic spectrum. Electromagnetic radiation can be described in terms of a stream of photons, which are massless particles each travelling in a wave-like pattern and moving at the speed of light. Each photon contains a certain amount of energy, and all electromagnetic radiation consists of these photons. Gamma-ray photons have the highest energy in the EMR spectrum and their waves have the shortest wavelength. Scientists measure the energy of photons in electron volts (eV). X- ray photons have energies in the range 100 eV to 100,000 eV (or 100 keV). Gamma-ray photons generally have energies greater than 100 keV.  Gamma radiation, unlike alpha or beta, does not consist of any particles, instead consisting of a photon of energy being emitted from an unstable nucleus. Having no mass or charge, gamma radiation can travel much farther through air than alpha or beta, losing (on average) half its energy for every 500 feet. Gamma waves can be stopped by a thick or dense enough layer material, with high atomic number materials such as lead or depleted uranium being the most effective form of shielding. What is the difference between gamma rays and X-rays? The key difference between gamma rays and X-rays is how they are produced. Gamma rays originate from the settling process of an excited nucleus of a radionuclide after it undergoes radioactive decay whereas X-rays are produced when electrons strike a target or when electrons rearrange within an atom. Gamma rays are external emitters that penetrate biological materials easily and produce their insidious effects without being taken internally. Alpha and beta particles are internal emitters; their damage to organisms is greatest when taken internally. 2.3.8 Penetration power of radiation There are large differences in penetrating ability depending on the type of radiation (α-, β- or γ-radiation). The illustration below gives you an idea about the penetration of the radiation from the radioactive sources. Alpha particles travel but a few centimeters, and can be stopped by a layer of dead skin, they are dangerous because they produce a large amount of local ionization which can cause mutations disrupting cell processes. Beta particles are high speed electrons. While much smaller than alpha particles, they are able to travel up to a couple of centimeters in living tissue, giving up their energy over a large path. Beta particles, like alpha particles can damage tissue, and like alpha particles, can cause mutations that affect the functioning of cells. Gamma rays and X-rays has the ability to penetrate tissue. Several feet of concrete or a few inches of lead are required to stop them. 2.3.9 Sources of information on radionuclides There are several sources of information providing useful summaries of the properties of radionuclides. One is a chart of the nuclides, a portion of which is shown in Figure (2.8). Every stable or radioactive nuclide is assigned a square on the diagram. Isotopes occupy horizontal rows and isotones occupy vertical columns. Isobars fall along descending 45-degree lines. Basic properties of each nuclide are listed in the boxes. Also shown in the figure is a diagram indicating the transformations that occur for various decay modes. A chart of the nuclides is particularly useful for tracing through a radioactive series. Fig. (2.8): Portion of a chart of the nuclides. Vertical axis = atomic number; horizontal axis = neutron number.