DOW University of Health Sciences Nuclear Physics Lecture 8, Week 8 PDF

Week 8 Lecture 8 Thursday 20-6-24 Saturday 22-6-24 OP,RCC,ST,PS & OPTO Lecturer: Syed Zeeshan ul hassan  What is Radioactivity Due to nuclear instability, an atom’s nucleus exhibits the phenomenon of Radioactivity. Energy is lost due to radiation that is emi...

Week 8 Lecture 8 Thursday 20-6-24 Saturday 22-6-24 OP,RCC,ST,PS & OPTO Lecturer: Syed Zeeshan ul hassan  What is Radioactivity Due to nuclear instability, an atom’s nucleus exhibits the phenomenon of Radioactivity. Energy is lost due to radiation that is emitted out of the unstable nucleus of an atom. Two forces, namely the force of repulsion that is electrostatic and the powerful forces of attraction of the nucleus, keep the nucleus together. These two forces are considered extremely strong in the natural environment. The chance of encountering instability increases as the size of the nucleus increases because the mass of the nucleus becomes a lot when concentrated. That’s the reason why atoms of Plutonium, Uranium are extremely unstable and undergo the phenomenon of radioactivity. March 1, 1896: Henri Becquerel Discovers Radioactivity. In one of the most well-known accidental discoveries in the history of physics, on an overcast day in March 1896, French physicist Henri Becquerel opened a drawer and discovered spontaneous radioactivity. Henry Becquerel discovered radioactivity by accident. A Uranium compound was placed in a drawer containing photographic plates, wrapped in a black paper. When the plates were examined later, it was found that they were exposed! This exposure gave rise to the concept of Radioactive decay. Radioactivity can be seen in such forms Gamma Decay (Photons having high energy are emitted) Beta Decay (Emission consists of Electrons) Alpha Decay (Emission consists of Helium nucleus)  Laws of Radioactivity Radioactivity is the result of the decay of the nucleus. The nucleus’s decay rate is independent of temperature and pressure. Radioactivity is dependent on the law of conservation of charge. The physical and chemical properties of the daughter nucleus are different from the mother nucleus. The emission of energy from radioactivity is always accompanied by alpha, beta, and gamma particles. The rate of decay of radioactive substances is dependent on the number of atoms that are present at the time. Units of Radioactivity Curie and Rutherford are the units of radioactivity. 1C = 3.7 × 104 Rd is the relationship between Curie and Rutherford. Uses of Radioactivity Some radioactivity uses are provided in the points below. Americium-241 is an alpha emitter and is used for domestic smoke detectors in the United States.  The alpha particles given out by the Americium sample ionize the air in the chamber of the smoke detector leading to a small current in the chamber. When smoke enters the chamber, it causes a drop in current causing the alarm to go off. Although Alpha particles have a very short range, they are devastating when in close contact. Alpha emitters, when swallowed, come in close contact with tissue and are deadly in such circumstances and is therefore used in assassination attempts by radiation poisoning. Advantages and Disadvantages of Radioactivity Advantages of radioactivity Gamma rays are used to kill cancerous cells and hence used in radiotherapy. Cobalt-60 is used to destroy carcinogenic cells. Gamma rays are used in scanning the internal parts of the body. Gamma rays kill microbes present in food and prevent it from decay by increasing the shelf life. Age of the rocks can be studied using radioactive radiations by measuring the argon content present in the rock. Disadvantages of radioactivity High dosage of radioactive radiation on the body might lead to death. Radioactive isotopes are expensive.  Differentiate between natural and artificial radioactive elements. The elements with atomic number 82-92 are found to radiate spontaneously in nature so they are known as natural radioactive elements. Whereas the elements that are produced in the laboratory by the bombardment of particles are called artificial radioactive elements. These are generally the elements with atomic number less than 82. Natural radiation comprises cosmic radiation and the radiation arising from the decay of naturally occurring radionuclides. The natural radionuclides include the primordial radioactive elements in the earth's crust, their radioactive decay products, and radionuclides produced by cosmic-radiation interactions. Alpha, Beta and Gamma Rays During radioactivity, particles like alpha, beta & gamma rays are emitted by an atom, due to unstable atom trying to gain stability. Hence, the atoms eventually decay by emitting a particle that transforms when they are unstable and transforms the nucleus into a lower energy state. This process of decaying continues till the nucleus attains a stable stage. There exist three major types of radiations emitted by the radioactive particles namely: These radiations are released from the nucleus of an atom. Their behavior differs from one another, though all the three causes some ionization and carry some penetration power. Let’s discuss the properties of beta, alpha and gamma one by one.  Properties of radioactive rays Alpha Rays Alpha rays are the positively charged particles. Alpha-particle is highly active and energetic helium atom that contains two neutrons and protons. These particles have the minimum penetration power and highest ionization power. They can cause serious damage if get into the body due to their high ionization power. They are capable of ionizing numerous atoms by a short distance. It is due to the fact that the radioactive substances that release alpha particles are required to be handled after wearing rubber gloves. Beta Rays Beta particles are extremely energetic electrons that are liberated from the inner nucleus. They bear negligible mass and carry the negative charge. A neutron in the nucleus splits into a proton and an electron on the emission of a beta particle. Hence, it is the electron that is emitted by the nucleus at a rapid pace. Beta particles have a higher penetration power when compared to alpha particles and can travel through the skin with ease. Beta particles can be dangerous and any contact with the body must be avoided, though their ionization power is low. Gamma Rays The waves arising from the high-frequency end of the electromagnetic spectrum that has no mass are known as gamma rays. They hold the highest power of penetration. They are the most penetrating but least ionizing and very difficult to resist them from entering the body. The Gamma rays carry a large amount of energy and can also travel via thick concrete and thin lead. Derivation of number of atoms present at any time.  Binding Energy Binding energy is typically defined as the smallest amount of energy that is required to remove a particle from a system of particles. In other words, it is the energy that is used to separate a system of particles into single units. We study about binding energy mostly in atomic physics and chemistry as well as in condensed matter physics. In nuclear physics, the term binding energy is used to describe separation energy. Binding energy is necessary to split subatomic particles in atomic nuclei or the nucleus of an atom into its components, namely: neutrons and protons or collectively known as nucleons. The binding energy of nuclei is a positive value because every nucleus needs net energy to isolate them into neutron and proton. Binding energy is also applicable to atoms and ions bound together in crystals. B.E = ( M – A ) C2 Types of Binding Energy Binding energy is of several types, and each operates over a different distance and energy scale. An important point to note here is that if the size of a bound system is small, then its associated binding energy will be higher Electron Binding Energy – Ionization Energy Electron binding energy, which is commonly known as ionization energy, is the energy required to remove an electron from its atomic orbital. The electron binding energy is mostly derived as a result of the electromagnetic interaction that occurs between the electron and the nucleus, the other electrons of the atom, molecule or solid that is usually mediated by photons. Atomic Binding Energy The atomic binding energy of the atom is the energy that is used to break down an atom into free electrons and a nucleus. We can say that it is the sum of the ionization energies of all the electrons that belong to a particular atom. The atomic binding energy is derived from the electromagnetic interaction between the electrons with the nucleus, which is mediated by the photons. Nuclear Binding Energy Nuclear binding energy is basically the energy required to dismantle a nucleus into free unbound neutrons and protons. It is the energy equivalent of the mass defect, the difference between the mass number of a nucleus and its measured mass.  Mass Defect The nuclear binding energy holds a significant difference between the nucleus’s actual mass and its expected mass depending on the sum of the masses of isolated components. Hence, energy and mass are related based on the following equation: Where c is the speed of light. In nuclei, the binding energy is so high that it holds a considerable amount of mass. The actual mass is less than the sum of individual masses of the constituent neutrons and protons in every situation because energy is ejected when the nucleus is created. This energy consists of mass which is ejected from the total mass of the original components and is called a mass defect. This mass is missing in the final nucleus and describes the energy liberated when the nucleus is made. Mass defect is determined as the difference between the atomic mass observed (Mo) and expected by the combined masses of its protons (mp, every proton has a mass of 1.00728 AMU) and neutrons (mn, 1.00867 AMU). Applications Binding energy is also applied in determining whether fusion or fission will be favourable. For elements that are lighter than iron-56, the fusion releases energy since the nuclear binding energy rises with the hike in mass. Elements that are heavier than iron-56 release energy on fission since the lighter elements consist of higher binding energy. Hence, there exists a peak at iron-56 according to the nuclear binding energy curve.  Nuclear Fission Nuclear fission is a nuclear reaction in which the nucleus of an atom is bombarded with low-energy neutrons which splits the nucleus into smaller nuclei. An abundant amount of energy is released in this process. Nuclear fission reactions are used in nuclear power reactors since it is easy to control and produces large amounts of energy. When uranium-235 is bombarded with slow-moving neutrons, the heavy nucleus of the uranium splits and produces krypton-89 and barium-144 with the emission of three neutrons.  Nuclear Fusion Nuclear fusion is a reaction that occurs when two or more atoms combine together to form to a single heavier nucleus. An enormous amount of energy is released in this process, much greater than the energy released during the nuclear fission reaction. Fusion occurs in the sun where the atoms of (isotopes of hydrogen, hydrogen-3, and hydrogen-2) deuterium and tritium combine in a high-pressure atmosphere with extremely high temperatures to produce an output in the form of a neutron and an isotope of Helium. Also, the amount of energy released in fusion is way greater than the energy produced by fission.  Chain reaction A chain reaction refers to a process in which neutrons released in fission produce an additional fission in at least one further nucleus. This nucleus in turn produces neutrons, and the process repeats. The process may be controlled (nuclear power) or uncontrolled (nuclear weapons). Fusion reaction at stars and Sun  Activity The product of decay constant and number of parent nuclei is called Activity. The number of nuclei disintegration per second. A=λ N Where A is the Activity. The activity of a radioactive element is a positive number which represents the decaying ability of substance subjected to extrinsic conditions. Binding fraction Binding fraction of Nucleus is the binding energy per nucleon.binding energy of nucleus the measure of the stability of nucleus against splitting up into its constituent. The nuclei with largest binding per nucleon and most tightly bounded are those in the middle region of the periodic table. Their binding energy per nucleon is over 8Mev F = B.E\ A  What are Radioisotopes Radioisotopes are radioactive isotopes of an element. They can also be defined as atoms that contain an unstable combination of neutrons and protons, or excess energy in their nucleus. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferred to one of its electrons to release it as a conversion electron; or used to create and emit a new particle (alpha particle or beta particle) from the nucleus. During those processes, the radionuclide is said to undergo radioactive decay.These emissions are considered ionizing radiation because they are powerful enough to liberate an electron from another atom. Uses of Radioisotopes Radioactive isotopes have many useful applications. In medicine, for example, cobalt-60 is extensively employed as a radiation source to arrest the development of cancer. The most widely used radioactive pharmaceutical for diagnostic studies in nuclear medicine. Different chemical forms are used for brain, bone, liver, spleen and kidney imaging and also for blood flow studies. Used to locate leaks in industrial pipe line and in oil well studies. Other radioactive isotopes are used as tracers for diagnostic purposes as well as in research on metabolic processes. When a radioactive isotope is added in small amounts to comparatively large quantities of the stable element, it behaves exactly the same as the ordinary isotope chemically; it can, however, be traced with a Geiger counter or other detection device. difference between an isotope and a radioactive isotope? Isotopes are atoms of the same element that have different numbers of neutrons but the same number of protons and electrons. Radioactive (unstable) isotopes have nuclei that spontaneously decay over time to form other isotopes.