CHAPTER 30 NUCLEAR PHYSICS Dr. Arshad Kamal (Assistant Professor) Department of Preparatory Year College of Engineering and Islamic Architecture Umm-Al Qura University, Makkah Kingdom of Saudi Arabia Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Topics 30.1 Radioactivity 30.2 Half-Life Dr. Arshad Kamal Department of Preparatory Year Umm-Al Qura University, Makkah

Department of Preparatory Year Umm-Al Qura University, Makkah Before we start.... At the end of this presentation, you will be a genious about these following issues (at least I hope so ) : Radioactive (a type of exponentional) Decay Law Concept of Half- life How to solve half-life problems Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

What is Nuclear Physics? Nuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most infamous application of nuclear physics was probably the development of the atom bomb in the 1940s, but nuclear physics has many more applications, including highly beneficial ones. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those in nuclear medicine and magnetic resonance imaging, ion implantation in materials engineering and radiocarbon dating in geology and archeology. Dr. Arshad Kamal Department of Preparatory Year Umm-Al Qura University, Makkah

Early Pioneers in Radioactivity Rutherford: Discoverer Alpha and Beta rays 1897 Roentgen: Discoverer of X-rays 1895 The Curies: Discoverers of Radium and Polonium 1900-1908 Becquerel: Discoverer of Radioactivity 1896 Dr. Arshad Kamal Department of Preparatory Year Umm-Al Qura University, Makkah

Department of Preparatory Year Umm-Al Qura University, Makkah Basic Properties of Nucleus The nucleus is made up of protons and neutrons, collectively known as nucleons. A proton has a positive electrical charge equal in magnitude to the electronic charge and a mass about 1840 times that of the electron. Neutrons are about 0.1% more massive than protons. They are electrically neutral. A nucleus is specified by its atomic number Z and its mass number A. Z is the number of protons, and A is the total number of nucleons. Neutron number N = A-Z. The mass number A=Z+N determines approximately the mass of the nucleus in atomic mass units. Dr. Arshad Kamal Department of Preparatory Year Umm-Al Qura University, Makkah

Department of Preparatory Year Umm-Al Qura University, Makkah A particular nucleus defined by A and Z is known as a nuclide. The standard notation for nuclei is represented by U92238 This nucleus has 238 nucleons, out of which 92 are protons and 146 are neutrons. U is the chemical symbol for the 92nd element, Uranium. Nuclear species, or nuclides, which have the same atomic number but different neutrons numbers are called isotopes. Three distinct types of forces play important roles in nuclei. Nuclei are held together by very strong, short-ranged nuclear forces among the nucleons. Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Department of Preparatory Year Umm-Al Qura University, Makkah Electrical forces are smaller in magnitude, but they become progressively more important as the number of protons in the nucleus increases. The weak interactions are responsible for the beta decay process. In the beta decay, neutrons in nuclei are converted into protons as they emit electrons and neutrinos. Gravitational forces are the weakest forces so they are not important in nuclear physics. Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Department of Preparatory Year Umm-Al Qura University, Makkah What do we mean by Radioactivity? Radioactive decay is the process in which an unstable atomic nucleus loses energy by emitting radiation in the form of particles or electromagnetic waves. There are numerous types of radioactive decay. The general idea: An unstable nucleus releases energy to become more stable The phenomenon of self-emission of radiation is called radioactivity and the substances which emit these radiations are called radioactive substances. All the naturally occurring elements with atomic number Z greater than 82 (i.e. heavier than lead) are radioactive . Examples: Radium, Polonium, Thorium, Uranium, Actinium, etc. Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Department of Preparatory Year Umm-Al Qura University, Makkah The radiations from a radioactive material such as Uranium were made to pass through a strong magnetic field as shown in the figure. The direction of the magnetic field is perpendicular to the plane of the paper and it is represented by crosses (x) in the diagram. Under the action of magnetic field, the beam/radiation separated into three distinct parts as shown in the figure. These three kinds of radiations were named Alpha (α), Beta (β) and Gamma (γ) radiations. Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Department of Preparatory Year Umm-Al Qura University, Makkah Where do these particles come from ? These particles generally come from the nuclei of atomic isotopes which are not stable. The decay chain of Uranium produces all three of these forms of radiation. Let’s look at them in more detail… Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Alpha Particles (a) Uranium Thorium U238 Th234 + a (4He) Note: This is the atomic weight, which is the number of protons plus neutrons Alpha Particles (a) Uranium U238 Thorium Th234 + p n n p a (4He) 92 protons 146 neutrons 90 protons 144 neutrons 2 protons 2 neutrons Note: The 226 refers to the atomic weight, which is the equal to the number of protons plus neutrons -The alpha-particle (a) is a Helium nucleus which are positively charged. - relatively easy to stop, can only penetrate light materials such as paper - large size - double positive charge It’s the same as the element Helium, with the electrons stripped off !

Yes, the same neutrino we saw previously Beta Particles (b) Carbon C14 Nitrogen N14 + e- 6 protons 8 neutrons 7 protons 7 neutrons electron (beta-particle) We see that one of the neutrons from the C14 nucleus “converted” into a proton, and an electron was ejected. The remaining nucleus contains 7p and 7n, which is a nitrogen nucleus. In symbolic notation, the following process occurred: n  p + e ( + n ) Note that in beta decay, the atomic mass not change, since the neutron and proton have nearly the same mass… Yes, the same neutrino we saw previously -faster than alpha particles, penetrate light materials, 10 mm of wood, 3 mm of Al

Gamma particles (g) Gamma rays are high-frequency electromagnetic radiations(i.e. photons which do not carry any charge. In much the same way that electrons in atoms can be in an excited state, so can a nucleus. Neon Ne20 Neon Ne20 + 10 protons 10 neutrons (lowest energy state) gamma 10 protons 10 neutrons (in excited state) very fast and able to penetrate most materials such a 7 cm of lead can be harmful to our bodies, suffer structural damage - It is NOT visible by your naked eye because it is not in the visible part of the EM spectrum.

Gamma Rays Neon Ne20 Neon Ne20 + The gamma from nuclear decay is in the X-ray/ Gamma ray part of the EM spectrum (very energetic!)

How do these particles differ ? Mass* (MeV/c2) Charge Gamma (g) Beta (b) ~0.5 -1 Alpha (a) ~3752 +2 * m = E / c2

The three main decays are Alpha, Beta and Gamma Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Where are the Sources of Radioactivity? Naturally Occurring Sources: Radon from the decay of Uranium and Thorium Potassium -40 – found in minerals and in plants Carbon 14 – Found in Plants and Animal tissue Manmade Sources: Medical use of Radioactive Isotopes Certain Consumer products –(e.g. Smoke detectors) Fallout from nuclear testing Emissions from Nuclear Power plants Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Radioactivity – Is it a Health Problem? The Alpha, Beta and Gamma particles all add energy to the body’s tissues. The effect is called the Ionizing Energy. It can alter DNA. Even though Alpha particles are not very penetrative if the decaying atom is already in the body (inhalation, ingestion) they can cause trouble. While gamma has the highest penetrating power, so gamma radiations are more harmful. When they fall on the human body, they kill the living tissues, cause radiation burns and can induce cancer. Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Radioactivity – Is it a Health Problem? Biological effects of nuclear radiation Short term recoverable effects, Long term irrecoverable effects and Genetic effects The first two effects are limited to the individuals who are actually exposed to the radiations, while the third effect appears in the later generations. Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Department of Preparatory Year Umm-Al Qura University, Makkah Half-Life (T) is the time required for half a sample of radioactive nuclei to decay impossible to predict the moment any nucleus will decay nuclear decay is random process but it is possible to predict the time it will take for half the nuclei to decay different radioactive isotopes have different half lives varying from nanoseconds to billions of years can predict the approximate age of a sample Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Department of Preparatory Year Umm-Al Qura University, Makkah Radioactive Decay Law Suppose at time t = 0, there are N0, nuclei, then one half-life later at t= T, an average of N0 /2 will remain. At t=2T when two half life have elapsed, N0 /4, nuclei will be left. At t = 3T, N0 /8 will be left, and so on. When the elapsed time is not an exact integer multiple of the half life, we can find the number of nuclei remaining as follows: The change ∆N in the number of nuclei N present occurring in a short time ∆t is proportional to N and ∆t, so we can write ∆N = - λN ∆t The negative sign is included because the number N of radioactive nuclei decreases with time due to decays. Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Department of Preparatory Year Umm-Al Qura University, Makkah What was that?!!! In the previous equation you have seen a symbol like: λ λ is a constant of proportionality, called the decay constant It differs according to the isotope it is in. The greater λ is, the greater the rate of decay This means that the greater λ is, the more radioactive the isotope is said to be. Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Department of Preparatory Year Umm-Al Qura University, Makkah Still confused about the equation... Don’t worry! If you are still confused about why this equation is like this, here is some of the important points.... Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Department of Preparatory Year Umm-Al Qura University, Makkah Exponential decay formula Figure shows a plot of N versus t which is the radioactivity decay curve. When t = T, N/ N0 = ½, according to half-life definition, When t = 2T, N/ N0 = ¼; ………….values of N/ N0 can be read from the graph or calculated using an electronic calculator. Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Department of Preparatory Year Umm-Al Qura University, Makkah How to calculate half-life? The decay constant and half-life has the relationship : λ = ln(2)/T = 0.693/T Example 30.1 Iodine 131 is used in the treatment of thyroid disorders. Its half-life is 8.1 days. If a patient ingests a small quantity of I-131 and none is excreted from the body, what fraction N/ N0 remains after 8.1 days, 16.2 days, 60 days? Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Department of Preparatory Year Umm-Al Qura University, Makkah Some representative half-lives are given in Table 30.1 Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal

Department of Preparatory Year Umm-Al Qura University, Makkah Homework Problems: 30.1, 30.2, 30.3, 30.5 and 30.7 Department of Preparatory Year Umm-Al Qura University, Makkah Dr. Arshad Kamal