CHAPTER 1: Atom and Luminescence

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1 PREPARED BY: MR KAMARUL AMIN BIN ABDULLAH SCHOOL OF MEDICAL IMAGING FACULTY OF HEALTH SCIENCES PHYSICS FOR RADIOGRAPHERS 2 CHAPTER 1: Atom and Luminescence

2 LEARNING OUTCOMES At the end of the lesson, the student should be able to:- Define what is atomic structure and its theory. Differentiate between mass number and atomic number. Explain the fluorescence, phosphorescence and thermo luminescence in medical imaging. Slide 2 of 24

3 OUTLINES INTRODUCTION 1.1 Centuries of Discovery 1.4 Atomic Nomenclature Greek Atom Dalton Atom 1.5 Combinations of Atoms Thomson Atom Bohr Atom 1.6 Radioactivity Radioisotopes 1.2 Fundamental Particles Radioactive Half Life 1.3 Atomic Structure 1.7 Types of Ionizing Radiation Electron Arrangement Electron Binding Energy 1.8 Luminescence Slide 3 of 24

1.1 Centuries of Discovery 1.1.1 Greek Atom It states all matter composed of four substances: earth, water, air, and fire.

4 1.1 Centuries of Discovery Greek Atom It states all matter composed of four substances: earth, water, air, and fire. All matter is a combination of these four substances in various proportions with modification of wet, dry, hot, and cold. The Greeks used the term atom (indivisible) to describe the smallest part of the four substances. Figure 1 The ancient Greek Slide 4 of 24

5 1.1 Centuries of Discovery Figure 2: Symbolic representation of the substances and essences of matter as viewed by the ancient Greek. Slide 5 of 24

1.1 Centuries of Discovery 1.1.2 Dalton Atom In 1808, John Dalton (English school teacher) published that the elements could be classified according to integral values of atomic mass.

6 1.1 Centuries of Discovery Dalton Atom In 1808, John Dalton (English school teacher) published that the elements could be classified according to integral values of atomic mass. An element was composed of identical atoms that reacted the same way chemically. E.g. all O 2 atoms were alike but very different from atoms of any other element. Figure 3 John Dalton Slide 6 of 24

1.1 Centuries of Discovery 1.1.2 Dalton Atom (Continued) The physical combination was visualized as being an eye-and hook affair. The size and number were different for each other.

7 1.1 Centuries of Discovery Dalton Atom (Continued) The physical combination was visualized as being an eye-and hook affair. The size and number were different for each other. The Dalton s work has triggered a Russian scholar (Dmitri Mendeleev) to arrange the elements in order that resulted in the first periodic table of elements. Figure 4 The difference between Greek and Dalton. Slide 7 of 24

1.1 Centuries of Discovery 1.1.3 Thomson Atom In the late 1890s, J.

He described the atom as a plum pudding, where the plums represented negative electric charges

8 1.1 Centuries of Discovery Thomson Atom In the late 1890s, J.J. Thomson concluded that electrons were an integral part of all atoms. He described the atom as a plum pudding, where the plums represented negative electric charges (electrons) and the pudding was a shapeless mass of uniform positive electrification. Figure 5 J.J. Thomson Figure 6 The model of Thomson Slide 8 of 24

1.1 Centuries of Discovery 1.1.3 Thomson Atom (Continued) The number of electrons and positive charges are

However, in 1911, Ernest Rutherford disproved Thomson s model and he introduced the nuclear model as atom

9 1.1 Centuries of Discovery Thomson Atom (Continued) The number of electrons and positive charges are equal as atom was known as neutral. However, in 1911, Ernest Rutherford disproved Thomson s model and he introduced the nuclear model as atom contains a small, dense, positively charged center surrounded by electrons. The center is known as nucleus. Figure 7 Ernest Rutherford Figure 8 Rutherford s Atomic Model Slide 9 of 24

1.1 Centuries of Discovery 1.1.4 Bohr Atom In 1913, Niels Bohr improved Rutherford s model.

10 1.1 Centuries of Discovery Bohr Atom In 1913, Niels Bohr improved Rutherford s model. Bohr s model was a miniature solar system in which the electrons revolved about the nucleus in orbits (energy levels). Figure 9 Niels Bohr As similar to the nuclear model, it has electrons that revolve in fixed and well defined orbits about the nucleus. Figure 10 Bohr Model Slide 10 of 24

11 1.1 Centuries of Discovery Figure 11: Through the years, the atom has been represented by many symbols. Slide 11 of 24

12 1.2 Fundamental Particles The fundamental particles of an atom are the electron, the proton, and the neutron. The atom can be viewed as miniature solar system. Electrons carries one unit of negative electric charge. (mass 9.1 x kg). Because atomic particle is extremely small, its mass is expressed in atomic mass unit (amu) for convenience. 1 amu = 1 ½ of mass a carbon -12 atom. [electron (amu) = amu] Nucleus contains nucleons (protons and neutrons). Mass of proton is x kg and the neutron is x kg. Proton carries one unit of positive charge while Neutron carries no charge (neutral). Slide 12 of 24

13 1.2 Fundamental Particles Figure 12: An atom. Figure 13: The proton, electron and neutron. Figure 14: The THREE elements in an atom. Slide 13 of 24

14 1.3 Atomic Structure The atom is essentially empty space. The number of protons determines the chemical elements and the neutrons are neutral charge. If the atoms have the same number of protons but differ in the number neutrons are called isotopes. Electrons can exist in only in certain shells which represent different electron binding energies or energy levels. Electron orbit shells have been identified as K, L, M, N, and so forth to show the different energy levels from the closest to the farthest to the nucleus. The total number of electrons in the orbital shells is equal to the number of protons in the nucleus. If it has extra, it will be removed of ionization. Ionization is the removal of an orbital electron from an atom. Slide 14 of 24

15 1.3 Atomic Structure Figure 15: The nucleus consists of protons and neutrons, which are made of quarks bound together by gluons. Slide 15 of 24

16 1.3 Atomic Structure Electron Arrangement The number of electrons that can exist in each shells increases with the distance of the shell from nucleus. The maximum number of electrons per shell can be calculated using this formula [2n 2 ], where n is the shell number. The number of electrons in the outermost shell of an atom is always limited to eight electrons. No outer shell can contain more than eight electrons. Slide 16 of 24

17 1.3 Atomic Structure Electron Binding Energy (E b ) It is the strength of attachment of an electron to the nucleus. The closer the an electron is to the nucleus, the more tightly it is bound. K-shell electrons have higher binding energies than L, M, N and so forth. The greater the total number of electrons in an atom, the more tightly each is bound. The larger and more complex the atom, the higher is the E b for electrons in any given shell. Figure 16: An example of binding energy for atom of Tungsten. Slide 17 of 24

18 1.4 Atomic Nomenclature Element Atomic Number (Z) Atomic Mass Number (A) Isotopes Isobar Isotone Isomer It is indicated with chemical symbols. e.g. Ca, H, Be The chemical properties of an element are determined by the number and arrangement of electrons. The number of protons. The number of protons and neutrons Atoms that have the same atomic number but different atomic mass numbers. Atomic nuclei that have the same atomic mass number but different atomic numbers. Atoms that have the same numbers of neutrons but different numbers of protons. The same atomic number and atomic mass number. Slide 18 of 24

19 1.5 Combinations of Atoms Molecule = combination of atoms of various elements. Example 1: Four atoms of Hydrogen (H2) and two atoms of oxygen (O2) can combine to form two molecules of water. 2H 2 + O 2 2H 2 O Example 2: An atom of sodium (Na) can combine with an atom of chlorine (Cl) to form a molecule of sodium chloride (NaCl), Na + Cl NaCl Slide 19 of 24

20 1.5 Combinations of Atoms Compound = A chemical compound is any quantity of one type of molecule. Example: Sodium, Hydrogen, Carbon, and Oxygen atoms can combine to form a molecule of sodium bicarbonate (NaHCO 3 ). Slide 20 of 24

21 1.6 Radioactivity Radioactivity is the emission of particles and energy in order to become stable. Figure 17: The emission of particles or energy by an unstable element. Slide 21 of 24

22 1.6 Radioactivity Radioisotopes Many factors affect nuclear stability. When the nucleus contains too few or too many neutrons, the atom can disintegrate radioactively, bringing the number of neutrons and protons into a stable and proper ratio. Radioisotopes are the isotopes that have radioactivity. It can be artificially produced in machines such as particle accelerators or nuclear reactors. Also, a few elements have naturally occurring radioisotopes as well.. Slide 22 of 24

23 1.6 Radioactivity Figure 18: Radioisotopes can decay and result in emission of alpha, beta particles or gamma rays. Slide 23 of 24

24 1.6 Radioactivity Radioactive Half-Life Radioactive matter is not here one day and gone the next. Rather, radioisotopes disintegrate into stable isotopes of different elements at a decreasing rate, so that the quantity of radioactive matter never reaches zero. Radioactive material is measured in curies (Ci). {1 Ci is equal to 3.7 x Bq} Half Life (T1/2) of radioisotopes is the time required for a quantity of radioactivity to be reduced to one-half of its original value. Slide 24 of 24

25 1.7 Types of Ionizing Radiation It can be classified into TWO categories: a) Particulate radiation b) Electromagnetic radiation Although of ionizing radiation acts on biological tissue in the same manner, there are fundamental differences between different types of radiation according to the mass, energy, velocity, charge, and origin. Slide 25 of 24

26 1.7 Types of Ionizing Radiation Particulate Radiation There are TWO types of particulate radiation which are alpha particles and beta particles. Both are associated with radioactive decay Alpha particle Is a helium nucleus that contains two protons and two neutrons. Its mass is 4 amu and carries 2 units of +ve electric charge. Travels with high velocity through matter but in short range. Slide 26 of 24

27 1.7 Types of Ionizing Radiation Beta particle Is an electron emitted from nucleus of a radioactive atom. Light particles with atomic mass number is zero. Carry 1 unit of ve or +ve charge. Originate in the nuclei of radioactive atoms. Positive beta particles are positrons. Travels in longer range than alpha particle. Slide 27 of 24

28 1.7 Types of Ionizing Radiation Electromagnetic Radiation X-rays and gamma rays are forms of electromagnetic ionizing radiation. Often called photons which no mass and no charge. They travel at the speed of light (c = 3 x 10 8 m/s). Gamma rays emitted from the nucleus of radioisotopes and are usually associated with alpha and beta particles. X-rays are produced outside the nucleus in the electron shells. Both have unlimited of travel range in matter. Slide 28 of 24

29 1.8 Luminescence Introduction Any material that emits light in response to some outside stimulation is called a luminescent material or phosphor. The emitted visible light is called luminescence. A number of stimuli, including electric current (fluorescent light), biochemical reactions (the lightning bug), and x-rays (a radiographic intensifying screen), cause luminescence in materials. In radiography, the intensifying screen, absorption of a single x-ray causes emission of thousands of light photons. Slide 29 of 24

30 1.8 Luminescence Principle When a luminescent material is stimulated, the outer shell electrons are raised to excited energy levels. Then, it creates a hole in the outer-shell electron, which is an unstable condition of atom. The hole is filled when the excited electron returns to its normal state. This transition is accompanied by the emission of a visible light photon. Luminescent materials emit light of a characteristic color. Three types of luminescence have been identified in medical imaging modalities: fluorescence, phosphorescence, and thermoluminescence. Slide 30 of 24

31 1.8 Luminescence Fluorescence Emission of electromagnetic radiation. Emitted only while the phosphor is stimulated. Usually visible light, caused by excitation of atoms in a material, which then re-emit almost immediately (within about 10 8 seconds). i.e. It ceases as soon as the exciting source is removed. Slide 31 of 24

32 1.8 Luminescence Phosphorescence Emission of light from a substance exposed to radiation. Persisting as an afterglow after the exciting radiation has been removed. The phosphor continues to emit light after stimulation. Requires additional excitation to produce radiation and may last from about 10-3 second to days or years, depending on the circumstances. Slide 32 of 24

33 1.8 Luminescence Thermo luminescence It is phosphorescence triggered by temperatures above a certain threshold. Heat is not the primary source of the energy, only the trigger for the release of energy that originally came from another source. Slide 33 of 24

34 1.8 Luminescence Luminescence Fluorescence Phosphorescence Immediate Short Time Long Time Thermo luminescence Slide 34 of 24

35 1.8 References No. REFERENCES 1 Ball, J., Moore, A. D., & Turner, S. (2008). Essential physics for radiographers. Blackwell. 2 Bushong, S. C. (2008). Radiologic science for technologists. Canada: Elsevier. Slide 35 of 24

36 Activity Define or otherwise identify the following: a) Photon Answer b) The Rutherford atom Answer c) Positron Answer d) Nucleons Answer e) Radioactive Half Life Answer f) Alpha Particle Answer g) Beta particle Answer Slide 36 of 24

37 Activity Who developed the concept of the atom as a miniature solar system? Answer List the fundamental particles within an atom. Answer Describe the difference between alpha and beta emission. Answer Slide 37 of 24

38 Activity Define luminescence Answer What are types of luminescence? Answer Slide 38 of 24

39 SUMMARY As a miniature solar system, the Bohr atom set the stage for the modern interpretation of the structure of matter. Atom is the smallest part of an element, and molecule is the smallest part of a compound. Three fundamental particles: proton, electron, neutron. Some atoms have the same number of protons and electrons but different number of neutrons, different atomic mass. These are isotopes. Radioactivity : some atoms contain too many or too few neutrons in the nucleus that can disintegrate. Two types of particulate radiation: alpha and beta particles. Half-life: time required of radioactivity to be reduced to one-half its original value. Electromagnetic radiation: x-rays and gamma rays. Slide 39 of 24

40 NEXT SESSION PREVIEW CHAPTER 2: ELECTROMAGNETIC RADIATION Slide 40 of 24

41 APPENDIX FIGURE Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 SOURCE 2QLfw4/s1600/John+Dalton.jpg /Thomson's_Model.gif Slide 41 of 24

42 APPENDIX FIGURE Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16 Figure 17 Figure 18 SOURCE Bwc/s1600/proton.jpg tomic/basicstructure/atmparts.gif onelectronlight.jpg c_k3_0.jpg Isotopes.jpg Slide 42 of 24

Atomic Theory. Democritus to the Planetary Model

Atomic Theory. Democritus to the Planetary Model Atomic Theory Democritus to the Planetary Model Democritus Greek philosopher (460-370 BCE) Believed in the philosophy of materialism With Leucippus, they though that matter can not be divided infinitely.