To understand the structure and properties of atoms and the interactions and mechanism among atoms. The current research: An increasingly detailed description of the structure of electronic shells of atoms and their excitations The interactions between atoms and radiation fields The interactions of atoms among themselves due to collisions in the gas phase and during the formation of molecules The physical principles lead to the formation of condensed phases from single atoms, and their properties.
where σ = (r1 + r2 )2 π. In terms of the particle number density n, ∆N = − nA∆xσ N. A
After integration, we have N = N0 e−nσL where N0 is the initial number of α-particles. The number of deflected α-particles is correspondingly Nscatt = N0 (1 − e−nσL ). The product nσ = α is also denoted as the scattering coefficient and σ as the total interaction cross section.
where NA = (6.022045 ± 0.000005) × 1023 mole−1 . Example One mole of the carbon isotope
12 C 6
is 12 grams.
Yuyiu Lam
Atomic Physics
Atom Nucleus
Introduction Properties
Yuyiu Lam Atomic Physics
Atom Nucleus
Introduction Properties
Bragg diffraction
W. L. Bragg and W. H. Bragg in 1913 discovered that crystalline solids produced surprising patterns of reflected X-rays. They found that these crystals, at certain specific wavelengths and incident angles, produced intense peaks of reflected radiation (Bragg peaks). In W. L. Bragg’s model, the crystal is considered as a set of discrete parallel planes separated by a constant parameter d. The incident X-ray would produce a Bragg peak if their reflections off the various planes interfered constructively.2
Technology
Quantum electronics: Lasers, navigation, geodetics Medical technology: Radiation effects Communications technology: Laser techniques, ionosphere
Atomic Physics
Atom Nucleus
Outline
1
Atom Introduction Properties
2
Nucleus Scattering
Yuyiu Lam
Atomic Physics
Atom Nucleus
Introduction Properties
Atomic physics
Atom Nucleus
Atomic Physics
Yuyiu Lam
Department of Physics, Jinan University Guangzhou 510632 China m@ Version 1.0
September 8, 2010
Yuyiu Lam
Atom Nucleus
Introduction Properties
Cross section
To determine the size of an atom in a gas, we may measure the interaction cross section (scattering cross section) between the collisions of atoms. Suppose we use a beam of α-particles for the gold foil experiment. Let r1 , r2 be the radii of α and gold atoms, ∆x and L be the variable of thickness and the thickness of the gold leaf, n be the number density of the gold foil, then the probability of a collision P = Area of all the interaction cross sections in the volume of the beam Total area A
Applications
Determination of units: Fundamental constants Space research: Earth and planetary atmospheres, weather Environment: Detection of pollutants Energy problems: New methods of energy production
Yuyiu Lam
Atomic Physics
Atom Nucleus
Introduction Properties
Avogadro number
One mole of a substance is as many grams as the relative atomic weight. One mole of any substance contains the same number NA (Avogadro number) of atoms or molecules. Therefore, Mass of an atom = Mass of 1 mole of substance NA
Rutherford then proposed a planetary model in which a cloud of electrons surrounded a small, compact nucleus of positive charge.
Yuyiu Lam Atomic Physics
Yuyiu Lam Atomic Physics
Atom Nucleus
Introduction Properties
Atom
Atom, the smallest unchangeable component of a chemical element, is a fundamental building block of matter, consisting of Electron is an elementary particle, assuming with a point-like structure, carrying negative electric charge −e. Nucleus is composed of neutrons and protons, which are not elementary particles and in general called nucleons. A proton carries positive electric charge with magnitude e, and a neutron is electrically neutral. Quark Proton and neutron are in fact made up of quarks which are elementary particles of matter. Each type of quarks carries electric charge with magnitude e/3.
Yuyiu Lam
Atomic Physics
Atom Nucleus
Introduction Properties
Atomic physics for:
Physics
Solid state physics: Ideal and defect structures Chemical physics: Formation of molecules, chemical reactions Astrophysics: Atomic spectroscopy Plasma physics: Excitation mechanisms Biophysics: Complex molecular structures Geophysics: Earth’s magnetic fields