Nuclear and Atomic Physics

1. Nuclear and Atomic Physics Intro: Atomic Structure and a little history.  Print off the topic outline for this unit by tomorrow!

2. Early atomic structure… • JJ Thomson: “Plum pudding” model • An atom is a mixture of positive and negative charges Image from: http://abyss.uoregon.edu/~js/21st_century_science/lectures/lec11.html

3. Atomic Structure • 1909: Rutherford—Worked with Geiger and Marsden on the gold foil experiment • A very thin gold foil was placed in the center of a chamber that had photo detecting material on its inner surface. • Alpha particles (essentially a helium nucleus) were shot toward the gold foil • The particles were detected on the inner surface of the chamber, and any scattering was noted

4. Image from: http://bhs.smuhsd.org/science-dept/marcan/apchemistry/structure_ML_MD.html

5. Gold foil experiment… • Results: • Most alpha particles were detected at very small scattering angles (essentially went through the foil, but were deflected) • Some larger-angle scattering occurred, sometimes large enough that the alpha particle seemed to reflect back nearly on its original path. • Conclusion: • Atoms have a dense, positively charged center (the nucleus) and the electrons must be in the space surrounding the nucleus

6. Rutherford’s Atom… • “Planetary model”: Image from: http://abyss.uoregon.edu/~js/ast123/lectures/lec04.html

7. Planetary Model • Massive, positively charged nucleus • Electrons orbited much like planets around the Sun • The Coulombic (electrostatic) force of attraction between the positive protons and the negative electrons kept the electrons in orbit

8. Planetary Model—problems… • According to a theory of electromagnetism, accelerating charges will emit energy in the form of electromagnetic radiation • Radiating energy would cause the electron to have a lower total energy and therefore would cause it to orbit a smaller distance from the nucleus… • Electron would spiral in and crash into the nucleus

9. Neils Bohr—atomic postulates • Studied Hydrogen atom • Determined that there are certain defined energy states in which the electron can exist • In one of these states, the electron will not radiate its energy and will remain in a stable orbit • Energy can only be lost if the electron transitions into a state of lower energy

10. Emission and Absorption Spectra • All elements will emit light in characteristic colors when heated • Scottish physicist Thomas Melville—first to study emitted light (1726-1753) • Heat source = flame • Passed emitted light through prism • Pattern produced was significantly different than white light passed through spectrum • Bright line spectrum

11. Emission and Absorption spectra give the same “fingerprint” for an element, but in different ways

12. Alpha particles directed at a thin gold foil will most likely... • pass directly through the foil with no deflection • be reflected straight back from the solid foil • pass through the foil wil a small amount of deflection • be deflected at large angles as it passes through the foil

13. Large deflections of alpha particles in Geiger and Marsden's scattering experiment suggested... • atoms consist of a small negative nucleus surrounded by protons • atoms consist of a small positive nucleus surrounded by electrons • atoms consist of a small neutral nucleus surrounded by electrons and protons • atoms consist of a large positive mass with embedded electrons

14. The Rutherford model of the atom has the positive charge... • spread uniformly throughout the atom's volume • circling the nucleus as positive electrons • concentrated in a central nucleus • none of the above

15. Isotopes • Nuclear isotopes (a.k.a nuclides) have specific nuclear notation: Z = atomic number (= # protons) A = mass number ( Nucleon number) (= #protons + # neutrons) X = chemical symbol of the element

16. Isotopes • Most elements have more than one isotope (although not always a stable one!) • Isotopes are atoms of the SAME ELEMENT with DIFFERENT numbers of NEUTRONS • Atomic number is ALWAYS the same for any isotope—only the mass number (nucleon number) changes • Evidence for neutrons using isotopes: there is no other way to logically explain the difference in mass for various atoms of a particular element.

17. Isotope Practice:

18. Nuclear Interactions • Fundamental Forces

19. Fundamental Forces… • Evidence: • Strong nuclear force: • protons do stay together in stable nuclei, even though the electromagnetic forces between them would suggest they would repel • Weak nuclear force: • evidence suggested during beta decay (where a neutron disintegrates into a proton and an electron…)

20. Radioactive Decay • Discovered in 1896 by Antoine Henri Becquerel • Inspired by discovery of X-rays, wanted to know connection between those and fluorescent or phosphorescent materials • Experiment: • Photographic paper wrapped in black paper to keep out light… • Salt samples (such as Uranium) placed on the covered paper • Also exposed the wrapped paper to sunlight for several hours…

21. Results: • Photographic plate was NOT exposed due to the sunlight • Outlines of the uranium sample clearly visible on plate: • THEN manipulated: • Temperature • Amount of light • Other physical and chemical changes • NO EFFECT! • Ionizing Radiation—because rays could ionize gas molecules

22. Radioactive Decay • Marie Curie (and husband, Pierre)—followed Becquerel’s experiments to look for other substances with the same properties as Uranium… • Isolated Thorium (~1898) • Discovered Radium and Polonium…won Nobel Prize in Chemistry (1903) • 1899—Rutherford discovered that Uranium emits 2 kinds of radiation (“alpha and beta rays”) • 1900—gamma rays discovered as a 3rd type of radiation by Paul Villard

23. Types of Radiation • Ionizing power: the ability of radiation to knock electrons out of orbit when they collide with another atom. • Alpha particles (a) • Helium nucleus • Charge = +2e (the same as 2 protons) • Mass = 4u (1u = mass of a nucleon) • Type of energy: all kinetic • velocity ~ 0.05c • Penetration: stopped by a sheet of paper • Range: a few centimeters • Ionizing power—largest of the 3 types of radiation…very dangerous if ingested!

24. Beta Particles (b) • Very fast moving electron—emitted from the nucleus • Charge = -1e • Mass = 1/1850 u • Energy = all kinetic • (velocity up to 99% speed of light) • Penetration: will be stopped by a few mm of aluminum • Range: a few meters through air • More penetrating than Alpha particles, but less ionizing.

25. Gamma Rays (g) • High energy electromagnetic radiation • (very high frequency, very short wavelength) • Charge = neutral • Mass = 0 • Energy: Photon Energy (proportional to the frequency of the ray) • Velocity = speed of light (c) • Penetration: can be stopped by several cm of lead or by a meter or more of concrete • Range: there is no maximum range • Lowest ionizing power of the 3 types of radiation

26. Particles can be identified based on how they interact with a magnetic field: • Alpha particles will curve slightly • Beta particles will be deflected significantly, and in the opposite direction from alpha • Gamma rays—no charge, so no deflection at all