Nuclear and Modern Physics The Baker Event of Operation Crossroads was a 21 kiloton underwater nuclear weapons effects test conducted at Bikini Atoll (1946). Note the ships that are visible in the photo. Source - http://chemistry.about.com/od/photogalleries/ig/Nuclear-Tests-Photo-Gallery/Baker-Event---Crossroads.htm
Back in 1895, Wilhelm Roentgen discovered a kind of ray that could travel through solid wood or flesh and yield photographs of living people's bones.
Only a few months later, in 1896, a scientist known as Henri Becquerel accidentally discovered that uranium samples could emit radiation.
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Becquerel observed that uranium emitted its rays whether the uranium was solid or pulverized, pure or in a compound, wet or dry, or whether exposed to light or heat.
Becquerel also concluded that as the percentage of uranium in a mineral increased, the intensity of the emitted radiation increased as well.
Using an instrument developed by Pierre Curie and his older brother Jacques, Marie Curie made careful measurements of uranium samples.
She patiently confirmed Becquerel’s observations and conclusions.
She hypothesized that the emission of rays by uranium could be related to the structure of the uranium atom.
Combined with Thomson’s discovery, Curie’s hypothesis led scientists to look for an underlying structure to the “uncuttable” atom.
Marie Curie studied all of the known elements to see if any of them were radioactive.
To describe the behavior of uranium and thorium she invented the word RADIOACTIVITY (based on the Latin word for ray).
Pierre Curie became intrigued. Together, they worked tenaciously to isolate other radioactive elements.
By passing radioactive rays between charged plates, Rutherford determined that there were different kinds of radiation.
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Later, a radiation was discovered that did not move toward either plate. This was called GAMMA radiation.
In 1909, Rutherford decided to test Thomson’s Plum Pudding Model. He set up the classic GOLD FOIL experiment.
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Rutherford expected most of the ALPHA radiation to go through the gold foil with only a tiny amount of deflection (caused by electrons).
Instead, he found that although most ALPHA radiation went straight through the gold foil, some was deflected straight back!!!
Rutherford eventually concluded that the atom was: • Mostly empty space AND • Had a dense + nucleus
Nuclear Equations In all cases, nuclear equations are written by conserving mass (to the precision used) and charge.
_____ Decay _ 238 234 92U 90Th + ___ Notice that with nuclear changes, new elements (like Thand ___) are formed!!!
Alpha Decay 4 238 234 92U 90Th + 2He Notice that with nuclear changes, new elements (like Thand He) are formed!!!
Alpha Decay 238 234 4 92U 90Th + 2He Because alpha particles are massive, they can be stopped by paper or even skin. But don’t eat an alpha emitter!
Beta Decay 14 _ 0 6C ___+ -1e An electron is released by an unstable nucleusin beta decay.
Beta Decay 14 14 0 6C 7N + -1e Beta particles can pass through paper, but are stopped by thin pieces of wood or metal.
Positron Emission 8 8 0 5B 4Be + +1e An electron’s anti-particle is released by an unstable nucleus in positron emission.
Half-Life Is the time required for exactly one-half of a sample of a radioactive isotope to decay.
Half-Life t / T Fraction Remaining = (1/2) T = The half-life of an isotope t = The time which has passed
Half-Life The fact that half-lives are so consistent allows us to date the origin of materials quite accurately.
Chapter 27 Early Quantum Theory and Models of the Atom
Units of Chapter 27 • Discovery and Properties of the Electron • Planck’s Quantum Hypothesis; Blackbody Radiation • Photon Theory of Light and the Photoelectric Effect • Energy, Mass, and Momentum of a Photon • Compton Effect • Photon Interactions; Pair Production
Units of Chapter 27 • Wave-Particle Duality; the Principle of Complementarity • Wave Nature of Matter • Electron Microscopes • Early Models of the Atom • Atomic Spectra: Key to the Structure of the Atom • The Bohr Model • de Broglie’s Hypothesis Applied to Atoms
27.1 Discovery and Properties of the Electron In the late 19th century, discharge tubes were made that emitted “cathode rays.”
27.1 Discovery and Properties of the Electron It was found that these rays could be deflected by electric or magnetic fields.
27.1 Discovery and Properties of the Electron By accelerating the rays through a known potential and then measuring the radius of their path in a known magnetic field, the charge to mass ratio could be measured: (27-1) The result is