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Unformatted text preview: Recap Astronomical Observations Recap Astronomical Observations What can be observed and measured through the optical telescopes ? Why is a shirt red ? Why is it black ? Why is the sky blue ? (explain in terms of reflection/refraction) What is the origin of the black lines in a star’s light spectrum ? Explain the meaning of a redshifted star spectrum. What is an interferometer ? Describe nonoptical astronomy. Describe fixed, movable radiotelescopes. Describe satellitebased detectors. Describe gravitational detectors. Observing the MicroUniverse Observing the MicroUniverse The microscopes Observing atoms and beyond Particleantiparticle collisions Particle accelerators Detectors The Microscope The Microscope A microscope is equivalent to an optical telescope plus the system to illuminate the observed objects. With a microscope one can observe very small objects, down to the size of the light wavelength (thousands of atomic units). Electronic Microscope Electronic Microscope Probability waves Electrons and all other particles in the microuniverse move with a certain speed and behave like waves because of the uncertainty principle in the quantum world. Louis de Broglie showed in 1926 how to calculate particles probability waves. Their size is inverse proportional to their speed. An electronic microscope uses very fast electrons instead of visible light and can therefore see deeper inside matter. Observing the structure of molecules
X rays or electrons Fluorescent Screens One slit experiment: - one maximum Double slit experiments: pattern of dark and light bands – They are the result of the interference of the associated waves X rays or electrons Waves interference Diffraction of X rays or electrons can produce a clear map of the atoms in a molecule or in a crystal. Observing the Structure of Observing the Structure of Atoms Observing atoms requires probability waves of the atomic size. The first experiment which observed the inside of atoms was performed in 1911 by E. Rutherford. He bombarded a metal target with alpha particles. From the distribution of the scattered particles he was able to see that atoms have a heavy positive nucleus (many particles did not change their trajectory, but a few were bounced back). He was the first to propose a planetary model for the atom: a heavy positive nucleus with electrons moving around like planets around the Sun. Observing Subnuclear Structures Observing Subnuclear Structures Observing the structure of matter at nuclear or subnuclear scale is similar to Rutherford’s experiment except that : The smaller the target the higher the energy of the projectiles accelerators are needed. The higher the projectile energy the smaller its probability wave The higher the impact energy the more complex impact phenomena will be. Instead of a simple scattering process the target can break into pieces; the interpretation of the whole process becomes more complicated and computerized detectors are needed. Creating new particles Creating new particles During collisions at high energies the projectile energy can be used to create new pairs of particles and antiparticles The preferred experiment for producing new particles is the use of a supercollider accelerating in separate rings particles and their antiparticles. An antiparticle is identical with a particle except its electric charge (ex. positron/electron, antiproton/proton). When a particle collides with an antiparticle both are annihilated and in their place nature creates gamma rays. The energy of these gamma rays is the sum of the particles kinetic energies plus the energy equivalent of their masses. Nature will promptly use the annihilation gamma rays to produce particleantiparticle pairs. It will produce all possible pairs with massenergy smaller or equal to the annihilation energy. Theory and experiment will compare the probabilities with which these pairs are created. Types of Accelerators Types of Accelerators Types of projectiles: electrons, protons and their antiparticles Types of accelerator : linear or circular. Advantages and disadvantages. The Supercollider is a double circular accelerator Energies measured in GeV (billions of electronvolts) Examples of Particle Examples of Particle Accelerators
Linear accelerator of electrons •Stanford (SLAC) 22 GeV Supercolliders p-p •Brookhaven 400 GeV •CERN (ISR) 31 GeV Proton Synchrotrons (circular) p-p •Serpuhovo 3,000 GeV •Fermi 1,000 GeV •Fermi (DSPS) 1,000 GeV •CERN 7,000 GeV •Fermi (SPS) •CERN (SPS) 450 GeV 450 GeV e--e+
•CERN (LEP) 200 GeV •Hamburg(PETRA) 20 GeV •Stanford(SPEAR) 4.5 GeV Particle Accelerators at CERN Particle Accelerators at CERN Particles Detectors Particles Detectors The analysis of all particles produced in a collision is even more complicated due to the fact that an experiment has to monitor many simultaneous collisions. Only computerized detectors can do this job. One of the most famous detectors is Gargamelle, built in 1976 at CERN. This bubble chamber contains 1000 tones of liquid freon kept under pressure at a temperature just over the boiling point. Lowering the pressure one obtains a state when charged particles created bubbles of vapors. For each short flux of incident particles a pump lowers the pressure and cameras take pictures from all angles. The flux of incident particles, the pump and the cameras have to work in perfect synchronization. The pictures are then analyzed by sophisticated computer programs. ...
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This note was uploaded on 05/03/2011 for the course NATS 1740 taught by Professor Hall during the Spring '10 term at York University.
- Spring '10