seeing the subatomic stephen miller saturday morning physics october 11, 2003
TRANSCRIPT
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Seeing the Subatomic
Stephen MillerSaturday Morning Physics
October 11, 2003
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Particles
AtomElectrons, Protons, Neutrons
PhotonsLight Electricity & magnetism
Particles emitted as Radiation in Radioactive Decay
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A Photon DetectorYour Eye
• Eye measures– Energy of photon = Color – Number of photons =
Brightness– Position = Spot on retina– Time - see objects moving
• Image processing in brain
• Photon→Chemical Reaction →Electrical Pulse
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Radioactivity & Radiation
Radioactive nuclei decay
Different kinds of radiation emitted
“Alpha” particle = 2protons+2neutrons
“Beta” particle = electron
“Gamma” ray = high energy photon
Neutrons
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First Radiation Detector
Ruined photographic plates
Plates stored in drawer next to uranium
Even without exposure to light, plates were fogged
Becquerel discovers radioactivity 1896
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An Electron DetectorYour T.V. Screen
Electron beam causes phosphorescent screen to emit light
Thomson discovers electron(1897)
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Electrons in Atoms
Electrons can occupy fixed energy levels
Electron absorbing photongets boosted to higher level
Electron can fall to lower level by emitting a photon (assuming space is available)
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Phosphorescence
Phosphorescence occurs when electrons changing energy levels emit visible light (Same as fluorescent light)Electron gets boosted to a higher level by collision withParticle – electron, alpha, or gamma ray
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Scintillator
• Modern detectors uses scintillating plastic or crystal
• Same principal of particle energy conversion to visible light
• Use Phototube to convert light to electrical signal
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IonizationParticle knocks electron away from atom
Atom now has net positive charge (ion)
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Geiger Counter
• Ionization– Electrons separated from atoms
• Electric field accelerates electrons– High voltage wire attracts
electrons
• Accelerated electrons ionize others
• Avalanche of charge created• Pulse of electrons hits wire and
makes electrical pulse
• Disadvantages:– Only counts events – No measurement of energy or
direction
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Another Look at Radioactivity
• Rate of counts – radioactivity of source
• Penetration of radiation
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Cloud Chamber
• Gas vapor in saturated state• Radiation ionizes atoms• Condensation forms along
path of ionization
• Shows direction of particle motion
• Shows paths of multiple particles
• Invented 1911
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Cosmic Rays
Radiation from outer spacecreates shower of particlesDiscovered 1912 by Victor Hess
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Cloud Chamber Discoveries
Cosmic ray studies Anti-matter
Strange particles
Heavy electrons (muons)
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Anti-Matter
Positively charged “electron” called positron
Discovered 1932
photon → electron+positron “pair creation”
electron + positron → photons “annihilation”
E=mc2
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Bubble Chamber
• Boiling forms along path of ionization
• Analyze photograph of bubbles • Disadvantages:
– Cannot handle high rate of events
– Data analysis time consuming
• Curvature of particles due to externally applied magnetic field
• A whole “zoo” of particles discovered
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Scale of Matter
Smallest known constituents of matter are quarks and electrons
Radioactivity
Cosmic Ray Studies
Particle accelerators
Higher Energy Particles Smaller distance scales
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Elementary Particles
Zoo of Particles
Combination of quarks andAnti-quarks
New kinds of quarks discoveredin collisions created at particle accelerators
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Drift Chamber
• Ionization detector– Similar to Geiger counter– Thousands of sense wires
• Gives precise trajectory of particle
• Operates inside a magnetic field– curvature of the particle paths
depends on their momentum
• Instrumented for electronic readout
• Measure thousands of events/second
• Only measures charged particles
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Drift Chamber
Connect the Dotsto find tracks
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Energy Measurement
• Measure energy of particle • Essential for detecting Neutral
particles – neutrons don’t ionize gas in
drift chamber • Use scintillating plastic
(sandwiched between lead plates) to produce light
• The brightness of light produced in the plastic is proportional to the energy
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Detecting Particles
Measure Momentum, Energy, Pattern of energy
Computer program used to find patterns
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Integrated Detector
• Combine different kinds of detectors
• Usually cylindrical in shape• Inner layer is drift chamber• Followed by calorimeters• Outer layer of muon detectors
• Particle accelerator used to create new particles at center of detector
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CDF Detector
Collaboration of about 500 physicists including U of M physicists – like me.
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CDF Detector
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Atlas Experiment
• Currently being built at CERN (Europe)
• U of M building muon detectors
• Drift tubes similar to Geiger counter
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Fermilab Accelerator
Accelerators collide matter and anti-matter
CDF and D0 detectors measure results of collision
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Collisions
Look for new kinds of particles
Matter & Anti-Matter annihilateEnergy converts back into new kinds of particles
Massive Particles decay in a shower of lighter particles
Top and anti-Top quarks 170x more massive than protonDecays in a billionth trillionth of a second into electrons and lighter quarks
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Event
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Other Questions
• How does one measure top quarks since they decay before entering the detector?– Answer given at next Saturday morning Physics lecture – Oct 18
• Besides cosmic rays which go through our bodies at a rate of 1/second, there billions of other particles going through our body each second. What are they? How do we know they are there?– Answer given at the 3rd Saturday morning Physics lecture – Oct
25
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What to Remember
• Subatomic particles lose energy when colliding with electrons in atoms
• To “see” the particles simply convert this energy to another form
• Scintillation/Phosphorescence – energy converted to visible light
• Ionization – energy converted to electric pulse
• High energy particles require big detectors to capture all the energy