Seeing the Subatomic Stephen Miller Saturday Morning Physics October 11, 2003

Particles Atom Electrons, Protons, Neutrons Photons Light Electricity & magnetism Particles emitted as Radiation in Radioactive Decay

A Photon Detector Your 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

Radioactivity & Radiation Radioactive nuclei decay Different kinds of radiation emitted “Alpha” particle = 2protons+2neutrons “Beta” particle = electron “Gamma” ray = high energy photon Neutrons

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

An Electron Detector Your T.V. Screen Electron beam causes phosphorescent screen to emit light Thomson discovers electron (1897)

Electrons in Atoms Electrons can occupy fixed energy levels Electron absorbing photon gets boosted to higher level Electron can fall to lower level by emitting a photon (assuming space is available)

Phosphorescence Phosphorescence occurs when electrons changing energy levels emit visible light (Same as fluorescent light) Electron gets boosted to a higher level by collision with Particle – electron, alpha, or gamma ray

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

Ionization Particle knocks electron away from atom Atom now has net positive charge (ion)

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

Another Look at Radioactivity Rate of counts – radioactivity of source Penetration of radiation

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

Cosmic Rays Radiation from outer space creates shower of particles Discovered 1912 by Victor Hess

Cloud Chamber Discoveries Cosmic ray studies Anti-matter Strange particles Heavy electrons (muons)

Anti-Matter Positively charged “electron” called positron Discovered 1932 photon → electron+positron “pair creation” electron + positron → photons “annihilation” E=mc 2

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

Scale of Matter Smallest known constituents of matter are quarks and electrons Radioactivity Cosmic Ray Studies Particle accelerators Higher Energy Particles Smaller distance scales

Elementary Particles Zoo of Particles Combination of quarks and Anti-quarks New kinds of quarks discovered in collisions created at particle accelerators

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

Drift Chamber Connect the Dots to find tracks

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

Detecting Particles Measure Momentum, Energy, Pattern of energy Computer program used to find patterns

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

CDF Detector Collaboration of about 500 physicists including U of M physicists – like me.

CDF Detector

Atlas Experiment Currently being built at CERN (Europe) U of M building muon detectors Drift tubes similar to Geiger counter

Fermilab Accelerator Accelerators collide matter and anti-matter CDF and D0 detectors measure results of collision

Collisions Look for new kinds of particles Matter & Anti-Matter annihilate Energy 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 proton Decays in a billionth trillionth of a second into electrons and lighter quarks

Event

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 3 rd Saturday morning Physics lecture – Oct 25

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