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Announcements

Announcements.

vera-giles
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Announcements

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  1. Announcements Friday is entirely devoted to Q&A. Come armed with questions !Exam Monday: ~23-25 questions, short answer, or fill in the blank.I will let you drop any 4. You must choose which ones to drop.Pick up question sheet and answer the questions…these thingsare what I want you to know for the exam. HW solutions for today’s HW will be posted in the library tomorrow morning. You must turn in your HW by then to receive any credit.

  2. Particle Accelerators & Detectors A Detector at Fermilab Fermilab

  3. e e e e Accelerating Charges +3000 V +1000 V +2000 V 0 V • After this, the electron has an energy of … 3000 [eV]. • So, to get to 5 MeV, for example, we would need 5 million volts !!! • This is highly impractical ! • So what do we do?

  4. e e e e -1000 V -1000 V +1000 V +1000 V +1000 V +1000 V -1000 V -1000 V +1000 V +1000 V -1000 V -1000 V +1000 V +1000 V -1000 V -1000 V Linear Accelerator • The voltage is “switched” back & forth at just the right timeso that the electron is always accelerated toward the next plate ! • Note, we only need 1000 V or so, not 1 million volts. If it passesthrough 1000 of these plates, it will have gained 1000 times theenergy of a single pair ! • In this way, we can accelerate an electron to high energy.

  5. e e Linear Accelerator Could be ~1 km, easily ! Vroomm! Accelerating plates

  6. e e -1000 V +1000 V +1000 V -1000 V Circular Accelerator Electron “ramps up”to full energy over manyturns! Accelerating cavities(many of them !!!) Circular Accelerator CESRCornell Electron Storage Ring Linear Accelerator

  7. Circular Accelerator • Particles go round & round.With each turn, they gainmore & more energy becauseof the accelerating cavities. • Particles are kept in a circle by powerful magnetswhich bend their direction !!! • The magnets “bending power”has to increase as the particlesenergy increases (big challenge).. • The energy limit is restrictedby our ability to keep them goingin a circle… Circular Accelerator CESRCornell Electron Storage Ring

  8. Particle Acceleration • We are able to accelerate electrons because they have electriccharge, and are attracted to a “plate” which has a high POSITIVEvoltage. • Using similar principles, we can also accelerate positively charged particles, like protons. • You just need to flip the positive & negative voltages !!!. • We can therefore also accelerate positrons (positively chargedelectrons). All the voltages are just reversed !! • So, we only know how to accelrate things which have electriccharge ! • How do we create positrons?

  9. e Creating Antiparticles e - this way target e+ e- e- Photonsare unaffectedby a magnet e+ e- e+ this way Using magnets, the negative electrons can be bent one wayand the positrons bent the other way, thus “separating” them fromeach other ! Once separated, the positrons can be “focused” and accelerated !

  10. Circular Accelerator positrons CESRCornell Electron Storage RingE ~ 5 [MeV]per beam Because electrons and positrons have the samemass, but opposite charge,they can both be accelerated in the same circularaccelerator !! electrons

  11. Collision !!!!! Around the collisionpoint, we build adetector to detect theparticles coming out.Using these detectors, we measure: 1. Momentum2. Type of particle3. Charge+ many other quantities Collisionpoint! CLEO positrons CESRCornell Electron Storage RingE ~ 5 [MeV]per beam electrons

  12. e+ e- p p0 p+ p0 Boom p p - p0 Detector There are several concentric layers to this detector. Each layer serves a specific function: 1. Tracking – map out the flight path of the particle 2. Calorimeter – measure the energy of photons 3. Particle identification: detectors capable of distinguishing pions from protons from kaons, etc (I won’t cover this)…

  13. As charged particle passes through gas, it ionizes the gas.This creates “free” electrons which are attracted toward the 1500 V wireGenerates a voltage pulse !You know thatthis wire “saw”a charged particle ! Tracking ~1 mm sep. Box filled with gas,perhaps Argon. A charged particle, likea proton, or p+ Wires at+1500 V

  14. x x x x x x x x x x x x x x x x x x x x “Reconstructed” Trajectory using“pattern recognition” programsi.e., find the patterns !! Tracking Continued wires If you have many layers you can see the tracks by looking at the wires which were “hit”10 layers shown here “Hit” wires Particles with oppositecharge will bend inopposite directions !The two particles here have opposite charge!

  15. Shower Calorimeters measure energyby converting nearly all the photons energy into either electrons or “flashes of light”which can be detected Calorimetry Useful in detecting photons(electrons also) ! Side view of calorimeter High density material like LEAD !interspersed with detectors

  16. Detecting Particles • Tracking: Detectors are inside a HUGE magnet. Measure charge by the direction the particle curves Measure momentum by how much it curves. If it curves alot  low momentum If it only curves a little  high momentum • Calorimetry Measure energy of photons, electrons also! • Particle Identification: I skipped this… Allows you to tell what kind of particle it is…

  17. Actual e+e- Collision at Cornell’s Collider E ~ 5 [GeV] forthe colliding e+ and e- Hadrons which are chargedand are “bent”by a magneticfield Side view ofDetector

  18. VIDEO(VCR)

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