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 things
are 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.
Particle Accelerators & Detectors
e e e e
0V +1000 V +2000 V +3000 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?
e e e e
The voltage is “switched” back & forth at just the right time
so that the electron is always accelerated toward the next plate !
Note, we only need 1000 V or so, not 1 million volts. If it passes
through 1000 of these plates, it will have gained 1000 times the
energy of a single pair !
In this way, we can accelerate an electron to high energy.
Could be ~1 km, easily !
Electron “ramps up”
to full energy over many
turns! Accelerating cavities
(many of them !!!)
Particles go round & round.
With each turn, they gain
more & more energy because
of the accelerating cavities.
Circular Accelerator Particles are kept in a
circle by powerful magnets
CESR which bend their direction !!!
Storage Ring The magnets “bending power”
has to increase as the particles
energy increases (big challenge)..
The energy limit is restricted
by our ability to keep them going
in a circle…
We are able to accelerate electrons because they have electric
charge, and are attracted to a “plate” which has a high POSITIVE
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 charged
electrons). All the voltages are just reversed !!
So, we only know how to accelrate things which have electric
How do we create positrons?
e - this way
e+ are unaffected
by a magnet
e+ this way
Using magnets, the negative electrons can be bent one way
and the positrons bent the other way, thus “separating” them from
each other !
Once separated, the positrons can be “focused” and accelerated !
E ~ 5 [MeV]
Because electrons and
positrons have the same
electrons mass, but opposite charge,
they can both be accelerated
in the same circular
Around the collision
point, we build a Collision
detector to detect the point!
Using these CESR
detectors, we Cornell Electron
measure: Storage Ring
E ~ 5 [MeV]
1. Momentum per beam
2. Type of particle
+ many other
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)…
As charged particle
passes through gas,
it ionizes the gas.
This creates “free”
electrons which are
attracted toward the
1500 V wire
~1 mm sep. Generates a
voltage pulse !
You know that
this wire “saw”
a charged particle !
Box filled with gas,
perhaps Argon. A charged particle, like
Wires at a proton, or p+
x x If you have many
x x layers you can see
x x the tracks by looking
at the wires which
x x were “hit”
x x 10 layers shown here
Particles with opposite
“Reconstructed” Trajectory using charge will bend in
“pattern recognition” programs opposite directions !
i.e., find the patterns !! The two particles here
have opposite charge!
Useful in detecting photons
(electrons also) !
Side view of calorimeter
Calorimeters measure energy
by converting nearly all the
photons energy into either
electrons or “flashes of light”
which can be detected
High density material like LEAD !
interspersed with detectors
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
Measure energy of photons, electrons also!
Particle Identification: I skipped this…
Allows you to tell what kind of particle it is…
Actual e+e- Collision at Cornell’s Collider
e e qq hadrons
E ~ 5 [GeV] for
e+ and e-
and are “bent”
by a magnetic
Side view of