# Particle Accelerators & Detectors

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Particle Accelerators & Detectors

A Detector
at Fermilab
Fermilab
Accelerating Charges

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?
Linear Accelerator

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
+1000V
-1000 V
-1000 V
+1000 V

 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.
Linear Accelerator

Could be ~1 km, easily !
e                               e

Vroomm!
Accelerating plates
Circular Accelerator
Electron “ramps up”
to full energy over many
turns!                                          Accelerating cavities
(many of them !!!)

+1000 V
-1000 V
Circular Accelerator
CESR
Cornell Electron
Storage Ring
-1000 V
+1000V

e
Linear Accelerator
Circular Accelerator
 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 !!!
Cornell Electron
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…
Particle Acceleration
 We are able to accelerate electrons because they have electric
charge, and are attracted to a “plate” which has a high POSITIVE
voltage.

 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
charge !

 How do we create positrons?
Creating Antiparticles
e - this way

target      e+
e-
e                                   e-
Photons
e+                          are unaffected
e-
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 !
Circular Accelerator

positrons
CESR
Cornell Electron
Storage Ring
E ~ 5 [MeV]
per beam
Because electrons and
positrons have the same
electrons                       mass, but opposite charge,
they can both be accelerated
in the same circular
accelerator !!
Collision !!!!!
Around the collision
point, we build a     Collision
detector to detect the point!
particles coming
out.
positrons
Using these                             CESR
detectors, we                      Cornell Electron
measure:                            Storage Ring
E ~ 5 [MeV]
1. Momentum                           per beam
2. Type of particle
3. Charge
electrons
+ many other
quantities
Detector
p0
p
p+

p0
e+                                                                   e-
p
p-

p0

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)…
Tracking
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+
+1500 V
Tracking Continued
wires

x                                                               x    If you have many
x                                                       x        layers you can see
x                                               x
x                                       x                the tracks by looking
x                               x
x                       x
at the wires which
x               x                            were “hit”
x       x
x    x
x   x                                  10 layers shown here
“Hit” wires
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!
Calorimetry
Useful in detecting photons
(electrons also) !

Side view of calorimeter
Shower

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
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…
Actual e+e- Collision at Cornell’s Collider
e  e  qq  hadrons
   
E ~ 5 [GeV] for
the colliding
e+ and e-
which are
charged
and are “bent”
by a magnetic
field

Side view of
Detector
VIDEO
(VCR)

```
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