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Detectors and
Accelerators
How do you find these particles, anyways?
High Energy, Please.
We have seen that the electromagnetic interaction allows for the process where an
electron -positron pair create a new particle-antiparticle out of the vacuum.
e+
e-
High Energy, Please
To create this particle-antiparticle pair, energy is needed.
E=mc2 is the rest energy for each particle. This is the energy
needed to create the particle at rest.
If the particle and antiparticle each has a kinetic energy of EK after it is
created, then the total energy that must be supplied is
E =2(mc2 + EK)
The very minimum energy needed is
E = 2mc2
This energy must come from the rest energy and the kinetic energy of the electron and
positron before the collision.
Resolution
It is clear that if we wish to produce particle of large mass, then very large
amounts of kinetic energy are needed.
We also know that in order to ‘see’ something very small, the object
‘thrown’ at it, must have a wavelength of a comparable size. Otherwise,
the wave is not deflected; it just continues along its original path.
Since the constituents of the particles are assumed to be very small, the
wavelength used must be very small.
For example, if a photon is used to detect a particle
that has a radius of ~10-15m, it must have a energy of
Visible Light has E ~ 10’s eV
Gamma Radiation: E ~ MeV
6.6 ´10-34 ´ 3´10 8
E=
=
l
10 -15
E » 20 ´10 -11
hc
E » 10 9 eV
E » 1GeV
Accelerators
To get these high energies, particle accelerators are used. We will look at three types.
1) The linear accelerator
2) The cyclotron
3) The synchrotron
3) The synchrotron accelerates
the particles like the cyclotron
but they follow a path of
constant radius.
1) In the linear accelerator, particles are
accelerated along a straight path by
electric fields.
2) The cyclotron accelerates
particles through a potential
difference. Magnets are used to
curve the path of the particles back
to the gap. The path of the particles’
gets bigger and bigger.
Linacs
Linear Accelerators, or linacs, consist of a series of evacuated tubes.
An alternating voltage is applied across the gaps between consecutive tubes. The
charged particles accelerate every time they move from one tube to another.
The length of the tubes increases so that the particles spend an equal time in each tube.
The beam can be accelerated to hit a fixed target or two beams can
be accelerated along the same straight line from opposite ends and
made to collide with each other.
Cyclotron
The cyclotron consists of two D shapes, with a gap between them, placed in a
uniform magnetic field.
A charged particle is accelerated from the center. A magnetic field
pushes the particle back toward the gap RHR #3)
When the particle reaches the gap, the polarity of the voltage is switched so that the
particle accelerates through the potential difference.
Remember: you discovered that the period of the particle’s rotation is constant.
The radius increases but the velocity also increases proportionally.
qvB = mv2/r
T = 2πm/(qB)
Where v = 2πr/T
Cyclotron frequency: f = qB/(2πm)
The Synchrotron
Protons move along a
circular path of fixed
radius.
Accelerating region
+
_
A
B
electromagnet
Electric field
B
Acceleration takes place here
A
The positive particles will accelerate across the gap. The electric potentials in the
gaps must be carefully established by carefully timing the arrival of the beam at
every gap. The period of the electric field must be synchronous with the beam.
Fc = Fm
Mv2/R = qvB
R = mv/qB
R = _E_
qBc
As the particles move closer to the
speed of light, their energy
becomes
E2 = (pc)2 +(mc2)2
E2 = (pc)2
E = pc
(When the momentum is very large,
we may ignore its rest energy)
The magnetic field must be constantly increasing as the particle’s energy increases
(that’s why they’re electromagnets).
Storage Rings
Two oppositely charged beams can be made to move around the same ring in
opposite directions. This is called a storage ring.
The two beams could be made to collide at set places and at specific times.
CERN’s LEP (large electron-positron) collider had a circumference of
27 km and 100 m underground.
The W and Z bosons were discovered there.
The LHC has now replaced the dismantled LEP.
Radiation
We have seen that accelerating charges emit radiation (hence the need for the
Bohr and quantum models of the atom).
In the cyclotron and the synchrotron, the charged particles are
continuously accelerated by the magnetic fields.
There is a large amount of radiation
(energy) lost due to this acceleration.
This radiation is called: synchrotron
radiation.
Electrons have a much smaller mass than protons which means if they have
the same energy, the electrons move faster. The acceleration (v2/r) depends
on speed and therefore the electrons would radiate (and lose) a lot more
energy than the protons.
Most synchrotrons use protons.
Available Energy
Consider a target of mass M bombarded by a particle of rest mass ‘m’ and total energy E.
The energy available to create new particles in this case is given by the formula
EA2 = 2Mc2E + (Mc2)2 + (mc2)2
Insert waving hand here
This equation comes from special relativity.
EA is an invariant quantity like ‘c’: All frames measure this energy.
Available Energy
Ex.
p + π-  Λ0 + K0
p (938 MeV)
Λ0 (1100 MeV)
π- (140MeV)
K0 (500 MeV)
What is the minimum kinetic energy of the proton in order to produce
these particles in a collision with the stationary pion π-?
E is a min when all particles created are at rest.
We need 1100 MeV + 500 MeV = 1600 MeV.
EA2 = 2Mc2E + (Mc2)2 + (mc2)2
This is the EA.
Solve for E.
(1600)2 = 2(140)E+(140)2 +(938)2
E = 5930 MeV.
This is the total energy of the
proton: it is its rest energy
plus its kinetic energy.
KE = 4990 MeV
Comparing Accelerators
The antiproton was discovered in a collision of two protons, one of
the at rest, according to the reaction:
p + p  p + p +p + p
a. What is the minimum kinetic energy to
which the proton must be accelerated?
b. Compare this with the kinetic energy
needed in a storage ring synchrotron.
In the synchrotron, each proton is
accelerated to the same KE.
EA = 4 (938 MeV) = 3752 MeV
EA2 = 2Mc2E + (Mc2)2 + (mc2)2
37522 = 2(938)E +2 (938)2
E = 6566 MeV
KE = 5630 MeV
EA still equals 3752 MeV.
This comes from the total
energy of both protons. This
means each proton has 1876
MeV of energy.
KE = 1876 MeV – 938 MeV = 938 MeV
Comparing Accelerators
Accelerator
Advantages
Disavantages
Linac
Have lower energy losses
due to synchrotron
radiation
A very long accelerator is
required to reach very
high energies.
No control of collision
time.
Cyclotrons
Compact size, low cost
Good for biomedical
research
Only uses fixed target.
Limit to energy since it is
dependent on the
magnet size
Synchrotrons
Can accelerate particles
to very high energies
(massive particles can be
produced).
Collisions can be
controlled with storage
rings.
High proportion of
energy lost due to
synchrotron radiation.
Low probability of
collisions.
Detectors
Not only do we need to detect the particles created in the collisions, but there needs
to be ways of measuring their momentum, electric charge, mass, velocity and
energy.
Particle detectors around placed around the point of the collision and have a layered
structure. Each layer has a specific function in the detection process.
The tracking chamber is the layer closest to the collision point. It records the path
of the particles.
Bubble chambers were used originally. You know ALL about bubble chambers.
Detectors
Now the wire chamber or proportional wire chamber are used.
These chambers can digitized the information so a computer can reconstruct the
path of the particles.
The wire chamber has a number of wires immersed in a gas. The wires
are kept at different potentials.
The ions and electrons created by the charged
particle as it moves through the gas collect at
the wires.
Their arrival at a particular point on the wire is recorded as a current.
The electrons or ions take a certain time to drift to the nearest wire. This time is
recorded and used to calculate the precise location where the electron or ion was
created and its momentum .
Detectors
The next layer is the electromagnetic calorimeter.
When charged particles in a given medium move faster than light in
that medium, they give off radiation.
This radiation can be detected using the photoelectric effect.
The current of photoelectrons is very small so it is amplified with photomultipliers.
These detectors can detect the presence of a single photon!
Knowledge of the intensity of the radiation allows
for the determination of the speed of the charged
particle that caused the radiation.
Knowing the particle’s momentum and now its speed, we can calculate its mass.