Particle accelerators
Agen-689
Advances in Food Engineering
Accelerators
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Accelerators solve two problems for
physicists:
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Since all particles behave like waves, physicists
use accelerators to increase a particle's
momentum, thus decreasing its wavelength
enough that physicists can use it to poke inside
atoms.
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The energy of speedy particles is used to create
the massive particles that physicists want to study
How do accelerators work?
“ Basically,
an accelerator takes a
particle, speeds it up using
electromagnetic fields, and bashes the
particle into a target or other particles
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Surrounding the collision point are
detectors that record the many pieces of
the event.
How to obtain particles to
accelerate?
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Electrons: Heating a metal causes electrons to
be ejected.
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A television, like a cathode ray tube, uses this
mechanism.
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Protons: They can easily be obtained by
ionizing hydrogen.
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Antiparticles: To get antiparticles:
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first have energetic particles hit a target.
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Then pairs of particles and antiparticles will be
created via virtual photons or gluons.
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Magnetic fields can be used to separate them.
Accelerating particles
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Accelerators speed up charged particles by
creating large electric fields which attract or
repel the particles.
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This field is then moved down the
accelerator, "pushing" the particles along.
Accelerating particles
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In a linear accelerator the field
is due to traveling
electromagnetic (E-M) waves.
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When an E-M wave hits a
bunch of particles, those in
the back get the biggest
boost, while those in the
front get less of a boost.
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In this fashion, the particles
"ride" the front of the E-M
wave like a bunch of
surfers.
Accelerator design
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There are several different ways to design these
accelerators, each with its benefits and drawbacks.
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Fixed target: Shoot a particle at a fixed target.
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Colliding beams: Two beams of particles are
made to cross each other.
Accelerator design
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Accelerators are shaped in one of two ways:
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Linacs: Linear accelerators, in which the particle
starts at one end and comes out the other.
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Synchrotrons: Accelerators built in a circle, in
which the particle goes around and around and
around...
Fixed target experiment
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A charged particle such
as an electron or a proton
is accelerated by an
electric field and collides
with a target, which can
be a solid, liquid, or gas.
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A detector determines the
charge, momentum,
mass, etc. of the resulting
particles.
Fixed target experiment
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An example of this
process is Rutherford's
gold foil experiment, in
which the radioactive
source provided highenergy alpha particles,
which collided with the
fixed target of the gold
foil.
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The detector was the
zinc sulfide screen.
Colliding beam experiments
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Two beams of high-energy
particles are made to cross
each other.
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The advantage of this
arrangement is that both beams
have significant kinetic energy,
so a collision between them is
more likely to produce a higher
mass particle than would a
fixed-target collision (with the
one beam) at the same energy.
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Since we are dealing with
particles with a lot of
momentum, these particles
have short wavelengths and
make excellent probes.
Colliders
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Einstein's famous equation E=mc2
tells us that energy and mass are
equivalent.
Thus the energy of a particle beam
can convert into mass, creating a
fascinating wealth of additional
particles, many of them highly
unstable and not normally found in
nature.
However if the incoming beam is
simply slammed into a stationary
target, much of the projectile energy
is taken up by the target's recoil and
not exploitable.
Much more energy is available for
the production of new particles if two
beams traveling in opposite
directions are collided together.
How they work?
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something to accelerate
the particles,
something to bend
them,
something to focus
them,
a vacuum for them to
travel through
plus something to
house the whole lot
The basic principles
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All particle beams start from a particle source.
The simplest source is a hot wire, like the filament inside a light
bulb.
This is the kind of source used by television sets.
Negatively charged electrons boil off the wire, and accelerate in
a vacuum towards and through a positively charged electrode.
Electromagnetic fields then sweep the beam across the screen.
The points where the beam strikes the screen glow, building up
a picture.
A similar filament is also used in a linear electron accelerator
Linacs accelerate particles to much higher energies than a
television, but the principle is the same.
In a linac, particles accelerate from one electrode to the next,
gaining energy with each one they pass.
Television
Televisions use the same
principles as LINAC, but on a
much smaller scale.
“ Televisions and particle
accelerators have a lot in
common:
“ a particle source
“ accelerating electrodes
(televisions have one,
accelerators have many
more)
“ electromagnetic fields to
deflect the particles...
“ a particle detector (in a
television, this is the screen)
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Basic components
“ Accelerating
component
“ Bending component
“ Focusing components
“ The race track
The accelerating component:
The cavity
Charged particles receive
the energy needed to reach
a speed close to that of light
from sophisticated
accelerating cavities like the
one illustrated here.
“ These cavities store up
electrical energy, transferring
a small amount to the
particles each time they
pass.
“ They act like a short section
of linear accelerator.
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The bending component: The
dipole magnet
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Magnets called dipoles
are used to keep the
particles moving in a
circle.
Each time more energy
is pumped into the
particles, the magnetic
field has to be
increased to prevent
them from skidding off
the ring.
The focusing component: The
quadrupole and sextupole
Other magnets,
called quadrupoles
and sextupoles, are
used to keep the
particles tightly
packed within the
beam.
“ They work in much
the same way as
lenses do with light.
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The race track: The vacuum
chamber
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In particle accelerators,
to ensure that particles
are not lost by colliding
with molecules of air,
they travel inside a
pipe, from which all the
air has been removed.
Vacuum pumps all
around the ring ensure
that there is even less
matter inside the beam
pipe than there is in
outer space.
The Large Electron Positron
accelerator
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The LEP is a collider.
Its 3368 magnets bend two particle
beams and keep them on orbit.
Where negatively charged electrons
bend one way, positively charged
positrons bend the other.
This allows LEP to circulate 90 GeV
beams of electrons and positrons in
opposite directions using the same
magnets.
The Super Proton Synchrotron
(SPS), uses the same technique to
circulate protons in one direction
and anti-protons in the opposite
direction.
Charged particles accelerators
“ To
induce nuclear reactions with
positively charged particles (protons,
alpha)
“ Particles must have sufficient KE to
overcome the barrier created by the
repulsion between the positive charges
of the particles and the nucleus
Charged particles accelerators
To achieve higher KE the particles have to be
ionized
“ These ions can be accelerated through a
potential difference thus acquiring some
additional KE
“ To obtain the desired KE:
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Production of the charged particles
Acceleration thru the required potential difference
Ion source – the principle
is bombarded
by energetic
electrons
“ The atoms of the
gas are ionized
“ Positive ions are
produced
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A gas
H2 Gas
B1
Hot
Filament
cathode
B3
B2
e Beam
anode
S1
S2
vacuum
H+ Ions
Ion source – the principle
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H2 flows into region above
filament
Electrons are accelerated to
an anode (dV over B1-B2 =
100 V)
Electrons passage thru the
gas cause ionization
Positive ions are extracted
by attraction to a negative
electrode (dV over S1-S2 =
1-10 kV) into the accelerator
region
Vacuum at beam extraction
is 10-4 Pa, ionization area 102 Pa
H2 Gas
B1
Hot
Filament
cathode
B3
B2
e Beam
anode
S1
S2
vacuum
H+ Ions
Single-stage accelerators
Developed by Cockcroft-Walton - 1932
“ The total potential produced from a highvoltage generator is imposed across the
accelerator
“ Between the source and the target
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Single-stage accelerator
“ Principles
“ The
total potential produced from high
voltage generator is imposed between the
ion source and the target
“ The KE of the particle is:
Ekin = nqV
# stages =1
Potential across acceleration gap
Charge of accelerated ions, C
Single-stage accelerator
Recently, small versions of the Cockcroft-Watson accelerator
“ Transformer-rectifier accelerators
“ Used for acceleration of electrons or acceleration of deuterons
for production of neutrons:
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3
1
H + H → He + n
2
1
4
2
Tritium targets are bombarded by accelerated deuterons
“ Tunneling of the Coulomb barrier results in good yield for this
reaction (even for 0.1 MeV)
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Single-stage accelerators
D2 molecules leak thru
a heated palladium foil
into the vacuum of the
ion source
“ There high frequency
electric field
decomposed the D2
molecules to form D+1
ions and electrons
“ Ions are extracted with
low negative potential
to enter the
acceleration tube with
2.5 keV KE
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Accelerator
tube
D2 Gas
Concentric electrodes
target
Ion source
100 kV
+<3 kV
Particle path
magnet
Radio
frequency
+100 kV
vacuum
Cooling
water
Electron extractor
High voltage generator
Single-stage accelerators
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The 100 kV is obtained
from a transform and
rectifier unit coupled to a
set of cylindrical
electrodes connected by a
resistor chain
The beam particles exit
the last electrode and drift
thru a short tube and
strike the target (titanium
with absorbed tritium)
The target is cooled by
water to minimize tritium
evaporation
Accelerator
tube
D2 Gas
Concentric electrodes
target
Ion source
100 kV
+<3 kV
Particle path
magnet
Radio
frequency
+100 kV
vacuum
Cooling
water
Electron extractor
High voltage generator
Single-stage accelerators
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With 100 keV and 0.5 mA
This accelerator can
produce 1010 n/s with 14
MeV
Can reduce KE to thermal
values (0.025 eV) by
placing water or paraffin
around the target
Flux of thermal neutron =
108 n/cm2
Production rate of
neutrons increases as the
beam energy and beam
current increases
Accelerator
tube
D2 Gas
Concentric electrodes
target
Ion source
100 kV
+<3 kV
Particle path
magnet
Radio
frequency
+100 kV
vacuum
Cooling
water
Electron extractor
High voltage generator
Van de Graaf accelerators (VdG)
“ Developed
by van de Graaf in 1931
“ Can provide beams of higher energy
than the single-stage C-W accelerators
“ The tandem-VdG can produce 20 MeV
protons and 30 MeV α-particles
“ VdG can also accelerate electrons and
positive ions of higher Z
Van de Graaf accelerators (VdG)
A rapidly moving belt
accumulates positive charge
as it passes an array of
sharp spray points
“ Which transfer electrons
from the belt to the spray
points
“ The positive charge on the
belt is continuously
transferred by the movement
of the belt away from the
ground
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Steel tank
target
Insulating supports
pulley
Accelerating tube
++++++++++++++++++++++++++
+
-
E1
belt
Ion source
pulley A
E2
Removable lid
Van de Graaf accelerators (VdG)
At the high-voltage terminal,
(a hollow metal sphere)
another set of spray points
neutralize the charges on the
belt by electrons emitted
from the spray points
“ This results in positive
charge to the sphere
“ The continuous process of
transferring positive charge
to the sphere can built a high
potential on the sphere
“
Steel tank
target
Insulating supports
pulley
Accelerating tube
++++++++++++++++++++++++++
+
-
E1
belt
Ion source
pulley A
E2
Removable lid
Van de Graaf accelerators (VdG)
The limit of the voltage that
can be accumulated in the
hollow electrode is
determined by the discharge
potential to the surrounding
housing
“ If it is insulated by some
pressurized gas (N2, CO2 of
SF6) about 16 MV can be
achieved
“ This can be used to
accelerate protons to energy
of about 15 MeV in a single
stage
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Steel tank
target
Insulating supports
pulley
Accelerating tube
++++++++++++++++++++++++++
+
-
E1
belt
Ion source
pulley A
E2
Removable lid
Van de Graaf accelerators (VdG)
“ The
energy of the beam produced by
the VdG generator is extremely precise
“ The current (10-100 µA) is less than
that of other accelerators
“ The beam current i (A) is:
Net charge of the beam particle
i = qI o = ezI o
Incident particle current (particles/s)
Particle charge (C)
Multi-stage accelerators
“ The
potential obtained from a high
voltage generator can be used
repeatedly in a multi-stage accelerator
process
“ The linear accelerator operates in this
principle
Wideroe Multi-stage
accelerators
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The accelerator tube consists of
a series of cylindrical electrodes
– drift tubes
The electrodes are coupled to a
radio frequency generator
The high voltage generator
gives a maximum voltage V
The voltage is applied to the
electrodes by the RF so that the
electrodes alternate in the sign
of the voltage at a constant
frequency
Ion source
Vacuum chamber
Ln
n-1
Drift tube
n
A
target
n+1
B V
~
RF oscillator
Wideroe Multi-stage
accelerators
If the particles arrive at the
gap between electrodes in
proper phase with the radio
frequency, the particles are
accelerated across the gap
“ They receive an increase ion
energy of qV (for n
electrodes = nqV)
“ Inside the drift tubes no
acceleration takes place
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Ion source
Vacuum chamber
Ln
n-1
Drift tube
n
A
target
n+1
B V
~
RF oscillator
LINAC
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Is a particle accelerator which accelerates charged particles electrons, protons or heavy ions - in a straight line.
Charged particles enter on the left and are accelerated towards
the first drift tube by an electric field.
Once inside the drift tube, they are shielded from the field and
drift through at a constant velocity.
When they arrive at the next gap, the field accelerates them
again until they reach the next drift tube.
This continues, with the particles picking up more and more
energy in each gap, until they shoot out of the accelerator on the
right.
The drift tubes are necessary because an alternating field is
used and without them, the field would alternately accelerate
and decelerate the particles.
The drift tubes shield the particles for the length of time that the
field would be decelerating.
Cyclotons
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The cyclotron is a
particle accelerator
conceived by Ernest O.
Lawrence in 1929, and
developed, with this
colleagues and
students at the
University of California
in the 1930s.
Cyclotons
A Cyclotron
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Consists of two large dipole
magnets designed to produce
a semi-circular region of
uniform magnetic field,
pointing uniformly downward.
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These are called Ds because
of their D-shape.
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The two D's are placed backto-back with their straight sides
parallel but slightly separated.
A Cyclotron
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An oscillating voltage is
applied to produce an electric
field across this gap.
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Particles injected into the
magnetic field region of a D
trace out a semicircular path
until they reach the gap.
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The electric field in the gap
then accelerates the particles
as they pass across it.
A Cyclotron
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The particles now have higher
energy so they follow a semicircular path in the next D with
larger radius and so reach the
gap again.
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The electric field frequency
must be just right so that the
direction of the field has
reversed by their time of arrival
at the gap.
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The field in the gap accelerates
them and they enter the first D
again.
A Cyclotron
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Thus the particles gain energy as
they spiral around.
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The trick is that as they speed up,
they trace a larger arc and so they
always take the same time to reach
the gap.
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This way a constant frequency
electric field oscillation continues to
always accelerate them across the
gap.
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The limitation on the energy that
can be reached in such a device
depends on the size of the magnets
that form the D's and the strength of
their magnetic fields.