Download Particle detectors Option J

Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
J.2.1 Explain the need for high energies in order
to produce particles of large mass.
J.2.2 Explain the need for high energies in order
to resolve particles of small size.
J.2.3 Outline the structure and operation of a
linear accelerator and of a cyclotron.
J.2.4 Outline the structure and explain the
operation of the synchrotron.
J.2.5 State what is meant by bremsstrahlung
(braking) radiation.
J.2.6 Compare the advantages and disadvantages of
linear accelerators, cyclotrons and
synchrotrons.
J.2.7 Solve problems related to the production of
particles in accelerators.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Explain the need for high energies in order to
produce particles of large mass.
●Recalling the mass-energy relationship we see
that from E = mc2 we obtain
m = E/c2
mass-energy equivalence
●Note that the bigger the mass of the particle we
desire to produce, the larger E must be.
EXAMPLE: Suppose you want to create a proton from
energy. How much energy must you provide?
SOLUTION: Particle creation must adhere to
conservation laws.
●Since the baryon number of a proton is +1, and
the baryon number of a photon is zero, an antiproton must also be created.
●Thus we need at least E = 2(938 MeV) = 1876 MeV.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Explain the need for high energies in order to
produce particles of large mass.
PRACTICE:
Find the energy needed to create an electron.
Then show that conservation of charge and
conservation of lepton number are both satisfied.
SOLUTION:
●The rest energy of an electron is 0.511 MeV.
●Thus we need E = 2(0.511 MeV) = 1.022 MeV to
create an electron, anti-electron pair.
2  e- + e+
CHARGE
0
-1
+1
LEPTON NUMBER
0
+1
-1
FYI
The minimum energy of any accelerator must be
twice the rest energy of the heaviest particle it
is designed to detect/create.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Explain the need for high energies in order to
produce particles of large mass.
●You may be asking yourself: “How do you get high
energy photons?”
●You can “harvest” high energy photons from two
sources:
1) The energy that is released by the
annihilation of matter/anti-matter particles
(that were created elsewhere).
2) The KINETIC ENERGY of the particles just
before they collide.
FYI
To maximize the amount of EK converted to the
photons’ energy the particle and anti-particle
are made to travel in opposite directions.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Explain the need for high energies in order to
produce particles of large mass.
PRACTICE: Accelerators use E-fields and B-fields
to control the speeds and paths of the charged
particle beams they accelerate. Explain how this
is advantageous when using matter, anti-matter
beams traveling in opposite directions.
SOLUTION:
●Matter and antimatter have opposite charges.
●Recall: FB = qvB sin , and FE = qE.
●Thus the directions of the magnetic force (given
by the right hand rule) and the electric force
depend on the signs of the charges the forces are
acting on.
●Since matter and antimatter have opposite signs,
the same fields will control both beams at once!
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Explain the need for high energies in order to
resolve particles of small size.
●Recall that the wavelength  of a photon of
energy E is given by E = hf = hc/.
 = hc/E
wavelength – energy relationship
●Clearly, the bigger the energy E the smaller the
wavelength .
●Recall also that the smaller the wavelength, the
better the resolution.
●Thus, the bigger the energy, the smaller the
particle that can be resolved.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Outline the structure and operation of a linear
accelerator and of a cyclotron.
●As the name implies, a
linear accelerator or
linac is a straight tube.
●The longer it is, the more
energy the accelerated particle
will have when it reaches the
end of the linac, at which
point it smashes into a target
at the end of the tube.
FYI
The accelerated particle is
charged. The alternating p.d. must be timed so
that the charge is repelled from the “behind”
tube and attracted to the “in-front” tube.
~
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Outline the structure and operation of a linear
accelerator and of a cyclotron.
PRACTICE: Explain why the
successive tubes
increase in length in the
linac.
SOLUTION:
●Since the particle is
accelerating, the distance
covered in a fixed time
interval increases.
●The tubes are designed so
that the frequency of the
alternating p.d. can remain
fixed.
FYI
An alternate design might vary the frequency and
keep all of the tube lengths equal.
~
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Outline the structure and operation of a linear
accelerator and of a cyclotron.
●The cyclotron accelerates
particles within two D-shaped
hollow containers, like the
+
ones shown here.
●Each D is connected to opposite
terminals of an alternating p.d.,
B
which accelerates the charged
particle from the center.
●A perpendicular magnetic field causes the
particles to followed a curved spiral until
reaching the outside circumference of the D’s.
●As the following slide will show, the period T
of the alternating p.d. is independent of the
radius of the particle’s trajectory…
~
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Outline the structure and operation of a linear
accelerator and of a cyclotron.
●In order for the particle of
mass m and charge q to
experience a centripetal force,
+
it must satisfy F = mac.
●The force F is caused by the
magnetic force F = qvB.
B
●But the centripetal acceleration ac = v2/r.
●Therefore F = qvB = mac so that qvB = mv2/r, or
qB = mv/r = m(2r/T)/r = 2m/T.
T = 2m/(qB)
period of a cyclotron
~
FYI
Note that f (= 1/T) does not depend on the
radius of the path. It depends only on m,q and B.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Outline the structure and operation of a linear
accelerator and of a cyclotron.
●To get an idea of the energy
capabilities of a cyclotron
consider the intermediate step
+
from the previous slide:
●From qB = mv/r we get v = qBr/m
so that
B
2
2
EK = (1/2)mv = (1/2)m(qBr/m) .
EK = q2B2r2/(2m)
energy of a cyclotron
FYI
The energy capabilities of the cyclotron are
proportional to the square of its radius.
Since creating large disks of vacuum within the
D’s is very difficult, the largest cyclotrons are
only able to produce energies in the MeV range.
~
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Outline the structure and explain the operation
of the synchrotron.
●The synchrotron is similar to a linac in that
acceleration occurs between tubes.
●The tubes in a synchrotron,
however, are all the same
length, and they are arranged
in a circle.
electric field
accelerates
●The charges are accelerated
beam
between the tubes by an
electric field applied across
two plates.
●The tubes are surrounded by a
magnetic field that causes the
charged particles to follow a curve.
magnetic field
bends beam
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Outline the structure and explain the operation
of the synchrotron.
●The alternating p.d. units are complex. They
must be synchronized precisely to the
accelerating particles (hence
the name synchrotron).
●The magnetic field strength
must also vary as the
electric field
particles pick up speed.
accelerates
beam
●The advantage of a
synchrotron over a linear
accelerator is that the
particle can make as many
circuits as needed to
accelerate it to any energy.
magnetic field
bends beam
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Outline the structure and explain the operation
of the synchrotron.
●Another advantage of the synchrotron over either
of the previous accelerators is that you can have
a matter beam, and
proton
an antimatter beam,
antiproton
circulating at the
same time in
opposite
directions.
●As we have
already discussed,
head-on collisions
maximize the energy
Fermilab
output of the
Batavia,
collisions.
Illinois.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Outline the structure and explain the operation
of the synchrotron.
●The synchrotron’s maximum energy capability is
limited by its curvature. The faster a particle
travels, the larger the centripetal forces the
magnetic field needs to provide. PLUS due to
relativistic effects, the faster a particle goes,
the more massive it
becomes-thus adding to
the difficulty of making
it turn in a circle.
●The largest synchrotron
in the world is at CERN,
in Geneva, and has a
circumference of 27 km.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
State what is meant by bremsstrahlung (braking)
radiation.
●When a charge is accelerated, it produces
electromagnetic radiation called bremsstrahlung
radiation.
●Given that an
acceleration is a
change in velocity
and that velocity
can change in its
magnitude (across the plates) or its direction
(the magnetic field tubes) we get radiation at
both locations in the synchrotron.
●Since the radius of curvature is so large, the
radiation from the tubes is relatively small
compared to that of the plates.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
State what is meant by bremsstrahlung (braking)
radiation.
●The radiation at the plates is in the X-ray
region of the spectrum, and is highly polarized.
●Because the X-ray
radiation is very
intense, very
parallel, and very
polarized, it is
well-suited for
X-ray diffraction and other experiments
investigating the properties of materials.
FYI
In fact, there are some synchrotrons that have
been built solely for X-ray diffraction and
materials physics.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Compare the advantages and disadvantages of
linear accelerators, cyclotrons and synchrotrons.
●Since there are no curves in the linear
accelerator, there is no radiation loss due to
direction change as there is in the synchrotron.
●In the linac and the cyclotron, if you miss the
collision at the end of the run, the projectiles
are lost, whereas in the synchrotron the
particles can continue to go around as many times
as is needed to effect a collision.
●The difficulty in constructing large enough
evacuated D’s and large-area magnetic fields
prevents the cyclotron from being a serious
contender in very high energy physics research.
●The cyclotron is the simplest to construct for
low-energy applications.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Solve problems related to the production of
particles in accelerators.
●If a particle and an antiparticle collide headon then we not only get the energy of
annihilation (the rest mass energy of the two
particles) but we harvest all of the original
kinetic energy of the particles before their
collision.
●If a particle collides with a stationary target
not all of the energy can be converted to new
particles. This is because to conserve momentum
the new particles must move in the same direction
as the original beam.
●Thus the energy needed to produce particles
striking a stationary target must exceed the
actual rest mass energy of the particles created.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Solve problems related to the production of
particles in accelerators.
●The minimum available energy Ea for particle
creation obtained by the collision of a particleprojectile of rest mass m and total energy E with
a stationary target of mass M is shown here:
Ea2 = 2Mc2E + (Mc2)2 +(mc2)2
energy available in
M = target rest mass.
collision of a
m = projectile rest mass.
moving mass m with
E = projectile total energy. stationary mass M
FYI
Notice that each factor has the units of
[energy]2. Don’t miss a square! Anywhere.
In general, the numbers are more manageable if
we keep the energies in MeV (or even GeV)…
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
Solve problems related to the production of
particles in accelerators.
Ea2 = 2Mc2E + (Mc2)2 +(mc2)2
energy available in
M = target rest mass.
collision of a
m = projectile rest mass.
moving mass m with
E = projectile total energy. stationary mass M
EXAMPLE: How much energy would be available if a
15 GeV proton collided with a stationary proton?
SOLUTION:
●m = M = 938 MeV c-2 = 0.938 GeVc-2. E = 15 GeV.
Ea2 = 2Mc2E + (Mc2)2 +(mc2)2
Ea2 = 2(0.938)(15) + [0.938]2 +(0.938)2 [GeV]2
Ea2 = 29.899688
[GeV]2
Ea = 5.5 GeV.
Option J: Particle physics
J2 Particle accelerators and detectors
Solve problems related to the production of
particles in accelerators.
●The E-field accelerates the charged
particles.
●The B-field makes the charged
particles turn and follow a circular
trajectory.
Option J: Particle physics
J2 Particle accelerators and detectors
Solve problems related to the production of
particles in accelerators.
●As v increases so does m, and the
centripetal force Fc = mac = mv2/r must also
increase to keep the particles moving in the
synchrotron’s circular path.
●Since FB = qvB = Fc, mv2/r = qvB  qB = mv/r.
●Thus r = mv/qB.
●Since r = CONST for a synchrotron, B must
increase with mv as the particle energy
increases.
Option J: Particle physics
J2 Particle accelerators and detectors
Solve problems related to the production of
particles in accelerators.
●Each beam should supply E = 1120 MeV
(half of 2240 MeV) in kinetic energy.
●We can use E = m0c2 + EK because the
annihilation provides some energy.
●EK = E - m0c2 = 1120 – 938 = 182 MeV.
Option J: Particle physics
J2 Particle accelerators and detectors
Solve problems related to the production of
particles in accelerators.
●For a stationary target
use Ea2 = 2Mc2E + (Mc2)2 + (mc2)2, where
Ea = 2240, M = 938, and m = 938. Then
22402 = 2(938)E + (938)2 + (938)2
E = 1740 MeV (E is total energy).
●EK = E - m0c2 = 1740 – 938 = 802 MeV.
Option J: Particle physics
J2 Particle accelerators and detectors
Solve problems related to the production of
particles in accelerators.
●EK = 182 MeV for proton, anti-proton headon collision.
●EK = 802 MeV for anti-proton hitting
stationary proton target.
●This is 802/182 = 4.4 times more energy.
●Thus you only need about a quarter of the
energy to produce the ,  pair using the
head-on synchrotron collision.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle accelerators
cyclotron
linac
detail
synchrotron
Option J: Particle physics
J2 Particle accelerators and detectors
Particle detectors
J.2.8 Outline the structure and operation of a
bubble chamber, the photomultiplier and the
wire chamber.
J.2.9 Outline international aspects of research
into high-energy particle physics.
J.2.10 Discuss the economic and ethical
implications of high-energy particle physics
research.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle detectors
Outline the structure and operation of a bubble
chamber, a photomultiplier and a wire chamber.
●The particles created in collisions must
somehow be detected. The bubble chamber was
one of the first detectors for such particles.
●When you pop the top of a soda, pressure is
suddenly released with a fizzing sound.
●If pressure is released the liquid reaches
its boiling point, and bubbles form. In the
case of soda, the bubbles are CO2.
●Each bubble forms at the site of an
impurity - if there were no impurities, bubbles
would not form automatically.
●Instead of water and CO2, a bubble chamber uses
hydrogen that has been cooled to the liquid state
just below its boiling point.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle detectors
Outline the structure and operation of a bubble
chamber, a photomultiplier and a wire chamber.
●The bubble chamber consists
of a liquid-hydrogen-filled
cylinder having one of its
faces made of glass.
●Incoming particles strike the
target and produce new particles.
●The piston moves outward,
lowering the liquid pressure.
●Bubbles form in the depressurized liquid hydrogen along
the path of the particles.
Stationary Target
●A magnetic field passing through the bubble
chamber ensures that the particles will have
curved trajectories if they are charged.
Piston
Option J: Particle physics
J2 Particle accelerators and detectors
Particle detectors
Outline the structure and operation of a bubble
chamber, a photomultiplier and a wire chamber.
●Some of the products of collision experiments
are gamma particles, which are of course highphotoenergy photons.
sensitive
material
●Gamma particles are very adept at ionizing
matter, but the act of ionization absorbs
50V
the photon, removing it from the picture.
dynode
100V
●Detection of a single photon being
absorbed is difficult, but it is made
150V
possible by a device called a
200V
photomultiplier.
●A single photon enters a photomultiplier
250V
through a small window and is absorbed by
photoa photosensitive plate, releasing an
multiplier
electron according to the photoelectric effect.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle detectors
Outline the structure and operation of a bubble
chamber, a photomultiplier and a wire chamber.
●In the photomultiplier are a cascade of dynodes,
each at a higher potential than the previous.
photo●Because of the acceleration caused by
sensitive
material
the p.d., more electrons are released
during each step in the cascade.
50V
●The end of the cascade has enough
dynode
electrons to create measurable current. 100V
FYI
150V
The Geiger counter uses a photomultiplier.
For very high-energy photons a photomul- 200V
tiplier doesn’t work. Instead a scintillator
250V
is used. A scintillator has a screen of
photophosphorescent material that glows when
multiplier
struck by a very high energy gamma photon.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle detectors
Outline the structure and operation of a bubble
chamber, a photomultiplier and a wire chamber.
wire
●If high-energy particles pass through a
chamber
gas, the gas particles become ionized.
Think of the high-energy particle as
“knocking” electrons off of the gas
atoms as it passes by.
●If the gas is between two wires which
A
have a p.d. applied to them, these
freed electrons travel to the
positive wire and away from the
R
V
negative wire, setting up a current.
●The voltage across a resistor which
the current is made to pass through
can then be digitally recorded.
●This device is called a wire chamber.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle detectors
Outline the structure and operation of a bubble
chamber, a photomultiplier and a wire chamber.
●So how does one get a 3D picture
(1,1,t1) (3,2,t2)
of particles? Simply set up an
1
array of wire chambers, and record 1 1 2
1 2
not only the place, but the time
3
3
2
2
4
4
the chambers detect a particle.
3
3
●The blue array tells us the left- 4
4
right coordinate of the particle.
●The red array tells us the up-down
coordinate of the particle.
●The timing from one double-grid to
the next tells us the forward-backward
coordinate of the particle.
FYI
3D images can then be computer generated!
Option J: Particle physics
J2 Particle accelerators and detectors
Particle detectors
Geiger
counter
bubble chamber
Large Hadron Collider
Option J: Particle physics
J2 Particle accelerators and detectors
Particle detectors
Outline international aspects of research into
high-energy particle physics.
●Because of the extreme expense in building and
operating high energy particle physics
installations, the larger ones have to be an
international collaboration to help share the
costs.
●Because of the international aspect of particle
research, there is a tendency of the academic
edifices of the world to come together, even in
times of war.
●The research is also transparent-no country can
be the “clearinghouse” of the information
generated by any international facility.
Option J: Particle physics
J2 Particle accelerators and detectors
Particle detectors
Discuss the economic and ethical implications of
high-energy particle physics research.
●People ask whether the cost of HEP is worth it.
●Theoretical physics stagnates without
experimental verification.
●Curiosity is a fundamental part of the human
mind.
●Sharing large research costs among many
countries encourages cooperation between
different cultures.
●Synchrotron radiation has a large range of
applications, including biology, medicine, and
technology.