PHYS 3446 – Lecture #13
Monday, Oct. 23, 2006
Dr. Jae Yu
1. Particle Detection
•
•
•
•
Scintillation Counters
Time of Flight
Cerenkov Counter
Calorimeters
2. Particle Accelerators
•
Monday, Oct. 23, 2006
Electrostatic Accelerators
PHYS 3446, Fall 2006
Jae Yu
1
Announcements
•
Research Day tomorrow – Rio Grande
–
–
–
•
9am – 4pm poster presentations
Our cloud chamber prototype will be in display from noon – 4pm
Need presenters to operate the chamber and man the poster
Next LPCC Workshop
–
Preparation work
•
•
–
•
CPB303 and HEP experimental areas
Quiz results
–
–
•
Class average: 68.5
Top score:96/90
Assignments
1.
2.
•
Lists of goals and items to purchase?
10am – 5pm, Saturday, Nov. 4
Derive Eq. 7.10
Carry out computations for Eq. 7.14 and 7.17
Due for these assignments is Monday, Oct. 30
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
2
Paper Template
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
3
Scintillation Counters
• Two types of scintillators
– Organic or plastic
•
•
•
•
Tend to emit ultra-violate
Wavelength shifters are needed to reduce attenuation
Faster decay time (10-8s)
More appropriate for high flux environment
– Inorganic or crystalline (NaI or CsI)
• Doped with activators that can be excited by electron-hole
pairs produced by charged particles in the crystal lattice
• These dopants can then be de-excited through photon
emission
• Decay time of order 10-6sec
• Used in low energy detection
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
4
Scintillation Counters – Photo-multiplier Tube
• The light produced by scintillators are usually too
weak to see
– Photon signal needs amplification through
photomultiplier tubes
• Gets the light from scintillator directly or through light guide
– Photocathode: Made of material in which valence electrons are
loosely bound and are easy to cause photo-electric effect (2 – 12
cm diameter)
– Series of multiple dynodes that are made of material with
relatively low work-function
» Operating at an increasing potential difference (100 – 200 V)
difference between dynodes
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
5
Scintillation Counters – Photo-multiplier Tube
• The dynodes accelerate the electrons to the next stage, amplifying the
signal to a factor of 104 – 107
• Quantum conversion efficiency of photocathode is typically on the order of
0.25
• Output signal is proportional to the amount of the incident light except for
the statistical fluctuation
• Takes only a few nano-seconds for signal processing
• Used in as trigger or in an environment that requires fast response
• Scintillator+PMT good detector for charged particles or photons or neutrons
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
6
Some PMT’s
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
Super-Kamiokande detector
7
Scintillation Detector Structure
HV PS
Scintillation
Counter
Light Guide/
Wavelength
Shifter
PMT
Readout
Electronics
Scope
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
8
Time of Flight
• Scintillator + PMT can provide time resolution of 0.1 ns.
– What position resolution does this corresponds to?
• 3cm
• Array of scintillation counters can be used to measure
the time of flight (TOF) of particles and obtain their
velocities
– What can this be used for?
• Can use this to distinguish particles with about the same momentum
but with different mass
– How?
• Measure
– the momentum (p) of a particle in the magnetic field
– its time of flight (t) for reaching some scintillation counter at a distance L from
the point of origin of the particle
– Determine the velocity of the particle and its mass
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
9
Time of Flight (TOF)
• TOF is the distance traveled divided by the speed of the particle,
t=L/v.
• Thus Dt in flight time of the two particle with m1 and m2 is
 1 1  L 1 1 
Dt  t2  t1  L       
 v2 v1  c   2 1 
• For known momentum, p,
– Since
E
m c 2
m c 2
m c 2





2
2

 m c
 m c  c pc
 m c
1
1
L  2 4
L  E2 E1 
2 2
2 4
2 2
m
c

p
c

m
c

p
c 
Dt  
 
2
1
2 

c  pc pc  pc 
L
L
• In non-relativistic limit,
Dt   m2  m1   Dm
p
p
• Mass resolution of ~1% is achievable for low energies
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
10
Cerenkov Detectors
• What is the Cerenkov radiation?
– Emission of coherent radiation from the excitation of atoms and
molecules
• When does this occur?
– If a charged particle enters a dielectric medium with a speed faster
than light in the medium
– How is this possible?
• Since the speed of light is c/n in a medium with index of refraction n, if the
particle’s >1/n, its speed is larger than the speed of light
• Cerenkov light has various frequencies but blue and ultraviolet
band are most interesting
– Blue can be directly detected w/ standard PMTs
– Ultraviolet can be converted to electrons using photosensitive
molecules mixed in with some gas in an ionization chamber
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
11
Cerenkov Detectors
1
• The angle of emission is given by cosqc   n
• The intensity of the produced radiation per unit length
of the radiator is proportional to sin2qc.
• For n>1, light can be emitted while for n<1, no light
can be observed.
• Thus, Cerenkov effect provides a means for
distinguishing particles with the same momentum
– One can use multiple chambers of various indices of
refraction to detect Cerenkov radiation from particles of
different mass but with the same momentum
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
12
Cerenkov Detectors
• Threshold counters
– Particles with the same momentum but with different mass will start
emitting Cerenkov light when the index of refraction is above a
certain threshold
– These counters have one type of gas but could vary the pressure in
the chamber to change the index of refraction to distinguish particles
– Large proton decay experiments use Cerenkov detector to detect the
final state particles, such as p  e+p0
• Differential counters
– Measure the angle of emission for the given index of refraction since
the emission angle for lighter particles will be larger than heavier
ones
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
13
Super Kamiokande
A Differential Water Cerenkov Detector
•Kamioka zinc mine, Japan
• 1000m underground
•40 m (d) x 40m(h) SS
•50,000 tons of ultra pure H2O
•11200(inner)+1800(outer)
50cm PMT’s
•Originally for proton decay
experiment
•Accident in Nov. 2001,
destroyed 7000 PMT’s
•Dec. 2002 resumed data
taking
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
14
Super-K Event Displays
Stopping m
Monday, Oct. 23, 2006
3m
PHYS 3446, Fall 2006
Jae Yu
15
Cerenkov Detectors
• Ring-imaging Cerenkov Counters (RICH)
– Use UV emissions
– An energetic charged particle can produce multiple UV distributed
about the direction of the particle
– These UV photons can then be put through a photo-sensitive
medium creating a ring of electrons
– These electrons then can be detected in an ionization chamber
forming a ring
– Babar experiment at SLAC uses this detector
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
16
Semiconductor Detectors
• Semiconductors can produce large signal (electron-hole pairs)
for relatively small energy deposit (~3eV)
– Advantageous in measuring low energy at high resolution
• Silicon strip and pixel detectors are widely used for high
precision position measurements
– Due to large electron-hole pair production, thin layers (200 – 300 mm)
of wafers sufficient for measurements
– Output signal proportional to the ionization loss
– Low bias voltages sufficient to operate
– Can be deposit in thin stripes (20 – 50 mm) on thin electrode
– High position resolution achievable
– Can be used to distinguish particles in multiple detector configurations
• So what is the catch?
– Very expensive  On the order of $30k/m2
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
17
DØ Silicon Vertex Detector
8
1 9
11
12 0
6
7
5
2
43
Barrels
1
1
1
2
F-Disks
H-Disks
Channels
387120
258048
147456
Modules
432
144
96
Inner R
2.7 cm
2.6 cm
9.5 cm
Outer R
9.4 cm
10.5 cm
26 cm
6
One Si detector
3
4
Monday, Oct. 23, 2006
Barrel
Disk
PHYS 3446, Fall 2006
Jae Yu
18
Calorimeters
• Magnetic measurement of momentum is not sufficient
for physics, why?
– The precision for angular measurements gets worse as
particles’ momenta increases
– Increasing magnetic field or increasing precision of the
tracking device will help but will be expensive
– Cannot measure neutral particle momenta
• How do we solve this problem?
– Use a device that measures kinetic energies of particles
• Calorimeter
– A device that absorbs full kinetic energy of a particle
– Provides signal proportional to deposited energy
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
19
Calorimeters
• Large scale calorimeter were developed during 1960s
– For energetic cosmic rays
– For particles produced in accelerator experiments
• How do high energy EM (photons and electrons) and
Hadronic particles deposit their energies?
– Electrons: via bremsstrahlung
– Photons: via electron-positron conversion, followed by
bremsstrahlung of electrons and positrons
– These processes continue occurring in the secondary
particles causing an electromagnetic shower losing all of its
energy
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
20
Electron Shower Process
Photon, 
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
21
Calorimeters
• Hadrons are massive thus their energy deposit via brem is
small
• They lose their energies through multiple nuclear collisions
• Incident hadron produces multiple pions and other secondary
hadrons in the first collision
• The secondary hadrons then successively undergo nuclear
collisions
• Mean free path for nuclear collisions is called nuclear
interaction lengths and is substantially larger than that of EM
particles
• Hadronic shower processes are therefore more erratic than
EM shower processes
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
22
Sampling Calorimeters
• High energy particles require large
calorimeters to absorb all of their
energies and measure them fully in
the device (called total absorption
calorimeters)
• Since the number of shower particles
is proportional to the energy of the
incident particles
• One can deduce the total energy of
the particle by measuring only the
fraction of their energy, as long as the
fraction is known  Called sampling
calorimeters
– Most the high energy experiments use
sampling calorimeters
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
23
How particle showers look in detectors
Hadron
EM
Monday, Oct. 23, 2006
PHYS 3446, Fall 2006
Jae Yu
24