Wednesday, Nov. 3, 2010PHYS 3446, Fall 2010 Andrew Brandt 1 PHYS 3446 – Lecture #16 Wednesday, Nov. 3, 2010 Dr. Andrew Brandt Particle Detection Time of

Wednesday, Nov. 3, 2010 PHYS 3446, Fall 2010 Andrew Brandt 1

PHYS 3446 – Lecture #16Wednesday, Nov. 3, 2010

Dr. Andrew Brandt

Particle Detection• Time of Flight• Cerenkov Counter• Silicon• Calorimeter

• Review Project assignmentsHW 7 due Wedsday

Project Assignments and tentative agendaWednesday Dec. 1• Solar Neutrino Deficit Baral-Lord• Long baseline neutrino experiments (neutrino mass) Butler-Mayfield• G-2 experiments Ouyang-Wright• HERA experiments: diffraction/large rapidity gaps Gray-WooFriday Dec. 3• Quark-Gluon Plasma (RHIC) Byrd-PryorMonday Dec. 6• Higgs Boson Theory Bridges-Corbin• Standard Model+Beyond the Standard Model Higgs Boson Searches at Dzero Absher-

Dean-Shumate• Supersymmetry Theory Ibarra• Blackhole/Extra Dimension Searches at ATLAS Contreras-LaRoque

Wednesday, Nov. 3, 2010 2PHYS 3446, Fall 2010 Andrew Brandt

Project Details• 16 minute presentation+4 mins for questions (preferably powerpoint) will be done on Dec. 1, 3, 6 as outlined on

previous slide. Let me know right away if some conflict. You should provide me with an outline and list of refs by Nov. 15 (10% of grade). By Nov. 22, you should provide me with a decent draft (10%), in order to get feedback in time to make modifications It is important to get an early start in case you have some questions about the project as it will be a significant part of your grade. At least 3 sources including original paper (and not including wikipedia); should answer most if not all of these questions as applicable. Suggested split: intro/detector results/data analysis

1) what is signature involved, are there other signatures not used, why?For experimental talk 1) was experiment designed to find the result for your talk2) what detector was/will be used, describe detector’s and sub-detectors3) what was/will be importance of discovery4) who was involved names if small, institutions if large, ~how many people5) what was major source of uncertainty, statistical or systematic6) what were two most important systematic errors7) what was largest backgroundFor theory talk1) How well motivated is the theory—are there other alternative models that the LHC (or Tevatron) is sensitive to?


Project Grading Issues1) What was the subject?

2) Did the presenters describe the detectors or theory adequately?

3) Did they describe the importance and/or method of the discovery/theory?

4) Did they describe the measurement or calculation, errors and backgrounds if applicable?

5) Was the talk interesting and comprehensible? Excessive jargon?

6) Was the time management good?

7) What overall grade would you give them if you were the prof? Why?



• 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?

• To distinguish particles with the similar 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—this gives the velocity– from the momentum and velocity of the particle can determine its mass

Time of Flight


• What is 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 local 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 with some gas in an ionization chamber

Cerenkov Detectors

n=1 n>>1

Cerenkov Effect

Use this property of prompt radiation to develop a fasttiming counter

particle



• TOF is the distance traveled divided by the speed of the particle, t=L/v.

• Thus t in flight time of the two particle with m1 and m2 is

• For known momentum, p,– Since

• In non-relativistic limit,

• Mass resolution of ~1% is achievable for low energies

Time of Flight (TOF)

2 1t t t

2 1E ELtc pc pc

t

2 4 2 2 2 4 2 22 12

Lm c p c m c p c

pc

2 1

1 1Lv v

2 1

1 1L

c

1

1

2

2

mc

mc

2mc

m c c

E

pc

2 1

Lm m

p

Lm

p


• The angle of emission is given by • The intensity of the produced radiation per unit length

of the radiator is proportional to sin2c.• For n>1, light (Cerenkov Radiation) will be emitted

while for n<1, no light is observed.• One can use multiple chambers of various indices of

refraction to detect Cerenkov radiation from particles of different mass but with the same momentum

Cerenkov Detectors1

cos c n


• 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+0

• 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

Cerenkov Detectors


Super-K Event Displays

Stopping 3


• Ring-imaging Cerenkov Counters (RICH)– Use UV emissions– An energetic charged particle can produce multiple UV photons

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 used this type of detector

Cerenkov Detectors


HW6 (due Mon 11/8)• 7.2, 7.3, 7.4, 7.5• Look up Cerenkov Radiation and find (and write down) the full

formula. What is the dependence on wavelength? How many photons would you expect from a 1 cm long quartz bar with n=1.5 for a wavelength range of 100 to 300 nm? 300-400? 400-700? 700-1000??


• Semiconductors can produce large signals (electron-hole pairs) for relatively small energy deposit (~3 eV)– Advantageous in measuring low energy at high resolution

• Silicon strip and pixel detectors are widely used for high precision position measurements (solid state MWPC)– Due to large electron-hole pair production, thin layers (200 – 300 m)

of wafers sufficient for measurements– Output signal proportional to the ionization loss– Low bias voltages sufficient to operate (avoid recombination)– Can be deposited in thin stripes (20 – 50 m) 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

Semiconductor Detectors


DØ Silicon Vertex Detector234

9 811

1

67

5

1012

1

6…...

12

34

Barrels F-Disks H-Disks

Channels 387120 258048 147456

Modules 432 144 96

I nner R 2.7 cm 2.6 cm 9.5 cm

Outer R 9.4 cm 10.5 cm 26 cm

One Si detector

BarrelDisk


• 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

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– electron and photon interactions result in an electromagnetic

shower that continues until all the initial energy is deposited

Calorimeters


Electron Shower Process

Photon,


• 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 length and is substantially larger than that of EM particles

• Hadronic shower processes are therefore more extended and erratic than EM shower processes (also may produce neutrinos, so Hadronic energy resolution worse than EM)

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 a fraction of their energy, as long as the fraction is known Called sampling calorimeters– Most the high energy experiments use

sampling calorimeters

Sampling Calorimeters

Wednesday, Nov. 3, 2010 PHYS 3446, Fall 2010 Andrew Brandt21

Principles of Calorimeters

Total absorption calorimeter: See the entire shower energy

Sampling calorimeter: See only some fraction of shower energy

For EM

Absorber plates

visE fE

For HAD visE fE

0

0 0

vis

visabs vis

XE

X X

0

0 0

vis

visabs vis E

Documents

Wednesday, Nov. 3, 2010PHYS 3446, Fall 2010 Andrew Brandt 1 PHYS 3446 – Lecture #16 Wednesday, Nov. 3, 2010 Dr. Andrew Brandt Particle Detection Time of