1. PHYS 3446 – Lecture #17 Wednesday ,April 4, 2012 Dr. Brandt
Time of Flight
Cerenkov Counter
Silicon
Calorimeter
HW6 TBA this afternoon due next Wednesday 11th
Bonus for finishing project in April
Meet Ian at lab at your normal lab time
Wednesday,April 4, 2012
PHYS Andrew Brandt

2. Scintillation Counters – Photo-multiplier Tube
The amount light produced by scintillators is usually too weak to see
Photon signal needs amplification through photomultiplier tubes
Light can pass directly from scintillator to PMT or else through a light guide
Photocathode: Made of material in which valence electrons are loosely bound and subject to photo-electric effect
Series of multiple dynodes that are made of material with relatively low work-function
Operate at an increasing potential difference (100 – 200 V) difference between dynodes
Wednesday,April 4, 2012
PHYS Andrew Brandt

3. Scintillation Counters – Photomultiplier Tube
The dynodes accelerate the electrons to the next stage, amplifying the signal by a factor of 104 – 107
Quantum conversion efficiency of photocathode is typically on the order of 25%, but newer photocathodes can reach 50%, and specialized devices greater than 80%
Output signal is proportional to the amount of the incident light except for statistical fluctuations
Takes only a few nano-seconds for signal processing
Used as trigger or in an environment that requires fast response
Scintillator+PMT good detector for charged particles

4. Some PMT’s Super-Kamiokande detector Wednesday,April 4, 2012
PHYS Andrew Brandt

5. Scintillation Detector Structure
HV PS
Scintillation
Counter
Light Guide/
Wavelength
Shifter
PMT
Readout
Electronics
Oscilloscope
Wednesday,April 4, 2012
PHYS Andrew Brandt

6. Time of Flight
Scintillator + PMT can provide time resolution of 0.1 ns.
What position resolution does this correspond 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
Wednesday,April 4, 2012

7. 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
For known momentum, p,
Since
In non-relativistic limit,
Mass resolution of ~1% is achievable for low energies
Wednesday,April 4, 2012
PHYS Andrew Brandt

8. Cerenkov Detectors What is Cerenkov radiation? When does this occur?
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 b>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
Wednesday,April 4, 2012
PHYS Andrew Brandt

9. Cerenkov Effect n=1 n>>1 particle
Use this property of prompt radiation to develop a fast
timing counter
Wednesday,April 4, 2012
PHYS Andrew Brandt

10. Cerenkov Detectors The angle of emission is given by
The intensity of the produced radiation per unit length of the radiator is proportional to sin2qc.
For bn>1, light (Cerenkov Radiation) will be emitted while for bn<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
Wednesday,April 4, 2012
PHYS Andrew Brandt

11. Cerenkov Detectors Threshold counters Differential 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
Wednesday,April 4, 2012
PHYS Andrew Brandt

12. Super-K Event Displays
Stopping m 3m
Wednesday,April 4, 2012
PHYS Andrew Brandt

13. Cerenkov Detectors 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
Wednesday,April 4, 2012
PHYS Andrew Brandt

14. Semiconductor Detectors
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 mm) 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 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
Wednesday,April 4, 2012
PHYS Andrew Brandt

15. DØ Silicon Vertex Detector2 3 4 9 8 11 1 6 7 5 10 12
…...
One Si detector
Disk
Barrel
Wednesday,April 4, 2012
PHYS Andrew Brandt