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 3446 Andrew Brandt

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 3446 Andrew Brandt

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

Some PMT’s Super-Kamiokande detector Wednesday,April 4, 2012 PHYS 3446 Andrew Brandt

Scintillation Detector Structure HV PS Scintillation Counter Light Guide/ Wavelength Shifter PMT Readout Electronics Oscilloscope Wednesday,April 4, 2012 PHYS 3446 Andrew Brandt

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

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 3446 Andrew Brandt

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 3446 Andrew Brandt

Cerenkov Effect n=1 n>>1 particle Use this property of prompt radiation to develop a fast timing counter Wednesday,April 4, 2012 PHYS 3446 Andrew Brandt

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 3446 Andrew Brandt

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 3446 Andrew Brandt

Super-K Event Displays Stopping m 3m Wednesday,April 4, 2012 PHYS 3446 Andrew Brandt

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 3446 Andrew Brandt

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 3446 Andrew Brandt

DØ Silicon Vertex Detector 2 3 4 9 8 11 1 6 7 5 10 12 …... One Si detector Disk Barrel Wednesday,April 4, 2012 PHYS 3446 Andrew Brandt