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UT Arlington PHYS 3446 - Lecture Notes

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PHYS 3446 Lecture 18 Wednesday Oct 29 2008 Dr Andrew Brandt 1 Particle Detection Scintillation Counters Time of Flight Cerenkov Counter Grades are posted please review them Lowest 1 Q and 1 HW dropped Olsen colloquium on Higgs and Mass at 4pm cookies at 3 30 Wednesday Oct 29 2008 PHYS 3446 Fall 2008 Andrew Brandt 1 Scintillation Counters Ionization produced by charged particles can excite atoms and molecules in the medium to higher energy levels The subsequent de excitation process produces light that can be detected and provide evidence for the traversal of the charged particles Scintillators are material that can produce light in visible part of the spectrum Wednesday Oct 29 2008 PHYS 3446 Fall 2008 Andrew Brandt 2 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 Wednesday Oct 29 2008 PHYS 3446 Fall 2008 Andrew Brandt 3 Scintillation Counters Photo multiplier Tube The light produced by scintillators are 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 Oct 29 2008 PHYS 3446 Fall 2008 Andrew Brandt 4 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 0 25 Output signal is proportional to the amount of the incident light except for statistical fluctuation 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 Wednesday Oct 29 2008 PHYS 3446 Fall 2008 Andrew Brandt 5 Some PMT s Wednesday Oct 29 2008 PHYS 3446 Fall 2008 Andrew Brandt Super Kamiokande detector 6 Scintillation Detector Structure HV PS Scintillation Counter Light Guide Wavelength Shifter PMT Readout Electronics Oscilloscope Wednesday Oct 29 2008 PHYS 3446 Fall 2008 Andrew Brandt 7 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 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 Oct 29 2008 PHYS 3446 Fall 2008 Andrew Brandt 8 Cerenkov Detectors 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 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 Oct 29 2008 PHYS 3446 Fall 2008 Andrew Brandt 9 Cerenkov Effect n 1 n 1 particle Use this property of prompt radiation to develop a fast timing counter Time of Flight TOF 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 1 1 L 1 1 Dt t2 t1 L v2 v1 c b2 b1 For known momentum p Since E gmc 2 1 1 gmc 2 2 pc b b gmc gm b c c L E2 E1 L m2 c 4 p 2 c 2 m 2 c 4 p 2 c 2 Dt 1 pc 2 2 c pc pc L L In non relativistic limit Dt m2 m1 Dm p p Mass resolution of 1 is achievable for low energies Wednesday Oct 29 2008 PHYS 3446 Fall 2008 Andrew Brandt 11 Cerenkov Detectors 1 The angle of emission is given by cosqc b n The intensity of the produced radiation per unit length of the radiator is proportional to sin2 c For n 1 light CR 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 Wednesday Oct 29 2008 PHYS 3446 Fall 2008 Andrew Brandt 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 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 Wednesday Oct 29 2008 PHYS 3446 Fall 2008 Andrew Brandt 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 Original experiment designed for proton decay search Accident in Nov 2001 destroyed 7000 PMT s Dec 2002 resumed data taking Wednesday Oct 29 2008 PHYS 3446 Fall 2008 Andrew Brandt 14 Super K Event Displays Stopping Wednesday Oct 29 2008 3 PHYS 3446 Fall 2008 Andrew Brandt 15 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


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UT Arlington PHYS 3446 - Lecture Notes

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