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UT Arlington PHYS 3446 - Lecture Notes

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Monday, Oct. 23, 2006 PHYS 3446, Fall 2006Jae Yu1PHYS 3446 – Lecture #13Monday, Oct. 23, 2006Dr. Jae Yu1. Particle Detection• Scintillation Counters• Time of Flight• Cerenkov Counter• Calorimeters2. Particle Accelerators• Electrostatic AcceleratorsMonday, Oct. 23, 2006 PHYS 3446, Fall 2006Jae Yu2Announcements• 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• Lists of goals and items to purchase?• 10am – 5pm, Saturday, Nov. 4– CPB303 and HEP experimental areas• Quiz results– Class average: 68.5– Top score:96/90• Assignments1. Derive Eq. 7.102. Carry out computations for Eq. 7.14 and 7.17• Due for these assignments is Monday, Oct. 30Monday, Oct. 23, 2006 PHYS 3446, Fall 2006Jae Yu3Paper TemplateMonday, Oct. 23, 2006 PHYS 3446, Fall 2006Jae Yu4• 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 detectionScintillation CountersMonday, Oct. 23, 2006 PHYS 3446, Fall 2006Jae Yu5• 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 dynodesScintillation Counters – Photo-multiplier TubeMonday, Oct. 23, 2006 PHYS 3446, Fall 2006Jae Yu6• 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 neutronsScintillation Counters – Photo-multiplier TubeMonday, Oct. 23, 2006 PHYS 3446, Fall 2006Jae Yu7Some PMT’sSuper-Kamiokande detectorMonday, Oct. 23, 2006 PHYS 3446, Fall 2006Jae Yu8Scintillation Detector StructureScintillation CounterPMTReadout ElectronicsHV PSLight Guide/WavelengthShifterScopeMonday, Oct. 23, 2006 PHYS 3446, Fall 2006Jae Yu9• 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 massTime of FlightMonday, Oct. 23, 2006 PHYS 3446, Fall 2006Jae Yu10• 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 m1and m2is• For known momentum, p,–Since• In non-relativistic limit,• Mass resolution of ~1% is achievable for low energiesTime of Flight (TOF)21tt t∆= − =21EELtcpc pc⎛⎞∆= − =⎜⎟⎝⎠t∆=24 22 24 22212Lmc pc mc pcpc⎡⎤+− +⎢⎥⎣⎦2111Lvv⎛⎞−=⎜⎟⎝⎠2111Lcββ⎛⎞−⎜⎟⎝⎠1β=1β×22mcmcγγ=22mcmcγβγ=2mcmccγβγ=⋅Epc()21Lmmp−=Lmp∆Monday, Oct. 23, 2006 PHYS 3446, Fall 2006Jae Yu11• 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 β>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 chamberCerenkov DetectorsMonday, Oct. 23, 2006 PHYS 3446, Fall 2006Jae Yu12• The angle of emission is given by • The intensity of the produced radiation per unit length of the radiator is proportional to sin2θc.•For βn>1, light can be emitted while for βn<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 momentumCerenkov Detectors1coscnθβ=Monday, Oct. 23, 2006 PHYS 3446, Fall 2006Jae Yu13• 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 Æ e+π0• 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 onesCerenkov DetectorsMonday, Oct. 23, 2006 PHYS 3446, Fall 2006Jae Yu14Super KamiokandeA 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


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UT Arlington PHYS 3446 - Lecture Notes

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