PHYS 3446 – Lecture #15Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15HW7 (due Weds 11/10)Monday, Nov. 1, 2010 PHYS 3446, Fall 2010 Andrew Brandt 1PHYS 3446 – Lecture #15Monday, Nov. 1, 2010Dr. Andrew Brandt/Mark SosebeeParticle Detection•MWPC•Scintillation Counters•Time of Flight•Cerenkov CounterMonday, Nov. 1, 2010 2 PHYS 3446, Fall 2010 Andrew Brandt•Depending on the magnitude of the electric field across the medium different behaviors are expected–Recombination region: Low electric field–Ionization region: Medium voltage that prevents recombination–Proportional region: large enough HV to cause acceleration of ionization electrons and additional ionization of atoms–Geiger-operating region: Sufficiently high voltage that can cause large avalanche if electron and ion pair production that leads to a discharge–Discharge region: HV beyond Geiger operating region, no longer usableIonization Detectors as f(HV) [review]Flat!!!Flat!!!Monday, Nov. 1, 2010 3 PHYS 3446, Fall 2010 Andrew Brandt•Gaseous proportional counters operate in high electric fields ~104 V/cm.•Typical amplification of factors of ~105•Use thin wires ( 10 – 50 m diameter) as anode electrodes in a cylindrical chamber geometry•Near the anode wire where the field is strongest, secondary ionization occurs giving a multiplicative effect and a large voltage on the wire nearest to the passing particle•Sensitive to voltage variation not suitable for energy measurement•Typically used for tracking device Proportional CountersMonday, Nov. 1, 2010 4 PHYS 3446, Fall 2010 Andrew Brandt•G. Charpak et al. developed a proportional counter into a multiwire proportional chamber–One of the primary position detectors in HEP•A plane of anode wires positioned precisely w/ about 2 mm spacing, which are sandwiched between cathode planes (or wires) Multi-Wire Proportional Chamber (MWPC)Cathode planesMonday, Nov. 1, 2010 5 PHYS 3446, Fall 2010 Andrew Brandt•These structures can be enclosed to form one plane of a detector•Multiple layers can be placed in a succession to provide three dimensional position information and improved resolutionMulti-Wire Proportional Chambers (MWPC)Monday, Nov. 1, 2010 6 PHYS 3446, Fall 2010 Andrew BrandtMomentum Measurement•A set of MWPC planes placed before and after a magnetic field can be used to obtain the deflection angle which in turn provides the momentum of the particle•Drift chambers have ~constant electric field in each cell in the direction transverse to normal incidence, so ionized electrons drift along field lines at slow and constant velocity, so from time of signal can measure position •Typical position resolution of drift chambers are on the order of 200 m.sinR L q=sinLBzec qF=qvB/c=mv2/Rp=Rbze/cp =Monday, Nov. 1, 2010 PHYS 3446, Fall 2010 Andrew Brandt 7•Ionization detector that operates in the Geiger range of voltages•For example, consider an electron with 0.5MeV KE that loses all its energy in the counter•Assume that the gaseous medium is helium with an ionization energy of 42eV. •Number of ionization electron-ion pair in the gas is•If a detector operates as an ionization chamber and has a capacitance of 1 nF, the resulting voltage signal is •In Geiger range, the expected number of electron-ion pair is of the order 1010 independent of the incoming energy, giving about 1.6V pulse heightGeiger-Muller Countersn =V =QC=neC=4 1991.2 10 1.6 101 10CF--� � ���62 10 V-�60.5 1042eVeV��12,000Monday, Nov. 1, 2010 PHYS 3446, Fall 2010 Andrew Brandt 8•Advantages–Simple construction–Insensitive to voltage fluctuation–Useful for detecting radiation•Disadvantages–Insensitive to the type of radiation–Due to large avalanche have a long recovery time (~1ms), so cannot be used in high rate environmentFeatures of Geiger-Muller CountersMonday, Nov. 1, 2010 PHYS 3446, Fall 2010 Andrew Brandt 9•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 spectrumScintillation CountersMonday, Nov. 1, 2010 PHYS 3446, Fall 2010 Andrew Brandt 10•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, Nov. 1, 2010 PHYS 3446, Fall 2010 Andrew Brandt 11•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 dynodesScintillation Counters – Photo-multiplier TubeMonday, Nov. 1, 2010 PHYS 3446, Fall 2010 Andrew Brandt 12•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 Scintillation Counters – Photomultiplier TubeMonday, Nov. 1, 2010 PHYS 3446, Fall 2010 Andrew Brandt 13Some PMT’sSuper-Kamiokande detectorMonday, Nov. 1, 2010 PHYS 3446, Fall 2010 Andrew Brandt 14Scintillation Detector StructureScintillation CounterPMTReadout ElectronicsHV PSLight Guide/WavelengthShifterOscilloscopeMonday, Nov. 1, 2010 PHYS 3446, Fall 2010 Andrew Brandt 15•Scintillator + PMT can provide time resolution of 0.1 ns.
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