PHYS 3446 – Lecture #12AnnouncementsSlide 3Particle DetectionSlide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14A Schematics of a Drift ChamberSlide 16Slide 17Slide 18Slide 19Slide 20Slide 21Scintillation MaterialsSlide 23Wednesday, Oct. 18, 2006 PHYS 3446, Fall 2006Jae Yu1PHYS 3446 – Lecture #12Wednesday, Oct. 18, 2006Dr. Jae Yu1. Particle Detection•Ionization Detectors•MWPC•Scintillation Counters•Time of FlightWednesday, Oct. 18, 2006 PHYS 3446, Fall 2006Jae Yu2Announcements•Next LPCC Workshop–Preparation work•Each group to prepare lists of goals and items to purchase by next Monday, Oct. 23–10am – 5pm, Saturday, Nov. 4–CPB303 and HEP experimental areasWednesday, Oct. 18, 2006 PHYS 3446, Fall 2006Jae Yu3•Subatomic particles cannot be seen by naked eyes but can be detected through their interactions within matter•What do you think we need to know first to construct a detector?–What kind of particles do we want to detect?•Charged particles and neutral particles–What do we want to measure?•Their momenta•Trajectories•Energies•Origin of interaction (interaction vertex)•Etc–To what precision do we want to measure?•Depending on the answers to the above questions we use different detection techniques Particle DetectorsWednesday, Oct. 18, 2006 PHYS 3446, Fall 2006Jae Yu4Particle DetectionInteractionPointelectronphotonjetmuonneutrino -- or any non-interacting particle missing transverse momentumÄBScintillating FiberSilicon TrackingCharged Particle TracksCalorimeter (dense)EM hadronicEnergyWire ChambersMagnetMuon TracksWe know x,y starting momenta is zero, butalong the z axis it is not, so many of our measurements are in the xy plane, or transverseWednesday, Oct. 18, 2006 PHYS 3446, Fall 2006Jae Yu5•Measures the ionization produced when an incident particles traverses through a medium•Can be used to–Trace charged particles through the medium–Measure the energy loss (dE/dx) of the incident particle•Must prevent re-combination of an ion and electron pair into an atom after the ionization•Apply high electric field across medium–Separates charges and accelerates electronsIonization DetectorsWednesday, Oct. 18, 2006 PHYS 3446, Fall 2006Jae Yu6•Basic ionization detector consists –A chamber with an easily ionizable medium•The medium must be chemically stable and should not absorb ionization electrons•Should have low ionization potential (I ) To maximize the amount of ionization produced per given energy–A cathode and an anode held at some large potential difference–The device is characterized by a capacitance determined by its geometryIonization Detectors – Chamber StructureWednesday, Oct. 18, 2006 PHYS 3446, Fall 2006Jae Yu7•The ionization electrons and ions drift to their corresponding electrodes, to anode and cathode–Provide small currents that flow through the resistor–The current causes voltage drop that can be sensed by the amplifier–Amplifier signal can be analyzed to obtain pulse height that is related to the total amount of ionizationIonization Detectors – Chamber StructureCathode: NegativeAnode: PositiveWednesday, Oct. 18, 2006 PHYS 3446, Fall 2006Jae Yu830cmx30cm D-GEM Detector SignalSignal from Cs137 SourceWednesday, Oct. 18, 2006 PHYS 3446, Fall 2006Jae Yu9•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 – HVFlat!!!Flat!!!Wednesday, Oct. 18, 2006 PHYS 3446, Fall 2006Jae Yu10•Operate at relatively low voltage (in ionization region of HV)•Generate no amplification of the original signal•Output pulses for minimum ionizing particle is small•Insensitive to voltage variation•Have short recovery time Used in high interaction rate environment•Response linear to input signal •Excellent energy resolution•Liquid argon ionization chambers used for sampling calorimeters•Gaseous ionization chambers are useful for monitoring high level of radiation, such as alpha decayIonization CountersWednesday, Oct. 18, 2006 PHYS 3446, Fall 2006Jae Yu11•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•Multiplication occur near the anode wire where the field is strongest causing secondary ionization•Sensitive to the voltage variation not suitable for energy measurement•But used for tracking device Proportional CountersWednesday, Oct. 18, 2006 PHYS 3446, Fall 2006Jae Yu12•G. Charpak et al. developed a proportional counter in a multiwire proportional chamber–One of the primary position detectors in HEP•A plane of anode wires positioned precisely w/ about 2 mm spacing•Can be sandwiched in similar cathode planes (in <1cm distance to the anodes) using wires or sheet of aluminumMulti-Wire Proportional Chambers (MWPC)Cathode planesWednesday, Oct. 18, 2006 PHYS 3446, Fall 2006Jae Yu13•These structures can be enclosed to form one plane of the detector•Multiple layers can be placed in a succession to provide three dimensional position informationMulti-Wire Proportional Chambers (MWPC)Wednesday, Oct. 18, 2006 PHYS 3446, Fall 2006Jae Yu14Momentum MeasurementssinL R q= �sinR L q=p =RBzec=sinLBzec q•A set of MWPC planes placed before and after a magnetic field can be used to obtain the deflection angle which in turn provides momentum of the particle•Multiple relatively constant electric field can be placed in each cell in a direction transverse to normal incident Drift chambers•Typical position resolution of proportional chambers are on the order of 200 m.Wednesday, Oct. 18, 2006 PHYS 3446, Fall 2006Jae Yu15A Schematics of a Drift ChamberPrimary Ionization createdElectrons and ions drift apartSecondary avalanche occursWednesday, Oct. 18, 2006 PHYS 3446, Fall 2006Jae Yu16•Ionization detector that operates in the Geiger range of voltages•For example, let’s
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