PHY357 Lecture 11 Experimental Methods Accelerators Particle Interactions Particle Detectors Full experiment (eg. ATLAS)
Introduction! Several different general classes of particle detectors (sensors)! Ionisation chambers charged particle trajectories! Scintillation detectors direct detection of photons! Semiconductor detectors very precise charged particle trajectories! Cerenkov detectors particle identification e/m! Calorimeters absorb energy of particles (Total E measurement) Reference: W. Leo, «Techniques for Nuclear and Particle Physics Experiments»
Ionisation Chambers! Detect passage of charged particles by measuring the total ionisation (electrons and ions) produced in a detection medium! Medium can be gas, liquid or solid! Provide an external electric field to separate ions and electrons after the passage of the particle and guide them to electrodes! Charges drift towards electrodes and are amplified during readout! Number of charges given by N I = -de/dx d/w! de/dx given Behte-Bloch, d is thickness of medium, W is mean ionisation potential (~30 ev in gas)
Ionisation regimes! Different operating regimes for a ionisation detector! Depends on Electric field in the medium (related to Voltage applied)
Ionisation regimes! Recombination regime! Weak electric field: Ions and electrons can recombine before drifting! Small fraction of ions collected! used to calibrate other detectors! Ionisation regime! Field large enough to drift all charges to the electrodes! Advantage: Signal collected proportional to energy deposited! Disadvantage: Number of charges quite small! specialised amplifiers needed to measure signal! Examples: Liquid Argon calorimeters, Silicon detectors! Proportional regime! Fields so high (E ~ 10 4 V/cm) charges accelerate towards electrodes! Secondary ionisation (amplification) possible N total = N 0 e αd (e αd ~ 10 4 ++)! Most gas detectors operate here simplifies readout electronics
Proportional Regime! Disadvantage: ionisation deposited less well measured! Large fluctuations in primary ionisation enhanced by amplification! Mostly use these to measure the position of particles! Drift chambers, Proportional wire chambers (PWC)! Relativistic charged particles actually lose relatively little of their energy to ionisation (few MeV/cm)! can measure their trajectory at many points before they stop (calorimeters usually stop particles )
February 12, 2018 UA1 Central Drift Chamber
Geiger Counters! For even larger fields produce avalanche ionisation! Multiplication occurs throughout the sensitive volume! Secondary photons produced in atom de-excitation also contribute! Can even produce audible signals (click!)! Avalanche stops when ionisation counteracts applied electric field! Detector is insensitive until charges clear
Scintillators! In a transparent medium, charged particle can excite an atom! Light emitted when atom dexcites: produces fluorescence! Photons produce signals in photosensitive detectors! Scintillating media produce very rapid signals! Rise time is typically 1ns (or less)! Faster than ionisation signal! Can be used to make fast-decisions! Trigger
Scintillating materials! Examples of scintillating materials include:! Organic materials (Plastic, liquids, crystals)! About 100 ev to create a photon, 400 nm light emitted! Inorganic crystals: NaI, PbWO 4, BGO,! Impurities added to crystal engineered to fluoresce! 20-200 ev needed to create a photon, but 500nm light possible! Photo detectors typically adapted to wavelength emitted! Exploit photo-electric effect to convert photon to charge! Usually a thin Alkali layer on surface of photo-detector! Efficiency of 25% good! Depends on photon wavelength
February 12, 2018 Collecting Scintillation Photons! Photons must escape scintillation medium to get to photodetectors! Number of photons exiting characterised by exponential attenuation! N(x) = N0e-x/λ where λ is typically 1m (or more)! Reduce losses at edges of scintillators (total internal reflection)! Wrap scintillators in reflective foil or sputter metal on surface! Also use wavelength shifters at end of scintillator change photon wavelength for transport and optimal conversion to charge
Photomultipliers! Once photon converted to charge on photocathode! Typically have a series of electrodes (10-14 stages)! Made of an optimal metal (low work function)! When energetic electrons arrive liberate many additional charges! Amplifies signal by as much as 10 7 after 14 stages! Scintillator efficiency depends on:! Photo-creation threshold for the medium! Attenuation length of light in the scintillator! Loss of photons at edges! Quantum efficiency of photo-detector
February 12, 2018 Scintillator Examples Super Photomultiplier CDF Calorimeter (Fermilab) " Super Kamioka Neutrino Detector #
Semiconductor Detectors! A low gain example of an ionisation detector! Charged particle doesn t actual ionise the material! Creates electron-hole pairs in the semi-conductor band gap! Only need about 3 ev to create pairs in silicon (vis 30 ev in gas)! Advantages! Very good energy resolution! Compact solid! Pattern with tiny electrodes! Can be very thin (small X 0 )! Mechanically stable! Disadvantages! Expensive! Fragile (also degrades with radiation)
Semiconductor Detector Applications! Measuring the energy of particles! Can make precise de/dx measure! Limited thickness (~1mm)! Limited areas (10 cm 2 )! Measuring the position of charged particles! Use semiconductor lithography to make precise collection electrodes! Microstrip detectors (50-100 µm pitch)! All experiments since early 1990s have one! Pixel detectors (50 x 400 µm readout)! First used in LHC experiments! Charged Coupled Devices (CCD) can be even smaller, but slower AMS detector (Space station)
Semiconductor Detectors February 12, 2018
Cerenkov Radiation! When a particle moves through a medium at v > c/n! It is moving faster than light can travel in that medium! A superposition of EM fields created along a particular direction: Θ C! Charged particle polarises medium as it passes! Medium can t reconfigure itself faster than c/n! Real photons radiated at Cerenkov angle: Θ C! EM analog to bow shock wave along track of a plane traveling faster speed of sound in air
Cerenkov Light γ rays from fission travelling faster than c/n in water
Cerenkov Threshold! Produced in any transparent medium! Energy loss is negligible (100x less than scintillation light)! But there is a threshold! No light produced if β<1/n! Right at threshold first light is produced in direction particle is traveling! Exploit this threshold to distinguish charged particles with same p but different m! Threshold given by: m th = p 1 β 2 th β th = p n 2 1! Particles heavier than m th will not emit Cerenkov radiation
Ring Imaging Cerenkov Detectors! Can actually produce/detect rings of photons! Opening angle of cone proportional to β Muon ring in SuperKamiokande
The SuperK Experiment Super-Kamiokande
Calorimeters! Measure the total energy of particles by absorbing them! Typically sample a fraction of the energy they deposit! Spatial segmentation allows to measure many particles at once! Incident particles initiate a shower (EM or hadronic) in detector! The shape, size and composition of shower depends on! Incident particle! Material used to make up the detector! Sample the energy in the form of! Heat, ionisation, photo-excitations, Cerenkov radiation! Total signal proportional to energy of incident particle
Calorimeter Systems! Constructed to cover all solid angle around collision point! Measure the energy of charged and neutral particles! Also measure energy flow from hadronisation of quarks (jet)! EM showers characterised by! Radiation length (X 0 )! Critical Energy (E c )! Moliere shower radius (R m = 21MeV/E c ) CMS ATLAS
Calorimeter Systems! Depth segmentation separates! EM showers (early)! Hadron showers (later) λ had >> X 0! Often two separate calorimeters
Sampling Calorimeters! Alternate layers of absorber (passive material) and readout (active material)! Measure only a fraction of the energy deposited/absorbed! Requires calibration to determine sampling fraction CDF Hadron Calorimeter
Summary! Looked at several different particle detector techniques! Ionisation chambers! Scintillation detectors! Semiconductor detectors! Cerenkov detectors! Calorimeters! Next see how these come together in an overall experiment