Particle Detectors - PHY743 - Detect charged particles (e, , , K, …) Detect the neutral particles (n and ) or separate neutral from charged Determine the time reference Determine the position reference Determine the energy deposition Identify the type of particle Tools and instrument to scope and sense the microscopic elements

Detectors Based on EM radiations I Excitation and followed by de-excitation Example – Scintillation by charged particles Crystal (fast, good optics, less rad. damage, but expensive) Plastic (fast, easy to shape, much cheaper for large volume) Wave length shifter (Ultraviolet to blue/green) Light output proportional to energy loss of the charged particle depending on radiator thickness and particle energy/momentum (if below minimum ionizing energy), i.e. Light intensity I  X and dE/dx character of the particle Charged particle EM energy () Molecular electron ex. g.s.  (light), collected by detector ~10-9 sec ex. g.s.

Detectors Based on EM radiations I – Cont. Nuclear collision followed by scintillation Medium – Organic (plastic) scintillator (CH), i.e. rich amount of H Need large thickness to compensate low collision cross section (thus low efficiency) Neutron detection – Head on collision Undetectable neutron (n) Stationary proton (p) in the radiator Recoil n Knocked out p that causes scintillation by the radiator

Detectors Based on EM radiations I – Cont. Direct absorption or Compton scattering of low energy  Example – Ge-detector (heavy crystal) E < ~2 MeV Low rate capability (long signal process time) Low efficiency Extremely high energy resolution, E ~ 3 keV for 1 MeV  Example – BGO-detector (crystal) Higher  energy detection range than Ge-detector Less energy resolution than Germanium scintillator Both of them very expansive

Detectors Based on EM radiations I – Cont. Basic Structure of a Scintillation Detector Scintillator: Convert energy loss to photons Light guide: Guide photons to Photomultiplier (PMT) (Issues: Cross section matching and collection efficiency – Ultra Violate Transmitting Lucite) PMT: Convert photons to photo-electrons then amplifying them by ~105 to 108 times (Issue: Response function, gain, rise/transit times, and linearity for different applications) Scintillator (Radiator) PMT Light Guide

Detectors Based on EM radiations I – Cont. Illustration of Signal Process by a Single Scintillation Detector Analog signals: SL and SR, i.e. V(t). [Si  Total energy loss, SL/SR  position (low precision). Issue: Signal/Noise Ratio] Discriminator: Generate (0.6-0.8V with adjustable width ~10-100 ns) logic signal Ti at the time Si passes Vth. Ti registers the detection of charged particle and references the detection time Coincidence of TL and TR – removes noise signals from PMT’s (Accidental coincidence rate: RLRRT) Average of TL and TR: Meaning time – More precise detection time TL /TR  position (better than SL /SR ration, still low precision – mms to cms) Used as a VETO to separate charged and neutral particles Charged Particle TL SL SR TR Disc Vth Disc Vth

Detectors Based on EM radiations I – Cont. Example of Scintillation Detector for Charged Particles S1.AND.S2 defines a charged particle detection (Real or Accidential - Rate) Overlap configuration gives fast position determination (Limited precision but fast – high rate tracking) Time of Flight (TOF) = T2 – T1 determines  { = L/(cTOF)} (Separation of particles with different masses) Ti – Time of particle detection S1 Plane S2 Plane Charged particle with known or within a range of momentum L – Length of particle trajectory

Detectors Based on EM radiations I – Cont. Example of Scintillation Detector for dE/dx to separate none minimum ionizing particles within a momentum range Example of Scintillation Detector as VETO and Neutron Detection TOF Thick S2 dE/dx Thin S1 Front TOF vs dE/dX can separates particles with different masses within a momentum range (Range Detector) Incident p n beam Vetoed VETO n Counter Ray Identified as n Scint. Stack TOF for p

Detectors Based on EM radiations I – Cont. Example for Low Energy Photon Detection Precision measurement of EM transitions of nuclei BGO Ge Light Guide PMT BGO Ge Low energy  High energy  background

Detectors Based on EM radiations II Čerenkov radiation (charged particle)=v/c > o(speed of light in medium)=1/n Č radiation EM radiation Well formed wave front  A B cos = 1/(n) Take place when n  1 Well formed wave front Č radiation

Detectors Based on EM radiations II – Cont. Features of Čerenkov radiation Instantaneous (direct EM radiation by charged particles) Well defined orientation: Cos = 1/(n) for n  1 Radiation energy  4 Low radiation power in general. Number of total photoelectrons from PMT photo cathode can be expressed as: NPE = ALSin2  – Radiation angle w.r.t. particle trajectory L – Length of particle trajectory in the radiator A – Characteristic constant that depends on light collection efficiency and quantum efficiency of the PMT. The typical A is about 100. Application for Particle Identification

Detectors Based on EM radiations II – Cont. Gas Čerenkov Detector (n  1) Typically used in hadronic beam line Size is very large (large L to compensate small ) PMT for particles with lower  (or higher mass) PMT for particles with higher  (or lower mass) Charged particle w/ small momentum and angular ranges Gas cylinder Tilted reflection mirror

Detectors Based on EM radiations II – Cont. Threshold Čerenkov Detector Low n solid radiator: Aerogel (n=1.01 to 1.06) Čerenkov Radiator (n) For particle with n > 1 there is a Č from radiator, light diffused to PMT’s No Č from the radiator for particles with n < 1 PMT Light diffusion box

Detectors Based on EM radiations II – Cont. Threshold Č Detector (Total Internal Reflection) Light absorption material Radiator with c Particle with high  ( > c) Particle with low  ( < c) PMT Radiation is absorbed (thus mass) threshold is set by critical angle c Suitable for near normal incident , small angular & momentum spread Radiation is transmitted by total internal reflection to PMT’s

Detectors Based on EM radiations II – Cont. Radiation Energy ( Sin2) Threshold Detector Using 2-D  vs light output size to separate particles with different masses Čerenkov Radiator (n) Light diffusion box PMT Particle with lower  Smaller total light output Particle with higher  Larger total light output

Detectors Based on EM radiations III Pair (e+e-) Production by Photon () Annihilation of e+ or e- Continued process til E < 1.022 MeV – EM Shower  Nucleus e+ e- Generate scintillation or Č radiation Heavy crystal is better E min. > 1.022 MeV  e+ or e- Nucleus Two real photons (2) Heavy material is better

Detectors Based on EM radiations III – Cont. Shower Counter by Pb-Glass (Crystal) Radiation length and radiator thickness Total energy measurement and precision Particle Identification (PID) PMT on each optically separated unit Pb-Glass Array A shower produces large overall light output by Č radiations ,e+, or e- Heavy charged particles Č radiation without shower process

Detectors Based on EM radiations III – Cont. Shower Counter by Pb-Scint. Sandwich Thin Pb plates sandwiched by scintillators PMT Light guide A shower from Pb produces large overall light output by Scintillation ,e+, or e- Heavy charged particles Scintillation (dE/dx) without shower process Other Type: Pb-Scint. Fiber Detector Less energy resolution but cheap in cost Large size for high energy leptons

Your Exercise A pair of scintillation counters forms a TOF hodoscope. If overall timing resolution is  = 100ps, what is the minimum path length needed to have a 4 separation for + and K+ at p=1.2GeV/c. A Č radiator has n=1.5. A screen that views the Č radiation ring located 5cm behind the radiator. When +, K+, and p travel through them in the normal direction with p=1.3 GeV/c, what are the inner image ring diameters for the three types of particle? If you have 1mm Pb plates and 5mm thick scintillator plates, to build a Pb-Scint. Sandwich shower counter with 10 radiation lengths (to ensure absorption of total energy), what will be the detector overall thickness?

Detectors Based on Ionization Ionization by charged particle Energy of ions in presence of electrostatic field Secondary ionization and avalanche (r0) Overall charge gain Charged Particle Free Electron Electric Field Ion Ionization Charged Particle Gas Molecule + - - Ion + Ion Charged Particle

Detectors Based on Ionization – Cont. The simplest example: Geiger Counter Ionization gas: Argon Quench gas: Halogens Detect ionization particles or photons by a short CASCADE (gas ionization) effect that gives an electric pulse HV: Enough for CASCADE but not continued “breakdown” Commonly used as radiation monitor for  particle, -ray, and even X-ray Cathode Tube Gas mixture Anode Wire Ionization particle or photon Electronic Counter Amplifier

Detectors Based on Ionization – Cont. Multi-Wire Proportional Chamber – Drift Chamber Charged Particle & Initial Ionization (Few Pairs) + - Electric Field Lines Cross Section View of One Coordinate Plane Cathode Foils at -HV One Cell Avalanche takes place near the sense wire (r ~ 0) Sense Wires at Ground Field Wires at -HV Gas Mixture: Ar (Ionization) + Ethane (Quench)

Detectors Based on Ionization – Cont. Gas Mixture Argon – Maximize the ionization rate Ethane – Larger molecule, collision rate for constant velocity, quench Drift Time Basic Electronics Position Drift Time Slope - Velocity Sense wire Pre-Amplifier Long distance bus Amplifier - Discriminator To DAQ

Detectors Based on Ionization – Cont. Initial ionization: 2-10 pairs (path length & pressure) Positive ions: >2000 times slower than negative ions Detector rate capability and efficiency depends on the + charge collection and gas recover/refresh speed Negative ions (electrons): Accelerate alone the field direction and gain kinetic energy Continued secondary ionization (amplification ~10) Consecutive “collisions” and ionizations make the drift velocity near constant Avalanche near sense wire: charge amplification ~103 Overall charge gain: ~104 (an electric pulse) Drift time gives the position Resolution:   0.2-0.3 mm (single cell)

Detectors Based on Ionization – Cont. Basic Position Determination by Drift Chamber Off-Set Planes to Remove Left-Right Ambiguity T0 references ZERO drift TDC - T0 gives the position -HV PMT Disc. TDC Start Single Drift Chamber Plane Amp Disc. TDC Stop Left – Right ambiguity X - Plane X’ - Plane TL + TR = Constant

Detectors Based on Ionization – Cont. Track Particle Trajectory by Multiple Planes 2 Fitting to Determine a Straight Trajectory Line Two Separated Sets X, X’ U, U’ V, V’  Each set measures x 6 times and y 4 times Z

Detectors Based on Ionization – Cont. Example: Application of Cylindrical Drift Chamber Solenoid York Iron Constant Axesial B Field Cylindrical Chambers Inner Fast Detectors Circular Motion Due to Transverse Momentum

Detectors Based on Ionization – Cont. Other Type of Gas Chamber – MWPC Small wire spacing and gap (w/o field wires) Position determined by wire position – low precision Faster (smaller cells and none-constant drift velocity) Can be 100 times higher rate per wire than DC Cheaper than other high rate tracking devices TDC Amp Disc Anode on ground Field Lines Single MWPC Plane - HV on the cathode

Detectors Based on Ionization – Cont. Other Type of Gas Chamber – Vertical DC (VDC) Large Gap – Vertical/constant v for secondary charge drift Measure several drift times for each particle Provide position and angles at the same time Particles must incident with angles High precision and effective but low rate capability Field Lines TDC Amp Disc Anode on ground Single VDC Unit - HV on the cathode

Detectors Based on Ionization – Cont. High Rate Chamber – Gas Electron Multiplier (GEM) Still Gas Ionization and Avalanche, again, but… A different way to get an intense electric field, Without dealing with fragile tiny wires, and Release + ions much faster -V ~400v 0.002” GEM To computer http://gdd.web.cern.ch/GDD/

Detectors Based on Ionization – Cont. Solid State Tracking Detector – Silicon Strip Detector (SSD) Fast, thus high rate capability Fine pitch, thus high precision Compact Much More expansive for large size Radiation Damage

Effect to Particles by Detectors Other Than Detection Energy Loss – such as dE/dx effect Multiple (Coulomb) Scatterings MCS theory is a statistical description of the scattering angle arising from many small interactions with atomic electrons. MCS alters the direction of the particle. Most important at low energy.  is particle speed, z is its charge, and X0 is the material’s Radiation Length.   

Put It All Together: A Detector System Example by Hall C HMS

Summary of Particle Detectors Detect Particles by Letting them Interact with Matter within the Detectors. Choose appropriate detector components, with awareness of the effects the detectors have on the particles. Design a System of Detectors to provide the measurements we need.