Introduction: detector tasks

Particle DetectorsTools of High Energy and Nuclear PhysicsGoal: to detect Individual Particlesand reconstruct their 4-vectorsThanks toH. Fenker (Jlab) and B. Surrow (MIT)

Introduction: detector tasks • Position measurement: localize hits of a charged particle (eg: wire chambers, segmented scintillators or calorimeters) • Momentum measurement: by measuring the particle deflection in a magnetic field (eg: magnetic spectrometer) • Energy measurement: deposition of energy in a localized volume (eg: calorimeters) • Particle identification: mass and charge of the particle (eg: Cerenkov detectors) • Triggering: select events of interest • Data acquisition system: readout of an event and storage after positive decision.

Introduction: detector performances • Time • Response time: time which is required to produce a signal after the passage of a particle in the detector (from ~10 ns to ~100 ms). • Deadtime: time that must elapse following the passage of a particle before the detector is ready for the next particle. • Efficiency That is (event registered)/(events emitted by source) The efficiency is the product of at least two quantities: • Intrisic efficiency: (event registered)/(events impinging on detector) • Geometric efficiency: (event impinging on detector)/(event emitted by source) • Resolution/Accuracy

Avalanche multiplication Detectors that see the electrons Wire chambers Time Projection Chamber Gas Electron Multiplier Detectors that see the light Scintillators Cerenkov Detectors Calorimeters Example of position detectors performance

Outline of class Interactions of Particles with Matter Charged particles Photons Using the Interactions:Particle Detectors Avalanche multiplication Detectors that sense Charge Detectors that sense light Aside: magnetic spectrometers Putting it all together

Charged particles: Energy and trajectories get degraded as they pass through matter: Ionization Bremsstrahlung Cherenkov radiation Photons: Flux gets decreased as they go through matter. All or nothing interactions Photo-electric effects Compton scattering Pair production Interaction of particles with matter

Outline of class Interactions of Particles with Matter Charged particle Photons Using the Interactions:Particle Detectors Avalanche multiplication Detectors that sense Charge Detectors that sense light Aside: magnetic spectrometers Putting it all together

Charged particles- Ionization Atomic electron is knocked free from the atom. The remaining atom is now an ion or left in an excited state (will decay by emitting a photon) Ionization Ion Free Electron Charged Particle Electric Field

Ionization: Bethe-Bloch formula where , , relate to particle speed, z is the particle’s charge.. The other factors describe the medium (Z/A, I, ), or are physical constants. dE/dx units is MeV cm2/g E= dE/dx * Dx / A value to keep in mind is 2 MeV.g-1.cm2 

Charged Particles: Bremsstrahlung Radiation of real photons in the Coulomb field of a nuclei of the absorber. Photon Electron Nucleus • Define X0 the radiation length: length during which the particle looses a fraction e of its initial energy • Critical energy (Ec) is the energy at which Bremstrahlung loss equal ionization loss Ec(e- Cu)=20 MeV Ec(- Cu)=800 GeV

Charged particles: Multiple Coulomb Scattering(both for ionization and radiation) Detectors scatter particles even without much energy loss… 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 in average. Most important at low energy.  is particle speed, z is its charge, X0 is the material’s Radiation Length.    0

Charged particles- Cerenkov radiation The electric field of a particle has a long-range interaction with material as it passes through a continuous medium. It does create a shock wave that causes the material to emit light when it’s speed is large enough v = c > c/n where n is the index of refraction of the material Also the light is emitted at the angle  = cos-1 (1/n) Photon energy ~ few eV (UV to visible) (not an efficient way to loose energy)  1/n

Cerenkov light produced by fuel elements of a nuclear power plant.

Interaction of photon with matter Photo-electric effect Compton scattering Pair production

Interaction of photon with matter Total probability for  interaction: Probability  per unit length or total absorption coefficient Such that the flux of photon is Absorption length= (absorption coeff)-1

Interactions of Particles with Matter - Summary When particles pass through matter they usually produce either free electric charges (ionization) or light (photoemission). Most “particle” detectors actually detect the light or the charge that a particle leaves behind. In all cases we finally need an electronic signal to record.

Outline of class Interactions of Particles with Matter Charged particle Photons Using the Interactions:Particle Detectors Aside: Avalanche multiplication Detectors that sense Charge Detectors that sense light Aside: magnetic spectrometers Putting it all together

Particle Detectors… Avalanche Multiplication When a particle passes through matter, it creates just a few electrons/ions or photons But the best we can do is to detect signals of the order of nV in a 100 Ohm resistor which correspond to I= U/R= 10-9 /100 = 10-11 A = 108 electrons. s-1 Which means: The detectors need to amplify the charge produced by particles going through matter. By giving the charges a push, we can make them move fast enough so that they ionize other atoms when they collide. After this has happened several times we have a sizeable free charge that can be sensed by an electronic circuit.

Particle Detectors: amplification with the photo multiplier Secondary Emission Energetic electrons striking some surfaces can liberate MORE electrons. Those, in turn, can be accelerated onto another surface … so on. Photoelectric Effect: A photon liberates a single electron • PMTs are commercially produced and very sensitive. • One photon --> up to 108 electrons! • Fast! …down to ~ few x 10-9 seconds.

Particle Detectors: amplification with the E-Field • V is the voltage • rc radius of the outer cathode plane • ra radius of the inner anode • Close to the wire the E field is very • Intense, the charged particle is • Accelerated and • as a result creates secondary • Particles.

Particle Detectors… Gas Electron Multiplier (GEM) Gas Ionization and Avalanche Multiplication again, but… … a different way to get an intense electric field, … without dealing with fragile tiny wires --V GEM ~400v 0.002” To computer http://gdd.web.cern.ch/GDD/

Dense Material => Lots of Charge. Typically no Amplification Particle Detectors: bypassing the amplification (new methods) • Semiconductor • Silicon • Diamond Strips Pixels Drift • Noble Liquid • Liquid Argon Calorimeter Electrons are knocked loose in the silicon and drift through it to electronics. 0.001” 0.012” Readout strips may be VERY NARROW Signals to Computer

Avalanche multiplication Detectors that see the electrons Wire chambers Time Projection Chamber Detectors that see the light Scintillators Cerenkov Detectors Calorimeters Particle detectors

Particle Detectors: Gas Filled Wire Chamber Let’s use Ionization and Avalanche Multiplication to build a detector… Make a Box. Fill it with some gas: noble gases are more likely to ionize than others. Use Argon. Insert conducting surfaces to make an intense electric field: The field at the surface of a small wire gets extremely high, so use tiny wires. Attach electronics and apply high voltage. We’re done!!

Straw Tube Tracker for the COSY-TOF Experiment Julich Institute (Germany)

Central Drift Chamber (Hall D Jlab)

Straw chambers: better resolution y X 2D solution: The wire touched gives X position info The time of transit between the two amplis gives the Y position info

Multi-Wire Gas Chamber stop start • Best 1D solution: • Use an external trigger to start a clock • Measure the time it takes for the electron to drift from the initial ionization to the wire. Resolution ~ 10 mm

TPC: 3D position information. • Time Projection Chamber (TPC): Drift through a Volume • Just a box of gas with • Electric Field and • Readout Electrodes • Readout elements only on one surface. • Ionization Electrons drift to Surface for • Amplification • Charge Collection • Readout Electrode Position gives (x,y) • Time of Arrival gives (z). Y Readout electrodes Z Particle track X Cathode Anode (gain)

Avalanche multiplication Detectors that see the electrons Wire chambers Time Projection Chamber Detectors that see the light Scintillators Cerenkov Detectors Calorimeters Particle detectors

Particle Detectors… Cerenkov Counter If b=v/c > 1/n, there will be light. If not, there won’t. Can be used for trigger system If you know the momentum of the particle: can be used for PID Cerenkov Counters – sensitive to  TRD Counters – sensitive to   = v/c = p/E = (1- 2)-1/2 = E/m Momentum (GeV/c)

Light is emitted at the angle  = cos-1 (1/n) Rich detector : Ring imaging Cherenkov

Scintillators • Scintillation Counters are probably the most widely used detectors in Nuclear and High Energy Physics. • Scintillator material are special material that • emit light when traversed by energetic particles and • can shift the wavelength of this light • to be harnessed by PMTs • They can be solid, liquid (even gas) • They can be molded in all kind of shapes

Solids Liquid Saint Gobin Inc

Sample experiment MIT/Bates Detecting scintillation In Air.

Particle Detectors: Calorimeter Used to measure energy. Based on Bremsstrahlung effect Suppose an initial photon of energy E0 After t radiation lengths, the average energy of secondary is: The shower stops when The number of secondaries is The energy resolution of the calorimeter therefore goes as EM (or hadronic) calorimeters are used because: - They can detect charged or neutral particle - They produce a really fast signal (10-100 ns) that is ideal for making trigger decision

Alice’s ZDC calorimeter CERN Hall A/DVCS calorimeter (50*30 cm2) JLAB

Particle Detectors:aside: magnetic spectrometer Nature lets us measure the Momentum of a charged particle by seeing how much its path is deflected by a magnet. Just as light of different wavelength is bent differently by a prism... (x2,y2) (x1,y1) Magnet

MAMI (Germany)

IN frame Usually a “short” target OUT frame usually equipped with a Vertical Drift Chamber (VDC) Central trajectory Four quantities measured in the IN frame and 4 in the OUT/VDC frame. The best is to design the magnet to be insensitive to xIN and with <x|> as the main coefficient

Putting it all Together:A Detector SystemHall C/JLAB

Putting it all Together:A Detector System

QWEAK: conceptual overview • Elastic e-p scattering on liquid hydrogen target • Toroidal magnet to provide momentum dispersion • Collimator system to select elastic events only • Lower energy inelastic events bent outside of the detector acceptance

Introduction: detector tasks