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HEP detectors M. Cobal. HEP detectors. ..rather a system of thousand of specialized sensors the interactions of particles with matter is used to get independent measurements of position, energy, momentum All this info has to be put together to reconstruct what happened.
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HEP detectors M. Cobal
HEP detectors ..rather a system of thousand of specialized sensors • the interactions of particles with matter is used to getindependent measurements of position, energy, momentum • All this info has to be put together to reconstruct what happened A detector is NOT a big camera to take pictures…
Charged particles (p, ions etc..) interact with gases, liquids, • amorphous solids and crystals • These interactions produce electrical or optical signals in these • materials wich betray the particles passage • Neutral particles are detected indirectly through secondary • particles: • a) photons produce free electrons (Compton or photoelectric • effects) or e+e- pairs • b) neutrons and neutrinos produce charged particles through • interaction with nuclei
Particle Detectors Many types of particle detectors: • Tracking devices – coordinate measurement • Time resolution counters • Particle identification devices • Spectrometers • Calorimeter – momentum and energy measurement
Here are some of the different detectors • Scintillators: provide fast time information, but have only moderate spatial resolution • -Gaseous counters: covering large areas (wire chambers) provide • good spatial resolution. Used in combination with magnetic fields to • measure momentum • Semiconductor counters: have a very good energy and spatial • resolution • Cerenkov counters and counters based on transition radiation: • used for particle identification • Calorimeters: measure the total energy at very high energy
Position measurement • Role of an Inner Tracker • Measure charged tracks with minimal perturbation • low mass detector small energy losses • small number of primary interactions • large fluctuations in deposited energy • Quantities of interest: • momentum (including sign) • angles (2) defining initial direction • point of origin (vertex) • Quality of data is vastly improved by magnetic tracking • muon, electron and tau identification • measuring momentum requires a magnetic field
Main principle: ionization products are either visualized (as in photoemulsion) or collected on electrodes to produce a computer- readable signal Basic requirements of high energy experiments: • high spatial resolution (~100 mm) • possibilities to register particles at the proper moment of time and with the high enough rate (good triggering) To fulfil the latter, electronic signal pick-up is needed. This is the reason why photoemulsion and bubble chambers were abandoned.... • Modern tracking detectors fall in two major categories: a) Gaseous detectors (“gas chambers”) resolution ~100-500 mm b) Semiconductor detectors ( ~few mm)
Bubble chambers Millions of pictures of collisions taken and studied one by one,,
Proportional & drift chambers • The simplest proportional • chamber: • A conducting chamber, filled with a gas mixture as cathode • A wire inside serves as an anode • Gas mixture adjustement: nr of secondary electrons caused by the • primary ionization to the nr of primary ion pairs • (~105 /pair for voltages of 104-105 V/cm) • Several anode wires -> measure coordinate (Multi-Wire chambers)
Electronic detectors Bubble chambers: too slow devices 1968:Georges Charpakat CERN invent the Multiwire Proportional Chamber Nobel Prize in 1992 Chamber with gas + wires under high voltages The particle ionizes the gas Charges collected from closest wire electrical signal Faster and can be Processed with computers
At very low voltages, charges begin to be collected, but recombination is still the dominant process Ionization modeat higher voltage, full charge collection begins Multiplication at a certain theshold voltage VT the electric field close to the surface of the anode is large enough to begin process of multiplication Proportional modeincreasing V0 above VT results in gains > 104 with detected charge proportional to primary deposited charge Limited proportionality at even higher voltages proportionality is gradually lost consequence of the E field distorsions due to space charge around the anode Geiger modethe region of limited proportionality eventually ends in a region of saturated gain – same size of signal indipendent of original deposition
Detector gas mixtures • Avalanche multiplication occurs in all gases • BUT..experimental requirements limit choises • low working voltage • stable operation at high gain • good proportionality • high rate capability • long lifetime • fast recovery
Main component: noble gas (e.g. Argon) • allows multiplication at relatively low E field • does not have molecules, produces only elastic scattering (little loss of energy) • Ar gives more primary ionization than He or N (Kr and Xe give even more, but expensive) • Counter full of Ar does not give stable operations • during avalanche process many excited Ar atoms decay emitting UV g’s (11.6 eV for Ar) • UV g’s strike cathod and eject photoelectrons which gives rise to another avalanche continuous discharge
Quenching gases • Chamber filled with pure Ar suffers such breakdown at low gain Polyatomic gases have many non-radiative vibrational and rotational excited states over a wide energy range If chamber contains a fraction of such a gas, its molecules will absorb energy from excited argon atom by colliding with it or dissociating it into smaller molecules Since temission >> tcollision UV g emission is `quenched’ • Presence of quencing gas can give enormous increase in stable obtainable gain Common property of hydrocarbon, alcohol families
Alternative to MWPC drift chambers • Ionization electrons produced along the particle passage arrive to the pick-up anode at different times • Knowing (from other detectors) the moment of particle’s arrival and field in the chamber, one can calculate coordinates of the track • Streamer detectors : wire chambers in which secondary ionization is not limited and develops into moving plasmas - streamers • If H.V. pulse is long enough, a spark will occur which is achieved in spark chambers
Semiconductor detectors • In semiconducting materials, ionizing particles produce electron- • hole pair, and the number of these pairs is proportional to energy • loss by particles • Equipping a slice of silicon with narrow pickup conducting strips, • and subjecting it to a high voltage, one gets a detector, analogous • to MWPC, with far better resolution • However semiconductor detectors have rather limited lifetimes • due to radiation damage.
Spectrometers • Momenta of particles are measured by the curvature of the track • in a magneticfield • Spectrometers are tracking detectors placed inside a magnet, • providingmomentum information • In collider experiments, no special spectrometers are arranged, • butall the tracking setup iscontained inside a solenoidalmagnet
Muon chambers Drift Cell Anode wire 3.6 kV Electrodes 1.8 kV • Overimposedlayers with independentcells • Resolution ~200 mm • Groups of orthogonallayersallow the reconstruction of a tri-dimensionalsegment Cathode -1.2 kV Drift lines . . . . . . . . . . Chamber (side view) . . . . . . CMS Barrel: 250 chambers, 172000 celle . .
Scintillation counters Signal passage of particles through a setup and measure the “time of flight” (TOF), scintillation counters are widely used. • Scintillators are materials (crystal or organic) in which ionizing • particles produce visible light without losing much of its energy. • The decay times of the fastest (organic) scintillator are ~ 1 ns • Inorganic(i.e: sodium iodide): • Doped with activator centers. Ionizing particles traversing the • crystal produce free electrons & holes, which move around until • captured by an activator center. This is transformed in an excited • state and decay with emission of light (in the visible egion) • Organic: • Mechanism is excitation of molecular levels which decay with • emission of light in the UV
The conversion of the light in the blue regionisdone via fluorescent • excitation of dyemoleculesknownaswavelengthshifters, mixed • to the primaryscintillator • The light from the scintillatorslabtravels down it by internal • reflection. At the border of the slabitiscollected by plastic light • guide or by fibres and sent to a photomultiplier • Photomultiplier • Photocathodecoated with alkalimetals, whereelectrons are liberated by photoelectriceffect • Electronstravel to a chain of secondaryemissionelectrodes (dynodes) atlarger and Largerpotentials. • ~ 4 secondaries are emitted per incident electron, amplificationfactors of 108 are achieved with 14 dynodes. • Transit time: ~50 ns
Particle Identification • Knowing momentum of particles is not enough to identify them, • complementary information is needed • For low energy particles TOF counters can provide this • complementary data • dE/dx depends on particle mass for energies below ~ 2 GeV. • Very reliable particle identification device: Cherenkov counters • In certain media, energetic charged particles move with velocities • higher than the speed of light in these media • Excited atoms along the path of the particle emit coherent photons • at a characteristic angle qc to the direction of motion:
