Detectors & Beams Giuliano Franchetti and Alberica Toia Goethe University Frankfurt GSI Helmholtzzentrum für Schwerionenforschung Pro-seminar Winter Semester 2015-16 DPG Spring Meeting Giuliano Franchetti & Alberica Toia2015 Heidelberg 23-27 March 1
Organization Language: English Lecture: Wednesday 09:00-11:00 Phys 02.114 Credits: 4CP Office hours: tbd on demand 2
Info: Email and Website E-Mail: G.Franchetti@gsi.de A.Toia@gsi.de Website: https://webdocs.gsi.de/~alberica/lectures/db_ws1516.html 3
Introduction General bases of measurements and detectors 4
Measurements in heavy ion collisions Interaction of particle with matter Particle detectors Measurement of charged particles Charge and momentum deflection in the magnetic field Mass (particle identification) - specific energy loss - velocity (flight time, Cherenkov radiation,...) - Total energy - penetration calorimetry (measurement of muons) Measurement of neutral particles Reconstruction from measurements of the decay products calorimetry photon conversion Examples from real experiments 5
General demands on particle detectors Particle detection Momentum or energy measurement Particle identication: electron - pion - kaon... Reconstruction of the invariant mass of decay products m2inv = ( i pi )2, four-momenta Missing Mass" or Missing Energy" for undetected particles like neutrinos Sensitivity to lifetime or decay length 6
Interaction of particles with matter Energy loss by ionization (heavy particles) Interaction of electrons Energy loss by Ionization Bremsstrahlung Cherenkov effect Transition radiation Interaction of photons Photo-effect Compton scattering Pair Production 7
Particle Detectors Gas detectors Semiconductors Scintillation counters Electromagnetic Calorimeters Hadronic Calorimeters (Detection of neutral particles: neutrons and neutrinos) 8
Momentum Measurements For fixed B and q the momentum p is proportional to the radius of curvature 9
Forwards spectrometers: Deflection (z,y) mainly fixed target experiments or forward spectrometers Cylindrical detectors: Deflection (x,y) - Solenoid: + Large homogeneous field inside - Weak opposite field in return yoke - Size limited by cost - Relatively large material budget - Toroidal + Field always perpendicular to p + Rel. large fields over large volume + low material budget - Non-uniform field - Complex structural design Momentum Resolution Sagitta method degrades linearly with momentum Improves linearly with B field Improves quadratically with radial extension of detector circle fit through measurement points along the track with point resolution x for each point 10
Tracker Technologies 2 major technologies are used for tracking detectors Gaseous detectors Ionization in gas (creation of electron-ion pair) O(100 e/cm) Not sufficient to create significant signal height above noise for standard amplitudes Gas amplification needed to reach sufficient signal height above noise Silicon detectors Ionization (creation of electron-ion pair) in solid state material O(100 e/ m) No amplification needed 11
Gas detectors 12
Ionization chamber no gas gain, charges move in electric field and induce signal in electrodes. 2 electrodes form parallel plate capacitor. Proportional Counter gas amplification with a gas gain in vicinity of wire Multi-wire proportional chamber MWPC planar arrangement of proportional counters without separating walls Drift chamber needs well defined drift field introduction of additional field wires in between anode wires. number of anode wires can be reduced wrt MWPC at improved spatial resolution (but affected by diffusion) Time Projection Chamber 3-dimensional measurement of a track (mostly) cylindrical detector central HV cathode + MWPCs at the endcaps of the cylinder electrons drift in homogenous electric fields towards MWPC, where arrival time and point and amount of charge are continuously sampled + complete track determination good momentum measurement + relatively few wires (mechanical advantage) + since also charge is measured: particle identication via de/dx + drift parallel to B transverse diffusion suppressed - drift time: relatively long - tens of microseconds - large data volume 13
Semiconductor detectors 14
Position measurement with semiconductor detectors segmentation of readout electrodes into strips, pads,pixels -tracking of particles close to primary vertex before multiple scattering -identification of secondary vertices Micro-strip detectors principle and segmentation see above typical pitch 20-50 m width of charge distribution = 10 m signal in 300 m Si: = 25 000 e for minimum ionizing particles order 100 channels/cm2 Silicon drift detectors potential inside wafer, analog to gaseous drift chambers: charge carriers drifting in well-defined E-field measurement of drift time position of ionizing track typical drift time: a few s for 5-10 cm Pixel detectors principle: like micro-strips, but 2-dimensional segmentation of p+ contacts: 'pixel` each pixel connected to bias voltage and readout electronics + 2-dim information like double-sided micro-strip, but more simultaneous hits per detector + low capacity and thus low noise good S/N - large number of read-out channels expensive, large data volume - pixel contacts are complicated typical pixel areas 2000 m2 order 5000 channels/cm2 15
Particle Identification 16
Special Signatures 17
Energy loss by ionization 18
Energy loss by ionization at low energies / velocities decrease as approx. ~1/ 2 up to > 1 broad minimum at =3-4 1-2 MeV cm2/g `minimally ionizing particle' logarithmic rise and `Fermi plateau' caused by growth of the transverse component of the E field of the particle with γ more pronounced for gases (50%) than for solids (10%) very low velocities (v < v cannot electron be treated this way) 19
Interaction of electrons Energy loss by ionization 20
Bremsstrhalung 21
Interaction of photons with matter 22
Total absorption cross-section Total cross-sections: σ = σf + σc + σp Multiply by number of atoms per volume unit N: μ= Nσ= N a ρσ A Na Avogadro constant, A atomic mass, ρ material density μ total absorption coefficient inverse value of mean free path of photon at material di = -μ I dx Equation for decreasing of photon number: I e x I0 23
Electron Photon Showers Combined effect of pair production and brammstrhalung for high energy photons - photon (E0) converts in matter into e+ e- pair (E0/2) - e+ and e- then emit energetic bremmstrhalung photons creation of an electromagnetic shower continues until the energy of the e+e- produced drops below the critical energy Ec when they lose energy by atomic collisions rather than bremmstrhalung Number of cascade particles increases geometrically: N(t) = 2t Mean energy of particles ε is: (t ) E E t 2 N (t ) Multiplication continues up to critical energy EC Maximal number of particles NMAX at deepness tmax N MAX ~ N MAX ~ E N (t MAX ) EC E N (t MAX ) EC tmax ~ ln Є 0 4 1 Radiation length of material X0 : distance over which electron energy is reduced E E/e due to radiation loss only 1 2 3 4 1/2 1/4 1/8 Width of electromagnetic shower Moliere radius R0 ~ cm (about 90% of shower energy is inside) t N(t) ε(t)/e 24
EM Calorimeter 2 different types of detectors homogeneous calorimeter (lead glass, lead tungstate, barium fluoride) Sampling calorimeter (scintillator + absorber) as hadronic calorimeter ( 10 times the interaction length) Energy resolution Optical fiber collects light Bremsstrahlung effective cross-section decreases with 1 / m4 only applicable for the measurement of e ±, γ 25
Cherenkov effect 26
Cherenkov detectors Threshold Cherenkov radiation observed > thr separation for a given momentum RICH measurement of c in medium with known n optics such that photons under certain focus on a ring of radius r 27
Transition Radiation Emission angle θ ~ 1 / the direction of the particle trajectory ω~ ω Emission of X-ray quanta (kev) P for > 1000 more boundary layers Interference Yield per boundary layer is proportional to α = 1/137 for the emission of a photon ~ 100 boundary layers are necessary! 28
Transition Radiation Detectors 29
Time of Flight 30
The time a relativistic particle, traveling at velocity v, covers a path of length L is: where E and pc are the particle energy and momentum where t0=l/c is the time taken by a particle traveling at the speed of light 31
Resistive parallel chambers 32
ALICE TOF: large coverage and high granularity Particle ID in high multiplicity environment... - Large array to cover whole ALICE barrel: 160 m2-100 ps time resolution - Highly segmented: 160,000 channels of size 2.5 x 3.5 cm2 to cope with very high multiplicity events TOF 33
Muon Measurements Momentum measurement: easy Track reconstruction in the magnetic field Identification: difficult de / dx: difficult to separate pions (mμ mπ) Tome of Flight: difficult to separate pions (mμ mπ) Cherenkov: difficult to separate pions (mμ mπ) TR: TR not produced by muons Calorimetry: no showers production by muons Identification of decay products: rejected because muon to long-lived (τ = 2.2 microseconds) Absorber technology: muons penetrating more than other particles (except neutrinos) absorber thickness of several 34
Comparison different PID methods for K/ separation 35
Decay Particles Invariant Mass Combinatorial background Event topology 36
Invariant Mass 37
Event topology Impact Parameter: Prolongation of a track to the primary vertex. Distance between primary vertex and prolongation is called impact parameter. If this number is large the probability is high that the track comes from a secondary vertex. 38
Impact parameter resolution 39
Examle: ALICE Silicon Tracker 40
Real Life Examples: multi-detector complex 41
ALICE: A Large Ion Collider Experiment 42
ATLAS: A Toroidal LHC ApparatuS 43
ATLAS: A Toroidal LHC ApparatuS 44
CMS: Compact Muon Solenoid 45
CMS: Compact Muon Solenoid 46
Extra 47
Forward Spectrometers Deflection in (y-z) plane 48
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ALICE 50
Momentum Resolution Worsen momentum resolution 51
Magnets for 4 Detectors Solenoid Toroid + Large homogeneous field inside - Weak opposite field in return yoke Size limited by cost - Relatively large material budget Examples: Delphi: SC, 1.2 T, 5.2 m, L 7.4 m L3: NC, 0.5 T, 11.9 m, L 11.9 m CMS: SC, 4 T, 5.9 m, L 12.5 m + Field always perpendicular to p + Rel. large fields over large volume + Rel. low material budget - Non-uniform field - Complex structural design Example: ATLAS: Barrel air toroid, SC, ~1 T, 9.4 m, L 24.3 m 52
Solenoidal and Toroidal fields at colliders Deflection in (x-y) plane 53
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Sagitta method 55
Sagitta/radius obtained by a circle fit through measurement points along the track with point resolution x for each point Resolution: degrades linearly with momentum Improves linearly with B field Improves quadratically with radial extension of detector 56
Toroidal fields 57
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Energy Loss de/dx Relativistic rise: due to Landau tail large overlap many measurements of de/dx likelihood 59
Bremsstrhalung 60
Photo Effect If E is high enough (Eγ > BeK binding energy on K-shell) the photo-effect will pass almost only on these electrons K(L,M)Edges: drop where K(L,M)-electrons are no longer available 61
Compton Scattering Compton Scattering Scattered photon energy Reflected electron energy We assumed: 1) scattering on free electron (E γ>>be) 2) electron is in the rest Giuliano Franchetti Albericatransferred Toia Distribution of & energy to electrons 62
Pair Production 63
Cherenkov detectors 64
/K/p separation with several Cherenkov thresholds 65
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Ring Imaging Cherenkov (RICH) 67
Example: CERES RICH event display 68
Example: K/p separation at p=200gev 69
DIRC: Detection of Internally Reflected Cherenkov light 70
Time of flight method 71
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Resistive parallel chambers 74
Multi-gap resistive plate chambers 75
ALICE TOF: large coverage and high granularity Particle ID in high multiplicity environment... - Large array to cover whole ALICE barrel: 160 m2-100 ps time resolution - Highly segmented: 160,000 channels of size 2.5 x 3.5 cm2 to cope with very high multiplicity events TOF 76