Time Projection Chamber (Signal creation, energy loss, PID) Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 1
Motivation • Standard detector courses describe to some extent how the signal is created and how the dE/dx variables are calculated. • They do not include however interdependencies that occur inside the gas chamber and that might influence the signal. • We would like to go deeper and describe all the processes that influence our measurements down to 0.2 % precision, which is our goal for dE/dx measurements in the ALICE TPC The Flammarion Engraving Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 2
Time Projection Chamber Energy loss The Flammarion Engraving Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 3
Time Projection Chamber Energy loss Transport The Flammarion Engraving Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 4
Time Projection Chamber Energy loss Transport Gas gain The Flammarion Engraving Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 5
Time Projection Chamber Energy loss Transport Gas gain Signal formation The Flammarion Engraving Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 6
Time Projection Chamber Energy loss Transport Gas gain Signal formation PID The Flammarion Engraving Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 7
Detection mechanisms • Detection interaction with the medium • Charged particles lose energy Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 8
Energy loss Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 9
Energy loss Focus: the processes that influence the PID measurements & performance. Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 10
Bethe Bloch formula Bethe Bloch formula for heavy particles Density dependent [S/ ρ ] [MeV/g/cm 2 ] - don’t undergo: Bremsshtrahlung, emission of Cherenkov radiation; - nuclear reactions are extremely rare - elastic scattering off nuclei is less common compared to: INELASTIC COLLISIONS WITH ATOMIC ELECTRONS Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 11
Bethe Bloch formula Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 12
Bethe Bloch formula More or less the same for different materials (except for the Hydrogen) Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 13
Bethe Bloch formula Leads to a slow rise of ionization losses with the particle momentum (accounting for relativistic flattening of the electric field of the incoming particle) More or less the same for different materials (except for the Hydrogen) Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 14
Bethe Bloch formula Maximum energy transfer Leads to a slow rise of to an electron ionization losses with (limited by E-p the particle momentum conservation laws) (accounting for relativistic flattening of the electric field of the incoming particle) More or less the same for different materials (except for the Hydrogen) Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 15
Bethe Bloch formula Maximum energy transfer Leads to a slow rise of to an electron ionization losses with (limited by E-p the particle momentum conservation laws) (accounting for relativistic flattening of the electric field of the incoming particle) More or less the same for different materials (except for the Hydrogen) Density effect factor (polarization of the medium) Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 16
Bethe Bloch formula Differences between heavy charge particles and electrons in matter: Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 17
Bethe Bloch formula Differences between heavy charge particles and electrons in matter: • electrons are light and collide with other atomic electrons - large angle multiple scattering - large energy losses possible - indistinguishability (in a quantum sense) - electrons are relativistic at nuclear energies Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 18
Bethe Bloch formula Differences between heavy charge particles and electrons in matter: • electrons are light and collide with other atomic electrons - large angle multiple scattering - large energy losses possible - indistinguishability (in a quantum sense) - electrons are relativistic ar nuclear energies • electrons emit radiation as they lose energy - Bremsshtrahlung - Cherenkov radiation Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 19
Bethe Bloch formula Bethe Bloch formula for electrons Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 20
Cherenkov radiation Already included in the Bethe Bloch formula! Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 21
Cherenkov radiation Already included in the Bethe Bloch formula! • Density effect – result of polarization and screening of distant atoms from the charged particle; more important at higher energies and more important for dense media • As the charged particle crosses through the material, polarization is induced and then relaxes to zero after the particle has passed through EM oscillation • the coherent sum at the shock wavefront, when the conditions are possible for Cherenkov emission, is part of the density effect calculation • the dE/dx for the Cherenkov portion of the energy loss is only about 1% of the typical dE/dx value (in condensed materials) Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 22
Bremsstrahlung (photon emission by an electron accelerated in Coulomb field of nucleus) Within ALICE ITS & TPC: low material budget (~10% of Bremsstrahlung – dominant radiation length for normal process for E > 10-30 MeV incident particles) Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 23
Bremsstrahlung Low momenta c≈0.144 (ITS) • Bremsshtrahlung leads to deterioration of the momentum resolution. • So, the objective is to decrease the radiation length reduce the material budget as much as possible (RUN3 goal) • Typical momentum resolution at low momenta is about 1%. • For electrons, this effect is much bigger than the multiple scattering. Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 24
Energy loss Typical range of E for the operation of HEP TPC’s Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 25
Energy loss Relativistic rise Region of minimum Fermi Plateau ionization approximately independent Energy loss of the material basically depends on the ratio Z/A Ex: gases (TPC, TRD) dense media (Silicon detector, ITS) Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 26
Energy loss Pion/Kaon separation requires a dE/dx resolution of < 5% • Energy loss affected by Landau distribution • A few measurements necessary • Truncated mean (samples with 40% highest values are ignored) Simultaneous measurement of p and dE/dx defines m, the particle identity True in the first approximation… Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 27
Ionization Particle trajectory in gaseous media Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 28
Ionization Particle trajectory Primary in gaseous media ionization Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 29
Ionization Particle trajectory Primary Secondary in gaseous media ionization ionization Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 30
Ionization Particle trajectory Primary Secondary in gaseous media ionization ionization ALICE TPC (Ne:CO 2 ) Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 31
Ionization Electrons above threshold do not contribute to the observed energy loss Energy loss how much the particle loses Energy deposit restricted energy loss (within cutoff ~1cm) Cutoff ~ 30 keV (1cm) So, you can measure it if the energy loss is localized. What we really reconstruct primary electrons in a small neighbourhood along the track. Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 32
Electron attachment Leads to a decrease of signal drift length mean free path • Probability for an electron drifting to be captured by O 2 molecule is 1% per 1m drift per 1 ppm of O 2 . Signal attenuation ~ 2.5% for the full drift length Particle tracking and identification at high rates WS 16/17 Bogdan Blidaru Page 33
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