Detectors in Nuclear and Particle Physics

Transcription

1 Detectors in Nuclear and Particle Physics Prof. Dr. Johanna Stachel Department of Physics und Astronomy University of Heidelberg July 1, 2014 J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

2 7. Particle Identification 7 Particle Identification Time of Flight Measurement Specific Energy Loss Transition Radiation Cherenkov Radiation J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

3 Particle identification - parameters in general, momentum of a particle measured in a spectrometer and another observable are used to identify the species velocity - time-of-flight τ 1/β - Cherenkov threshold β > 1/n - transition radiation γ e/π energy loss - de dx z2 ln aβγ β2 energy measurement - calorimeter (chap. 8) E = γm 0 c 2 T = (γ 1)m 0 c 2 (for p, n, nuclei) E dep = γm 0 c 2 + m 0 c 2 (for p, n,...) J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

4 Special signatures photon - total energy in crystal or electromagnetic sampling calorimeter + information on neutrality neutron - energy in calorimeter or scintillator with Li, B, or 3 He + information on neutrality muon - only de/dx in thick calorimeter, penetrates thick absorber K 0, Λ, Ξ, Ω,... - reconstruction of m inv of weak decay products neutrino - only weak interaction with detector material, either as charged or neutral current J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

5 Time of Flight Measurement 7.1 Time of flight τ time difference between two detectors with good time resolution: start and stop -counter typically scintillator or resistive plate chamber, also calorimeter (neutrons) coincidence set-up or put all stop-signals into TDC (time-to-digital converter) with common start or stop from beam or interaction scintillator 1 scintillator 2 particle photomultiplier discriminator PC: signal display stop start time to digital converter time J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

6 Time of Flight Measurement for known distance L between start and stop counter, time-of-flight difference of two particles with masses m 1,2 and energies and E 1,2 : ( ) 1 t = L c t = τ 1 τ 2 = L c 1 β 1 β 2 ( ) 1 1 (m 1 c 2 /E 1 ) (m 2 c 2 /E 2 ) 2 limiting case E pc m 0 c 2 t = Lc 2p 2 (m2 1 m2 2 ) require e.g. t 4σ t separation K/π at L = 3 m for σ t = 300 ps up to p = 1 GeV/c Cherenkov counter or RPC s scintillator + PM σ t 40 ps σ t 80 ps J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

7 Time of Flight Measurement Difference in time-of-flight for L = 1 m 10 p/α Δ t [ns] 1 K/p π/k p/d μ/π e/π best achievable for 4σ p [GeV/c] but of course distance L can be larger $$ detector area for a given acceptance J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

8 Time of Flight Measurement particle identification (PID) via time-of-flight at moderate momenta mass resolution: p = βγm with rest mass m, β = L/τ (here exceptionally c = 1 for short notation) ( τ m 2 = p 2 2 ) L 2 1 ( τ δ(m 2 2 ) ) = 2pδp L τδτ p2 L }{{} 2 2 δl L }{{} 3 p2 τ 2 }{{} m 2 /p 2 use p2 τ 2 L 2 = m 2 + p 2 = E 2 = 2m 2 δp p ( σ(m 2 ) = 2 m 4 ( σp p + 2E 2 δτ τ 2E 2 δl L ) 2 + E 4 ( σ τ τ ) 2 + E 4 ( σ L L ) 2 ) 1 2 usually σ L L σ p p σ τ τ σ(m 2 ) 2E 2 σ τ τ error in time measurement dominates J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

9 Time of Flight Measurement Resistive plate chambers: gas detector for precise timing measurement (material taken from talk by C. Williams on ALICE TOF) how to get a good timing signal from a gas detector? where is the problem? normally signal generated in vicinity of anode wire, timing determined by drift of primary ionization clusters to this wire, signal consists of cathode a series of avalanches spread over interval of order of 1 µs anode cathode no way to get precision (sub-nanosecond) timing idea: go to parallel plate chamber (high electric field everywhere in detector) clusters start to avalanche immediately induced signal sum of all simultaneous avalanches cathode but in practice this is not so... anode J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

10 Time of Flight Measurement cathode anode electron avalanche according to Townsend N = N 0 e αx only avalanches that traverse full gas gap will produce detectable signals only clusters of ionization produced close to cathode important for signal generation. avalanche only grows large enough close to anode to produce detectable signal on pickup electrodes. if minimum gas gain at 10 6 (10 fc signal) and maximum gain at 10 8 (streamers/sparks produced above this limit), then sensitive region first 25% of gap time jitter time to cross gap gap size/drift velocity so a) only a few ionization clusters take part in signal production b) gap size matters (small is better) J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

11 Time of Flight Measurement first example: Pestov chamber (about 1970) 40 years ago Y. Pestov realized importance of size Pestov chambers gas gap of 100 µm gives time resolution 50 ps, first example of resistive plate chamber glass electrode and metal electrode cathode 100 µm gap anode 12 atmospheres Pestov glass generally, excellent time resolution 50 ps or better! but long tail of late events mechanical constraints (due to high pressure) non-commercial glass no large-scale detector ever built J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

12 Time of Flight Measurement how to make real life detector? a) need very high gas gain (immediate production of signal) b) need way of stopping growth of avalanches (otherwise streamers/sparks will occur) answer: add boundaries that stop avalanche development. These boundaries must be invisible to the fast induced signal - external pickup electrodes sensitive to any of the avalanches from this idea the Multi-gap Resistive Plate Chamber was born J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

13 Time of Flight Measurement Multi-gap Resistive-Plate Chamber Signal electrode Cathode -10 kv (-8 kv) (-6 kv) (-4 kv) (-2 kv) stack of equally-spaced resistive plates with voltage applied to external surfaces (all internal plates electrically floating) pickup electrodes on external surfaces (resistive plates transparent to fast signal) internal plates take correct potential initially due to electrostatics but kept at correct potential by flow of electrons and positive ions - feedback principle that ensures equal gain in all gas gaps Anode 0 V Signal electrode J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

14 Time of Flight Measurement Internal plates electrically floating! Cathode -10 kv (-8 kv) Flow of positive ions (-6 kv) (-4 kv) (-2 kv) Flow of electrons and negative ions Anode 0 V in this example: 2 kv across each gap (same E field in each gap) since the gaps are the same size - on average - each plate has same flow of positive ions and electrons (from opposite sides of plate) thus zero net charge flow into plate. STABLE STATE J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

15 Time of Flight Measurement What happens if a plate is at a wrong voltage for some reason? Cathode -10 kv (-8 kv) -6.5 kv (-6 kv) (-4 kv) (-2 kv) Low E field - low gain High E field - high gain decreased flow of electrons and increased flow of positive ions net flow of positive charge. This will move the potential on this plate more positive than 6.5 kv (i.e. towards 6 kv) Anode 0 V feedback principle that automatically corrects potentials on the resistive plates stable situation is equal gains in all gas gaps J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

16 Time of Flight Measurement ALICE TOF prototypes Entries/50 ps STRIP 10 H.V kv 10 gaps of 220 micron Uncorrected time spectrum σ = 66 ps minus 30 ps jitter of timing scintillator = 59 ps 200 indeed one gets sub 50 ps time resolution Entries/50 ps Time with respect to timing scintillators [ps] STRIP 10 H.V kv time spectrum after correction for slewing σ = 53 ps minus 30 ps jitter of timing scintillator = 44 ps Time with respect to timing scintillators [ps] J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

17 Time of Flight Measurement test of pre-production strip: cm 2 read-out plane segmented into cm 2 pads Efficiency [%] ADC bins Applied differential voltage [+- kv] Resolution (ps) ADC bins Applied differential voltage [+- kv] ADC bins pedestal peak of charge spectra well separated from zero no sign of streamers but how precise do these gaps of 250 µm have to be? J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

18 Time of Flight Measurement gain not strongly dependent on gap size - actually loose mechanical tolerance - but why? Smaller gap Larger gap Higher electric field Lower electric field higher Townsend coefficient higher gas gain but smaller distance for avalanche lower gas gain lower Townsend coefficient lower gas gain but larger distance for avalanche higher gas gain with the gas mixture used (90% C 2 F 4 H 2, 5% SF 6, 5% isobutane) and with 250 µm gap size these two effects cancel and gap can vary by ±30 µm J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

19 Time of Flight Measurement Cross section of double-stack MRPC ALICE TOF 130 mm active area 70 mm Flat cable connector Differential signal sent from strip to interface card honeycomb panel (10 mm thick) PCB with cathode pickup pads double stack each stack has 5 gaps (i.e. 10 gaps in total) 250 µm gap with spacers made from fishing line external glass plates 0.55 mm thick internal glass plates (0.4 mm thick) PCB with anode pickup pads resistive plates off-the-shelf soda lime glass 400 µm internal glass 550 µm external glass 5 gas gaps of 250 micron resistive coating 5 MΩ/square M5 nylon screw to hold fishing-line spacer connection to bring cathode signal to central read-out PCB PCB with cathode pickup pads Honeycomb panel (10 mm thick) Silicon sealing compound J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

20 Time of Flight Measurement TOF with very high granularity needed! Hits in inner tracker TPC hits The red hits/track corresponds to a single particle (π in this case) Hits in TOF array array to cover whole ALICE barrel m 2 and 100 ps time resolution highly segmented - 160,000 channels of size cm 2 gas detector is only choice! J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

21 Particle Identification EMCAL Time of Flight Measurement HMPID TOF TRD TOF array arranged as a barrel with radius of 3.7 m, divided into 18 sectors 20º TPC 10º ITS TOF TRD PHOS cross section of ALICE detector J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

22 Time of Flight Measurement 54.6º 3.6º 15.8º 3.6º º TRD TOF along beam direction each sector divided into 5 modules i.e = 90 modules in total 160 m 2 and 160,000 channels I. P Central module 7.9º Outer module Intermediate module J. Stachel (Physics University Heidelberg) Detectorphysics July 1, /

23 Time of Flight Measurement 0º Central module 0.5º 1.6º 2.7º 3.7º 4.8º 5.9º 6.9º 7.9º 8.5 cm cm 1º 3.2º 5.3º 7.4º 99 cm 114 cm 2.1º 4.3º 6.3º Intermediate module 8.2º 9.3º 10.3º 11.4º 12.4º 13.4º 14.5º 15.5º 16.5º 17.5º 18.5º 19.5º 20.5º 21.5º 22.5º 23.4º 24.4º 25.4º 26.3º 27.3º 9.8º 11.9º 13.9º 16º 18º 20.1º 22.1º 8.7º 10.8º 12.9º 14.9º 17º 19.1º cm 21.1º 23º 23.9º 24.9º 25.9º 26.8º 147 cm Outer module 27.3º 28.2º 29.2º 30.1º 31.0º 31.9º 32.8º 33.7º 34.6º 35.4º 36.3º 37.1º 37.9º 38.8º 39.6º 40.4º 41.2º 42.0º 42.8º 43.5º 44.3º 27.8º 28.7º 29.6º 30.5º 31.5º 32.3º 33.3º 34.2º 34.9º 35.8º 36.6º 37.4º 38.3º 39.2º 40.1º 40.8º 41.6º 42.4º 43.1º 43.9º cm J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / cm

24 Time of Flight Measurement ALICE TOF time resolution TOF β 1 π π 0.8 K for full system one gets 80 ps resolution K p p p Pb s NN = = TeV TeV s NN 5/03/ p Pb minimum bias ALI PERF p (GeV/c) J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

25 Specific Energy Loss 7.2 Specific energy loss use relativistic rise of de/dx - but is that possible with Landau fluctuations? effective way to suppress fluctuations: make many measurements of de/dx and truncate large energy-loss measurements 1.6 e 300 p normalised energy loss μ π K p momentum p [GeV/c] mean energy loss relative to minimum ionization, normally only µ/π separation excluded number of events π signal amplitude (in arbitrary units) energy loss distribution for 600 MeV/c π and p in Si (3mm) p > p min.ion for protons J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

26 Specific Energy Loss normally, due to Landau tail, very large overlap of, e.g., pion and kaon 0.3- K π 1 cm Ar - CH ¹ 10 ² K 1 cm Ar - CH 4 (80 : 20 ) 0.2 (80 : 20) p = 50 GeV/c 10 ³ 10 ⁴ p = 50 GeV π 10 ⁵ 10 ⁶ ⁷ 10 ⁸ 10 ⁹ energy loss ΔE [kev] 10 ¹⁰ 10 ¹¹ energy loss ΔE [kev] truncated mean method: many measurements and truncation to the 30 50% highest de/dx values for each track J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

27 Specific Energy Loss Alternative: likelihood -method for several de dx -measurements probability that pion produces a signal x: p i π (x) for each particle measurements x 1... x 5 probability for pion: P 1 = probability for kaon: P 2 = P π = P 1 = P 2 = pπ i (x i ) i=1 5 pk i (x i ) i=1 P 1 P 1 + P 2 } (see example on the right) P π = 99.8% K π 1 cm Ar - CH 4 (80 : 20) p = 50 GeV/c energy loss ΔE [kev] pk i pπ i J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

28 Particle Identification μ π K ρ TPC de /dx (arb. units) energy loss de/dx (in arbitrary units) 36 Specific Energy Loss K 180 p d /05/2011 Pb-Pb snn = 2.76 TeV e e π p (GeV/c) ALI PERF , momentum p [GeV/c] multiple energy loss measurement in TPC (TPC/Two-Gamma collaboration, LBNL 1988) ALICE TPC σ(de/dx)/de/dx=6% (Ne/C02 /N2 ) record: 3% have been reached (NA49 at SPS with Ar/CH4, larger cells) J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

29 Transition Radiation 7.3 Transition Radiation effect: see chapter 2, particles with Lorentz factor γ 1000 emit X-ray photon when crossing from medium with one dielectric constant into another, probability of order α per boundary crossing average energy of transition radiation photons [kev] number of events π e 15 GeV electron momentum p [GeV/c] e mean energy of transition radiation photon as function of electron momentum energy deposit [kev] energy loss distribution for 15 GeV e, π in transition radiation detector J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

30 recorded charge Q Particle Identification Transition Radiation Transition radiation detector TRD (schematic) potential wire Gas volume anode wire (+ HV) radiator cathode (-HV) backing electrode δ-electron Q threshold particle principle of separating ionization energy loss from the energy loss from emission of transition radiation photons energy loss (excitation, ionization) plus transition radiation number of events dn/dn π e 15 GeV number of ionization clusters N distribution of number of clusters above some threshold for 15 GeV e, π J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

31 Transition Radiation e/π separation in a transition radiation detector traditionally, two methods for electron discrimination total energy loss cluster counting method novel type: ALICE TRD makes use of spatial information of TR absorption last hour if time pion contamination Total energy loss charge cluster method Q threshold = 4 kev electron acceptance e/π separation at 15 GeV in a Li-foil radiator. J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

32 Cherenkov Radiation 7.4. Cherenkov radiation real photons emitted when v > c/n v < c/n induced dipoles symmetric, no net dipole moment v > c/n induced dipoles not symmetric non-vanishing dipole moment illustration of the Cherenkov effect AB = tβc AC = t c n cos θ c = 1 βn simple geometric determination of the Cherenkov angle θ c J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

33 Cherenkov Radiation threshold effect: radiation for β > 1/n, asymptotic angle θ c = arc cos 1 βn number of Cherenkov photons per unit path length in interval λ 1 λ 2 (see Chapter 2) dn γ dx = 2παz 2 λ2 in case of no dispersion (n const. in interval) dn γ dx λ 1 ( 1 1 n 2 β 2 = 2παz 2 sin 2 θ c λ 2 λ 1 λ 1 λ 2 ) dλ λ 2 (z = charge in e) application of Cherenkov radiation for separation of particles with masses m 1, m 2 at constant momentum (say m 1 < m 2 ) to distinguish: particle 1 above threshold β 1 > 1/n particle 2 at most at threshold β 2 = 1/n or n 2 = γ2 2 γ in λ = nm range, lighter particle with γ radiates dn γ dx = 490 sin 2 θ c = 490 (m 2c 2 ) 2 (m 1 c 2 ) 2 p 2 c 2 use sin 2 θ c = 1 cos 2 θ c = 1 γ2 2 1 β 2 1 γ2 2 photons per cm 1 γ γ 2 1 J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

34 Cherenkov Radiation for radiator of length L in cm and quantum efficiency q of photocathode N = 490 (m 2c 2 ) 2 (m 1 c 2 ) 2 p 2 c 2 and for threshold at N 0 photoelectrons L = L q N 0 p 2 c 2 490[(m 2 c 2 ) 2 (m 1 c 2 ) 2 ] q (cm) defines the necessary length of the radiator required detector length [m] e/π π/k K/p momentum p [GeV/c] required detector length for N 0 = 10 and q = 0.25 J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

35 Cherenkov Radiation π/k/p separation with Cherenkov detector: use several threshold detectors p [GeV/c] Particle γ 1/β 10 π K p condition for no radiation: β < 1 n 1 β > n or C1 C2 C3 π K p π : C1 C2 C3 pion trigger K : C1 C2 C3 kaon trigger p : C1 C2 C3 proton trigger aerogel n = neopentane n= Ar - Ne n = principle of particle identification by threshold Cherenkov counters (x represents production of Cherenkov photons) J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

36 Cherenkov Radiation Differential Cherenkov detectors selection of velocity interval in which then actually velocity is measured accept particles above threshold velocity β min = 1/n detect light for particles between β min and a value β t where by total reflection light does not propagate into (air) light guide cos θ c = 1 nβ the critical angle for total reflection: sin θ t = 1 n β-range cos θ t = 1 n < β < 1 n n 2 radiator θ Al- mirror working principle of a differential Cherenkov counter Ze air light guide photomultiplier example: diamond n = < β < 0.454, i.e. β = 0.04 window selected if optics of read-out such that chromatic aberrations corrected velocity resolution β/β = 10 7 can be reached principle of DISC (Discriminating Cherenkov counter) J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

37 Cherenkov Radiation Ring Imaging Cherenkov counter (RICH) I optics such that photons emitted under certain angle θ form ring of radius r at image plane where photons are detected. spherical mirror of radius R S projects light onto spherical detector of radius R D. particle 1 focal length of spherical mirror: f = R S 2 place photon detector in focus: R D = R S /2 Cherenkov light emitted under angle θ c radius of Cherenkov ring at detector: r = f θ c = R s 2θ c 1 β = n cos(2r/r S ) interaction point detector surface R D r Cherenkov radiator particle 2 spherical mirror with radius R S working principle of a RICH counter photon detection: - photomultiplier - multi-wire proportional chamber or parallel-plate avalanche counter filled with gas that is photosensitive, i.e. transforms photons into electrons. e.g. addition of TMAE vapor (CH 3 ) 2 N) 2 C = C 5 H 12 N 2 E ion = 5.4 ev) - or CsI coated cathode of MWPC (ALICE HMPID or hadron blind detector HBD in PHENIX) J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

38 Cherenkov Radiation example: K/π separation at p = 200 GeV/c 10 3 π K photons detected in MWPC filled with He(14%), TEA (triethyl-amine, 3%), CaF 2 entrance window (UV transparent) Cherenkov ring radius r [mm] J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

39 Cherenkov Radiation event displays - CERES RICH 1 electron produces about 10 photons J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

40 Cherenkov Radiation CERES Electron Identification with TPC and RICH RICH π rejection vs. efficiency π rejection via TPC de/dx combined rejection - e.g. at 1.5 GeV/c at 67% e-efficiency π rejection J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

41 Cherenkov Radiation { DIRC Detection of Internally Reflected Cherenkov Light collection and imaging of light from total internal reflection (rather than transmitted light) optical material of radiator used in 2 ways simultaneously: - Cherenkov radiator - light guide for Cherenkov light trapped in radiator by total int. reflection advantage: photons of ring image can be transported to a detector away from path of radiating particle intrinsically 3d, position of hit θ c, φ c and time long. position example: BABAR at SLAC PMT + Base 10,752 PMT's - rectangular radiator from fused silica n=1.473 radiation hard, long attenuation length, low chromatic dispersion, excellent optical finish possible - surrounded by nitrogen n stand-off box filled with water n=1.346 (close to radiator) mm Thickness (35.00 mm Width) Mirror Bar Track Trajectory Wedge Purified Water Bar Box Window Light Catcher PMT Surface Standoff Box NIM A538 (2005) 281 { 4.9 m 4 x 1.225m Bars glued end-to-end 1.17 m A6 J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

42 Cherenkov Radiation Transmission or Reflectivity or Q.E Water transmission (1.1m) Mirror reflectivity Internal reflection coeff. (365 bounces) Epotek transmission (25µm) ETL 9125 quantum efficiency (Q.E.) PMT Q.E. PMT window transmission Predicted Total photon detection efficiency Wavelength [nm] θ C (mrad) e µ π K p p Lab (GeV/c) kaons can be separated up to 4 GeV/c BABAR physics: decays of B 0 to study CP violation b-tagging (78 % of B 0 K + + X) golden channel for CP: B 0 J/ψ + φ and φ K + + K J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372

43 Cherenkov Radiation Comparision different PID methods for K/π separation π / K - separation RICH Cherenkov threshold Cherenkov Time-of-flight de/dx multiple de/dx DISC Cherenkov transition radiation momentum p [GeV/c] illustration of various particle identification methods for K/π separation along with characteristic momentum ranges. interaction point radiator iron absorber timeof-flight counters threshold Cherenkov counters transition radiation detector muon chamber multiple de/dx and momentum measurement a detector system for PID combines usually several methods J. Stachel (Physics University Heidelberg) Detectorphysics July 1, / 372