Transition radiation detectors

Transcription

1 Particle Identification de/dx measurement Time of flight Cherenkov detectors Κ π µ p Transition radiation detectors Christian Joram V/

2 Particle Identification Particle Identification Particle identification is an important aspect of high energy physics experiments. Some physical quantities are only accessible with sophisticated particle identification (B-physics, CP violation, rare exclusive decays). One wants to discriminate: π/k, K/p, e/π, γ/π 0. The applicable methods depend strongly on the interesting energy domain. Depending on the physics case either ε xx or ε xy has to be optimized: Efficiency: ε xx = N tag x N x ε xx Misidentification: ε xy = x tag N y N y Rejection: Rxy = ε xx ε xy ε xy The performance of a detector can be expressed in Qx Qy terms of the resolving power Dx, y = σ Q Christian Joram V/

3 Specific energy loss Particle ID using the specific energy loss de/dx de dx p = m 0 βγ ln β ( β γ ) Simultaneous measurement of p and de/dx defines mass m, hence the particle identity e µ π K p π/k separation (σ) requires a de/dx resolution of < 5% Average energy loss for e,µ,π,k,p in 80/0 Ar/CH 4 (NTP) (J.N. Marx, Physics today, Oct.78) But: Large fluctuations + Landau tails! Christian Joram V/3

4 Specific energy loss (backup) Improve de/dx resolution and fight Landau tails Chose gas with high specific ionization Devide detector length L in N gaps of thickness T. Sample de/dx N times (B. Adeva et al., NIM A 90 (990) 5) wire 4 wires L: most likely energy loss A: average energy loss (M. Aderholz, NIM A 8 (974), 49) Don t cut the track into too many slices! There is an optimum for each total detector length L. calculate truncated mean, i.e. ignore samples with (e.g. 40%) highest values Also pressure increase can improve resolution, but reduced rel. rise due to density effect! Christian Joram V/4

5 Specific energy loss Example ALPEPH TPC Gas: Ar/CH 4 90/0 N samples = 338, wire spacing 4 mm de/dx resolution: 4.5% for Bhabhas, 5% for m.i.p. s log scale! linear scale! Christian Joram V/5

6 Specific energy loss de/dx can also be used in Silicon detectors Example DELPHI microvertex detector (3 x 300 µm Silicon) E (a.u.) log p [GeV/c] E (a.u.) log p [GeV/c] Christian Joram V/6

7 Time of flight Particle ID using Time Of Flight (TOF) t = L βc m = p c L start t stop Combine TOF with momentum measurement Mass resolution ( p = m βγ ) 0 dm dp dt = + γ m p t + dl L t TOF difference of particles at a given momentum L = c β = L c ( ) Lc + m c / p + m c / p ( m m ) β p t for L = m path length σ t = 300 ps π/k separation up to GeV/c Christian Joram V/7

8 Time of flight Example: CERN NA49 Heavy Ion experiment detail of the grid Small, but thick scint. 8 x 3.3 x.3 cm Long scint. (48 or 30 cm), read out on both sides TOF requires fast detectors (plastic scintillator, gaseous detectors), approporiate signal processing (constant fraction discrimination, corrections + continuous stability monitoring. Christian Joram V/8

9 Time of flight System resolution of the tile stack From γ conversion in scintillators L = 5 m T rel. = T / T π NA49 combined particle ID: TOF + de/dx (TPC) Christian Joram V/9

10 Interaction of charged particles Remember energy loss due to ionisation There are other ways of energy loss! A photon in a medium has to follow the dispersion relation c n c k c ω = πν = π = k ω = 0 ε = n λ n ε ε v r,m 0 h ω, hk θ ē Re ε Im ε schematically! regime: effect: optical absorptive X-ray Cherenkov radiation ionisation transition radiation ω For soft collisions + energy and momentum conservation real photons: ω r v k r = v k ω cos θ cosθ = = = vk nβ Emission of Cherenkov photons if β Christian Joram V/0 β ε n

11 Cherenkov detectors Cherenkov radiation Cherenkov radiation is emitted when a charged particle passes a dielectric medium with velocity β β thr = n n : refractive index wave front l light =(c/n) t θ l part =βc t cosθ C θ C = with n = n( λ) nβ βthr θ n θ max = arccos n = C 0 threshold saturated angle (β=) Number of emitted photons per unit length and unit wavelength interval dn/dλ d N dxdλ d N dxdλ πz = λ λ α β n with c λ = = ν πz = λ hc E α sin θ C d N = const. dxde dn/de λ Ε Christian Joram V/

12 Cherenkov detectors medium n θ max (β=) N ph (ev - cm - ) air isobutane water quartz Energy loss by Cherenkov radiation small compared to ionization ( %) Number of detected photo electrons N p. e. α = Lsin θ Q E c ε ( ) ε h E E N0 = 370 ev i i ( E)dE cm ε total E = E - E is the width of the sensitive window of the photodetector (photomultiplier, photosensitive gas detector...) E Example: for a detector with ε E = 0. ev L = o and a Cherenkov angle of θ C = 30 one expects 8 photo electrons N p. e. = total cm Christian Joram V/

13 Cherenkov detectors Particle ID with Cherenkov detectors Detectors can exploit... N ph (β): threshold detector (do not measure θ C ) θ(β): differential and Ring Imaging Cherenkov detectors RICH Threshold Cherenkov detectors N n = n β ( + m p ) radiator medium principle mirror particle Example: study of an Aerogel threshold detector for the BELLE experiment at KEK (Japan) Goal: π/k separation β kaon ; light yield (a.u.) PM β kaon n= n= n= p kaon [GeV/c] Christian Joram V/3

14 Cherenkov detectors The Belle Detector edited by S. Mori KEK Progress Report Christian Joram V/4

15 Cherenkov detectors Ring Imaging Cherenkov detectors (RICH) RICH detectors determine θ C by intersecting the Cherenkov cone with a photosensitive plane (J. Seguinot, T. Ypsilantis, NIM 4 (977) 377) requires large area photosensitive detectors, e.g. wire chambers with photosensitive detector gas PMT arrays θ C = arccos nβ = arccos n E p p + m = arccos n p π/k π/k/p K/p n =.8 C 6 F 4 liquid DELPHI n =.008 C 5 F gas π/h π/k/p K/p σ β β = tan θ σ θ Detect N photons (p.e.) σ θ σ p. e. θ N p. e. minimize σ θ maximize N p.e. Christian Joram V/5

16 Cherenkov detectors Principle of operation of a RICH detectors DELPHI RICH radiators + photodetector C 5 F (40 cm, gas) C 4 F 0 (50 cm, gas) spherical mirror Photodetector TMAE-based A RICH with two radiators to cover a large momentum range. π/k/p separation GeV/c: DELPHI and SLD C 6 F 4 ( cm, liquid) (W. Adam et al. NIM A 37 (996) 40) Two particles from a hadronic jet (Z-decay) in the DELPHI gas and liquid radiator + hypothesis for π and K Christian Joram V/6

17 Cherenkov detectors The mirror cage of the DELPHI Barrel RICH (88 parabolic mirrors) Christian Joram V/7

18 Cherenkov detectors Marriage of mirror cage and central detector part of the DELPHI Barrel RICH. Christian Joram V/8

19 Cherenkov detectors Performance of DELPHI RICH (barrel) in hadronic Z decays Liquid radiator gas radiator π K p π K p (E. Schyns, PhD thesis, Wuppertal University 997) Christian Joram V/9

20 Cherenkov detectors (backup) Photo detectors for RICH counters Gas based detectors Admix photosensitive agent (TEA, TMAE) to detector gas example DELPHI: TPC like detector (CH 4 + C H 6 + TMAE) - 54 kv MWPC quartz box Drawbacks: ^ C cone x-y projection z from drift time - slow response because of long drift times (many µs) - λ thr (TMAE) = 0 nm => only sensitive to UV light example ALICE: A MWPC with CsI deposited cathode pads Printed Circuid Board (PCB) mm mm cadhode pads (8x8mm ), CsI deposited sense wires, 4 mm spacing cathode mesh 70 mm collection mesh quartz window (mm) radiator (C 6 F 4 ) Fast response (<00 ns), but still only sensitive to UV light Christian Joram V/0

21 Cherenkov detectors (backup) Vacuum based detectors Photomultiplier tubes, Hybrid Photo Diodes. example HERMES (DESY) 3870 PMTs, Philips XP9/UV, Ø 3/4 + light funnels 3D view of one half detector ca..5 m Double radiator C 4 F 0 gas Aerogel gas ring Online Event Display aerogel ring σ θ ~ 8 mrad N p.e. (aerogel) = 8 N p.e. (C 4 F 0 ) = Christian Joram V/

22 Cherenkov detectors (backup) The DIRC of BaBar (asymmetric B-factory at SLAC) Detector for Inernally Relfected Cherenkov light P. Cole et al, NIM A 343 (999) 456 I. Adam et al., SLAC-PUB-8345 Bucking coil Support tube for barboxes containing fused silica bars each Standoff Box 075 PMTs in water Magnetic Shield Mirrors e + (3. GeV) e - (9.0 GeV) e + Support tube (Al) e - Quartz Barbox exploded view of the DIRC Standoff box Assembly flange Compensating coil Christian Joram V/

23 Cherenkov detectors (backup) Basic principle of the DIRC Transport of C-light by internal reflection in quartz bar. Detection by PMT array. Stand off tank filled with water for ref. index matching. N quartz =.47, θ crit. = 43º, θ c max = 47º T = 99.9 % / m R = % ~ bounces loss ~0-0% = f(θ dip ) Christian Joram V/3

24 Cherenkov detectors (backup) DIRC: Event reconstruction is a bit more difficult! Cherenkov ring is decomposed into disjointed segments. σ θ = 9.8 mrad, (7.0 mrad from Ø PMT =8 mm, 5.4 mrad from dn/de). N p.e. ~ 30 π/k separation of 3. σ at p = 3 GeV/c Christian Joram V/4

25 Transition radiation detectors Transition radiation detectors (there is an excellent review article by B. Dolgoshein (NIM A 36 (993) 434)) TR predicted by Ginzburg and Franck in 946 Electromagnetic radiation is emitted when a charged particle traverses a medium with a discontinuous refractive index, e.g. the boundaries between vacuum and a dielectric layer. A (too) simple picture medium vacuum A correct relativistic treatment shows that (G. Garibian, Sov. Phys. JETP63 (958) 079) electron Radiated energy per medium/vacuum boundary W = αhω 3 p γ W γ Only high energy e ± will emit TR. Identification of e ± ω p = Nee ε m 0 e plasma frequency hω p 0eV (plastic radiators) Christian Joram V/5

26 Transition radiation detectors Number of emitted photons / boundary is small N ph W hω α 37 Need many transitions build a stack of many thin foils with gas gaps X-rays are emitted with a sharp maximum at small angle θ γ TR stay close to track Emission spectrum of TR Typical energy: hω 4 hω photons in the kev range p γ Simulated emission spectrum of a CH foil stack Christian Joram V/6

27 Transition radiation detectors TR Radiators: stacks of CH foils are used hydrocarbon foam and fiber materials Low Z material preferred to keep re-absorption small ( Z 5 ) R D R D R D R D sandwich of radiator stacks and detectors minimize re-absorption TR X-ray detectors: Detector should be sensitive for 3 E γ 30 kev. Mainly used: Gaseous detectors: MWPC, drift chamber, straw tubes Detector gas: σ photo effect Z 5 gas with high Z required, e.g. Xenon (Z=54) Intrinsic problem: detector sees TR and de/dx Pulse height ( cm Xe) de/dx 00 e - TR (0 kev) 500 e - t Discrimination by threshold Christian Joram V/7

28 ATLAS Transition Radiation Tracker A prototype endcap wheel. X-ray detector: straw tubes (4mm) (in total ca !) Xe based gas TRT protoype performance Pion fake rate at 90% electron detection efficiency: π 90 =.58 % Christian Joram V/8

29 Particle Identification Summary: A number of powerful methods are available to identify particles over a large momentum range. Depending on the available space and the environment, the identification power can vary significantly. A very coarse plot. TOF de/dx RICH TR e ± identification π/k separation p [GeV/c] Christian Joram V/9