Particle Detectors A brief introduction with emphasis on high energy physics applications

Transcription

1 Particle Detectors A brief introduction with emphasis on high energy physics applications TRIUMF Summer Institute 2006 July Lecture I measurement of ionization and position Lecture II scintillation and photo detection calorimetry Lecture III particle identification detector systems Michel Lefebvre Physics and Astronomy University of Victoria

2 Lecture II Scintillation and photo detection Organic scintillators Inorganic scintillators Scintillator readout Photo detectors Calorimetry Interactions of electrons and photons in matter Electromagnetic and hadronic showers Electromagnetic and hadronic calorimeters Example: LAr accordion EM calorimetry TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/2

3 Scintillation and photo detection Energy deposition by ionizing particles production of scintillation light (luminescence) Scintillators are multi purpose detectors trigger counters veto counters time of flight measurement tracking detectors calorimetry others! Inorganic and organic scintillators Inorganic: high light output, but slow Organic: fast, but lower light output TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/3

4 Organic scintillators Plastic scintillators are the most widely used Ionization by charged particles generate optical photons (blue green) densities: between 1.03 and 1.20 g cm -3 typical yield:1 photon per 100 ev of deposited energy MIP in 1cm thick material yields photons Response not linear with ionization density Birks factor dl = L Decay times a few ns dx 1+ kb de dx de dx rise times much faster (allows sub-ns timing) light decay can depend on exciting particle, allowing ID Production to desired shape, low cost also scintillating fibres TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/4

5 Organic scintillators Scintillation mechanism scintillation, followed by resonant dipole-dipole interaction with fluor; strong coupling increases fluor shifts wavelength to where the base is more transparent; shortens the decay time of scintillation and increases yield Particle Data Group Cautions: aging and handling, radiation damage,... they have low Z, being mainly made of H and C low γ detection efficiency ( only Compton effect). but high neutron detection efficiency via (n,p) reactions TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/5

6 Organic scintillators TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/6

7 Inorganic scintillators Inorganic crystalline scintillators Ionization by charged particles NaI, CsI, BaF 2, Bi 4 Ge 3 O 12, PbWO 4,... High density and high Z well suited for detection of charged particle and γ densities: between ~4 and ~8 g cm -3 high de/dx high conversion efficiency for electrons and γ often with very high light output often more than two time constants fast recombination from active centers (ns to µs) delayed recombination due to trapping ( 100 ns) TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/7

8 Inorganic scintillators light ouput has strong temperature dependence (From Harshaw catalog) PWO4 final polished cristal for CMS calorimetry PbWO 4 BGO TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/8

9 Inorganic scintillators Liquid noble gases (LAr, LKr, LXe) also two time constants few ns and 100 to 1000 ns, but same wavelegth A excitation ionization A* A + collision with g.s. atoms excited molecule A 2 * A 2 + de-excitation and dissociation A A A 2 * UV 130nm (Ar) 150nm (Kr) 175nm (Xe) Joram ionized molecule recombination e - TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/9

10 Scintillator readout light guides transfer by total internal reflection (and reflector) fish tail adiabatic wavelength shifter (WLS) bars WLS green small air gap Photo detector blue (secondary) scintillator UV (primary) primary particle TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/10

11 Scintillator readout optical fibres light transport by total internal reflection typ. 25 µm core polystyrene n=1.59 typically <1 mm cladding (PMMA) n=1.49 dω = 4π multi-clad fibres for improved aperture and absorption length dω = 4π 5.3% in one direction 3.1% in one direction attenuation length > 10 m for visible light n 1 n 2 core polystyrene n=1.59 θ cladding (PMMA) n= µm fluorinated outer cladding n= µm TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/11

12 Scintillating fibre tracker scintillating plastic fibres many advantages high geometrical flexibility; fine granularity; low mass fast response (ns if fast readout; suitable for triggering) UA2 upgrade scintillating fibre tracker (1987!) Ansorge et al, NIM A265 (1988) Alitti et al, NIM A279 (1989) TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/12

13 Scintillating fibre tracker UA2 upgrade SFD Ansorge et al, NIM A265 (1988) Alitti et al, NIM A279 (1989) TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/13

14 Photo detectors Convert light into detectable electronics signal use photoelectric effect to convert photons to photoelectrons require high sensitivity quantum efficiency = Q.E. = N p.e /N γ many type of photodetectors gas based devices (RICH) vacuum based solid state thresholds TEA TMAE,CsI UV ~15-20% for PMT ~70% for Si photo diodes visible bialkali GaAs... multialkali E (ev) Joram λ (nm) TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/14

15 Photo detectors Photoelectric effect in photocathodes 3-step process photo inonization of molecule electron propagation through cathode escape of electron back into the vacuum Semitransparent photocathode Opaque photocathode γ glass γ e - PC e - PC substrate Joram TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/15

16 Vacuum based photo detectors Photo multiplier tubes (PMT) photo emission from photocathode (PC) secondary emissions from dynodes dynode gain: g = 3 to dynodes with g = 4: total gain G = ē γ (Philips Photonic) TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/16

17 Vacuum based photo detectors Energy resolution of PMT determined mainly by the fluctuation of the number of secondary electrons emitted from the dynodes counts 1 p.e. Single photons. Pulse height spectrum of a PMT with Cu-Be dynodes. 1 p.e. 2 p.e. Pulse height spectrum of a PMT with negatice electron affinity (NEA) dynodes. 3 p.e. noise H. Houtermanns, NIM 112 (1973) 121 Philips Photonic Pulse height Pulse height TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/17

18 Vacuum based photo detectors Multi anode PMT example: Hamamatsu R5900 series up to 8x8 channels size 28x28 mm 2, active area 41% bialkali PC: Q.E. = 20% at 400 nm gain G 10 6 recent example: Hamamatsu flat panel PMT active area > 90% TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/18

19 Solid state photo detectors Photo diodes (PD) high Q.E. but no gain (G = 1) p layer must be very thin (< 1 µm); visible light absorbs rapidly in Si Q.E. 80% at λ = 700 nm good for read out of scintillators Avalanche photo diodes (APD) avalanche multiplication high reverse bias V 100 to 200V high internal field leads to avalanche p p n p n = i (intrinsic) n + h e avalanche h e E TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/19

20 Hybrid photo detectors Hybrid photo diodes (HPD) photocathode (like in PMT) photo-electron acceleration silicon detector (pixel, strip, pads) photocathode focusing electrodes electron single photoelectron detection with high resolution V G = e V W = kev ev 3 Commercial HPD (DEP PP0270K) with slow electronic (2µs shaping time) C.P. Datema et al. NIM A 387(1997) 100 n p.e. peeks! silicon sensor background from e - backscattering from Si surface TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/20

21 Calorimetry Energy measurement by total absorption works for charged and neutral particles spatial reconstruction particle identification capability Measured particle is lost (destructive method) Basic mechanism electromagnetic or hadronic showers Detector response is proportional to E not always true for hadronic showers Energy converted into ionization and/or excitation of matter TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/21

22 Interactions of electrons in matter Ionization and bremsstrahlung Particle Data Group critical energy E c = 6.9 MeV TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/22

23 Interactions of electrons in matter bremsstrahlung photon produced in the Coulomb field of the 13 absorber nuclei r e = cm 2 2 de 2 2 me 4 N Z ln 183 E A z re E 2 1/3 2 = α dx A m Z m important for electrons and ultra relativistic muons for electrons de = dx E X X N Z = α r A A e ln E Ee X TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/23 x = 183 1/3 Z 1 radiation length (normally in g/cm 2 ) See also table on page II/33

24 Interactions of electrons in matter critical energy bremsstrahlung loss = ionization loss for electrons solid and liquid E c for muons Z MeV E E m = 207 E e µ 2 e c c c m e gas E c 710 Z MeV TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/24

25 Interactions of photons in matter Photo-electric effect + γ+ A = e + A energy-momentum conservation requires a third partner close by photo effect releases electrons mainly from the inner most atomic shell (K) K photo-e Zr ε 7 e re Z σ = π α π α 3ε Compton scattering e e λ=λ 1 cosθ γ+ =γ+ ( ) atomic e 1 atomic σ ln Compton = Zσ ε ε Compton σ Compton ε C ε ε E mc e γ 2 TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/25

26 Interactions of photons in matter Pair production + γ+ A = ee + A In the Coulomb field of a nucleus or electron. σ pair ε 1 7 A 1 9 N X A Only possible if E γ 2m e of course. ε 1 9 λpair X 7 Mass attenuation coefficient µ I γ = I e µ x normally µ in cm 2 /g and x in g/cm 2 µ=µ +µ +µ + photo-e Compton pair N A A µ i = σi ε E mc e γ 2 TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/26

27 Interactions of photons in matter 2 m e c 2 2 m e c 2 Particle Data Group min is 14.2 b/atom TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/27

28 Electromagnetic showers Simple model multiplication electron shower in a cloud chamber with lead absorbers (Joram) Only consider bremsstrahlung (e ± ) and pair production (γ) assume E γ large. Multiplication stops when average shower particle energy = critical energy. At this stage the e ± will mainly lose energy through ionization and the multiplication will stop. This happens before the photons have less than 2m e c 2 energy. TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/28

29 Electromagnetic showers Let k be the number of steps needed for the multiplication to stop. E = E C The corresponding maximum number of particles in the shower is then k n= 0 2 k n k+ 1 k 2 = = 2 Each step takes place over a characteristic length: X o for e ± and 9X o /7 for γ. Assume X o for both. Then k In practice, ionization energy losses and fluctuations lowers somewhat this number TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/29 E E = 1 ln E = 1.44ln E ln 2 EC EC ln E 1.1 X max EC k = X = ln E 0.3 EC C for electrons for photons This is the number of X o needed to stop shower multiplication. This corresponds to shower max. Grows like lne. Kleinknecht

30 Electromagnetic showers Simple model attenuation Complicated mixture of ionization losses, radiation losses and Compton scattering. But the electromagnetic shower attenuation seems to follow the minimum attenuation for photons. The corresponding photon mean free path is λ = γ N A A ρσ The attenuation follows exponentially with a mean free path of λ γ. So to attenuate down to 95%, one needs 3λ γ. Therefore to contain 95% of a shower energy we need an absorber thickness of k X ρ + 3λ TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/30 min γ This is actually close to experimental results! For 100 GeV electrons on lead this gives 20X o = 11cm γ

31 Electromagnetic showers Transverse shower development 95% containment in a cylinder of radius 2R M Molière radius R M 21 E MeV = X C For 100 GeV electron in lead this gives 1.7 cm 6 GeV/c ē C. Fabjan, T. Ludlam, CERN-EP/82-37 TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/31

32 Nuclear interactions Inelastic nuclear processes determines the interaction of energetic hadrons p, n, π, K,... Z,A p π 0 π + ρ n N ln(e o ) Excitation and breakup of nucleus transverse momentum about 0.35 GeV/c nucleus fragments and secondary particles cross section depends ~only on A 0.7 σ σ A σ 35 hadronic absorption length λ A a = N σ inelastic mb in g/mol A inelastic TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/32 A

33 Interaction and hadronic absorption lengths Material Z A ρ [g/cm 3 ] X 0 [g/cm 2 ] λ a [g/cm 2 ] Hydrogen (gas) (g/l) Helium (gas) (g/l) Beryllium Carbon Nitrogen (gas) (g/l) Oxygen (gas) (g/l) Aluminium Silicon Iron Copper Tungsten Lead Uranium TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/33

34 Hadronic shower many processes involved more complex (and larger) than EM showers hadronic component 0 π γγ electromagnetic component large energy fluctuations limited σ(e) E E m m 0.85 E E GeV for 2.6 ± π GeV for p Grupen TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/34

35 Calorimeter types Homogeneous calorimeters the detector is the absorber good energy resolution limited spatial resolution, especially longitudinal only used for electromagnetic calorimetry Sampling calorimeters the detector and absorber are separated only sample part of the shower additional sampling fluctuations affect energy resolution limited energy resolution good spatial resolution TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/35

36 Homogeneous calorimeters Two main types Scintillation crystals Glass blocks, through Cherenkov radiation Scintillation crystals Scintillator Density [g/cm 3 ] X 0 [cm] Light Yield γ/mev (rel. yield) τ 1 [ns] λ 1 [nm] TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/36 Rad. Dam. [Gy] Comments NaI (Tl) hydroscopic, fragile CsI (Tl) (0.49) Slightly hygroscopic CSI pure Slightly (0.04) hygroscopic BaF (0.13) BGO PbW light yield =f(t) Relative light yield: rel. to NaI(Tl) readout with PM (bialkali PC) Joram example: CMS electromagnetic calorimeter uses PbWO 4 example: L3 used BGO for its EM calorimeter

37 Homogeneous calorimeters Cherenkov radiators Cherenkov radiation produced when a charged particle travels in a dielectric (index of refraction n) at a velocity greater than the speed of light in that dielectric (c/n). Material Density [g/cm 3 ] X 0 [cm] n Light yield [p.e./gev] (rel. p.e.) SF-5 Lead glass ( ) SF Lead glass ( ) PbF ( ) λ cut [nm] Rad. Dam. [Gy] Relative light yield: rel. to NaI(Tl) readout with PM (bialkali PC) Comments 10 3 Not available in quantity Joram example: OPAL used lead glass for its barrel and end-cap calorimeters TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/37

38 Sampling calorimeters Sample a fraction of the shower variety of detectors used MWPC warm liquids TMP (tetramethylpentane) TMS (tetramethylsilane) liquid noble gase elements LAr (mainly), LKr, LXe scintillators, fibres silicon detectors detectors absorbers TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/38

39 Sampling calorimeters TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/39

40 Electromagnetic calorimeters Use EM shower results: number of particle at shower max is prop. to E o total track length of e ± is prop. to E o Linearity if the calorimeter signal is proportional to the e ± track length if the EM shower is completely contained in the calorimeter then the calorimeter signal is proportional to E o TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/40

41 Electromagnetic calorimeters Intrinsic limit to energy resolution total number of track segments N EC energy resolution improves with incident energy ( E) σ( N) 1 1 σ E N N E spatial resolution also scales the same way General parametrization σ( ) E = a b c E E E E stochastic or sampling term constant term comes from inhomogeneity, bad calibration, non-linearity noise term including electronic and pileup noise TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/41

42 Hadronic calorimeters Sampling calorimeters EM and hadronic energy depositions are measured with different efficiencies m 1 e/h ratio " / e" 1 ( 1 h/e E π = ) E 0 if e/h = 1, the calorimeter is compensating non compensations worsen the energy resolution Grupen TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/42

43 Liquid argon calorimetry LAr ionization chamber + U id ( t) R i d () t dq = dt due to drift i considering only the electron drift and assuming RC >> t d id () t = i 1 t t d t d typical values: T = 87 K, ρ = 1.40 g/cm 3 X 0 = ρ 13.9 cm W = 23.6 ev U 0 = 2 kv over 2 mm gaps t d 450 ns (compare to LHC 25 ns!) Particle Data Group TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/43

44 Liquid argon calorimetry Accordion design novel hermetic design used for all ATLAS EM calorimetry Lead-steel absorbers multilayer readout boards LAr is radiation hard RD3 collaboration, NIM A309 (1991) angle change with radius to keep LAr gap constant thickness M. Lefebvre TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/44

45 Liquid argon calorimetry Accordion EM calorimeter beam test results σ ( E) E ATLAS TDR = a E b / E c RD3 collaboration, NIM A309 (1991) TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/45

46 Lecture II: Questions Question II.1 Use the numbers shown on slide II/11 to obtain the solid angle aperture (in one direction) quoted, namely 3.1% and 5.3%. Question II.2 Using the lectures notes, obtain the result quoted at the bottom of slide II/30, namely that 100 GeV electron EM showers in lead are 95% contained in a depth of 20Xo = 11cm. TRIUMF Summer Institute 2006, Particle Detectors Michel Lefebvre, Victoria II/46