Introduction Overview of detector systems Sources of radiation Radioactive decay Cosmic Radiation Accelerators Content Interaction of Radiation with Matter General principles Charged particles heavy charged particles electrons Neutral particles Photons Neutrons Neutrinos Definitions Detectors for Ionizing Particles Principles of ionizing detectors Gas detectors Principles Detector concepts
Content Semiconductor detectors Semiconductor basics Sensor concepts Different detector materials Readout electronics Scintillation detectors Calorimeters General characteristics Organic materials Inorganic materials Light output response Velocity Determination in Dielectric Media Cerenkov detectors Cerenkov radiation Cerenkov detectors Transition Radiation detectors Phenomenology of Transition Radiation Detection of Transition Radiation Complex Detector Systems Particle Identification with Combined Detector Information Tracking
Lecture 9 Scintillating Detectors and Calorimeters
Scintillation photodetector Energy deposition by ionizing particle production of scintillation light (luminescense) Scintillators are multi purpose detectors: calorimetry time of flight measurement tracking detector (fibers) trigger counter veto counter Two material types: Inorganic and organic scintillators
Organic materials sp 2 -hybridisation: 2p x and 2p y mix with s-orbital -orbital p z remains unchanged π-orbital
Pi electron energy levels Organic scintillators: Monocrystals or liquids or plastic solutions Monocrystals: naphtalene, anthracene, p-terphenyl. Liquid and plastic scintillators They consist normally of a solvent + secondary (and tertiary) fluors as wavelength shifters. Fluorescence 10-8 - 10-9 sec peak ~ 320 nm ~10-11 sec non-radiative transition ~ 10-6 sec (Förster transf.) Phosphorescence 10-4 sec Fast energy transfer via non-radiative dipole-dipole interactions (Förster transfer). shift emission to longer wavelengths longer absorption length and efficient read-out device
Wavelength shifting no self-absorption also used for light re-direction
Organic scintillators Practical organic scintillators uses a solvent + large concentration of primary fluor + smaller concentration of secondary fluor +... The emitted wavelength is always longer or equal to the incident wavelength. The difference is absorbed as heat in the atomic lattice of the material.
Organic scintillators have low Z (H,C) Low density (< 2 g/cm 3 ) Low g detection efficiency (practically only Compton effect). But high neutron detection efficiency via (n,p) reactions.
Inorganic Crystalline Scintillators The most common inorganic scintillator is sodium iodide activated with a trace amount of thallium [NaI(Tl)]. Energy bands in impurity activated crystal often 2 time constants: fast recombination (ns - µs) from activation center delayed recombination due to trapping (100 ms)
BaF 2 fast and slow signals 200ns/square 2ns/square
Inorganic Crystalline Scintillators Strong dependence of the light output and the decay time with temperature. * * Bismuth germinate Bi 4 Ge 3 O 12 is the crystalline form of an inorganic oxide with cubic eulytine** structure, colourless, transparent, and insoluble in water. ** From the Greek eulitos = "easily liquefiable", in allusion to its low melting point.
Inorganic scintillators PbWO 4 ingot and final polished CMS ECAL scintillator crystal from Bogoroditsk Techno-Chemical Plant (Russia).
Liquified Noble Gases: LAr, LXe, LKr Also here one finds 2 time constants: from a few ns to 1 ms. from C. D'Ambrosio, Academic Training, 2005
Common materials Density (g/cm 3 ) λ emiss (nm) #photon /MeV (ns) NaI(Tl) 3.7 410 40000 230 hygrosc. CsI(Tl) 4.5 560 45000 1100 hygrosc. BGO 7.1 480 8000 300 BaF 2 4.9 220 / 310 2300/ 10000 0.8 / 630 CeF 3 6.2 320 5500 27 rad. hard plastic 1.03 430 10000 2 5 = good = bad easy handling
Light collection
Light collection
light transport by total internal reflection typ. 25 mm Optical fibers core polystyrene n=1.59 cladding (PMMA) n=1.49 n 1 typically <1 mm n 2 n arcsin 2 69. 6 d % n1 4 3.1 in one direction d 4 5.3% and absorption length: l>10 m for visible light multi-clad fibres for improved aperture core polystyrene n=1.59 cladding (PMMA) n=1.49 25 mm fluorinated outer cladding n=1.42 25 mm
Optical fibers for tracking Scintillating plastic fibers Capillary fibers, filled with liquid scintillator Planar geometries (end cap) Circular geometries (barrel) (R.C. Ruchti, Annu. Rev. Nucl. Sci. 1996, 46,281) High geometrical flexibility Fine granularity Low mass Fast response (ns) (if fast read out) first level trigger a) axial b) circumferential c) helical
Scintillating fiber tracking (H. Leutz, NIM A 364 (1995) 422) Charged particle passing through a stack of scintillating fibers (diam. 1mm) Hexagonal fibers with double cladding. Only central fiber illuminated. Low cross talk!
Photon Detectors Purpose: Convert light into detectable (electronic) signal Principle: Use photoelectric effect to convert photons (g) to photoelectrons (pe) Standard requirements: High sensitivity, usually expressed as: quantum efficiency: QE(%) radiant sensitivity S(mA/W): Low intrinsic noise Low gain fluctuations High active area N pe N g QE(%) 124 S( ma/ W ) l( nm)
Photon detectors Photon detectors Main types of photon detectors: gas-based vacuum-based solid-state hybrid Photoemission threshold W ph of various materials TEA TMAE,CsI Ultra Violet (UV) Visible Bialkali Infra Red (IR) Multialkali GaAs 12.3 4.9 3.1 2.24 1.76 1.45 E [ev] 100 250 400 550 700 850 l [nm]
The photoelectric effect 3-step process: absorbed g s impart energy to electrons (e) in the material; energized e s diffuse through the material, losing part of their energy; e s reaching the surface with sufficient excess energy escape from it; ideal photo-cathode (PC) must absorb all g s and emit all created e s Semi-transparent PC Opaque PC Optical window g e - Vacuum e - g Substrate
Energy-band model in semi-conductor PC Standard model NEA material e - Photoemission threshold W ph Negative electron affinity E A g energy E g h Band gap E G (Photonis) Electron affinity E A E g h W ph E G E A Wph E G
QE s of typical photo-cathodes Photon energy E g (ev) 12.3 3.1 1.76 1.13 GaAsP GaAs Ag-O-Cs CsTe (solar blind) Bialkali Multialkali (Hamamatsu) Bialkali: SbKCs, SbRbCs Multialkali: SbNa 2 KCs (alkali metals have low work function)
Transmission of optical windows
Scintillator-Photomultiplier system (in-)organic material scintillation light light guide transmission scint. to tube photomultiplier signal amplification
Photomultiplier tubes (PMTs) Basic principle: Photo-emission from photocathode Secondary emission (SE) from N Dynodes: dynode gain g 3 50 (function of incoming electron energy E) total gain M: Example: 10 dynodes with g = 4 M = 4 10 10 6 M N i 1 g i
Secondary Electron Emission Approximately the same as the Photo Electric Effect. On electron impact, energy is transferred directly to the electrons in the secondary electron emission material allowing a number of secondary electrons to escape. Since the conducting electrons in metals hinder this escape, insulators and semiconductors are used. Materials in common use are: Ag/Mg, Cu/Be and Cs/Sb. Use has also been made of negative affinity materials as dynodes, in particular GaP.
SE coefficient d Counts Counts SE coefficient d Gain fluctuation of PMT s Mainly determined by the fluctuations of the number m(d) of secondary e s emitted from the dynodes; GaP(Cs) NEA dynodes E A <0 Poisson distribution: P ( m) d m d e m! d Standard deviation: d 1 m d d d fluctuations dominated by 1 st dynode gain; CuBe dynodes E A >0 (Photonis) E energy 1 pe 2 pe 1 pe 3 pe (H. Houtermanns, NIM 112 (1973) 121) (Photonis) Noise (Photonis) Pulse height E energy Pulse height
Dynode configurations of PMT s Traditional Position-sensitive Mesh Metall-channel (fine-machining techniques) PMT s are in general very sensitive to magnetic fields, even to earth field (30-60 µt). Magnetic shielding required.
The Micro Channel Plate (MCP) (Hamamatsu) Continuous dynode chain Pb-glass Pore : 2 mm Pitch: 3 mm Kind of 2D PMT: + high gain up to 5 10 4 ; + fast signal (transit time spread ~50 ps); + less sensitive to B-field (0.1 T); - limited lifetime (0.5 C/cm 2 ); - limited rate capability (ma/cm 2 ); (Burle Industries)
Hybrid Photo Detector Photo Multiplier Tube - dynodes and anode + Silicon Sensor = HPD n + p + n + - + - + - photocathode Hybrid Photo Diode focusing electrodes electron V [Kinetic energy of the impinging electron] [work to overcome the surface] Electron-hole pairs = [Silicon ionization energy] ~ 4-5000 electron-hole pairs Good energy resolution silicon sensor
But Electronic noise, typically of the order of 500 e Back scattering of electrons from Si surface: 20% of the electrons deposit only a fraction o <1 of their initial energy in the Si sensor. continuous background (low energy side) Hybrid Photo Detector 3 parameters: - - <n pe > - Si Si 0.18 back scattering probability at E 20 kv C. D Ambrosio et al. NIM A 338 (1994) p. 396.
Solid-state photon detectors Photodiodes: P(I)N type p layer very thin (< 1 µm), as visible light is rapidly absorbed by silicon High QE(80% at 700 nm) No gain: cannot be used for single photon detection Avalanche phtodiode: High reverse bias voltage: typ.100-200 V due to doping profile, high internal field and avalanche multiplication High gain: typ. 100-1000
Light absorption in Silicon
Special photo diodes APD SPAD Avalanche PhotoDiode Bias: slightly below breakdown Linear-mode: it s an amplifier Gain: limited < 1000 Single-Photon Avalanche Diode Bias: well above breakdown Geiger-mode trigger device Gain huge Passive quenching by serial resistor at output (simple but slow ~ 200 ns) Active quenching via additional CMOS circuitry faster
APD/SPAD quantum efficiency
Triggering device Scintillation is fast perfect for triggering on particle beam e.g. finger counters, veto panels, etc. often used in test beams
Calorimeters
Calorimeter Types Homogeneous calorimeters: detector = absorber good energy resolution limited spatial resolution (particularly in longitudinal direction) only used for electromagnetic calorimetry Sampling calorimeters: detectors and absorber separated only part of the energy is sampled. limited energy resolution good spatial resolution used both for electromagnetic and hadron calorimetry
Homogeneous calorimeters Two main types: Scintillators (crystals) Scintillator Density [g/cm 3 ] Cherenkov radiators X 0 [cm] Light Yield g/mev (rel. yield) 1 [ns] l 1 [nm] Rad. Dam. [Gy] Comments NaI (Tl) 3.67 2.59 4 10 4 230 415 10 hydroscopic, fragile CsI (Tl) 4.51 1.86 5 10 4 (0.49) 1005 565 10 Slightly hygroscopic CSI pure 4.51 1.86 4 10 4 10 310 10 3 Slightly (0.04) 36 310 hygroscopic BaF 2 4.87 2.03 10 4 0.6 220 10 5 (0.13) 620 310 BGO 7.13 1.13 8 10 3 300 480 10 PbW0 4 8.28 0.89 100 10 440 10 4 light yield =f(t) 10 530 Material Scintillator crystals or glass blocks (Cherenkov radiation). photons. Readout via photomultiplier, -diode/triode Density [g/cm 3 ] X 0 [cm] n Light yield [p.e./gev] (rel. p.e.) SF-5 Lead glass 4.08 2.54 1.67 SF-6 5.20 1.69 1.81 Lead glass PbF 2 7.66 0.95 1.82 2000 l cut [nm] Rad. Dam. [Gy] 10 2 10 2 Comments 10 3 Not available in quantity
Example ECAL - homogeneous OPAL Barrel + end-cap: lead glass + pre-sampler Principle of pre-sampler or preshower detector 10500 blocks (10 x 10 x 37 cm 3, 24.6 X 0 ), PM (barrel) or PT (end-cap) readout. ( E) E 0.06 E 0.002 Spatial resolution (intrinsic) 11 mm at 6 GeV Sample first part of shower with high granularity. Useful for g/ 0, e/g, e/ discrimination. Usually gas or, more recently, Si detectors
Sampling calorimeters Sampling calorimeters = absorber + detector MWPC, streamer tubes warm liquids (TMP = tetramethylpentane, TMS=tetramethylsilane cryogenic noble gases: mainly LAr scintillators, scintillating fibres, silicon detectors Shashlik readout
Energy resolution of a calorimeter T E E 0 Ntotal C T det E E 0 X 0 C F( ) T ( E) ( T E T det E E ) cut C det 1 1 total number of track segments total track length T det detectable track length (above energy E cut ) E 0 holds also for hadron calrimeters More general: ( E) E a E b c E Also spatial and angular resolution scale like 1/ E stochastic term (see above) constant term inhomogenities bad cell inter-calibration non-linearities Quality factor! noise term electronic noise radioactivity pile up
Sampling calorimeters Sampling fluctuations N T d det F( ) E E c X d 0 detectors absorbers ( E) E N N 1 F( ) Ec E d X 0 Pathlength fluct. + Landau fluct. wide spread angular distribution of (low energy) e In thin gas detector layers the deposited energy shows typical Landau tails d
Hadronic cascades A hadron calorimeter shows in general different efficiencies for the detection of the hadronic and electromagnetic components h and e : R E E h h h The fraction of the energy deposited hadronically depends on the energy response of calorimeter to hadron shower becomes nonlinear e e
Hadronic cascades How to achieve compensation? increase h : use Uranium absorber amplify neutron and soft photon component by fission + use of hydrogeneous detector high neutron detection efficiency decrease e : combine high Z absorber with low Z detectors. Suppressed low energy photon detection ( Z 5 ) offline compensation: requires detailed fine segmented shower data event by event correction.
Example ECAL - sampling ATLAS electromagnetic Calorimeter Accordion geometry absorbers immersed in Liquid Argon Liquid Argon (90K) + lead-steal absorbers (1-2 mm) + multilayer copper-polyimide readout boards Ionization chamber. 1 GeV E-deposit 5 x10 6 e - Accordion geometry minimizes dead zones. Liquid Ar is intrinsically radiation hard. Readout board allows fine segmentation (azimuth, pseudorapidity and longitudinal) acc. to physics needs Test beam results:
Example HCAL sampling CMS Hadron calorimter Cu absorber + scintillators 2 x 18 wedges (barrel) + 2 x 18 wedges (endcap) 1500 T absorber Scintillators fill slots and are read out via fibres by HPDs Test beam resolution for single hadrons E E 65% 5% E