PhotomultipliersPhotomultipliers:: EyesEyes forfor youryour ExperimentExperiment
extension of our senses new technology understand tools on fundamental level
new knowledge in correct interpretation fundamental processes of results
Example: photomultipliers today: mature and versatile technology vast range of applications … and still gets improved by innovations Stephan Eisenhardt Edinburghinburgh, 23.07.2003 University of Edinburgh BasicBasic PrinciplePrinciple
Photo emission from photo cathode Q.E. = Np.e./Nphotons
Electron collection – Focusing optics – optimise efficiency – minimise transient
time spread Equi-potentials and trajectories in a fast input system Secondary emission from dynodes: Electric potential Electron multiplication dynode gain: g(E) = 3…50
Total gain: G = Π gi e.g. 10 dynodes with g=4 10 6 Schematic of Photomultiplier Tube G = 4 ≈ 10 (Philips Photonic) Edinburgh, 23.07.2003 Stephan Eisenhardt 2 AA BriefBrief HistoryHistory
1887: photoelectric effect discovered by Hertz 1902: first report on a secondary emissive surface by Austin et al. 1905: Einstein: "Photoemission is a process in which photons are converted into free electrons." 1913: Elster and Geiter produced a photoelectric tube 1929: Koller and Campbell discovered compound photocathode (Ag-O-Cs; so-called S-1) 1935: Iams et al. produced a triode photomultiplier tube (a photocathode combined with a single-stage dynode) 1936: Zworykin et al. developed a photomultiplier tube having multiple dynode stages using an electric and a magnetic field 1939: Zworykin and Rajchman developed an electrostatic-focusing type photomultiplier tube 1949 & 1956: Morton improved photomultiplier tube structure
commercial phase, but still many improvements to come
Edinburgh, 23.07.2003 Stephan Eisenhardt 3 ExtensionExtension ofof VisionVision
Human eye: Photomultiplier:
Spectral sensitivity: 400 Spatial resolution: ~100 lines/mm (2500dpi) 2mm…50cm Intensity range: O(1016γ/mm2s) (daylight) 1γ … ~108γ/mm2s single photon sensitivity (~1mA anode current for 1’’ tube) after adaptation Life time: O(105C/mm2 70yrs) O(10C) for semi-transparent cathode Edinburgh, 23.07.2003 Stephan Eisenhardt 4 PhotoemissionPhotoemission semitransparent opaque 2-step process: photocathode photocathode γ – photo ionisation γ glass – escape of electron into vacuum PC e- - e substrate Multi-reflection/interference due to PC high refractive index bialkali: n(λ = 442 nm) = 2.7 Q.E. difficult to measure often only an effective detection glass efficiency determined: PC + internal reflection from a metallic surface vacuum + collection of the photoelectrons + electronic theshold… Edinburgh, 23.07.2003 Stephan Eisenhardt 5 SpectralSpectral ResponseResponse In the band model: Photo electric effect hν [eV] Alkali Photocathode – hν > Eg + EA + dE/dx –E= hν -W -dE/dx γ: E = hν e- e Spectral response: E [eV] – Quantum efficiency e N p.e. Pν 1 QE(λ) = = (1− R) PS ( ) Nλ α 1+1 αl R = reflection coefficient α = γ absorption coefficient l = e−mean escape length Pν = excitation probability PS = surface transition probability – Cathode sensitivity I λ [nm]QE(λ) S()λ = C = [A/W] P()λ 1240 Edinburgh, 23.07.2003 Stephan Eisenhardt 6 AlkaliAlkali PhotocathodesPhotocathodes (Philips Photonic) semitransparent photocathodes Q.E. Bialkali : SbK2Cs, SbRbCs Multialkali : SbNa2KCs S(λ) [mA/W] Solar blind : CsTe (cut by quartz window) T [%] LiF (arb. units) nsitivity λ [nm] human eye λ [nm] transmittance of window materials human se (shape only) Edinburgh, 23.07.2003 Stephan Eisenhardt 7 PhotocathodePhotocathode ThicknessThickness Blue light is stronger absorped than red light! semi-transparent cathodes best compromise for the thickness λ [nm] ] of the PC: -1 0.4 [cm photon absorption length λA(Eph) α electron escape length λE(Ee) coefficient λA =1/α n o Q.E. of thick cathode: 5 -1 α≈4 · 10 cm red response λΑ ≈ 25 nm absorpti blue response Q.E. of thin cathode: 5 -1 α≈1.5 · 10 cm blue response λ ≈ 60 nm Α red response hν [eV] Edinburgh, 23.07.2003 Stephan Eisenhardt 8 AlkaliAlkali PhotocathodePhotocathode ProductionProduction evaporation of metals in high vacuum Example: SbK2Cs – < 10-7 mbar simplified sequential bialkali process -9 – < 10 mbar H2O partial pressure T 100 % – no other contaminants (CO, CxHy...) Sb T = 70 °C bakeout of process chamber dSb ≈ 5 nm (>150oC) and substrate (>300oC) 85 % @ 632 nm t condensation of vapour and chemical Sb reaction on entrance window T = 160 °C Maximum Q.E. @ 400 nm K Q.E. ≈ 3 - 5 % evaporation re-evaporation Sb K Sb 3 t T = 160 °C Maximum Q.E. condensation @ 400 nm reaction Q.E. ≈ 15 - 20 % crystal growth Cs t K3Sb K2SbCs relatively simple technique Edinburgh, 23.07.2003 Stephan Eisenhardt 9 PhototubePhototube FabricationFabrication internal: external: CERN/LHCb evaporation and encapsulation facility Indium seal Hot glass seal Edinburgh, 23.07.2003 Stephan Eisenhardt 10 SemiconductorSemiconductor PhotocathodesPhotocathodes ☺ high Q.E. and spectral width ☺ negative electron affinity complex production molecular beam epitaxial growth ] % Q.E. [ 10-10 mbar (Hamamatsu) λ [nm] a) grow PC on crystalline substrate b) create interface layer c) fuse to entrance window a) b) c) d) d) etch substrate away Edinburgh, 23.07.2003 Stephan Eisenhardt 11 SecondarySecondary EmissionEmission alloy of alkali or earth-alkali and noble metal secondary e- dynodes alkaline metal oxidises → insulating coating k – Large gain Gain δ = a(∆V ) with k = 0.7...0.8 ∆V – Stability for large currents – Low thermal noise ⎛ − eΦ ⎞ I = aT 2 exp⎜ ⎟ ⎝ kT ⎠ n me−m Statistical process: Poisson distribution P(n,m) = m! electron multiplication – Spread (RMS) σ n 1 n = = – Largest at 1st and 2nd dynode n n n typically 10 … 14 dynode stages linearity limits: bleeder chain – pulse: space charge scheme of external circuit – DC: photo current << bleeder current for dynode potentials Edinburgh, 23.07.2003 Stephan Eisenhardt 12 ClassicClassic DynodesDynodes uniformity: photoelectron collection efficiency: best photoelectron simple design collection efficiency good for large PC ∅ good uniformity venetian blind ☺☺ ☺box and grid ☺ excellent linearity compact good time resolution fast time response fast time response linear focusing ☺☺circular cage sensitive to Earth B-field (30-60µT)! no spatial resolution Edinburgh, 23.07.2003 Stephan Eisenhardt 13 SensitivitySensitivity toto MagneticMagnetic FieldsFields linear focusing venetian blind z y x B||z B||z B||y B||y B||x B||x ±50 µT ± 200 µT Earth field: 30-60 µT ± 0.5 Gauss 2 Gauss requires µ-metal shielding Edinburgh, 23.07.2003 Stephan Eisenhardt 14 ModernModern DynodesDynodes B-field immunity up to 1.2T B-field spatial resolution via segmented anode good uniformity poor e- collection efficiency! foil ☺☺ proximity mesh excellent linearity microchannel plate ☺ metal channel excellent time resolution: ☺ ☺ good e- collection efficiency transit time spread: 50ps excellent time characteristics stable gain …up to 20mT low cross-talk pixels Edinburgh, 23.07.2003 Stephan Eisenhardt 15 MultianodeMultianode PMTPMT Position sensitive PMT – 8x8 metal channel dynode chains in one vacuum pixel active area envelope (26x26 mm2) – segmented anode: 2x2 mm2 – active area fraction: 48% UV glass window Bialkali photo cathode MaPMT window & electron focusing segmented anode – QE = 22…25% at λ = 380 nm Gain – 3.105 at 800 V Uniformity, Crosstalk pulse charge – much improved Applications: – medical imaging – HERA-B, LHCb: Ring Imaging Cherenkov counters p.e. probability Relative distance [0.1 mm] Edinburgh, 23.07.2003 Stephan Eisenhardt 16 PMTPMT CharacteristicsCharacteristics Fluctuations single photon events to oscilloscope (50Ω) – number of secondary electrons MaPMT – Poisson distribution Saturation – space charge 5mV – large photon current 1ns Non-linearity – at high gains Stability – drift, temperature dependency – fatigue effects Monitoring Sensitive to magnetic fields – Earth: 30-60 µT requires µ-metal shielding charge integration pulse height spectrum Edinburgh, 23.07.2003 Stephan Eisenhardt 17 PulsePulse HeightHeight SpectrumSpectrum first dynode gain: δ = 25 first dynode gain: δ = 4 – clear separation of nγ signals – broad signal distributions – Gaussian shape – Poisson shape for npe=1,2 – Poisson for γ probability – significant signal loss below threshold cut fit to data MaPMT 1 photoelectron 2,3 photoelectrons (Gilmore) pulse amplitude pulse amplitude Photo conversion possible at 1st dynode – Gain reduced by δ1 contradiction when HV is changed!! –explains databetween – this data is contaminated with noise and 1 p.e. incompletely sampled charge pulses Edinburgh, 23.07.2003 Stephan Eisenhardt 18 ConclusionConclusion know your tools don’t fool yourself with immature conclusions always cross-check as far as possible photomultipliers are a mature and versatile technology suitable for a vast range of applications in light detection: – Spectral sensitivity: 110 flat panel PMT: next generation MaPMT development goes on… Edinburgh, 23.07.2003 Stephan Eisenhardt 19 Super-Kamiokande:Edinburgh, 23.07.2003 20’’ tubes Stephan Eisenhardt 20