By Blake D. Leverington . Introduction . Inorganic Scintillators . Excitons and Activators . Stokes Shift and the Franck-Condon Principle . Organic Scintillators . The Foerster Resonant Energy Transfer (FRET) Mechanism . The LHCb Scintillating Fibre Tracker Heidelberg HighRR Seminar B. Leverington 8/2/2017 2 “Scintillation is a luminescence induced by ionizing radiation in transparent dielectric media.” Taken from: Inorganic Scintillators for Detector Systems Physical Principles and Crystal Engineering Second Edition Paul Lecoq • Alexander Gektin • Mikhail Korzhik Heidelberg HighRR Seminar B. Leverington 8/2/2017 3 . Detectors for radioactive particles . The modern era of particle physics began with Rutherford using Zinc Sulfide to observe alpha particles in 1899. Heidelberg HighRR Seminar B. Leverington 8/2/2017 4 Path length . Neutrons and hadrons . Nuclear interactions (pi0 showers, beta or gammas, knock-on electrons) Particle Energy decreases . Ionization of atoms (stripped electrons) e- e- e-e- e- . Electrons and photons e- e- e- e- e- . Bremsstrahlung, pair production (high E) electromagnetic shower . photo-absorption (low to med E) . Compton scattering dominates below 1 MeV . Lots of “hot” electrons produced . Convert this to a signal! Heidelberg HighRR Seminar B. Leverington 8/2/2017 5 . Inorganic Scintillators . Organic Scintillators . Crystals (hundreds, such as PbW04) . Crystals (anthracen, naphthalene, stilbene, etc) . Liquids (in organic solvents, benzene ring needed) . High Z atoms . Plastic (organic fluors in a base polymer) . High density (3-8 g/cm3) . Good for compact calorimeters . Lower Z / density (1-2 g/cm3) . High light yield (10-100 kph/MeV) . Lower light yield (1-10 kph/MeV) . Good resolution for calorimetry . Fast ns decay times . Slower ns – us decay times . Good timing resolution . Can be bad for timing resolution . afterglow . Cheaper . Expensive (production) . Shapeable/machinable . Hygroscopic (will reduce light yield) . Independent of temperature (-60 -- +20) . Temp dependent Light yield (%/degree) . Gases (nitrogen + noble gases) . Glasses (boron silicates) Heidelberg HighRR Seminar B. Leverington 8/2/2017 6 Unwanted . In the case of ordered materials like crystals, <keV electrons begin to couple with the lattice Mobility of electrons . Some electrons are ionized and free(E>Eion) . Some electrons are excited from the valence band to the conductance band (or just above and relax) (E>~Eg) . Some loosely bind with a hole and form an exciton (E<~Eg) Image from Heidelberg HighRR Seminar B. Leverington 8/2/2017 https://crystalclear.web.cern.ch/crystalclear/scintmechanisms.html 7 . The crystal must contain luminescent centers to be a scintillator . Intrinsic . Extrinsic . Beware of traps (defects) – reduces conversion efficiency, delays emission . Once the electrons or energy can travel, high probability of absorption by a luminescent center . Relaxes to its ground state -> photon emission S.E. Derenzo, Scintillation Counters, Photodetectors and Radiation Spectroscopy, IEEE Short Course Radiation Detection and Measurement, 1997 Nuclear Science Symp. Heidelberg HighRR Seminar B. Leverington 8/2/2017 8 . The crystal must contain luminescent centers to be a scintillator . Intrinsic . “involve electron-hole recombination; free, self- trapped (PbW04), and defect-trapped exciton luminescence; constituent transition group or post- transition group ion fluorescence; core-valence band transitions; or charge transfer transitions Many paths to within a molecular complex” –Derenzo NIM 2002 luminescence . Internal defects allow for exciton recombination (eg. BGO) . Extrinsic . Defects and Impurities . Doped with ions (activator) . Combination of activator ions and with impurities . Beware of traps (defects) – reduces conversion efficiency, delays emission . Once the electrons or energy can travel, high probability of absorption by a luminescent center S.E. Derenzo, Scintillation Counters, Photodetectors and Radiation . Relaxes to its ground state -> photon emission Spectroscopy, IEEE Short Course Radiation Detection and Measurement, 1997 Nuclear Science Symp. Heidelberg HighRR Seminar B. Leverington 8/2/2017 9 . The crystal must contain luminescent centers to An exciton is a loosely bound (0.01 eV) e-h pair be a scintillator that can move within the crystal; integer spin . Intrinsic . “involve electron-hole recombination; free, self- trapped (PbW04), and defect-trapped exciton luminescence; constituent transition group or post- transition group ion fluorescence; core-valence band transitions; or charge transfer transitions within a molecular complex” –Derenzo NIM 2002 . Internal defects allow for exciton recombination (eg. BGO) . Extrinsic . Defects and Impurities . Doped with ions (activator) . Combination of activator ions and with impurities . Beware of traps (defects) – reduces conversion efficiency, delays emission . Once the electrons or energy can travel, high probability of absorption by a luminescent center S.E. Derenzo, Scintillation Counters, Photodetectors and Radiation . Relaxes to its ground state -> photon emission Spectroscopy, IEEE Short Course Radiation Detection and Measurement, 1997 Nuclear Science Symp. Heidelberg HighRR Seminar B. Leverington 8/2/2017 10 . Time to excited state is very fast . (femtoseconds) . Radiative emission from Singlet states . Fluorescence . Fast (nanoseconds) . Radiative emission from Triplet states . Phosphorescence . Slow (microseconds) . Quenching will compete with these processes . Relaxation to ground state through vibrational modes Heidelberg HighRR Seminar B. Leverington 8/2/2017 11 푝ℎ 106 퐿푌 = 푁푒−ℎ푆푄 = 푆푄 푀푒푉 훽퐸푔 푁푒−ℎ = 푛푢푚푏푒푟 표푓 푒 − ℎ 푝푎푟푠 퐸푔 = 푏푎푛푑 푔푎푝 푒푛푒푟푔푦 (NaI(Tl)) has a light output ∼ 40,000 photons per MeV of 훽 = 표푛푠푎푡표푛 푒푛푒푟푔푦 푐표푛푣푒푟푠표푛 푒푓푓. energy deposit. (typically ∼ 2-7) • Q (Tl+) ∼ 1 • Rise time is fairly fast (τR ∼ 10 ns). 푆 = 푐푎푟푟푒푟 푡푟푎푛푠푓푒푟 푒푓푓푐푒푛푐푦 • Slow rise times are from slow carriers (least well known: material, temperature, impurity • the decay time is rather slow (τ ∼ 250 ns). dependent) D • some transitions are forbidden (phosphorescence) 푄 = 푞. 푒. 표푓 푙푢푚푛푒푠푐푒푛푡 푐푒푛푡푒푟 Heidelberg HighRR Seminar B. Leverington 8/2/2017 12 Photon re-absorption? Heidelberg HighRR Seminar B. Leverington 8/2/2017 13 . Franck-Condon principle (semi-classical) . The electronic transitions are much faster than the nuclear motions 푓푠 . new vibrational level must be Excitation 퐸0,0 퐸1,2 instantaneously compatible with the nuclear 푝푠 positions and momenta of the vibrational Relaxation 퐸 퐸 level of the molecule 1,2 1,0 . transitions between vibrational sublevels 푛푠 ν=0 and ν=2 and ν=1→ν=0 are favoured Radiative 퐸1,0 퐸0,2 over ν=0→ν=0 . ℎ휐 < Eabs (Stokes Shift) = longer wavelength Heidelberg HighRR Seminar B. Leverington By Samoza, CC BY-SA 3.0, 8/2/2017 14 https://commons.wikimedia.org/w/index.php?curid=33461268 Inorganic Scintillator Absorption and Emission spectra & Transmission 2007 IEEE Nuclear Science Symposium Conference Record N49-1, Rihua Mao et al. Heidelberg HighRR Seminar B. Leverington 8/2/2017 15 . Rise time, τr , due to transfer time from . Timing resolution dependent on the arrival vibration modes to luminescent centres time, t1st ,of the first photon . Decay time, τd , due to emission and quenching of luminescent centres 휏푑 휏푑 푡 푠푡 = for 휏푟 < 1 푁푝ℎ 푁푝ℎ −푡 −푡 휏 휏 퐼 푡 = 퐼0(푒 푑 − 푒 푟) 2휏푟휏푑 휏푑 푡 푠푡 = for 휏푟 > 1 푁푝ℎ 푁푝ℎ τr τd From “Inorganic Scintillators for Detector Systems,” Paul Lecoq Heidelberg HighRR Seminar B. Leverington 8/2/2017 16 Benrachi et al. NIMA281(1989)137-142 95 MeV alphas τf = 800 ns . For CsI(Tl), the pulse shape has two τs = 4000 ns distinct components, fast and slow −푡 −푡 ℎ푓 휏 ℎ푠 퐼 푡 = 푒 푓 + 푒 휏푠 휏푓 휏푠 . τf is dependent on ionization density . Excitation to states with forbidden decays more likely . τs is independent ℎ . 푅 = 푠 increases with ionization (ℎ푠+ℎ푓) density Heidelberg HighRR Seminar B. Leverington 8/2/2017 17 INDRA collaboration Heidelberg HighRR Seminar B. Leverington 8/2/2017 18 . Refractive index: . Radiation and Interaction length . Coupling efficiency to photodetectors . Shower containment improves energy . High (1.5-2) resolution . Transmission of emitted photons . Optical transparency of the crystal . Temperature dependence . Density: . Wavelength matching to photodetector . Compactness = cost * volume quantum efficiency . Small Moliere radius (high density, lower Z) . RM~X0 (Z + 1.2)/37.74). Smaller EM shower radius = better separation Heidelberg HighRR Seminar B. Leverington 8/2/2017 19 Complex interaction of luminescent centres results in multiple decay time components A more extensive HTML table is available at: http://scintillator.lbl.gov Heidelberg HighRR Seminar B. Leverington 8/2/2017 20 . At higher temperatures, the vibrational states widen . Simple case: Overlapping ground states result in reabsorption (quenching) and loss of light yield . Improves the timing, however 2007 IEEE Nuclear Science Symposium Conference Record N49-1, Rihua Mao et al. J.B. Birks, The Theory and Practice of Scintillation Counting, New York, 1964 Heidelberg HighRR Seminar B. Leverington 8/2/2017 21 granularity . Mass resolution through 2 photon decay channel (i.e. H->gamma gamma) . Want ∂E/E = 0.5% for 100 GeV photons statistical noise . a=2% is achievable for modern homogenous EM calorimeters 훿퐸 1 constant ~ 퐸 √푁 . B<0.5% is hard . Need X0~25 for 100GeV photons . The shower is contained in the readout cluster . Temperature dependence can increase Shower Non- Inter- bF; also production variation leakage uniformity calibration