Scintillators Scintillation Detector • Scintillation detectors are widely used to measure radiation. • The detectors rely on the emission of photons from excited states. – Counters – Calorimeters 1. An incident photon or particle ionizes the medium. 2. Ionized electrons slow down causing excitation. 3. Excited states immediately emit light. 4. Emitted photons strike a lightsensitive surface. 5. Electrons from the surface are amplified. 6. A pulse of electric current is measured. Energy Collection • Counters need only note that some energy was collected. • For calorimetery the goal is to convert the incident energy to a proportional amount of light. – Losses from shower photons – Losses from fluorescence xrays Compton Peak • For incident photons, Compton scattering transfers energy to electrons. • The recoil energy: • This is an important effect for photon measurement below a few MeV. • Has a maximum at q = 180°: T h 0 x(1 cosq ) 1 x(1 cosq ) x 2h 0 x h 0 T 1 2 x h 0 mec 2 / 2 2 • For photons in keV: T h 0 2 h 0 256 h 0 me c 2 Photon Statistics Typical Problem • Gamma rays at 450 keV are absorbed with 12% efficiency. Scintillator photons with average 2.8 eV produce photoelectrons 15% of the time. • What is the energy to produce a measurable photoelectron? • How does this compare to a gas detector (W-value)? Answer • The total energy of scintillation is 450 x 0.12 = 54 keV. – 5.4 x 104 / 2.8 = 1.93 x 104 photons produced – 1.93 x 104 x 0.15 = 2900 photoelectrons produced • The equivalent W-value for the scintillator is: – 450 keV/2900 = 155 eV/pe – W-value in gas = 30 eV/ip Inorganic Scintillators • Fluorescence is known in many natural crystals. – UV light absorbed – Visible light emitted • Artificial scintillators can be made from many crystals. – Doping impurities added – Improve visible light emission Band Structure conduction band h impurity excited states impurity ground state valence band • Impurities in the crystal provide energy levels in the band gap. • Charged particles excites electrons to states below the conduction band. • Deexcitation causes photon emission. – Crystal is transparent at photon frequency. Jablonski Diagram • Jablonski diagrams characterize the energy levels of the excited states. – Vibrational transitions are low frequency – Fluoresence and phosphoresence are visible and UV • Transistions are characterized by a peak wavelength lmax. Time Lag 10-12 s S1 10-15 s S0 10-7 s • Fluorescence typically involves three steps. – Excitation to higher energy state. – Loss of energy through change in vibrational state – Emission of fluorescent photon. • The time for 1/e of the atoms to remain excited is the characteristic time t. Crystal Specs • Common crystals are based on alkali halides – Thallium or sodium impurities • Fluorite (CaF2) is a natural mineral scintillator. • Bismuth germanate (BGO, Bi4Ge3O12) is popular in physics detectors. Crystal t (ns) NaI(Tl) 250 CsI(Tl) 1000 CsI 16 ZnS(Ag)110 CaF2(Eu) 930 BGO 300 lmax(nm) output 415 100 550 45 315 5 450 130 435 50 480 20 www.detectors.saint-gobain.com Tracking Detector • Iarocci tubes used in tracking are arranged in layers. • Hits in cells are fit to a track. – Timing converted to distance from wire – Fit resolves left-right ambiguity Organic Scintillators absorption emission • A number of organic compounds fluoresce when molecules are excited. • The benchmark molecule is anthracene. – Compounds are measured in % anthracene to compare light output R. A. Fuh 1995 Pi-Bonds • Carbon in molecules has one excited electron. – Ground state 1s22s22p2 – Molecular 1s22s12p3 • Hybrid p-orbitals are p-orbitals. – Overlapping p-orbitals form bonds – Appears in double bonds Excited Rings p-bonds are most common in aromatic carbon rings. • Excited states radiate photons in the visible and UV spectra. – Fluorescence is the fast component – Phosphorescence is the slow component • At left: π-electronic energy levels of an organic molecule. S0 is the ground state. S1, S2, S3 are excited singlet states. T1, T2, T3 are excited triplet states. S00, S01, S10, S11 etc. are vibrational sublevels. Plastics • Organic scintillators can be mixed with polystyrene to form a rigid plastic. – Easy to mold – Cheaper than crystals • Used as slabs or fibers Transmission Quality • Scintillator is limited by the transmission efficiency. – It’s not clear • The attenuation length cannot be too long for the application. Liquids • Organic scintillators can be mixed with mineral oil to form a liquid. – Circulate to minimize radiation damage – Fill large volume Waveshifter • • Photons from scintillators are not always well matched to photon detectors. – Peak output in UV-blue – Peak detection efficiency in green light. Wavelength shifting fibers have dyes that can absorb UV and reemit green light. • Fibers can be bent to direct light to detectors.