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Radiation Sensors Zachariadou K. | TEI of Piraeus Radiation Sensors Part-IV Scintillation Sensors Part-IV Scintillation Sensors The course is largely based on : G. F. Knoll, “Radiation detection and measurement” ; 3rd ed., New York, Wiley, 2000 Gordon Gilmore & John D. Hemingway, “ Practical Gamma-Ray Spectrometry”; Willey , 21008 Richard Fernow. “Introduction to experimental particle physics”, Cambridge University Press, 1986 Christian Joram “CERN Summer Student Lectures on Particle Detectors” 2003 C. Zorn, “Instrumentation in High Energy Physics” World Scientific, 1992 Claude LeRoy, P. G. Rancoita “ Principles Of Radiation Interaction In Matter And Detection” Scintillation Sensors Main components: Based on a material (Scintillator) that produces a pulse of light shortly after the passage of a particle Scintillator The light produced is propagated through light guides and directed on the face pf a photomultiplier tube Light guide Photolectrons emitted from the cathode of the tube are amplified to give a fast electronic pulse Photo multiplier tube Scintillation Sensors They emit light when irradiated promptly (<10-8s) Fluorescence delayed (>10-8s) Phosphorescence A good scintillator: its material converts as large a fraction as possible of the incident radiation energy to prompt fluorescence Scintillation Sensors principles Detect ionizing radiation by the scintillation light produced Scintillation material requirements: Convert the kinetic energy of charged particles into detectable light with high scintillation efficiency Linearity: the light yield should be proportional to deposited energy The medium should be transparent to the wavelength of its own emission for good light collection The material should be of good optical quality and able to be manufactured in large sizes The index of refraction should be near that of glass (~1.5) to permit efficient coupling of the scintillator light to a photomultiplier tube or other light sensor Scintillator types Organic (Plastic or Liquid solutions) (They yield less light but they are faster) Liquid • Economical • messy Solid • Fast decay time (ns to μs) • long attenuation length Inorganic (They yield more light but they are slower) NaI, CsI • Excellent gamma resolution • Best light output • Best linearity • Slow decay time (μs) • Expensive BGO •High density •compact Inorganic: high density crystals , high Z Organic (hydrogen content) : gamma spectroscopy beta and fast neutron detection Medical imaging Inorganic Crystal Scintillators Crystals of alkali metals (iodides) NaI(Tl) CsI(Tl) CaI(Na) LiI(Eu) CaF2(Eu) Sodium iodide activated with thallium [NaI(Tl)], coupled to Photomultiplier tubes (PMTs) and operated in pulse mode, is used for most nuclear medicine applications Bismuth germanate (BGO) is coupled to PMTs and used in pulse mode as detectors in most PET scanners Inorganic Crystal Scintillators Scintillation Mechanism NaI, CsI. BaF2.. Inorganic Crystal Scintillators Scintillation Temperature Dependence The light output shows strong temperature dependence Inorganic Scintillators Liquid Noble gases Scintillation mechanism: Inorganic Scintillators Lead tungstate crystals used in the CMS electromagnetic calorimeter : Organic Scintillators Monocrystals: aromatic compounds (consist of planar molecules made of benzene rings) examples:Toluene, Anthracene The potential energy of a molecule (which comes from electronic, vibrational and translational energies) changes with interatomic distance. Light production in organic scintillators is the result of molecular transitions. The radiation raises the molecule to an excited stat (A1) The molecule returns to the ground state by losing energy through vibrations (heat) to move to point B1 and then by emitting a photon to point B0. The transition is rapid (10-8 s), and results in a measurable photon of energy different than the incident one. Energy Liquid of plastic: mixtures of compounds that consist of a solvent (highest concentration) and solute (s). Examples of solvents benzene, toluene, and p-Xylene EA1 Excited state A1 Ground state EB1 EB0 EA0 B1 Bo Ao Inter-atomic distance Organic Scintillators Emitted light is in the UV range Shift emission to longer wavelengths Longer absorption length and efficient read out device Wavelength shifting: Scintillator readout In a plastic scintillator for 3keV ionization energy deposited one photon is produced. This results to 500 photons per cm of scintillator material This low intensity scintillation light has to be optically coupled to a photomultiplier for amplification and transformation to an electrical signal Light guides Wavelength shifter bars Photomultiplier tubes Light guides Goals: To match (rectangular) scintillator to (circular) Photomultiplier tube To optimize light collection for applications Fish tail light guide Wavelength shifter bars Collect the light at a location distant from the scintillator A wavelength shifter absorbs the light and re-emits it at a larger wavelength assuring its transfer over a relative long distance The choice of the material of a WLS : to match the emission spectrum to the absorption spectrum of the photocathode of the PMT Photomultiplier tubes (PMT) PMTs perform two functions: Conversion of ultraviolet and visible light photons into an electrical signal Signal amplification, on the order of millions to billions Photomultiplier tubes (PMT) Main Components A PMT consists of an evacuated glass tube containing a photocathode typically 10 to 12 electrodes called dynodes (which eject additional electrons after being struck by an electron. Multiple dynodes result in 106 or more signal enhancement Anode-collector (accumulates all electrons produced from final dynode) Resistor The collected current passes through a resistor to generate voltage pulse Photomultiplier tubes Electrons emitted by the photocathode are attracted to the first dynode and are accelerated to kinetic energies equal to the potential difference between the photocathode and the first dynode When these electrons strike the first dynode, about 5 electrons are ejected from the dynode for each electron hitting it These electrons are attracted to the second dynode, and so on, finally reaching the anode Reflector Glass Emitted electron Dynodes (secondary e- emission) +200V Anode +600V +50V Coaxial out Scintillation photon +400V Photocathode +800V Gain ~ 106 - 107 Photomultiplier tubes Principle of detection E hf W In the photocathode the photons are converted into photoelectrons via the photoelectric effect: E=kinetic energy of the photo-electron f= frequency of the incident photon W=working function The photocathode must be made of a low W material to maximize the emission of photolectrons Quantum efficiency of the PMT’s photocathode is the probability for an electron creation per striking photon number of emitted photoelectrons QE( ) number of incident photons Typical value: 20-25% Photomultiplier tubes The total amplification of the PMT is the product of the individual amplifications at each dynode If a PMT has ten dynodes and the amplification at each stage is 5, the total amplification will be approximately 10,000,000 The amplification can be adjusted by changing the voltage applied to the PMT Photomultiplier tubes The intensity (I) of light transported in the scintillator and the light guide is attenuated by atomic absorption and scattering in the reflecting surface. The reduction is a function of the distance l and the light wavelength λ: I (l , ) I (0, )e l I ph ( ) The number of photoelectrons produced by the photocathode: Ι(0,λ)=initial light intensity Iph(λ)=Photon attenuation length @ λ n pe n ph I ( L, )QE( ) f ( )dλ Nph=number of photons produced in the scintillator of length L F(λ)=fraction of light trapped Photomultiplier tubes A numerical example: • Scintillator thickness=1cm • Light output=25% •1 MeV deposited energy corresponds to the production of 104 γ •Α minimum ionizing particle produces 2 MeV/cm 2X104 γ are emitted in 1cm of the scintillator I (l , ) I (0, )e l I ph ( ) Assume • quantum efficiency 20% • Optical collection efficiency 50% The number of photoelectrons follow Poisson statistics If the attenuation length =250cm the intensity loss is 0.4% in 1 cm The number of photoelectrons coming off the photocathode: Npe=360 n nen P ( n) n! Photomultiplier tubes A numerical example: Anode: 0 V Cathode: -1400V 2200V Multiplication factor M between the first stage of the n-dynodes and the anode: For M=107 the charge collected at the anode is: M 1 2 3 n Q eM 1.6 10 19 C 10 7 1.6 pC Typically the charge is collected within 5μs the current in the anode is: dq i 0.32mA dt Commonly Used Scintillators Density [g/cm3] Emission Max [nm] Decay Constant (1) Refractive Index (2) Conversion Efficiency (3) Hygroscopic NaI(Tl) 3.67 415 0.23 ms 1.85 100 yes CsI(Tl) 4.51 550 0.6/3.4 ms 1.79 45 no CsI(Na) 4.51 420 0.63 ms 1.84 85 slightly CsI undoped 4.51 315 16 ns 1.95 4-6 no CaF2 (Eu) 3.18 435 0.84 ms 1.47 50 no 6LiI (Eu) 4.08 470 1.4 ms 1.96 35 yes 6Li glass 2.6 390 - 430 60 ns 1.56 4-6 no 4.64 390 3 - 5 ns 1.48 5-7 yes Material CsF (1) Effective average decay time For g-rays. (2) At the wavelength of the emission maximum. (3) Relative scintillation signal at room temperature for g-rays when coupled to a photomultiplier tube with a Bi-Alkalai photocathode. Commonly Used Scintillators Material Density [g/cm3] Emission Maximum [nm] Decay Constant (1) Refractive Index (2) Conversion Efficiency (3) Hygros copic BaF2 4.88 315 220 0.63 ms 0.8 ns 1.50 1.54 16 5 no YAP (Ce) 5.55 350 27 ns 1.94 35 - 40 no GSO (Ce) 6.71 440 30 - 60 ns 1.85 20 - 25 no BGO 7.13 480 0.3 ms 2.15 15 - 20 no CdWO4 7.90 470 / 540 20 / 5 ms 2.3 25 - 30 no Plastics 1.03 375 - 600 1 - 3 ms 1.58 25 - 30 no (1) Effective agerage decay time For g-rays. (2) At the wavelength of the emission maximum. (3) Relative scintillation signal at room temperature for g-rays when coupled to a photomultiplier tube with a Bi-Alkalai photocathode. Compare Scintillators Choice of a certain scintillation crystal in a radiation detector depends strongly on the application. Material Important Properties Major Applications NaI(Tl) Very high light output, energy resolution CsI(Tl) Noon-hygroscopic, rugged, long wavelength emission Particle and high energy physics, general radiation detection, photodiode readout, phoswiches CsI(Na) High light output, rugged Geophysical, general radiation detection CsI undoped Fast, non-hygroscopic, radiation hard, low light output Physics (calorimetry) CaF2(Eu) Low Z, high light outut b detection, a, b phoswiches CdWO4 Very high density, low afterglow, radiation hard DC measurement of X-rays (high intensity), readout with photodiodes, Computerized Tomography (CT) Plastics Fast, low density and Z, high light output Particle detection, beta detection good General scintillation counting, health physics, environmental monitoring, high temperature use ScintillatorsNeutron detection Neutrons do not produce ionization directly in scintillation crystals Neutrons can be detected through their interaction with the nuclei of a suitable element. In 6LiI(Eu) crystals neutrons interact with 6Li nuclei to produce an alpha particle and 3H which both produce scintillation light that can be detected. Timing applications Application: measurement of time intervals at the nanosecond level System that uses of the excellent timing capabilities of scintillators: time of flight (TOF) A time of flight (TOF) detector can discriminate between a lighter and a heavier elementary particle of same momentum using their time of flight between two scintillators. The first of the scintillators activates a clock upon being hit while the other stops the clock upon being hit. Time of flight difference: P=momentum E=energy x x t v bc pc b E x((mc ) 2 p 2 )1 / 2 t pc pc mc pc 2 2 2 2 Consider two particles with same momentum p and different masses Timing applications-cont t12 t 22 x 2 ((m1c) 2 p 2 ) ( pc) 2 x 2 ((m2c) 2 p 2 ) ( pc) 2 x 2 (m12 m22 ) p2 t12 t22 (t1 t2 )(t1 t2 ) x 2 (m12 m22 ) t1 t2 (t1 t2 ) p 2 For high momentum (e.g. p>1 GeV/c for p’s): t1+t2=2t and x/tc x(m12 m22 ) x(m12 m22 ) t1 t2 1667 psec/meter 2 2 2cp p Timing applications Measure the Flight Time between two Scintillators (TOF) Stop Start Disc Disc TDC Particle Trajectory Literature Crismatec, “Catalogue of Scintillation Detectors”, Saint-Gobain (1992); C. D’Ambrosio et al., “Low dose-rate irradiation set-up for scintillating crystals”, NIM A, V. 388,1-2, (1997); C. D’Ambrosio et al., “A HPMT based set-up to characterize scintillating crystals”,NIM A, V. 434, 2-3, (1999); M. Moszynski, “Inorganic scintillation detectors in γ-ray spectrometry”, NIM A, V.505, 1-2, (2003); J. B. Birks, “Scintillation counters”, Pergamon Press, (1954) London; I. B. Berlmann, “Handbook of fluorescence spectra of aromatic molecules” ; 2nd ed.,Academic Press, (1971) New York H. Leutz,”Scintillating Fibres”, NIM A, V. 364, (1995) 422; RD7, DRDC Status Reports, CERN, Geneva; ATLAS Technical Design Report, CERN, (1999); C. D’Ambrosio and H. Leutz, “Hybrid photon detectors” NIM A, V.501, 2-3, (2003);