25 de dx Scintillation detectors excitation L25.pdf P627 YK 3/14/2012 detectable photons also by UV, or molecular collisions, chem. reactions, bubbles. etc. Detector building requirements (sometimes controversial): High conversion efficiency Linearity Transparency to own light collection efficiency Fast time response (pulse) Convenient material for custom applications: high/low Z, different sizes etc. Low radiation damage effect Matching with glass interface Non-organic, e.g. NaI(Tl) crystal (standard): ~ 25 ev/ e.g. BGO crystal (Bi 4 Ge 3 O 12 ) ~ 300 ev/ Organic, e.g. anthracene C 14 H 10 crystal (standard): ~ 61 ev/ Plastic e.g. based on polystyrene (or PMMA) ~ 100 ev/ Liquid Scint., Loaded Scint., etc. Classical book: J.B. Birks, The theory and practice of scintillation counting, Pergamon Press, Oxford 1964; See also PDG; G. Knoll, Chapter 8. 1
Luminescence-Phosphorescence-Fluorescence 2
t I I0e t Absorption/ emission Stokes shift ~ ns ~ ms larger Absorption / emission Stokes shift Stokes shift 3
Energy/excitation transfer Energy transfer in binary systems via molecular collisions, Forster mechanism. With sufficient number of fluor scintillator components wavelength shifting is possible 4
Energy/excitation transfer E.g. in KamLAND Liquid Scintillator: Dodecane 80% Pseudocumene 20% 1.5 g/l PPO Initial excitation Energy transferred by emission and reabsorption, and by molecular collisions Forster mechanism. Detectable PPO emission PPO excitation PPO emission KamLAND LS: Efficiency ~ 70% of anthracene; ~ 100 ev per detectable ; Emission components: ~60% with ~ few nsec ~30% with ~ 20 nsec ~10% with longer ~375 nm Scintillator emission spectrum 5
In scintillation detectors all kinds of de-excitations are possible leading to quenching Birks law: dl dx L 0 1 de k B dx de dx ; L light output kb Birks coefficient L 0 constant 6
(Visible Energy)/(Real Energy) Stopping Power, MeV/g/cm2 Birks k B in KamLAND scintillator 3000 2500 2000 SRIM: in KamLAND LS with H/C=1.969 and =0.78 g/cm 3 1500 1000 500 electronic nuclear 0 0.001 0.01 0.1 1 10 100 Energy, MeV 0.09 kb=0.006 0.0075 0.0078 0.008 0.08 0.07 0.06 0.05 0.04 0 1 2 3 4 5 6 7 8 9 10 Real Energy, MeV L 0 dl = ò ( kb ) dx E a de de dx 7
Measured -response in KamLAND is non-linear: Two sources of non-linearity: (a) Birks quenching (same k B for all particles e,, p, (?) (b) Emission of Cherenkov radiation that can be detected directly in the visible range or from UV range in a process of absorption - reemission Lvis = L int ( k ) + R * L E g real Sc B Cher can be calculated if n( ) is known 8
Properties of organic/plastic scintillators (from Knoll) 9
Timing properties of some fast plastic scintillators -t t -t t1 ( ) I = I e -e 0 1 -t t 0 ; t-decay, t -level population (8.10) I = I f( t) e (8.11) De-excitation timing can be different for different primary particles pulse-shape discrimination (PSD) methods used e.g. neutron/gamma discrimination Problem: propose the scheme of measurement and the procedure that would separate the neutrons and gammas by PSD in the scintillation detector 10
Light transport, total internal reflection, gluing, wrapping Due to Liouville's theorem, the total area of the cross-section along a light guide cannot be reduced without light losses. For changes in direction a maximum bending (minimal bending radius) should be chosen according to the relation ( ) 2 n 2-1 ³ d 2 r + 1 where: d diameter of fiber (or light guide), r bending radius, n relative refractive index. With a radius chosen according to the relation given above, all light entering the plane front surface of a light guide is transported due to total reflection. Superior reflector wrap: ESR (Enhanced Specular Reflector) VM2000 film by 3M [St. Paul, MN] http://www.osti.gov/bridge/purl.cover.jsp?purl=/957053-3bkjbw/957053.pdf Twisted guides PMT 11
in experiment D0 at Fermilab 12
Scintillating and wavelength shifting fibers (SF and WLSF) 13
WLS fibers use in NOvA experiment at Fermilab 14
Inorganic scintillators produced by industry 15
Inorganic scintillators (crystals) Scintillation mechanism is different since there is no big molecules with vibrational degrees of freedom. Crystals are dielectrics or insulators with quite large gap between valence and conducting bands. Usually large excitations are involved, e.g. 7 ev that corresponds to 170 nm UV light. NaI (crystal) is an excellent scintillator; but water solution of NaI is not a scintillator. 16
Inorganic Scintillation Properties: http://scintillator.lbl.gov/ 17
Properties of inorganic scintillators (from Knoll) 18
PDG 19
Emission time of NaI(Tl) and BGO 20
NaI(Tl) detector performance 21
Temperature dependence of light yield for some inorganic crystals Noble gas scintillators Light emission of noble gases can be enhanced in the presence of electric filed! 22