Inorganic Scintillators Yeongduk Kim Sejong University 2004 6

Inorganic Scintillators Yeongduk Kim Sejong University 2004. 6. 18

Scintillation Detectors Inorganic crystals are used for dark matter search, double beta decay, sattellite gamma measurements, accelerator calorimeters, and neutrino physics. The type of nuclei in crystal is important depending on physics.

Inorganic scintilltors – – Photons are produced by recombination of “exciton” Exciton is “bound” state of electron and hole Pure crystal And Doped crystal

Scintillation mechanism (very complex) Still no first principle solution : Refer to Birk’s & “ physical processes in inorganic scintillators”, P. A. Rodnyi Particle excite a valence band electron to produce electron and hole – Recombination to produce an exciton – Capture by trap – Independent diffusion Exciton diffuse in the lattice and – Capture by luminescence center – – excitation energy transferred to luminescence center Thermal equilibrium (part of energy go to phonons) Capture by lattice luminescence center Capture by a quenching center

Emission spectra of inorganic scintillator Pure crystal : large overlap between absorption spectra and emission spectra Doped crystal : energy of luminescence center < excitation energy Emission spectra is more “red”, so overlap is small. Self transparent.

Temperature dependence of inorganic scintillator Generally light output increases for non-doped scintillator as temperature decreases

Doping effects l Light output light output of inorganic scintillator grows as doping concentration but it saturates.

Light collection in scintillator l Produced lights(photons) should be collected efficiently L : attenuation length n : refractive index

Photosensor l Photons from scintillator are measured photosensor.

photomultiplier 3 currents flows thru PMT l “Signal” current l “Base” current l “Dark” current Q : -2000 V를 걸어준 10 dynode를 가지는 PMT가 cathode와 1 st dynode, 그리고 각 dynode사이에 같은 300 kΩ의 저항열로 되는 base를 연결했을때, PMT에서 발생 하는 열은 몇 Watt인가 ? !!! You have to check the current when you supply HV to PMT !!!

Quantum efficiency l Radiant cathode sensitivity Typically < 30%, have to match the emission lights from scintillator

Use of pmt l l l PMT entrance window is important For Cherenkov detector, the Quartz(fused silica) window preferable Liquid Xe : Ma. F 2

Example of intermediate size scintillator spectrum with 2 different energy gamma source Q : can you think over the origin of each peaks ?

Atomic effects – 광전효과는 간단하지 않다. l X-ray excape peak : when photoelectric absorption usually from K-shell electrons. x-ray emission from L shell l If this x-ray escape from the detector, Emeasured=Egamma-Ex-ray Q 3 : Can you see the escape peak with Cs-137 source(662 ke. V) ?

Where to buy scintillating crystal ? l l Most inorganic scintillators are imported. Major crystal companies 1) 2) 3) 4) 5) 6) l Amcrys-H www. amcrys-h. com Russia Crystal Gm. Bh www. crystal-gmbh. com Germany Hilger Crystals www. hilger-crystals. co. uk UK STCRI www. isc. kharkov. com/STCRI Russia Crismatec www. bicron. com France Shanghai www. siccas. com China Crystal growing method 1) 2) Chochralski : melted and uplifted as rotating Bridgmann : sealed in quarz tube

How to prepare detector l Wrapping is important : tyvek, aluminized foil, teflon 1) l Generally Teflon is the best reflector. Surface : polishing or sanding 1) Polishing 이 항상 좋은것은 아니다.

Practice with scintillators l l l Decay time measurement with digital oscilloscope(DSO) Try to utilize DSO as much as possible in crystal tests. Tests on Cs. I(Tl), GSO crystals.