Inorganic Scintillators

Yeongduk KimSejong 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 crystalAnd 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

Light outputlight output of inorganicscintillator grows asdoping concentrationbut it saturates.

Light collection in scintillator

Produced lights(photons) should be collected efficientlylxeLxL /

0)( L : attenuation length

0

11sinn

nC

n : refractive index

Photosensor

Photons from scintillator are measured photosensor.

photomultiplier

3 currents flows thru PMT “Signal” current “Base” current “Dark” current

Q : -2000V 를 걸어준 10 dynode 를 가지는 PMT 가 cathode 와 1st dynode, 그리고 각 dynode 사이에 같은 300kΩ 의 저항열로 되는 base 를 연결했을때 , PMT 에서 발생하는 열은 몇 Watt 인가 ? !!! You have to check the current when you supply HV to PMT !!!

Quantum efficiency Typically < 30%, have to match the emis

sion lights from scintillatorphoton

electron

N

NQ

]/[1240

)()(

)(

)()(

WAnm

hc

e

P

IS k

Radiant cathode sensitivity

Use of pmt

PMT entrance window is important For Cherenkov detector, the Quartz(fused silica) window prefer

able Liquid Xe : MaF2

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 . X-ray excape peak :

when photoelectric absorption usually from K-shell electrons. x-ray emission from L shell

If this x-ray escape from the detector, Emeasured=Egamma-Ex-ray

Q3 : Can you see the escape peak with Cs-137 source(662 keV) ?

Where to buy scintillating crystal ?

Most inorganic scintillators are imported. Major crystal companies

1) Amcrys-H www.amcrys-h.com Russia2) Crystal GmBh www.crystal-gmbh.com Germany3) Hilger Crystals www.hilger-crystals.co.uk UK4) STCRI www.isc.kharkov.com/STCRI Russia5) Crismatec www.bicron.com France6) Shanghai www.siccas.com China

Crystal growing method 1) Chochralski : melted and uplifted as rotating2) Bridgmann : sealed in quarz tube

How to prepare detector

Wrapping is important : tyvek, aluminized foil, teflon1) Generally Teflon is the best reflector.

Surface : polishing or sanding1) Polishing 이 항상 좋은것은 아니다 .

Practice with scintillators

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