inorganic scintillators - university of...

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Inorganic ScintillatorsInorganic scintillators are inorganic materials (usually crystals) that emit light in response to ionizing radiation

NaI is the protypical exampleScintillation mechanism is different than for organic scintillatorsInorganic scintillators have higher Z and higher density (4-8 g/cm3 versus ~1 g/cm3) than organic scintillators

Higher Z and density translates into higher photon conversion efficiency and stopping power

Uses include calorimetry in particle physics, gamma ray spectroscopy, and medical/biological imaging

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Inorganic ScintillatorsThe physical processes leading to scintillation in inorganic materials are complex and are dependent on the specific scintillatorA good picture to start with is that there is a core valence band and a conduction bandGeneral steps to scintillation are

Initial electron-hole production and secondary production from the initial excitation energyThermalizationTransport and localization of electrons, holes, and excitonsExcitation and de-excitation of the luminescent centers

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Inorganic Scintillators

Mechanism of luminescence in some inorganic scintillators

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Excitation/ionizationCauses the creation of (hot) electrons in the conduction band and (deep) holes in the inner core band

RelaxationOn a very short time scale (~1 fs), a large number of secondary electronic excitations occur

Radiative decay (secondary x-rays), Auger electrons, and inelastic electron-electron scattering

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ThermalizationElectrons and holes thermalize by making intraband transitions and by phonon production Electrons end up at the bottom of the conduction band and holes end up at the top of the valence bandOccurs on ~ 1 ps time scale

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Inorganic ScintillatorsTransport/localization

Occurs on ~1-500 ns time scaleElectrons and/or holes migrate through the material and become trapped by impurities or activator ions, sometimes repeatedly, sometimes sequentiallyCoulomb attraction can cause electrons and holes to form excitons or self-trapped excitons (STE) Holes can become self-trapped (between two anions – called VK centers)

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Inorganic ScintillatorsLuminescence

During the localization stage, luminescent centers can be excited by various mechanisms and subsequently de-excite by the emission of scintillation lightTransport and forbidden transitions can be relatively slow

Two broad types of materialsIntrinsic or self-activated

Luminescence is produced by part of the crystal structure itself

External or activatedMust add some impurity to create energy levels between valence and conduction band

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Inorganic ScintillatorsSome examples

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yield quantumt luminescen theis efficiency transfer theis

efficiency conversion hole-electron theis

3.210output Light

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QS

QSEE

nL

g

photons

β

βγ

⋅⋅==

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Inorganic ScintillatorsScintillation efficiency for 1 MeVphotons

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Inorganic ScintillatorsNaI(Tl)

High light output and widely used in gamma spectroscopy

BaF2Fastest known inorganic scintillator

BGO (Bi4Ge3O12)Used in x-ray tomography and PET

LSO (Lu2O3-SiO2(Ce))Used in PET

PbWO4High density and radiation hard but low light yieldUsed in CMS EM calorimeter

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NaI(Tl)

Discovered by Hofstadter in 1948 but still a standard in gamma ray spectroscopy today

Activator is thallium (Tl) at 10-3 mole fraction+ Excellent light yield+ Relatively small non-linearity in energy response- Hydroscopic (must be sealed)- Damage from mechanical or thermal shock- Slow (τ ~ 230 ns (90%) and 0.15 s (10%))

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NaI(Tl)Tl+ is a well-known luminescent center because of its 5d106s2 configurationAlso the hole mobility is very small which increases the rise time of the luminescenceThe excited states are P states which means the luminescence is spin-forbidden i.e. slow decayBecause fluorescence occurs through the activator sites in the forbidden band, NaI will be transparent to scintillation lightVery efficient transfer to activator sites results in high light output

38k photons per MeV of energy deposited

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NaI(Tl)The band structure for NaI (Tl) looks something like

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NaI(Tl)In NaI, here are some of the trapping mechansims

And here are SOME of the recombination mechanisms

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NaI(Tl)

Light output is well-matched to a PMT

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CsI(Tl)

CsI(Tl) needles used in digital radiography

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BaF2

An example of intrinsic emission

The very fast transitions in BaF2 and CsF are due to an intermediate transition between the valence and core bandsActually there are two components of light: one with τ ~ 0.6 ns and one with τ ~ 630 ns

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BGO

Bismuth germanate (Bi4Ge3O12)+ High density (7.13g/cm3) and high Z (83) result in high probability for photoelectric absorption+ Rugged and not hydroscopic+ No afterglow (phosphorescence)- τ ~ 300 ns (90%) and 60 ns (10%)- Lower light yield (about 10-20% of NaI)

Finds widespread application in PET and CT scanners

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BGO

Another example of intrinsic emissionIn this case the luminescence center is one of the constituents of the crystal Ionization of Bi results in a hole in the 6s2

level and an excited electron in the 6s6p level of Bi3+

These self-trapped excitons give photons on recombination

BGO emission is well-matched to sensitivity of photodiodes

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LSOLutetium Oxyorthosilicate (Lu2O3-SiO2(Ce))

+Good light output (~75% of NaI)+Relatively fast (τ~47ns)+High density (7.4g/cm3)+Easily grown-Contains 176Lu which is radioactive!-Nonlinear response somewhat degrades energy resolution

Finds application in PET scanners

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Light output is strongly dependant on temperature

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Inorganic ScintillatorsProperties

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Properties

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Scintillator Comparison

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Inorganic scintillators have found wide application in HEP as calorimeters as they provide excellent energy resolution

Crystal Ball – NaIL3, CLEO, KTeV, BaBar, BELLE – CsICMS, ALICE - PWO

R&D on inorganic scintillators has been spurred in part by HEP

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Inorganic ScintillatorsStill an active area of R&D

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Gamma CameraThese images are made using gamma cameras

We will cover the details of these (and similar detectors) in upcoming lectures

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Gamma CameraA schematic of a standard gamma camera

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CMS EM Calorimeter

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CMS EM Calorimeter

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CMS EM Calorimeter

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CMS EM Calorimeter80,000 PWO crystals

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CMS EM Calorimeter

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Standard ModelSummary

HiggsMechanism

MassiveGaugeBosons

LocalGauge

Invariance

MassiveHiggs Boson

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Higgs Decay

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Higgs Decay

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CMS EM Calorimeter

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CMS EM Calorimeter

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CMS EM Calorimeter

PWO is relatively radiation hard for HEP

inorganic scintillators - university of...

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