Scintillatori Organici

In questi materiali, non cristallini, esiste una struttura di livelli atomici vibrazionali, ed elettronici(π electronic structure).

Sono proprietà intrinseche alla molecola, non dipendono dal suo stato fisico (solido, liquido o vapore)

A temperatura ambiente le molecole sono nello stato S00 (infatti T = 0.025 << 0.15)

Il processo di eccitazione/ionizzazione indotto dalla radiazione porta gli elettroni sui livelli S1x, S2x, S3x, …

Gli stati elettronici/vibrazionali decadono velocemente (~ ps) nello stato S10 via transizioni non radiative.

La radiazione pronta (prompt fluorescence) viene emessa nella transizione tra lo stato S10 e gli stati S0X con una legge di tipo esponenziale

La radiazione ritardata (delayed phosphorescence) viene emessa dopo una transizione intra-bande che porte gli elettroni nello stato di tripletto (~ ms)

Stati Vibrazionali (~ 0.15 eV)Stati Elettronici (~ 3-4 eV)10

/0 SstatolifetimeeII t == − ττ

Delayed

slow

Prompt

ns

RESUME

NO

radiation

Crossing back Triplett singlet: T0+T0 S*+So+phonons S*+ photons (delayed)

ev

10ev

rad. damage

Spin flip forbidden

Inorganic Scintillator• Ionisation excites electron conduction band

de-excitation via photoemisson or radiationless transition

• Large bandgap => slow process

• Impurities add centres (traps) in band gapIncrease transition rateSuitable doping can also increaselight output ( ex: CsI(tl), CsI(Na)Hols trapped at activate site ->recombination with e- emits photon

Example of LaBr3(Ce3+)

“Classical” Scintillation Mechanism – LaBr3(Ce3+)

• Ionic Bonding / Transitions Dominate• Transfer of Excitation from Host Ions to Activator

Position

Energy

Lu Lu Ce Lu Lu

electron

hole Position

Energy

Lu Lu Ce Lu Lu

electron

hole Position

Energy

Lu Lu Ce Lu Lu

electron

hole Position

Energy

Lu Lu Ce Lu Lu

electron

holePosition

Energy

Lu Lu Ce Lu Lu

electron

hole

Scintillation Photon

Energy

Ce 5d

Ce 4fLattice Valence Band

Lattice Conduction Band

Band Gap

• Small band gap• Ce 4f-5d levels in band gap, close to lattice energy• Good lattice transport & lattice → Ce transport• THERE and onlt THERE Transition is spin-parity allowed (decay lifetime is

short, quenching reduced).• Atomic diameter similar to heavy metal ions (“fits” into lattices of dense host

compounds).• Not radioactive (no background signal).

Position

Energy

Conduction Band (Empty

electron

holeValence Band (Filled)

Impurity Band (Part Full

Position

Energy

Conduction Band

electron

holeValence Band

Position

Energy

Conduction Band

electron

holeValence Band

Position

Energy

Conduction Band

holeValence Band

Scintillation Photon

In Inorganic Scintillators

Effetto del drogaggio

Drogare un cristallo scintillatore organicoSignifica rendere differente lo spettro della luce emessa dallo spettro della luce assorbita.

La conseguenza più diretta del drogaggio quindi è la riduzione dell’autoassorbimento

Lo shift tra la luce di emissione e quella di assorbimento si chiama ‘Stokes Shift’

Scintillatori Inorganici

Diversamente dal caso degli scintillatori organici l’emissione di luce non è monocromatica

Anche per gli scintillatori inorganici sono presenti fenomeni di quenching e di dipendenza dal valore di dE/dx dell’emissione di luce

LaBr3

Scintillatori Inorganici

La ricerca di un ottimale scintillatore e ancora in corso !

Scintillator Detectors in general are very fast detectors !

Ricorda:Temperature dependance of scintillation.

Organic scintillators are not very much sensitive totemperature (in -60°C till +60°C degree--- light output only varies 5%)The π-electrons that built the singulett and triplett statesand the following singulett-singulett (faster)and triplett (triplett-to singulett) transmissions (slower)Are nearly independant to temperature

The anorganic Scintillators with their bandstructure(cristal NOT semiconductor !) show VERY strong temperatur dependance(semiconductor have to be cooled down )

Light output-response

Schema riasuntivo del processo di rivelazione

RESUME Scintillator

• Light emitted from ionisation in transport material( dE/dxloss)a) I(t) = I0e-t/τ ; τ : decay time, several component possibleb) τ>μs -> count rate<< MHz,

τ ∼ns -> count rate>> MHz,c) Wavelength of emission determine photosensor

• Two main material types of scintillatorsa) Inorganic scintillatorb) Organic scintillator

RESUME Scintillator charasteristics• Inorganic scintillators

a) Result of crystalline structureb) Large band gap, insulators c) High light output but relatively slow (~μsec)

d) Rather expensive, moderate size (~kg)

• Organic scintillatorsa) Molecular property of hidrocarbonsb) Moderate light output but fast (~ns)c) Cheap, large size (~ton)d) Liquid scintillator (LSC)

Plastic scintillator (PSC)Crystal (ex:anthracene, stibulene)

“Detection of scintillation lightthrough photo-electrons

amplification and detections of electrons”

Scintillator light output readout

(optical grease)

(need very very good)

Primary

Small Angles: photons escape

Light transmission

• Total reflectionsin θ> next/n, sin θ> 1/n, next = air

• Light guide• Optical grease (optical cement) • Light reflector

Teflon, Al, white paint

ScintillatorHas also toBe put in externalREFLECTOR (Teflon, Alu..)to Improve lightcollection

Lightguidemay havevery differentgeometry and form,should garanty goodTiming and low loss

TEFLON

Strong effect, all must betaken

PMT externalreflector

scintillator optical coupling

dynodes

photo cathode

electron multiplication

Nη

αδ1

δ2

δ3 δnNel

ideal case: Nel = α η N

Light detection

Photomultiplier Tube

Photomultiplier Tube (PMT)

PMT types

Venetian blind (old)

Box-and-grid

Focused linear structure

Circular grid

Gains - 107

PMTS

Detection principle and surrounding

Calculated with computer simulations(analytically and monte-carlo)

Einstein Photoeffect formula:Ekinetic_of emitted_electron=hν – Eworkfunction

Above threshold theEfficiency is not unity !Efficiency varies with frequencyOf light !

Quantum efficiency n(λ)=Number_of_photoelectrons_generated/Number_of_incident_photons_on_cathode(λ)

Typical “Photoresponse” of differnt PMT photocatode material

H2O

Lots of different Materials on the market, but everytimecompromise between, resolution, efficiency, timing, cost

NuclearStructure group,Milano

HECTOR@RISING NuclearStructure group,Milano

Fast versus Slow component helps to disentangle differnt particles: gamma from LCPDiagonal: gamma rays; below diagonal, LCP (alpha, proton) induced signalswith cuts to select gamma-rays coming from target.

Riccorda PMT:•Time response should be independent from the

Photocatode interaction position !

•Prevent scintillators from ambient light !This will possibly ddestroy the Photomultiplier-tube !

•In order to eliminate magnetic field influence on thePMT performance the whole PMT is embedded in mu-metal*.

(The shield should NOT be deformed !)

*mu-metal (VAC Vacuumschmelze, Hanau) lega magnetica dotata di alta permeabilità magnetica o µ (da cui il nome), costituita da:

* 80% Ni* 14.48% Fe* 5% Mo* 0.5% Si* 0.02% Cu