Scintillators

Scintillation Detector

• Scintillation detectors are widely used to measure radiation.

• The detectors rely on the emission of photons from excited states.

– Counters – Calorimeters 1.

2.

3.

4.

5.

6.

An incident photon or particle ionizes the medium.

Ionized electrons slow down causing excitation.

Excited states immediately emit light.

Emitted photons strike a light sensitive surface.

Electrons from the surface are amplified.

A pulse of electric current is measured.

Energy Collection

• Counters need only note that some energy was collected.

• For calorimetery the goal is to convert the incident energy to a proportional amount of light.

– Losses from shower photons – Losses from fluorescence x rays

Compton Peak

• For incident photons, Compton scattering transfers energy to electrons.

• This is an important effect for photon measurement below a few MeV.

• The recoil energy:

T



h

1 

x

( 1  0

x

( 1   cos cos q q ) )

x



h

 0

m e c

2 • Has a maximum at q = 180°:

T

 2

h

 0

x

1  2

x



h

 0   0 2 

m e c

2 / 2 • For photons in keV:

T



h

   0 2 0  256

Photon Statistics

• • • Typical Problem Gamma rays at 450 keV are absorbed with 12% efficiency. Scintillator photons with average 2.8 eV produce photoelectrons 15% of the time.

What is the energy to produce a measurable photoelectron?

How does this compare to a gas detector (W-value)?

• • Answer The total energy of scintillation is 450 x 0.12 = 54 keV.

– – 5.4 x 10 4 photons produced 1.93 x 10 4 / 2.8 = 1.93 x 10 x 0.15 = 2900 photoelectrons produced 4 The equivalent W-value for the scintillator is: – – 450 keV/2900 = 155 eV/pe W-value in gas = 30 eV/ip

Inorganic Scintillators

• Fluorescence is known in many natural crystals.

– – UV light absorbed Visible light emitted • Artificial scintillators can be made from many crystals.

– Doping impurities added – Improve visible light emission

conduction band

h

 impurity excited states impurity ground state valence band

Band Structure

• Impurities in the crystal provide energy levels in the band gap.

• Charged particles excites electrons to states below the conduction band.

• Deexcitation causes photon emission.

– Crystal is transparent at photon frequency.

Jablonski Diagram

• Jablonski diagrams characterize the energy levels of the excited states.

– Vibrational transitions are low frequency – Fluoresence and phosphoresence are visible and UV • Transistions are characterized by a peak wavelength l max .

S 1 S 0 10 -15 s 10 -12 s 10 -7 s

Time Lag

• Fluorescence typically involves three steps.

– Excitation to higher energy state.

– Loss of energy through change in vibrational state – Emission of fluorescent photon.

• The time for 1/

e

of the atoms to remain excited is the characteristic time t .

Crystal Specs

• • • Common crystals are based on alkali halides – Thallium or sodium impurities Fluorite (CaF 2 ) is a natural mineral scintillator.

Bismuth germanate (BGO, Bi 4 Ge 3 O 12 ) is popular in physics detectors.

Crystal t (ns) NaI(Tl) 250 CsI(Tl) 1000 CsI 16 ZnS(Ag)110 CaF 2 (Eu) 930 BGO 300 l max (nm) output 415 100 550 315 450 435 480 45 5 130 50 20 www.detectors.saint-gobain.com

• Iarocci tubes used in tracking are arranged in layers.

• Hits in cells are fit to a track.

– Timing converted to distance from wire – Fit resolves left-right ambiguity

Tracking Detector

absorption emission

Organic Scintillators

• A number of organic compounds fluoresce when molecules are excited.

• The benchmark molecule is anthracene.

– Compounds are measured in % anthracene to compare light output R. A. Fuh 1995

• Carbon in molecules has one excited electron.

– – Ground state 1s Molecular 1s 2 2s 2 1 2s 2p 2 3 2p 2 • Hybrid p-orbitals are p -orbitals.

– Overlapping p -orbitals form bonds – Appears in double bonds

Pi-Bonds

Excited Rings

• • p -bonds are most common in aromatic carbon rings.

Excited states radiate photons in the visible and UV spectra.

– Fluorescence is the fast component – Phosphorescence is the slow component At left: π-electronic energy levels of an organic molecule. S 0 is the ground state. S 1 , S 2 , S 3 are excited singlet states. T 1 , T 2 , T 3 excited triplet states. S 00 , S 01 , S 10 , S 11 vibrational sublevels. are etc. are

Plastics

• Organic scintillators can be mixed with polystyrene to form a rigid plastic.

– – Easy to mold Cheaper than crystals • Used as slabs or fibers

Transmission Quality

• Scintillator is limited by the transmission efficiency.

– It’s not clear • The attenuation length cannot be too long for the application.

• Organic scintillators can be mixed with mineral oil to form a liquid.

– Circulate to minimize radiation damage – Fill large volume

Liquids

Waveshifter

• Photons from scintillators are not always well matched to photon detectors.

– – Peak output in UV-blue Peak detection efficiency in green light.

• Wavelength shifting fibers have dyes that can absorb UV and reemit green light.

• Fibers can be bent to direct light to detectors.