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. An incident photon or particle ionizes the medium.

2. Ionized electrons slow down causing excitation.

3. Excited states immediately emit light.

4. Emitted photons strike a light-sensitive surface.

5. Electrons from the surface are amplified.

6. 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:

• Has a maximum at = 180°:

• For photons in keV:

)cos1(1)cos1(0

xxhT 2

0

cmhx

e

2/21

22

0

200

cmhh

xxhT

e

2560

20

hhT

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 104 / 2.8 = 1.93 x 104

photons produced– 1.93 x 104 x 0.15 = 2900

photoelectrons produced• 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

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.

conduction band

valence band

himpurity excited states

impurity ground state

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 max.

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 .

10-15 s

S1

S0

10-12 s

10-7 s

Crystal Specs

www.detectors.saint-gobain.com

• Common crystals are based on alkali halides– Thallium or sodium

impurities• Fluorite (CaF2) is a natural

mineral scintillator.• Bismuth germanate (BGO,

Bi4Ge3O12) is popular in physics detectors.

Crystal (ns) max(nm) output

NaI(Tl) 250 415 100CsI(Tl) 1000 550 45CsI 16 315 5ZnS(Ag) 110 450 130CaF2(Eu) 930 435 50

BGO 300 480 20

Tracking Detector

• 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

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

absorption

emission

R. A. Fuh 1995

Pi-Bonds

• Carbon in molecules has one excited electron.– Ground state 1s22s22p2

– Molecular 1s22s12p3

• Hybrid p-orbitals are -orbitals.– Overlapping -orbitals

form bonds– Appears in double bonds

Excited Rings

• -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. S0 is the ground state. S1, S2, S3 are excited singlet states. T1, T2, T3 are excited triplet states. S00, S01, S10, S11 etc. are vibrational sublevels.

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.

Liquids

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

radiation damage– Fill large volume

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.