Scintillators - Northern Illinois University

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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 lightsensitive 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 xrays
Compton Peak
• For incident photons, Compton
scattering transfers energy to
electrons.
• The recoil energy:
• This is an important effect for
photon measurement below a
few MeV.
• Has a maximum at q = 180°:
T
h 0 x(1  cosq )
1  x(1  cosq )
x

2h 0 x
h 0 
T

1  2 x h 0  mec 2 / 2
2
• For photons in keV:
T
h 0 2
h 0  256
h 0
me c 2
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
conduction band
h
impurity excited states
impurity ground state
valence band
• 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 lmax.
Time Lag
10-12 s
S1
10-15 s
S0
10-7 s
• 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 (CaF2) is a natural
mineral scintillator.
• Bismuth germanate (BGO,
Bi4Ge3O12) is popular in
physics detectors.
Crystal t (ns)
NaI(Tl) 250
CsI(Tl) 1000
CsI
16
ZnS(Ag)110
CaF2(Eu) 930
BGO 300
lmax(nm) output
415
100
550
45
315
5
450
130
435
50
480
20
www.detectors.saint-gobain.com
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
absorption
emission
• 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
Pi-Bonds
• Carbon in molecules has one
excited electron.
– Ground state 1s22s22p2
– Molecular 1s22s12p3
• Hybrid p-orbitals are p-orbitals.
– Overlapping p-orbitals form
bonds
– Appears in double 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. 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.

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