PHYS 352
Radiation Detectors II: Scintillation Detectors
Scintillation
• the physics definition of scintillation: the process by which ionization
produced by charged particles excites a material causing light to be emitted
during the de-excitation
• one of the most common detection techniques for nuclear radiation and
particles
• earliest use by Crookes in 1903 – a ZnS-coated screen scintillates when
struck by α particles
• Curran and Baker in 1944 – coated a photomultiplier tube with ZnS producing
the first scintillation counter that didn’t require the human eye
• the scintillation process differs in different materials (e.g. inorganic crystals,
organic liquids, noble gases and liquids, plastic scintillators); we’ll briefly
examine each type…
More Definitions
• when you excite a material (not thermally) and it subsequently gives off light,
that is luminescence
• how it’s excited determines the type of luminescence (e.g.
photoluminescence, chemiluminescence, triboluminescence)
• fluorescence is photoluminescence or scintillation (i.e. excitation produced by
ionizing radiation) that has a fast decay time (ns to μs)
• phosphorescence is the same, only with a much slower decay time (ms to
seconds)
Stokes Shift
• an important, general concept to keep in mind for all scintillators
• emitted photons are at longer wavelengths (smaller energies) than the energy
gap of the excitation – called the “Stokes shift”
• the processes that produce the Stokes shift are different in different
scintillating materials
• this allows the scintillation light to propagate through the material
• emitted photons can’t be self-absorbed by exciting the material again
from Wikipedia
Characteristics of Different Scintillators
• light yield: high efficiency for converting ionization energy to light output
[photons/MeV]
• emission spectrum: best if it overlaps with spectral response of light detector
(e.g. PMT or photodiode have different spectral range of peak sensitivity)
• decay time: how long it takes the excited states to de-excite and give off light
• can be different for alphas and betas
• because depends on ionization density
• density and Z: determine response to γ, e−
and other electromagnetic processes
Scintillation in Inorganic Crystals (e.g. sodium
iodide, NaI crystals)
• the scintillation mechanism requires the crystal band structure; you can’t
dissolve NaI in water or melt these crystals and get scintillation
• most are impurity activated
• luminescence centres are associated with the activator sites in the lattice
• interstitial, substitutional, excess atoms, defects
excitation
electron
hole
Doped versus Exciton Luminescence in Crystals
• for doped crystals: the decay time primarily depends on the lifetime of the
activator excited state
• examples of doped crystals: NaI(Tl), CsI(Tl), CaF2(Eu)
• in crystals with exciton luminescence: e-h pairs stay somewhat bound to
each other forming an exciton
• the exciton moves together in the crystal
• impurities or defects (w/o activator) → site for recombination
• example of exciton luminescence: BGO
(bismuth germanate Bi4Ge3O12)
BGO from Shanghai typical NaI(Tl) detector
Institute of Ceramics in Queen’s undergraduate lab
CsI(Tl) from BaBar
Roma group
Self-Activated Scintillating Crystals
• chemically pure crystal has luminescence centres (probably interstitial) due to
stoichiometric excess of one of the constituents
• example: PbWO4 and CdWO4
• extra tungstate ions are the activator centres
PbWO4 crystals for the CMS ECAL at the LHC
from Wikipedia
Comparison of Inorganic Crystals
from Particle Data
Group, Review of
Particle Detectors
CaF2(Eu) 3.18
CdWO4 7.9
940
14000
435
475
1.47
2.3
50
40
no
no
Comparison of Emission Spectra from Different
Inorganic Crystals
light yield compared to NaI(Tl) from previous table is
over the spectral response range of bi-alkali PMT
 some crystals emit at longer wavelengths and are better
matched to Si photodiode spectral response
 e.g. CsI(Tl) with a photodiode would be 145% of NaI(Tl)
Organic Scintillators
• the scintillation mechanism is determined by the chemistry and physics of the
benzene ring
• an organic scintillator will thus scintillate whether it’s in a crystal form, is a
liquid, a gas, or imbedded in a polymer
• all organic scintillators in use employ aromatic molecules (i.e. have a benzene
ring)
• chemical bonds in the benzene ring:
• σ-bonds are in the plane, bond angle 120°, from sp2 hybridization
• π-orbitals are out of the plane; they overlap and the π-electrons are
completely delocalized
benzene from Wikipedia
Scintillation in Organic
Molecules
• after absorption of a photon or excitation
by ionization, the molecule undergoes
vibrational relaxation to S10
• the excited S10 state decays radiatively
to vibrational sub-levels of the ground
state; the S10 lifetime is ~ns
• thus the fluorescence emission spectrum
is roughly a “mirror image” of the
absorption spectrum (same spacing)
• emitted photons have less energy than
S00-S10 – that’s the important Stokes
shift
excited triplet state can’t decay to ground
• no S2-S0 emission; internally de-excite in state (angular momentum selection rules)
→ results in delayed fluorescence and
picoseconds (non-radiatively)
phosphorescence
Absorption and Emission Spectra
• mirror images of each other
• individual vibrational states are thermally broadened and smear together
Types of Organic Scintillator
single crystal
anthracene
liquid
scintillator
plastic
scintillator
Comparison: Organic versus Inorganic Scintillator
•
137Cs
spectrum in NaI crystal
same in plastic scintillator
Figure 4.13: Cs-137 spectrum taken with 2m X-Plank.
• the photopeak is not seen with plastic scintillator; that’s the Compton edge
• organic scintillators: low density; just C and H; photoelectric effect goes as Z4
General Properties of Scintillation Detectors
Figure 4.14: Na-22 spectrum taken with 2m X-Plank.
• scintillator (inorganic crystal, plastic, liquid) must be optically4-16coupled to the
light detector
• very frequently a photomultiplier tube is used though photodiodes (or
avalanche photodiodes) have been used also as the light sensor
• when there is a lot of scintillation light, the high gain, high sensitivity of a
SCINTILLATION DETECTORS
PMT might not be required
x particle energy converted to visible light in a scintillating material
• acrylic light guides are sometimes
used to couple scintillator to
x light sensed by photomultiplier tube and converted into electrical
photomultiplier tube – work by
total internal reflection
pulse
SCINTILLATOR
J - ray
PHOTOMULTIPLIER
dynodes
elight
electrons
vacuum
Reflector
Glass envelope
anode
electrical
pulse
Energy Resolution of Scintillation Detectors
• Poisson statistics of the number of scintillation photons emitted per MeV
energy deposited
• NaI is about 25 eV per photon (40,000 photons/MeV)
• plastic scintillator is about 100 eV per photon (10,000 photons/MeV)
• Fano factor for scintillators F = 1
• unlike gas detectors and semiconductor detectors (we will study those
next) where F < 1 and the resolution is better than sqrt(N)
• hand-waving reason: the ionization energy and excitation energy produced
by the interactions degrade almost continuously down to the main excited
state via coupling to vibrational states (in the molecule or to phonons in
the crystal lattice) so it is quite a good approximation that there is a single
independent statistical quantity that determines the average energy
required to be deposited to get one scintillation photon emitted
When to Use Inorganic? When to use Organic?
• for spectroscopy, use inorganic (or better still semiconductor)
• NaI has high density, high Z, good photopeak
• for timing applications, use organic
• most common inorganic scintillating crystals have longer decay times than
organic scintillators which have scintillation lifetimes of ~few ns
• e.g. for cosmic ray detector, use sheets of plastic or tanks of liquid to
cover a large area cheaply (compared to many crystals) since you might
not care about energy resolution since a cosmic ray crossing your detector
deposits a known amount of energy but rather care when the cosmic ray
hit your detector
• or use time-of-flight spectrometry to determine a particles mass/
momentum/energy
• for neutron detection, having C and H is favourable hence use an organic
Scintillating Fibres
• the fibre core is
• glass with activator
PMMA first cladding
fluorinated PMMA second cladding
• plastic scintillator
• can take a bundle of scintillating fibres and observe which fibre lights up
when radiation interacts or a particle traverses the bundle
polystyrene core fibres
from Saint-Gobain Crystals
glass fibers from Pacific
Northwest National Lab
Scintillation in Noble Gases/Liquids
• scintillation mechanism is again different
• noble gases/liquids are monatomic but excited atoms can form dimers
(excited dimer or excimer)
• e.g.
Ar2*
• the excited dimer is either in a singlet or triplet state
• singlet state is fast (6 ns for argon)
• triplet state is slow (1.6 μs for argon)
• it decays by photon emission with photon energy less than what’s needed to
excite the monomer – Stokes shift
• hence, transparent to its own scintillation light
• high light yield: e.g. 40,000 photons/MeV for argon, 50,000 photons/MeV for
xenon