1 scintillators one of the most widely used particle detection techniques ionization ->...
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ScintillatorsOne of the most widely used particle
detection techniques Ionization -> Excitation -> Photons ->
Electronic conversion -> AmplificationVariety of uses in EPP
Calorimetry Tracking detectors Time-of-flight measurements Trigger and veto counters
And other fields Medical imaging detectors (SPECT, PET, CT,
…) Gamma ray spectroscopy Homeland security
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Scintillators
Two types Organic
Crystal, liquid, plastic (most widely used in particle physics)
Lower light output but faster Inorganic
Crystal, glass Higher light output but slower
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Organic ScintillatorsIn general,
+Fast (ns or better time resolution) +Relatively large signal (using PMT or
SSPM ) +Simple, machinable, robust +Variety of shapes +Pulse shape discrimination between
neutrons and photons (NE213) -Poorer position and energy resolution
than other detector types -Sensitive to neutrons
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Organic ScintillatorsNotes
Some organic substances, such as those containing aromatic rings, release a small fraction of excitation energy as photons Polystyrene (PS) or polyvinyltoluene (PVT)
With the addition of a fluor to the base plastic (PS or PVT), the Forster mechanism (FRET) becomes the predominant mode of energy transfer
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Organic ScintillatorsNotes
The Forster mechanism (FRET) is a non-radiative transfer of energy between two molecules over long distances (10-100 A)
It arises because of an interaction between the electric fields of the dipole moments of donor and acceptor atoms
FRET has a number of applications including photosynthesis and DNA sequencing
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Organic ScintillatorsNotes
Base solvent is usually PVT or PS (something with aromatic rings)
The base can produce UV photons itself however the addition of a primary fluor (1% by weight) provides an additional mode of energy transfer from base to fluor Shorter decay time (2 to 20 ns) More light
The primary fluor often does not have good emission wavelength or attenuation length characteristics so a second fluor is added (at a fraction of percent by weight) as a wavelength shifter
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Organic ScintillatorsLuminescence
Radiation emitted by an atom or molecule after energy absorption
Fluorescence Radiation emitted from the lowest singlet
vibrational level of an excited state Generally true that a molecule will undergo
internal conversion to the lowest vibrational level of its lowest excited state, regardless of the initial excited singlet state
~ 10-7 – 10-9 sPhosphorescence
Radiation emitted from the lowest triplet vibrational level of an excited state, after intersystem crossing
~ 10-4 – 10 s
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Organic ScintillatorsCrystals
Not used much but anthracene (C14H10) has the highest scintillation efficiency (light output / energy deposited) of all organic scintillators
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Organic ScintillatorsLiquids
Base is usually toluene, xylene, benzene Typical concentration of primary fluor (e.g.
PBD) is 3g of solute/liter of solvent +Arbitrary shapes +Radiation resistant +Can be loaded with B, Li or Pb, Sn for n or
gamma detection +Pulse height discrimination -Toxic -Messy -Impurities can render useless
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Organic ScintillatorsPlastic
Solvent is usually PVT or PS Typical concentration of first fluor is 10g of
solute / l of solvent +Fast +Relatively inexpensive +Easily machined or extruded into fibers +Can be loaded -Ages or crazes with time -Subject to radiation damage -Attenuation length (1-3m) can be a problem
for large counters -No pulse height discrimination
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Rules of ThumbFor plastic scintillators
Density is about 1 g/cm3
Photon yield is about 1 photon / 100 eV of energy deposited Thus a 1 cm thick scintillator traversed by
a mip (e.g. muon) yields about 2 x 104 photons
Collection and transport efficiency will reduce the yield
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Birk’s LawPlastic scintillators do not respond
linearly to ionization density Both in light output and decay time
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Pulse Shape Discrimination In most scintillators, fluorescence is
dominated by one time constant (tf ~ 1 ns)
However some scintillators (e.g. NE213) have a substantial slower time component as well (ts~100 ns)
The fraction of light that appears in the slow component often depends on particle type (dE/dx loss rate) In NE213 there are more long-lived T1
excitations for neutrons compared to photons
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