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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 ScintillatorsOrganic scintillators produce light

by

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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 ScintillatorsOrganic scintillators produce light

by

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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 ScintillatorsEnergy levels for organic

scintillators look like

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Solvent

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Scintillators

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1313

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Organic Scintillators

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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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Range

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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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Birk’s Law

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Birk’s Law

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Birk’s LawkB values

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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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phononsSSTT 0111

sf ttt BeAeI //

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Pulse Shape Discrimination

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Pulse Shape Discrimination

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ADC value with long digitizing gate

ADC (short)/ADC (long)

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DZero Pixel Counters

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DZero Pixel Counters

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Homeland SecurityNeutron

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Homeland Security

Comparison of performance and cost of a few gamma ray detectors