Transcript phys586-lec03

Scintillators
 One of the most widely used particle
detection techniques
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Ionization -> Excitation -> Photons -> Electronic
conversion -> Amplification
 Variety of uses in EPP
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Calorimetry
Tracking detectors
Time-of-flight measurements
Trigger and veto counters
 And other fields
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Medical imaging detectors (SPECT, PET, CT, …)
Gamma ray spectroscopy
Homeland security
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Scintillators
Two types
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Organic
 Crystal, liquid, plastic (most widely used in
particle physics)
 Lower light output but faster
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Inorganic
 Crystal, glass
 Higher light output but slower
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Organic Scintillators
In general,
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+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 Scintillators
Organic scintillators produce light by
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Organic Scintillators
Notes
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Some organic substances, such as those
containing aromatic rings, release a small
fraction of excitation energy as photons
 Polystyrene (PS) or polyvinyltoluene (PVT)
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With the addition of a fluor to the base
plastic (PS or PVT), the Forster mechanism
(FRET) becomes the predominant mode of
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energy transfer
Organic Scintillators
Notes
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The Forster mechanism (FRET) is a nonradiative 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 Scintillators
Notes
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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 Scintillators
Organic scintillators produce light by
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Organic Scintillators
 Luminescence
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Radiation emitted by an atom or molecule after
energy absorption
 Fluorescence
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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
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t ~ 10-7 – 10-9 s
 Phosphorescence
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Radiation emitted from the lowest triplet
vibrational level of an excited state, after
intersystem crossing
t ~ 10-4 – 10 s
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Organic Scintillators
Energy levels for organic scintillators
look like
Solvent
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Scintillators
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Organic Scintillators
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Organic Scintillators
Crystals
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Not used much but anthracene (C14H10)
has the highest scintillation efficiency (light
output / energy deposited) of all organic
scintillators
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Organic Scintillators
 Liquids
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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 Scintillators
 Plastic
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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 Thumb
For plastic scintillators
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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 Law
 Plastic scintillators do not respond linearly to
ionization density
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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 Law
 kB 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) I  Ae t / t f  Be t /t s
 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 T  T  S  S  phonons
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Pulse Shape Discrimination
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Pulse Shape Discrimination
ADC value
with long
digitizing
gate
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ADC (short)/ADC (long)
DZero Pixel Counters
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DZero Pixel Counters
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Homeland Security
Neutron
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Homeland Security
Comparison of performance and cost of
a few gamma ray detectors
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