phys586-lec03
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Transcript phys586-lec03
Scintillators
One of the most widely used particle
detection techniques
Ionization -> Excitation -> Photons -> Electronic
conversion -> Amplification
Variety 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 Scintillators
In 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 Scintillators
Organic scintillators produce light by
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Organic Scintillators
Notes
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
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energy transfer
Organic Scintillators
Notes
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
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
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
t ~ 10-7 – 10-9 s
Phosphorescence
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
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
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
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
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
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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1
1
0
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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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