Transcript Applications of Caesium Iodide in High Energy Physics

Simon Jolly
Brunel University
Applications of Caesium
Iodide in High Energy Physics
Applications of Caesium Iodide
in High Energy Physics
Simon Jolly
Brunel University
What is Caesium Iodide?
 Alkali Halide Scintillator
 Grown from crystal seed
 Fantastic light output
 Properties enhanced by dopants
(Thallium or Sodium) - CsI(Tl)
Unfortunately CsI(Tl) is...
 Prone to radiation damage
 A Poison
A Caesium Iodide crystal
from the BaBar calorimeter
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Applications of Caesium Iodide
in High Energy Physics
Simon Jolly
Brunel University
What is Poison?
 Piss-poor US rock/metal
 Big hair (4 counts)
 Still living in the 80’s
 Leather and Denim
fetishists
We proved in tests…
…music has same artistic
value as a Polo mint
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Applications of Caesium Iodide
in High Energy Physics
Simon Jolly
Brunel University
Poison’s artistic merit
Poison vs. Polo Mint
Jimi Hendrix
Comfortably
mediocre
Artistic Merit
(log scale)
Krankies
Nowt
Polo Mint
Poison
Arsecockfuck
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Simon Jolly
Brunel University
Applications of Caesium Iodide
in High Energy Physics
What is a scintillator?
All solids give off energy as thermal radiation, normally in the infrared range.
Some solids have the special capability of emitting ultra-violet and visible
radiation - when the
g
excitation of the crystal is
by charged particles or
eephotons, the luminescent
g
material is called a
+
e
g
scintillator. Energy loss
e+
in scintillators is through
g
ethe usual shower
deposition process.
eee+
g
Radiation
Lengths c0
0
1
2
3
e5
Applications of Caesium Iodide
in High Energy Physics
Simon Jolly
Brunel University
The process of scintillation
Energy absorption from shower is by sensitiser. Photon emission is
by activator. In some scintillators, a dopant is added to the lattice as
a second activator to hinder re-absorption of photons - this is the role
of the Thallium in CsI(Tl).
Electrons
Excitation
Conduction
Band EC
Photon
absorption
Photon
emission
De-excitation
Holes
Exciton
Band EE
Exciton
creation
Exciton remains
bound until
de-excited
Valence
Band EV
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Applications of Caesium Iodide
in High Energy Physics
Simon Jolly
Brunel University
Caesium Iodide - the John Shaft of scintillators
Not only is CsI(Tl)
the black private
dick that’s a sex
machine to all the
chicks, it is also the
cat that won’t cop
out when there’s
danger all about.
=
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Applications of Caesium Iodide
in High Energy Physics
Simon Jolly
Brunel University
Applications of CsI(Tl)
3 main areas that utilise CsI(Tl) scintillator:
 Electromagnetic Calorimetry
 Medical: PET and CAT scanners
 Industrial applications: x-ray scanners (scintillator on
CCD, PMT or pin-diode)
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Applications of Caesium Iodide
in High Energy Physics
Simon Jolly
Brunel University
CsI(Tl) for Calorimetry
CsI(Tl) very good for calorimetry at BaBar energies. CsI(Tl) is also used
for the electromagnetic calorimeter of the Belle experiment in Japan also looking at B-physics.
The BaBar
Detector
Cross-section of
BaBar Calorimeter
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Applications of Caesium Iodide
in High Energy Physics
Simon Jolly
Brunel University
Medical Applications
2 main types: Positron Emission Tomography (PET) and Computed Axial
Tomography (CAT). PET scans use positron emission from radioactive
isotopes; CAT scans rely on direct x-ray detection.
PET image
CAT image
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Applications of Caesium Iodide
in High Energy Physics
Simon Jolly
Brunel University
Commercial Uses of CsI(Tl)
Primary commercial use of scintillators is x-ray scanners, such as those
used in airport baggage scanners e.g. Hilger Crystals produce CsI(Tl)
arrays for Rapiscan baggage scanners.
CsI(Tl) strip
mounted on a
pin-diode array
Some freak’s luggage
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Applications of Caesium Iodide
in High Energy Physics
Simon Jolly
Brunel University
What’s so pimp about CsI(Tl)?
 High light output and efficiency
 Light output peak at 565nm (good for PMT’s)
 Light output enhanced by second activator
(Thallium) - less re-absorption
 Moderately dense (4.5 g/cm3 - PbWO4 is just over
8 g/cm3) but fine for MeV-GeV energies
 Slightly hygroscopic so crystals are water tuneable
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Applications of Caesium Iodide
in High Energy Physics
Simon Jolly
Brunel University
Why CsI(Tl) doesn’t ask for trouble,
but ends up getting glassed
2 main phenomena which affect performance of CsI(Tl):
 Radiation Damage
 Afterglow
Both phenomena are affected by doping concentration
and crystal growing conditions; both hinder performance
of crystal.
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Simon Jolly
Brunel University
Applications of Caesium Iodide
in High Energy Physics
Radiation Damage: what’s the beef?
Continuous exposure to ionising radiation leads to the formation of colour
centres (F-centres) - these colour centres absorb light in the blue/green,
hence the reddish-brown hue of damaged/old crystals.
Cs+
F-centre
formation in
CsI(Tl)
I-
Scattered g
Incident g
e
Captured
electron
Scattered
I- ion
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Applications of Caesium Iodide
in High Energy Physics
Simon Jolly
Brunel University
Afterglow in CsI(Tl)
Afterglow in CsI(Tl) is not properly understood. Short term (ms) afterglow is
important: high afterglow leads to smearing of measurements and degrades
energy resolution.
Light output
from a CsI(Tl)
crystal with
significant
afterglow
100ms
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Applications of Caesium Iodide
in High Energy Physics
Simon Jolly
Brunel University
Understanding Afterglow
System currently under construction for measuring
afterglow in 1-inch CsI(Tl) plugs.
PC running
LabVIEW
Incident
x-rays
BNC
Adapter
Box
Crystal
and holder
1.25 MHz
ADC Card
PMT
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Applications of Caesium Iodide
in High Energy Physics
Simon Jolly
Brunel University
And to conclude...
Caesium Iodide:
not as accurate as
it could be...
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