Applications of Caesium Iodide in High Energy Physics

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Applications of Caesium Iodide in High Energy
Physics
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Applications of Caesium Iodidein High Energy
Physics
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 Iodidein High Energy
Physics
What is Poison?

• Piss-poor US rock/metal
• Big hair (4 counts)
• Still living in the 80s
• Leather and Denim fetishists

We proved in tests music has same artistic
value as a Polo mint
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Applications of Caesium Iodidein High Energy
Physics
Poisons artistic merit
Jimi Hendrix
Comfortably mediocre
Artistic Merit (log scale)
Krankies
Nowt
Arsecockfuck
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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 excitation of the crystal is
by charged particles or photons, the luminescent
material is called a scintillator. Energy loss
in scintillators is through the usual shower
deposition process.
g
e-
e-
g
e
g
e
g
e-
e-
e-
e
g
Radiation Lengths c0
e-
3
2
1
0
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Applications of Caesium Iodide in High Energy
Physics
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
Conduction Band EC
Exciton Band EE
Excitation
Photon emission
Photon absorption
Exciton creation
Exciton remains bound until de-excited
De-excitation
Valence Band EV
Holes
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Applications of Caesium Iodide in High Energy
Physics
Caesium Iodide - the John Shaft of scintillators
Not only is CsI(Tl) the black private dick thats
a sex machine to all the chicks, it is also the
cat that wont cop out when theres danger all
about.

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Applications of Caesium Iodide in High Energy
Physics
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
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
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
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 freaks luggage
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Applications of Caesium Iodide in High Energy
Physics
Whats so pimp about CsI(Tl)?

• High light output and efficiency
• Light output peak at 565nm (good for PMTs)
• 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
Why CsI(Tl) doesnt 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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Applications of Caesium Iodide in High Energy
Physics
Radiation Damage whats 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.
I-
Cs
F-centre formation in CsI(Tl)
Scattered g
Incident g
e
Captured electron
Scattered I- ion
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Applications of Caesium Iodide in High Energy
Physics
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
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Applications of Caesium Iodide in High Energy
Physics
Understanding Afterglow
System currently under construction for measuring
afterglow in 1-inch CsI(Tl) plugs.
PC running LabVIEW
Incident x-rays
1.25 MHz ADC Card
BNC Adapter Box
Crystal and holder
PMT
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Applications of Caesium Iodide in High Energy
Physics
And to conclude...
Caesium Iodide not as accurate as it could be...