Simon Jolly
Brunel University

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
2

Applications of Caesium Iodide in High Energy Physics
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
Simon Jolly
Brunel University
3

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
Arsecockfuck
Poison
4

Applications of Caesium Iodide in High Energy Physics
Simon Jolly
Brunel University
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 e photons, the luminescent material is called a scintillator. Energy loss g e + e g in scintillators is through the usual shower e e + g deposition process.
e e g e +
Radiation
Lengths c
0
0 1 2 3 e -
5

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 Conduction
Band E
C
Exciton
Band E
E Excitation
Photon absorption
Photon emission Exciton creation
De-excitation
Exciton remains bound until de-excited
Valence
Band E
V Holes
6

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.
=
7

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)
8

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
9

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
10

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
11

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/cm 3 - PbWO
4
8 g/cm 3 ) but fine for MeV-GeV energies is just over
 Slightly hygroscopic so crystals are water tuneable
12

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.
13

Applications of Caesium Iodide in High Energy Physics
Simon Jolly
Brunel University
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 + I -
F-centre formation in
CsI(Tl)
Incident g
Scattered g e
Captured electron
Scattered
I ion
14

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
15

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.
Incident x-rays
PC running
LabVIEW
BNC
Adapter
Box
1.25 MHz
ADC Card
Crystal and holder
PMT
16

Applications of Caesium Iodide in High Energy Physics
And to conclude...
Simon Jolly
Brunel University
Caesium Iodide: not as accurate as it could be...
17