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Scintillation detectors
hotonsectable pdetexcitationdx
dE
Detector building requirements (sometimes controversial):
High conversion efficiency Linearity Transparency to own light collection efficiency Fast time response (pulse) Convenient material for custom applications:
high/low Z, different sizes etc. Low radiation damage effect Matching with glass interface
Non-organic, e.g. NaI(Tl) crystal (standard): ~ 25 eV/e.g. BGO crystal (Bi4Ge3O12) ~ 300 eV/
Organic, e.g. anthracene C14H10 crystal (standard): ~ 61 eV/Plastic e.g. based on polystyrene (or PMMA) ~ 100 eV/Liquid Scint., Loaded Scint., etc.
also by UV, or molecular collisions, chem. reactions, bubbles. etc.
Classical book: J.B. Birks, The theory and practice of scintillation counting, Pergamon Press, Oxford 1964;See also PDG; G. Knoll, Chapter 8.
25L25.pdfP627 YK3/14/2012
1
Luminescence-Phosphorescence-Fluorescence
2
teItI 0
~ ns ~ mslarger
Absorption/emission “Stokes shift”
Absorption / emission “Stokes shift”
Stokes shift
3
Energy transfer in binary systems via molecular collisions,“Forster mechanism”. With sufficient number of fluor scintillator components “wavelength shifting” is possible
Energy/excitation transfer
4
Energy transferred by emission and re-absorption, and by molecular collisions—
Forster mechanism.
Detectable PPO emission
Dodecane80%
Pseudocumene20%
1.5 g/lPPO
E.g. in KamLAND Liquid Scintillator:
Initial excitation
~375 nmKamLAND LS:Efficiency ~ 70% of anthracene; ~ 100 eV per detectable ;Emission components: ~60% with ~ few nsec~30% with ~ 20 nsec~10% with longer
Energy/excitation transfer
PPO excitation
PPO emission
Scintillator emission spectrum
5
In scintillation detectors all kinds of de-excitationsare possible leading to “quenching”
;dxdEk
dxdEL
dx
dL
B 1
0Birks law:L – light outputkB – Birks coefficientL0 – constant 6
0.001 0.01 0.1 1 10 100
Energy, MeV
0
500
1000
1500
2000
2500
3000
Sto
ppin
g P
ow
er,
Me
V/g
/cm
2
SRIM: in KamLAND LS with H/C=1.969 and =0.78 g/cm3
0 1 2 3 4 5 6 7 8 9 10
Real Energy, MeV
0.04
0.05
0.06
0.07
0.08
0.09
(Vis
ible
Ene
rgy)
/(R
eal E
nerg
y)
kb=0.006 0.0075
0.008
0.0078
Birks’ kB in KamLAND scintillator
nuclear
electronic
0
( )BE
dL dEL k
dx dE dxa
= ⋅ò
7
Measured -response in KamLAND is non-linear:
int( )visSc B Cher
real
LL k R L
E g= + *
Two sources of non-linearity:(a) Birks’ quenching (same kB for all particles e, , p, …(?)(b) Emission of Cherenkov radiation thatcan be detected directly in the visible range orfrom UV range in a process of absorption - reemission
can be calculated if n() is known
8
Properties of organic/plastic scintillators (from Knoll)
9
Timing properties of some fast plastic scintillators
De-excitation timing can be different for different primary particles
pulse-shape discrimination (PSD) methodsused e.g. neutron/gamma discrimination
Problem: propose the scheme of measurement and the procedure that would separate the neutrons and gammas by PSD in the scintillation detector
( )10 1
0
; -decay, -level population (8.10)
( ) (8.11)
t t
t
I I e e
I I f t e
t t
t
t t- -
-
= -
= ⋅ ⋅
10
Light transport, total internal reflection, gluing, wrapping
Due to Liouville's theorem, the total area of the cross-section along a light guide cannot be reduced without light losses. For changes in direction a maximum bending (minimal bending radius) should be chosen according to the relation
where: d – diameter of fiber (or light guide), r – bending radius, n – relative refractive index.With a radius chosen according to the relation given above, all light entering the plane front surface of a light guide is transported due to total reflection.
( )22 1 2 1n d r- ³ +
Twistedguides
PMT
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Superior reflector wrap: ESR (Enhanced Specular Reflector) VM2000 film by 3M [St. Paul, MN]http://www.osti.gov/bridge/purl.cover.jsp?purl=/957053-3bKJBw/957053.pdf
in experiment D0 at Fermilab
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Scintillating and wavelength shifting fibers (SF and WLSF)
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WLS fibers use in NOvA experiment at Fermilab
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Inorganic scintillators produced by industry
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Inorganic scintillators (crystals)
Scintillation mechanism is different since there is no big molecules with vibrational degrees of freedom.
Crystals are dielectrics or insulators with quite largegap between valence and conducting bands. Usually large excitations are involved, e.g. 7 eV that corresponds to 170 nm UV light.
NaI (crystal) is an excellent scintillator;but water solution of NaI is not a scintillator.
16
Inorganic Scintillation Properties: http://scintillator.lbl.gov/
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Properties of inorganic scintillators (from Knoll)
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PDG
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Emission time of NaI(Tl) and BGO
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NaI(Tl) detector performance
21
Noble gas scintillators
Light emission of noble gases can be enhanced in the presence of electric filed !
Temperature dependence of light yield for some inorganic crystals
22