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

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Energy transfer in binary systems via molecular collisions,“Forster mechanism”. With sufficient number of fluor scintillator components “wavelength shifting” is possible

Energy/excitation transfer

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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

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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

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Properties of organic/plastic scintillators (from Knoll)

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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- -

-

= -

= ⋅ ⋅

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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.

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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

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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

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