Organic scintillator

Scintillator Detectors

Electrons formed in ionization process
are NOT the same giving the electronic signals !!!

= phosphorescence
Light is produced by
deexcitations of
molecules
Phosphorescence is a property of
many crystals and organic materials

ZnS: the precursor of modern scintillator counters
In 1903 W. Crookes demonstrated in England his
“spinthariscope” for the visual observation of individual
scintillations caused by alpha particles impinging upon a ZnS
screen. In contrast to the analogue methods of radiation
measurements in that time the spinthariscope was a singleparticle
counter, being the precursor of scintillation counters
since. In the same period F. Giesel, J. Elster and H. Geitel in
Germany also found that scintillations from ZnS represent single
particle events. This paper summarises the historical events
relevant to the advent of scintillation counting.
“2003: a centennial of spinthariscope and scintillation counting”
Z. Kolar et al., App. Rad. And Isot. 61 (2004)261

Organic scintillator
[Solid or liquid: haromatic hydrocarbons (benzene, …) ]
Excited electrons are the ones NOT strongly
involved in the bonding of the material (π electrons)
Low Z
Low efficiency
# γ/keV ∼ 8-10
0.1 eV
1 ps
1 eV
τ ∼ 10 ns
Rise time
Δτ ∼ 0.1 nsec
Fluorecence: 10 -8 s
(FAST)
π electrons energy levels
Singlet
Spin=0
absorption
GS = S 00
Phosphorescence: 10 -6 s
Emission after intra-band transition (SLOW)
fluorescence
Triplet
Spin=1
phosphorescence

⇒ in Organic scintillators
Absorption and Emission
occur at different wave-length
at room temperature
all electrons are in S 00

Inorganic scintillator
[Solid crystals: NaI, CsI, BGO, BaF 2 , LaBr 3, … ]
Excited electrons beween atomic states
(from valence band to conducting band)
NaI
4 eV
τ ∼ 230 ns
Rise time
Δτ ∼ 10 nsec
High Z
High efficiency
# γ/keV ∼ 40
[⇒ 4 times better than plastic]
1 part/10 3
NaI(Tl), CsI(Na), …
Doping material is used to minimize
re-absorbtion from the crystal,
since emitted light has lower
energy than energy-gap.

Similar effec in Organic Sintillator

Charged Particles identifications
Organic scintillators
energy levels
singlet triplet
absorption
fluorescence
phosphorescence
prompt fluorescence
(from singlet state):
~ few ns
stilbene
C 14 H 12
the slow component (τ ~ ms)
due to delayed phosporescence
(from triplet state)
is larger for particles with large dE/dx
light yield
S = scintillator efficiency
kB = fitting constant

Inorganic Scintillators: CsI(Tl), BaF 2 , …
Light output:
L(
t)
=
h
τ
f
f
⎛⎛
⎜⎜
t
exp −
⎜⎜
⎝⎝ τ f
⎞⎞ ⎛⎛ ⎞⎞
⎟⎟
hs
t
+ exp⎜⎜
⎜⎜−
⎟⎟
⎟⎟
⎟⎟
⎠⎠ τ s ⎝⎝ τ s ⎠⎠
Sum of two exponential functions:
fast & slow components
1. τ s independent of particle nature
2. R = h s /(h f +h s ) increases with decreasing
ionisation density
3. τ f increases with decreasing ionisation density
è� it is possible to identify different particles
N.B. CsI have been used at first for particle studies:
- less fragile than NaI
- good particle discrimination
L slow
CsI(Tl)
α particle
E α =95 MeV
τ f = 800 ns
τ s = 4000 ns
L fast

Organic vs. Inorganic

Big Disadvantage: Hygroscopic

Relative
Light output
Temperature effect
Organic scintillators:
independent of temperature between -60° and 20°
Inorganic scintillators:
Strong dependence on temperature
Temperature

Use of light Pipe:
- coupling with photodetector
- need to locate photodetector
away from scintillator (magnetic field ..)

(ε ∼ 30%)

From Dynodes
From Anode
Output Signals

ε =
# photoelectrons generated
# incident photons on cathode
Photocathod
(ε ∼ 30%)

Different types of PMT
G ∼ δ n
δ ∼ 3-5
emission probability
of secondary electrons
n ∼ 10

Another Dynode configuration: Micro Channel Plate
Advantages: 1. fast timing 20ps (short distance, high field)
2. tollerate high magnetic fields
3. position sensitive

Secondary
Emission coefficient
[if electrons are released in random directions
Only few will reach the surface ⇒ reduced gain]
Material: semiconductors
2-3 eV needed to release an electron
Linearity and Stability is required

# γ/keV ∼ 40 �

Energy resolution
Never achieved in practice, due to various sources of electronic noise