6 - INFN

Scintillator Detectors
Electrons formed in ionization process
are NOT the same giving the electronic signals !!!
= phosphorescence
Phosphorescence is a property of
many crystals and organic materials
Light is produced by
deexcitations of
molecules
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)
π electrons energy levels
Triplet
Spin=1
Singlet
Spin=0
Low Z
Low efficiency
# γ/keV ∼ 8-10
Fluorecence: 10-8 s
GS = S00
(FAST)
Phosphorescence: 10-6 s
Emission after intra-band transition (SLOW)
phosphorescence
fluorescence
Rise time
Δτ ∼ 0.1 nsec
absorption
0.1 eV
1 ps
1 eV
τ ∼ 10 ns
⇒ in Organic
scintillators
Absorption and Emission
occur at different wave-length
at room temperature
all electrons are in S00
Inorganic scintillator
[Solid crystals: NaI, CsI, BGO, BaF2, LaBr3, …]
Excited electrons beween atomic states
(from valence band to conducting band)
NaI
4 eV
τ ∼ 230 ns
1 part/103
NaI(Tl), CsI(Na), …
Rise time
Δτ ∼ 10 nsec
High Z
High efficiency
# γ/keV ∼ 40
[⇒ 4 times better than plastic]
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
phosphorescence
triplet
fluorescence
absorption
singlet
stilbene
C14H12
prompt fluorescence
(from singlet state):
~ few ns
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), BaF2, …
Light output:
hf
⎛ t
L(t ) = exp⎜ −
⎜ τ
τf
⎝ f
CsI(Tl)
⎞ hs
⎛
⎞
⎟ + exp⎜ − t ⎟
⎜ τ ⎟
⎟ τ
⎝ s ⎠
⎠ s
α particle
Eα=95 MeV
τf = 800 ns
τs = 4000 ns
Sum of two exponential functions:
fast & slow components
2. R = hs/(hf+hs) increases with decreasing
ionisation density
3. τf increases with decreasing ionisation density
Lslow
1. τs independent of particle nature
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
Lfast
Organic vs. Inorganic
Big Disadvantage: Hygroscopic
Temperature effect
Organic scintillators:
independent of temperature between -60° and 20°
Inorganic scintillators:
Strong dependence on temperature
Relative
Light output
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
Photocathod
ε =
# photoelectrons generated
# incident photons on cathode
(ε ∼ 30%)
Different types of PMT
G ∼ δn
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