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
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 single- particle 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.
ZnS: the precursor of modern scintillator counters
“2003: a centennial of spinthariscope and scintillation counting”
Z. Kolar et al., App. Rad. And Isot. 61 (2004)261
Ra
ZnS
Organic scintillator
[Solid or liquid: haromatic hydrocarbons (benzene, …) ]
0.1 eV 1 ps
1 eVt ~ 10 ns Rise time Dt ~ 0.1 nsec
Low Z
Low efficiency
# g/keV ~ 8-10
p electrons energy levels
absorption fluorescence phosphorescence
Singlet Spin=0 Excited electrons are the ones NOT strongly
involved in the bonding of the material ( p electrons )
GS = S00
Fluorecence: 10
-8s (FAST)
Phosphorescence: 10
-6s
Emission after intra-band transition (SLOW)
Triplet
Spin=1
Þ in Organic scintillators Absorption and Emission
occur at different wave-length
at room temperature
all electrons are in S
00Inorganic scintillator
[Solid crystals: NaI, CsI, BGO, BaF
2, LaBr
3, …]
High Z
High efficiency
# g/keV ~ 40
[Þ 4 times better than plastic]
Excited electrons beween atomic states (from valence band to conducting band)
NaI
4 eVt ~ 230 ns Rise time Dt ~ 10 nsec
1 part/10
3NaI(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
the slow component (t ~ ms) due to delayed phosporescence
(from triplet state)
is larger for particles with large dE/dx energy levels
absorption fluorescence phosphorescence
singlet triplet
prompt fluorescence
(from singlet state):
~ few ns
light yield
S = scintillator efficiency kB = fitting constant
stilbene
C
14H
12Inorganic Scintillators: CsI(Tl), BaF
2, …
a particle E
a=95 MeV
tf = 800 ns ts = 4000 ns
÷÷ ø çç ö
è æ -
÷ +
÷ ø ö ç ç
è æ -
=
s s
s f
f
f
t h t
t h
L ( ) t exp t t exp t
Light output:
Sum of two exponential functions:
fast & slow components
1. ts independent of particle nature
2. R = hs/(hf+hs) increases with decreasing ionisation density
3. tf increases with decreasing ionisation density è
it is possible to identify different particles
CsI(Tl)
L
fastL
slowN.B. CsI have been used at first for particle studies:
- less fragile than NaI
- good particle discrimination
Organic vs. Inorganic
Big Disadvantage: Hygroscopic
Organic scintillators
:independent of temperature between -60° and 20°
Inorganic scintillators
:Strong dependence on temperature
Temperature effect
Relative Light output
Temperature
Use of light Pipe:
- coupling with photodetector - need to locate photodetector
away from scintillator (magnetic field ..)
(e ~ 30%)
Output Signals
From Anode
From Dynodes
Photocathod
(e ~ 30%)
# photoelectrons generated
# incident photons on cathode
e =
G ~ d
nd ~ 3-5
emission probability of secondary electrons
n ~ 10
Different types of PMT
Material: semiconductors
2-3 eV needed to release an electron
Linearity and Stability is required
[if electrons are released in random directions Only few will reach the surface Þ reduced gain]
Secondary
Emission coefficient
Another Dynode configuration: Micro Channel Plate
Advantages: 1. fast timing 20ps (short distance, high field) 2. tollerate high magnetic fields
3. position sensitive
# g/keV ~ 40 à
