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 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.
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ZnS “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 (p electrons) p electrons energy levels
Singlet Triplet Spin=0 Spin=1 1 eV 0.1 eV t ~ 10 ns 1 ps Rise time Dt ~ 0.1 nsec absorption fluorescence phosphorescence
-8 GS = S00 Low Z Fluorecence: 10 s Low efficiency (FAST) Phosphorescence: 10-6 s # g/keV ~ 8-10 Emission after intra-band transition (SLOW) Þ 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 1 part/103 4 eV NaI(Tl), CsI(Na), … t ~ 230 ns Rise time Dt ~ 10 nsec
Doping material is used to minimize re-absorbtion from the crystal, since emitted light has lower High Z energy than energy-gap. High efficiency
# g/keV ~ 40 [Þ 4 times better than plastic] Similar effec in Organic Sintillator
Charged Particles identifications
Organic scintillators stilbene energy levels C14H12 singlet triplet
the slow component (t ~ ms) absorption fluorescence
phosphorescence due to delayed phosporescence (from triplet state) is larger for particles with large dE/dx
prompt fluorescence light yield (from singlet state): S = scintillator efficiency ~ few ns kB = fitting constant Inorganic Scintillators: CsI(Tl), BaF2, …
Light output: CsI(Tl) a particle h æ t ö h æ t ö Ea=95 MeV L(t) = f expç- ÷ + s expç- ÷ ç ÷ ç ÷ t f t f t s è t s ø è ø tf = 800 ns ts = 4000 ns Sum of two exponential functions: fast & slow components
1. ts independent of particle nature
2. R = hs/(hf+hs) increases with decreasing ionisation density slow L 3. tf 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 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 ..) (e ~ 30%) From Dynodes
From Anode Output Signals Photocathod
e = # photoelectrons generated # incident photons on cathode
(e ~ 30%) Different types of PMT
G ~ dn d ~ 3-5 emission probability of secondary electrons
n ~ 10 Secondary Emission coefficient
[if electrons are released in random directions Only few will reach the surface Þ reduced gain]
Material: semiconductors 2-3 eV needed to release an electron
Linearity and Stability is required 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 Energy resolution
Never achieved in practice, due to various sources of electronic noise Cited by Knoll Book