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