are NOT the same giving the electronic signals !!!

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

s (FAST)

Phosphorescence: 10

s

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

Inorganic scintillator

[Solid crystals: NaI, CsI, BGO, BaF

, LaBr

]

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

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

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

H

Inorganic Scintillators: CsI(Tl), BaF

, …

a particle E

=95 MeV

t_f = 800 ns t_s = 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. t_s independent of particle nature

2. R = h_s/(h_f+h_s) increases with decreasing ionisation density

3. t_f increases with decreasing ionisation density è

it is possible to identify different particles

CsI(Tl)

L

L

N.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

d ~ 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

Energy resolution

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

Cited by Knoll Book