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34 4 Scintillation detectors
describe the quenching effect, the ”Birks formula” [29]
dL/dx =
S · dE/dx
. (4.4)
1 + kB · dE/dx
He introduced the quenching factor k and BdE/dx, which describes
the density of excitation centres along the track. kB is also referred to
as the Birks factor. Likewise the α/β-ratio is quoted, which is related to
the Birks factor as long as the Birks formula is valid for this scintillator.
It is the ratio of the light yield for α particles to the light yield for electrons
of the same energy.
The quenching factor and α/β-ratio depend on the crystal and the conditions
of the experiment. For doped materials the strength of quenching
is related to the amount and kind of dopant.
The α/β ratio is 0.66 - 0.67 for NaI:Tl and CsI:Tl and 0.2 for BGO. The
light output of inorganic scintillators depends generally less on the particle
energy than the light output of organic scintillators, which can
have quenching factors of 10 [27, 28, 29].
Figure 4.6: Quenching factors for α-particles in CsI:Tl and calculated
quenching curve with kB = 2.3 · 10 −3 gMeV −1 cm −2 for ∆t = 5 µs [31]
The time ∆t of the scintillation signal collection also determines the
measured quenching factors. The amplitudes of different scintillation
light components depend on dE/dx. As these components have
often different decay times, different fractions of these components
are collected in different ∆t. For α particles in CsI(Tl) with energies
between ≈ 0 and 10 MeV the Birks factor was determined to be
1.1×10 −3 gMeV −1 cm −2 for ∆t = 1µs and to 2.3·10 −3 gMeV −1 cm −2 for