Development of a Liquid Scintillator and of Data ... - Borexino - Infn

5.1 Pulse Shape Discrimination in Liquid Scintillator
track are quenched by competing reactions before they have an opportunity to transfer their
energy to the fluors which will emit the photons. Hence, an alpha particle will produce, on average
the same number of photons as an electron with about one tenth of its energy. Quenching
results in a non-linear relation between the deposited energy and the emitted scintillation light.
This relation can be parametrized as
Ä
Ü
Ü
Ü
(Birk’s law)
is a characteristic magnitude of the quenching (in Birks model, is the fraction of
Ü
’damaged’ molecules along the particle track; is the fraction of these molecules which cause
quenching). The alpha quenching in the scintillator mixtures tested in the CTF has been measured
on laboratory samples for various alpha energies (see table 5.1). The pulse height of
the alpha particles was compared to the pulse height of betas with known energies (Compton
scattering of gamma rays). The ratio of the alpha energy to the ‘beta -equivalent’, measured
energy is defined as the quenching factor. From these measurements, the value of can be
calculated:
È ÈÈÇ Ð ¦ ¡
È ÌÈ Ð ×ÅË ÑÐ ¦ ¡
ÑÅÎ
ÑÅÎ
The relative intensity of the fast and the slow scintillation components also depends on the
energy loss per unit path length, since the slow component is less affected by quenching. Thus,
the relative intensity of the slow component is larger for alpha particles than for electrons,
thus allowing to discriminate between the two types of radiation on the basis of the decay time
distribution.
Element «-energy (MeV) measured energy (keV) quenching factor
È ÈÈÇ Ð
Rn 5.49 ¦ ¦
Po 6.00 ¦ ¦
Po 7.69 ¦ ¦
Po 5.30 ¦ ¦
È ÌÈ Ð ×ÅË ÑÐ
Rn 5.49 ¦ ¦
Po 6.00 ¦ ¦
Po 7.69 ¦ ¦
Po 5.30 ¦ ¦
Table 5.1: Quenching factors for alpha particles with different energies for scintillator mixtures
based on PXE and PC. Measured on small samples [Nef96].
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