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

5 Particle Identification with a Neural Network
Figure 5.1: electronic energy levels of an organic molecule. S is the ground state. S are
excited singlet states (spin 0), T are excited triplet states (spin 1). Taken from [Bir64].
absorption threshold. The scintillator is thus transparent to its own scintillation emission. The
typical radiative lifetime of the -singlet state S is of the order of s. Radiative
transitions from T to S Ü are called phosphorescence. Due to the forbidden nature of the
transition, the radiative lifetime of T can range up to a few seconds. Phosphorescence does not
play a significant role in the scintillation process. The T state decays instead by the process
of triplet annihilation:
Ì Ì Ë Ë phonons
The molecule in S subsequently decays to the ground state with the same wavelength spectrum,
but with a time delay that is determined by the rate of collisions between T exited
molecules. This process is called delayed fluorescence. In some scintillators, this slow component
can last up to 1 s.
5.1.2 Response to Different Radiations
Only a small fraction Õ of the energy of the incident particle or radiation is transformed
into light. For radiation with a low ionization density (electrons, ­-rays), the relation between
the differential energy loss per path length Ü of the incident particle and the resulting
luminiscence energy per path length ÄÜ is linear:
Ä
Ü
¡
Ü
The higher specific ionization of heavier particles (protons, alpha particles, ...) results in a
lower scintillation yield as more of the energy goes into nonlight-producing processes. Compared
to an electron track, nearly 90 % more of the excited molecules produced along an alpha
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