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

5 Particle Identification with a Neural Network
5.3 Results
5.3.1 Simulated Data
In a Monte Carlo simulation, 20 000 alpha and beta events were generated all over the Inner
Vessel, with random energies from 0 to 3 MeV. Fig. 5.6 shows a typical simulated alpha and a
beta pulse for the CTF2 geometry. The time spectrum recorded was binned in such a way to
reproduce the flash ADC output (2.5 ns sampling time, 8 bit depth).
The input values for the NN were:
- 100 bins of the normalized flash ADC spectrum (250 ns);
- the radial distance from the center (reconstructed position), normalized to the Inner
Vessel radius;
- the energy of the event, in MeV.
The NN had 102 neurons in the input layer, 102 neurons in the hidden layer, and 1 neuron in
the output layer, giving ¢ weights Û. The output parameter of the
net is called nn¬ and can take values between 0 and 1. Ideally it should be 1 for beta events
and 0 for alpha events. If nn¬ is between 0 and 0.5, the event is classified as alpha event; if
nn¬ is larger than 0.5, the event is classified as beta event. The training set consisted of 8000
events. After each 10 events the weights were updated using the backpropagation algorithm.
The validation set consisted of 8000 events (different from the training set). The results of
the NN after 5000 generations are shown in in fig. 5.7 for events with energies greater than
0.2 MeV. From 3629 beta events only 46 were misidentified as alpha events. From 3607 alpha
events 42 were misidentified as beta events. This means, that for 98.7 % beta identification
efficiency, 98.8 % of the alpha events are rejected.
For a comparison I also calculated the tail-total-ratio from the flash ADC spectrum with the
tail starting after 47.5 ns (see fig. 5.8). For the same beta identification efficiency (98.7 %) with
this method, I also got the same alpha rejection efficiency (98.8 %).
The neural network has the same discrimination capability as the standard tail-to-total ratio,
but it cannot beat this simpler and more straightforward technique.
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