Event identification

Once the clusters have been created, the Saffron2 code applies a set of iden-tification algorithms to tag the clusters with their type of interaction. The three types that are differentiated here are successively muons, nuclear sig-nals (NS) and electromagnetic sigsig-nals (ES). The algorithms mainly use the cluster lengths and the amplitude and/or integral of the included waveforms to perform the identification.

4.3. EVENT RECONSTRUCTION 83 Muonsare highly energetic particles, from atmospheric origin. Because of their high energy, they can traverse through multiple detection cells in which they deposit large amounts of energy and leave a clear track. For these tracks, three types are identified:

• Type 0: clipping muons, that cross only one or two cubes at the edge of the detector.

• Type 1: vertical muons, that cross many cubes, but only one detector plane.

• Type 2: muons that traverse multiple detector planes and deposit en-ergy on a large number of detector channels.

Type 1 (2) muons are selected by requiring 11 or more channels in the x-or (and) y-direction, with a detected signal above 200 ADC (∼6.5 PA). The muon track is then reconstructed by combining the horizontal and vertical channels in one (type 1) or multiple (type 2) detector planes, as shown in figure 4.3. The type 0 muons currently have no specific identification proce-dure implemented and the corresponding clusters will therefore end up in the electromagnetic signal selection, that is explained below.

The total muon signal rate in the Phase I detector is about 250 Hz on average.

CHAPITRE 3. RECONSTRUCTION DES ÉVÉNEMENTS

Figure3.19 – exemples de traces induites par un muon traversant un module SoLid (10×16×16 cubes) pour les trois types définis. La droite noire est le parcour du muon, induisant des dépôts d’énergie dans les cubes représentés en rouge. Les canaux touchés sont affichés en rouge sur les faces extérieures du module de détection. Gauche : Trace de type 0, correspondant à un muon traversant traversant peu de cubes sur une arête du détecteur. Centre : Trace de type 1 où un muons traverse uniquement un seul plan. En partant de la projection des cubes, il demeure une ambiguïté représentée par les deux droites noires. Droite : Trace de type 2. Le muons traverse plusieurs plans, il est ainsi possible de reconstruire sa trace sans ambiguité.

Figure3.20 – Evènement associé au passage d’un muon après reconstruction. Gauche : signaux provenant des différents canaux impliqués. Les fibres horizontales et verticales sont représentées en bleu et orange. Droite : projection des canaux touchées. La droite rouge représente la paramétri-sation de la trace reconstruite. Les croix rouges représentent les points d’entrées et de sorties.

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Figure 4.3: Examples of reconstructed muons of type 0, type 1 and type 2 in one module of the SoLid Phase I detector. The black line shows the reconstructed track through the detector. For type 1 muons, that only trigger fibres in one detector plane, two possible tracks can be reconstructed and it is not possible to resolve this ambiguity.

84 CHAPTER 4. DETECTOR RESPONSE AND EVENT RECONSTRUCTION Nuclear signals (NS)are related to the scintillations in the ZnS of the neu-tron detection screens. NS events thus include the IBD neuneu-trons of interest, but also events induced by neutrons from other processes orα-particles from radioactive decay (cf. section 5.2.2). In a NS candidate cluster, that contains neutron triggered waveforms, the detector cube with the highest recorded number of peaks is first selected. For a further selection, the integral-on-amplitude (IonA) value is evaluated, plus the asymmetry of the energy de-posits over the different fibres of the cube. The IonA parameter exploits the fact that NS are mainly characterised by a long scintillation time and multi-ple waveform peaks, leading to a large integral for relatively low amplitudes.

The discrimination power of the asymmetry parameter originates in the po-sition of the four WLS fibres w.r.t. the neutron detection screens in a detector cell (cf. figure 2.5). For the vertical fibre that is fully touching the punched Li-screen, for example, other signal intensities are expected than for the re-maining three fibres that have very little or no contact with a neutron screen.

A detailed explanation of the NS selection procedure can be found in refer-ence [82]. The NS rate in the SoLid Phase I detector is about 15 Hz.

Electromagnetic signals (ES)are generated by the interactions in the PVT cubes of particles such as photons (γ’s), electrons and positrons. We should note that also muons belong to this category, but they are treated separately since they generally deposit a much larger amount of energy over a larger amount of cubes. To first order, the ES are selected as those events that remain after the muon and NS selection. However, since ES are characterised by a very short pulse shape, only clusters with a duration of less than 75 samples are kept. In addition, a signal threshold of 80 ADC (∼ ^{30 keV) or} 144 ADC (∼ 50 keV) is applied, depending on the processing settings. The resulting rate of ES events is of the order of 100 kHz.

4.3.4 Event reconstruction

Once the different signal types are identified, a set of event properties, such as their time, position and energy have to be reconstructed.

For an ES event, the timing is closely related to the trigger time of the event. Its time can be determined very precisely, by exploiting the short and peaked signal shape. For the time reconstruction of an NS event, one can not simply rely on the trigger time, since the PoT requirement is mostly only reached in the tail of the signal. In general, the ES and NS time are therefore determined in the clustering algorithm and are simply taken as the start time

4.4. ENERGY ESTIMATOR 85