The SoLid detector is designed to be highly segmented and is therefore con-structed out of many small detector cells. The identical cubical cells are in-strumented with two types of scintillator for the antineutrino detection. One is polyvinyl toluene (PVT), a solid plastic that is machined into (5×5×5) cm3 blocks. The second scintillator consists of thin sheets of 6LiF:ZnS(Ag) deposited on a plastic, reflective backing. Here, the 6Li is used for neutron capture:
n+6Li→ 3H+4He+4.78 MeV. (2.7) The neutron detection sheets are cut into squares of (5×5) cm2 and are placed on two adjacent faces of a PVT cube.
PVT is rich in protons, which makes it a very effectiveνe target. When an IBD reaction takes places, the resulting positron almost instantly deposits all of its energy in the plastic scintillator, creating a prompt and pulsed scintil-lation signal. It will then annihilate and create two γ’s of 511 keV, that also cause a scintillation.
The IBD neutron first thermalises, while elastically scattering through the PVT, until it gets captured in one of the inorganic scintillator sheets2. The triton and alpha particle that are produced in the neutron capture reaction (eq. 2.7), are energetic enough to cause excitation of the electrons in the ZnS crystal. The de-excitation of these states, in its turn, results in the neutron scintillation signal. Due to the time taken for the neutron to scatter before capture, this second scintillation signal is delayed with respect to the signal from the positron. The typical time interval between the positron and neu-tron signals ∆t is about 60 µs. In general, the IBD neutrons do not travel a large distance in the PVT cubes before they are captured by the 6Li and so the neutron signal is usually seen in the same or one of the neighbouring cubes as the positron signal. In addition, because of the finite lifetime of the ZnS excited states, which is significantly larger than the decay time of the PVT scintillator, the resulting waveforms of positron and neutron signals will have a distinct shape. Comparing the waveforms can thus help in discrimi-nating positrons from neutrons [67]. Figure 2.4 sketches the described SoLid antineutrino detection concept.
2The PVT also acts as a neutron moderator. Simulations have shown that a neutron scatters through the PVT over a period of maximally hundreds of microseconds, travelling at most about 15 cm or 3 cubes from the antineutrino interaction point before they are captured in a lithium screen.
2.3. SOLID DETECTOR CONCEPT 31
Figure 2.4: Illustration of the topology of an IBD event in the SoLid detector (top) and the corresponding scintillation signals (bottom).
32 CHAPTER 2. THE SOLID EXPERIMENT To preserve the position information of the scintillation signals, each de-tector cell is optically isolated by a wrapping of reflective Tyvek paper. The light pulses are transported from the respective cubes to the photodetector by wavelength shifting (WLS) fibres. These fibres fit through (5×5) mm2 grooves, that are machined in 4 different faces of each plastic scintillator cube, as illustrated in figure 2.5. The raster lay-out of the fibres enables position reconstruction, since any specific combination of two horizontal and two ver-tical fibres points to a unique cube. The fibres shift the wavelength of the blue PVT (ZnS(Ag)) scintillation light of 425 nm (450 nm) to green light with a wavelength of 500 nm. This longer wavelength lies in the optimal response region of the silicon photomultipliers (SiPMs) that are placed at one end of each WLS fibre and translate the photons to an electronic signal. At the other end of the fibres an aluminium mirror is placed to reflect as much light as possible towards the SiPM side. Both the mirror and SiPM are placed in a 3D-printed plastic housing to ensure a good connection with the WLS fibre, see figure 2.5.
The type of photomultiplier used for the SoLid detector is a multi-pixel photon counter (MPPC) with a surface of (3×3)mm2, that consists of 3600 pixels. Each pixel detects photons and amplifies the signal based on the principle of charge avalanche in Geiger mode.3 The total current coming from the MPPC is the sum of the currents from its individual pixels and is therefore proportional to the number of pixels that detected photons and triggered subsequent avalanches. The scintillation light produced in the detector and transported to the MPPCs is thus measured in units of pixel avalanches (PA).
The MPPC signals are then amplified and digitised by the external readout electronics. The analogue-to-digital conversion (ADC) happens with 14 bit resolution, and a 40 MHz sampling frequency, resulting in waveform samples of 25 ns each. The amplification or gain is set such that one PA corresponds to ∼32 ADC counts.
Table 2.1 gives an overview of all key components of the SoLid detector, including references to the data sheets provided by the producers.
3Photons of high energy create charge carriers when they hit a silicon pixel. In Geiger mode, a high enough electric potential is maintained such that the electrons produce an avalanche in which they are multiplied [68]. For MPPCs this multiplication factor is about 106and the typical photon detection efficiency (PDE) is roughly 35%.
2.3. SOLID DETECTOR CONCEPT 33
Figure 2.5: Technical illustration of the specifications of a SoLid detector cell (left) and the configuration of the readout instruments (right). All values are in mm and are summarised in table 2.1.
Table 2.1: Summary of the SoLid detector materials and their respective dimensions.
For each component, the producers and product code plus a reference are added as well.
Detector material Dimensions [mm] Producer (product type) PVT 49.8×49.6×49.3 ELJEN (EJ200) [69]
6LiF:ZnS(Ag) surf.: 49.2×49.2 Scintacor (ND) [70]
” thick.: 0.225 (+ 0.225)* ”
Tyvek thickness: 0.3 Dupont (1082D) [71]
WLS fibres 3×3×922.4 St.-Gobain (BCF-91A) [72]
SiPM 3×3 Hamamatsu (S12572-050P) [73]
*Most detector cells are equipped with lithium screens that have a plastic backing that is 0.225 mm thick. The cells on the detector edge, however, have an older version of the screens, without backing. Cf. section 2.5.
34 CHAPTER 2. THE SOLID EXPERIMENT