Commissioning and operation of the readout system for the SoLid neutrino detector

Abstract

A : The SoLid experiment aims to measure neutrino oscillation at a baseline of 6.4 m from the BR2 nuclear reactor in Belgium. Anti-neutrinos interact via inverse beta decay (IBD), resulting in a positron and neutron signal that are correlated in time and space. The detector operates in a surface building, with modest shielding, and relies on extremely efficient online rejection of backgrounds in order to identify these interactions. A novel detector design has been developed using 12800 5 cm cubes for high segmentation. Each cube is formed of a sandwich of two scintillators, PVT and 6 LiF:ZnS(Ag), allowing the detection and identification of positrons and neutrons respectively. The active volume of the detector is an array of cubes measuring 80x80x250 cm (corresponding to a fiducial mass of 1.6 T), which is read out in layers using two dimensional arrays of wavelength shifting fibres and silicon photomultipliers, for a total of 3200 readout channels. Signals are recorded with 14 bit resolution, and at 40 MHz sampling frequency, for a total raw data rate of over 2 Tbit/s. In this paper, we describe a novel readout and trigger system built for the experiment, that satisfies requirements on: compactness, low power, high performance, and very low cost per channel. The system uses a combination of high price-performance FPGAs with a gigabit Ethernet based readout system, and its total power consumption is under 1 kW. The use of zero suppression techniques, combined with pulse shape discrimination trigger algorithms to detect neutrons, results in an online data reduction factor of around 10000. The neutron trigger is combined with a large per-channel history time buffer, allowing for unbiased positron detection. The system was commissioned in late 2017, with successful physics data taking established in early 2018.

Performance of the SoLid reactor anti-neutrino detector

“…4 (Left). The averaged light yield measured with the 22 Na in the detector was 96 PA/MeV (this figure does not take into account a 20% cross talk of the MPPCs [9]) for a stochastic energy resolution of 12% at 1 MeV. The linearity of the energy response of the detector have been verified for a subset of cubes in the detector and is within a few percent in the [0.5 -4] MeV region, see Fig.…”

“…Example of neutron waveform (black)[9]. The dashed lines show the zero suppression threshold.The value of the neutron trigger variable (i.e number of peaks in the rolling time window), using algorithm parameters optimised for physics mode, is shown in blue.…”

Performance of the SoLid reactor neutrino detector

“…Hence neutron captures on the 6 Li will produce much longer waveforms composed of several pulses of light as shown in figure 1(Right). SoLid exploits the characteristics of these NS signals for trigger and PSD discrimination [7].…”

Status of the SoLid experiment at the BR2 reactor

“…Three different triggers are implemented in the SoLid read-out system; a neutron trigger, which uses a pulse shape discrimination (PSD) algorithm, a threshold trigger, which is based on signal coincidences in crossing X and Y fibres with energies above 2 MeV, and a random trigger that reads out the full detector with a rate of 1 Hz [7]. The waveforms that pass the trigger are processed offline and are first clustered together in time.…”