SoLAr: a novel LAr TPC to detect MeV-scale neutrinos

SoLAr is a proposed liquid argon time projection chamber experiment that will be able to measure the “hep branch” of solar neutrinos. The scintillation light and the deposited charge signals are collected on the same readout plane that contains a combined grid of silicon photon multipliers and charge collection pixels. The combined light and charge readout allows for millimeter-scale resolution and light and charge matching. First prototypes were successfully operated in Bern, CH in cosmic-rays. The conceptual design of SoLAr, the advantages, and the first results of the SoLAr prototypes will be discussed in details in this proceeding.


Introduction
An outstanding issue in neutrino physics and astrophysics is the observation of neutrinos generated from the fusion of protons and 3 He in the Sun.The process, named the hep branch [1], is described as: 3  +  → 4  +  + +   .
Its direct measurement would impact solar modeling and models for stellar evolution in astrophysics.However, the first direct detection has still to happen.The SNO collaboration sets an upper limit for the hep neutrino flux [2].
A possible detection method of solar hep neutrinos is through the usage of liquid argon time projection chambers (LAr TPCs).Previous studies [3][4][5] determined that low energy sensitive liquid argon detectors with high masses  (100 kton) have the potential to detect the hep neutrinos and neutrinos from supernova bursts.The SoLAr collaboration has been formed to develop a multipurpose LAr TPC technology to study solar neutrinos as well as neutrino beam events [6].More precisely, SoLAr proposes the usage of a dual readout anode plane containing both the charge and light readout, creating the possibility to do online 3D reconstruction, which means an instantaneously localized readout for small energy deposits.The charge collection gives information for a native 3D track reconstruction and an efficient light collection on the anode plane is needed for a high position and energy resolution of the track.
Two SoLAr prototype LAr TPCs were built: SoLAr prototype-v1 and SoLAr prototype-v2.SoLAr prototype-v1 is the first small LAr TPC with light and charge collecting readout systems on the anode plane.It collected cosmic-rays without interruption for about 24 hours.SoLAr prototype-v2 was designed to investigate in more details the interplay and effects of the dual readout with a larger anode plane compared to SoLAr prototype-v1.SoLAr prototype-v2 was running for about seven days and collected cosmic-rays and data from 60 Co decays.
The motivation to detect low energy particles and to identify hep neutrinos are described in section 2. Section 3 presents the SoLAr prototype-v1 and its operation.Prototype SoLAr-v2 is introduced in section 4.This section shows the first reconstructed track of SoLAr-v2.An outlook of the SoLAr collaboration is given in section 5.

Solar neutrinos in LAr TPCs
The goal of SoLAr is to measure the low energy hep neutrinos in a LAr TPC for the first time.The new concept from SoLAr, to collect scintillation light and deposited charges on a double pixelated readout anode plane, should create the possibility to perform a localized online event triggering and a native 3D reconstruction of MeV up to GeV scale particles.The detection and identification of hep neutrinos needs a precise energy reconstruction in the low energy regime because they can be distinguished in the solar neutrino flux from different neutrino production sources in the sun in a small energy band in the higher energy range of the solar neutrino energy spectrum only [7].For low energies the small cross section of neutrinos [8] requires a correct and efficient detection and identification of particles in a very large mass detector.
A crucial aspect to enable the detection of low-energy neutrinos is the understanding and mitigation of the background.The hep neutrinos will be identified with flavor tagging of the neutrino by detecting the final state electron and the gamma's from the de-excitation of the 40 K * nucleus from the electron neutrino interaction in liquid argon, that is described as: The ability to discriminate small energy depositions from neutrinos is very dependent on the calibration of the low energy MeV electron scale over the full detector.
A LAr TPC sensitive to low energy particles can be used for a broad physics program, as it will also be sensitive to low energy supernova bursts, 8 B solar fusion branch neutrinos, nucleon decays, and will open the possibility to do more neutrino oscillation parameter related physics.

SoLAr prototype-v1
The basic idea of SoLAr is to combine two readouts, one to collect deposited charge carriers and another to measure scintillation light, into a double pixelated readout on the anode plane.Unlike most of the current LAr TPCs, which detect the 128 nm scintillation light with some wavelength-shifting stages before the actual detection, SoLAr will detect the light with a grid of vacuum ultra violet (VUV) silicon photon multipliers (SiPMs).Each VUV SiPM is surrounded by charge collecting pixels.
For the SoLAr prototype-v1 a set-up with a grid of ceramic packaged Hamamatsu VUV SiPMs with pins (S13370-6050CN) [9], that are able to detect the 128 nm scintillation light directly, is chosen.For each SiPM the full light waveform is individually collected.The charge carriers are collected on the grounded gold-plated copper pixel pads that are readout with LArPix chips [10].The design of the first prototype is chosen to be a TPC with an anode plane of 7 × 7 cm 2 and a drift length of 5 cm with 16 VUV SiPMs and 4 LArPix chips.A picture of the anode plane is shown in figure 1.To avoid the interference of the SiPM pins and the LArPix chip mounted on the back of the anode, a stack of three Printed Circuit Board (PCB) layers soldered together is used.A Computer Aided Design (CAD) drawing of the PCB stack is shown in figure 2. On the first and innermost PCB layer of the TPC are the gold-plated copper pads, used as charge collecting pixels, with holes for the SiPMs.This PCB layer is labelled in figure 2 as "first PCB layer".The second PCB layer is needed to mount the SiPMs, so that the surface of the SiPMs is on the same level as the charge pixels.On this board the pin lines are routed away from the original pin location, creating the possibility to mount the LArPix chips on -2 -  the third and outermost PCB.On that third PCB the SiPM bias-voltage-traces and signal-traces are routed to the connector on the edge of the PCB.This set-up has the possibility to float the SiPMs on a different voltage level with respect to the voltage level on the pixels.Later this could be used to guide the drifting electrons away from the SiPMs on the charge collecting pixels.
Figure 3 shows the open SoLAr prototype-v1 TPC.The electric field is formed with resistors and strips on the wall.The TPC is fixed on a plastic stick, which is then mounted to the top flange of the cryostat.Figure 3  The combination of charge pixel detectors and light VUV SiPMs on the anode plane is not trivial.Apart from possible cross talks between the readout systems and potentially an inefficient or an insufficient detection of drift electrons or scintillation light, there is the possibility that the electric field gets disturbed.For a first estimation of the behavior of the electric field a numerical computation for different heights of pixels in relation to the SiPMs is performed with COMSOL [11].Results of the numerical computation are shown in figure 4. The electric field gets homogeneous further away from the anode plane.The different heights between SiPMs and charge pads matter for the electric -3 -  field close to the anode plane because the field lines should end on the pixels to assure that all the drift electrons are collected.The best performance, when most of the field lines end on a pixel, is obtained for pixels that are higher up than the SiPMs, as shown in figure 4 in the bottom numerical computation.More studies to estimate the impact on lowered SiPMs for the light collection efficiency are being performed.An option considered to optimize the electron collection on the pixels is to -4 -  float the SiPM surface on a negative voltage level with respect to the pixels.This could help to drift the electrons away from the SiPM onto the charge collecting pixels.
The SoLAr prototype-v1 was used to measure cosmic muon tracks by collecting both light and charge on the anode plane for the first time.For this run a continuous measurement of cosmic muons for about 24 hours was successfully performed in October 2022.During the operation no breakdown of either the charge or light readout occurred.For a limited amount of time runs with different floating voltage level of the SiPMs with respect to the voltage level of the pixels were performed.
In addition to proving the engineering concept, the charge-light matching capability of the SoLAr concept was verified with the detection and reconstruction of cosmic-ray muons.Figure 5 and figure 6 show a combined charge-light event display, that was obtained from the SoLAr prototype-v1 cosmics -5 - The amount of collected data with different floating voltage levels of the SiPMs is limited to a maximum of 20 minutes per voltage level.These data are used to perform a first study on the effect of different float voltage levels on the collected photoelectrons.Figure 7 shows the mean amount of collected photoelectrons calculated from the integrated waveform for the different floating voltage levels of 0 V, −25 V, −50 V, −75 V and −100 V with respect to the voltage level of the pixels.As expected, the measurement shows no significant change in the light collection for different floating voltage levels.

SoLAr prototype-v2
After the operation of SoLAr prototype-v1, a second prototype, SoLAr prototype-v2, was built with new Surface Mount Device (SMD) Hamamatsu VUV SiPMs and operated during July of 2023.The design of the SoLAr prototype-v2 consists of an active anode plane of about 30 × 30 cm 2 with 64 VUV SiPMs (S13370-2221) and space for 64 LArPix chips.Thanks to the development and availability of SMD VUV SiPMs it is possible to have both, the charge pixels and the SiPMs mounted on a single multi-layer PCB.The left picture in figure 8 shows the anode plane of SoLAr prototype-v2.The anode plane was used in a 30 × 30 × 30 cm 3 TPC.The TPC is mounted to the top flange of the cryostat as it is shown in the picture on the right of figure 8.The new anode plane makes it possible to reduce and test cross talks of the different readout systems further.For this run the anode plane was instrumented with 64 SiPMs and 20 LArPix chips.The 20 LArPix chips are put in a 5 × 4 matrix, resulting in an active area of about 200 cm 2 .The larger anode plane gives the possibility to measure longer muon tracks and, -6 -therefore, to better asses the performance of this technology.A medium sized cryogenic system, that allows to cool and filter the argon, is used for this run.SoLAr prototype-v2 was able to collect data for several days, with most of the time devoted to collect cosmic-ray events.The performance of the detector at very low-energy is studied using a dedicated run when a 60 Co source was used to produce 1.2 and 1.3 MeV gamma-rays.The run also investigates the accumulation of charge on the SMD SiPMs.-7 -

Outlook for SoLAr
The aim of SoLAr is to demonstrate construction and operation of a LAr TPC that is able to cover three orders of magnitude in energy, from MeV to GeV.To this end a road map is envisaged with prototype testing, performance studies and realisation of dedicated technologies.Two initial prototypes were built and they demonstrated with a simple reconstruction technique that the concept of SoLAr works.The two runs create the possibility to do first detector performance analysis.This will inform the direction of focus and improvements required for the next prototypes.Once more LArPix chips will become available SoLAr could run prototype-v2 with 64 LArPix again to increase the amount of data as well as active area.
In the near future more research and development TPCs will be built.The new prototypes will use and optimize the latest technologies.The idea is to develop the "SoLAr cell": a VUV SiPM with four charge collecting pads on top of it.This would create the possibility to keep the charge pixels homogeneously distributed over the full anode plane.The left CAD drawing in figure 11 shows the conceptual design idea for a SoLAr cell.Other ideas, such as alternative charge readout chips and light readout systems are also being considered.A desirable feature of our readout system is the possibility to achieve localized online triggering of the charge readout thanks to the detection of scintillation light by SiPMs on the anode.
In a medium term it is planed to build a cosmic-ray shielded few-ton scale LAr TPC underground, 1100 m overburden at Boulby, U.K., demonstrating the technology.The two anode planes will consist of 80 30 × 30 cm 2 readout tiles, each with 10404 pixels and 289 SiPMs.The right CAD drawing in -8 - figure 11 shows the first design for the few-ton scale LAr TPC.The goal of this demonstrator will be to realise the first flavor tagged solar neutrinos measurement in liquid argon.The experiment at Boulby will be extremely valuable to prove the design of SoLAr as a multi-purpose LAr TPC with superior low-energy performance with respect to the current state-of-the-art LAr TPCs for neutrino physics.A successful experiment with the SoLAr concept will make it an excellent candidate for the fourth far detector module of DUNE, the so-called "Module of Opportunity".
Apart from building the next steps for the detector development, the collaboration is studying the design and performance of SoLAr with dedicated Monte Carlo simulations.These studies are crucial to understand background sources, develop mitigation strategies and reconstruction techniques to finally establish the SoLAr sensitivity to solar neutrinos.All these studies, combined with the experience gained in the prototyping runs, will guide the design of the SoLAr detector.

Conclusion
SoLAr prototype-v1, that was collecting data in October 2022, can be summarized as a great success as it demonstrated the capability of measuring charged particles in a LAr TPC by collecting charge and light on the anode plane.The first muon tracks with light and charge matching were measured and reconstructed.There was no significant change in the average number of collected photoelectrons, calculated from the integrated waveform, observed for different floating voltage levels on the SiPMs.Further studies for the electric field homogeneity, the scintillation light and drift electron collection efficiency are being performed.
SoLAr prototype-v2 was operated in July of 2023 and took cosmic-ray and 60 Co decay photon data.Longer muon tracks were measured by collecting light and charge on the anode plane.This time the newly available SMD VUV SiPMs are used for the light collection.The first muon tracks including delta rays are reconstructed.These data will be used to do low energy resolution studies.The low energy 60 Co decay photons will help to investigate the performance of the LAr TPC at the MeV scale.

Figure 1 .
Figure 1.The picture shows the top view of the anode plane.The gold-plated pixels collect the drift electrons and the silver squares are the VUV SiPMs, surrounded by the black ceramic package.

Figure 2 .
Figure 2. The CAD drawing shows the three PCBs that are stacked together to form the anode plan of the SoLAr prototype-v1.
also shows the hanging TPC.Two pre-amplifiers for the light readout are mounted slightly above the TPC.Thin flexible PCBs are used to route the SiPM bias-voltage-traces and signal-traces to the pre-amplifiers.Figure3also shows a CAD drawing of the TPC inside of the cryostat.

Figure 3 .
Figure 3.The left picture shows the open SoLAr prototype-v1 TPC.The middle of this figure represents a picture of the TPC hanging on top of the open cryostat.The black cables are thin flexible PCBs.The right side shows a CAD drawing of the TPC mounted inside of the cryostat.

Figure 4 .
Figure 4. Three different scenarios for the height of SiPMs in relation to the pixels are shown in the three images.The top left numerical computation shows the electric field shape if the charge pads and SiPMs are on the same level.The right top numerical computation represents the field in the case that the charge pads are lower than the SiPMs.The bottom numerical computation shows the behavior of the electric field if the charge pads are raised higher than the SiPMs.

Figure 5 .
Figure 5.The two plots show a reconstructed muon track collected in SoLAr prototype-v1.The left image indicates the amount of collected charge on the pixels with the smaller colored squares.The squares surrounded in black indicate the VUV SiPMs.The color represents the amount of collected photoelectrons calculated from the integrated waveform.The right plot shows the collected light waveform for the corresponding SiPM.

Figure 6 .
Figure 6.The two plots show a reconstructed muon track collected in SoLAr prototype-v1.The left image indicates the amount of collected charge on the pixels with the smaller colored squares.The squares surrounded in black indicate the VUV SiPMs.The color represents the amount of collected photoelectrons calculated from the integrated waveform.The right plot shows the collected light waveform for the corresponding SiPM.

Figure 7 .
Figure 7.The plot shows the mean amount of collected photoelectrons for different floating voltage levels of one SiPM with respect to the voltage level of the pixels.

Figure 8 .
Figure 8.The left picture shows the anode plane of the SoLAr prototype-v2.The right image displays the TPC mounted on the flange.

Figures 9
Figures 9 and 10 show the reconstruction of three cosmic muons obtained from a first analysis of our data.They display on the left side the top view of the 5 × 4 active charge read out area.The colored black surrounded boxes represent the SiPMs, where the color indicates the amount of collected photoelectrons calculated from the integrated waveform for each SiPM.The smaller red squares indicate the amount of charge collected on the corresponding pixel.The center image represents the location of the reconstructed track in the drift direction () with the anode at  equal to 0. The 3D reconstruction of the track in figure 9 is shown in the rightmost image.

Figure 9 .
Figure 9. Reconstruction of a muon event with at least two delta rays.The left plot shows the top view of the active anode plane.The colors indicate the amount of collected charge on the pixels for the smaller squares and the amount of collected photoelectrons calculated from the integrated waveform for each SiPM in the black surrounded squares.The middle plot shows the drift distance of the track from the anode plane, where the anode plane is located at  = 0.The right plot shows a 3D reconstruction of the event.

Figure 10 .
Figure 10.Reconstruction of different muon events.The left plot shows the top view of the active anode plane.The colors indicate the amount of collected charge on the pixels for the smaller squares and the amount of collected photoelectrons calculated from the integrated waveform for each SiPM in the black surrounded squares.The right plot shows the drift distance of the track from the anode plane, where the anode plane is located at  = 0.

Figure 11 .
Figure 11.The left CAD drawing shows the conceptual design idea for a SoLAr cell: a VUV SiPM with four charge collecting pads on top.The right CAD displays the preliminary design of the Boulby, U.K. mid scale few-ton scale LAr TPC.