GanESS: detecting coherent elastic neutrino-nucleus scattering with noble gases

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Introduction
Predicted in the 70s, coherent elastic neutrino-nucleus scattering (CENS) was first measured a few years ago [1].In this process low energy neutrinos (few tens of MeV) interact coherently with an atomic nucleus as a whole, through the weak current channel, as long as the coherent condition |Q| < 1/R is satisfied, with |Q| being the momentum transfer and R the radius of the nucleus.As a result of the low value of the weak mixing angle for protons, the coupling ends up being effectively proportional to the squared number of neutrons (N 2 ) in the target nucleus.The process is of utmost relevance to deepen our knowledge of a large number of neutrino properties.For example, current measurements of this interaction have already been used in nuclear structure studies to improve bounds on Non-Standard neutrino Interactions [2].Improved CENS measurements can also be used to better determine the neutrino electromagnetic properties [3], constrain the weak mixing angle [4] and test both dark matter [5,6] and sterile neutrinos models [7].Finally, CENS can be used to monitor nuclear proliferation thus honing the detection techniques of this process may potentially lead to technological applications.
The most intense neutron spallation source in the coming years, the European Spallation Source (ESS), will produce ∼2.53 × 10 23 neutrinos per year, almost a factor 10 improvement over the Spallation Neutron Source (Oak Ridge National Laboratory, U.S.A.), where CENS was first detected.The ESS will provide first protons on target at 2 MW and 800 MeV in 2025 with the goal to increase it to 5 MW and 2 GeV in the following years (not sooner than 2027).As described in [8,9], the combination of compact detectors and the large neutrino flux produced by the ESS will allow CENS measurements only limited by few percent systematics and not by statistical fluctuations.Moreover, given its pulsed nature, non-beam induced backgrounds can be characterized during the beam-off periods allowing for background subtraction in the data analysis.

GanESS: gaseous detector for neutrino physics at the ESS
The GanESS experiment will make use of a high pressure noble gas time projection chamber (TPC) with electroluminiscence amplification to take advantage of the golden opportunity offered by the ESS to study the CENS process and the related physics topics.As illustrated on figure 1, the chamber will have a symmetrical configuration with a central cathode and two amplification regions, one at each side of the chamber.Both primary and secondary scintillation (S1 and S2) will be detected -1 -by light sensors located in front of the electroluminiscence zone.The proximity of the sensors to the amplification region will allow to reconstruct the transverse position of the interactions, while the longitudinal position will be given by the time difference between S1 and S2.The envisioned design for GanESS consists of a 60 cm diameter and length cylindrical TPC, with two drift volumes of 30 cm.With this geometry, the chamber will be able to accommodate ∼20 kg of Xe at 20 bar.With this mass ∼7,000 CENS events will be detected at a distance of 20 m from the ESS target (figure 2, left) with a threshold of 1 keVnr.
The technology offers several interesting characteristics for CENS searches.First, it permits operation with different noble gases with minimal changes in the setup.Consequently, GanESS will provide CENS measurements in different targets being Xe, Kr and Ar the ones under consideration given their higher cross-section (as it depends on N 2 ) and density.Lighter gases are not considered as the expected CENS rate is rather poor.The use of different targets significantly boosts the physics potential of the experiment as it allows to reduce degeneracies of NSI parameters [8].An example of this is illustrated on figure 2, right.A second advantageous characteristic of the experiment is given by electroluminiscence amplification.An electroluminiscence yield of a few hundred photons per cm has already been observed at ∼10 bar in gaseous xenon in a detector of a similar size to that of GanESS [11].Such yield allows to relax enormously the light collection efficiency requirements for single electron detection.With a few percent light collection efficiency the potential detection threshold can thus be as low as the energy needed for single electron production, slightly above 20 eVee.It should be noted such threshold is only possible in "S2-only" detection mode which will require to reconstruct the longitudinal position based on the S2 width [10].S1 detection will result in a much higher threshold and will be exploited to characterize and understand the backgrounds as well as to calibrate the detector.
The previous traits are shared with dual-phase liquid noble time projection chambers, broadly used in dark matter searches.However operation of such detectors at shallow depths, where neutrino sources are available, is hampered by charge trapping in the inter-phase between liquid and gas which diminishes the detection threshold and limits the overall performance of the detector.This situation does not occur on single-phase detectors which makes gaseous noble gases TPCs more suitable for CENS searches.In spite of their obvious advantages, gaseous detectors have not been considered -2 -for CENS detection before, mostly because their relatively low density could limit the event rate in a moderate-size detector.Fortunately, the high neutrino fluxes available at the ESS solve this issue and allow fully exploiting the advantages of the technology. and    [2].The exclusion region for a gas TPC detector operating with two different nuclei (separate runs, each of them for 3 years) is shown, as well as for a configuration where the detector is filled with each of the two gases during half of the total data taking period (1.5 years running with 132 Xe, 1.5 years with 40 Ar).The dashed lines show the allowed regions at 90% CL, as obtained in [2] using COHERENT data.

GaP: the gaseous prototype
While high pressure noble gas TPCs have been largely developed and employed in the context of neutrinoless double  decay searches [11], its use for low energy searches has been non-existent.Therefore, full response characterization in the few keV range will be needed to fully assess the capability and performance of these detectors for CENS.
The Gaseous Prototype (GaP), shown on figure 3, has been built to completely understand the detection technique in the low energy regime.Several aspects will be evaluated in the prototype, such as the electroluminiscence yield at higher pressures for the different gases, alternative sensor solutions or realistic estimates of the detection threshold.Still, the main goal of the prototype is to determine the quenching factor (QF) of the different gases under consideration as it is currently unknown in the energy region of interest for CENS searches.A proper understanding of the QF is of utmost importance to fully understand any observations in the GanESS detector.The characterization will be done at different pressures up to 50 bar, which is the maximum pressure that the vessel withstands.
The GaP vessel houses a vertical 2 cm drift TPC with a 0.9 cm gap for electroluminiscence.Seven Hamamatsu R7378A PMTs [12] are located above the electroluminiscence region, with a TPB-coated quartz window between the amplification region and the sensors, in order to shift the wavelength of the scintillation light of the medium from the VUV band to the blue band, where the detection efficiency is significantly higher.This is specially relevant in the case of Ar as its light peaks at 128 nm, a wavelength the employed PMTs are not sensitive to.
-3 - The detector started operations on summer 2023, with argon at 9.5 bar.An 83 Kr source, coupled to the gas system, has been used to calibrate the detector.83 Kr is a common source used for low energy calibration, given its small energy deposition (∼41.5 keV) and its natural gaseous state which results in an homogeneous distribution within the detector.In addition to the krypton source, a 241 Am  source was placed inside the detector, at the cathode.
The light collection achieved in this configuration has been rather limited, well below 1% according to simulations.The limited efficiency combined with a single channel trigger and the long decay time of Ar scintillation, above 1 μs, has harmed the trigger efficiency resulting in reduced sensitivity.Consequently, only 241 Am events were detected (an example is shown on figure 4).A series of improvements have been envisioned and an upgrade of the detector is scheduled to be completed before the end of 2023.First, the PMTs will be directly coated with TPB and the quartz window will be removed.This will considerably increase the solid angle between the TPB re-emission points and the PMTs photocathode.A further boost on the light collection efficiency will come from the installation of a TPB-coated teflon light tube.Moreover, the PMTs will be located much closer to -4 -the electroluminiscence region.Second, the DAQ system is being updated to trigger on the sum of all channels and increase trigger efficiency in the low-light regime.Third, the drift region is being enlarged from 2 cm to ∼10 cm, to increase the rate of interacting events within the active volume, (figure 3).The combination of these changes is expected to allow for few-keV efficient triggering and an extensive data-taking period is anticipated to start in early 2024.
Once the light collection efficiency has been maximized and the system has been fully understood, a thorough nuclear recoil response characterization campaign will take place.Three different neutron sources will be employed to calibrate the detector at different neutron energies.First, a 252 Cf source will be used to evaluate neutron response and investigate electron recoil/nuclear recoil discriminators.A lead brick located in front of the source will block  emissions (figure 3).Second, quasi monochromatic neutron recoil signals will be obtained using a neutron gun and a backing detector with n- discrimination.The backing detector selects neutrons scattered at a given angle.The recoil energy can be calculated from the angle and neutron initial energy.Finally, 88 Y/Be and 124 Sb/Be photoneutron sources will be used to produce neutrons of ∼152 keV and ∼24 keV respectively and study their recoils in the gas.

Conclusions
The ESS will be the largest low-energy neutrino source in the near future.As such, it has the potential to be the leading facility for CENS detection.The GanESS experiment will deploy a high pressure noble gas time projection chamber to take advantage of the golden opportunity provided by the ESS.With ∼20 kg of Xe several thousands of CENS events per year are expected in GanESS, which will allow to tackle several of the physics topics unlocked by CENS detection.
A strong R&D program focused in understanding the low-energy response of noble gas time projection chambers is being developed to guarantee the success of GanESS.A small prototype, GaP, has been built to characterize the technique in that energy regime, with a strong focus on understanding the quenching factor of the different noble gases that will be used in GanESS: Ar, Kr and Xe.GaP is in early commissioning and a series of upgrades focused on increasing the light collection and maximizing the detection efficiency will be performed with the goal of providing first physics results during 2024.

Figure 1 .
Figure 1.Concept of the GanESS detector: a symmetric detector with two active regions of ∼30 cm length by ∼60 cm diameter.Two sensor planes equipped with photomultiplier tubes will provide large collection efficiency.

Figure 2 .
Figure 2. Left: estimated CENS number of events as a function of the nuclear recoil energy () in GanESS (20 kg xenon) installed 20 m away from the ESS target for 3 years data-taking.Right: reproduced from [8], with permission from Springer Nature.Increase in sensitivity to non-standard neutrino-quark interactions parameters   and   [2].The exclusion region for a gas TPC detector operating with two different nuclei (separate runs, each of them for 3 years) is shown, as well as for a configuration where the detector is filled with each of the two gases during half of the total data taking period (1.5 years running with 132 Xe, 1.5 years with 40 Ar).The dashed lines show the allowed regions at 90% CL, as obtained in[2] using COHERENT data.

Figure 3 .
Figure 3. Left: close-up of the GaP TPC.The full body of the TPC is shown prior to mounting the PMTs and TPB-coated window (between PMTs and upper mesh).Right: designed setup for 252 Cf calibration.The source will be located at the bottom of the vessel with a lead brick just above it.Although not shown in the image, a PTFE teflon light tube will be installed.

Figure 4 .
Figure 4. Waveform of an 241 Am event acquired in GaP using 5 PMTs (each channel represented by a coloured line).