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SOX: search for short baseline neutrino oscillations with Borexino

The Borexino detector has convincingly shown its outstanding performances in the low energy regime through its accomplishments in the observation and study of the solar and geo neutrinos. It is then an ideal tool to perform a state of the art source-based experiment for testing the longstanding hypothesis of a fourth sterile neutrino with ~ eV2 mass, as suggested by several anomalies accumulated over the past three decades in source, reactor, and accelerator-based experiments. The SOX project aims at successively deploying two intense radioactive sources, made of Cerium (antineutrino) and Chromium (neutrino), respectively, in a dedicated pit located beneath the detector. The existence of such an ~ eV2 sterile neutrino would then show up as an unambiguous spatial and energy distortion in the count rate of neutrinos interacting within the active detector volume. This article reports on the latest developments about the first phase of the SOX experiment, namely CeSOX, and gives a realistic projection of CeSOX sensitivity to light sterile neutrinos in a simple (3+1) model.


Introduction
The standard three-neutrino oscillation paradigm, associated with small squared mass splittings ∆m 2 0.1eV 2 , has been successfully built up over the past decades with various techniques using solar, atmospheric, long baseline and medium baseline reactor neutrino experiments. However, this well-established picture might suffer from anomalous results both reported in the ν e appearance channels [1]. Those anomalies could jointly be interpreted as the existence of at least an additional fourth sterile neutrino, with mixing parameters ∆m 2 new 0.1eV 2 and sin 2 (2θ new ) ∼ 0.1. The goal of the SOX experiment [2] is to definitively address the anomalous deficits reported in the detected neutrino count rates at short baselines in the the Gallium neutrino [3,4,5] and reactor antineutrino experiments [6], respectively. It consists in deploying an intense MeV neutrino source beneath the Borexino detector, and searching for both meter-scaled spatial and energy oscillation patterns in the detected neutrino spectrum, pushing the concept of neutrino "oscillometry" a step further compared to any previous neutrino oscillation experiments [7].
2. CeSOX: the first phase of the SOX experiment The first phase of the SOX experiment will make use of an intense antineutrino generator (CeANG) deployed at the Borexino detector at the end of 2016. As such,ν e will be detected through the inverse beta decay (IBD) reaction, which has a 1.8 MeV energy threshold and is almost free of any backgrounds.

The Borexino detector
The Borexino detector [8] is an ultra low background neutrino detector, initially designed for the observation and study of solar and geo neutrinos. It is located deep underground under a 3800 meter water equivalent overburden at the Laboratori Nazionali del Gran Sasso (LNGS). As shown by figure 1, the detector conceptual design relies on nested spherical concentric volumes, which protect and gradually shield the innermost detection volume against radioactivity and cosmic ray particles. Going inward, the first sub-volume is contained in a stainless steel tank (R=9 m) and is filled up with ultrapure water acting both as a radioactivity shielding and a cosmic muon veto. The second sub-volume is contained in a stainless steel sphere (R=6.85 m), equipped with 2214 PMTs viewing the innermost central part of the detector, and is filled up with non-scintillating and transparent oil, also acting as a shielding against radioactivity. Finally, the third sub-volume is the inner detection volume (R=4.25 m) and is made of 300 tons of liquid scintillator contained in a transparent nylon vessel. The detector energy and vertex reconstruction performances are respectively 5% at 1 MeV and 15 cm. A detailed discussion about the choice of the best pair of radioactive isotopes for making aν e generator in a source-based experiment can be found in [9].

Source related technical challenges and requirements
The production and transport of a 144 Ce-basedν e generator up to LNGS in Italy is technically and administratively challenging. This section summarizes the most important technical milestones that have to be completed to successfully perform phase 1 of the SOX experiment.

CeANG production
The production of the CeANG can be realized by isolating and extracting 144 Ce from spent nuclear fuel (SNF). The Russian SNF reprocessing company "Federal State Unitary Enterprise Mayak Production Association", hereafter PA Mayak, has been identified to be the only facility able to deliver such a source with PBq scale activity. Cerium separation and extraction, together with final source packaging and certification, takes several months. The first step of the Cerium extraction is a standard reprocessing of SNF, called the Purex® process, leading to extraction of Uranium, Plutonium and Neptunium and formation of a so-called a Purex raffinate which consists of fission and corrosion products, as well as lanthanides and actinides. In a second step, the Purex raffinate is purified from Strontium, Cesium and chemical elements after corrosion; and a feed solution consisting of lanthanides and actinides is formed after a double oxalate precipitation procedure. In a third step, Cerium fraction is separated from the feed solution using a complexing displacement chromatography method (see [9] and references therein). The final stage of the source production consists in calcining, pressing and inserting into a stainless steel capsule the Cerium oxyde. The final product will be made of a few kilograms of CeO 2 containing a few tens of grams of 144 Ce. The source delivery will happen in December 2016.

CeANG shielding
The identified 144 Ce − 144 Pr beta decaying pair for making the CeANG also emits gamma rays, that have to be shielded against both for biological protection purposes and to avoid sourceinduced backgrounds in the Borexino detector. The shielding material and dimensions are mostly driven by the 2.185 MeV gamma ray emitted with 0.7% intensity. Tungsten heavy alloys offer the best compromise among all the high density materials. A cylindrical tungsten heavy alloy shielding, with density larger than 18.0 g cm −3 , 60 cm height and diameter, has been designed and is currently being manufactured at Xiamen Honglu Inc., China. Such dimensions ensure a 19-cm thick shielding and a 10 12 gamma-ray attenuation factor along any directions. The corresponding radiation dose at contact for a 100 kCi (3.7 × 10 15 Bq) source is less than 1 µSv/h, compelling with the safety regulations imposed by the International Agency for Atomic Energy (IAEA), French nuclear safety authority (ASN), as well as Italian nuclear safety authority for radioactive material transportation and handling. The shielding will be delivered early 2016.

CeANG transportation and deployment at LNGS
As for any radioactive material transportation, a certified container has to be used to ship the CeANG from PA Mayak up to LNGS. The so-called TN-MTR cask, jointly developed by CEA Le Havre up to LNGS. The complete transportation solution takes less than a month, and will limit the source activity loss below 5%. The expected delivery date at LNGS is end of December 2016.

CeANG characterization
A full and precise CeANG characterization is a fundamental requirement to achieve the best sensitivity over the light sterile neutrino mixing parameter space preferred by the short baseline anomalies. This section only deals with the CeANG activity measurement and the shape of the beta/neutrino spectrum. For the interested reader, reference [9] discusses other source specifications, such as source radioactive impurity content, which is especially important to control to avoid source-induced backgrounds in the SOX experiment.
3.4.1. Activity measurement A significant gain in the SOX sensitivity can be achieved if the source activity is known within O(1%) precision (see section 4 for more details). Two calorimetric devices are currently being built and calibrated at CEA-Saclay and TUM/Genova, respectively, for measuring the heat power released by the CeANG. Although slightly differently designed, those two calorimeters work on the same principle, which consists in measuring the temperature in and out of a water loop circulating around the source. The temperature difference is related to the heat power released by the source, and hence on its activity through a power-to-activity conversion factor P = 215.6 ± 1.3 W/PBq (itself depending on the shape of the 144 Ce and 144 Pr beta spectra, see next subsection). Each calorimeter device has been carefully designed to minimize leaks, by using supsension platforms + insulations materials to limit conductive losses, a vaccum vessel to prevent any convection losses and a multilayer insulation + vaccum vessel thermalization system to avoid heat radiation. Characterization and calibration of these setups are currently on-going, using electrical sources and an Aluminium mock-up shielding.

Beta spectrum measurement
The precision of the source activity measurement and the expected IBD interaction rate within the Borexino detector strongly depends on the shape of the CeANG beta spectrum. 144 Ce and 144 Pr beta spectra both present non-unique forbidden transitions, for which the spectral shape theoretical predictions are uncertain at the few % level. Furthermore, past beta spectrum measurements show discrepancies at the 10-15% level, which translates into a 10% uncertainty on the predicted IBD rate and would then spoil the calorimetric measurement. New measurements of the 144 Ce and 144 Pr beta spectrum shapes are thus necessary. Two setups, which use different techniques, have been developed for the 144 Ce and 144 Pr beta spectra measurement. A first setup uses a cylindrical plastic scintillator coupled to two high quantum efficiency photo-multiplier tubes, whereas a second setup uses a multiwire chamber in coincidence with a plastic scintillator to measure beta rays and suppress gamma-ray background. These setups are currently running and been calibrated at CEA-Saclay. A 5% uncertainty on the shape of the 144 Ce and 144 Pr beta spectra is expected, improving the prediction of the IBD rate within the desired specifications.

Projected sensitivity of CeSOX
The projected sensitivity of the CeSOX experiment to sterile neutrinos in the framework of a (3+1) model is shown on figure 2, in light of the two source-related systematic uncertainties discussed in the previous section. The baseline scenario for the sensitivity calculations is a 100 ν e disappearance channel anomalies is covered at 95% C.L. Figure 2 also illustrates the gain in sensitivity by using the rate information brought by an activity measurement (solid curves on the left panel of figure 2) additionally to the shape information (dashed curve on the left panel of figure 2). The right panel of figure 2 shows the influence of the 144 Pr beta spectrum shape uncertainty, and confirms that a 5% precision for the beta spectrum measurement can be targeted without significantly degrading the sensitivity contours.

Systematics related to the⌫ e source
Source activity: • needed for rate analysis

Conclusions
The SOX experiment, which aims at testing the neutrino count rate anomalous deficits observed in short baseline experiments in the ν e disappearance channel, will soon start the first phase of its science program in December 2016. A PBq scale activityν e source, made of 144 Ce, will be deployed underneath the Borexino detector . In this article, the most important milestones of the CeSOX project have been reviewed, with a special emphasis on the technical challenges that have to be addressed for the source production and transportation. Last but no least, a full characterization of the CeANG is simultaneously on-going in the SOX collaboration, which aims at minimizing the source-related systematic uncertainties. In light of these studies, it has been shown that the projected CeSOX sensitivity covers most of the light sterile neutrino parameter space preferred by the short baseline anomalies.