Final Report Summary - 4TH-NU-AVENUE (Search for a fourth neutrino with a PBq anti-neutrino source)
The majority of neutrino oscillation results are explained by the three-neutrino formalism. But a bunch of anomalies in short-baseline oscillation data can be interpreted by invoking a fourth neutrino. This new state would be separated from the three standard neutrinos by a squared mass difference >0.1 eV2. This new - sterile - neutrino would not be sensitive to standard model interactions but mix with the others, with a Baseline/Energy ratio of about 1 m/MeV. Such a scenario calling for new physics beyond the standard model needs to be tested with new data. This is the purpose of the deployment of a PBq of 144Ce-144Pr next to a large liquid scintillator detector, such as KamLAND (CeLAND) and Borexino (CeSOX).
An antineutrino source uses the beta decay process to produce a non-monoenergetic neutrino spectrum. Antineutrinos allow the use of Inverse Beta Decay (IBD) as detection process. At a few MeV’s it has the advantage of a higher cross section with respect to neutrino scattering off electrons, by roughly one order of magnitude. Furthermore, the time and space coincidence between positron and neutron allow a very effective tagging of the process, leading to much easier background rejection. The main drawback is the 1.8 MeV energy threshold requiring a high Q-value beta decay. Since the period and the Q-value are strongly anticorrelated for beta decay, this requirement leads to nuclei with a period shorter than the day, preventing the effective production and use of an antineutrino source based on a single isotope. The solution relies on the use of a cascade of two beta decays, the father having a long period (a month or year scale) and the daughter having a Q-value above the IBD threshold, as high as possible to maximize the event rate.
We investigated both the CeLAND and CeSOX experiments in order to use 4 PBq of 144Ce in KamLAND and/or Borexino. Cerium was chosen because of its high Q, its 4% abundance in fission products of uranium and plutonium, and finally for engineering considerations related to its possible extraction of rare earth from regular spent nuclear fuel reprocessing followed by customized column chromatography. This can be done at PA Mayak (Russia). We overcame all technical problems related to the production of a compact >3 PBq 144Ce antineutrino generator, at a high purity level (negligible other neutron and gamma emitters).
We investigated the logistic issues (cask and routes) for transporting the Cerium antineutrino generator from the production site to the detector site. It has been a major concern due to the necessary time required to certify transport containers that we identified to be suitable for this operation. This is a drawback for deploying a 4 PBq 144Ce source in KamLAND (Japan) in the near future. Therefore the KamLAND site was not selected for realizing promptly the experiment. Since transportation to Italy is easier we concentrated all our efforts to deploy 4 PBq of 144Ce next to the Borexino detector, in collaboration with our Borexino-SOX colleagues.
The goal was to deploy the 144Ce radioisotope 8.3 m away from the Borexino detector center and to search for an oscillating pattern in both event spatial and energy distributions that would determine neutrino mass differences and mixing angles unambiguously. The source shall fit inside a <15 cm capsule, to consider the Cerium volume as a point-like source. 144Ce has a low production rate of high-energy γ rays (> 1 MeV) from which the detector must be shielded to limit background events. We designed and constructed a tungsten alloy shielding that was manufactured in China. Backgrounds are of two types, those induced by the environment or detector, and those due to the source (attenuated by a 19 cm tungsten alloy shielding). We optimized the experimental design to minimize backgrounds through comprehensive Monte Carlo simulations.
Prior to the data taking the activity of the Cerium antineutrino generator is measured via calorimetric measurements. This calibration is of prime importance for the success of the experiment. A calorimeter is currently was designed, constructed, and tested at CEA, then at the LNGS laboratory.
Safety studies were carried out to perform the CeSOX experiment in agreement with all international regulations. But early 2018 the SOX project was canceled due to the impossibility of realizing the source with the required characteristics. The source producer was eventually not able to deliver the generator with the required characteristics. Indeed the generator could not reach the required activity required to perform the experiment in a competitive way.
An antineutrino source uses the beta decay process to produce a non-monoenergetic neutrino spectrum. Antineutrinos allow the use of Inverse Beta Decay (IBD) as detection process. At a few MeV’s it has the advantage of a higher cross section with respect to neutrino scattering off electrons, by roughly one order of magnitude. Furthermore, the time and space coincidence between positron and neutron allow a very effective tagging of the process, leading to much easier background rejection. The main drawback is the 1.8 MeV energy threshold requiring a high Q-value beta decay. Since the period and the Q-value are strongly anticorrelated for beta decay, this requirement leads to nuclei with a period shorter than the day, preventing the effective production and use of an antineutrino source based on a single isotope. The solution relies on the use of a cascade of two beta decays, the father having a long period (a month or year scale) and the daughter having a Q-value above the IBD threshold, as high as possible to maximize the event rate.
We investigated both the CeLAND and CeSOX experiments in order to use 4 PBq of 144Ce in KamLAND and/or Borexino. Cerium was chosen because of its high Q, its 4% abundance in fission products of uranium and plutonium, and finally for engineering considerations related to its possible extraction of rare earth from regular spent nuclear fuel reprocessing followed by customized column chromatography. This can be done at PA Mayak (Russia). We overcame all technical problems related to the production of a compact >3 PBq 144Ce antineutrino generator, at a high purity level (negligible other neutron and gamma emitters).
We investigated the logistic issues (cask and routes) for transporting the Cerium antineutrino generator from the production site to the detector site. It has been a major concern due to the necessary time required to certify transport containers that we identified to be suitable for this operation. This is a drawback for deploying a 4 PBq 144Ce source in KamLAND (Japan) in the near future. Therefore the KamLAND site was not selected for realizing promptly the experiment. Since transportation to Italy is easier we concentrated all our efforts to deploy 4 PBq of 144Ce next to the Borexino detector, in collaboration with our Borexino-SOX colleagues.
The goal was to deploy the 144Ce radioisotope 8.3 m away from the Borexino detector center and to search for an oscillating pattern in both event spatial and energy distributions that would determine neutrino mass differences and mixing angles unambiguously. The source shall fit inside a <15 cm capsule, to consider the Cerium volume as a point-like source. 144Ce has a low production rate of high-energy γ rays (> 1 MeV) from which the detector must be shielded to limit background events. We designed and constructed a tungsten alloy shielding that was manufactured in China. Backgrounds are of two types, those induced by the environment or detector, and those due to the source (attenuated by a 19 cm tungsten alloy shielding). We optimized the experimental design to minimize backgrounds through comprehensive Monte Carlo simulations.
Prior to the data taking the activity of the Cerium antineutrino generator is measured via calorimetric measurements. This calibration is of prime importance for the success of the experiment. A calorimeter is currently was designed, constructed, and tested at CEA, then at the LNGS laboratory.
Safety studies were carried out to perform the CeSOX experiment in agreement with all international regulations. But early 2018 the SOX project was canceled due to the impossibility of realizing the source with the required characteristics. The source producer was eventually not able to deliver the generator with the required characteristics. Indeed the generator could not reach the required activity required to perform the experiment in a competitive way.