Periodic Reporting for period 1 - BOLD (A background-free experiment to discover the nature of neutrinos based on single Barium Atom Light Detection)
Okres sprawozdawczy: 2021-03-01 do 2022-08-31
We know that the Universe is made almost exclusively of matter. However, the Big Bang theory predicts that the early Universe contained the same amount of matter and antimatter particles. This prediction is consistent with the “small Big Bangs” that form in proton collisions at CERN's giant LHC accelerator, where a symmetrical production of particles and antiparticles is always observed. A possible mechanism points to the existence of heavy neutrinos that were its own antiparticle, and therefore, could decay into both matter and antimatter. After all the matter and antimatter in the Universe were annihilated (with the exception of a small excess), the result would be a cosmos made only of matter.
It is possible to demonstrate that the neutrino is its own antiparticle by observing a rare type of nuclear process called neutrinoless double beta decay. This process can occur in some rare isotopes, such as xenon-136. The NEXT experiment looks for these decays using high pressure gas chambers.
So far, NEXT was focused on observing the characteristic signal emitted by the two electrons resulting in the mentioned decay, but this signal is extremely weak and could be eventually masked by the background noise due to the ubiquitous natural radioactivity. If in addition to observing the two electrons, the barium ionized atom, which results from xenon disintegration, is detected, we would have the unequivocal signal we are looking for, and the experimental evidence that the neutrino is indeed its own antiparticle.
The possibility was proposed by David Nygren in 2016 and, in 2020, a collaboration between Fernando Cossio and Juan José Gómez Cadenas, published in the prestigious journal Nature, managed to demonstrate that it is possible to capture the barium atom with a molecule capable of forming a supramolecular complex with it and to provide a clear signal when this occurs.
The goal of the Synergy-2020 NEXT-BOLD project is to design, develop and build a new generation of the NEXT detector with the capability to detect the barium ion, based on a molecular fluorescent indicator and advanced microscopy techniques. This experiment would have a great potential to discover if the neutrino is its own antiparticle, which would allow to answer the fundamental questions about the origin of the universe.
It is possible to demonstrate that the neutrino is its own antiparticle by observing a rare type of nuclear process called neutrinoless double beta decay. This process can occur in some rare isotopes, such as xenon-136. The NEXT experiment looks for these decays using high pressure gas chambers.
So far, NEXT was focused on observing the characteristic signal emitted by the two electrons resulting in the mentioned decay, but this signal is extremely weak and could be eventually masked by the background noise due to the ubiquitous natural radioactivity. If in addition to observing the two electrons, the barium ionized atom, which results from xenon disintegration, is detected, we would have the unequivocal signal we are looking for, and the experimental evidence that the neutrino is indeed its own antiparticle.
The possibility was proposed by David Nygren in 2016 and, in 2020, a collaboration between Fernando Cossio and Juan José Gómez Cadenas, published in the prestigious journal Nature, managed to demonstrate that it is possible to capture the barium atom with a molecule capable of forming a supramolecular complex with it and to provide a clear signal when this occurs.
The goal of the Synergy-2020 NEXT-BOLD project is to design, develop and build a new generation of the NEXT detector with the capability to detect the barium ion, based on a molecular fluorescent indicator and advanced microscopy techniques. This experiment would have a great potential to discover if the neutrino is its own antiparticle, which would allow to answer the fundamental questions about the origin of the universe.
During the initial period of the project the activities have been focused on the development of the essential components that will allow to build the Ba tagging system (Figure 1 illustrates how these components will be integrated in the detector):
1) Development of Fluorescent Bicolor Indicators (FBI sensors). Several types of organometallic compounds have been developed. These molecules will constitute the core of the chemical sensor, as their fluorescence spectra will change when trapping a Ba2+ ion. The engineering aims to obtain molecules where the fluorescence peaks of chelated and non-chelated species are shifted as much as possible, so that the background emission of molecules which have not trapped the Ba2+ ion does not totally overcome the signal of the single molecule which has trapped the Ba2+. To this respect, one of the molecules already developed (identified as G2) displays spectral features which allow for a first realization of the sensors. Important advances have also been done in defining a unified synthetic scheme for functionalisation of the surfaces, so that the FBI molecules can be attached to the sensor surface. This work has been carried out by EHU.
2) Fundamental understanding of the sensor physics and, in particular, how the molecules interact with the sensor substrate, and how this affects the ion capture mechanism and the fluorescence spectra of the FBI. This has been possible thanks to the application of complementary surface science techniques (X-ray Photoemission Spectroscopy, Surface Tunnelling Microscopy, and electronic structure calculations) and has led to the demonstration of the chelation in vacuum of FBI indicators by Ba2+ ions, once sub-monolayers of molecules are deposited onto a technologically relevant substrate. This achievement (described in a landmark paper, already accepted for publication by Nature Communications, whose preprint can be found at arXiv:2201.09099) constitutes a major milestone, as it validates the concept and enables the immediate application to the ultra-low background ββ0ν detection experiment. This work has been carried out by MPC and DIPC.
3) Development of subsystems for the detector, and in particular an optical set up for single molecule fluorescence characterisation, a Ba ion source and a robotic arm adapted to work inside the high-pressure Xe vessel. The optical set up has been built under normal atmospheric conditions, and will be subsequently adapted to work in vacuum. The Ba source will also be part of the vacuum set up, which will allow to very limited amount of Ba2+ ions onto the sensor, in order to validate detection of fluorescence signals emitted by a single chelated molecule. Since the development of a sensor with a surface equal to that of the Xe detector plane is not economically nor technologically viable, the proposed alternative solution is to implement a robotic arm that could displace a smaller surface sensor towards the zone where the Ba2+ is predicted to land (this can be done once the double-beta signal is detected in the, opposite, tracking plane, as described in the Figure). Several conceptual designs have been considered, and the use of several cartesian robot arms has been deemed to be the best adapted for the final implementation. This work has been carried out by BGU, DIPC and UV.
4) Development of the Energy-Tracking Detector prototype (pETD). A simulation framework has been set up for studying the potential performances of a detector made of only silicon photomultipliers, ensuring that the electronics requirements and the physics requirements are reachable and identifying potential constraints. A wide range of simulations for several different detector geometries has been carried out in order to identify the best configurations for a pETD. Two fundamental elements of the system (the electronics readout for the pETD, and a new meta-lens for 175nm wavelength) have been developed, and their performance is currently being evaluated. This work has been carried out by HARVARD, UMAN and UPV.
1) Development of Fluorescent Bicolor Indicators (FBI sensors). Several types of organometallic compounds have been developed. These molecules will constitute the core of the chemical sensor, as their fluorescence spectra will change when trapping a Ba2+ ion. The engineering aims to obtain molecules where the fluorescence peaks of chelated and non-chelated species are shifted as much as possible, so that the background emission of molecules which have not trapped the Ba2+ ion does not totally overcome the signal of the single molecule which has trapped the Ba2+. To this respect, one of the molecules already developed (identified as G2) displays spectral features which allow for a first realization of the sensors. Important advances have also been done in defining a unified synthetic scheme for functionalisation of the surfaces, so that the FBI molecules can be attached to the sensor surface. This work has been carried out by EHU.
2) Fundamental understanding of the sensor physics and, in particular, how the molecules interact with the sensor substrate, and how this affects the ion capture mechanism and the fluorescence spectra of the FBI. This has been possible thanks to the application of complementary surface science techniques (X-ray Photoemission Spectroscopy, Surface Tunnelling Microscopy, and electronic structure calculations) and has led to the demonstration of the chelation in vacuum of FBI indicators by Ba2+ ions, once sub-monolayers of molecules are deposited onto a technologically relevant substrate. This achievement (described in a landmark paper, already accepted for publication by Nature Communications, whose preprint can be found at arXiv:2201.09099) constitutes a major milestone, as it validates the concept and enables the immediate application to the ultra-low background ββ0ν detection experiment. This work has been carried out by MPC and DIPC.
3) Development of subsystems for the detector, and in particular an optical set up for single molecule fluorescence characterisation, a Ba ion source and a robotic arm adapted to work inside the high-pressure Xe vessel. The optical set up has been built under normal atmospheric conditions, and will be subsequently adapted to work in vacuum. The Ba source will also be part of the vacuum set up, which will allow to very limited amount of Ba2+ ions onto the sensor, in order to validate detection of fluorescence signals emitted by a single chelated molecule. Since the development of a sensor with a surface equal to that of the Xe detector plane is not economically nor technologically viable, the proposed alternative solution is to implement a robotic arm that could displace a smaller surface sensor towards the zone where the Ba2+ is predicted to land (this can be done once the double-beta signal is detected in the, opposite, tracking plane, as described in the Figure). Several conceptual designs have been considered, and the use of several cartesian robot arms has been deemed to be the best adapted for the final implementation. This work has been carried out by BGU, DIPC and UV.
4) Development of the Energy-Tracking Detector prototype (pETD). A simulation framework has been set up for studying the potential performances of a detector made of only silicon photomultipliers, ensuring that the electronics requirements and the physics requirements are reachable and identifying potential constraints. A wide range of simulations for several different detector geometries has been carried out in order to identify the best configurations for a pETD. Two fundamental elements of the system (the electronics readout for the pETD, and a new meta-lens for 175nm wavelength) have been developed, and their performance is currently being evaluated. This work has been carried out by HARVARD, UMAN and UPV.
All concepts of detectors for double-beta decay experiments currrently being developed face the problem of discriminating the extremelly rare signal among the tenths of thousands of similar signals which are due to other processes taking place in the detector (impact of muons, decay of radioactive impurities in the materials, etc). These signals are typically named as background. The identification of the ion resulting from the double beta decay provides an univoque confirmation of the event. This is an inmensely challenging task as it concerns detection of a single ion. The work carried out so far has validated the concept of using a chemical sensor where molecules are engineered to trap this ion, and to show a large change in their fluorescence spectra when this happens.