The ultimate goal of this project is to build a proof-of-concept of the BOLD apparatus (e.g a detector which includes a molecular target capable of detecting the Ba2+ ion emitted in the double beta decay of Xe-137. To achieve this objective, the project in several parallel projects, namely: a) development and optimisation of molecular sensors; b) development and characterisation of the molecular target; c) development of the laser system and microscopy to scan the molecular target; d) development of the steering system(s) to bring the Ba2+ ion to the target(s) area; e) development of the host detector (a NEXT-like TPC), and f) development of the delayed trigger, to ensure that the two electrons and the Ba2+ ion emitted in the double beta decay are observed in (delayed) coincidence.
The development of the project, so far, has advanced each one of the lines outlined above, as we briefly outline below:
\begin{enumerate}
\item {\em Development and optimisation of molecular sensors}: Our baseline are the so-called 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 realisation 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.
\item {\em Development and characterisation of the molecular target}. One of the cutting edge technological developments of this project is the development of {\em molecular targets}, that is arrangement of sensors immobilised in a suitable support, which need to operate in a dry atmosphere (gas xenon at high pressure). Many different elements need to be studied here, including choice of the support, layer density, functionalisation technique to fix the molecules to the substrate, control of surface backgrounds, etc. Extensive work has been conducted by UPV/EHU (functionalisation, study of different sensors), CFM (surface characterisation of sensors), and DIPC (characterisation of the molecular targets via SMFI studies).
\item {\em Laser and microscopy}. This is another major challenge, since the need to detect Single Molecule Fluorescence Indicators (SMFI) imposes using high numeric aperture objectives which need, therefore, to operate at high pressure. At the same time, fast, wide-field microscopy is needed to scan in a reasonable time relatively large areas. The microscopy project is being developed by DIPC, which is developing timing and wide-field vacuum microscopy (vacuum or light gas pressure are dry media, equivalent from the optical point of view to pressurised noble gas) and UTA, which has developed a high-pressure system.
\item {\em Steering systems}. The expected impact position of the Ba2+ ion can be predicted from the reconstruction of the event topology with a resolution of about 1 cm. A prior, one could instrument the cathode of the BOLD detector with molecular targets, and then move a microscope to the impact region and scan a region of about 1 cm2 searching for a chelated molecule. In practice, none of this possibilities is realistic. The cathode surface of the final BOLD detector will be of the order of 4 m2, too large to be fully covered by molecular targets, and scanning a region of 1 cm2 would take too much time.
The first problem can be solved by developing a movable system which is able to bring a molecular target to the region of predicted impact. The second problem can be tackled by developing concentrators, such as the so called RF carpets, able to focus the incoming ion from 1 cm2 to a surface area of 1 mm2 or less.
Both solutions are technologically difficult. DIPC is working in the movable system, but the full development of such system requires firs the construction of the baseline BOLD demonstrator. UTA is leading the development of RF carpets with initial, promising results.
\item {\em Host Detector}. The full Barium Tagging system must be integrated in a host detector. DIPC and Manchester University (MU) are collaborating to develop such detector, that we call BOLD-DEMO. The detector is a xenon TPC, which includes four major subsystems. a) a Barrel Fibre Detector (BFD), that provides a fast measurement of the energy of the system (from the primary scintillation in the gas), b) a Dense Silicon Plane (DSP), that provides an accurate measurement of the energy of the system and permits the reconstruction of the topology; c) The Ba2+ sensor, integrated in the cathode; and d) the delayed trigger that uses the information provided by the BFD and DSP to provide a trigger and a prediction of the impact point and impact time of the Ba2+ ion. BOLD-DEMO is currently under construction. In parallel to this effort, MU, in collaboration with the University Polytechnic of Valencia (UPV) is developing an ASIC for the readout of the DSP (a must for the delayed trigger).
\item {\em Proof of concept}. Two additional systems, called SABRA and RITA are being developed as a collaboration between the three main groups (DIPC, UPV/EHU and MU) and BGU in Israel. The SABRA system is based on a Ba2+ gun capable of sending thermal ions to a test molecular target. The Ba2+ gun is already commissioned and in initial operation. The RITA system is based on sending Ra2+ ions to a molecular target. The Ra2+ ions, produced by a radioactive source, are excellent proxies of Ba2+ and will permit the calibration of the future BOLD detector. Both systems are expected to operate in 2025.
\end{enumerate}
