The main objective of the ISOTTA project is to create the conditions for the safe procurement (in terms of radiopurity of the final source) of a large amount (at the 100 kg – 1 ton scale) of isotopically enriched material for the performance of a next generation 0ν-DBD experiment.
Such a purchase represents a considerable investment (of the order of 10 – 100 M€, depending on the quantity and on the isotope), which is however an unavoidable step to face the challenges posed by the investigation of the inverted hierarchy region of the neutrino mass pattern.
The investment for the enriched materials needs to be timely, and should come along with the full maturation of the technologies which enable to realize a 0ν-DBD experiment at the ton scale. This necessity imposes the 0ν-DBD scientific community to create now the sophisticated tools required to validate the level of radiopurity and/or the efficacy of the purification techniques, in view of the performance of a search with the desired sensitivity. The development of these tools implies a considerable R&D activity, which will be at the centre of the present program.
The general objective stated above can be broken down in several sub-objectives:
- Overview of the isotope producers that have the capability to provide materials in an amount adequate to the realization of 0n-DBD experiments;
- Overview of possible innovative technologies for the isotope production;
- Procurement of samples of isotopically enriched materials from different producers to perform trace analysis on radioactive isotopes;
- Development of techniques and facilities for the characterization of the radiopurity of existing and/or procured samples with standard methods (nuclear and mass spectroscopy);
- Development of techniques and facilities for the characterization of the radiopurity of existing and/or procured samples through innovative methods, which prefigure at small scale the structure of future Double Beta Decay detectors;
- Direct investigation of the radiopurity of the enriched materials already cast in the form of the sources (bolometric absorbers, thin foils, crystal scintillators, semiconductor crystals for charge collection) foreseen for the future experiments;
- Development of methods to purify enriched isotopes by using combination of chemical (recrystallization) and physical (vacuum distillation, filtration, zone melting) approaches;
- A side, but not minor, activity will consist in testing the radio-purity of elements that will form the final detector, even if not coinciding properly with the source, with special attention to the surface.
The list of the isotopes under investigation will comprise 82Se, 100Mo, 116Cd, 130Te and possibly 150Nd. These materials have all a high Q-value for the Double Beta Decay transition, larger than 2.5 MeV, and are relevant for several future 0ν-DBD experiments, both in Europe (SuperNEMO, CUORE, LUCIFER, COBRA) and outside (SNO+, DCBA). Another isotope that will be analysed is 106Cd. This nuclide is one of the few interesting β+β+ emitters, thanks to the high Q-value (2770 KeV). This decay channel is important because the ratio between the two equivalent decay modes EC/EC and EC/β+ may identify the mechanism of the 0ν-DBD (neutrino mass or right-handed currents in weak interactions).
Two important isotopes (76Ge and 136Xe) will not the main subjects of our investigations, for obvious reasons: 76Ge has been already produced in large amounts with the desired radiopurity level for the experiments Heidelberg-Moscow, IGEX and GERDA, while 136Xe is or will be used in the experiments NEXT, EXO-200 and KamLAND-ZEN in gaseous or liquid form, and therefore with more straightforward and safe purification techniques. In addition, the results of the running EXO-200 experiment (which contains 200 kg of Xe enriched in 136Xe at 90%) will provide a formidable test bench whose sensitivity to radiopurity is beyond the scope of this project.
However, the proponents are persuaded that the present program is relevant for all the Double Beta Decay community, since there is a general consensus that a coordinated 0ν-DBD program imposes the study of many isotopes simultaneously, in order to provide the necessary cross-checks in case of hints for discovery.