Recent developments in fundamental physics have stressed the importance of experiments carried out without accelerators on rare events, like the interactions of neutrinos or other slightly interacting particles or rare nuclear decays. One of the challenges of these experiments is the reduction of the natural background coming from interactions of cosmic rays with the consequent need to install the detectors in an underground laboratory. In addition one has to reduce the background produced by natural radioactivity present in the environment or even in the detector itself. This led sometimes to unexpected developments in the instrumentation to detect tiny presences of radioactive, and sometimes even non-radioactive nuclei, at a level never reached before. This could allow for exciting applications in totally different fields such as environmental physics or archaeometry. We report here the case of Roman lead procured and analysed for the experiment, CUORE, on rare events, whose study is also very promising for investigations in archaeology.
Lead plays an important role as a material to shield against environmental radioactivity due to its high density and atomic number together with reasonable mechanical properties and acceptable cost. This role is however hindered by the unavoidable natural presence of Pb-210, which undergoes beta decay, with consequent emission of gamma and Bremsstrahlung radiation. Fortunately, since its lifetime is of 22.3 years, it has totally disappeared in ancient lead after extraction, even if it was obviously present at the time of excavation. Our search was stimulated by the discovery near the Mal di Ventre island (Oristano) of a Roman “navis oneraria magna” specialised in transport of lead and sunk near the Sardinian coast. On the basis of an agreement between the Archaeological Superintendence of Cagliari and INFN, about 1000 ingots of lead of 33 kg each were recovered. About 150 of them were given to INFN to be used as shielding material in the Gran Sasso Laboratory, with the requirement to save the beautiful Roman inscription (”cartiglio”) on their top.
The inscription (”cartiglio”) on one of the ingots. The part of the bricks that is adorned with inscriptions will be removed and conserved, whereas the remaining part will be cleaned of incrustations and melted to construct the shield for the experiment CUORE (INFN). | Recovery of one of the ingots (Credit: INFN).
In order to ensure against the presence of 210Pb we applied a very sensitive detecting method: the bolometric one. If a dielectric and diamagnetic crystal is kept at very low temperature its heat capacity becomes so low that even the tiny energy delivered in the form of heat by a charged particle increases it temperature that can be measured by a suitable “thermometer”. 210Pb decays in 210Bi and then in 210Po, which can be easily detected due to its sharp line. Lead has the nice property that it is possible to construct it with a bolometer capable to detect its own radioactivity. In our experiment we were able to show that Roman lead had a 210Pb activity of less than 4 mBq/kg, lower by almost five orders of magnitude with respect to modern lead and by two orders of magnitude with commercially available very expensive “Pb-210 free” lead.
With a part of this lead, we were able to shield CUORICINO, a pilot experiment in view to construct the much larger CUORE to search for a very rare event, the neutrino-less double beta decay, which if found would allow a measurement of the neutrino mass.
Unfortunately the amount of Roman lead obtained was not sufficient to shield CUORE and in addition we were not sure that this material, while free from 210Pb, could not contain some radioactive contamination by uranium and thorium. For this aim samples of Roman lead were recently exposed to the LENA reactor of the University of Pavia to undergo the so-called neutron activation analysis. The result was excellent: the presence of these elements was found to be less than a part over a trillion: Roman lead was found to be also as good or better than any other available lead. As a consequence of these findings, from a collaboration involving INFN and the Archaeological Superintendence of Cagliari, with the support of the Ministry of Cultural Heritage, rise the agreement to receive a further 120 ingots, sufficient in addition to the previous ones to fully shield CUORE, with the gladly accepted condition to apply our advanced instrumentation to determine the archeological origin of the Roman lead.
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About Neutrino-less Double Beta Decay
In contrast with all known fundamental particles, which have a distinct antiparticle with the same characteristics (mass and spin) but opposite electric charge, being neutral the neutrino could be a hypothetical type of fundamental particle called the Majorana particle, that is the anti-particle of itself.
The only way to gain credit for this hypothesis is to study the Neutrino-less Double Beta Decay, a rare process during which a nucleus turns into another by transforming two neutrons into protons and emitting two electrons, without any neutrinos produced. In the more common “simple” double beta decay the same transformation occurs by emitting two antineutrinos jointly with the two electrons, as required by the conservation laws. So, neutrino-less double beta decay would be only possible in the case of a neutrino and an antineutrino being the same particle (a ‘’Majorana’’ particle), occurring in this case as a repetition process (n->p+e-+anti-ν; ν(=anti-ν)+n->p+e-). The possibility to detect this rare phenomenon means both the opportunity to determine the neutrino mass and to know whether the neutrino has a singular nature or not: any answer to this last question will give certainly give a chance to a new Standard Model. | The energy spectrum with different lead shields shows the importance of the use of Roman lead as a shielding material: 1) Common lead 2) Low activity lead 3) Roman lead. The curves 1 and 2, obtained with shields of present (common lead) and of special and very expensive industrial lead (Low activity lead), are quite above the one for Roman lead.(INFN).
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The National Laboratories of Gran Sasso recently received 120 2.000-year-old archaeological lead bricks, recovered undersea 20 years ago. The Roman lead, which albeit very low original radioactivity has been reduced by approximately 100,000 times after 2000 years, is already employed to shield experiments of extreme precision such as the CUORE experiment.
