2.000 years-old Roman Lead for physics Print E-mail

ImageThe 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.


Recovery of one of the ingots /Credit: 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.

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.
2.000 years-old Roman Lead for physics
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).

Submitted by Ettore Fiorini, INFN-Milano Bicocca
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