Archaeology vs. Physics: Conflicting roles for old lead

The study of archaeology has long been carried out using tools from the physics lab. Among these are carbon-14 dating, thermoluminescence dating, x-ray photography, x-ray fluorescence elemental analysis, CAT and MRI scanning, ground-penetrating sonar and radar, and many others. What is less well known is that archaeology has also made substantial contributions to physics. This is the story of old lead; why it is important to physics, and what ethical problems it presents to both sciences.

An ancient shipwreck

In about 50 AD, a ship set sail from Cadiz in Spain carrying cargo to Italy (probably to Rome). Having passed through the Straits of Gibraltar, the ship hugged the coastline, a course wholly different from the usual open-sea direct course that would normally be taken. The ship sank in 25 m (82 ft) of water not far off the coast of Villajoyosa, about 15 km (9 mi) NE of Alicante in Spain, after perhaps 500 km (300 mi) of sailing. Its cargo included hundreds of amphorae of garum (the Roman version of Worcestershire sauce) and about two thousand bars of lead each weighing about 33 kg (52 lb). When discovered in 2000, the remnants of the 36 m (120 ft) long ship were named the Bou Ferrer shipwreck.

The expense of examining such a wreck using proper archaeological techniques was considerable. When Ettore Fiorini, a nuclear physicist at the University of Milan-Bicocca, read about the find, he offered the National Archaeological Museum of Cagliari in Sardinia the financial support of the Italian National Institute for Nuclear Physics (INFN) in excavating the vessel and its cargo. In return, a portion of the antique lead (amounting to less than 15 percent, or about 9 metric tons) would be turned over to INFN for use in physics experiments. Archaeological support always being excruciatingly tight, Cagliari agreed to the bargain.

What's all this about lead, then?

With regular lead available at about US$2,000 per metric ton, supporting a major archaeological operation might seem like a very expensive way to purchase lead. But therein lies the rest of the story.Lead, offering as it does a convenient combination of density and formability, is the first line of defense for radiation shielding. However, newly smelted lead contains a radioactive lead isotope, Pb-210, which is generated in the decay of U-238. While the uranium and other radioactive elements are largely removed during the smelting process, the Pb-210 remains, producing a low-level radioactive decay (about 200 decays per kilogram per second) that restricts the ability of the most sensitive nuclear and particle physics experiments to function.

Pb-210, however, has a 22.3 year half-life. When lead bars have lain underwater for 2,000 years, all of the Pb-210 has decayed, leaving "Roman lead" (or old lead) with a radioactive level roughly 100,000 times lower than is found in new lead.

The use of old lead for shielding increases the sensitivity of our most delicate experiments by orders of magnitude, an increase that is crucial when looking for a reaction that sheds light on new physics. Lead recovered from roofs, old plumbing, and even stained glass windows has been used, but Roman lead from a shipwreck is the best you can find.

The Standard Model, neutrinos, and new physics

Physics, at its heart, is the struggle to understand how the Universe works. At this moment in time, the vast majority of experimental data is consistent with the predictions of the Standard Model. However, there are definite signs that the Standard Model is incomplete. One of these is the discovery of neutrino oscillations, from which one can infer that neutrinos must have a (very small) mass, and hence travel slower than light. Why does this matter? All known neutrinos have a left-handed spin, and all known antineutrinos have a right-handed spin, as shown by numerous experiments that show parity violation. This appears to be the only difference between neutrinos and antineutrinos.

In relativity theory, if you move faster than a left-handed object, it can look like a right-handed object. Applied to neutrinos, this essentially allows you to turn a neutrino into an antineutrino (and vice versa) depending on how fast you run. This would represent a huge violation of the foundations of relativity, which state that a physical event does not depend on who is observing it. What actually happens in the world isn't supposed to depend on whether or not you stand on your head.

A somewhat less drastic option than throwing away relativity would be if neutrinos were Majorana fermions, particles that are their own antiparticles. Although no fundamental particle is currently known to be a Majorana fermion, there do exist certain collective excitations in solids that behave in this manner. Essentially what you get here is that a neutrino is now a superposed mixture of a left-handed and a right handed particle, and they flip back and forth at a rate determined by their mass. The principles of relativity are now satisfied, as our rapidly moving observer now sees a neutrino regardless of his speed.

On the prowl for Majorana neutrinos

The most likely experiment to test if neutrinos are Majorana fermions or not is to search for a type of nuclear decay called neutrinoless double beta decay (NDBD).

Beta decay is the emission of an electron (or a positron) from an atomic nucleus, as shown in the upper left figure above. Double beta decay is also a well-known mode of nuclear decay, in which a pair of beta decays occur at once, converting, for example, a Tellurium-130 atom into a doubly-charged Xe-130 ion, two free electrons, and two electron antineutrinos. Because of the number of particles that must cooperate to get this decay, it is very rare. In fact, the half-life of Te-130 is about a billion trillion years.

Neutrinoless double beta decay is essentially the same process, but can only occur if the neutrino is its own antineutrino. In this case, two neutrons turn into two protons, two electrons, and two electron antineutrinos, but then the antineutrinos mutually annihilate through the interaction that gives the antineutrinos mass, so that in the end no neutrinos or antineutrinos are emitted. If this occurs, the total energy released by the NDBD (2.527 MeV) is contained in the kinetic energy of the product Xe-130 nucleus and the beta rays emitted therefrom, and will quickly be absorbed by the surrounding material in the form of heat.

Past experiments have not observed NDBD, but have established that its half-life is greater than a few ten trillion trillion years, more than 10,000 times longer than the half-life typical of double beta decay. The reason for this lengthy half-life is that the chances for two antineutrinos to annihilate within an atomic nucleus, even if they are Majorana fermions, is very small.

If neutrinos are Majorana fermions, then NDBD will occur at some rate, and not otherwise, making a search for NDBD a good thumbs up-thumbs down experiment for Majorana neutrinos. Even a null result can be used to gain an upper limit on the electron neutrino mass, which combined with other experimental results might let Majorana neutrinos be ruled out.

Ignoring everything else

The expected rate of NDBD reactions in presently planned experiments is measured in no more than a few events per year – perhaps as low as a few events per decade. The biggest problem in detecting such rare events is to adequately shield the detectors against spurious signals; if background radiation triggers the detectors ten times a second, it becomes difficult to detect a real signal that may occur once or twice a year. Most such experiments (NDBD, proton decay, dark matter detection, and the like) are carried out far underground, so that a huge mass of earth and rock shields the detectors against cosmic rays, and the decay of isotopes formed by interaction of the cosmic rays with nuclei in the soil.

The Cryogenic Underground Observatory for Rare Events (CUORE) is an NDBD experiment currently being built at the Italian Gran Sasso National Laboratory. It is located beneath 1,500 m (5,000 ft) of granite that absorbs all but one in a million of the cosmic rays that try to pass through the Gran Sasso mountain. The goal of the CUORE shielding is to reduce spurious detection events which might be confused with NDBD events to a level of a few in the entire apparatus per year. This will allow detection of NDBD with a half-life as long as a few hundred trillion trillion years, about ten times longer than the previous limit.

While Gran Sasso itself does an excellent job of shielding against cosmic radiation, the granite of the mountain is itself radioactive, containing traces of uranium, thorium, and potassium-40. This (particularly the radon gas from the decay chains of the uranium and thorium) is a source of radioactive contamination and background radiation against which the experimental facilities must be protected.

It is also necessary to construct the particle detectors so that they do not contain materials with low levels of radioactivity. You may want to rethink a decision to mount detector crystals in a carbon fiber structure when remembering that the carbon-14 in a kilogram of carbon emits about 200 beta particles per second.

The CUORE detector consists of about 750 kg (1,650 lb) of TeO2 crystals, which are housed in a container measuring about 40 cm (16 in) in diameter and 100 cm (39 in) tall. The detector is surrounded by a 3 cm (1.2 in) thick layer of Roman lead, and two 10 cm (3.9 in) thick layers of lead, an inner layer of low-activity (but not archaeological) lead and an outer layer of new lead. This arrangement is designed to make as effective use of the Roman lead as possible, allowing the experiment to be carried out using only about half a metric ton of lead. This experiment can't be done without old lead.

Back to the shipwreck

Where do you find an old lead shop? Most of the old lead being used by physicists is archaeological in origin, and hence protected by law and treaty. The cream of the crop for old lead is from shipwrecks, as the water covering them also filters out much of the cosmic radiation that can form additional radioisotopes within the lead. Given that protecting our cultural heritage and seeking new knowledge are both important, what does one ethically decide when the two are in conflict? Do you destroy relics to gain new information about the workings of the Universe, or preserve the relics at the cost of such knowledge?

One must acknowledge that both sides of this conundrum are sensitive to the issues being raised by the other. The story of CUORE is a good illustration of this.

The lead bars from the Bou Ferrer shipwreck were not simply melted down and recast. Rather, considerable precautions were taken to lose as little information as possible. First, only the most damaged ingots were selected for transfer to INFN.

All Roman lead bars include stamped characters identifying the smelter, the miner, and often the mine from which the lead came. The material in which these symbols were stamped was removed from the rest of the ingot, and stored for archaeological reference. Not only is the source information vital to working out the flow of trade for the Empire, but keeping the actual stampings also allows the lead of any given bar to be analyzed.

In the end, however, the bars were melted down destroying in the process any residual archaeological value.

International law is silent

Making archaeological information unavailable for future generations of scholars is not considered good archaeological practice. While there is little settled law about legal protection for shipwrecks, the 2001 UNESCO Convention on the Protection of the Underwater Cultural Heritage does include provisions to protect our underwater cultural heritage, but primarily from commercial exploitation and loss (less politely called looting). There are also numerous statements to the effect that scientific research is encouraged, but the context makes it clear that large-scale diversion and destruction of artifacts to support physics research was not what the drafters had in mind. The 1982 United Nations Convention on the Law of the Sea makes only one relevant comment, in Article 149:

"All objects of an archaeological and historical nature found in the Area shall be preserved or disposed of for the benefit of mankind as a whole"

The tension between heritage and progress is hidden in the phrase "for the benefit of mankind as a whole." As Yogi Berra said: "It's tough to make predictions, especially about the future." Would the lead bars now being destroyed for use in physics experiments have aided in the discovery of a major historical truth about humanity? Will those physics experiments discover something that allows mankind to make a huge jump forward? Neither assertion can be taken for granted. The most probable situation is that neither the cultural value nor the value to physics of these lead bars will be enormous, in the long run, leaving us with the ethical dilemma of deciding how to judge such conflicts between two apparently beneficial paths.

Where to from here?

Until the international community comes to some form of guidance on regulating the non-commercial use of archaeological artifacts, I believe that negotiation of one-off agreements between organizations who are sensitive to the needs and ethical issues of the other (as well as their own) is most likely to allow mankind to establish better boundaries while avoiding disastrously mismanaged scenarios to past or future cultural heritage. The video below gives an underwater look at the Bou Ferrer shipwreck. We'll let University of Birmingham Professor Elena Perez Alvaro, who has written a paper on the subject (PDF), have the last word:"The study of sunken vessels is essential to history because entire continents have been discovered, colonized, invaded and defended by sea. The salvage of this material should be done under the surveillance of an archaeological team. On the other hand, not so far in the future, the development and operation of new sciences and technologies – make it likely that further investigation, development and use of the underwater cultural heritage for other purposes may occur. Compromise does not equal defeat; sometimes, it is the only path to success. Guidelines are necessary ‘for the benefit of humankind’."