Physics Experiment Will Use Lead From a Roman Shipwreck

April 16, 2010 11:15 PM

Roman ingots to shield particle detector. "Around four tonnes of ancient Roman lead was yesterday transferred from a museum on the Italian island of Sardinia to the country's national particle physics laboratory at Gran Sasso on the mainland. Once destined to become water pipes, coins or ammunition for Roman soldiers' slingshots, the metal will instead form part of a cutting-edge experiment to nail down the mass of neutrinos." [Via]
posted by homunculus (22 comments total)

Oh the streets of Rooooome,
are fiiiillllled with rubble...
posted by flapjax at midnite at 11:16 PM on April 16, 2010 [3 favorites]

Swords into ploughshares. Interesting read, thanks.
posted by Blazecock Pileon at 11:18 PM on April 16, 2010

Low-alpha lead is kind of cool. Talk about an unexpectedly valuable material for treasure hunters.
posted by Mitrovarr at 11:37 PM on April 16, 2010

I imagine the poor Sardinian lead wholesaler circa the fall of Rome who got stuck with 4 tons of lead he couldn't offload. His wife probably taunted him about it until the day he died. I can just hear it now:

"When are you going to get that useless shit out of the shed, Publicus? Well, have you called the Visigoths? It's not going to move itself, you know" and "Why don't you put it out by the curb with a 'free lead' sign on it."
posted by kuujjuarapik at 11:56 PM on April 16, 2010 [18 favorites]

See also "pre-1945" or "non-radioactive" steel, recovered from sunken warships at Scapa Flow (and possibly elsewhere, but there are a lot of 'em at Scapa, in quite shallow water) and used to make radiation-sensitive instruments. As with lead, atmospheric nuclear explosions have rendered all "modern" steel very slightly radioactive.

(This really does stretch the definition of "very slightly", though. It's almost like that apocryphal bottle of vermouth that was strapped to the side of one of the early nuclear tests, so that ever since you've been able to make the perfect dry martini by putting cold gin in a glass and holding it up in the air.)
posted by dansdata at 12:33 AM on April 17, 2010 [8 favorites]

(Well, actually probably not "as with" the lead, now that I read the article properly. :-)
posted by dansdata at 12:36 AM on April 17, 2010

There's some more information in one of my AskMe questions, Are satellites built from old metals to avoid post-A-bomb radiation levels? I got some very interesting replies, some from people who actually work with these radiation detector setups. In particular, pseudonick wrote:

If you gave me hunks of steel made in 1920, 1965, and 2009, I could rank them in order of age using the germanium detector I mentioned above, just by the relative levels of fallout material.

Others commented that the sheer age of the steel -- the time since the iron and carbon were purified and separated from other radioactive elements in the ore -- was probably a big factor.

The consensus was that the satellites themselves are almost certainly built from aluminium and/or titanium, but that old metals - I was specifically asking about steel from WWI battleships - could well have been used in shielding for on-board radiation detectors, confirming something that a friend of mine half-remembered hearing on a tour of a European Space Agency facility.

So there are chunks of WWI battleship -- and maybe even roman lead -- floating around in space, contributing to cutting edge research.

One thing I still don't understand, though, is this:
After it is extracted from the ground, however, lead-210 decays into more stable isotopes, with the concentration of the radioactive isotope halving every 22 years. The lead in the Roman ingots has now lost almost all traces of its radioactivity.

Surely the lead has been decaying since the atoms were originally formed? I don't understand why a couple of thousand years at the bottom of the sea leaves it less radioactive than the same couple of thousand years at the bottom of a mine. The only mechanism I can think of is if lead ore is almost always mixed with uranium/polonium/etc. which would provide a steady supply of unstable lead isotopes. Doesn't sound very plausible to me, though. Does anyone know the answer?
posted by metaBugs at 1:10 AM on April 17, 2010 [1 favorite]

(This really does stretch the definition of "very slightly", though. It's almost like that apocryphal bottle of vermouth that was strapped to the side of one of the early nuclear tests, so that ever since you've been able to make the perfect dry martini by putting cold gin in a glass and holding it up in the air.)

So that's why that works.
posted by TwelveTwo at 1:11 AM on April 17, 2010

The consensus was that the satellites themselves are almost certainly built from aluminium and/or titanium, but that old metals - I was specifically asking about steel from WWI battleships - could well have been used in shielding for on-board radiation detectors, confirming something that a friend of mine half-remembered hearing on a tour of a European Space Agency facility.

That's odd. Isn't outer space basically throwing charged ions — radiation, and lots of it — at satellites anyway? If the background radiation of the metal shield is known ahead of this, can't this "error" just be subtracted from the radiation sensor signal as residual noise?
posted by Blazecock Pileon at 1:26 AM on April 17, 2010

That's odd. Isn't outer space basically throwing charged ions — radiation, and lots of it — at satellites anyway? If the background radiation of the metal shield is known ahead of this, can't this "error" just be subtracted from the radiation sensor signal as residual noise?

Not if the noise level is much higher then what you're trying to measure. Radiation is intrinsically random, so it basically looks like noise. Think of a photograph at a high ISO setting, there's no way to extract detail if you don't know if it's true detail or noise from the camera.

As far as radiation from space, that's what the shielding is supposed to prevent.
posted by delmoi at 1:51 AM on April 17, 2010

Surely the lead has been decaying since the atoms were originally formed? I don't understand why a couple of thousand years at the bottom of the sea leaves it less radioactive than the same couple of thousand years at the bottom of a mine.

Pure guess here, but I suspect purifying the lead not only removes other impurities that cause lead-210 to form, as you surmise, but also concentrates the radioactive lead into a smaller volume, accelerating decay by chain reaction.
posted by Malor at 2:40 AM on April 17, 2010

If the background radiation of the metal shield is known ahead of this, can't this "error" just be subtracted from the radiation sensor signal as residual noise?

Taking what delmoi said a step or two further, remember that these sensors detect individual photons. It's like trying to detect a candle from 10 miles away; you can see better if you're in a very, very dark room. Spontaneous radiation from the shielding functions as a local 'light' source, outright hiding the interesting signal with unwanted noise.

They're detecting down to individual events, and each and every spontaneous emission from the shielding can potentially look like data. Get enough of that noise, and they simply can't pull the signal out anymore.

I don't have hard knowledge, but it wouldn't shock me if many stars out there have been detected by fewer than ten photons.
posted by Malor at 2:46 AM on April 17, 2010 [1 favorite]

Lead ore is frequently found in conjunction with various radioactive elements, commonly uranium, thorium, radium and polonium, and the isotopic ratios of these to lead (and of lead's own isotopes, most of which are the end results of decay chains in the earth) are valuable dating and origin-identification aides.

When lead is refined, the major radioactive source remaining is 210Pb, which has a half-life of 22 years. (There are natural sources of low-radioactivity lead ore, but I don't know how common they are. Not very.) This is a common problem in electronics, where alpha particles are unwelcome guests.

(I tried to get chapter and verse on this. The first eight papers I found would have cost me:

£17.25 ($26.54)
$30
$35
$30
37.71 euro ($50.89)
£20 ($30.78)
$43
------------
$246.21

And we wonder why school students use Wikipedia.)
posted by Devonian at 3:35 AM on April 17, 2010 [3 favorites]

The only mechanism I can think of is if lead ore is almost always mixed with uranium/polonium/etc. which would provide a steady supply of unstable lead isotopes. Doesn't sound very plausible to me, though. Does anyone know the answer?

I don't know why you find this implausible: it is entirely correct. Statistically speaking, uranium is about as common as lead in the earth's crust.

Pure guess here, but I suspect purifying the lead not only removes other impurities that cause lead-210 to form, as you surmise, but also concentrates the radioactive lead into a smaller volume, accelerating decay by chain reaction.

Not sure I understand what you have in mind, but it doesn't sound very likely: Lead-210 decays by beta emission: except in some pretty exotic scenarios, the decay rate isn't really affected by anything.
posted by Dr Dracator at 3:39 AM on April 17, 2010 [1 favorite]

Well, you're probably right then, I was assuming there'd be something like the uranium acceleration of decay, without knowing the actual details. I defer to your greater expertise.
posted by Malor at 4:01 AM on April 17, 2010

I don't understand why a couple of thousand years at the bottom of the sea leaves it less radioactive than the same couple of thousand years at the bottom of a mine. The only mechanism I can think of is if lead ore is almost always mixed with uranium/polonium/etc. which would provide a steady supply of unstable lead isotopes. Doesn't sound very plausible to me, though. Does anyone know the answer?

Previously.

Years ago I read an interesting article about this in The Sciences magazine (New York Academy of Sciences), which stated that the machines that mine and process the ore are contaminated enough by atmospheric nuclear tests to interfere with the neutrino experiments.
posted by weapons-grade pandemonium at 8:26 AM on April 17, 2010

The "previously" link should go to my comment, but it keeps getting borked.
posted by weapons-grade pandemonium at 8:27 AM on April 17, 2010

Sounds like these physicists are about to be "led" to some interesting conclusions! Thank you, I'm here all week.
posted by No-sword at 8:29 AM on April 17, 2010

The background physics to this story is pretty interesting. First, the article calls this a neutrino experiment, so let's start with some basics about neutrinos. Neutrinos are extremely light particles (long thought to be massless) produced through the weak interaction and only through the weak interaction (I'm ignoring gravity, and you should too). This is to be contrasted with the charged leptons (electrons, muons, and taus), which can be produced through weak and electromagnetic (EM) interactions, and the hadrons (quarks and such), which are produced through strong interactions (as well as weak and EM). This fact makes neutrinos very hard to detect for two reasons 1) events involving weak interactions are extremely rare at low energies, and 2) events involving the other forces are extremely common, and many of these can look like weak interaction events in an experiment. So for any neutrino experiment, you need to look for rare signal events amongst a huge background of garbage. This means you want to eliminate background as much as possible (through shielding and low temperature operation, for example), and then try to understand the remaining background as well as you can so that you can make cuts in your data (i.e. look for specific types of events that give the best signal above background).

However, your run-of-the-mill neutrino experiments, which are concerned with neutrino oscillations, don't typically use this level of shielding. It's just too expensive and you don't need it (although yes, it could help). Instead, you can build your experiment in an old mine, use a veto shield, limit your attention to particles originating from your known source, and then deal with background statistically. Ideally the source is one you created, such as in a particle accelerator or a nuclear reactor, as this offers extra control of the signal (among other things). (Alternatively, you can use a well-understood and plentiful natural source (e.g. the Sun). In fact, it was a solar neutrino experiment that first demonstrated that neutrinos have mass.)

So what's different about this neutrino experiment? Well, they're not looking for neutrinos, they're looking for the absence of neutrinos. Specifically, they want to see if double-beta decay can take place without the production of neutrinos. Now, weak interaction events themselves are rare, and now we're talking effectively about looking for two events happening at the same time. Moreover, the "source" in these experiments isn't something you can really control--it's just block of material that undergoes radioactive decay. You're stuck with its natural decay rate and the only way you could increase the signal is to get more of the stuff. So, shielding is extraordinarily important. Additionally, high temperatures would kill the experiment because of noise from phonons, so they have to run at near absolute zero. To give some numbers, for a typical atom used in these detectors, you'd have to wait somewhere around 10^28 years (see Table 1) before it would undergo such a decay. CUORE hopes to use a few thousand moles of tellurium dioxide, so that brings observation within reach over a reasonable run-time of a few years, assuming adequate background controls.

Now for the "Why do we care?" question. One thing it could get us is an absolute scale for neutrino masses. (Oscillation experiments measure differences in neutrino squared masses). But the real reason people care about this is that observation of neutrinoless double-beta decay would instantly answer a fundamental question about neutrinos: do they have "Majorana masses" (that is, are they their own anti-particles)? Neutrinos are the only known particles that could have such a mass. The other fermions all have electric charge; their anti-particles come with opposite charge and so are clearly distinct. The existence of a Majorana mass is of great theoretical interest for several reasons: 1) it would imply lepton number violation, and could thus help explain the matter/anti-matter asymmetry of the Universe, 2) a Majorana mass allows for a simple and elegant explanation for why neutrino masses are so incredibly tiny through the "seesaw mechanism,", which 3) is important in many grand-unification scenarios. As much hype as their is surrounding the LHC, my feeling is that neutrino experiments like these may offer an equally powerful microscope for examining the structure of physics.

I'm sure this overview has left out a lot, but to sum up there's a ton of really fundamental physics going on behind the scenes of this story--physics that hopes to answer questions about how we came to be and what the fundamental building blocks of nature really are. We've long gained insight into these questions from the preserved recordings of the earliest attempts at answering them. I find it pretty neat that we can also gain material support for the effort from the ancient world.
posted by dsword at 10:21 AM on April 17, 2010 [19 favorites]

MetaBugs, I wondered the same thing. Others have answered your question, but this link has a handy table that summarizes the decay series of 238-U. It generates 2 unstable Pb isotopes (among a whole bunch of other stuff) on its way to its final fate as stable 206-Pb.

Homunculus, thanks again for a nifty post. Actually, when I see a post with your byline I get a sinking feeling, knowing that my endless to-do list will once again get pushed aside ... Plus your posts seem to attract really interesting comments like dsword's (awesome, love the high-level genuine-article real-deal geeks around here), and here it is noon already and I haven't accomplished anything all day. *sigh*
posted by Quietgal at 12:04 PM on April 17, 2010 [1 favorite]

An archaeological mystery in a half-ton lead coffin
posted by homunculus at 11:17 AM on April 18, 2010

and here it is noon already and I haven't accomplished anything all day. *sigh*

My evil plan is working!
posted by homunculus at 11:18 AM on April 18, 2010