It's practically a cliche these days to compare scientific pursuits to unraveling a mystery, but it's an accurate description of the ongoing search for neutrinos. Neutrinos, the so-called "ghost particles," rarely interact with other particles, which makes them extremely difficult to detect.
Physicists have generally relied on building large underground laboratories, like the Sudbury Neutrino Observatory (SNO) or Japan's SuperKamiokande facility. SNO, for example, is outfitted with giant vats of heavy water, surrounded in turn by light water, and lots of photomultipliers to detect significant "events." When a neutrino collides with a proton, the reaction produces a tiny flash of light, called "Cherenkov radiation." Unfortunately, this also produces a lot of background "noise"; it's easy to get a "false positive," so to speak, or to miss one of the telltale signals of neutrinos.
Sean, a.k.a., the Spousal Unit, tells me that from a theoretical perspective, such detectors should be observing neutrinos from distant supernovae all the time, yet such events remain quite rare. How can we get rid of all that pesky background noise so we can see what we've been missing? Well, during his recent trip to a science conference in Melbourne, Sean heard about a relatively new strategy in which the vats would be filled with 100 tons of an element called gadolinium, and helpfully emailed me the details, knowing I'm a neutrino fan.
It's not a common element: it belongs to the family of rare earth metals; its name derives from Finnish chemist Johan Gadolin. (The first spectroscopic evidence for gadolinium was achieved by a Swiss chemist named Jean Charles Galissard de Marignac.) But it has a very useful property: it's a terrific absorber of neutrons. That's why gadolinium is commonly used as a shielding material in neutron radiography and in nuclear reactors, not to mention for magnetic resonance imaging. And compounds of gadolinium are now being used to make phosphors for color televisions.
That last application might be one reason why using gadolinium for neutrino detection might soon be a reality. See, in the past, the element has been ridiculously expensive, costing on the order of $4000 per kilogram. (A neutrino detector would need 100 tons of gadolinium; you do the math!) But the growing demand for the element means that countries like China have built up a prosperous business around rare-earth metals. That translates into lower prices, on the order of $5 per kilogram. And that means places like SNO and SuperKamiokande might indeed be able to fill their big detector vats with gadolinium, which is sensitive not just to protons, but also neutrons, making it easier to distinguish actual neutrino events from the background noise.
Gadolinium also has a personal significance. One of my favorite artists, Nash Hyon, has an entire series devoted to the elements of the periodic table, one of which is called "Gadolinium." Her late husband died of a brain tumor years ago, and she used one of his old MRI images as the basis for her painting. I first saw it at an exhibit in 2002 and it haunted me thereafter. When I finally met Hyon personally in 2007, and we were touring her home studio, there it was, along with "Cesium," "Lithium," "Tin," and other elemental siblings. We ended up buying the painting, which now graces our living room.
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