Carbon Cages Shorten Lifespans

If you take carbon atoms and arrange a sheet of them in a hexagonal pattern, you’ll make graphite. If you intersperse your hexagons with pentagons, much like a soccer ball, you’ll make a hollow sphere known as a buckyball. The shape requires 60 carbon atoms, hence the symbol C60.

First time I ever heard that term was from an Australian physics professor, and I thought it was an Aussie term for a soccer ball…turns out the molecule is named for Richard Buckminster Fuller, discoverer of hollow carbon microstructures known as fullerenes.

Anyway, so we have these buckyballs. What to do with them? Well, some physicists have been using them as cages for their pet beryllium-7 atoms. A recent experiment by Japanese physicists, led by T. Ohtsuki, shows that beryllium-7 lasts longer when it is around other beryllium-7 atoms than when it is isolated in a buckyball.a

7Be is a radioactive isotope of beryllium. Radioactive nuclei decay by catching or releasing particles and becoming other nuclei in the process. 7Be decays by snatching one of its orbiting electrons, transforming one of its protons into a neutron. Then it is 7Li, a stable isotope of lithium.

Among other 7Be atoms, a sample of 7Be needs about 53.3 days for half of it to become 7Li. Inside a buckyball cage, this half-life is only 52.5 days.

It should be noted that the half-life for 7Be alone was taken at room temperature while the trapped 7Be was measured at about -450°F (-268°C if you’re outside the US and 5K if you’re savvy). In any case, the trapped beryllium was pretty darn cold, only slightly warmer than absolute zero.

At first, I wondered if the temperature difference accounted for the difference in half-lives. However, it seems that the half-life of the cold 7Be should be longer than its warm counterpart, not shorter. Feel free to share your thoughts.

As interesting as this half-life business is, what really drew me to this research was the question how did they get a beryllium-7 atom into a buckyball?.

First, they had to make 7Be, and they did this by shooting protons at lithium carbonate. The lithium will catch the proton and kick out a neutron. The newly formed 7Be has some extra energy, so it uses some of that up before it “penetrates into the C60 cage.”b

Apparently it just walks right in and doesn’t walk back out again. There was also mention of an electron changing hands (from the beryllium to the buckyball), so perhaps that’s enough to keep the beryllium inside. I’ll be sure to update if I get a better explanation.

a T Ohtsuki et al. “Radioactive decay speed-up at T=5K: The electron-capture decay rate of 7Be encapsulated in C60.” Submitted to Physical Review Letters.
b T. Ohtsuki et al., Phys. Rev. Lett. 77, 3522 (1996).

…Buckminster. It must have been awful for him, back in the third grade.

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