Name: Named for the planet Neptune, which takes its name from the Roman god of the sea.

Atomic mass: (237).

History: Discovered in 1940 by Edwin M. McMillan and Philip H. Abelson at the University of California, Berkeley.
Occurrence: Trace amounts found in uranium ore.
Appearance: Silvery solid.

Behavior: Radioactive.

Uses: Neptunium-237 is used in neutron-detection instruments.
Day 1. Okay, I thought, neptunium sounds like a cool element. It's a transuranium, which gives it a certain je ne sais quoi, and it's sandwiched between uranium and plutonium--both heavy hitters in the periodic table. And it had to have been named for the Roman god of the sea: Evoke an image of a fierce Neptune, trident in hand, flowing mane of hair from a broad forehead. I don't know much, er, anything, about neptunium, but I can research. I can write 750 words on neptunium.

Day 2. Procrastinate a bit; work through a couple of items on my "Don't forget to do this" Post-it. What did I do before Post-its? Isn't there something I forgot to put on my Post-it that I really do need to do? No system is infallible.

Day 3. Commiserate with other C&EN staffers writing element essays. Mull possible leads, angst over illustrations. But even as I'm empathizing, I'm confident. Cocky even. I've got Neptune in my back pocket.

Day 4. Start researching. Not much in any of the reference books. Not much online either. There's not much to be said about neptunium, evidently. I may be in trouble. And my cockiness is gone: Neptunium was named for the planet Neptune. Drat. Wait--the planet was probably named for the god. That's a stretch, though.

Day 5. No writing yet. No brilliant gestalt of the finished essay.

Day 6. Researching Neptune is not actually procrastinating. I'm finding interesting images; one might work as an illustration. Maybe I can focus on parallels between the element and the planet, making the naming of the element providential. I hope there are parallels. Found a website for a band named Neptune--a scrap metal band. The band members play instruments they made from scrap metal, and they've got photos of the instruments and the band on their site. Maybe I can tie them into neptunium as well--Nep-tune-ium? Nope.

Day 7. Time to see what I've got:

Well, you can't find neptunium in seawater or the atmosphere, and it's now known to be present in only trace amounts in uranium ore. The only other place to find the element is in nuclear facilities or research labs.

Neptunium was the first transuranium element discovered, so as Neptune follows Uranus in the lineup of planets, Np is next to U in the periodic table. It's no surprise that plutonium is next in the actinide series.

But back to neptunium's first appearance. Working at the University of California, Berkeley, in 1940, Edwin M. McMillan bombarded a uranium target with cyclotron-produced neutrons. He noted that some unusual -rays were emitted, indicating the presence of a new isotope. Philip H. Abelson, working with McMillan, proved in May 1940 that the -rays came from a new element. They announced their discovery in a paper published in Physical Review later that year. Because of national security concerns, research on neptunium continued in secret; in 1944, the first pure compound (NpO2) was made using a few milligrams of the element. Scientists working toward a nuclear weapon found that plutonium was much better than neptunium for their purposes, so Np research moved to the back burner.

CRITICAL CARE This nickel-clad neptunium sphere was used to experimentally determine the critical mass of Np at Los Alamos National Lab last year.
So far, it's pretty dry going, but I've found some more interesting facts:

Sources differ on the number of isotopes neptunium has, but 17 is the most frequently given number, and they are all radioactive. McMillan and Abelson produced 239Np, which has a half-life of about 2.4 days. Among the isotopes, the range of stability is amazing: 232Np has a half-life of 13 minutes, whereas 237Np's is 2.14 million years!

237Np, a by-product of plutonium production from 238U in nuclear reactors, can be used in neutron-detection instruments.

Because 237Np does exist and is fissile, researchers at Los Alamos National Lab set about determining exactly how much neptunium would be required to sustain a fission chain reaction, thus to make a nuclear bomb. This quantity is called the critical mass, and in a funny coincidence, the apparatus used to conduct the experiment from which the calculation is made is called the "Planet." It took 12 years to reach the point of needing the Planet, but in October 2002, LANL announced "Neptunium criticality achieved."

So what is 237Np's critical mass? Secondary sources give it as about 60 kg; for comparison, the critical mass of 239Pu is 10 kg, and 235U's is 50 kg. But the 237Np that's produced is not pure, and it's very difficult to separate out the pertinent isotope. Still, the International Atomic Energy Agency monitors 237Np.

Day 8. Add item to my "Under no circumstances do this again" Post-it: Volunteer to take on a project without doing some research first.

Robin Giroux is C&EN's assistant managing editor for editing and production. She oversees the unit of talented editors who make sure that the stories produced by C&EN staff actually appear on page.


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