Chemistry in its element: californium


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Chris Smith

This week, let's go surfing.

Brian Clegg

What comes to mind when you think of California? Surfing and the Beach Boys? Hollywood and Governor Schwarzenegger? The University of California at Berkeley has ensured that California also has its place in the periodic table with element 98, the tenth of the actinides, californium.

Although it seems perfectly sensible to celebrate the location where it was discovered, californium's name was, in fact, a failure for the team behind its production. Glen T. Seaborg and his co-workers had named americium to parallel the lanthanide above it in the periodic table, europium. They went on to name curium and berkelium in a way that was also derived from the equivalent lanthanide. So, for instance, the actinide berkelium was named after Berkeley because the lanthanide above it, terbium, was named after the Swedish village Ytterby where it was quarried.

When it came to californium, an artificial element first produced in 1950, the equivalent lanthanide would be dysprosium, which comes from the Greek for 'hard to get.' After some head-scratching, Seaborg and his team gave up on the search for an equivalent and just went for the location of the lab. They had already discarded a list of names including cyclotronium and cyclonium, after the device used in producing the first californium, along with the more than a little cheesy radlabium, reflecting the team's origins as part of the radiation laboratory or rad lab.

They did, though, manage a neat bit of rationalization, arguing that they paralleled dysprosium's 'hard to get' meaning because 'the searchers for another element a century ago found it difficult to get to California.' This referred to the state's inaccessibility during the nineteenth century gold rush.

The first isotope of californium produced was californium 245, with a half life of just 44 minutes. The team battered a target of curium with alpha particles using a cyclotron, an early type of particle accelerator still in use today, particularly in medical applications. The cyclotron accelerates charged particles using electrodes that switch rapidly between attracting and repelling as the particles spiral around a circular chamber until they collide with a target. In this case the collision produced californium and a spare neutron.

The most stable of californium's 20 or so produced isotopes is californium 251, which has a half life of 898 years, though many of the isotopes have half-lives measured in minutes. It's most often made now by starting with berkelium 249 and adding neutrons in a nuclear reactor. Although this is a purely artificial element here on earth, it may exist in space as one of the many by-products of supernovas.

When it comes to practical uses, this slivery substance is an excellent neutron emitter. This makes it handy for kick-starting nuclear reactors, where a high neutron flow is required to get the chain reaction going. It also means that, in principle, californium would make effective small scale nuclear weapons, requiring as little as five kilograms of californium 251 to achieve critical mass - about half the amount of plutonium required for a bomb - but in practice it is so fiddly to produce that even at this scale it is unlikely to be used.

As well as providing the starter for reactors, small amounts of californium have also found their way into a number of devices requiring a flow of neutrons, whether it is specialist detectors or radiotherapy, as a last resort for some cancer treatments where gentler sources have failed.

Perhaps californium's most common application is in moisture gauges used in potential oil wells. These detectors fire fast neutrons through the material to be tested. Hydrogen nuclei, typical of those in water and oil, tend to slow down the neutrons, so a slow neutron detector can be used to search for telltale hydrogen. The neutrons from californium can also be used in prospecting for silver and gold, using a technique called neutron activation analysis which bombards an area to be tested with neutrons and searches for the gamma rays emitted from the bombarded substance, with a characteristic signature.

In the end, though, it's californium's name that remains most significant. Perhaps, to parallel dysprosium's 'hard to get', it should have been lethium, from the latin for 'lying hidden' - but maybe that sounds too like lithium. Shortly after californium was first produced, the name was the subject of a running joke between its discoverers and the New Yorker magazine.

The magazine observed that the discoverers had missed a trick. It commented that 'California's busy scientists will undoubtedly come up with another atom or two one of these days, and the university might well have anticipated that. Now it has lost forever the chance of immortalizing itself in the atomic tables with some such sequence as universitium (97), offium (98), californium (99), berkelium (100).' Spelling out 'University of California, Berkeley,' across the table.

The discoverers fired back that the problem with calling elements 97 and 98 universitium and offium was the appalling possibility that some New Yorker could discover 99 and 100 and name them newium and yorkium. The New Yorker staff claimed already to be at work on these elements. but as yet all the journalists had achieved was to think up the names.

As it is, we can never be quite sure if 'californium' refers to the state or the university - and it is hard to produce - so in these respects, at least, californium parallels dysprosium as an element that's 'hard to get'.

Chris Smith

Well in that case, if we're naming things after other things that are hard to get hold of, how about a taxi in rush hourium or, worse still perhaps, what about a James Blunt CD you can tolerateium? That would be my suggestion. That was Brian Clegg, with this week's element Californium. Next time, it's over to Sarah Staniland.

Sarah Staniland

I always find the question 'what's your favourite element' a difficult one. There are several front runners for vastly varying reasons; however, always a top contender has to be cobalt because it excels in several important character traits: Cobalt has amazing beauty and strength, as well as great cooperation.

Chris Smith

I thought she was talking about me for a minute there. That's Sarah Staniland from Leeds University who will be here next week with the story of cobalt. Do try and join us. Thanks for listening, I'm Chris Smith, and goodbye.


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