Some background...
From
http://www.rsc.org/periodic-table/element/98/californium (see the podcast transcript)
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.
On the dark side, to say the least, even though there are legitimate uses for Californium, it would make formidable dirty bomb or site (0r individual--think polonium) specific irradiation material as well because it is such a good neutron emitter.
