Wednesday, 11 August 2010

Superheavy in Berkeley

I'm in Berkeley, California, attending the Nuclear Structure 2010 conference. There have been a few talks on superheavy elements (roughly those heavier than found on the earth, so heavier than Uranium). This is hardly any wonder since Berkeley is home of the Lawrence Berkeley National Laboratory, where superheavy element creation was pioneered.

Krzysztof Rykaczewski presented a talk about the recently-announed discovery of element 117. Like all superheavy nuclei, it is made by reacting together two lighter nuclei: In this case Calcium-48 (20 protons, 28 neutrons) and Berkelium-249 (97 protons, 152 neutrons). This is the most obvious choice, since Calcium-48 is the most neutron rich stable light nucleus that there is and then one needs to match with the right number of protons in the other nucleus to make the one you're interested in. The tough thing about this is that Berkelium has a half life of around 320 days and is itself a superheavy nucleus that has to first be made in a lab. They made it at Oak Ridge, Tennessee, where they placed (also superheavy, or at least transuranic) Americium (element 95, widely used in household smoke detectors) and Curium (element 96) samples in a nuclear reactor for 250 days, where they absorbed neutrons and underwent beta decay until heavier elements had been created, including the Berkelium, which was separated by chemical means.

They made a total of 22mg of Bk-249 which they then turned into a target which they shipped to Russia (which turns out to require quite a bit of paperwork) to the nuclear physics lab at Dubna. Here they installed the Berkelium target onto which the Ca-48 beam impinged. They had a total of 3g of Ca-48 to work with. It's not as rare as Berkelium, but it's pretty rare, and Russia have it all. They ran the experiment for several months, and in that time made a positive identification of the isotopes of Z=117 with N=176 and N=177. When there has been independent verification of the discovery, the group will be invited to name the new element.

As my Institute of Physics Branch colleague Alby notes, the same group are now in a position to name element 114.

The paper announcing the element, in Physical Review Letters, is available (to subscribers). Details below:

Oganessian, Y., Abdullin, F., Bailey, P., Benker, D., Bennett, M., Dmitriev, S., Ezold, J., Hamilton, J., Henderson, R., Itkis, M., Lobanov, Y., Mezentsev, A., Moody, K., Nelson, S., Polyakov, A., Porter, C., Ramayya, A., Riley, F., Roberto, J., Ryabinin, M., Rykaczewski, K., Sagaidak, R., Shaughnessy, D., Shirokovsky, I., Stoyer, M., Subbotin, V., Sudowe, R., Sukhov, A., Tsyganov, Y., Utyonkov, V., Voinov, A., Vostokin, G., & Wilk, P. (2010). Synthesis of a New Element with Atomic Number Z=117 Physical Review Letters, 104 (14) DOI: 10.1103/PhysRevLett.104.142502