Trends in isotope discovery

It all started innocently enough with a review article I wrote in 2004 about the nuclear driplines, which described the exploration of the most neutron- and proton-rich isotopes (Thoennessen 2004). The article included tables listing the first observation of each isotope along the proton- and neutron-dripline. The idea to expand this list to cover all isotopes lingered for a few years until in 2007 I mentioned it to an undergraduate student as a possible research project. At the beginning we did not appreciate the magnitude of the project; after all, there are more than 3000 isotopes presently known. However, with the help of many undergraduate students performing elaborate literature searches and carefully judging the merits of the individual papers we continued, even though we extrapolated that the project would take about 10 years to reach completion.

We have described details of the discovery of each isotope in short paragraphs, arranged by elements, which are published in a series of articles in Atomic Data and Nuclear Data Tables. In a summary table, the first author, year, journal, laboratory, country and method of discovery are presented. Now, only four years after we started, the project is almost completed. We finished the initial discovery assignment for all isotopes and are currently finalizing the paragraphs for the last four elements: actinium, thorium, protactinium and uranium.

The master table of all elements is a rich source of interesting information. Along the way it has been fascinating to see how not only the physics and technology changed over time, but also the style of the papers. For example, the average number of authors per article increased from 1.1 in 1930 to 16.4 in 2000.

One piece of information – the number of isotopes discovered by the different laboratories around the world as a function of time – was recently highlighted by a Nature News article and has drawn a lot of attention over the past few weeks (Samuel Reich 2011). The article reveals the labs and individuals that have discovered the largest number of new isotopes. The results show that while Lawrence Berkeley National Laboratory leads by almost a factor of two, other laboratories in Japan and Europe – most notably GSI in Germany – have made most of the new discoveries in the past couple of decades. A graph displaying the number of isotopes discovered per laboratory as a function of time was featured as the “Trendwatch” in a recent issue of Nature (Trendwatch 2011). The graph seems to indicate that the top five laboratories are Berkeley, Cavendish, GSI, RIKEN, and JINR in Dubna; however, RIKEN was included only because of the large number of recent discoveries. But in reality, CERN’s ISOLDE has played a pioneering role in the discovery of isotopes, especially with the “isotope-online” technique, and ranks number five on the list.

Now why is the information contained in the database significant? The discovery of isotopes has a long history beginning with the discovery of radioactivity of uranium (later identified as ²³⁸U) by Becquerel in 1896. The discovery of new isotopes is closely linked to developments of new techniques and new accelerators (Thoennessen and Sherrill 2011). Creating and detecting new isotopes is the first prerequisite to being able to study them, automatically putting the laboratories that produce the most exotic isotopes in the best position for doing the most exciting science with these isotopes. The techniques to produce, separate and identify these isotopes are also critical to make and deliver clean beams of less exotic isotopes at higher intensities, which can then be used to explore the properties of these nuclei. The recent conference on Advances in Radioactive Isotope Science, ARIS 2011, highlighted not only the tremendous interest in the field and the most recent advances in physics but also the technical developments making these experiments with exotic isotopes possible (ARIS 2011 charts the nuclear landscape).

The data presented in the Trendwatch indicate that the balance of power pushing the field forward has shifted away from the US. The article did not stress that 2010 was the most productive year for the discovery of isotopes. For the first time more than 100 isotopes were discovered in a single year. This points to a renaissance of the field, which is driven by the start of a new accelerator system in RIKEN, Japan, and new technical developments at GSI. During the past 20 years, most new isotopes were discovered at projectile fragmentation facilities, thus the next major step will be the new accelerators currently being designed at the Facility for Antiprotons and Ion Research (FAIR) at GSI and the Facility for Rare Isotope Beams (FRIB) at Michigan State University in the US. FRIB is absolutely critical for the US to play a leading role in nuclear physics in the future.