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IntroductionThe search for new nuclides has been one of the driv-

ing forces for discoveries of new phenomena in nuclear physics for over a hundred years. In order to understand the nuclear force acting between neutrons and protons it is nec-essary to explore a large range of possible configurations of neutrons and protons forming a specific nuclide. The first step in studying properties of these nuclides is always their formation in the laboratory. Over the years new discover-ies have been made with the innovation of new techniques, primarily new accelerators or significant improvements in accelerator techniques.

A few years ago we began a project to document the discovery of all isotopes of all known elements, which is being published in a series of papers in Atomic Data and Nuclear Data Tables. The first paper on the discovery of the cerium isotopes was published in 2009 [1]. The definition of what constitutes a nuclide or an isotope of a specific ele-ment is not well defined [2]. While for very neutron-rich nuclides the timescale differences between the lifetimes of β– decaying nuclides and nuclides unstable with respect to neutron emission allows for a precise definition of the neutron “drip-line” the situation is more complicated for proton-rich nuclides. Nuclei that are unbound with respect to proton emission can have significant lifetimes because of the Coulomb barrier so that in some cases β+-decay or electron-capture can compete or even dominate the decay.

In order to avoid an arbitrary lifetime for the definition of the existence of a nuclide, all nuclides where distinctive, identifying features have been observed were included. These include very short-lived resonance states, for exam-ple the di-neutron where a scattering length of –16.4 ± 1.9 fm [3] was measured demonstrating an attractive interac-tion between the two neutrons.

At the other end of the mass spectrum, the recognition for the discovery of a new element is regulated by the In-ternational Union of Pure and Applied Chemistry (IUPAC) and the International Union of Pure and Applied Physics (IUPAP). A joint working group (JWP) recommends the acceptance of new elements based on strict criteria [4,5]. Such rigorous and elaborate reviews are not practical for

the documentation for the discovery of nuclides where less stringent criteria were applied: (1) clear identification, ei-ther through decay-curves and relationships to other known nuclides, particle or γ-ray spectra, or unique mass and Z-identification, and (2) publication of the discovery in a ref-ereed journal. Thus the discovery of a specific isotope of an element does not necessarily imply the discovery of the element itself.

With these criteria, 3104 nuclides were discovered until the end of 2011.

Compilation of DiscoveriesThe project was recently completed with the acceptance

of the last paper on the discovery of the astatine, radon, francium, and radium isotopes. An overview of the project and an up-to-date publication status can be found at Ref. [6]. The project resulted in a large database with detailed information about the discoveries. For each nuclide it in-cludes the production method, year, laboratory, and country of discovery as well as well as all authors of the first pub-lication.

In a preliminary analysis the relationship between the number of discoveries and the development of new accel-erators and techniques over time was shown [7]. The evo-lution of the chart of nuclides can be followed in a short video [8].

In addition to the 3104 nuclides included in the compi-lation presently 25 nuclides have been observed but have only been reported in conference proceedings or internal reports.

LaboratoriesOver 120 different laboratories in 25 countries contrib-

uted to the discovery of the nuclides and the top 25 are listed in Table 1. Four laboratories—Berkeley, GSI, Cambridge, and Dubna—account for almost half of all discoveries. The dominance of these laboratories can be directly related to the innovation and development of new production mecha-nisms [9,10].

The Discovery of the Nuclides

Michael Thoennessen

National Superconducting Cyclotron Laboratory and Department of Physics & Astronomy, Michigan State University, East Lansing, MI 48824, USA

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1970s mostly utilizing fusion-evaporation, spallation, and deep-inelastic reactions. More recently, Dubna has been the dominant laboratory for the discovery of new elements that automatically resulted in the observation of new nuclides. The construction of the velocity filter SHIP utilizing fusion-evaporation reactions and later the fragment separator FRS for projectile fragmentation/fission reactions resulted in the leading position of the Gesellschaft für Schwerionen-forschung (GSI) in Darmstadt, Germany, for the last three decades. In addition to these four laboratories, the new ra-dioactive ion beam facility RIBF at RIKEN (also included in Figure 1) came recently online and has already produced a significant number of new nuclides.

The number of facilities that have discovered new nu-clides has decreased from a maximum of 55 in the 1960s to only 15 during the last ten years. This demonstrates the increased complexity needed to reach further and further away from the stable nuclides.

It should be mentioned that the last time period in Figure 1 covers only 2 years (2010 and 2011) in which already 117 nuclides were discovered compared to 180 during the whole previous decade (2000–2009).

CountriesNot surprisingly, the countries that discovered the most

nuclides correspond to the countries of the leading labo-ratories. Overall 25 different countries contributed to the discovery of all nuclides, and they are listed in Table 2. The top five countries: the United States, Germany (including West Germany from 1949–1990), the United Kingdom, USSR/Russia, and France account for over 80% with the

Figure 1. Number of nuclides discovered per decade at the top four laboratories, Berkeley, GSI, Cambridge, and Dubna. In addition, RIKEN is included because of the sig-nificant number discovered during the last decade.

Rank Laboratory Nucl. % Years

1 Berkeley 635 20.39 1928–2010 2 GSI 372 11.95 1977–2011 3 Cambridge 222 7.13 1913–1940 4 Dubna 220 7.06 1957–2010 5 CERN 119 3.82 1965–2009 6 Argonne 116 3.73 1947–2006 7 GANIL 85 2.73 1985–2005 8 Oak Ridge 78 2.50 1946–2006 9 Orsay 73 2.34 1959–198910 RIKEN 71 2.28 1972–201011 MSU 62 1.99 1967–201112 Los Alamos 54 1.73 1948–199013 Chicago 45 1.45 1920–195614 Brookhaven 44 1.41 1952–198615 Grenoble 40 1.28 1965–199516 Ohio State 35 1.12 1941–1960 Studsvik 35 1.12 1971–1993 Jyväskylä 35 1.12 1972–2010 Berlin 35 1.12 1907–200020 McGill 34 1.09 1900–198121 Amsterdam 29 0.93 1934–197622 Mainz 26 0.83 1950–197623 Lanzhou 23 0.74 1993–200424 Harwell 22 0.71 1949–1971 Rochester 22 0.71 1937–1972 U. of Michigan 22 0.71 1937–1969

Table 1. Top 25 of laboratories. The total of number of nuclides discovered, the percentages, and the range of years when nuclides were discovered are listed.

Figure 1 shows the number of nuclides discovered at these four laboratories as a function of time. Aston’s mass spectrographs at the Cavendish Laboratory in Cambridge, UK led the efforts in the 1920s and 1930s. Berkeley was the leading laboratory for the next three decades due to signifi-cant accelerator developments. Beginning with Lawrence’s first cyclotron accelerating light particles accelerator physicists at Berkeley built new accelerators with increas-ing beam energies higher and the capability to accelerate heavy ions leading to the discovery of almost 40% of all isotopes during this time period. Berkeley also pioneered the target- (spallation) as well as projectile fragmentation reactions. The Joint Institute for Nuclear Research JINR at Dubna made significant contributions in the 1960s and

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United States contributing almost half (42%) of all nuclides discovered so far.

Figure 2 displays the number of nuclides discovered per decade for these five countries. In addition, Japan is also shown demonstrating the most recent contributions. Dur-ing the 1940s and 1950s about 80% of all nuclides were discovered in the United States. However, during the last decade (2002–2011) the United States fell behind Germany and Japan with less than 20%.

While the number of individual facilities producing new nuclides dropped dramatically over the last 50 years the overall interest of the international community remained strong. The number of countries discovering new nuclides has remained fairly constant around 12 during the last 90

years with a maximum of 16 during the 1960s and 1970s. During the last decade (2002–2011) 9 different countries reported the observation of new nuclides.

AuthorsThe discovery of the nuclides was published in 1,508

papers by almost 900 different first authors and over 3,300 different coauthors.

Four researchers coauthored papers discovering over 200 nuclides. G. Münzenberg from GSI leads the list with 218 nuclides followed by H. Geissel (GSI, 210), F.W. Aston (Cambridge, 207), and P. Armbruster (GSI, 201). Münzen-berg’s research covered a remarkablely broad range from the very light (unbound) 12Li produced in proton removal reactions with radioactive beams to 277Cn formed in fu-sion–evaporation reactions. The impressive careers of G. Seaborg (LBL, 94) and A. Ghiorso (LBL, 117) spanned 57 and 48 years of discovering nuclides, respectively.

The change from small single author papers to large collaborations working at large facilities that occurred in nuclear physics in general is also reflected in the number of authors per paper in the discovery papers. Figure 3 dem-onstrates the rapid increase over the last few years. For ex-ample, Aston was the single author of 38 of his 39 papers reporting his discoveries of stable nuclides in the 1920s and 1930s. In contrast, the recent observation of 47 nuclides at RIBF at RIKEN was coauthored by 60 researchers [11].

It is thus also not surprising that Aston leads the list of first-author publications (207), followed by M. Bernas

Figure 2. Number of nuclides discovered per decade at the top five countries: the United States, Germany, the United Kingdom, USSR/Russia, and France, as well as Japan. Japan is included because of the large number discovered during the last decade.

Rank Country Nucl. % Years

1 USA 1311 42.09 1907–2011 2 Germany 492 15.79 1898–2011 3 UK 300 9.63 1900–1994 4 USSR/Russia 245 7.87 1957–2010 5 France 214 6.87 1896–2005 6 Switzerland 129 4.14 1934–2009 7 Japan 126 4.04 1938–2010 8 Sweden 62 1.99 1945–1993 9 Canada 61 1.96 1900–199810 Finland 37 1.19 1961–201011 Netherlands 36 1.16 1934–197612 China 26 0.84 1991–200413 Belgium 17 0.55 1967–199114 Denmark 14 0.43 1935–199415 Argentina 12 0.39 1954–196316 Italy 11 0.35 1934–201017 Austria 6 0.19 1936–1966 Israel 6 0.19 1972–197919 Norway 2 0.06 1956 India 2 0.06 1935 Australia 2 0.06 1985–198822 New Zealand 1 0.03 1968 Brazil 1 0.03 1956 Poland 1 0.03 1934 Hungary 1 0.03 1973

Table 2. List of all countries where nuclides were discovered. The total of number of nuclides discovered, the percentages, and the range of years when nuclides were discovered are listed.

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(110), T. Ohnishi (49), and Yu. Ts. Oganessian (47), who performed their experiments at GSI, RIKEN, and Dubna, respectively.

The number of nuclides reported per paper is another indicator for the changing technologies involved in the dis-covery of nuclides and is shown in Figure 4. The number is fairly constant around the overall average of two nuclides per paper. The increase during the 1920s is due to Aston’s mass spectrograph experiments, which allowed him to measure and report several elements fairly quickly. The recent rapid increase is due to the discoveries at projectile fragmentation facilities that are able to identify many nuclides simultane-ously at a given setting of the fragment separator. The most new nuclides reported in a single paper were published by M. Bernas et al. with 58 and was based on an experiment performed at the fragment separator at GSI [12].

SummaryThe documentation of the discovery of nuclides is an

ongoing project. While most of the discoveries are not con-troversial some cases are certainly debatable. Some of the assignments can change when new measurements demon-strate that earlier reported results were incorrect.

Complete tables of the data presented here as well as fur-ther details and links to all discovery papers can be found at Ref. [6]. Comments, corrections, and suggestions are always welcome and should be sent to thoennessen@nscl.msu.edu.

AcknowledgmentsThis work was supported by the National Science Foun-

dation under grant Numbers PHY06–06007 and PHY11–02511 (NSCL), PHY07–54541 and PHY10–62410 (REU), the High School Honors Science Program (HSHSP) pro-gram, and the Professorial Assistantship Program of the Honors College (PAPHC) at MSU.

I thank the many undergraduates who worked on the compilation of individual nuclides: S. Amos (REU, 2009), A. Bury (HSHSP, 2008), J. Claes (MSU, 2010), A. Frit-sch (REU, 2008), C. Fry (REU, 2011), K. Garofali (MSU, 2010), J.Q. Ginepro (PAPHC, 2007–2008), J.L. Gross (MSU, 2009–2011), M. Heim (REU, 2008), J. Kathawa (MSU, 2010–2011), E. May (PAPHC, 2010–2011), D. Mei-erfrankenfeld (MSU, 2009), A. Nystrom (REU, 2010), A. Parker (REU, 2010), R. Robinson (MSU, 2010–2011), A. Schuh (REU, 2008), A. Shore (REU, 2008), T. Szymanski (MSU, 2009)

References 1. G. Q. Ginepro, J. Snyder, and M. Thoennessen, At. Data Nucl.

Data Tables 95 (2009) 805. 2. M. Thoennessen, Rep. Prog. Phys. 67 (2004) 1187. 3. R.P. Haddock et al., Phys. Rev. Lett. 14 (1965) 318. 4. B.G. Harvey et al., Science 193 (1976) 1271. 5. IUPAC Transfermium Working Group, Pure Appl. Chem. 63

(1991) 879. 6. http://www.nscl.msu.edu/~thoennes/isotopes 7. M. Thoennessen and B.M. Sherrill, Nature 473 (2011) 25. 8. http://www.youtube.com/watch?v=oRkc521no94 9. E. S. Reich, Nature News, 4 October 2011. doi: 10.1038/

news.2011.57110. Trend Watch: Isotope ranking reveals leading labs, Nature

478 (2011) 160.11. T. Ohnishi et al., J. Phys. Soc. Japan 79 (2010) 073201.12. M. Bernas et al., Phys. Lett. B 415 (1997) 111.

Figure 3. Number of authors per paper.

Figure 4. Number of nuclides per paper.