Case Study 11.2: The Cold Fusion vs. Hot Fusion Race — GSI, Dubna, and RIKEN

Three Laboratories, Two Methods, One Periodic Table

The story of superheavy element discovery in the late 20th and early 21st centuries is inseparable from the story of three laboratories: GSI in Darmstadt, Germany; JINR in Dubna, Russia; and RIKEN in Wako, Japan. Each laboratory brought a different experimental philosophy, different accelerator capabilities, and different national scientific traditions to the same question: how far can the periodic table be extended?

The competition among these three labs — sometimes acrimonious, often mutually stimulating, and ultimately complementary — drove some of the most remarkable experimental achievements in modern nuclear physics.

GSI: The Cold Fusion Pioneers

The Gesellschaft fur Schwerionenforschung (Society for Heavy-Ion Research), founded in 1969 in Darmstadt, was designed from the ground up for heavy-ion physics. Its centerpiece was UNILAC, the Universal Linear Accelerator, which could accelerate any ion from carbon to uranium. In the 1970s and 1980s, a team led by Peter Armbruster and Gottfried Münzenberg developed the cold fusion approach to superheavy element synthesis.

The GSI philosophy was methodical and conservative. Each new element was identified through its complete alpha-decay chain — a sequence of alpha decays connecting the unknown parent to a known, well-characterized daughter nucleus. The identifying signature was not just the alpha energies but the correlation in time and position between successive decays: the parent nucleus was implanted in a specific pixel of a position-sensitive silicon detector, and each daughter nucleus decayed in the same pixel, with characteristic energy and time intervals.

Between 1981 and 1996, GSI discovered six elements:

• Element 107 (bohrium), 1981: $^{54}$Cr + $^{209}$Bi $\rightarrow$ $^{262}$Bh + n
• Element 108 (hassium), 1984: $^{58}$Fe + $^{208}$Pb $\rightarrow$ $^{265}$Hs + n
• Element 109 (meitnerium), 1982: $^{58}$Fe + $^{209}$Bi $\rightarrow$ $^{266}$Mt + n
• Element 110 (darmstadtium), 1994: $^{62}$Ni + $^{208}$Pb $\rightarrow$ $^{269}$Ds + n
• Element 111 (roentgenium), 1994: $^{64}$Ni + $^{209}$Bi $\rightarrow$ $^{272}$Rg + n
• Element 112 (copernicium), 1996: $^{70}$Zn + $^{208}$Pb $\rightarrow$ $^{277}$Cn + n

The cross sections decreased from nanobarns (elements 107-109) to picobarns (elements 110-112). For element 112, only two atoms were observed in weeks of beam time. It was clear that cold fusion had reached its practical limit: extending to Z = 113 and beyond would require prohibitively long irradiation times.

The GSI contribution to the field extended beyond the discoveries themselves. Münzenberg and colleagues developed the velocity filter SHIP (Separator for Heavy Ion reaction Products), which became the model for recoil separators worldwide. The systematic identification methodology — requiring complete decay chains correlated in time and position — set the standard for how element discovery claims would be evaluated by the IUPAC/IUPAP Joint Working Parties.

JINR Dubna: The Hot Fusion Revolution

The nuclear physics tradition at JINR (Joint Institute for Nuclear Research) in Dubna stretches back to the 1950s. Georgy Flerov — who had independently suggested the idea of the nuclear chain reaction during World War II and later discovered spontaneous fission — built the Dubna nuclear physics program into one of the world's strongest. His student and successor, Yuri Oganessian, would transform the field.

Oganessian recognized in the 1970s that the cold fusion approach, while brilliant, had a fundamental limitation: the cross sections for producing elements beyond Z = 112 would be unmeasurably small. He proposed an alternative: use $^{48}$Ca as a projectile and actinide targets. The key insight was that $^{48}$Ca's doubly magic structure would enhance the fusion probability, while its large neutron excess would produce compound nuclei closer to the N = 184 shell closure.

The Dubna approach used the U400 cyclotron to accelerate $^{48}$Ca and the DGFRS (Dubna Gas-Filled Recoil Separator) to separate the reaction products. The DGFRS used hydrogen gas at low pressure to equilibrate the charge states of the recoiling ions, enabling efficient separation by magnetic rigidity regardless of the initial charge-state distribution. This was a simpler and more efficient design than the velocity filters used at GSI, though it provided less precise particle identification.

Between 1998 and 2010, Oganessian's team at Dubna, often in collaboration with American laboratories (Lawrence Livermore National Laboratory, Oak Ridge National Laboratory), produced elements 113-118. The experimental tour de force included:

• Element 114 (1998): $^{48}$Ca + $^{244}$Pu
• Element 116 (2000): $^{48}$Ca + $^{248}$Cm
• Element 115 (2003): $^{48}$Ca + $^{243}$Am
• Element 113 (2003): as daughter of element 115
• Element 118 (2006): $^{48}$Ca + $^{249}$Cf
• Element 117 (2010): $^{48}$Ca + $^{249}$Bk

The cross sections were in the range of 0.5-10 pb — remarkably, the same order of magnitude as the cold fusion cross sections for lighter elements. This meant that hot fusion could produce elements that cold fusion could not.

The Dubna claims were initially met with skepticism in some quarters of the international community. The decay chains of elements 114-116 did not always terminate in known nuclei (some ended in spontaneous fission of previously uncharacterized isotopes), making independent verification more difficult. But confirmation experiments at GSI, Berkeley, and elsewhere gradually established the Dubna results beyond reasonable doubt.

RIKEN: The Japanese Entry

Japan's entry into the superheavy element race came through RIKEN (Rikagaku Kenkyusho, the Institute of Physical and Chemical Research) in Wako, near Tokyo. The RIKEN program was led by Kosuke Morita, a meticulous experimentalist who had studied at GSI and brought the cold fusion methodology back to Japan.

RIKEN's approach was to extend cold fusion to element 113 using the reaction $^{70}$Zn + $^{209}$Bi $\rightarrow$ $^{278}$Nh + n. This was the same philosophy as GSI — cold fusion with a lead or bismuth target — but pushed to its absolute limit. The cross section was predicted to be in the femtobarn range, meaning that detecting even a single atom would require years of beam time.

RIKEN's GARIS (Gas-filled Recoil Ion Separator) was optimized for this search, and the RIKEN linear accelerator (RILAC) provided the intense $^{70}$Zn beam. The experiment began in September 2003.

The first atom of element 113 was observed on July 23, 2004. The decay chain was stunning: $^{278}$Nh decayed by alpha emission through a sequence of six alpha decays, terminating in the known nucleus $^{254}$Md (mendelevium-254, whose properties had been measured independently). The complete chain, correlated in time and position in the detector, provided an unambiguous identification.

The second atom was observed on April 2, 2005. Then came a long drought. The RIKEN team continued the search for more than seven years without another event — a test of patience and perseverance that is almost without parallel in experimental physics.

The third atom was finally observed on August 12, 2012. This event was particularly important because the decay chain followed a different path than the first two, ending in $^{254}$No (nobelium-254) through an alpha-decay branch that had not been observed before. The observation of two distinct decay pathways from the same parent nucleus provided compelling evidence that the observed nucleus was indeed $^{278}$Nh.

The Priority Question

When IUPAC and IUPAP convened their Joint Working Parties (JWP) to evaluate the discovery claims for elements 113-118, the question of priority for element 113 was the most contested.

Dubna's claim: Element 113 had been observed as early as 2003, as the alpha-decay daughter of element 115 ($^{288}$Mc $\rightarrow$ $^{284}$Nh + $\alpha$). Dozens of $^{284}$Nh atoms had been identified through their position in the decay chains of Mc. However, the decay chain from $^{284}$Nh itself ended in spontaneous fission of a previously unknown isotope, so the identification rested on the internal consistency of the decay chains rather than on a connection to known nuclei.

RIKEN's claim: Only three atoms of $^{278}$Nh had been observed, but the decay chains terminated in well-known nuclei ($^{254}$Md and $^{254}$No). The identification was therefore grounded in the measured properties of established nuclei, providing an unambiguous connection between the unknown parent and the known daughters.

The JWP, in its 2015 report, awarded the discovery of element 113 to RIKEN. The reasoning emphasized the importance of linking the unknown to the known — a decay chain that terminates in a well-characterized nucleus provides stronger evidence than one that terminates in spontaneous fission of an uncharacterized isotope, regardless of how many events are observed. This decision was significant: it was the first element discovery credited to a laboratory in Asia.

The Complementarity of Approaches

In retrospect, the cold fusion and hot fusion approaches were not competitors but complements. Cold fusion, with its low excitation energies and clean 1n evaporation channels, provided the most unambiguous identifications for elements in the Z = 107-113 range. Hot fusion, with its higher cross sections and access to more neutron-rich products, was the only viable route to elements 114-118 and beyond.

The competition between GSI, Dubna, and RIKEN drove innovation in all three programs. GSI's rigorous identification standards forced Dubna to strengthen its evidence. Dubna's hot fusion breakthrough forced GSI and RIKEN to recognize that cold fusion had reached its limits. RIKEN's patient, precise methodology demonstrated that cold fusion could contribute even in the femtobarn regime.

The field has now entered a new phase. The seventh row of the periodic table is complete. The three original laboratories — joined by new programs in China (IMP Lanzhou), France (GANIL), and the United States (Argonne, FRIB) — are now competing and collaborating to extend the table into the eighth row. The next element, 119, will be the first of a new era.

The Human Element

Behind the technical achievements are human stories worth remembering.

Yuri Oganessian (born 1933) spent his entire career at JINR Dubna. He conceived the $^{48}$Ca + actinide approach in the 1970s, but it took nearly 25 years of accelerator development, target preparation, and separator design before the first superheavy element (Fl) was produced in 1998. He was 65 years old. By the time element 118 was named oganesson in 2016, he was 83.

Kosuke Morita launched the RIKEN search for element 113 knowing that the cross section was in the femtobarn range — meaning that success would require years of continuous operation and perhaps only a handful of atoms to show for it. The nine-year gap between the experiment's start (2003) and the decisive third event (2012) tested not just scientific methodology but human resolve.

Gottfried Münzenberg and Peter Armbruster at GSI pioneered techniques in the 1970s-80s that remain the foundation of superheavy element research. Münzenberg's SHIP separator and Armbruster's systematic approach to element identification shaped how the entire field evaluates discovery claims.

The quest for superheavy elements is, at its core, a human endeavor — driven by curiosity about the limits of nuclear matter, sustained by decades of patience, and rewarded by the knowledge that the periodic table, the most iconic diagram in science, can be extended by human ingenuity.

The Role of Confirmation Experiments

A crucial and sometimes underappreciated part of the superheavy element story is the role of confirmation experiments — independent reproductions of a discovery at a different laboratory. IUPAC's criteria for recognizing the discovery of a new element have evolved over time, but they now strongly emphasize independent confirmation.

For elements 114 and 116, the initial Dubna observations (1998-2000) were confirmed by independent experiments at GSI (2009) and at Lawrence Berkeley National Laboratory (2009-2010). These confirmations were essential for building community confidence in the Dubna results. The confirmation experiments used the same reactions ($^{48}$Ca + $^{242,244}$Pu for Fl, $^{48}$Ca + $^{248}$Cm for Lv) but different separators, detectors, and analysis methods — ensuring that any systematic experimental artifact at Dubna would not be reproduced.

For element 113, the confirmation came through a different route. Dubna's observation of element 113 as the alpha-decay daughter of element 115 was cross-checked by independent experiments at GSI (2012) and at the Australian National University. But the RIKEN observation, with its decay chains terminating in known nuclei, was regarded as self-confirming — the measured properties of the daughter nuclei ($^{254}$Md, $^{254}$No) matched the known values from independent measurements, providing a built-in cross-check.

The confirmation process for elements 117 and 118 has been more limited, owing to the extreme difficulty and cost of the experiments. A confirmation of the element 117 decay chains was reported by GSI in 2014, using a second sample of $^{249}$Bk. Element 118 has not been independently confirmed at a second laboratory as of the mid-2020s, though the internal consistency of the three observed decay chains (with their connection to confirmed element 116 daughters) provides strong evidence.

The lesson for future discoveries — particularly elements 119 and 120 — is that the community will require not just initial observation but independent confirmation before new elements are recognized and named. This means that even after the first atom is produced, years of additional work may be needed before the periodic table is officially extended.

A Cross-Section Comparison

The following table summarizes the peak cross sections achieved for each new element discovery, illustrating the evolution of the field:

The cross-section drop from cold fusion Z = 107 (200 pb) to Z = 113 (0.02 pb) is four orders of magnitude. The hot fusion cross sections from Z = 114 to Z = 118 show a more gentle decline, from about 5 pb to 0.5 pb — one order of magnitude. The predicted cross section for Z = 119 represents another order-of-magnitude drop, testing the limits of current experimental sensitivity.

Discussion Questions

1. The JWP awarded the discovery of element 113 to RIKEN (3 atoms, chains ending in known nuclei) rather than Dubna (many more atoms, chains ending in spontaneous fission of unknown nuclei). Do you agree with this decision? What should the standard of evidence be for claiming the discovery of a new element?

2. The competition between GSI, Dubna, and RIKEN was sometimes contentious, particularly regarding naming rights for elements 104-106 during the Cold War era ("transfermium wars"). To what extent does competition benefit scientific progress in this field? What are the risks of overly competitive environments?

3. The synthesis of element 117 required the production of 22 mg of $^{249}$Bk at ORNL and its transport to Dubna — an international collaboration between the United States and Russia. What does this tell us about the relationship between basic science and international cooperation? How might geopolitical tensions affect future superheavy element research?

4. RIKEN's search for element 113 required 553 days of beam time over 9 years for 3 events. In an era of increasingly results-oriented funding, how do you justify an experiment with such a low event rate? What metrics, other than counting new elements, should be used to evaluate the success of such programs?