Current status and future potential of nuclide discoveries

In 1908, Rutherford and Geiger had identified the α-particle as helium [11] and in 1913 Thompson accepted in addition to neon with mass number 20 the presence of a separate neon substance with mass number 22 which represented the beginning of mass spectroscopic methods to identify isotopes as separate identities of the same element with different mass numbers [15]. The first 'mass spectra' were measured by Aston when he added focusing elements to his first 'positive ray spectrograph' in 1919 [16]. From 1919 to 1930 the number of known identified nuclides jumped from 40 to about 200 mostly due to Aston's work. The development of more sophisticated mass spectrographs by Aston [17, 18] and others [19–21] led to the discovery of essentially most of the stable nuclides [22].

In 1919 Rutherford discovered nuclear transmutation: 'from the results so far obtained it is difficult to avoid the conclusion that the long-range atoms arising from collision of α particles with nitrogen are not nitrogen atoms but probably atoms of hydrogen, or atoms of mass 2' [23]. He apparently observed the reaction ¹⁴N(α,p); however, it took six years before Blackett identified the reaction residue as the new nuclide ¹⁷O [24]. It took another seven years before in 1932 the discovery of the neutron by Chatwick [25] and the first successful construction of a particle accelerator by Cockcroft and Walton [26] led to the production of many new nuclides by nuclear reactions.

Cockcroft and Walton were able to prove the production of ⁸Be using their accelerator: '...the lithium isotope of mass 7 occasionally captures a proton and the resulting nucleus of mass 8 breaks into two α-particles...' [27]; Harkins, Gans and Newson produced the first new nuclide (¹⁶N) induced by neutrons (¹⁹F(n,α)) [28] and in 1934, Curie and Joliot observed artificially produced radioactivity (¹³N and ³⁰P)¹ in (α,n) reactions for the first time [29].

Also in 1934, Fermi claimed the discovery of a transuranium element in the neutron bombardment of uranium [30]. Although the possibility of fission was immediately mentioned by Noddack: 'It is conceivable that [...] these nuclei decay into several larger pieces' [31], even with mounting evidence in further experiments, Meitner, Hahn and Strassmann did not take this step: 'These results are hard to understand within the current understanding of nuclei' [32] and 'As chemists we should rename Ra, Ac, Th to Ba, La, Ce. As 'nuclear chemists' close to physics, we cannot take this step, because it contradicts all present knowledge of nuclear physics' [33]. After Meitner and Frisch correctly interpreted the data as fission in 1939 [34], Hahn and Strassmann identified ¹⁴⁰Ba [35] in the neutron induced fission of uranium. The first transuranium nuclide (²³⁹Np) was then discovered a year later by McMillan and Abelson in neutron capture reactions on ²³⁸U [36].

Light-particle induced reactions using α-sources, neutron irradiation, fission, and continuously improved particle accelerators expanded the chart of nuclei toward more neutron-deficient, neutron-rich and further transuranium nuclides for the next two decades. The number of nuclides produced every year continued to increase only interrupted by World War II. By 1950 the existing methods had reached their limits and the number of new isotopes began to drop. New technical developments were necessary to reach isotopes further removed from stability.

Although Alvarez demonstrated already in 1940 that it was possible to accelerate ions heavier than helium in the Berkeley 37-inch cyclotron [37], the next major breakthrough came in 1950 when Miller et al successfully accelerated detectable intensities of completely stripped carbon nuclei in the Berkeley 60-inch cyclotron [38]. Less than two months later Ghiorso et al reported the discovery of ²⁴⁶Cf in the heavy-ion fusion–evaporation reaction ²³⁸U(¹²C,4n) [39]. This represented the first correct identification of a californium nuclide because the discovery of the element californium claimed the observation of ²⁴⁴Cf [40] which was later reassigned to ²⁴⁵Cf [41].

With continuous increases of beam energies and intensities fusion–evaporation reactions became the dominant tool to populate and study neutron-deficient nuclei. The peak in the overall production rate of new nuclides around 1960 is predominantly due to the production of new neutron-deficient nuclides and new superheavy elements. Fusion–evaporation reactions are currently still the only way to produce superheavy elements. The discovery of new elements relies on even further improvements in beam intensities and innovations in detector technology.

The significant beam energy increases of light-ion as well as heavy-ion accelerators opened up new ways to expand the nuclear chart. In the spallation or fragmentation of a uranium target bombarded with 5.3 GeV protons, Poskanzer et al were able to identify several new neutron-rich light isotopes for the first time (¹¹Li, ¹²Be and ^14,15B) in 1966 [42]. Target fragmentation reactions were effectively utilized to produce new neutron-rich nuclides (see, for example, [43]) using the isotope separation on-line (ISOL) method. This technique was developed already 15 years earlier for fission of uranium by Kofoed-Hansen and Nielsen who discovered ⁹⁰Kr and ^90,91Rb [44].

The inverse reaction, the fragmentation of heavy projectiles on light-mass targets was successfully applied to produce new nuclides for the first time in 1979 by bombarding a beryllium target with 205 MeV/nucleon ⁴⁰Ar ions [45]. Projectile fragmentation began to dominate the production of especially neutron-rich nuclei starting in the late 1980s when dedicated fragment separators came online. For an overview of the various facilities, for example, the LISE3 spectrometer at GANIL [46], the RIPS separator at RIKEN [47], the A1200 and A1900 separators at NSCL [48, 49], and the FRS device at GSI [50] see [51]. In addition to these separators a significant number of nuclides were discovered at storage rings, see, for example, [52, 53].

The most recent increase in the production rate of new nuclides is predominantly due to new technical advances at GSI [53–55] and the new next generation radioactive beam facility RIBF [56] with the separator BigRIPS [57] at RIKEN.

It is interesting to follow the discovery of nuclides over the years as a function of isotopes (Z = constant), isotones (N = constant) and isobars (A = constant) as shown in the top, middle and bottom panels of figure 2, respectively.

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Unique characteristics of isotopes of elements from the radioactive decay chains were determined around 1900, and although the concept of isotopes was not established at that time these observations can be taken as the first identification of isotopes of these elements. For most of the elements up to Z = 60 the first isotope was discovered in the early 1920s with exception of the transition metals of the fifth period between niobium and palladium which were identified for the first time in the 1930s. Also, as mentioned earlier, isotopes of helium (⁴He or the α-particle [11]) and neon (^20,22Ne [15]) were discovered earlier and the neutron was discovered in 1932 [25].

Isotopes of the remaining stable elements were identified by the late 1930s. The last four missing elements below uranium were discovered by the identification of their specific isotopes. They were technetium (Z = 43) in 1938 [58], francium (Z = 87) in 1939 [59], astatine (Z = 85) in 1940 [60], and promethium (Z = 61) in 1947 [61]. Transuranium elements were then discovered starting in 1940 with the identification of neptunium (²³⁹Np) [36] at an approximately constant rate of about one element every three years (also see figure 5).

Plotting the year of discovery as a function of isotones reveals another pattern. In the light-mass region—approximately between chlorine and zirconium (N ∼ 20–50)—the even-N isotones were discovered around 1920 while it took about another 15 years before the odd-N isotones were identified. This is due to the significantly smaller abundances of the even-Z/odd-N isotones in this mass region. In contrast, the abundances are more equally distributed in the lanthanide region (N ∼ 80–110). While the advances in the discovery of new elements were fairly constant, the discovery of isotones displays a different pattern.

Although intense neutron irradiation of plutonium in the Idaho Materials Test Reactor did not discover any new elements, the successive neutron capture reactions produced many new isotones. In 1954 alone seven new isotones (N = 150–156) were discovered. However, in the following 40 years only one additional isotone was added per decade.

At Dubna hot fusion reactions were used to populate new elements leading to the discovery of 15 new isotones within one year (2004) up to the heaviest currently known isotone of N = 177. The recent discovery of element 117 and 118 did not push the isotone limit any further. It should be mentioned that the isotone N = 164 has not yet been identified (see also section 4.4).

The pattern of the discovery as a function of mass number up to A ∼ 200 shown in the bottom panel of figure 2 mirrors approximately the pattern of the isotones. Until 1937, when Meitner et al [32] discovered ²³⁹U, the discovery of radioactivity by Becquerel in 1896 [4] later attributed to ²³⁸U represented the heaviest nuclide. The missing (4n + 3) radioactive decay chain observed in 1943 by Hagemann et al [62] filled in the gaps at masses 213, 217, 221, 225 and 229. Currently the heaviest element (Z = 118) also represents the heaviest nuclide (A = 294).

Until the end of 2011 twenty-six nuclides had only been reported in conference proceedings or internal reports. Table 2 lists these nuclides along with the author, year, laboratory, conference or report and reference of the discovery. Most of them were reported at least ten years ago, so that it is unlikely that these results will be published in refereed journals in the future. Conference proceedings quite often contain preliminary results and it is conceivable that these results then do not hold up for a refereed journal.

A curious case is the reported discovery of ^155,156Pr and ^157,158Nd in the proceedings of RNB-3 in 1996 [96] where these nuclides were included as newly discovered in a figure of the chart of nuclides. The authors also stated: 'In this first experiment, 54 new isotopes were discovered, ranging from $^{86}_{32}{\rm Ge}$ to $^{158}_{60}{\rm Nd}$' [96]. However, in the original publication only 50 new isotopes were listed and there was no evidence for the observation of any praseodymium or neodymium isotopes [97]. A modified version of the nuclide chart showing these nuclei was included in two further publications [98, 99].

These two neodymium isotopes (^157,158Nd) have recently been reported (see section 5) by van Schelt et al [100] and Kurcewicz et al [101], respectively.

Another argument for not giving full credit for a discovery reported in conference proceedings are contributions from single authors (for example [102, 103]). These experiments typically involve fairly large collaborations and it is not clear that these single-author papers were fully vetted by the collaboration. Also everyone involved in the experiment and the analysis should get the appropriate credit.

The authors of the more recent proceedings and reports are encouraged to fully analyze the data and submit their final results for publication in referred journals.

The proton dripline has been crossed in the medium-mass region between antimony and bismuth (Z = 51–83) with the observation of proton emitters of odd-Z elements. Promethium is the only odd-Z element in this mass region where no proton emitters have been discovered yet. In these experiments the protons are detected in position sensitive silicon detectors correlated with the implantation of a fusion–evaporation residue after a mass separator. The high detection efficiency for these protons makes this method very efficient and nuclides far beyond the proton dripline with very small cross sections can be identified.

In contrast, for nuclides closer to the dripline proton emission is not the dominant decay mode due to the smaller Q-values for the proton decay. The identification of these nuclides is more difficult because of the lower detection efficiency for β- and γ-rays. In fact many of these nuclei were identified by β-delayed proton emission from excited states of the daughter nuclei. Thus, there are isotopes not yet discovered between the lightest β-emitters and the heaviest proton emitters for the odd-Z elements.

Figure 3 shows the medium-mass neutron-deficient region of the chart of nuclides. The thick (red) borders indicate proton emitters and the gray shades of the nuclides indicate the decade of discovery.

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Currently nine odd-Z nuclides (^118,119La, ^122,123La, ^132,133Eu and ^136,137,138Tb) fall into these gaps. For the tenth missing nuclide, ¹⁴³Ho, ¹⁴²Ho has already been identified by β-delayed proton emission [125]. In fact, decay properties of ¹⁴³Ho have also been measured but the results were only reported in an annual report [104].

There are three even-Z holes in this mass region: ¹²⁶Nd, ¹³⁶Gd and ¹⁵⁰Yb. In all three cases, the even more neutron-deficient nuclides were observed by the detection of β-delayed proton emission at the Institute of Modern Physics, Lanzhou, China (¹²⁵Nd [126], ¹³⁵Gd [127] and ¹⁴⁹Yb [128]).

The identification of ¹²⁶Nd, ¹³⁶Gd and ¹⁵⁰Yb in the fragmentation reaction of a 30 MeV/nucleon ¹⁹⁷Au beam has been reported only in a contribution to a conference proceeding [103]. The recent advances in beam intensities and detection techniques for fragmentation reactions (especially identification and separation of charge states) should make it possible to discover these and many more additional nuclides along and beyond the proton dripline in this mass region.

In contrast to the proton dripline the neutron dripline has not been reached for medium mass nuclides. The heaviest neutron-rich nuclide shown to be unbound is ³⁹Mg [129]. Most of the most neutron-rich nuclides have been produced in projectile fragmentation or projectile fission over the last 15 years. The nuclides are separated with fragment separators according to their magnetic rigidity (= momentum over charge of the nuclides which corresponds approximately to their A/Z) and identified by time-of-flight and energy-loss measurements.

Figure 4 displays the neutron-rich part of the chart of nuclides between argon and thorium (Z = 18–90) as a function of A/Z. It shows the A/Z ranges covered by the different experiments. The figure also includes the most recent measurement by Kurcewicz et al [101] (see section 5). If one considers that the location of the neutron dripline is predicted to be more or less constant at about 3.2 in this mass region, it is clear from the figure that it is still far away. The limits of the projectile fragmentation/fission method are currently determined by the small cross sections which can be overcome to an extend by improvements of primary beam intensities and/or larger acceptance separators. In the long term the method is limited by the limited availability of neutron-rich projectiles.

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The discovery of superheavy nuclides has always been special because it is directly related to the discovery of new elements. It is interesting to follow the evolution of element discovery and the discovery of nuclides. In the 1990 book 'The elements beyond uranium' Seaborg and Loveland showed the number of discovered transuranium elements and nuclides as a function of year [139]. Figure 5 displays an extension of these data until today. The number of discovered nuclides tracks closely the number of discovered elements with about 10 isotopes per elements.

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In addition to the efforts to discover elements 119 and 120 it is important to link the isotopes of the elements beyond 113 to known nuclides. Figure 6 shows the nuclear chart beyond nobelium. It shows the separation of the more neutron-rich nuclides up to Z = 118 produced in 'hot' fusion–evaporation reactions from the less neutron-rich nuclides up to Z = 113 which were predominantly produced in 'cold' fusion–evaporation reactions. No isotone with N = 164 has been observed so far which does not mean that this isotone line corresponds to the separation of the decay chains.

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Table 3 lists the ten currently observed unconnected decay chains. There are five even-Z and five odd-Z chains. The four chains starting at ²⁸²113, ²⁸⁵Fl, ²⁸⁸115 and ²⁹¹Fl bridge the N = 164 gap and end in ²⁷⁰Bh, ²⁶⁵Rf, ²⁶⁸Db and ²⁶⁷Rf, respectively. It should be mentioned that the odd-Z N–Z = 57 chain passes through the N = 164 isotone ²⁷¹Bh; however, the properties of this nuclide could not unambiguously be determined [95].

The decay chains cannot be connected to known nuclides by extending them to lower masses because they terminate in nuclides which spontaneously fission. The relationship has to be established by systematic features of neighboring isotopes for different elements. Thus the missing isotopes ^279−281113, ^278−280Cn, ^275−277Rg and ^274−276Ds as well as the other N = 164 isotopes ²⁷³Mt, ²⁷²Hs, ²⁷¹Bh, ²⁷⁰Sg and ²⁶⁹Db have to measured. In total there are 39 nuclides still to be discovered between already known light and heavy rutherfordium and Z = 113 nuclides.

In addition, there are a few gaps of unknown nuclides in the lighter (trans)uranium region. ²³⁹Bk and the two curium isotopes ^234,235Cm have yet to be discovered, although as mentioned in section 4.1 the curium isotopes have been reported in annual reports. Also three uranium isotopes are still unknown, ^220,221U and ²⁴¹U. It is especially surprising that the two lighter isotopes ^220,221U have not been observed because three even lighter isotopes (^217−219U) are known. ²²²U was formed in the fusion–evaporation reaction ¹⁸⁶W(⁴⁰Ar,4n) [140] and most of the other light uranium isotopes were formed in 4n or 5n reactions. Thus ^220,221U should be able to be populated and identified in ¹⁸⁴W(⁴⁰Ar,4n) and ¹⁸⁶W(⁴⁰Ar,5n) reactions, respectively.

As mentioned in section 3 the present definition of nuclides also includes very short-lived nuclides beyond the proton and neutron driplines. So far, these nuclides are only accessible in the light-mass region and characteristics of many of these nuclides up to magnesium beyond the proton dripline and up to oxygen beyond the neutron dripline have been measured.

The proton dripline has most likely been reached or crossed for all elements up to technetium (Z = 43). Table 4 lists the first isotope of elements between aluminum and technetium which has been shown to be unbound but which has not been identified yet or the first isotope for which nothing is known, so that in principle it still could be bound or could have a finite lifetime. With maybe the exception of scandium, bromine and rubidium where resonances have been already measured for ³⁸Sc [141], ⁶⁹Br [142] and ⁷³Rb [143], resonance parameters for at least one isotope of these elements should be in reach in the near future.

For elements lighter than aluminum at least one unbound isotope has been identified. Although not impossible it is unlikely that further nuclides will exist for which characteristic resonance parameters can be measured.

For neutron-rich nuclei characteristic properties of at least two isotopes beyond the neutron dripline have been identified for the lightest elements, hydrogen, helium and lithium. Neutron-rich nuclides between beryllium and magnesium which have been shown or expected to be unbound but have not been observed are listed in table 5. Most of these nuclides should be able to be measured in the near future. In fact, ¹⁶Be, ²⁶O and ²⁸F have been discovered recently (see section 5). The open question whether the (A − 3Z = 6) nuclei between fluorine and magnesium (³³F, ³⁶Ne, ³⁹Na and ⁴²Mg) should be answered in the near future with the available increased intensities of the RIBF at RIKEN [56]. Beyond aluminum the dripline has most likely not been reached yet with the observation that ⁴²Al is bound with respect to neutron emission [151].