Radiochim. Acta 2022; 110(6–9): 417–439
Contribution to “Diamond Jubilee of RCA”
Christoph E. Düllmann*, Michael Block, Fritz P. Heßberger, Jadambaa Khuyagbaatar,
Birgit Kindler, Jens V. Kratz, Bettina Lommel, Gottfried Münzenberg, Valeria Pershina,
Dennis Renisch, Matthias Schädel and Alexander Yakushev
Five decades of GSI superheavy element
discoveries and chemical investigation
https://doi.org/10.1515/ract-2022-0015 1 Introduction
Received January 20, 2022; accepted April 20, 2022;
published online May 16, 2022 The laboratory synthesis of artificial elements heavier than
uranium, theheaviest one found in largequantities onEarth,
Abstract: Superheavy element research has been a strong
has started about 80 years ago, and the synthesis of the first
pillar of the research program at GSI Darmstadt since its
transuranium element, 93Np, was reported in 1940 [1]. Thefoundation. Six new elements were discovered along with
race towards ever heavier elements is still ongoing, fueledby
many new isotopes. Initial results on chemical properties
the quest for an “island of stability” of superheavy nuclei
of the heaviest elements were obtained that allowed for
(SHN), the existence of which followed from calculations
comparing their behavior with that of their lighter homo-
based on the shell model of atomic nuclei, e.g., [2]. These
logs and with theoretical predictions. Main achievements
predicted that shell effects associatedwith the closure of the
of the past five decades of superheavy element research at
next nucleon shells beyond those at proton number Z = 82
GSI are described along with an outlook into the future of
superheavy element research in Darmstadt. and neutron number N = 126, giving rise to doubly magic208Pb, would lead to increased fission barrier heights [3].
Keywords: GSI Darmstadt; SHIP; superheavy elements; From about 1966 on, the next shell closures were generally
TASCA; transactinides. expected at Z = 114 andN= 184 [2], and predictions appeared
that suggested nuclei at and around the predicted doubly-
magic nucleus 298*Corresponding author: Christoph E. Düllmann 114 to have half-lives that are longer than, GSI
Helmholtzzentrum für Schwerionenforschung GmbH, Planckstr. 1, the age of the universe, cf., e.g., [4].
64291 Darmstadt, Germany; Department Chemie – Standort TRIGA, On the way towards exploring the limits of nuclear
Johannes Gutenberg-Universität Mainz, Fritz-Strassmann-Weg 2, stability, the elements up to 100Fm are accessible by
55128 Mainz, Germany; and Helmholtz-Institut Mainz, Staudingerweg neutron-capture reactions in research reactors [5]. Heavier
18, 55128 Mainz, Germany, E-mail: duellmann@uni-mainz.de
Michael Block, GSI Helmholtzzentrum für Schwerionenforschung ones can be produced in the laboratory in the collision of
GmbH, Planckstr. 1, 64291 Darmstadt, Germany; Department two nuclei. For this, projectile nuclei are accelerated to en-
Chemie – Standort TRIGA, Johannes Gutenberg-Universität Mainz, ergies sufficiently high to overcome the Coulomb repulsion
Fritz-Strassmann-Weg 2, 55128 Mainz, Germany; and Helmholtz- between the projectile and target nuclei, both of which are
Institut Mainz, Staudingerweg 18, 55128 Mainz, Germany
positively charged. Nuclear reaction products are separated
Fritz P. Heßberger, Jadambaa Khuyagbaatar, Birgit Kindler, Bettina
Lommel, Valeria Pershina, Matthias Schädel and Alexander by physical or chemical methods and are then analyzed for
Yakushev, GSI Helmholtzzentrum für Schwerionenforschung GmbH, the presence of the desired heavy products. The progress of
Planckstr. 1, 64291 Darmstadt, Germany the synthesis of the heaviest elements has thus been inti-
Jens V. Kratz, Department Chemie – Standort TRIGA, Johannes mately connected with the development of heavy ion ac-
Gutenberg-Universität Mainz, Fritz-Strassmann-Weg 2, 55128 Mainz,
Germany celerators and associated setups suitable for identifying the
Gottfried Münzenberg, GSI Helmholtzzentrum für rare heavy reaction products, predominantly fusion
Schwerionenforschung GmbH, Planckstr. 1, 64291 Darmstadt, products.
Germany; and Institut für Physik, Johannes Gutenberg-Universität Germany entered the race towards the superheavy ele-
Mainz, Staudingerweg 7, 55128 Mainz, Germany ments (SHE)– elementswhere all isotopes exist solely thanks
Dennis Renisch, Department Chemie – Standort TRIGA, Johannes
Gutenberg-Universität Mainz, Fritz-Strassmann-Weg 2, 55128 Mainz, to stabilizing nuclear shell effects – about five decades ago,
Germany; and Helmholtz-Institut Mainz, Staudingerweg 18, 55128 with the proposed accelerator being a central component.
Mainz, Germany Different competing proposals existed. Among them, the
Open Access. © 2022 Christoph E. Düllmann et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0
International License.
418 Ch.E. Düllmann et al.: Five decades of GSI superheavy element discoveries and chemical investigation
UNIversal Linear ACcelerator (UNILAC), designed by Chris- Chemical studies initially focused on isolating long-
toph Schmelzer, was chosen. Initially it was proposed to be lived isotopes from the complex nuclear reaction product
built at Karlsruhe. The UNILAC was designed to allow ac- mixture, before the focus shifted towards elucidating the
celeration of all elements up to 92U to velocities sufficiently chemical properties of the SHE. Figure 2 depicts the current
high to overcome the Coulomb barrier for any type of pro- Periodic Table of the Elements (PTE) with the SHE high-
jectile/target combination. In April 1969, the decision was lighted and with an emphasis on the elements studied at
taken to found the Gesellschaft für Schwerionenforschung GSI. The SHE start a new series in the 7th row of the PTE. In
(GSI), and Darmstadt was selected as the location. The GSI the first ones, from 104Rf to 112Cn, the 6d electron shell is
was founded on December 17, 1969, with two partners; the being filled. After Cn, filling of the 7p shells occurs in the
Federal Republic of Germany and the State of Hessen. The next six elements, from 113Nh through 118Og. Heavier ele-
name changed to GSI Helmholtzzentrum für Schwer- ments with Z = 119 and 120, waiting for their discovery, will
ionenforschung in 2008 to reflect GSI’s role within the belong to groups 1 and 2 of the PTE, accordingly, with the
Helmholtz Association of German research centers. His- filling of the 8s shell. Since relativistic effects (RE) on the
torical aspects that led to the foundation of GSI have been atomic orbitals (AOs) increase as Z2 down the groups of
described numerous times, see, e.g., references in [6]. chemical elements, it was predicted that their influence
Based on early predictions, cross sections of 100 mb should be very strong in SHEs. The fundamental aspects of
were expected, leading to production rates of 106 s−1. For relativistic quantum theory and the influence of RE on
the isolation of the produced superheavy nuclei, the chemical properties of SHEs are described in many publi-
Separator for Heavy Ion reaction Products (SHIP) [7] was cations, from pioneering works based on atomic relativistic
designed and built and was ready for first experiments calculations [9, 10], reaching today’s sophisticated molec-
when first UNILAC-beams became available in 1976. At that ular and solid-state calculations [11]. According to these
time, discoveries of all elements up to 106Sg were reported. predictions, properties of the earlier transition metal ele-
Chemical approaches were developed as well to isolate ments (104Rf through 108Hs), should be defined by the
new elements but also to study their chemical properties. valence 6d AOs. Their gradual relativistic destabilization
Over the course of 50 years since the construction of and expansion should result in the properties of SHEs being
GSI, six new elements as well as many new nuclei were in line with the trends in the groups of chemical elements,
discovered at this location, see Figure 1. however, with some peculiarities due to the large spin–orbit
Figure 1: Chart of nuclei of the heaviest elements. For elements up to 118Og, known isotopes are shown. Nuclides discovered at GSI are
indicated in orange boxes, and nuclides produced in experiments at GSI are indicated in yellow. Elements indicatedwith symbols in italics red
have been discovered at GSI, with the underlined isotope being the discovery isotope. Isotopes of elements 119 and 120, which have been
searched for but not discovered to date, are given with the mass number and element symbol in parentheses. The latter are based on the
definition of IUPAC (International Union of Pure and Applied Chemistry) [8].
Ch.E. Düllmann et al.: Five decades of GSI superheavy element discoveries and chemical investigation 419
Figure 2: Periodic table of the elements. Superheavy elements are shown in blue-bordered boxes. Elements discovered at GSI are marked by
orange-filled boxes, and those confirmed at GSI before their official approval by yellow-filled boxes. Elements with symbols given in red have
been chemically studied at GSI; in several cases, these were the first chemical studies of these elements. Elements 119 and 120 have not yet
been discovered.
(SO) effects on the 6d-AOs. At the end of the d series, in SHIPTRAP [20–22], where studies around the N = 152
groups 11 and 12, strong relativistic stabilization and neutron shell closure have been performed [23], employing
contraction of the 7s AOs are predominant over the 6d AO’s the relatively high cross sections associated with the
expansion and destabilization. This should result in an in- 48Ca + Pb fusion reactions. More recently, laser spectro-
crease in inertness and stability of these elements, however, scopic studies of the heaviest elements were established
also in line with the trends in the groups. For even heavier, [24], employing the RAdiation Detected Resonance Ioni-
7p elements, the influence of relativistic effects will be even zation Spectroscopy (RADRIS) technique [25]. Again the
more pronounced, particularly for 113Nh and 114Fl, due to the region of Fm-No around the N = 152 neutron shell closure
stabilization and contraction of the 7p1/2 subshell and huge was in the focus of the first experiments, which contribute
SO effects. Chemical studies conducted at GSI, both theo- information onnuclear [26] aswell as atomic [27] properties
retical and experimental ones, therefore, had the ambitious of the studied species. Comprehensive reviews, not limited
goals to verify those trends and predictions, as well as to to the work at GSI, are included in the 2015 special issue of
provide detailed information on the chemical behavior of “Nuclear Physics A” [28], which gives an overview of all
SHEs. relevant aspects of superheavy element research.
In the past decades, the portfolio of topics studied in This article describes five decades of (SHE) studies at
the heaviest elements has yet broadened. Optimizing and GSI, focusing on the experiments aimed at discovering new
extending the synthesis of heaviest elements, mostly by elements (Section 2) and at obtaining initial information
fusion-evaporation but also by other reactions [12, 13], re- about their chemical properties (Section 3). Besides the
mains at the core of the program, not least as a prerequisite projectile beam provided by the heavy-ion accelerator, a
for any study of these nuclei and elements. Similarly, stable layer of target nuclei able to withstand the intense
chemical studies form a second pillar [11, 14–16]. Also the beam is of prime importance for SHE synthesis. Targets of
study of the nuclear structure of heaviest elements, mainly stable isotopes and the quasi-stable 238U are produced in the
by decay spectroscopy, remains as a pillar [17–19], but was GSI target laboratory,whereas targets of (highly) radioactive
extended by a program on high-precision direct mass transuranium isotopes are prepared at the specialized
measurements in the double-Penning-trap system infrastructure at the Department of Chemistry’s TRIGA-Site
420 Ch.E. Düllmann et al.: Five decades of GSI superheavy element discoveries and chemical investigation
(the former Institute of Nuclear Chemistry) at Johannes A big step forward in the production of heavy elements
Gutenberg University (JGU) Mainz (Section 4). The manu- and deeper insights into the synthesismechanism came from
script closes with an outlook into the next decade(s) of su- studies of 238U on 248Cm performed at GSI in collaboration
perheavy element research in Darmstadt. with US national laboratories [35, 38]. Again, chemical sep-
arations were performed [16, 33]. The enhancement of the
formation of transcurium isotopes in the reaction of 238 U on
2 Search for superheavy elements 248Cm is shown in Figure 3. Cross sections for 100Fm, 99Es, and
98Cf are three to four orders of magnitude higher than in theand discoveries 238Uon 238U reactions [35, 38]. Evaporation calculations assign
the surviving heavy actinides to 3n and/or 4n evaporation
2.1 Early SHE searches and actinide channels from primary fragments. Together with data from
syntheses – reactions 238U + 238U, 238U + the 238U on 238U reaction, the results from the 238U on 248Cm
248Cm and 48Ca + 248Cm reaction provided anchor points that still serve as bench-
marks for theoretical calculations and extrapolations [39, 40].
2.1.1 Transfer reaction studies with a 238U beam on 238U
and 248Cm targets 2.1.2 The 48Ca + 248Cm reaction
Driven by the quest for SHE [29], in a joint venture of the With a completely different outcome, heavy and super-
nuclear chemistry group at GSI and the JGU’s Institute of heavy element programs based on reactions of 48Ca on
Nuclear Chemistry, the experimental program with 238U 248Cm were performed at GSI in two periods: a first one in
beams from GSI’s UNILAC started in 1976 [16] with great 1982–1983, following up on experiments previously per-
optimism that a massive exchange of nucleons in so-called formed in Dubna, Russia, and in Berkeley, USA; see [32] for
“damped collisions” would lead to the synthesis of SHE. All
attempts to discover SHE in reactions of 238U on 238U remained
unsuccessful [30–32]. Thereafter, in 1979, an international
collaboration began an intense search for SHE in reactions of
238U with 248Cm targets. Again, no SHE were identified,
pointing at cross sections being <40 pb for isotopeswith half-
lives between hours and several years [31, 32].
To explore the synthesis of heavy elements, 238U targets
were irradiated with 6.49–9.0 MeV/u 238U beams. Applying
liquid chromatography for chemical separation [33], cross
sections were measured for actinide isotopes up to 256Fm
[34, 35]. This yielded isotope distributions, i.e., curves of
cross sections versus mass number, for actinide elements
up to 100Fm. By exploiting the symmetry of the 92U on 92U
reaction and making use of cross sections measured for
complementary lighter elements, e.g., 84Po for 100Fm, de-
tails of the multi-nucleon transfer process were unraveled.
Cross sections for the surviving heavy actinides indicate
that they are produced in the low-energy tails of the
dissipated energy distributions, however, with a low-
energy cutoff at ≈35 MeV. Excitation functions show that
identical isotope distributions are populated independent
of the bombarding energy, indicating that the same bins of
excitation energy are responsible for the production
of these fissile isotopes. This provided a better under- Figure 3: Cross sections for the formation of target-like transcurium
238 248
standing of the synthesis of heavy actinides and trans- isotopes in the U + Cm reaction at ≤7.40 MeV/u in comparison
with data for the 238U + 238U reaction at ≤7.50 MeV/u; see [35] for
actinides in transfer reactions [35, 36], elucidated limitations more details. (Reprinted figure with permission from J.V. Kratz et al.,
(dynamical hindrance), and challenged diffusion model Phys. Rev. C 88, 054,615 (2013), https://doi.org/10.1103/PhysRevC.
calculations [37]. 88.054615. © 2013 by the American Physical Society).
Ch.E. Düllmann et al.: Five decades of GSI superheavy element discoveries and chemical investigation 421
a status report of all SHE searches up to that time. A more lives from minutes to hours [41]; see curves 3, 4, and 5 in
recent program began in 2005. Figure 4 and [16, 42] for experimental details.
At first, in a collaborative effort, groups from the GSI, At various bombarding energies of 40Ca and 48Ca ions
the JGU, and the Lawrence Berkeley National Laboratory on 248Cm targets, isotope cross sections of 86Rn through 92U
(LBNL) as the leading partners, carried out experiments not (below-target elements) and 97Bk through 100Fm (above-
only applying the Small Angle Separating SYstem (SASSY) target elements) were obtained in radiochemical experi-
at LBNL’s SuperHILAC accelerator and SHIP at the UNILAC ments performed at LBNL and GSI. The following insights
but also a variety of chemical separations. Due to insuffi- into the synthesis of above-target element isotopes were
cient sensitivity in this early period, SHE were not discov- derived from these experiments [42, 44]: (i) The maxima of
ered [41]; see Figure 4. the isotope distributions are located at only 2–3 mass
However, deeper insights to into the synthesis of heavy numbers larger for 48Ca than for 40Ca reactions. (ii) Shapes
elements in multi-nucleon transfer reactions were gained and half-widths of these distributions are similar to those
from cross section measurements of heavy actinide iso- observed earlier for reactions with lighter-element pro-
topes and their complementary below-target elements [42]. jectiles and the widths are smaller than the ones obtained
In 1982–1983, SHE fractionswere chemically separated from 238U-induced reactions [38]. (iii) While the excitation
manually at the LBNL. Thereafter, at the GSI, a new level of functions for 40Ca peak near the Coulomb barrier, those for
sophistication was reached by a continuous transport 48Capeak at≈20MeVabove thebarrier. (iv) In reactionswith
of nuclear reaction products with He/KCl-cluster jets to 48Ca, isotopes are produced essentially cold, i.e., with very
gas-phase chemistry devices and, as an essential step in low excitation energy of ≈(0–12) MeV for 99Es and 100Fm.
liquid-phase chemistry [14, 16], to the first version of After successfully using ARCA in 48Ca on 248Cm ex-
the Automated Rapid Chemistry Apparatus (ARCA) [43]. It periments, improved versions of ARCA were applied. The
drastically speeded up transport and separation times unique potential of 254Es targets was probed in searches for
giving access to nuclides with half-lives in the range of SHE [45, 46] and the synthesis of the most neutron-rich
minutes. In these experiments, cross section limits for SHE isotopes of the heaviest actinides [47]. 136Xe on 244Pu ex-
were obtained in the 0.1 nb range for nuclides with half- periments led to the discovery of 243Np and 244Np [48] and
confirmed the outstanding role of 136Xe as a projectile to
maximize the production of below-target neutron-rich
nuclides in transfer reactions.
2.2 Fusion-evaporation reactions at recoil
separators – SHE searches and
discoveries
The probability to form SHN in fusion-evaporation re-
actions is extremely low; state-of-the-art experimental
separation and detection techniques are required for suc-
cessful experiments [49]. A dedicated instrument, the
SHIP, has been built in the mid-1970s [7]. SHIP is a two-
stage velocity filter with separated electric and magnetic
fields (Figure 5a), which efficiently separates evaporation
residues (ER) from other ions recoiling from the target,
because of their significantly different kinematics [7]. ER
are implanted into a position-sensitive silicon detector to
Figure 4: Upper limit cross sections for SHE production in the 48Ca measure their radioactive decay [50]. Decays of the ER and
on 248Cm reaction in the energy range of 4.5–5.2MeV/u as a function their daughter nuclei can be correlated, allowing identifi-
of the half-life. Data are results from recoil separators (curves 1 and cation of new isotopes and even new elements based on
2) and from a variety of chemical methods (3–8); see [41] for details.
(Reprinted gure with permission from P. Armbruster et al., Phys. one single measured α-decay sequence [51].fi
Rev. Lett. 54, 406–409 (1985), https://doi.org/10.1103/ At SHIP, the elements with Z = 107 to Z = 112, bohrium
PhysRevLett.54.406. © 1985 by the American Physical Society). (107Bh), hassium (108Hs), meitnerium (109Mt), darmstadtium
422 Ch.E. Düllmann et al.: Five decades of GSI superheavy element discoveries and chemical investigation
Figure 5: The two recoil separators installed at GSI. Left: SHIP [7] in its upgraded version [49], total length 13 m, flight time of recoils 2 µs.
Right: Schematic drawing of TASCA [55], total length 3.5 m, flight time of recoils 0.6 µs. (Left panel: Reprinted figure with permission from S.
Hofmannet al., Rev.Mod. Phys. 72, 733–767 (2000), https://doi.org/10.1103/RevModPhys.72.733.©2000by the American Physical Society).
(110Ds), roentgenium (111Rg), and copernicium (112Cn) were experiments; e.g., prior to element 107 synthesis, isotopes
discovered using cold fusion-evaporation reactions using of the α-decay daughter Db were produced to test the
doublymagic 208Pb or its neighbor 209Bi as targets, irradiated experimental method and to create a safe basis for the
by 54Cr, 58Fe, 62,64Ni, and 70Zn beams [49]. parent-daughter correlation by studying the decay prop-
The discovery of 108Hs proofed the concept of SHEs, the erties of the expected daughter isotopes.
existence of nuclei solely by shell stabilization [3, 52]. The Element 107, later named bohrium, was the first
discovery of the region of shell-stabilized hexadecapole- element discovered at SHIP, based on observed six atoms
deformed nuclei centered at Z = 108 and N = 162 [53, 54] from the 54Cr + 209Bi reaction in February 1981 [51]. Figure 6
paved the way towards the present top of the chart of displays one of these α-decay chains from an implanted
nuclei. nucleus, together with two selected daughter chains from
In the late 2000s, the gas-filled TransActinide Separator the companion experiment. The lifetimes of individual
and Chemistry Apparatus (TASCA), see Figure 5b, was put nuclei show (expected) statistical fluctuations. As the
into operation [55]. Its magnets came from a post-separator range of α particles in silicon is larger than the implanta-
used behind SHIP in the late 1980s and early 1990s [56]. The tion depth of the nuclei, only about half of the α particles
TASCA is very efficient for the collection of SHN while back- are registered with full energy in the stop detector, leading
ground suppression is not as high as in SHIP. Application of to partly incomplete chains. The chains end in known α
the correlation method under larger background is possible decays of 250Fm, and 250Md and, thus, could be assigned to
thanks to double-sided silicon-strip detectors, which provide 258Db and 262Bh. These results were confirmed later [60].
thousands of pixels for determining the positions of implan- Next, element 109, meitnerium, was discovered in 1982
ted nuclei. Fast digital electronics allows resolving radioac- in the reaction 209Bi(58Fe,n)266Mt [51] via a single decay
tive decays occurring on sub-µs timescales [57].When TASCA sequence observed in a 19-day long experiment [61]. A nu-
became available for the search for new SHE, the elements clear-chemical study at FLNR [62] measured the long-lived
with Z= 113–118 hadalreadybeen claimed to bediscovered at 266Mt progeny 246Cf, supporting the assignment, which was
the GAs-filled Recoil Ion Separator (GARIS), operating at later also confirmed in further experiments at SHIP in 1988
RIKEN, Wako-shi, Japan (Z = 113) [58], and at the Dubna Gas- [63] and 1997 [64]. In the latter, 12 additional 266Mt atoms
Filled Recoil Separator (DGFRS) at the Flerov Laboratory for were observed. The nuclide features α-decay energies in the
Nuclear Reactions (FLNR), Dubna (Russia) (Z = 114–118) [59]. wide range of 10.5–11.8 MeV; this is a common feature of
Accordingly, the new element synthesis program at TASCA odd–odd SHN.
was dedicated to searches for elements Z = 119 and 120. Element 108, hassium, was discovered in 1984 in the
reaction 208Pb(58Fe,n)265Hs [65] based on three decay chains.
2.2.1 New SHE from SHIP The assignment was supported by a 208Pb(54Cr,xn) compan-
ion experiment to produce 259-261Sg. In 1986, even–even 264Hs
2.2.1.1 The first series: bohrium, hassium, andmeitnerium was observed in the 58Fe + 207Pb reaction [49]. The Hs exper-
The SHE program at SHIP started with element 105, now iments were performed after the Mt one. Theory predicted
named dubnium, and proceeded stepwise towards heavier spontaneous fission half-lives belowmicroseconds for even–
SHE. As the method was new, the first experiments to evenHs isotopes [66].Mt, via odd–odd 266Mt,was synthesized
create new elements were carried out as companion first. This was expected to have a long partial fission half-life
Ch.E. Düllmann et al.: Five decades of GSI superheavy element discoveries and chemical investigation 423
Figure 6: Left panel: A single-atom decay chain ending in the decay of 250Fm. Right panel: two examples of daughter chains. (Reprinted by
permission from Springer-Verlag Zeitschrift für Physik A, Identification of Element 107 by α correlation chains, G. Münzenberg et al. Z. Phys.
A 300, 107–108 (1981), © 1981).
due to an unpaired proton and an unpaired neutron, thus, the understanding of the cold-fusion reaction mechanism,
profiting twice from fission hindrance of unpaired nucleons. guiding the selection of optimum bombarding energies for
This was expected to enhance the fission half-life by about producing heavier nuclei.
106-fold. The unexpected observation of α decay for Hs, The synthesis of new elements was resumed in
especially for even–even 264Hs, together with the results for November 1994. The 269110 was the first isotope of element
even–even 260Sg, was afirst hint at a region of enhanced shell 110, later named darmstadtium. Within 13 days, three
stabilization of nuclei centered at Z = 108 and N = 162 [54]. atomswere observed in the 62Ni + 208Pb reaction [68]. Then,
Thiswas later confirmedand thenucleus locatedat the center the beam was switched to 64Ni to produce 271Ds [71]. The
of this region, 270Hs, was discovered in 2006 in Hs chemistry cross-section increased ≈6-fold compared to 269Ds; thus,
experiments [67] (Section 3.2). the search for element 111, roentgenium, was started using
the reaction 209Bi(64Ni,n)272Rg. In 18 days three decay events
of 272Rg were registered in December 1994 [72]. In January
2.2.1.2 The second series: darmstadtium, roentgenium,
1996, an attempt to produce element 112, copernicium, in
and copernicium
– the reaction
70Zn(208Pb,n)277Cn started. After 34 days of
Although the elements with Z = 107 109 were discovered in
irradiation, two decays attributed to 277Cn were published
the 1980s, shortcomings of the experimental method and set-
≈ – [73]. A reanalysis of the data performed five years laterup became evident: (a) quite low beam intensities of 100
9 −1 revealed that the first decay chain had been created200 nApart (1 nApart = 6.24·10 s ), (b) small transmission of
spuriously. However, in a follow-up experiment, a second
SHIP of <30% for the considered reactions, (c) limitations in
real 277Cn decay chain had been observed [74].
the detection efficiency of the detector system, and (d) insuf-
ficient knowledge of the production mechanism. These
limited proceeding to heavier elements accessible at expect- 2.2.2 Search for new elements 119 and 120 at SHIP and
edly lower rates but were overcome within a few years. A TASCA
high-charge-state injector consisting of an radiofrequency
quadrupole (RFQ) + interdigital H-mode (IH) acceleration After the discoveries of the elements 107Bh to 112Cn [49] it
structure equipped with an electron-cyclotron resonance became obvious that cold fusion-evaporation reactions
(ECR) ion source provided beam intensities >500 nApart [68]. will result in extremely low cross sections for the synthesis
The target position was shifted closer to the SHIP entrance, of yet heavier elements. This was since proven by the dis-
increasing the transmission by about two-fold [69]. The old covery of element 113 at GARIS, see Figure 7. About
‘stop’ detector comprising seven individual detectors was 550 days of beam time yielded the observation of three
replaced by a single 16-strip detector. A box of silicon de- atoms [58]. However, shell-stabilized nuclei can also be
tectors wasmounted, surrounding the backward hemisphere used in projectile-target combinations for SHE synthesis in
of the stop detector to measure escaping α particles [68]. hot-fusion reactions based on the doubly-magic 48Ca pro-
Excitation functions of the 208Pb(50Ti,1-2n)256,257Rf [70] and jectile and actinide targets. Such reactions had already
208Pb(58Fe,n)265Hs [64] reactions were measured to improve been considered for SHE synthesis in the 1970–1980s (see
424 Ch.E. Düllmann et al.: Five decades of GSI superheavy element discoveries and chemical investigation
be considerably higher. In experiments performed in 2007/
08, a cross-section limit of≈90 fbwas reached [81]. No decay
chain was observed that could be attributed to a Z = 120
isotope. In spring 2012, another attempt was undertaken at
SHIP, using the 54Cr + 248Cm reaction. In a ≈40-days long
irradiation, a cross-section limit of 0.58 pb was reached.
Again, no decay chain that could be interpreted to originate
from a Z = 120 isotope was observed [82, 83]. A third
experiment was performed at TASCA, in the reaction 50Ti +
249Cf. This is expected to be the most suitable one for Z = 120
synthesis from the reaction point of view [84]. In ≈39 days of
irradiation, a cross-section limit of 0.2 pb was reached [85].
No genetically correlated α-decay chain that could be
Figure 7: Experimentally measured maximum cross sections for the interpreted to originate from a Z = 120 isotope was observed.
production of elements with Z = 102–118 as well as lowest reported
upper limits for elementswithZ=119and120. The ts (solid:Cold fusion The fourth element 120 search experiment was conductedfi
reactions; short-dashed:Hot-fusion reactionswith lightprojectiles; long- during the search for element 119 (see below).
dashed: 48Ca-induced reactions) are shown to guide the eye.
2.2.2.2 Search for element 119
Section 2.1.2), albeit unsuccessfully. The sensitivity for So far, the last attempt at GSI to synthesize an element
such experiments had since been significantly improved at beyondOgwas a search for element 119 in the reaction 50Ti+
the DGFRS in the late 1990s. As a result, the elements 114– 249Bk at TASCA [86]. The 249Bk has a half-life of only
118 were discovered at FLNR [59]. 327.2 days. 12 mg of this isotope, produced at Oak Ridge
In 2006, 48Ca-beam based campaigns restarted at GSI. National Laboratory, Oak Ridge, USA [5], became available
First, 283Cn, which has six neutrons more than the 277Cn in 2012. Targets were prepared at JGU (see Section 4.2)
produced by cold fusion, was synthesized via the 48Ca + shortly after receiving the isotope. Irradiation started within
238U reaction at SHIP [75]. The permission to irradiate about one month after target production. In a long run of
transuranium targets at SHIP and at TASCA enabled the about four months, no signature of element 119 was
synthesis and study of 114Fl [76], 115Mc [77] and 117Ts [78] at observed. This resulted in a cross-section limit of 65 fb [86].
TASCA and of 116Lv at SHIP [79]. The successful synthesis of element 119 in a future experi-
The continuation of the synthesis of SHE beyond 118Og ment apparently requires a considerably higher sensitivity.
faces many experimental challenges. Fusion-evaporation re- In the course of the four-months experiment, 249Bk
actions with projectiles beyond 48Ca have to be used, due to continuously β− decayed into 249Cf. At the end, the 249Cf
insufficient amountsof isotopeswithZ>98 tomake targets [5]. content was about 35%. This provided a unique opportu-
nity to simultaneously search for element 120. The beam
2.2.2.1 Search for element 120 energy was slightly lower (about 6 MeV) than the one used
Four reactions, 64Ni + 238U, 58Fe + 244Pu [80], 54Cr + 248Cm, in the previous search experiment for element 120 in the
and 50Ti + 249Cf are considered to synthesize element 50Ti + 249Cf reaction. No α-decay chains that could be
Z = 120. Its discovery was attempted in three of them at GSI. attributed to element 120were observed, resulting in a ‘one
Theories predicting the next proton shell closure to occur at event’ cross section limit of 0.2 pb [86].
Z = 114 predict microsecond-half-lives for the Z = 120 iso-
topes produced in these reactions. This should still be long
enough to survive the flight through compact recoil sepa- 3 Chemical studies to classify new
rators, but detection requires a fast data-acquisition system,
prompting an essential upgrade for the focal planedetection elements in the periodic table
technique. To this end, a fast digital data acquisition system
was implemented prior to search for element 120 [57]. 3.1 From actinides to the transactinide
In 2007, SHIP was not yet approved for irradiations of element 106, seaborgium, Sg
highly radioactive targets, so the reaction 64Ni + 238U was
chosen for a first attempt, although the predicted cross In 1985, the nuclear chemistry program began to focus on
section was only ≈5 fb [39]. As such theoretical predictions studies of the influence of RE on chemical properties of the
are rather uncertain, the actual cross-section was hoped to heaviest elements and the architecture of the PTE [14–16,
Ch.E. Düllmann et al.: Five decades of GSI superheavy element discoveries and chemical investigation 425
87–89]. Prior experiments, performed in Berkeley and compounds for group-6 elements and their volatility
Dubna, had shown that 103Lr behaved as expected for the sequence of MoO2Cl2 > WO2Cl2 > SgO2Cl2 [94] motivated
last member of the actinide series and 104Rf as a group-4 probing their formation and volatility. Advances in experi-
element. Now, the group-5 element Db came into the mental techniques enabled studies with ≈10-s 265105 Sg synthe-
focus and was studied. ARCA II was built [16, 90] to study sized in the 22Ne + 248Cm reaction. Studies were carried out in
34-s 262Db, and also 27-s 263Db that was discovered in these liquid [97–99] and gaseous phases [97, 100, 101]. Nuclei,
experiments [89, 91, 92]. Db showed surprising chemical recoiling from the target, were thermalized in He, attached to
properties; simple extrapolations of chemical properties in aerosol particles and flushed to a quartz chromatography
the PTE did not yield trustworthy predictions anymore, but column within 3 s. Reactive gas was added to form volatile
the properties were correctly predicted theoretically on the species. The yield of Sg passing the isothermal column at
basis of fully relativistic calculations of group-5 complexes different temperatures was measured by registering Sg α
and explained by large SO effects on the 6d AOs causing a decay chains [102]. The interaction strength of the species
trend reversal in the group [88]. All experimental and with the column material was determined, see Figure 8 [97,
theoretical results show that Db has chemical properties 100].
typical for a group 5 element. The obtained volatility sequence of MoO2Cl2 >
The 106Sg is placed in group 6 of the PTE, beneath Mo WO2Cl2 ≈ SgO2Cl2 and adsorption enthalpies (−ΔHa) of 90 ± 3,
and W. Theoretical calculations [88] show that Sg has a 96 ± 1, and 98+2−5 kJ/mol, respectively, were the first thermo-
6d47s2 ground state electronic configuration and the chemical information on Sg [100], and were in line with ex-
oxidation state 6+ in aqueous phases and in compounds trapolations in group 6 and with relativistic theory
accessible in gas phase studies [93]. The stability and calculations [94]. Empirical correlations [103] yielded the
behavior of many chemical compounds have been pre- sublimation enthalpy of a hypothetical Sg metal; this would
dicted [88, 94, 95]. Sg is expected to behave similarly to its be equal to or even higher than that of W, making Sg one of
lighter homologs. the least volatile elements. Later, the formation of Sg oxide
After first work at FLNR [14], the chemical behavior of Sg hydroxide compounds was probed [101].
was investigated by international collaborations at GSI using The chemistry of Sg in aqueous solutions [97–99] was
ARCA II [90] for studies in the aqueous phase and theOn-Line studied on cation-exchange resin with ARCA II [14, 16, 90]. In
Gas chemistry Apparatus (OLGA) III [15, 96] in the gas-phase. thefirst experiments [97, 98], Sg eluted likeMoandW in0.1M
Theoretical results predicting stable dioxydichloride HNO −43/5 × 10 M HF, unlike U. Thus, Sg is hexavalent, like a
group-6 element. To shed more light on the formed com-
pound, a second – and until today last – experiment was
performedwith pure HNO3 [99]. Here, Sgwas not observed in
the W fraction, indicating that neutral or anionic oxyfluoride
complexes were formed in the first study. Sg is the heaviest
element studied in aqueous solution todate. The experiments
were supported by fully-relativistic theoretical studies on the
stability of oxidation states, complex formation and extrac-
tion from acidic solutions [88, 95].
The Sg chemistry experiments at GSI, including sub-
sequent ones on Hs (see Section 3.2), and experiments at
RIKEN yielded information on nuclear properties of 265Sg
and 266Sg; for a summary see Figure 9 and [67, 102,
104–106]. Isomeric states were found in 265Sg and 261Rf, and
the conclusion was drawn that 266Sg has not been observed
before and was first synthesized as a daughter of 270Hs [67].
Figure 8: Relative yield, yrel, of MO2Cl2 (M =Mo, W and Sg) breaking Recently, the gas-phase formation of volatile group-6
through the column in OLGA III as a function of the isothermal element carbonyl complexes opened up novel types of SHE
temperature in the column. Adapted from [100]. (Reprinted figure experiments. After exploratory studies [107] with fission-
with permission fromWILEY-VCH from A. Türler et al., Angew. Chem.
Int. Ed. 38, 2212–2213 (1999), https://doi.org/10.1002/(SICI)1521- produced Mo thermalized in CO-containing gas at the
3773(19990802)38:15<2212::AID-ANIE2212>3.0.CO;2-6. © TRIGA Mainz reactor [89, 108] and with W separated in
WILEY-VCH Verlag GmbH, D-69,451 Weinheim, 1999). TASCA at GSI, this technique was successfully applied in
426 Ch.E. Düllmann et al.: Five decades of GSI superheavy element discoveries and chemical investigation
using 269Hs produced via 26Mg on 248Cm. These differed
from previous gas-phase studies: (i) The rotating target
wheel ARTESIA [14] was used to allow highest beam in-
tensities; (ii) The chemical reaction ofHswithO2 in very dry
He-gas was performed in situ behind the target [113],
avoiding aerosol-jet transport; (iii) Instead of high-
temperature chromatography, the cryo online detector
(COLD), based on the technique developed at LBNL [114]
was used. It consists of a thermochromatography channel
formed by 36 pairs of silicon photodiodes kept at temper-
atures from −20 °C at the inlet to −170 °C at the exit [111].
Decay chains from 269Hs were observed in a narrow peak
[111, 112]; see Figure 10.
The observation of seven molecules of HsO4 and their
adsorption maximum at −44 °C, in comparison with −82 °C
for OsO4, shows that Hs forms a relatively stable, volatile
tetroxide [111]. HsO4 adsorbed at higher temperature than
OsO4, i.e., has a low volatility or high, negative adsorption
enthalpy. This is in excellent agreement with the −ΔHa
value of 45.4 ± 1 kJ/mol obtained from improved fully
relativistic 4-component density functional theory
(4c-DFT) model calculations [115]. Relativistic calculations
show that the trend in volatility and other properties
established by RuO4 and OsO4 is reversed when going to
HsO4. From Monte-Carlo simulations (solid lines in
Figure 9: Decay pattern for the chain 269Hs → 265Sga,b → 261Rfa,b → Figure 10) to the experimental data, the following values
257No. Yellow: α decay; green: spontaneous fission. The α-particle
energies are given in MeV. The relative intensities of the decay
branches are indicated by arrow thickness, and the intensity of the
60
most intense branch is given. Data are from [104–106]. -20
50 -40
joint studies on Sg(CO)6 at RIKEN [89, 109]. Again, Sg
behaved like a typical member of group 6. Theory works at -6040
GSI, predicting the behavior of carbonyl complexes of -80
group 6–9 elements [11, 110], were confirmed. 30 -100
20 -120
3.2 Element 108, hassium, Hs -140
10
-160
In 2001, the time was ripe for 108Hs [111]. Applying novel 0 -180
techniques for irradiation, separation, and detection, all Hs 1 2 3 4 5 6 7 8 9 10 11 12
chemistry experiments were conducted at GSI in interna- Detector number
tional collaborations. They yielded chemical information
Figure 10: Experimentally observed thermochromatogram of HsO4
on Hs, and also exciting nuclear results [112], see Figure 9, (full histogram) and of OsO4 (open histogram) as relative yields
including evidence for the new isotope 270Hs [67] – the first versus chromatography/detector number.Onedetector is 3 cm long.
nuclide located on the N = 162 neutron-shell – and a The dashed line shows the temperature profile. Solid lines represent
confirmation of the Cn discovery by reproducing 277Cn results of Monte Carlo-simulations of the
269HsO and 1724 OsO4
α-decay chains [74] from 269Hs onwards. migration along the temperature gradient assuming standard
adsorption enthalpies of −46.0 kJ/mol and −39.0 kJ/mol, respec-
The Hs homologs, Ru and Os, show a unique property: tively. Taken from [111]. (Reprinted figure from Ch.E. Düllmann et al.,
they exhibit oxidation state 8+, forming highly volatile Nature 418, 859–862 (2002), https://doi.org/10.1038/
tetroxides, attractive for gas-chemical studies of HsO4 nature00980, with permission from Springer Nature).
Relative Yield [%]
 Temperature [°C]
Ch.E. Düllmann et al.: Five decades of GSI superheavy element discoveries and chemical investigation 427
(on silicon nitride) were deduced: −ΔHa(HsO4) = (46 ± 2) kJ/ and contraction of the 7s and 7p1/2 AOs, making these or-
mol and −ΔHa(OsO4) = (39 ± 1) kJ/mol. bitals less accessible for chemical bonding [121]. A rela-
A second Hs chemistry experiment was performed tively low reactivity of these elements towards Au and
using CALLISTO [89, 116]. Again, tetroxides were formed in quartz, much lower than those of their 6th row homologs,
a recoil chamber. This timeH2O vaporwas added to the gas, was then predicted by fully relativistic periodic DFT cal-
which transported volatile products to detectors facing culations of their −ΔHa on these surfaces [122–124] (see
NaOH thin films. More than 50% of Os adsorbed opposite Figure 11). As a result, Fl should interact stronger with a
the first detector; one decay chain of Hs was detected in the gold surface than Cn, because of its active 7p AOs, while Cn
first, one in the second, and three in the third detector [116]. is a closed-shell atom and should be relatively volatile (but
Presumably, Hs deposits by forming a hassate(VIII) ac- not like Rn). In contrast, 113Nh and 115Mc should be chem-
cording to 2 NaOH + HsO4 → Na2[HsO4(OH)2]. The low ically more reactive, due to one and three valence p-elec-
statistics constrains conclusions about the reactivity of trons, respectively, in the ground states. This is also
HsO4 as compared to OsO4. However, Os peaking earlier confirmed by relativistic calculations of their −ΔHa on Au
than Hs may indicate that HsO4 is less reactive (more co- and quartz [125] (Figure 11).
valent) than OsO4 [89], which would agree with theory [11]. Relying on the expected high volatility and weak
For the first time, an acid-base chemical reaction was chemical reactivity, thermochromatography studies with
performed with HsO4 [116]. All known Hs properties agree COLD and COMPACT were performed for Cn and Fl, mainly
with its place in group 8. on Au surfaces [119, 126, 128–130]. A noble-gas like
Further thermochromatography experiments behavior was derived from the pioneering chemistry
addressing nuclear aspects were performed using the Cryo- experiment with Fl at the FLNR, based on the observation
Online Multidetector for Physics and Chemistry of Trans- of three Fl atoms [128]. The unambiguous identification of
actinides (COMPACT) array [67, 112]. Obtained highlights single Fl atoms suffered from substantial background,
include insights into the α decay of 269Hs into two 265Sga,b which can be efficiently suppressed by using a recoil
isomers, followed by α decays into two long-lived 261Rfa,b separator as a preseparator [131]. Preseparation is applied
states (Figure 9) as well as the discoveries of 270,271Hs and
266Sg [67, 106, 112]. The Hs isotopes were produced with
cross sections of 2–7 pb [106]. This showed for the first time
that isotopes produced in the 1–10 pb range can be
explored in chemical studies, which opened up a window
to SHE around Fl. The properties of the 270Hs decay chains
were confirmed at the DGFRS and the half-life of 270Hs was
measured to 7.6 s [117]. The rather long half-lives, espe-
cially of the Sg and Hs isotopes, confirm the region of
enhanced stability at around Z = 108 and N = 162 (see
Section 2.2.1).
3.3 Element 113, nihonium, Nh and element
114, flerovium, Fl
In the past 15 years, 112Cn and the main group elements
beyond have come into the center of attention [11, 118, 119].
Figure 11: Theoretical and experimental values of adsorption
There are long-standing predictions, e.g. [120], that these
enthalpy−ΔHa on quartz (left panel, black symbols and lines) andAu
elements should be volatile in the elemental state. This (right panel, red symbols and lines) surfaces for the heaviest
makes them amenable to gas phase chemical studies using elements of the groups 12–15. Theoretical values (from [122, 123,
setups similar as that used for the characterization of the 125]) are given by solid squares connected by solid lines for
highly volatile HsO [111]. Particularly interesting are Cn superheavy elements and by open squares connected by dashed4 112
and 114Fl in this respect. The reason for a high inertness of
lines for their homologs. Red star symbols and red arrows represent
experimental adsorption enthalpy values on Au surfaces or their
these elements is their closed and quasi-closed shell limits, respectively (from Refs. [126–130]). The grey marked areas
ground state electronic con gurations, 6d10fi 7s2 and represent the range, within which experimental values of −ΔHa can
7s27p 21/2 , respectively, and a large relativistic stabilization be measured to date in an SHE chemistry experiment.
428 Ch.E. Düllmann et al.: Five decades of GSI superheavy element discoveries and chemical investigation
at GSI in chemistry experiments with elements beyond Hs For the heavy-element program at GSI, two set-ups are
[129, 130, 132] using the TASCA separator. First, two Fl available: the SHIP separator and its ancillary setups,
atoms were registered on Au kept at room temperature, which focuses on physics experiments with stable targets,
suggesting a metallic Fl-Au interaction [129]. In follow-up and the TASCA separator, where primarily actinide targets
experiments, six further Fl atoms were observed [130], are used.
whereas several attempts at the FLNR did not result in the
observation of Fl [119]. The Fl events measured in experi-
ments at TASCA were distributed over two deposition 4.1 Targets of stable isotopes from the GSI
zones: five of themwere found on Au at room temperature, target laboratory
and three deposited at a low temperature on ice. An un-
ambiguous explanation for the measured distribution will For the majority of experiments on synthesis and investiga-
profit from additional data and extended theoretical work. tion of nuclear properties of SHE using stable targets, the
The 244Pu(48Ca,3-4n)288,289Fl and 243Am(48Ca,3n)288Mc most frequently used ones include enriched Pb isotopes,
reactions have similar cross sections, ≈10 pb [59]. The mostly 208Pb, as well as 209Bi. Since Pb and Bi have low
secondmember of the 288Mc decay chain is 284Nh (T1/2 ≈ 1 s), melting temperatures of 327 °C and 271 °C, respectively,
which is accessible for chemical studies after an α decay of increasing beam intensities that lead to larger energy depo-
the short-lived mother nuclide 288Mc (T1/2 ≈ 170 ms). How- sition are challenging. During the history in heavy-element
ever, adsorption studies with Nh are likely more chal- synthesis, major developments for the targets were necessary
lenging than with Fl due to the higher reactivity of Nh to ensure their durability under increasing beam intensity.
atoms [125], which more easily leads to their loss on any The size of the active target area was increased to distribute
surface they encounter before reaching the chromatog- the beam intensity over a target segment as wide as possible.
raphy column. Pioneering Nh adsorption studies were The production process had to be adapted to keep the ho-
performed at the FLNR [127]. A limit of −ΔHa on Au was mogeneity of the target layer. Additionally, the rotation fre-
derived from the observation of five events, however, at quency of the wheel was optimized to fit to the pulsed beam
high background conditions. Two attempts to measure structure of the UNILAC so that exactly the area of one target
the Nh adsorption behind a recoil separator were per- area is covered with one pulse, which has a typical duration
formed at the FLNR [133] and at TASCA [132], but no Nh of 5ms. In addition, the shapeof thebeam-spotwas refined to
events were observed. These results demonstrated distribute the intensity more equally perpendicular to the
additional challenges in studies of more reactive rotation axis. Figure 12 showsa comparisonof thenew targets
elements and called for the development of an advanced with the older version, each from the back and from the front
setup. The new miniCOMPACT detector array at TASCA, side, respectively, before irradiation [135]. A major develop-
which does not require any transport line between the ment to enhance the durability of the targetswas substituting
recoil transfer chamber and the detection setup [132], the metals by chemical compounds with higher melting
facilitates future gas-chromatography experiments on points. For lead and bismuth, the most suitable compounds
Nh and, possibly, even on Mc. were found to be PbS and Bi2O3 withmelting points of 1114 °C
and817 °C, respectively. For the PbS layers it turnedout that a
heating of the backing during the evaporation process was
4 Targets for SHE synthesis necessary to get a homogeneous compound layer [136].
Also, 238U is a frequently used target material; here
The SHE production is based on an intense heavy-ion beam several alternatives are available via physical vapor
impinging on a thin target, which has to withstand the deposition (PVD) methods. UF4 can be evaporated from a
beam-induced heating and structural modifications. The tantalum crucible. Metallic U and UO2 can be deposited
target material has to form a uniform layer with a well- with DC magnetron sputtering [137, 138]. While metallic U
defined layer thickness and sufficient mechanical stability and UF4 have melting points of 1132 °C and 1036 °C,
to avoid loss of material during the – sometimes many respectively, the melting points of UO2 (2865 °C) and UC
months long – irradiation. Therefore, alongside the (2375 °C) are significantly higher. In addition, metallic U
advancement of beamquality and intensity, the permanent oxidizes easily in (humid) air. Moreover, as most of the
improvement of the targets plays a key role to advance the commercially available metallic U has already some oxy-
field [134]. The target quality and the matching with gen impurity on delivery, further oxidation cannot not be
experimental requirements are essential for heavy-element hindered, even when the produced targets are kept in an
production. inert atmosphere. Therefore, only a target of a high-melting
Ch.E. Düllmann et al.: Five decades of GSI superheavy element discoveries and chemical investigation 429
especially of transplutonium elements, are therefore only
available in minute quantities [5], which limits the appli-
cable methods for target production. The “molecular
plating” (MP) method [141] fulfills the requirements of high
efficiency, target stability, the option to reprocess irradi-
ated targets, and is applicable to actinides. TheMP is based
on an electrochemical deposition of dissolved material
from an alcoholic solution by applying a constant current
between an anode and the supporting substrate, which is
biased as a cathode. By adapting, e.g., the applied current
and time, the method is capable to produce homogeneous
layers of various actinide elements from U up to Cf with
thicknesses up to about 1 mg/cm2 and yields of 90% or
more in a single deposition step [142–144]. In the past, thin
foils of Be, C, Ta or Pt have mostly been used as substrate
for the actinide layer. Currently ≈2.2 µm Ti foils are favored;
these offer a good compromise between a minimal thick-
ness to minimize the energy loss of the ion beam, and
sufficient mechanical and thermal resistance to survive the
208 target production process as well as the harsh conditionsFigure 12: Pb-targets on carbon backings for SHIP showing the
backing side (1 and 3 from above) and the target side (2 and 4 from during irradiation. Figure 13 shows an exemplary target
above), respectively, with an enlarged target area and improved wheel with four
249Bk segments, which were used for ex-
backing quality (upper two) compared to the previous target version periments on Ts [78] and the search for element 119 [86] at
(lower two) [135]. (Reprinted from B. Lommel et al., Nucl. Instrum. TASCA.
Meth. A 480, 16–21 (2002), “Improvement of the target durability for Key parameters of the production process and relevant
heavy-element production”, https://doi.org/10.1016/S0168-
layer parameters are determined by using various analytical
9002(01)02041-1, © 2002, with permission from Elsevier.).
uranium compound, which is stable in air, guarantees
for long lasting stability. Currently, however, obtaining
depleted material in high purity in the desired compound
form as a sputtering target is a major problem.
For all the targetmaterialsmentioned above, amorphous
Cwitha thicknessbetween30and40μg/cm2 is thebackingof
choice [139]. Additionally, the target layer is covered with a
thin C layer to minimize target losses from sputtering. The
lanthanides are also interesting target materials for spectro-
scopic investigations of lighter elements, and also for chem-
ical studies, because they lead to the lighter homologs of the
SHE that are produced with actinide targets. Here, nearly all
lanthanides are produced by thermal evaporation as
lanthanide fluoride on C backing. For the production of
actinide targets, different combinations of backings and
compounds were tested [140].
Figure 13: Assembled TASCA target wheel with four target
segments, containing a total amount of about 12 mg 249Bk,
deposited by molecular plating on 2 µm Ti backings [144]. The total
4.2 Actinide targets from the Mainz nuclear 249Bk ß−-activity was 6·1011 Bq at the beginning of irradiation [86].
chemistry lab (Reprinted by permission from Springer nature Customer Service
Centre GmbH: Nature Springer J. Radioanal. Nucl. Chem. 299,
1081–1804 (2014), “Preparation of actinide targets for the synthesis
In contrast to Pb and Bi and basically any natural isotope, of the heaviest elements”, https://doi.org/10.1007/s10967-013-
transuranium isotopes are produced artificially. Many, 2616-6, J. Runke et al., © 2014).
430 Ch.E. Düllmann et al.: Five decades of GSI superheavy element discoveries and chemical investigation
Figure 14: Photographies (top and bottom left) and SEM pictures (center and right) of a 500 μg/cm2 La target on a TASCA target frame.
techniques. These include deposition yield determination via 5 Outlook and perspectives
α and γ-spectroscopy, either of the finished target (direct
determination) or of the supernatant solution (indirect SHE research at the GSI will continue to address open key
determination). If stable or very long-lived nuclides are used, questions in the field like pinning down the exact location
γ-spectrometry of neutron-activated samples can provide and extension of the island of stability and exploration of the
similar information. Radiographic imaging provides qualita- limits of nuclear stability and of the PTE, as well as studying
tive information of the homogeneity of radioactive layers. how well the SHE fit into the structure of the PTE. The
Further microscopic techniques include atomic force micro-
connection of GSI to JGU has been strengthened with the
scopy (AFM), which reveals the morphology and roughness
foundation of the joint daughter Helmholtz Institute Mainz
of a target on a micrometer scale, and scanning electron
(HIM) on June 9, 2010. HIM is an outpost of GSI located on the
microscopy (SEM), which allows taking detailed pictures of
JGU campus. Accelerator development as well as SHE
representative structures on the target (see Figure 14). This
research are pillars of the HIM research mission. In a
helps benchmarking the influence of specific plating pa-
continuation of current research activities, which are much
rameters (e.g., used solvent, current density, deposition time)
on the produced layers. In combination with an energy- broader than the focus described in this article, a compre-
dispersive X-ray (EDX) detector, elemental analysis of the hensive program studying production, nuclear, chemical,
layer can be performed. and atomic properties of superheavy nuclides will form the
Although MP has been used for many decades in the core of the activities in Darmstadt in the next decades.
SHE community, comprehensive analytics of the properties Fusion-evaporation reactions with stables beams
of layers produced by this method and the chemical and remain key to access the region of highest Z and A as the
physical changes induced by extended exposures to intensities of radioactive beams are presently too low to
intense heavy-ion beams are still incomplete and have produce SHN. Producing elements beyond Og in fusion-
48
become a field of interest in the last decade [145, 146]. The evaporation reactions requires projectiles beyond Ca, as
aim is to get a better understanding of the MP process and no target material with Z > 98 is available in sufficient
the properties of the produced layers,which shall serve as a quantity. Based on theoretical predictions and on current
basis to advance target production methods. One avenue experimental upper limits, cross-sections of 0.01 pb or
goes into the direction of more modern electrochemical below are expected. Thus, a substantial increase in beam
approaches, known in literature for lanthanide chemistry intensity will be crucial to carry out promising search ex-
[147–149]. Such methods have the potential (i) to give ac- periments within acceptable experiment durations. More
cess to more beam-resistant targets, (ii) to provide layers of neutron-rich isotopes may become accessible in multi-
thicknesses in excess of 1 mg/cm2, (iii) to avoid unwanted nucleon transfer reactions [89].
structural inhomogeneities caused, e.g., by mud-cracking An order of magnitude higher beam intensity compared
effects, and (iv) to provide layers with a better defined to the UNILAC is expected from a future HElmholtz LInear
chemical structure. Further work addresses the backing. ACcelerator (HELIAC) [150], for which R&D activities are
Ch.E. Düllmann et al.: Five decades of GSI superheavy element discoveries and chemical investigation 431
Figure 15: Schematic layout of the HELIAC linear accelerator in the full SHE configuration. ECR, Electron-cyclotron resonance; RFQ, Radio-
frequency quadrupole IH, Interdigital H-mode cavity. Bunchers for longitudinal beam focusing are indicated by red boxes and doublets and
triplets of quadrupole magnets for transversal beam focusing by orange square boxes. Addition of a 4th cryomodule to reach higher energies
ismost relevant for applications outside of SHE research. Typical projectiles are indicated alongwith themaximum reachable energies for the
given charge states.
ongoing and first components are being commissioned. A first contact; particles and photons emitted in their decay
schematic of the HELIAC in an envisaged configuration with can efficiently be registered in the Si detectors and in
continuous wave beams for SHE research is depicted in closely positioned high-efficiency Ge detectors, as was
Figure 15. Activities towards a first, intermediate step, where recently demonstrated with a proof-of-concept prototype.
theoperationalparameterswill be similar to thoseofUNILAC, This technique will allow to explore simultaneously
are currently ongoing. chemical as well as physical properties of SHN, including
Higher beam currents necessitate further developments processes, whichwere previously almost inaccessible, e.g.,
of the target technology. Detailed studies of the properties fission fragment mass distribution from spontaneously
and performance of (actinide) targets from different pro- fissioning SHN. The new setups will provide more options
duction methods and the development of new target pro- for the direct identification of Z of new elements by the
duction technologies are under way at JGU. registration of α-X-ray correlations. Alpha-photon spec-
The recoil separators and detection systems are troscopy of the SHN will provide important nuclear struc-
constantly being upgraded for increased sensitivity and ture data. Recently, decay chains starting at Flwere studied
efficiency. State-of-the-art digital signal processing for in this way [154] using the TASISpec+ detector, an inter-
dead-time free data acquisition [57, 151] will be used and mediate upgrade of the “TASCA in Small Image mode
will be optimized for detecting low-energy conversion Spectroscopy” (TASISpec) setup [155] towards LUNDIUM. In
electrons [152] and electron-capture decays that will play these studies, the observed α-decay energies attributed to
an increasingly important role in SHN that are more even–even decay chains 116Lv-114Fl-112Cn do not show a
neutron-rich than the currently known ones and will be kink; this, though, is a typical signature for a shell closure.
located closer to the beta-stability line. The absence of a kink indicates that no pronounced shell
New spectroscopy setups are under construction. The effect from crossing Z = 114 is observed at N = 174 [89,154].
new LUNDIUM detector is a next-generation spectroscopy set- Future chemical studies at TASCAwill profit fromnext-
up based on separating ERs in a recoil separator and generation equipment [132] and higher beam intensity.
implanting them in a highly granular Si detector sur- More short-lived isotopes will come into reach by coupling
rounded by pixelized box detectors (cf. Section 2.2.1), fast chemistry setups with a novel Universal high-density
which are monitored by highly efficient multi-segment gas stopping Cell (UniCell) [156], promising access to ele-
photon detectors [153]. In contrast, in the so-called diffu- ments beyond Fl aswell as the still neglected elementswith
sion-controlled adsorption technique, ERs are extracted Z = 109–111.
from the separator, thermalized in a gas-filled volume and Further observables of SHE are probed in precision
flushed into a narrow channel built from Si detector arrays measurements by mass spectrometry with SHIPTRAP and
similar to those used for chemical studies of HsO4 and the by laser spectroscopy [157]; these techniques have reached
elements Cn and beyond (cf. Sections 3.2 and 3.3 and Db and Lr, respectively, and will be advanced to heavier
Figure 10). Non-volatile species adsorb quantitatively upon elements. Besides providing accurate masses, long-lived
432 Ch.E. Düllmann et al.: Five decades of GSI superheavy element discoveries and chemical investigation
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12. Heinz S., Devaraja H. M., Beliuskina O., Comas V., Hofmann S.,
Acknowledgments: The results and perspectives summa- Hornung C., Münzenberg G., Ackermann D., Gupta M.,
rized in this article would not have been possible without Henderson R. A., Heßberger F. P., Kindler B., Lommel B.,
Mann R., Maurer J., Moody K. J., Nishio K., Popeko A. G.,
the hard work of all staff within the GSI groups working in
Shaughnessy D. A., Stoyer M. A., Yeremin A. V. Synthesis of new
physics and chemistry of the heaviest elements and in transuranium isotopes in multinucleon transfer reactions using
associated groups including the ion source and accelerator a velocity filter. Eur. Phys. J. A 2016, 52, 278.
departments, the experiment electronics department, the 13. Di Nitto A., Khuyagbaatar J., Ackermann D., Andersson L.-L.,
target laboratory, the nuclear chemistry groups at JGU Badura E., Block M., Brand H., Conrad I., Cox D. M.,
Mainz, and more recently also at HIM. International DüllmannCh. E., Dvorak J., Eberhardt K., Ellison P. A., Esker N. E.,
Even J., Fahlander C., Forsberg U., Gates J. M., Golubev P.,
collaboration partners from all over the world made Gothe O., Gregorich K. E., Hartmann W., Herzberg R.-D.,
essential contributions to the experiments at GSI. We are Heßberger F. P., Hoffmann J., Hollinger R., Hübner A., Jäger E.,
grateful for the continuous strong support from the GSI Kindler B., Klein S., Kojouharov I., Kratz J. V., Krier J., Kurz N.,
directorate and for funding that superheavy element Lahiri S., Lommel B., Maiti M., Mändl R., Merchán E., Minami S.,
research obtained over the decades from a variety of Mistry A. K., Mokry C., Nitsche H., Omtvedt J. P., Pang G. K.,
Renisch D., Rudolph D., Runke J., Sarmiento L. G., Schädel M.,
funding agencies.
Schaffner H., Schausten B., Semchenkov A., Steiner J.,
Author contributions: All the authors have accepted Thörle-Pospiech P., Trautmann N., Türler A., Uusitalo J., Ward D.,
responsibility for the entire content of this submitted Wegrzecki M., Wieczorek P., Wiehl N., Yakushev A.,
manuscript and approved submission. Yakusheva V. Study of non-fusion products in the 50Ti + 249Cf
Research funding: None declared. reaction. Phys. Lett. B 2018, 784, 199–205.
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Chem. Int. Ed. 2006, 45, 368–401.
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