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The European physical journal / A 59(2), 14 (2023). doi:10.1140/epja/s10050-023-00919-7, Published by Springer, Heidelberg

A novel combination of advanced gas-chromatography and detection systems coupled to a buffer-gas cell was characterized on-line to allow gas-phase chemical studies of accelerator-produced short-lived α -decaying mercury, francium, and astatine isotopes. These were produced in 40Ar- and 48Ca-induced nuclear fusion–evaporation reactions, subsequently isolated in the recoil separators MARS at Texas A&M University, USA, and TASCA at GSI Darmstadt, Germany, before being thermalized in a buffer-gas-stopping cell. From the latter, the nuclear reaction products were extracted into gas-phase chromatographic systems, suitable for registering α -decaying short-lived radionuclides, such as isotopes of superheavy elements. Efficiencies of 21(3)% for 204-209Fr were reached for the extraction into the optimized miniCOMPACT gas-chromatography setup, indicating that this technique enables the identification of isotopes of volatile as well as non-volatile elements. These studies guide the path towards chemical investigations of superheavy elements beyond flerovium, which are out of reach with currently used setups.

In the heaviest elements, the instability of atomic nuclei against spontaneous fission leads to ever shorter nuclear half-lives. Upon falling below a timescale of ${10}^{\ensuremath{-}14}$ s, the border of existence of isotopes is crossed because this is the timescale on which the formation of atomic shells occurs. Analysis of the experimental data on the spontaneous fission half-lives of Rf isotopes in relation with their expected single-particle orbitals hint at a potentially abrupt decrease in half-lives of unknown neutron-deficient Rf isotopes with neutron numbers $l149$, which suggests that the isotopic border is already almost reached. However, this conjecture, which cannot be explained within the current knowledge, was directly related to uncertainty in the experimental data on $^{253}\mathrm{Rf}$. We revisited the decay of $^{253}\mathrm{Rf}$ and identified two fission activities, which are attributed to decays of the two different states with half-lives of $12.{8}_{\ensuremath{-}3.4}^{+7.0}$ ms and ${44}_{\ensuremath{-}10}^{+17}\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$. In addition, hitherto unknown $\ensuremath{\alpha}$ decay in $^{253}\mathrm{Rf}$, which is followed by $\ensuremath{\alpha}$ decay of the new isotope $^{249}\mathrm{No}$ with a half-life of ${15}_{\ensuremath{-}7}^{+74}$ ms, was observed. Based on our new data, no abrupt decreases in half-lives of the neutron-deficient No and Rf isotopes are expected, which is in line with theoretical predictions. Fission half-lives of the two different states in $^{253}\mathrm{Rf}$ are benchmark cases for the theoretical description of the single-particle orbital influence on the fission process.

The question of the number and origin of isomeric states in $^{256}\mathrm{Rf}$ arose from two independent experiments but remained unanswered for a decade. To shed light on this puzzle, we studied isomeric decay in $^{256}\mathrm{Rf}$ by measuring conversion electrons with fast fully digital electronics. $^{256}\mathrm{Rf}$ was produced in the fusion-evaporation reactions of $^{50}\mathrm{Ti}+^{207}\mathrm{Pb}$ and $^{50}\mathrm{Ti}+^{208}\mathrm{Pb}$ at the gas-filled recoil separator TransActinide Separator and Chemistry Apparatus. Among a total of 120 decays of $^{256}\mathrm{Rf}$, we detected 22 and 12 decays proceeding through one and two isomeric states. Half-lives of the low- and higher-lying states were assigned to be ${T}_{1/2}={14}_{\ensuremath{-}4}^{+6}$ and ${10}_{\ensuremath{-}3}^{+5}\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$, respectively. Population rates of these isomeric states were estimated to be $\ensuremath{\approx}18%$ and $g10%$, which are similar to those for two-quasiparticle $K$ isomeric states in this region of nuclei. The results, thus, confirm an earlier claim on the existence of multiple $K$ isomeric states in $^{256}\mathrm{Rf}$ and show their population rates are as high as in No isotopes. Suggestively, $K$ remains a good quantum number in isotopes of heavier elements like Sg and Hs, where yet unknown high-$K$ isomeric states still could exist. The present experimental results demonstrate that the ``triggerless'' measurement guarantees an efficient detection of delayed conversion electron signals. Thus, it is a very efficient method for the identification of isomeric states in experiments with one nucleus-at-a-time production rates.

Online gas-solid adsorption studies with single atom quantities of Hg, Tl, and Pb on SiO$_{2}$ and Au surfaces were carried out using short-lived radioisotopes with half-lives in the range of 4-49 s. This is a model study to measure adsorption enthalpies of superheavy elements Cn, Nh, and Fl. The short-lived isotopes were produced and separated by the gas-filled recoil separator TASCA at GSI. The products were stopped in He gas, and flushed into gas chromatography columns made of Si detectors whose surfaces were covered by SiO$_{2}$ or Au. The short-lived Tl and Pb were successfully measured by the Si detectors with the SiO$_{2}$ surface at room temperature. On the other hand, the Hg did not adsorb on the SiO$_{2}$ surface, but adsorbed on the Au surface. The results demonstrated that the adsorption properties of short-lived Hg, Tl, and Pb could be studied with this setup, and that this method is applicable to the experiment for Cn, Nh, and Fl., 半減期4$\sim$49秒の短寿命同位体を用いて、水銀(Hg), タリウム(Tl), 鉛(Pb)の単一原子レベルでのSiO$_{2}$及びAu表面への固体吸着挙動をオンライン気相実験により調べた。超重元素Cn, Nh, Flの吸着エンタルピー測定のためのモデル実験として実施した。短寿命同位体はドイツ重イオン研究所の反跳核分離装置TASCAを用いて生成・分離した。生成核はヘリウムガス中に捕集された後、SiO$_{2}$あるいはAuで表面を覆われたSi検出器で作製されたガスクロマトグラフィーカラムへと導入した。短寿命Tl及びPb同位体は室温においてSiO$_{2}$表面に吸着し、Si検出器で測定することに成功した。一方、HgはSiO$_{2}$表面には吸着せず、Au表面に吸着した。この結果より、本実験のセットアップを用いることで短寿命Hg, Tl, Pb同位体の吸着特性を調べることができると証明され、この手法がCn, Nh, Flの実験にも適用できることが確認された。

A nuclear spectroscopy experiment was conducted to study α-decay chains stemming from isotopes of flerovium (element Z=114). An upgraded TASISpec decay station was placed behind the gas-filled separator TASCA at the GSI Helmholtzzentrum fur Schwerionenforschung in Darmstadt, Germany. The fusion-evaporation reactions ^{48}Ca+^{242}Pu and ^{48}Ca+^{244}Pu provided a total of 32 flerovium-candidate decay chains, of which two and eleven were firmly assigned to ^{286}Fl and ^{288}Fl, respectively. A prompt coincidence between a 9.60(1)-MeV α particle event and a 0.36(1)-MeV conversion electron marked the first observation of an excited state in an even-even isotope of the heaviest man-made elements, namely ^{282}Cn. Spectroscopy of ^{288}Fl decay chains fixed Q_{α}=10.06(1) MeV. In one case, a Q_{α}=9.46(1)-MeV decay from ^{284}Cn into ^{280}Ds was observed, with ^{280}Ds fissioning after only 518 μs. The impact of these findings, aggregated with existing data on decay chains of ^{286,288}Fl, on the size of an anticipated shell gap at proton number Z=114 is discussed in light of predictions from two beyond-mean-field calculations, which take into account triaxial deformation.

In 2014 the first synthesis of a transactinide carbonyl complex – seaborgium hexacarbonyl – was reported. This was achieved in gas-phase chemical experiments in a beam-free environment behind the recoil separator GARIS. Extending this work to heavier elements requires more efficient techniques to synthesize carbonyl complexes as production rates of transactinide elements drop with increasing atomic number. A novel approach was thus conceived, which retains the benefit of a beam-free environment but avoids the physical preseparation step. The latter reduces the yields for products of asymmetric reactions such as those used for the synthesis of suitable isotopes of Sg, Bh, Hs and Mt. For this a series of experiments with accelerator-produced radioisotopes of the lighter homologues W, Re and Os was carried out at the tandem accelerator of JAEA Tokai, Japan. A newly developed double-chamber system, which allows for a decoupled recoil ion thermalization and chemical complex formation, was used, which avoids the low-efficiency physical preseparation step. Here, we demonstrate the feasibility of this newly developed method using accelerator-produced short-lived radioisotopes of the 5d homologues of the early transactinides.

Physics letters / B 820, 136601 (2021). doi:10.1016/j.physletb.2021.136601, Published by North-Holland Publ., Amsterdam

Journal of physics / Conference series 1643, 012181 (2020). doi:10.1088/1742-6596/1643/1/012181, Published by IOP Publ., Bristol

A search for production of the superheavy elements with atomic numbers 119 and 120 was performed in the 50Ti+249Bk and 50Ti+249Cf fusion-evaporation reactions, respectively, at the gas-filled recoil separator TASCA at GSI Darmstadt, Germany. Over four months of irradiation, the 249Bk target partially decayed into 249Cf, which allowed for a simultaneous search for both elements. Neither was detected at cross-section sensitivity levels of 65 and 200 fb for the 50Ti+249Bk and 50Ti+249Cf reactions, respectively, at a midtarget beam energy of Elab=281.5 MeV. The nonobservation of elements 119 and 120 is discussed within the concept of fusion-evaporation reactions including various theoretical predictions on the fission-barrier heights of superheavy nuclei in the region of the island of stability. peerReviewed

Nuclear structure of $^{243}\mathrm{Fm}$, $^{245}\mathrm{Fm}$, and their daughter nuclei were investigated via detection of their radioactive decays, $\ensuremath{\alpha}$, $\ensuremath{\gamma}$, and spontaneous fission. Measured $\ensuremath{\alpha}$-decay energies, half-lives, and branching ratios improve the literature data significantly. A signature for detection of the hitherto unknown $^{235}\mathrm{Cm}$ was found in the $\ensuremath{\alpha}$-decay chains from $^{243}\mathrm{Fm}$. Two groups of $\ensuremath{\alpha}$ events with average energies of 6.69(2) MeV and 7.01(2) MeV and with a half-life of ${T}_{1/2}={300}_{\ensuremath{-}100}^{+250}$ s are suggested to originate from $^{235}\mathrm{Cm}$. Tentative decay schemes for $^{243}\mathrm{Fm}$, $^{239}\mathrm{Cf}$, $^{235}\mathrm{Cm}$, and $^{245}\mathrm{Fm}$, $^{241}\mathrm{Cf}$ isotopes are suggested based on the present experimental data. Systematical trends of single-particle states in $N=141$ isotones of $Z=92--98$ nuclei are discussed.

The electron-capture decay followed by a prompt fission process was searched for in the hitherto unknown most neutron-deficient Md isotope with mass number 244. Alpha decay with $\ensuremath{\alpha}$-particle energies of 8.73--8.86 MeV and with a half-life of ${0.30}_{\ensuremath{-}0.09}^{+0.19}\text{ }\text{ }\mathrm{s}$ was assigned to $^{244}\mathrm{Md}$. No fission event with a similar half-life potentially originating from spontaneous fissioning of the short-lived electron-capture decay daughter $^{244}\mathrm{Fm}$ was observed, which results in an upper limit of 0.14 for the electron-capture branching of $^{244}\mathrm{Md}$. Two groups of fission events with half-lives of ${0.9}_{\ensuremath{-}0.3}^{+0.6}\text{ }\text{ }\mathrm{ms}$ and ${5}_{\ensuremath{-}2}^{+3}\text{ }\text{ }\mathrm{ms}$ were observed. The ${0.9}_{\ensuremath{-}0.3}^{+0.6}\text{ }\text{ }\mathrm{ms}$ activity was assigned to originate from the decay of $^{245}\mathrm{Md}$. The origin of eight fission events resulting in a half-life of ${5}_{\ensuremath{-}2}^{+3}\text{ }\text{ }\mathrm{ms}$ could not be unambiguously identified within the present data while the possible explanation has to invoke previously unseen physics cases.

Spontaneous fission properties of the isotopes $^{255}\mathrm{Rf}$, $^{256}\mathrm{Rf}$, and $^{258}\mathrm{Rf}$ produced in the reactions $^{50}\mathrm{Ti}+^{207}\mathrm{Pb}$, $^{50}\mathrm{Ti}+^{208}\mathrm{Pb}$, and $^{50}\mathrm{Ti}+^{209}\mathrm{Bi}$ were studied. The method of time and position correlations was used to identify spontaneous fission events. The correction to the energy deficit in measured total kinetic energy (TKE) determined on the basis of a study of $^{252}\mathrm{No}$ was applied to evaluate the $\overline{\mathrm{TKE}}$ of investigated rutherfordium isotopes. A signature which we assigned tentatively to bimodal fission was observed in TKE distributions of $^{255}\mathrm{Rf}$ and $^{256}\mathrm{Rf}$. Two new high-K isomeric states in $^{255}\mathrm{Rf}$ were identified by ER-CE-(CE)-$\ensuremath{\alpha}$/SF correlation search, with half-lives of ${38}_{\ensuremath{-}7}^{+12}$ and ${15}_{\ensuremath{-}4}^{+6}$ $\ensuremath{\mu}\mathrm{s}$ at excitation energies 1150--1450 and 900--1200 keV, respectively. Including those metastable states a tentative partial level scheme was proposed.

An upgraded TASISpec setup, with the addition of a veto DSSD and the new Compex detector-germanium array, has been employed with the gas-filled recoil separator TASCA at the GSI Helmholtzzentrum für Schwerionenforschung Darmstadt, to study flerovium (element 114) decay chains. The detector upgrades along with development of new analytical techniques have improved the sensitivity of the TASISpec setup for measuring α-photon coincidences. These improvements have been assessed with test reactions. The reaction 48Ca+206,207Pb was used for verification of experimental parameters such as transmission to implantation DSSD and target-segment to α-decay correlations. The reaction 48Ca+ nat Hf was used to produce several short-lived nuclei with multiple-α decay chains to investigate pile-up event deconvolution.

For characterization of targets and foils prepared at the target laboratory as well as for characterization of e.g. degrader or windows of internal customers, different analytical devices are available. Besides a lot of standard equipment, the target laboratory of GSI holds a 3D-measurement system (MicroProf®) equipped with optical sensors for measuring surface parameters as well as total thickness variations contact-free. In the paper the measuring principle including the possibilities and features of the MicroProf®-system are explained and some different applications are shown.

The isotopic distribution of nuclei produced in the 50Ti + 249Cf reaction has been studied at the gas-filled recoil separator TASCA at GSI Darmstadt, which separates ions according to differences in magnetic rigidity. The bombardment was performed at an energy around the Bass barrier and with the TASCA magnetic fields set for collecting fusion-evaporation reaction products. Fifty-three isotopes located “north-east” of 208Pb were identified as recoiling products formed in non-fusion channels of the reaction. These recoils were implanted with energies in two distinct ranges; besides one with higher energy, a significant low-energy contribution was identified. The latter observation was not expected to occur according to kinematics of the known types of reactions, namely quasi-elastic, multi-nucleon transfer, deep-inelastic collisions or quasifission. The present observations are discussed within the framework of two-body kinematics passing through the formation of a composite system.

International audience; In this work, we present results from the investigations of high-K isomers in 255Rf, 256Rf and 257Rf isotopes produced in 50Ti+207Pb and 50Ti+208Pb reactions. The method of time and position correlation search was used to identify ER–CE–α/SF events in the data. Previously known high-K states in 256Rf and 257Rf were confirmed. A search for high-K isomer in 255Rf was performed for the first time. Observation of ER–CE correlations with a lifetime of ≈ 30 µs indicates the presence of isomeric states also in this isotope.

The isotopes 255,256,258Rf were produced in the fusion-evaporation reactions 50Ti + 207,208Pb and 50Ti + 209Bi at GSI Darmstadt, using the velocity filter SHIP. Total kinetic energies of fragments from spontaneous fission for these isotopes were evaluated with a correction to pulse-height defect.

The heaviest currently known nuclei, which have up to 118 protons, have been produced in 48Ca induced reactions with actinide targets. Among them, the element tennessine (Ts), which has 117 protons, has been synthesized by fusing 48Ca with the radioactive target 249Bk, which has a half-life of 327 d. The experiment was performed at the gas-filled recoil separator TASCA. Two long and two short α decay chains were observed. The long chains were attributed to the decay of 294Ts. The possible origin of the short-decay chains is discussed in comparison with the known experimental data. They are found to fit with the decay chain patterns attributed to 293Ts. The present experimental results confirm the previous findings at the Dubna Gas-Filled Recoil Separator on the decay chains originating from the nuclei assigned to Ts.

After publication of the paper, the authors noticed some errors in the affiliations. Their correct version is given in this erratum.

Products of the fusion-evaporation reaction 48Ca + 243Am were studied with the TASISpec set-up at the gas-filled separator TASCA at the GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany. Amongst the detected thirty correlated α-decay chains associated with the production of element Z=115, two recoil-α-fission and five recoil-α-α-fission events were observed. The latter five chains are similar to four such events reported from experiments performed at the Dubna gas-filled separator, and three such events reported from an experiment at the Berkeley gas-filled separator. The four chains observed at the Dubna gas-filled separator were assigned to start from the 2n-evaporation channel 289115 due to the fact that these recoil-α-α-fission events were observed only at low excitation energies. Contrary to this interpretation, we suggest that some of these recoil-α-α-fission decay chains, as well as some of the recoil-α-α-fission and recoil-α-fission decay chains reported from Berkeley and in this article, start from the 3n-evaporation channel 288115.

The neutron-deficient isotopes 254,255Rf were produced in the fusion-evaporation reaction 50Ti + 206Pb at the gas-filled recoil separator TASCA. Decay properties of these nuclei were investigated by applying fast digital electronics. A search for isomeric states in both isotopes was performed by using the accompanying conversion electron emissions. Isomeric states with half-lives of 4(1) μs and >30 μs were measured for 254Rf and 255Rf, respectively, which confirm the findings at different separators. The present experimental results demonstrate the great potential of fast digital electronics for measurements of isomeric states in the heaviest nuclei, which are only producible in small quantities.

Uranium targets are very important for accelerator-based research of nuclear properties. Depending on the reaction to be studied and on the conditions during the experiments different restrictions on the target material have to be met; as for example, durability, melting temperature, reactivity or a possible contribution of the additional compounds present to the reaction. Therefore, we are developing processes to produce uranium targets in the elemental form as well as in different compounds. Here we report on the production and application of targets from metallic uranium, UF4 and UO2.

Acta physica Polonica / B Particle physics and field theory, nuclear physics, theory of relativity 51(1), 39 (2020). doi:10.5506/APhysPolB.51.39, Published by Inst. of Physics, Jagellonian Univ., Cracow

Superheavy elements are formed in fusion reactions which are hindered by fast nonequilibrium processes. To quantify these, mass-angle distributions and cross sections have been measured, at beam energies from below-barrier to 25% above, for the reactions of $^{48}\mathrm{Ca}$, $^{50}\mathrm{Ti}$, and $^{54}\mathrm{Cr}$ with $^{208}\mathrm{Pb}$. Moving from $^{48}\mathrm{Ca}$ to $^{54}\mathrm{Cr}$ leads to a drastic fall in the symmetric fission yield, which is reflected in the measured mass-angle distribution by the presence of competing fast nonequilibrium deep inelastic and quasifission processes. These are responsible for reduction of the compound nucleus formation probablity ${P}_{CN}$ (as measured by the symmetric-peaked fission cross section), by a factor of 2.5 for $^{50}\mathrm{Ti}$ and 15 for $^{54}\mathrm{Cr}$ in comparison to $^{48}\mathrm{Ca}$. The energy dependence of ${P}_{\mathrm{CN}}$ indicates that cold fusion reactions (involving $^{208}\mathrm{Pb}$) are not driven by a diffusion process.

Short-lived α-decaying nuclei "northeast" of 208Pb in the chart of nuclides were studied using the reaction 48Ca+243Am with the decay station TASISpec at TASCA, GSI Darmstadt. Decay energies and times from pile-up events were extracted with a tailor-made pulse-shape analysis routine and specific α-decay chains were identified in a correlation analysis. Decay chains starting with the even-even 220Ra and its odd-A neighbors, 219Fr, and 219,221Ra, with a focus on the 219Ra→215Rn decay, were studied by means of α-γ spectroscopy. A revised α-decay scheme of 219Ra is proposed, including a new decay branch from a previously not considered isomeric state at 17 keV excitation energy. Conclusions on nuclear structure are drawn from the experimental data, aided by Geant4 simulations and a discussion on theoretical calculations. (Less)

The production of the heaviest elements in fusion-evaporation reactions is substantially limited by very low cross sections, as fusion cross sections (including fusion-fission) are greatly reduced by the competing quasifission mechanism. Using the Australian National University Heavy Ion Accelerator Facility and CUBE detector array, fission fragments from the $^{48}\mathrm{Ti}+^{204,208}\mathrm{Pb}$ and $^{50}\mathrm{Ti}+^{206,208}\mathrm{Pb}$ reactions have been measured, with the aim to investigate how the competition between quasifission and fusion-fission evolves with small changes in entrance-channel properties associated mainly with the nuclear structure. Analysis of mass-distribution widths of strongly mass-angle-correlated fission fragments within the framework of the compound-nucleus fission theory demonstrates significant differences in quasifission (and therefore fusion) probabilities among the four reactions. The impact of nuclear structure on fusion highlights the importance of future radioactive beams.

For a special application, carbon-backings with a very flat surface, microscopically as well as macroscopically, were needed as backings for targets of enriched isotopes. However, betaine-sucrose routinely applied at GSI as parting agent for carbon deposition results in a microscopically rough surface which was not perfectly satisfying the experimental requirements. For these targets we investigated the carbon-backing quality in relation to the applied different parting agents and different deposition processes. In this paper we report on the yield, on the structure of the carbon layers and the deposited target layer of 208PbS, 206PbS, and 142NdF3 depending on the parting agent, the thickness and the deposition methods. We report on elastic scattering experiments with a 48Ti-beam demonstrating the influence of the structure of the carbon backing on the experimental results.

In deep inelastic multinucleon transfer reactions of 48 Ca + 248 Cm we observed about 100 residual nuclei with proton numbers between Z = 82 and Z = 100 . Among them, there are five new neutron-deficient isotopes: 216 U, 219 Np, 223 Am, 229 Am and 233 Bk. As separator for the transfer products we used the velocity filter SHIP of GSI while the isotope identification was performed via the α decay chains of the nuclei. These first results reveal that multinucleon transfer reactions together with here applied fast and sensitive separation and detection techniques are promising for the synthesis of new isotopes in the region of heaviest nuclei.

Physics letters / B 748, 89 - 94 (2015). doi:10.1016/j.physletb.2015.06.068, Published by North-Holland Publ., Amsterdam

Self-supporting layers of metal and metal oxides are the most desirable form of targets and films for heavy-ion applications. If the self-supporting film cannot be produced in the wanted size or thickness, then typically a thin, self-supporting carbon backing is employed. But nevertheless there are applications where carbon as a backing does not offer the optimal properties as a backing. Then very often plastic backings are wanted. Several plastics are available commercially as a standard backing with very good quality and homogeneity. We report on the deposition of metals and metal oxides on various backings, when a standard commercial alternative is not available.

We report on the development of processes for pretreatment, coating, and tempering of structured layers on diamond surfaces for detector applications. The different steps were developed and processed in close collaboration with the GSI detector laboratory and with feedback from the experiment. Beginning with thermal evaporation and electron beam evaporation we currently use magnetron sputtering as standard coating technique for diamonds in a system especially equipped for this purpose. The layers of choice are mostly chromium–gold or titanium–platinum–gold; sometimes also single layers of aluminum, copper or palladium. The application of the coated diamonds in a detector for the HADES-experiment is described.

Rapid in situ formation of metal carbonyl complexes with short-lived nuclides has been demonstrated to be feasible with recoiling ions formed in nuclear fusion and fission reactions. These carbonyl complexes are highly volatile and can be transported rapidly in a gas-stream to counting or chemistry devices. This method was already successfully applied in the chemical investigation of the superheavy element seaborgium (Z = 106) and appears promising for various fields of nuclear research. In this article, we give an overview on the current status of metal carbonyl complex studies with short-lived d-element isotopes.

After a successful production of isotopes of element 116 in the reaction 48Ca + 248Cm, the reaction 54Cr + 248Cm → 302120* was investigated at the velocity filter SHIP at GSI, Darmstadt, aiming to search for isotopes of element 120. One chain of events was observed, which is not created by chance with high probability. Parts of the chain agree with model predictions and measured data from a decay chain which could start at the isotope 299120. In a complementary study, model dependent shell-correction energies and related heights of fission barriers were deduced from measured Qα values. The results are compared with predictions of macroscopic-microscopic models. The consequences for calculations of crosssections are discussed.

International audience; Spontaneous fission of 255Rf, 256Rf and 258Rf was studied at SHIP in GSI Darmstadt. The isotopes were produced in fusion–evaporation reactions 50Ti + 207,208Pb and 50Ti + 209Bi (compound nuclei 257Rf, 258Rf and 259Db, respectively) and implanted into the focal plane detector of the SHIP setup. The deficit in the measured fragments energies was evaluated as a function of implantation depth of evaporation residues in the silicon detector. This correction was applied to obtain the mean total kinetic energies of 255Rf, 256Rf and 258Rf.

The new neutron-deficient nuclei 240 Es and 236 Bk were synthesised at the gas-filled recoil separator RITU. They were identified by their radioactive decay chains starting from 240 Es produced in the fusion–evaporation reaction 209 Bi( 34 S,3n) 240 Es. Half-lives of 6 ( 2 ) s and 22 − 6 + 13 s were obtained for 240 Es and 236 Bk, respectively. Two groups of α particles with energies E α = 8.19 ( 3 ) MeV and 8.09 ( 3 ) MeV were unambiguously assigned to 240 Es. Electron-capture delayed fission branches with probabilities of 0.16 ( 6 ) and 0.04 ( 2 ) were measured for 240 Es and 236 Bk, respectively. These new data show a continuation of the exponential increase of ECDF probabilities in more neutron-deficient isotopes.

We report on the in-situ synthesis of metal carbonyl complexes with short-lived isotopes of transition metals. Complexes of molybdenum, technetium, ruthenium and rhodium were synthesized by thermalisation of products of neutron-induced fission of 249Cf in a carbon monoxide-nitrogen mixture. Complexes of tungsten, rhenium, osmium, and iridium were synthesized by thermalizing short-lived isotopes produced in 24Mg-induced fusion evaporation reactions in a carbon monoxide containing atmosphere. The chemical reactions took place at ambient temperature and pressure conditions. The complexes were rapidly transported in a gas stream to collection setups or gas phase chromatography devices. The physisorption of the complexes on Au and SiO2 surfaces was studied. We also studied the stability of some of the complexes, showing that these start to decompose at temperatures above 300 ℃ in contact with a quartz surface. Our studies lay a basis for the investigation of such complexes with transactinides.

Fission fragment mass distributions were measured in heavy-ion induced fission of 238U. The mass distributions changed drastically with incident energy. The results are explained by a change of the ratio between fusion and quasifission with nuclear orientation. A calculation based on a fluctuation dissipation model reproduced the mass distributions and their incident energy dependence. Fusion probability was determined in the analysis. Evaporation residue cross sections were calculated with a statistical model for the reactions of 30Si+238U and 34S+238U using the obtained fusion probability in the entrance channel. The results agree with the measured cross sections of 263,264Sg and 267,268Hs, produced by 30Si+238U and 34S+238U, respectively. It is also suggested that sub-barrier energies can be used for heavy element synthesis.

Electron capture decay of 258 Db was measured directly via delayed coincidences between K X-rays and/or conversion electrons (from the electron capture decay), and spontaneous fission or \(\alpha\) decay of the daughter nuclide 258Rf. Delayed K X-rays were also measured in prompt coincidence with conversion electrons, showing that electron capture decay also populates excited levels in the daughter nucleus. Our data indicate electron capture decay from two states in 258Db, thus supporting the existence of two long-lived states in 258Db with some seconds half-life already known from \( \alpha\)-decay studies. The analysis of correlations between photons, conversion electrons and fission events resulted in a strong evidence for population of two isomeric states in 258Rf by the electron capture decay of 258Db, with half-lives of \(T_{1/2} = (15 \pm 10)\) μs and \(T_{1/2} = (2.4^{+2.4}_{-0.8})\) ms. A previously reported \(\alpha\) branch of 258Rf was confirmed. However, we obtained a significantly lower branching ratio \( b_{\alpha} = (0.049 \pm 0.016)\). The technique is discussed as a possible new method for Z-identification of odd-odd superheavy nuclei.

This special issue of the Journal of Radioanalytical and Nuclear Chemistry is dedicated to the 26th World Conference of the International Nuclear Target Development Society (INTDS 2012) held from August 19th to 24th, 2012 at the conference center Erbacher Hof in Mainz, Germany with special emphasis on targets for acceleratorbased research. The INTDS 2012 was co-organized by the Johannes Gutenberg-Universitat Mainz and the GSI Helmholtzzentrum fur Schwerionenforschung, Darmstadt. The 4th Target Fabrication Workshop (TFW), dedicated to targets for laser application and hosted by the Technical University Darmstadt, was held in parallel to the INTDS 2012 at the Erbacher Hof with common plenary sessions giving the participants of both events the opportunity of exchanging experiences and know-how. Accordingly, these proceedings contain some contributions presented at the 4th Target Fabrication Workshop. The series of INTDS conferences dates back to 1963 when the ‘‘First Symposium on Research Materials for Nuclear Measurements’’ was held sponsored by the Central Bureau for Nuclear Measurements (CBNM, recently IRMM—Institute for Reference Materials and Measurements) in Belgium. Since then researchers involved in target preparation share experience and discuss related problems during biennial conferences hosted alternatively in Europe, North America and Asia (www.intds.org). It dates back to 1978 and 1988 when the 7th and the 14th INTDS conference were held at Garching, organized by Ludwig-Maximilians-Universitat together with Technische Universitat Munchen and at Darmstadt, organized by GSI, respectively. Thus, it was delightful to meet again in Germany on the occasion of the 26th International INTDS conference. More than 70 participating scientists from 15 countries (Belgium, Canada, France, Germany, India, Italy, Japan, Korea, Poland, Romania, Russia, South Africa, Switzerland, UK, USA), met in Mainz presenting over 50 scientific contributions covering the wide field of ‘‘Targets for Accelerator-bases Research’’. Two complementary visits to GSI and the TRIGA Mainz research reactor operated by the Johannes Gutenberg-Universitat were included in the scientific program to introduce the institutions and laboratories. The conference comprised seven different sessions titled ‘‘Classical Accelerator Targets’’, ‘‘Laser& Accelerator Based Research’’, ‘‘RIB Targets’’, ‘‘Isotope Enrichment and Medical Radioisotopes’’, ‘‘Stripper’’, K. Eberhardt (&) Johannes Gutenberg-Universitat Mainz, 55128 Mainz, Germany e-mail: klaus.eberhardt@uni-mainz.de

Titanium-50 (50Ti) is an important and often requested ion beam for nuclear physics’ experiments. While natural titanium of very high purity is available in different forms; enriched material can only be bought as the dioxide or the tetrachloride. These compounds cannot be processed from currently available ion sources with a sufficient beam quality and sufficient beam intensity for a long time. We describe here the process of converting titanium dioxide into the metal, the material analysis of the starting material as well as of the reduced material. Despite varying contamination levels of silicon, chlorine, and tin in the primary materials, we obtained high yields of metallic titanium with different contamination levels. The obtained metallic 50Ti was applied at the accelerator UNILAC for the production of a high intensity ion beam for several month of beam time.

We report on the development of new materials for laser-ion acceleration applicable for the advanced mechanism of radiation-pressure-acceleration. These targets are ultra-thin with thicknesses of just a few nm. For several years, diamond-like carbon foils in this thickness range can be produced. An alternative material containing more than one ion-species has the potential to further improve the acceleration mechanism. The fabrication and characterization of self-supporting polymer-based targets will be described in this paper. Furthermore, we show the significant influence on a radiation-pressure induced acceleration process by experimental data.

The heaviest elements are synthesized in heavy-ion induced hot fusion reactions with various actinide targets. Because the actinide material is often available only in very limited amounts, a deposition method with high yields (~90 %) is needed. We report on the production of 244Pu, 243Am, 248Cm, 249Bk, and 249Cf targets on thin Ti backings by molecular plating. Different chemical purification steps using ion chromatographic techniques were applied for the purification of 249Cf and 244Pu. The deposition procedure applied for the production of ~0.4–0.8 mg/cm2 thick targets is described. The deposition yield was determined either by α-particle or γ-ray spectroscopy. Furthermore, neutron activation analysis has been applied in the case of 244Pu, 243Am, and 248Cm. Information about the spatial distribution and homogeneity of the target layer was obtained by radiographic imaging.

We report on the production and characterization of polymer-based ultra-thin (sub 10 nm) foils suited for experiments on laser-ion acceleration in the regime of radiation pressure acceleration. Beside the remarkable mechanical stability compared with commonly used diamond-like-carbon foils, a very homogeneous layer thickness and a small surface roughness have been achieved. We describe the technical issues of the production process as well as detailed studies of the mechanical stability and surface roughness tests. The capability of producing uniform targets of large area is essential for advanced laser-ion acceleration projects which are dealing with high repetition rate and extended measurement series, but might also be useful for other applications which require ultra-thin and freestanding substrates of high quality. (c) 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1355-1360