Spectroscopy along flerovium decay chains. II : Fine structure in odd-A289Fl

Fifteen correlated α-decay chains starting from the odd-A superheavy nucleus 289Fl were observed following the fusion-evaporation reaction 48Ca+244Pu. The results call for at least two parallel α-decay sequences starting from at least two different states of 289Fl. This implies that close-lying levels in nuclei along these chains have quite different spin-parity assignments. Further, observed α-electron and α-photon coincidences, as well as the α-decay fine structure along the decay chains, suggest a change in the ground-state spin assignment between 285Cn and 281Ds. Our experimental results, on the excited level structure of the heaviest odd-N nuclei to date, provide a direct testing groun…

Spectroscopic Tools Applied to Flerovium Decay Chains

Abstract 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 …

Recoil-α-fission and recoil-α–α-fission events observed in the reaction 48Ca + 243Am

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 ch…

Production and Decay of Element 114: High Cross Sections and the New NucleusHs277

The fusion-evaporation reaction Pu-244(Ca-48, 3-4n)(288,289)114 was studied at the new gas-filled recoil separator TASCA. Thirteen correlated decay chains were observed and assigned to the production and decay of (288, 289)114. At a compound nucleus excitation energy of E* = 39.8-43.9 MeV, the 4n evaporation channel cross section was 9.8(-3.1)(+3.9) pb. At E* = 36.1-39.5 MeV, that of the 3n evaporation channel was 8.0-(+7.4)(4.5) pb. In one of the 3n evaporation channel decay chains, a previously unobserved alpha branch in (281)Ds was observed ( probability to be of random origin from background: 0.1%). This alpha decay populated the new nucleus (277)Hs, which decayed by spontaneous fission…

Low-lying states in Ra219 and Rn215 : Sampling microsecond α -decaying nuclei

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 …

Ca48+Bk249Fusion Reaction Leading to ElementZ=117: Long-Livedα-DecayingDb270and Discovery ofLr266

The superheavy element with atomic number Z=117 was produced as an evaporation residue in the 48Ca+249Bk fusion reaction at the gas-filled recoil separator TASCA at GSI Darmstadt, Germany. The radioactive decay of evaporation residues and their α-decay products was studied using a detection setup that allowed measuring decays of single atomic nuclei with half-lives between sub-μs and a few days. Two decay chains comprising seven α decays and a spontaneous fission each were identified and are assigned to the isotope 294-117 and its decay products. A hitherto unknown α-decay branch in 270Db (Z=105) was observed, which populated the new isotope 266Lr (Z=103). The identification of the long-liv…

Spectroscopy along flerovium decay chains. III : Details on experiment, analysis, 282Cn, and spontaneous fission branches

Flerovium isotopes (element Z=114) were produced in the fusion-evaporation reactions 48Ca+242,244Pu and studied with an upgraded TASISpec decay station placed in the focal plane of the gas-filled separator TASCA at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany. Twenty-nine flerovium decay chains were identified by means of correlated implantation, α decay, and spontaneous fission events. Data analysis aspects and statistical assessments, primarily based on measured rates of various events, which laid the foundation for the comprehensive spectroscopic information on the flerovium decay chains, are presented in detail. Various decay scenarios of an excited state obse…

First superheavy element experiments at the GSI recoil separator TASCA: The production and decay of element 114 in thePu244(Ca48,3-4n) reaction

Experiments with the new recoil separator, Transactinide Separator and Chemistry Apparatus (TASCA), at the GSI were performed by using beams of Ca-48 to irradiate targets of Pb206-208, which led to the production of No252-254 isotopes. These studies allowed for evaluation of the performance of TASCA when coupled to a new detector and electronics system. By following these studies, the isotopes of element 114 ((288-291)114) were produced in irradiations of Pu-244 targets with Ca-48 beams at compound nucleus excitation energies around 41.7 and 37.5 MeV, demonstrating TASCA's ability to perform experiments with picobarn-level cross sections. A total of 15 decay chains were observed and were as…

Search for elements 119 and 120

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 pr…

Recoil-alpha-fission and Recoil-alpha-alpha-fission Chains Stemming from Element 115

GSI Scientific Report 2014 - GSI Report 2015-1

Study of non-fusion products in the 50Ti + 249Cf reaction

Physics letters / B B 784, 199 - 205 (2018). doi:10.1016/j.physletb.2018.07.058

Fusion reaction Ca48+Bk249 leading to formation of the element Ts ( Z=117 )

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 confi…

Study of non-fusion products in the Ti50+Cf249 reaction

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 ex…

Spectroscopy along Flerovium Decay Chains: Discovery ofDs280and an Excited State inCn282

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 eleme…

Decay and Fission Hindrance of Two- and Four-QuasiparticleKIsomers inRf254

Two isomers decaying by electromagnetic transitions with half-lives of 4.7(1.1) and 247(73) μs have been discovered in the heavy ^{254}Rf nucleus. The observation of the shorter-lived isomer was made possible by a novel application of a digital data acquisition system. The isomers were interpreted as the K^{π}=8^{-}, ν^{2}(7/2^{+}[624],9/2^{-}[734]) two-quasineutron and the K^{π}=16^{+}, 8^{-}ν^{2}(7/2^{+}[624],9/2^{-}[734])⊗8^{-}π^{2}(7/2^{-}[514],9/2^{+}[624]) four-quasiparticle configurations, respectively. Surprisingly, the lifetime of the two-quasiparticle isomer is more than 4 orders of magnitude shorter than what has been observed for analogous isomers in the lighter N=150 isotones. …

To identify the atomic number of superheavy nuclei produced in Ca-48-induced fusion-evaporation reactions, an experiment aiming at measuring characteristic X-rays is being prepared at GSI, Darmstadt, Germany. The gas-filled separator TASCA will be employed, sending the residues towards the multi-coincidence detector setup TASISpec. Two ion-optical modes relying on differing magnetic polarities of the quadrupole magnets can be used at TASCA. New simulations and experimental tests of transmission and background suppression for these two focusing modes into TASISpec are presented.

117番元素Ts合成のための48Ca+249Bk融合反応

We have performed an experiment to synthesize the element 117 (Ts) with the $^{48}$Ca+$^{249}$Bk fusion reaction. Four $\alpha$-decay chains attributed to the element 117 were observed. Two of them were long decay chains which can be assigned to the one originating from the $\alpha$ decay of $^{294}$Ts. The other two were short decay chains which are consistent with the one originating from the $\alpha$ decay of $^{293}$Ts. We have compared the present results with the literature data, and found that our present results mostly confirmed the literature data, leading to the firm confirmation of the synthesis of the element 117.