Gas-filled Recoil Separator Research Articles

Quasiparticle alignments andα-decay fine structure of175Pt

Excited states and decay properties of ${}^{175}$Pt have been investigated using the ${}^{92}$Mo${(}^{86}\mathrm{Sr},2pn)$ fusion-evaporation reaction. The JUROGAM I $\ensuremath{\gamma}$-ray spectrometer and the GREAT spectrometer were used in conjunction with the gas-filled recoil separator RITU for the measurement of the radiation at the target and focal plane positions, respectively. Two new band structures, assigned to be based on the ${I}^{\ensuremath{\pi}}=({{}^{7}{/}_{2}}^{\ensuremath{-}})$ ground state in ${}^{175}$Pt, have been established and the known yrast band has been extended up to ${I}^{\ensuremath{\pi}}=({{}^{49}{/}_{2}}^{+})$. Rotational properties of the excited states in ${}^{175}$Pt have been investigated within the cranked shell-model formalism. The low-frequency changes in the alignments of the positive- and negative-parity bands are interpreted as a sign of proton-pair excitations in the rotating core. Furthermore, the $\ensuremath{\alpha}$-decay measurements reveal a candidate for a fourth $\ensuremath{\alpha}$-decay branch in ${}^{175}$Pt, feeding a non-yrast state in ${}^{171}$Os.

αdecay of the new neutron-deficient isotope205Ac

The new neutron-deficient isotope ${}^{205}$Ac was synthesized in the complete-fusion reaction ${}^{169}$Tm(${}^{40}$Ca, $4n$)${}^{205}$Ac. The evaporation residues were separated in-flight by the gas-filled recoil separator SHANS in Lanzhou and subsequently identified by the $\ensuremath{\alpha}$-$\ensuremath{\alpha}$ position and time correlation method. The $\ensuremath{\alpha}$-decay energy and half-life of ${}^{205}$Ac were determined to be 7.935(30) MeV and 20${}_{\ensuremath{-}9}^{+97}$ ms, respectively. Previously reported decay properties of the ground state in ${}^{206}$Ac were confirmed.

Average charge states of heavy and superheavy ions passing through a rarified gas: Theory and experiment

The average charge states $\overline{q}$ of heavy and superheavy ions (atomic numbers $Z=80$--114) passing through He gas are studied experimentally and theoretically. Experimental data were measured at the gas-filled recoil separator, i.e., the TransActinide Separator and Chemistry Apparatus (TASCA) at GSI Darmstadt, for ion energies of a few hundred keV/u at gas pressures of 0.2 to 2.0 mbar. An attempt is made to describe experimental $\overline{q}$ values by means of atomic calculations of the binding energies and electron-loss and electron-capture cross sections. The influence of the gas-density effect is included in the calculations. The calculated $\overline{q}$ reproduce the experimental values for elements with $Z=80$--114 within 20%. A comparison with different semiempirical models is presented as well, including a local fit of high accuracy, which is often used in superheavy-element experiments to estimate the average charge states of heavy ions, e.g., at the gas-filled recoil separator TASCA. The $\overline{q}$ values for elements with $Z=115$, 117, 119, and 120 at He-gas pressure of 0.8 mbar are predicted.

High intensity target wheel at TASCA: target wheel control system and target monitoring

At GSI Darmstadt, the gas-filled recoil separator transactinide separator and chemistry apparatus (TASCA) is in operation for experiments with superheavy elements. It is optimized for hot-fusion reactions with actinide targets. The small cross sections of such reactions require the capability to accept highest beam intensities. The limited availability of some of the exotic actinide isotopes limits the size of target systems. To maintain target integrity during long experiments, automated target monitoring and control is necessary. Here, the TASCA target wheel system and the on-line target monitoring are described.

A gas-filled recoil separator, SHANS

A gas-filled recoil separator, SHANS (Spectrometer for Heavy Atom and Nuclear Structure), constructed at the Institute of Modern Physics in Lanzhou is reported. With the QDQQ magnetic configuration, where D and Q denote the dipole and quadrupole magnets respectively, the apparatus is used to study the heavy nuclei produced in the heavy-ion-induced fusion reactions. The helium gas is filled in the field region of the separator at a pressure of about 1mbar. The maximum magnetic rigidity of the dipole magnet is 2.9Tm. With the commissioning of the device, the performance of the separator in several fusion evaporation reactions is investigated.

α-decay studies of the francium isotopes198Fr and199Fr

Very neutron deficient francium isotopes have been produced in fusion evaporation reactions using ${}^{60}$Ni ions on ${}^{141}$Pr targets. The gas-filled recoil separator RITU was employed to collect the fusion products and to separate them from the scattered beam. The activities were implanted into a position sensitive silicon detector after passing through a gas-counter system. The isotopes were identified using spatial and time correlations between the implants and the decays. Two $\ensuremath{\alpha}$-particle activities, with ${E}_{\ensuremath{\alpha}}=7613(15)$ keV and ${T}_{1/2}$ $=$ (${15}_{\ensuremath{-}5}^{+12}$) ms and ${E}_{\ensuremath{\alpha}}=7684(15)$ keV and ${T}_{1/2}$ $=$ (${16}_{\ensuremath{-}5}^{+13}$) ms were identified in the new isotope ${}^{198}$Fr. In addition, the half-life and $\ensuremath{\alpha}$-particle energy of ${}^{199}$Fr were measured with improved precision. The measured decay properties deduced for ${}^{199}$Fr and ${}^{198}$Fr suggest that there is an onset of ground-state deformation at $N=112$ in the Fr isotopes.

Experimental studies of the249Bk + 48Ca reaction including decay properties and excitation function for isotopes of element 117, and discovery of the new isotope277Mt

Studies of superheavy nuclei produced in the ${}^{249}$Bk + ${}^{48}$Ca reaction were performed using the Dubna Gas Filled Recoil Separator. The cross section for the production of ${}^{293}$117 and ${}^{294}$117 isotopes was measured at five excitation energies of the ${}^{297}$117 compound nucleus ranging from 30 to 48 MeV and yielding maximum values of 1.1${}_{\ensuremath{-}0.6}^{+1.2}$ pb for the $3n$ and 2.4${}_{\ensuremath{-}1.4}^{+3.3}$ pb for the $4n$ reaction channels. Alpha emission from ${}^{281}$Rg competing with spontaneous fission ($\ensuremath{\alpha}$/SF decay probability 1:9) was observed for the first time leading to the identification of the new isotope ${}^{277}$Mt (${T}_{\mathrm{SF}}\ensuremath{\approx}5$ ms). The measured decay properties are in good agreement with those expected based on the properties of neighboring even-$Z$ and odd-$Z$ nuclei. The $\ensuremath{\alpha}$ energies and half-lives of odd-$Z$ isotopes observed in the ${}^{293}$117 and ${}^{294}$117 decay chains together with results obtained for lower-$Z$ superheavy nuclei demonstrate enhanced stability with increasing neutron number toward the predicted new magic number $N=184$.

Observation of superheavy nuclide 271Ds based on the Gas-filled separator at IMP

A gas-filled recoil separator was designed and built newly at Institute of Modern Physics (IMP) in Lanzhou. With the recent commissioning of the separator, the decay properties of 271Ds were studied via the 208Pb (64Ni, n) reaction at a beam energy of 313.3 MeV. One α-decay chain for 271Ds was established in total. The experimental result is consistent with the values reported in the literature.

Investigation of the243Am+48Careaction products previously observed in the experiments on elements 113, 115, and 117

Results from the production and decay properties of element 115 nuclei observed using the reaction ${}^{243}\mathrm{Am}+{}^{48}\mathrm{Ca}$ at various beam energies between November 1, 2010, and February 26, 2012, at the Dubna Gas Filled Recoil Separator are presented. This long-running experiment with a total beam dose of $3.3\ifmmode\times\else\texttimes\fi{}{10}^{19}$ and carried out in the excitation energy range ${E}^{*}=31$--47 MeV of the ${}^{291}$115 compound nucleus resulted in observation of three isotopes of element 115 with masses 287, 288, and 289. The 28 detected decay chains of ${}^{288}$115 show that this isotope is produced with the maximum probability at ${E}^{*}=34.0$--38.3 MeV with a corresponding cross section of ${\ensuremath{\sigma}}_{3n}=8.{5}_{\ensuremath{-}3.7}^{+6.4}$ pb. The four events attributed to the isotope ${}^{289}$115 that decays via a short $\ensuremath{\alpha}\ensuremath{\rightarrow}\ensuremath{\alpha}\ensuremath{\rightarrow}\text{SF}$ chain could be detected only at the lowest excitation energy ${E}^{*}=31$--36 MeV, in accordance with what could be expected for the 2$n$-evaporation channel of the reaction. The decay characteristics of this nuclide were established earlier (2010) and more recently (2012) in the reaction ${}^{249}$Bk(${}^{48}$Ca,4$n$)${}^{293}$117 and following $\ensuremath{\alpha}$ decay to ${}^{289}$115. At the energy ${E}^{*}=44.8\ifmmode\pm\else\textpm\fi{}2.3$ MeV we observed only a single long chain of the isotope ${}^{287}$115. The decay properties of nuclei starting at ${}^{288}$115 and ${}^{287}$115 isotopes obtained in the present work reproduce in full the results of the first experiment of 2003 that reported the discovery of elements 115 and 113. The excitation functions of the production of the isotopes of element 115 and observation of the isotope ${}^{289}$115 in cross-bombardment reactions with the targets of ${}^{243}$Am and ${}^{249}$Bk provide additional evidence of the identification of the nuclei of elements 115 and 113. The experiments were carried out using the ${}^{48}$Ca beam of the U400 cyclotron of the Flerov Laboratory of Nuclear Reactions, JINR.

The isomeric structure of 132Pr

The isomeric structure of the neutron deficient nucleus 132Pr, located in the rare-earth region of the nuclear chart, has been studied with the 98Mo(40Ar,5pn)132Pr reaction at beam energies of 150, 158 and 165 MeV. The experiment was performed at the University of Jyväskylä, Finland where the 40Ar beam was accelerated onto the target by the K130 cyclotron. The JUROGAM II HPGe detector array was employed in conjunction with the RITU gas-filled recoil separator. The focal-plane chamber housed a multi wire proportional counter and a position-sensitive silicon strip detector used for the implantation and identification of recoiling nuclei. The recoil-isomer tagging technique was used to correlate the delayed decays, measured in the Planar and Clover detectors of the GREAT spectrometer, with the known prompt transitions in 132Pr. Two new delayed transitions have been observed at energies of 102 and 118 keV. The corresponding X ray peaks are consistent with Pr Kα and Kβ X rays with energies of 35.63 and 40.91 keV, respectively. The half-life of the newly established isomeric state, from which the 102 and 118-keV transitions proceed, has been measured to be 2.5(3) μs.

RF power supplies for the RFQ cooler and buncher RFQ1L

In order to match directly the beams from the gas-filled recoil separator, a RFQ cooler and buncher RFQ1L has been designed to have a large radius (r0=60 mm), thus the RF power supply becomes not a trivial problem. We have successfully solved this problem by two different methods, and the RF power supplies specific to the RFQ1L have been developed and tested. Technical descriptions and test results are presented in this paper. It shows that the power supply with series resonance circuits should be used for the operation of the RFQ1L.

Study of the average charge states of 188Pb and 252,254No ions at the gas-filled separator TASCA

The average charge states of 188Pb and 252,254No ions in dilute helium gas were measured at the gas-filled recoil separator TASCA. Hydrogen gas was also used as a filling gas for measurements of the average charge state of 254No. Helium and hydrogen gases at pressures from 0.2mbar to 2.0mbar were used. A strong dependence of the average charge state on the pressure of the filling gases was observed for both, helium and hydrogen. The influence of this dependence, classically attributed to the so-called “density effect”, on the performance of TASCA was investigated. The average charge states of 254No ions were also measured in mixtures of helium and hydrogen gases at low gas pressures around 1.0mbar. From the experimental results simple expressions for the prediction of average charge states of heavy ions moving in rarefied helium gas, hydrogen gas, and in their mixture were derived.

Observation of the Superheavy Nuclide 271Ds

With the recent commissioning of a gas-filled recoil separator at Institute of Modern Physics (IMP) in Lanzhou, the decay properties of 271Ds (Z = 110) were studied via the 208Pb(64Ni, n) reaction at a beam energy of 313.3 MeV. Based on the separator coupled with a position sensitive silicon strip detector, we carried out the energy-position-time correlation measurements for the implanted nucleus and its subsequent decay α's. One α-decay chain for 271Ds was established. The α energy and decay time of the 271Ds nucleus were measured to be 10.644 MeV and 96.8 ms, which are consistent with the values reported in the literature.

Synthesis of the New Element with Z=117

The synthesis of the new chemical element with atomic number Z=117 is presented. The isotopes 293117 and 294117were produced in fusion reactions between 48Ca and 249Bk. The 249Bk was produced in the High Flux Isotope Reactor and chemically separated at Oak Ridge. Decay chains involving eleven new nuclei were identified by means of the Dubna Gas Filled Recoil Separator. The measured decay properties show a strong rise of stability for super-heavy nuclei toward N=184.

The SAGE spectrometer: A tool for combined in-beam γ-ray and conversion electron spectroscopy

The SAGE spectrometer allows simultaneous in-beam γ-ray and internal conversion electron measurements, by combining a germanium detector array with a highly segmented silicon detector and an electron transport system. SAGE is coupled with the ritu gas-filled recoil separator and the great focal-plane spectrometer for recoil-decay tagging studies. Digital electronics are used both for the γ ray and the electron parts of the spectrometer. SAGE was commissioned in the Accelerator Laboratory of the University of Jyväskylä in the beginning of 2010.

In-beam conversion-electron spectroscopy of180Hg

Excited states in {sup 180}Hg were populated using the {sup 147}Sm({sup 36}Ar,3n){sup 180}Hg reaction and studied by in-beam conversion-electron spectroscopy. Conversion electrons emitted at the target position were measured using the Silicon Array for Conversion Electron Detection (SACRED) spectrometer and tagged through the characteristic {alpha} decays of {sup 180}Hg detected in a position-sensitive silicon strip detector located at the focal plane of the gas-filled recoil separator Recoil Ion Transport Unit (RITU). Electron conversion of transitions previously assigned to {sup 180}Hg through in-beam {gamma}-ray spectroscopy studies was identified up to the 10{sup +}{yields}8{sup +} transition and the intensities of the conversion-electron transitions were found to be consistent with the previous multipolarity assignments. Evidence was also found for two highly converted transitions in {sup 180}Hg: a 167 keV transition is interpreted as the transition from the newly identified 2{sub 2}{sup +} state at 601 keV to the 2{sub 1}{sup +} state at 434 keV, while a 420 keV transition is assigned as the E0 decay from the 0{sup +} bandhead of the prolate-deformed configuration to the weakly deformed ground state.

An attempt to build a statistical model for a pips detector operated in a real-time mode

Basic approaches by K.-H. Schmidt and V.B. Zlokazov to estimate the probability of registered multichain event to be explained by random coincidences are considered. A specific feature of the long-term experiments aimed at the synthesis of superheavy nuclei with the Dubna Gas-Filled Recoil Separator is usage of a real-time mode for radical suppression of background products. In fact, this assumes that the first correlation group, namely, recoil-alpha correlation, stops target irradiation for a short time and the forthcoming signals are detected in more favorable background conditions. Due to the application of this detection mode the first recoil-alpha chain can be considered as a “starter” for the detection of the forthcoming alpha-particle signals with high background suppression. This fact is taken into account in the given PIPS detector statistical model.

Absolute transmission and separation properties of the gas-filled recoil separator RITU

The absolute transmission values of the RITU gas-filled recoil separator have been measured for several fusion evaporation products. Three different beams and several targets were used to create fusion evaporation residues in mass region A=170–190 mainly in neutron evaporation channels. Measured absolute transmission values from around 7% up to round about 90% have been observed for very asymmetric and symmetric reactions, respectively. For the reactions studied it was observed that the angular spread of products after the target and the angular acceptance of RITU determine the transmission probability relatively accurately. Magnetic rigidities and equilibrium charges of the product ions were seen to systemically decrease and increase, respectively, with increasing gas pressure. The comparison of observed equilibrium charges to various formulae is carried out. Clear minima of image width were found in case of asymmetric reactions as a function of gas pressure. Effects of the position of an additional movable beam stopper and the field strength of the first quadrupole magnet on transmission have been studied and are reported.

Eleven new heaviest isotopes of elementsZ=105toZ=117identified among the products ofBk249+Ca48reactions

The heaviest isotopes of elements Z = 117 to Z = 105, 294117, 293117, 290115, 289115, 286113, 285113, 282Rg, 281Rg, 278Mt, 274Bh, and 270Db, were identified by means of the Dubna gas-filled recoil separator among the products of the 249Bk + 48Ca reaction. The details of the observed six decay chains, indicating the production and decay of isotopes 293117 and 294117, are presented and discussed. The decay energies and resulting half-lives of these new nuclei show a strong rise of stability with increasing neutron number, validating the concept of the island of enhanced stability for superheavy nuclei.

Superheavy elements at GSI: a broad research program with element 114 in the focus of physics and chemistry

AbstractStudies of the superheavy elements form one of the pillars of the GSI research program. A unique combination of experimental facilities is installed at the GSI. Various topics, ranging from the synthesis of new elements, spectroscopy experiments to study the nuclear structures of the heaviest nuclei, highly accurate mass measurements beyond uranium, to chemical investigations of elements around element 114 and the synthesis of novel chemical superheavy element compound classes are being studied. This is complemented by fully relativistic quantum chemical calculations. As a recent highlight, the244Pu(48Ca,3–4n)288,289114 reaction was studied, leading to the observation of element 114 at the new gas-filled recoil separator TASCA.