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Data is presented on the radioactive decay of isotopes 256Rf and 249, 250No synthesized in complete fusion reactions 50Ti + 208Pb, and 48Ca + 204Pb, with subsequent evaporation of neutrons from the excited state of the composite nucleus. Experiments use high-intensity beams of accelerated heavy ions 48Ca and 50Ti from the U-400 cyclotron at the Flerov Nuclear Reactions Laboratory of the Joint Institute for Nuclear Research and the SHELS kinematic separator.

International audience; A great many experimental studies devoted to exploring in detail the properties of radioactive decay of the isotopes of transfermium elements by means of alpha, beta, and gamma spectroscopy have been performed over the past fifteen years at the Flerov Laboratory of Nuclear Reactions (Joint Institute for Nuclear Research, Dubna). High-intensity accelerated heavy-ion beams of ${}^{22}$Ne, ${}^{48}$Ca, ${}^{50}$Ti, and ${}^{54}$Cr from the U-400 cyclotron were used in experiments with the SHELS kinematic separator. An upgrade of the separator and detector system is planned in the future with the aim of improving the transportation efficiency for the nuclei under study and the detection efficiency for gamma rays.

International audience; Spontaneous fissions of the $^{252}$No and $^{254}$No nobelium isotopes have been studied in a series of experiments with the SHELS separator. These isotopes are produced by colliding a beam of $^{48}$Ca ions from the U-400 cyclotron with the $^{206}$Pb and $^{208}$Pb targets. The measured characteristics of the $^{252}$No and $^{254}$No spontaneous fissions include their half-lives, total kinetic energies of fission fragments, and prompt-neutron multiplicities. The average number of neutrons per $^{254}$No spontaneous-fission act is measured for the first time as 4.88 ± 0.53.

International audience; GABRIELA (Gamma Alpha Beta Recoil Investigations with the Electromagnetic Analyzer) is a detection system installed at the focal plane of the SHELS (Separator for Heavy Elements Spectroscopy) recoil separator for gamma and internal conversion electron spectroscopy of heavy and superheavy nuclei. GABRIELA has recently been upgraded. The characteristics of the new setup are presented using the Geant4 Monte Carlo simulation toolkit and validated against experimental results. The impact of summing on the gamma-ray and electron detection efficiencies is discussed.

International audience; The neutron yield from the spontaneous fission of the short-lived neutron-deficient$^{254}$Rf nucleus produced in the complete fusion reaction with the beam of multiply charged ions from the U400 accelerator at the Flerov Laboratory of Nuclear Reactions (FLNR), Joint Institute for Nuclear Research (JINR), is measured using the combined detection system of the SHELS velocity filter. The half-life and the branching ratio of$^{254}$Rf are also measured. The data on the multiplicity of prompt neutrons from spontaneous fission of$^{256}$Rf ($\bar {\nu }$ = 3.87 ± 0.34) are presented for the first time.

International audience; Experiments aimed at synthesizing and investigating the features of radioactive decay of isotopes of superheavy elements (Z ≥ 110) have been conducted in the Flerov Laboratory of Nuclear Reactions at the Joint Institute for Nuclear Research. Complete fusion reactions of an accelerated beam of$^{48}$Ca ions with transactinide targets with the subsequent evaporation of several neutrons from excited compound nucleus are typically used in these experiments. “Exotic” reaction channels with the evaporation of protons and several neutrons may be used to produce more neutron-rich isotopes of superheavy elements. The production cross sections were measured in experiments at the SHELS separator for Db and Rf isotopes produced in xn and pxn channels of the$^{50}$Ti +$^{209}$Bi →$^{259}$Db* reaction. The ratio of cross sections σ(xn)/σ(pxn) was on the order of 100.

The problem of studying the spontaneous fission of nuclei from the exotic region of short-lived neutron-deficient isotopes with Z ≥ 100 is considered. A combined detecting system is created in the focal plane of the VASSILISSA separator. The main part of this system is a neutron detector that allows us to measure the yields of spontaneous fission neutrons from nuclei produced in a total fusion reaction with heavy ion beams in the on-line mode. This procedure is used for the first time to acquire data on the emission of spontaneous fission neutrons from short-lived 244,246Fm isotopes. Data gathered earlier for 252No isotopes are refined in the scope of this work.

series of experiments performed in 2014–2015 using the SHELS separator and a beam of multicharged ions from the U-400 accelerator at the Flerov Laboratory of Nuclear Reactions (Joint Institute for Nuclear Research) in Dubna is described. In preparation for a large program of experiments with a beam of highly charged ions of 50Ti, an experiment is performed to study the spontaneous fissioning (SF) of short-lived neutron-deficient isotope 256Rf formed in complete fusion reaction50Ti + 208Pb = 256Rf + 2n. The half-life of 256Rf is measured along with the probability of its SF decay. The average number of neutrons per one event of 256Rf SF ( $$\bar v$$ = 4.47 ± 0.09) is obtained for the first time. The results from experiments to study the SF of short-lived neutron-deficient isotopes synthesized in complete fusion reaction 48Ca + 204Pb → 252No* are presented as well. Two short-lived activities attributed to the SF of the ground and isomeric states of 250No are revealed. The half-life, total kinetic energies of fission fragments, and neutron multiplicities are measured for the obtained short-lived nuclei. The average numbers of neutrons per one SF event are $$\bar v$$ = 4.38 ± 0.13 and $$\bar v$$ = 3.90 ± 0.20, respectively, for SF activity with t1/2 = 5.1 ± 0.3 and t1/2 = 36 ± 3 μs.

A high intensity ion beam of 50Ti ions was obtained using the ECR ion source on the U400 cyclotron. The experimental tests using accelerated 50Ti ions were performed with a modernized VASSILISSA separator (SHELS). Data has been obtained on the transmission coefficients of recoil nuclei synthesized in complete fusion reactions. Estimates from ion optical calculations performed in the design phase of the project of modernizing the separator are completely confirmed.

This study describes an experiment on investigating the properties of spontaneous fission of shortlived neutron-deficient nuclei synthesized in the reaction of complete fusion 48Ca + 204Pb = 252No*. The experiment is performed using the SHELS separator and the beam of multicharged ions at U-400 accelerator (LNR, JINR). Two activities undergoing spontaneous fission, which can be related to the ground and isomeric states of 250No nucleus, are registered. The half-lives, total kinetic energies of fission fragments, and neutron multiplicities are measured for the short-lived nuclei. The average number of neutrons per fission for the activity with t 1/2 = 5.1 ± 0.3 μs is = 4.38 ± 0.13 μs, and for nuclei with the half-life t 1/2 = 36 ± 3 μs it is xxxxx.

International audience; During the last 12 years using the GABRIELA (Gamma Alpha Beta Recoil Invetsigations with the Electromagnetic Analyser) detector set-up and kinematic recoil separators VASSILISSA / SHELS the experiments aimed to the gamma and electron spectroscopy of the Fm – Db isotopes, formed at the complete fusion reactions 48Ca + 207,208Pb → 255,256No*, 48Ca + 209Bi → 257Lr*, 22Ne + 238U → 260No*, 50Ti + 208Pb → 258Rf* and 50Ti + 209Bi → 259Db*, were performed at FLNR JINR.

International audience; In the last years we carried out several experiments aimed to investigate properties of short-lived SF isotopes. The neutron-deficient isotope 256Rf were produced in fusion-evaporation reactions using an 208Pb target and an intense 50Ti beam. Fusion-evaporation residues were separated by the SHELS separator (FLNR, Dubna) and implanted into a large-area double-sided silicon 48×48 strip detector surrounded by 3He-based neutron counters. The half-life and decay branching ratio of 256Rf is measured. The average number of neutrons per spontaneous fission of 256Rf is determined for the first time. Next we carried out an experiment aimed at investigating the properties of spontaneous fission of neutron deficient isotope 250No produced in the reaction with 48Cabeam and 204Pb-target. Earlier two spontaneous fission half-lives (t1/2 = 5.1±0.3 μs and t1/2 = 36±3 μs) originate from 250No were registered. The obtained excitation functions after the evaporation of 2 neutrons from the compound 252No nuclei have been compared with similar data obtained earlier. From the experimental data for the first time the average number of neutrons per spontaneous fission and TKE-spectra of fission fragments of both 250No activities was determined.

International audience; Spontaneous fission properties of the short-lived neutron-deficient 256Rf nucleus produced in thecomplete fusion reaction with a beam of multiply charged heavy 50Ti ions from the U-400 cyclotron (FLNR,JINR) are experimentally investigated. Its half-life and decay branching ratio are measured. The averagenumber of neutrons per spontaneous fission of 256Rf ( = 4.47 ± 0.09) is determined for the first time.

The detector system of the VASSILISSA separator has been complemented with an array of 54 3He neutron counters to study the multiplicity of prompt spontaneous-fission neutrons. The detection efficiency of the new neutron detector is 39%. The description of the detector system and the characteristics of 252No spontaneous fission, obtained in the test experiment in July of 2010, are presented.

A new setup composed of 32 pairs of semiconductor detectors, systems of data acquisition, storage, and processing, and a vacuum system has been developed at the Flerov Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research. The setup can be used for low-background measurements and is intended to study radioactive decays of relatively long-lived isotopes of heavy and superheavy elements in a geometry close to 4π.

The neutron-deficient isotope 246Fm , produced in the complete fusion reaction 40Ar$ + $208Pb , was investigated. The main goal of the experiment was to determine the neutron multiplicity at spontaneous fission of this isotope. For experiments aimed at the study of spontaneous fission of transfermium nuclei improvements in the focal plane detector system of recoil separator VASSILISSA have been made. A neutron detector consisting of 54 3He -filled counters has been mounted around the focal-plane detector chamber. From the experimental data the average number of neutrons per spontaneous fission of 246Fm was determined ( $ \bar{{\nu}}$ = 3.55±0.5) .

Two gas phase adsorption chemistry experiments aimed at the chemical characterization of element 112 using its isotope 283 112 have been performed at the Flerov Laboratory for Nuclear Reactions (FLNR) Dubna, Russia. The applied Insitu-Volatilization and On-line Detection (IVO) technique is a thermochromatographic system combining the determination of the deposition temperature of volatile elements on a surface along a temperature gradient with an efficient detection of the deposited species by event-by-event alpha and SF-fragment spectroscopy. Two possibilities to produce the isotope 283 112 were used: 1.) the direct production reaction 238 U( 48 Ca,3n) 283 112; 2.) the reaction 242 Pu( 48 Ca,3n), where the primary product 287 114, decays via alpha emission to 283 112 with a half-life of 0.5 s. The chemistry experiments were aimed at a chemical identification of 283 112 and an independent confirmation of its decay properties. In the direct reaction no decays related to 283 112 were observed. However, two decay chains unambiguously attributed to the decay of 283 112 were observed using the second production path. Previously reported observation of 283 112 and 279 Ds and their decay properties were confirmed. From its thermochromatorgaphic deposition first thermochemical data were deduced for element 112, unveiling it as a typical group 12 element.

For experiments aimed at the study of spontaneous fission of transfermium nuclei improvements in the focal plane detector system of recoil separator VASSILISSA have been made. The neutron detector consisting of 72 3He filled counters has been mounted around the focal plane detector chamber. In the first experiment the multiplicity of prompt neutrons emitted in spontaneous fission of 252No was measured.

Aiming at an increase in the identification ability and higher sensitivity in the experiments on the synthesis and study of the decay properties of superheavy nuclei, the separator VASSILISSA has been upgraded. After preparatory experiments, the experiment aiming at the synthesis of the isotope 283 112 in the reaction 48 Ca + 238 U was performed. The obtained results on the decay mode, half-life and production cross-section are in agreement with those obtained in our first experiment.

The upgraded separator VASSILISSA was used to confirm results of previous experiments on the synthesis of the heavy isotopes of the element 112 obtained in complete-fusion reactions of 48Ca and 238U. An additional reason was the non-observation of any events in the same reaction studied with the BGS separator at Berkeley. Limits of $280 \leq A \leq 286$ for the atomic mass number of the observed spontaneously fissioning isotope were measured. The obtained results on the decay mode, half-life and production cross-section are in agreement with those obtained in our first experiment.

Within the past 15 years, the recoil separator VASSILISSA has been used for the investigations of evaporation residues produced in complete fusion reactions induced by heavy ions. The study of decay properties and formation of cross-sections of the isotopes of elements 110, 112 and 114 was performed using high-intensity 48 Ca beams and 232 Th , 238 U , 242 Pu targets. For further experiments aimed at the synthesis of the superheavy element isotopes ( Z ⩾110) with the use of intense 48 Ca extracted beams, improvements in the ion optical system of the separator and the focal plane detector system have been made. The results from the test reactions and new results for the isotope 283 112 are presented.

Within the past twelve years, the recoil separator VASSILISSA has been used for the investigations of evaporation residues produced in heavy-ion-induced complete fusion reactions. In the course of the experimental work, fourteen new isotopes have been identified by the parent-daughter correlations in the region of elements with 92≤Z≤94. The study of the decay properties and formation cross sections of the isotopes of elements 110, 112, and 114 was performed using high-intensity 48Ca beams and 232Th, 238U, and 242Pu targets. At the beam energies corresponding to the calculated cross-section maxima of the 3n evaporation channels, the isotopes 277110, 283112, and 287114 were produced and identified. For further experiments aimed at the synthesis of the superheavy element isotopes (Z≥110) with the use of intensive 48Ca extracted beams, improvements in the ion optical system of the separator and the focal plane detector system have been made.

Heavy-ion fusion reactions 48Ca + 204Pb and 44Ca + 208Pb leading to the same compound nucleus 252No* were run in attempts to produce new neutron-deficient spontaneous-fission isotopes of 249,250No using the electrostatic separator VASSILISSA. Production cross-sections for the spontaneous-fission activities with the half-lives 5.6 and 54 μs observed in these reactions are compared with the measured ones for the well-known isotopes of 251 - 255No formed in the heavy-ion fusion reactions 48Ca + 206Pb and 48Ca + 208Pb. The obtained excitation functions for the reaction products formed after the evaporation of 1-4 neutrons from the corresponding compound No nuclei have been compared with similar data obtained earlier and results of statistical model calculations.

International audience; The results of the first experimental tests of the modernized VASSILISSA separator with the useof accelerated 22Ne ions are presented. Data has been obtained on the transmission coefficients of recoilnuclei synthesized in asymmetric combinations of the incident ion and target nuclei. Estimates from ion optical calculations performed in the design phase of the project of modernizing the separator are completelyconfirmed.2015

International audience; Investigations of complete fusion reactions leading to the synthesis of heavy and superheavy nuclei have been an important part of the experimental program at the Flerov Laboratory of Nuclear Reactions (JINR, Dubna). For synthesis and decay studies of heavy nuclei, a project of modernization of the existing experimental set up VASSILISSA is now underway. The results of the first experimental tests with the modernized VASSILISSA separator are presented, in particular the experimentally measured transmission coefficients for recoil nuclei synthesized in asymmetric reactions with a Ne-22 beam. These results completely confirm the ion optical calculations performed during the design stage of the modernization.

The first attempt to chemically identify one of the recently discovered long-lived isotopes of superheavy elements, namely 283112 (3 min, SF), made at FLNR, Dubna is reported. The nuclide was produced by fusion of accelerated 48Ca with a target of natural U, which contained some Nd to simultaneously produce also short-lived Hg nuclides. According to test experiments with Hg, the expected lighter homologue of element 112, both elements can be isolated from the products of the bombardment in metallic state, and transported from the target in flowing He gas to a detection system, where spontaneous fission and alpha decays 49-s 185Hg) were registered using PIPS detectors. The surface of the detectors was covered with a thin layer of Au or Pd which ensured the detection of Hg with high efficiency due to its chemisorption on these surfaces. While about three SF events could be expected, not a single one was detected at an upper limit for the production cross section of about one picobarn. This may point to a "Rn-like" rather than "Hg-like" behavior of element 112. A new experiment is planned, in which both a "Rn-like" and a "Hg-like" species could be detected.

Since the past 10 years, the recoil separator VASSILISSA has been used for the investigations of evaporation residues produced in heavy-ion-induced complete fusion reactions. In the course of experimental work in the region of the elements with 92⩽Z⩽94 , 14 new isotopes have been identified by the parent–daughter correlations. For further experiments aimed at the synthesis of the superheavy element isotopes ( Z⩾110 ) with intensive 48 Ca extracted beams, improvements in the focal plane detector system have been made. As a result, energy and position resolutions of the detectors have been significantly improved, the detection efficiency for the α -particles, emitted from the implanted into the focal plane detector recoil nuclei, has been increased to 85%. The decay properties of the new isotope of element 112 and the isotope of the new element 114 with mass 287 have been measured in two-nuclei decays.

The stability of heavy nuclides, which tend to decay by α-emission and spontaneous fission, is determined by the structural properties of nuclear matter. Nuclear binding energies and lifetimes increase markedly in the vicinity of closed shells of neutrons or protons (nucleons), corresponding to ‘magic’ numbers of nucleons; these give rise to the most stable (spherical) nuclear shapes in the ground state. For example, with a proton number of Z = 82 and a neutron number of N = 126, the nucleus 208Pb is ‘doubly-magic’ and also exceptionally stable. The next closed neutron shell is expected at N = 184, leading to the prediction of an ‘island of stability’ of superheavy nuclei, for a broad range of isotopes with Z = 104 to 120 (refs 1, 2). The heaviest known nuclei have lifetimes of less than a millisecond, but nuclei near the top of the island of stability are predicted to exist for many years. (In contrast, nuclear matter consisting of about 300 nucleons with no shell structure would undergo fission within about 10−20 seconds.) Calculations3,4,5 indicate that nuclei with N > 168 should already benefit from the stabilizing influence of the closed shell at N = 184. Here we report the synthesis of an isotope containing 114 protons and 173 neutrons, through fusion of intense beams of 48Ca ions with 242Pu targets. The isotope decays by α-emission with a half-life of about five seconds, providing experimental confirmation of the island of stability.

The reaction 48Ca+238U was investigated at the recoil separator VASSILISSA at the Flerov Laboratory of Nuclear Reactions, JINR Dubna, in attempts to synthesize new isotopes of element 112. The experiments were performed at two beam energies resulting in excitation energies of the compound nucleus of 33 and 39 MeV. The collected beam dose was 3.5 × 1018 and 2.2 × 1018, respectively. Two spontaneous fission events were observed at the lower beam energy, which tentatively were assigned to the new neutron rich isotope 283112 produced in the reaction 238U(48Ca,3n)283112. The measured cross-section is (5.0+6.3−3.2) pb and the half–life is (81+147−32) s. No event was observed at the higher beam energy resulting in the upper cross–section limit of 7.3 pb.

The production cross sections and excitation functions for the neutron deficient isotopes have been measured in the and reactions for n and (4-7)n evaporation channels of the de-excitation of the compound nuclei and , respectively. This study considers in addition the de-excitation via the n evaporation channels of the compound nucleus formed in the reaction. The production cross sections of formed after evaporation of n and n from the and compound nuclei produced in the and reactions were also measured respectively. The evaporation residues emerging from the target were separated in-flight from the projectiles and background reaction products by the electrostatic recoil separator VASSILISSA (Yeremin A V et al 1994 Nucl. Instrum. Methods A 350 608). The investigation regards the U and Fm compound nuclei in the 40-80 MeV excitation energy range. For the analysis of the evaporation cross sections the advanced statistical model (D'Arrigo A et al 1994 Phys. Lett. B 334 1) calculations were used. The angular-momentum dependence of the shell correction to the fission barrier, and the effects of the nuclear viscosity and dynamical deformation for these fissile excited nuclei are considered. The values at the initial steps of the de-excitation cascade for the U and Fm compound nuclei were derived from the measured excitation functions and discussed from the point of view of the consequences for the fission-process dynamics.

224U nuclei were populated in fusion-evaporation reactions using a 206Pb target and an intense 22Ne beam. Fusion-evaporation residues were separated by the new separator SHELS at the FLNR, Dubna and implanted into a large-area double-sided silicon strip detector. Position- and time-correlated alpha decays were used to identify evaporation residues. A new $ \alpha$ -decay line at 8095(11) keV was observed in this work and assigned as the decay from 224U to the first excited 2+ in the daughter nucleus 220Th. Coincident photons were also observed allowing to unambiguously determine the excitation energy of the first excited 2+ state in 220Th to be 386.5(1) keV and not 373.3(1)keV as previously reported. The half-life of 224U was measured to be 396(17)μs.

The production cross sections were measured for the xn, pxn, and αxn de-excitation channels of the 212Rn compound nuclei formed at excitation energies of 80–160 MeV in the reaction 22Ne + 190Os. The maximum cross sections obtained for the xn (x = 8–12), pxn (x = 8–11) and αxn (x = 8–11) reaction channels were fitted satisfactorily with the computer code HIVAP using its standard parameter set except that the single scaling parameter C = 0.65 was implemented to adjust the values of the liquid-drop fission barriers predicted by the Cohen-Plasil-Swiatecki (CPS) model. Additionally, an analysis of a large set of data on the formation cross sections of neutron-deficient Z = 83–92 nuclei showed that the whole body of data obtained for the nuclear region notable for the large variations of both the liquid-drop barrier and the shell effects, can be described well with practically the same parameter C = 0.55–0.68. One may take this observation as an indication that, though the CPS model overestimates the liquid-drop fission barriers, it reproduces well the relative behaviour of these barriers in the whole range of neutron-deficient Z = 83–92 nuclei.

Within the past seven years the electrostatic separator VASSILISSA [A.V. Yeremin et al., Nucl. Instr. and Meth. A 274 (1989) 528; A 350 (1994) 608] has been used for the investigations of evaporation residues (ER) produced in heavy-ion induced complete fusion reactions. Two types of such studies were of specific interest to us. These are the synthesis of heavy (Z ≥ 100) nuclides and investigations of heavy-ion fusion reactions leading to formation of neutron-deficient nuclei with Z ≥ 82 and high excitation energy ( 60 MeV ≤ E ∗ ≤ 200 MeV ). In the course of the experimental work the optimization of the ion-optical and other systems of the separator including the improvements in the focal plane detector system has been made. As a result, the separation efficiency values ranging from 3 to 30% were achieved for ER's produced in the reactions with heavy ions from 12C to 48Ti. The suppression factors > 1019 for the full energy beam particles and > 104 for multinucleon transfer reaction products were achieved. The results of the performed experiments illuminating the separator performance and future plans of the separator upgrade are briefly presented.

For experiments aimed at the study of spontaneous fission of transfermium nuclei improvements in the focal plane detector system of recoil separator VASSILISSA have been made. A neutron detector consisting of 54 3He-filled counters has been mounted around the focal-plane detector chamber. The reaction 48Ca + 206Pb = 2n + 252No is used for tuning the separator settings and calibrating the detector system with the spontaneous fission of the 252No. The average neutron number per 252No spontaneous fission event is as large as ν̅ = 4.06 ± 0.12. The short-lived heavy isotopes 244,246Fm, produced in the complete fusion reactions 40Ar + 206,208Pb, are investigated. The average number of neutrons per spontaneous fission of 244,246Fm from the experimental data were (ν̅ = 3.3 ± 0.3) and (ν̅ = 3.55 ± 0.50), respectively. Both values are determined for the first time.

The neutron detector with 3He -filled counters placed in the focal plane of the VASSILISSA separator is used for measuring the average number and determining the multiplicity distribution of prompt neutrons from the spontaneous fission of heavy short-lived isotopes. The test reaction $\ensuremath \mathrm{{}^{48}Ca}+\mathrm{{}^{206}Pb}=2{\rm n}+\mathrm{{}^{252}No}$ is used for tuning the separator settings and calibrating the detector system with the spontaneous fission of the 252No . The average neutron number per 252No spontaneous fission event is as large as $\ensuremath \bar{\nu}=4.06 \pm 0.12$ . The short-lived heavy isotope 244Fm , produced in the complete fusion reaction 40Ar + 206Pb , is investigated. The average number of neutrons per spontaneous fission of 244Fm from the experimental data ( $\ensuremath \bar{\nu}=3.3 \pm 0.3$ is determined for the first time.

Within the last five years the kinematic separator VASSILISSA [A.V. Yeremin et al., Nucl. Instr. and Meth. A 274 (1989) 528; and A.V. Yeremin et al., Preprint JINR E15-90-347, Dubna 1990] has been used for investigations of neutron deficient evaporation residues (ER) produced in heavy ion fusion reactions. In the course of this work optimization of the ion-optical and other systems of the separator including improvements of the focal plane detector system have been made. As a result, the separation efficiency values ranging from 3 to 30% were achieved for ERs produced in the reactions with heavy ions ranging from 16O to 40Ar. The suppression factors >1019 for the full energy beam particles and >104 for multinucleon transfer reaction products were achieved. The results of the performed experiments illuminating the separator performance are briefly presented.

New high-statistics data have been obtained on the decay properties of 253 No and its daughters using the reaction 207 Pb( 48 Ca, 2n) 253 No. This was made possible thanks to an improved transmission of fusion–evaporation residues through the VASSILISSA recoil separator and an increased efficiency of the GABRIELA detector setup. The decay schemes of 253 No and 249 Fm have been revisited. The known level scheme of 249 Fm has been confirmed, including a new level at 669 keV excitation energy. The observation of L X-rays in coincidence with the α decay of 249 Fm gives additional support to the ground-state configuration of 1 / 2 + [ 631 ] instead of 5 / 2 + [ 622 ] for 245 Cf. In both 249 Fm and 245 Cf, the interpretation of the data has been checked by comparing experimental α -particle and γ -ray spectra with realistic simulations of the decay cascades and of the interaction of particles and photons in the detectors of GABRIELA. The population of a 0.7 ms isomeric state attributed to 253 No is confirmed by an α -tagged calorimetric measurement and the corresponding γ and electron decay spectra are presented. Possible evidence for more than one isomer is given and a tentative partial decay scheme is discussed in the light of the available experimental data, systematics and theoretical expectations.

International audience; An IN2P3-JINR collaboration has launched a project called GABRIELA at the Flerov Laboratory for Nuclear Reactions (FLNR) within the Joint Institute for Nuclear Research (JINR) in Dubna (Russia). The goal of the project is to perform gamma-ray and internal conversion electron spectroscopy of heavy nuclei produced in fusion-evaporation reactions and transported to the focal plane of the recoil separator VASSILISSA. During five experimental campaigns of GABRIELA, the detection system has gained in sensitivity and new spectroscopic information has been obtained for 249Fm, 251Fm, 253No and 255Lr.

The new neutron deficient isotope 217U was produced in the bombardment of the 182W target with 40Ar ions and identified using a recoil-α-α correlation method. The α-decay energy and the half-life of 217U were determined to be 8005 ± 20 keV and 15.6 −5.7 +21.3 ms, respectively.

International audience; Performed work on the modernization of the detecting system of the separator VASSILISSA. In order to improve the efficiency of registration of conversion electrons were installed new DSSD, containing 48 strips on each side. © 2010 American Institute of Physics

For experiments aimed at the study of spontaneous fission of transfermium nuclei improvements in the focal plane detector system of recoil separa tor VASSILISSA have been made. The neutron detector consisting of 54 3He ‐filled counters has been mounted around the focal plane detector chamber. The multiplicity of prompt neutrons emitted in spontaneous fission of 246 Fm was measured.

International audience; Complete fusion (CF) and incomplete fusion (ICF) reactions were studied with the beams of loosely bound 6He and 6Li bombarding 166Er and 165Ho targets. Experiments were carried out to test an approach exploiting the measured intensities of γ rays emitted at the transitions between the yrast‐band levels of reaction products formed after the termination of neutron evaporation. Partial waves feeding the CF [165Ho(6Li,5n)166Yb, 166Er(6He,6n)166Yb] and ICF [165Ho(6Li,α3n)164Er, and 166Er(6He,α4n)164Er] reaction channels were revealed from the obtained γ‐ray data. © 2009 American Institute of Physics

In 2004, the GABRIELA (Gamma Alpha Beta Recoil Investigation with the Electromagnetic Analyzer VASSILISSA) collaboration started a twofold scientific program at the FLNR (Dubna) laboratory : (a) the systematic study of the behaviour of single particle states within isotopic and isotonic chains with Z = 100–104 as N varies from 152 to 162 and (b) the study of isotopes for which very little spectroscopic information is known. After discussing the Physics motivations and the experimental setup, some new results concerning the structure of 249Fm and 253No nuclei, and some preliminary results on the decay of 217Pa decay are presented.

We report about R&D of the new separator for spectroscopic studies which we plan to install at the beam of the modernized U400R FLNR cyclotron and which will allow to realize new possibilities of the JINR—IN2P3 collaboration project GABRIELA [1, 2].

The production of 283112 in 48Ca induced nuclear reactions was investigated using physical and chemical separation techniques. In the reaction 48Ca on 238U, four events were registered at the SHIP velocity filter. The mean atomic mass of the evaporation residues (EVR)

Presently available experimental information on spontaneous fission of transfermium elements mainly concerns partial half ‐ lives. In addition, for Fm and No isotopes and for a few Md, Lr and Rf isotopes the total kinetic energy (TKE) and mass distributions of fission fragments from spontaneous fission were also accurately measured. Recoil in — flight separator VASSILISSA is widely used for the synthesis and study of decay properties of heavy and superheavy nuclei. It is planned in the future to measure the TKE and multiplicity distribution of prompt neutrons in spontaneous fission of other No. Now, the search of alternative methods of the neutrons detection was realized. During the search, with colleagues from Skobeltsyn Institute of Nuclear Physics (SINP MSU), the properties of some inorganic scintillators, was investigate.

International audience; Detailed spectroscopic information of excited nuclear states in deformed transfermium nuclei is scarce. Most of the information available today has been obtained from investigations of fine structure $\alpha$ decay. Although $\alpha$ decay gives access to hindrance factors and lifetimes which are strongly correlated to shell/subshell closures and the presence of isomers, only the combined use of $\gamma$ and conversion electron spectroscopy allows the precise determination of excitation energy, spin and parity of nuclear levels. In particular, the study of odd isotopes gives crucial information on the single particle states available at and around the Fermi surface. In the following, the preliminary results of decay studies using $\alpha - \gamma$ and $\alpha - \beta$ coincidences at the focal plane of the VASSILISSA recoil separator are presented.

International audience; A JINR — IN2P3 collaboration project named “GABRIELA” aimed at the nuclear spectroscopy of transfermium elements using the recoil separator VASSILISSA was launched in 2004 at JINR in Dubna. In the close future the FLNR cyclotron U400M will go through a major upgrade with the goal to deliver heavy ion beams at the energy close to the Coulomb barrier in a new experimental area. Here we report about R&D of the new separator for spectroscopic studies which we plan to install at the beam of the modernized accelerator and which will allow to realize new possibilities of the GABRIELA project.

With the aid of the Geant4 Monte Carlo simulation package a new detection system has been designed for the focal plane of the recoil separator VASSILISSA situated at the Flerov Laboratory of Nuclear Reactions, JINR, Dubna. GABRIELA (Gamma Alpha Beta Recoil Investigations with the Electromagnetic Analyser VASSILISSA) has been optimised to detect the arrival of reaction products and their subsequent radioactive decays involving the emission of alpha- and beta-particles, fission fragments, gamma- and X-rays, and conversion electrons. The new detector system is described and the results of the first commissioning experiments are presented., 24 pages, Submitted to NIM A