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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.

Isomeric states in $^{256}\mathrm{No}$ were investigated using internal conversion electron and $\ensuremath{\gamma}$-ray spectroscopy with the GABRIELA detection system at the focal plane of the SHELS recoil separator, at the Flerov Laboratory for Nuclear Research (FLNR, JINR, Dubna). The nuclei of interest were produced using the highly asymmetric fusion-evaporation reaction $^{238}\mathrm{U}(^{22}\mathrm{Ne},4n)^{256}\mathrm{No}$. The emission of internal conversion electrons and $\ensuremath{\gamma}$ rays occurring between a $^{256}\mathrm{No}$ implantation and a subsequent $\ensuremath{\alpha}$-decay event were studied, resulting in the observation of high-$K$ isomerism in this nobelium isotope. The nature of the isomeric states is discussed in terms of possible two- and four-quasiparticle structures.

The experiments of detailed study of No and Rf isotopes radioactive decay properties in complete fusion reactions 50 Ti+ 208 Pb and 48 Ca+ 208,206,204 Pb with subsequent neutron evaporation from the excited compound nucleus at the kinematic separator SHELS were performed in FLNR JINR. The data of the 256 Rf decay properties and preliminary data of 250 No decay properties are presented.

The decay properties of the ground state and excited states of $^{257}$Rf have been investigated with the detector array GABRIELA at the FLNR, Dubna. The electromagnetic decay of a new excited state in $^{253}$No has been observed. The state lies 750 keV above the ground state and is favourably populated in the alpha decay of the low-lying spin isomer of $^{257}$Rf. It decays to the 9/2$^-$ ground state by an M1 transition and is assigned the 11/2$^-$[725] Nilsson configuration. The presence of this state suggests a possible reinterpretation of the decay of the high-K isomer in $^{253}$No., Comment: 7 pages, 4 figures

International audience; In the reaction of complete fusion of $^{48}$Ca-ions with a $^{204}$Pb target, a new α line, related to the decay of a new, previously unknown isotope ($^{249}$No), has been detected in an experiment carried out at the SHELS separator. The α decay energy (E$_{α}$ = 9129 keV) and the half-life (T$_{1/2}$ = 38.1 ± 2.5 ms) are measured using the GABRIELA detector array. The SF branching ratio was estimated (b$_{SF}$ ≤ 0.2%).

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.

Evaporation residue cross sections for the $^{16}\mathrm{O}+^{203,205}\mathrm{Tl}$ reactions were measured at laboratory beam energies in the range of 82--113 MeV using a gas-filled separator. Transmission efficiency of the separator was estimated using a calibration reaction $^{16}\mathrm{O}+^{197}\mathrm{Au}$ and by simulating the evaporation residues angular distributions. Statistical model calculations were performed for both the measured systems. These calculations overestimate the experimental evaporation residue cross sections. This could be attributed to the presence of noncompound nuclear fission. An estimation of noncompound nuclear fission contribution was carried out. Comparison with neighboring systems shows that a slight change in the entrance channel or the compound nucleus properties makes a large difference in evaporation residue cross sections.

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.

The analysis of fission events following the implantation of evaporation residues produced in the fusion reaction of 50Ti and 209Bi at different bombarding energies has revealed 5 millisecond decays, which are attributed to the spontaneous fission of proton-evaporation channels. The average cross sections for proton evaporation are found to be ∼100 and 10 times smaller than the largest neutron-evaporation channel cross section at the same excitation energy. These results suggest that the proton evaporation channel, albeit weak, may represent a realistic alternative to synthesize new, more neutron rich super heavy nuclei. Keywords: Fusion-evaporation reaction, Proton evaporation, Super heavy nuclei, Cross section

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.

International audience; In the last years we carried out several experiments aimed to investigate properties of short-lived SF isotopes. The neutron-deficient isotopes of nobelium were produced in fusion-evaporation reactions using 206,208Pb targets and an intense 48Ca-beam. Fusionevaporation residues were separated by the SHELS separator and implanted into a largearea double-sided 48×48 strip silicon detector surrounded by 3He-based neutron counters. Half-lives and decay branching ratios for 252,254No isotopes were measured. The average number of neutrons per spontaneous fission of 254No determined for the first time.

The GABRIELA [1] set-up is used at the FLNR to perform detailed nuclear structure studies of transfermium nuclei. Following the modernization of the VASSILISSA separator (SHELS) [2] the GABRIELA detection system has also been upgraded. The characteristics of the upgraded detection system will be presented along with results from some recent electronics tests., Comment: 6 pages, 3 figure, 1 table uncorrected pre-print of EXON 2018 conference proceedings

The structure of low-lying excited states in $^{251}\mathrm{Fm}$, populated by the $\ensuremath{\alpha}$ decay of $^{255}\mathrm{No}$, has been investigated by means of combined $\ensuremath{\gamma}$ and internal conversion electron spectroscopy. The values for the internal conversion coefficients for the $1/{2}^{+}\ensuremath{\rightarrow}5/{2}^{+}$ and $5/{2}^{+}\ensuremath{\rightarrow}9/{2}^{\ensuremath{-}}$ transitions have been measured. The determined $M2/E3$ mixing ratio and lifetime for the $5/{2}^{+}$ decay to the ground state allowed to determine the corresponding reduced transitions strengths of $B(E3)=18(6)$ W.u. and $B(M2)=3.0(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ W.u. These results, as well as the results of previous studies in $N=151$ isotopes, are compared to theoretical calculations beyond the mean-field approach, including the first QRPA calculations using the Gogny D1M parametrization for such heavy odd-$N$ nuclei. The comparison points to the importance of accounting for the octupole vibrations for a proper understanding of the low-lying nuclear structure of some of the heaviest elements.

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.

Previously reported preparation method for Pd-based intermetallic targets (Usoltsev, et al., 2012) [1] has been successfully applied for producing two stationary 243Am/Pd targets. Both targets have been irradiated at the U-400 cyclotron at Flerov Laboratory of Nuclear Reactions Dubna (Russian Federation) using high intensity beams (up to 0.83 μApart) of 48Ca18+. Alpha-particle spectroscopy and light microscopy allowed for a comprehensive characterization of the intermetallic targets before and after irradiation. A natNd/Pd intermetallic target and a solely electroplated 243Am/Ti target were similarly investigated for comparison.

The adsorption behavior of 197Hg and 183–185Hg on red amorphous selenium (red a-Se) and trigonal selenium (t-Se) was investigated experimentally by off-line and on-line gas chromatographic methods, in preparation of a sensitive chemical separation and characterization of the transactinides copernicium (Cn, Z = 112) and flerovium (Fl, Z = 114). Monte-Carlo simulations of a diffusion controlled deposition were in good agreement with the experimental results, assuming as interaction limits −ΔH ads red a-Se (Hg) > 85 kJ/mol, and −ΔH ads t-Se (Hg)

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 setup for studying the physicochemical properties of superheavy elements in experiments involving gas transport systems is described. The setup is composed of four detecting modules with semiconductor detectors; systems of data acquisition, storage, and processing; a cooling system for semiconductor detectors; and a vacuum system. In each detecting module, there are two oppositely located four-strip semiconductor detectors. The detecting system is capable of detecting with a high efficiency α particles and spontaneous fission fragments produced in decays of superheavy elements.

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) .

6 pages; International audience; Isomeric states in 253No have been investigated by conversion-electron and gamma-ray spectroscopy with the GABRIELA detection system. The 31 micro second isomer reported more than 30 years ago is found to decay to the ground state of 253No by the emission of a 167 keV M2 transition. The spin and parity of this low-lying isomeric state are established to be 5/2+. The presence of another longer-lived isomeric state is also discussed.

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.

Element 112 was discovered at the Heavy Ion Research Laboratory in Darmstadt, Germany in 1996. A decade on, and some of its chemical properties have now been determined. Irradiation of plutonium-242 with intense calcium-48 beams for three weeks produced two atoms of element 112 (not yet officially named, but commonly called ununbium), and that's enough to do some chemistry on if you are quick. Chemically ununbium behaves as a typical element of the group 12 in the periodic table (which it shares with Zn, Cd and Hg). It is very volatile and forms a metallic bond with a gold surface. An experiment has scrutinized two atoms of element 112, finding that it is very volatile and forms a metallic bond with a gold surface. These characteristics establish element 112 as a typical element of group 12. The heaviest elements to have been chemically characterized are seaborgium1 (element 106), bohrium2 (element 107) and hassium3 (element 108). All three behave according to their respective positions in groups 6, 7 and 8 of the periodic table, which arranges elements according to their outermost electrons and hence their chemical properties. However, the chemical characterization results are not trivial: relativistic effects on the electronic structure of the heaviest elements can strongly influence chemical properties4,5,6. The next heavy element targeted for chemical characterization is element 112; its closed-shell electronic structure with a filled outer s orbital suggests that it may be particularly susceptible to strong deviations from the chemical property trends expected within group 12. Indeed, first experiments concluded that element 112 does not behave like its lighter homologue mercury7,8,9. However, the production and identification methods10,11 used cast doubt on the validity of this result. Here we report a more reliable chemical characterization of element 112, involving the production of two atoms of 283112 through the alpha decay of the short-lived 287114 (which itself forms in the nuclear fusion reaction12 of 48Ca with 242Pu) and the adsorption of the two atoms on a gold surface. By directly comparing the adsorption characteristics of 283112 to that of mercury and the noble gas radon, we find that element 112 is very volatile and, unlike radon, reveals a metallic interaction with the gold surface. These adsorption characteristics establish element 112 as a typical element of group 12, and its successful production unambiguously establishes the approach to the island of stability of superheavy elements through 48Ca-induced nuclear fusion reactions with actinides.

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.

Summary We present results of the second experiment on the chemical identification of element 112. Similar to the first test in 2000, we aimed at the production of the spontaneously fissioning 283112 nuclei with T 1/2≈3min. A natU3O8 (with some Nd) target, 2mg of U/cm2 thick, was bombarded with 233-MeV 48Ca ions (the energy in the middle of the target layer). The nuclei recoiling from the target were thermalized in flowing helium. The target chamber was connected through a 25m long capillary to detectors of α-particles and fission fragments. All the equipment and detectors were kept at ambient temperature. According to the test experiments, of all the heavy elements produced in the bombardment, only Hg, Rn and At could be transported to the detectors. The first detecting device was similar to that used earlier – an assembly of 8 pairs of PIPS detectors coated with gold. Here one would observe the decay of element 112 atoms if they like Hg adsorbed on gold. The atoms which were not retained and freely passed through the PIPS detectors entered a new, flow-through ionization chamber, 5000 cm3 in volume, optimized for detecting fission fragments. The PIPS detectors and the ionization chamber were placed inside a large assembly of 3He – filled neutron counters to detect prompt neutrons from the fission events. In 22.5 days, a beam dose of 2.8×1018 ions was accumulated. More than 95 of the simultaneously produced α-active 185Hg (T 1/2=49 s) were found deposited already on the first pair of PIPS detectors; meanwhile, all the PIPSs did not detect any fission event. In the ionization chamber, eight fission events were observed in coincidence with neutron counts while the expected background was insignificant. Hence, the spontaneous fissions of the volatile activity can be conclusively attributed to the decay of element 112 produced in the fusion reaction 48Ca+ 238U, and formerly observed in Dubna physical experiments. Evaluation of the experimental data in terms of the adsorption enthalpies indicates much weaker interaction of element 112 with Au than that of Hg. One can conclude that in the given chemical environment, element 112 behaves like Rn rather than like Hg. The formation cross section of 283112 estimated from the data amounts to several pb. The experiments were carried out at the Flerov Laboratory of Nuclear Reactions at JINR in November–December 2001.

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.