Search

The new neutron-deficient isotope $^{249}$No was synthesized for the first time in the fusion-evaporation reaction $^{204}$Pb($^{48}$Ca,3n)$^{249}$No. After separation, using the kinematic separator SHELS, the new isotope was identified with the GABRIELA detection system through genetic correlations with the known daughter and granddaughter nuclei $^{245}$Fm and $^{241}$Cf. The alpha-decay activity of $^{249}$No has an energy of 9129(22)$~$keV and half-life 38.3(2.8) ms. An upper limit of 0.2\% was measured for the fission branch of $^{249}$No. Based on the present data and recent information on the decay properties of $^{253}$Rf and aided by Geant4 simulations, the ground state of $^{249}$No is assigned the 5/2$^+$[622] neutron configuration and a partial decay scheme from $^{253}$Rf to $^{245}$Fm could be established. The production cross-section was found to be $\sigma$(3n)=0.47(4) nb at a mid-target beam energy of 225.4 MeV, which corresponds to the maximum of the calculated excitation function. Correlations of the $^{249}$No alpha activity with subsequent alpha decays of energy 7728(20) keV and half-life $1.2_{-0.4}^{+1.0}$ min provided a firm measurement of the electron-capture or $\beta^{+}$ branch of $^{245}$Fm to $^{245}$Es. The excitation function for the 1n, 2n and 3n evaporation channels was measured. In the case of the 2n-evaporation channel $^{250}$No, a strong variation of the ground state and isomeric state populations as a function of bombarding energy could be evidenced.

An experiment on the study of the $^{246}$Fm spontaneous fission was conducted using the SHELS separator. The isotope was synthesized in the complete fusion reaction of $^{40}$Ar beam ions and $^{208}$Pb target nuclei. The neutron yields of $^{246}$Fm spontaneous fission ($\overline{\nu} = 3.79\pm0.30$, $\sigma^{2}_{\nu} = 2.1$) were obtained using the SFiNx detector system. The multiplicity distribution of emitted prompt neutrons was restored using the Tikhonov method of statistical regularisation ($\overline{\nu}_{r} = 3.79\pm0.20$, $\sigma^{2}_{\nu r} = 2.8$). The spontaneous fission branching ratio ($b_{SF} = 0.061\pm0.005$) and the half-life ($T_{1/2} = 1.50^{+0.08}_{-0.07}$ s) of the isotope were determined. The experimental data were compared with scission point model predictions. Excellent convergence was observed in the average number of neutrons per spontaneous fission process. However, the forms of the experimental and model prompt neutron multiplicity distributions differ significantly.

An analysis of recent experimental data [J. Khuyagbaatar et al., Phys. Rev. C 104, L031303 (2021)] has established the existence of two fissioning states in $^{253}$Rf: the ground state and a low-lying isomeric state, most likely involving the same neutron single-particle configurations as in the lighter isotone $^{251}$No. The ratio of fission half-lives measured in $^{253}$Rf was used to predict the fission properties of the 1/2$^{+}$ isomeric state in $^{251}$No and draw conclusions as to the stability against fission of even lighter Rf systems. This paper focusses again on the fission properties of $^{253}$Rf and their impact on the stability of other neutron deficient isotopes, using new and improved data collected from two experiments performed at the Flerov Laboratory of Nuclear Reactions in Dubna, Russia. Two fission activities with half-lives of 52.8(4.4)$~\mu$s and 9.9(1.2) ms were measured in the case of $^{253}$Rf, confirming the results of J. Kkuyagbaatar et al. A third state, at much higher excitation energy, was also observed through the detection of its electromagnetic decay to the 52.8$~\mu$s state. This observation leads to the opposite quantum-configuration assignments for the fissioning states as compared to the ones established by J. Khuyagbaatar et al., namely that the higher-spin state has the shortest fission half-life. This inversion of the ratio of fission hindrances between the low and high-spin states is corroborated in the isotone $^{251}$No by the non observation of any substantial fission branch from the low-spin isomer.

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

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

$Background:$ Previous measurements of $\beta$-delayed neutron emitters comprise around 230 nuclei, spanning from the $^{8}$He up to $^{150}$La. Apart from $^{210}$Tl, with a minuscule branching ratio of 0.007\%, no other neutron emitter is measured yet beyond $A=150$. Therefore new data are needed, particularly in the heavy mass region around N=126, in order to guide theoretical models and to understand the formation of the third r-process peak at $A\sim195$. $Purpose:$ To measure both, $\beta$-decay half-lives and neutron branching ratios of several neutron-rich Au, Hg, Tl, Pb and Bi isotopes beyond $N=126$. $Method:$ Ions of interest are produced by fragmentation of a $^{238}$U beam, selected and identified via the GSI-FRS fragment separator. A stack of segmented silicon detectors (SIMBA) is used to measure ion-implants and $\beta$-decays. An array of 30 $^3$He tubes embedded in a polyethylene matrix (BELEN) is used to detect neutrons with high efficiency and selectivity. A self-triggered digital system is employed to acquire data and to enable time-correlations. The latter are analyzed with an analytical model and results for the half-lives and neutron-branching ratios are derived using the binned Maximum-Likelihood method. $Results:$ Twenty new $\beta$-decay half-lives are reported for $^{204-206}$Au, $^{208-211}$Hg,$^{211-216}$Tl,$^{215-218}$Pb and $^{218-220}$Bi, nine of them for the first time. Neutron emission probabilities are reported for $^{210,211}$Hg and $^{211-216}$Tl. $Conclusions:$ The new $\beta$-decay half-lives are in good agreement with previous measurements in this region. The measured neutron emission probabilities are comparable or smaller than values predicted by global models like RHB+RQRPA., Comment: 17 pages, 21 figures, will be submitted at PRC

The $\beta$-delayed neutron emission probabilities of neutron rich Hg and Tl nuclei have been measured together with $\beta$-decay half-lives for 20 isotopes of Au, Hg, Tl, Pb and Bi in the mass region N$\gtrsim$126. These are the heaviest species where neutron emission has been observed so far. These measurements provide key information to evaluate the performance of nuclear microscopic and phenomenological models in reproducing the high-energy part of the $\beta$-decay strength distribution. In doing so, it provides important constraints to global theoretical models currently used in $r$-process nucleosynthesis., Comment: Submitted to Phys. Rev. Lett

The rapid neutron nucleosynthesis process involves an enormous amount of very exotic neutron-rich nuclei, which represent a theoretical and experimental challenge. Two of the main decay properties that affect the final abundance distribution the most are half-lives and neutron branching ratios. Using fragmentation of a primary $^{238}$U beam at GSI we were able to measure such properties for several neutron-rich nuclei from $^{208}$Hg to $^{218}$Pb. This contribution provides a short update on the status of the data analysis of this experiment, together with a compilation of the latest results published in this mass region, both experimental and theoretical. The impact of the uncertainties connected with the beta-decay rates and with beta-delayed neutron emission is illustrated on the basis of $r$-process network calculations. In order to obtain a reasonable reproduction of the third $r$-process peak, it is expected that both half-lives and neutron branching ratios are substantially smaller, than those based on FRDM+QRPA, commonly used in $r$-process model calculations. Further measurements around $N\sim126$ are required for a reliable modelling of the underlying nuclear structure, and for performing more realistic $r$-process abundance calculations., Comment: Nuclear Physics in Astrophysics VI, Lisbon 2013, Conference Proceedings

Fusion-evaporation in the $^{124}$Sn+$^{136}$Xe system is studied using a high intensity xenon beam provided by the Ganil accelerator and the LISE3 wien filter for the selection of the products. Due to the mass symmetry of the entrance system, the rejection of the beam by the spectrometer was of the order of $5times10^8$. We have thus performed a detailed statistical analysis to estimate random events and to infer the fusion-evaporation cross sections. No signicant decay events were detected and upper limit cross sections of 172 pb, 87 pb and 235 pb were deduced for the synthesis of $^{257}$Rf, $^{258}$Rf and $^{259}$Rf, respectively., Comment: 7 pages, 4 figures

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., Comment: 24 pages, Submitted to NIM A

Production cross sections and excitation functions of neutron deficient isotopes of U, Pa and Th have been measured in the reaction of 22Ne + 208Pb. The comparison of experimental production cross sections of uranium isotopes with the neutron number 126 <= N <= 134 to the values calculated using statistical model of nuclear deexcitation has been carried out. There is shown, that none of the commonly used methods of introduction of shell effect influence on the production cross sections of evaporations residues allows to reproduce unambigiously used set of experimental data. Simple semiempirical method of introduction of shell effect influence in fission channel is proposed and good agreement with production cross sections of neutron deficient isotopes in the region from Bi to U is reached., Comment: 10 pages, 4 figures. We re-publish a contribution to Proc. of Int. Conf. DANF'96 (Casta-Papiernicka, Slovakia), Dubna 1996, p. 129 (with 2 extra figures), Part of M.V.'s PhD thesis. A fission barrier systematics is introduced which was further used in the model of fusion-quasifission competition (see nucl-ex/0302012)

The effects of the entrance channel and shell structure of reacting nuclei on the experimental evaporation residues have been studied by analysing the 40Ar+176Hf, 86Kr+130,136Xe, 124Sn+92Zr and 48Ca+174Yb reactions leading to the 216Th* and 222Th* compound nuclei. The measured excitation function of evaporation residues for the 124Sn+92Zr reaction was larger than that for the 86Kr+130Xe reaction. The experimental values of evaporation residues in the 86Kr+136Xe reaction were about 500 times larger than that in the 86Kr+130Xe reaction. These results are explained by the initial angular momentum dependence of the fusion excitation functions calculated in framework of the dinuclear system concept and by the differences in survival probabilities calculated in framework of advanced statistical model. The dependencies of the fission barrier and the \Gamma_n / \Gamma_f ratio on the angular momentum of the excited compound nucleus are taken into account., Comment: 13 pages, 9 (eps) figures, uses revtex4, submitted to Phys.Rev. C