Prompt neutrons accompanying the spontaneous fission of 250^{250}No

International audienceThis paper describes the experiment conducted on the SHELS (Separator for Heavy Element Spectroscopy) separator [1] aimed at studying the properties of spontaneous fission of 250No. A separator detection system consists of a time-of-flight system and SFiNx (Spontaneous Fission, Neutrons and x-rays) detection system [2] is described. The SFiNx detection system consists of an assembly of double-sided Si detectors, around which 116 proportional neutron counters filled with 3He are placed. During experiment, 1366 events of spontaneous fission of 250No were registered. The average number of neutrons per one fission (⁠v¯ = 4.24 ± 0.13) and the distribution of neutron multiplicities were obtained, and the half-lives of the 250No were measured for the ground T1/2 = (3.89 ± 0.17) µs and isomeric states T1/2 = (39.14 ± 3.34) µs

Prompt neutron emission in the spontaneous fission of

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 ($${\overline{\nu }} = 3.79\pm 0.30$$, $$\sigma ^{2}_{\nu } = 2.1$$) were obtained using the SFiNx detector system. The multiplicity distribution of emitted prompt neutrons was restored using the Tikhonov method of statistical regularisation ($${\overline{\nu }}_{r} = 3.79\pm 0.20$$, $$\sigma ^{2}_{\nu r} = 2.8$$). The spontaneous fission branching ratio ($$b_{SF} = 0.061\pm 0.005$$) 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. An agreement 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

Study of the production and decay properties of neutron-deficient nobelium isotopes

International audienceThe 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

Alpha-decay spectroscopy of 257^{257}Rf

International audienceThe decay properties of states in $^{257}$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 from a low-lying isomeric state in $^{257}$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. Due to the favoured nature of the $\alpha$-decay the 11/2$^-$[725] Nilsson configuration is also assigned to the first excited state of $^{257}$Rf, lying at 74 keV