arXiv subjects

B. Gall

Publications and source records attributed to B. Gall.

12 recordsLinked to original sources

arXiv2026

The electromagnetic decay of $^{250m}$No and the stability of neutron deficient Rf isotopes

The electromagnetic decay of the $\approx$40 $\mu$s isomer of $^{250}$No has been investigated using the \textsc{Geant4} toolkit for the simulations of the interaction of particles through matter. It is concluded that the decay does not follow the pattern established in the lighter isotones, where the isomer decays directly to members of the ground state rotational band. An alternative scenario is proposed. The implications on the location of the isotopic border for neutron deficient Rf isotopes are discussed.

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arXiv2025

Structure studies of 257Db through combined {\alpha}, {\gamma} and internal-conversion-electron spectroscopy

This work reports on the study of the decay properties along the $^{257}$Db decay chain using the GABRIELA setup. The first observation of a high-K isomer in $^{257}$Db is presented. In addition, an unreported $\alpha$-decay branch in $^{249}$Md has been evidenced, allowing to constrain the differences in energy of the $\alpha$-decaying levels in $^{249}$Md, $^{253}$Lr and $^{257}$Db. Finally, the combination of the observed fine structure $\alpha$-decay from the high-spin state in $^{257}$Db with the first observation the internal decay in $^{253}$Lr requires a revision of level and decay scheme. In particular, a change of parity for the high-spin state from 9/2$^{+}$ to 9/2$^{-}$ in the $^{257}$Db is suggested, and the implications of such a change are also discussed.

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arXiv2022

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

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.

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arXiv2022

Spontaneous fission of 246Fm

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.

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arXiv2022

Fission properties of $^{253}$Rf and the stability of neutron-deficient Rf isotopes

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.

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arXiv2021

First observation of high-$K$ isomeric states in $^{249}$Md and $^{251}$Md

Decay spectroscopy of the odd-proton nuclei $^{249}$Md and $^{251}$Md has been performed. High-$K$ isomeric states were identified for the first time in these two nuclei through the measurement of their electromagnetic decay. An isomeric state with a half-life of $2.8(5)$ ms and an excitation energy $\geq 910$ keV was found in $^{249}$Md. In $^{251}$Md, an isomeric state with a half-life of $1.4(3)$ s and an excitation energy $\geq 844$ keV was found. Similarly to the neighbouring $^{255}$Lr, these two isomeric states are interpreted as 3 quasi-particle high-$K$ states and compared to new theoretical calculations. Excited nuclear configurations were calculated within two scenarios: via blocking nuclear states located in proximity to the Fermi surface or/and using the quasiparticle Bardeen-Cooper-Schrieffer method. Relevant states were selected on the basis of the microscopic-macroscopic model with a deformed Woods-Saxon potential. The most probable candidates for the configurations of $K$-isomeric states in Md nuclei are proposed.

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arXiv2021

Alpha-decay spectroscopy of $^{257}$Rf

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.

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arXiv2020

In-beam gamma-ray and electron spectroscopy of $^{249,251}$Md

The odd-$Z$ $^{251}$Md nucleus was studied using combined $\gamma$-ray and conversion-electron in-beam spectroscopy. Besides the previously observed rotational band based on the $[521]1/2^-$ configuration, another rotational structure has been identified using $\gamma$-$\gamma$ coincidences. The use of electron spectroscopy allowed the rotational bands to be observed over a larger rotational frequency range. Using the transition intensities that depend on the gyromagnetic factor, a $[514]7/2^-$ single-particle configuration has been inferred for this band, i.e., the ground-state band. A physical background that dominates the electron spectrum with an intensity of $\simeq$ 60% was well reproduced by simulating a set of unresolved excited bands. Moreover, a detailed analysis of the intensity profile as a function of the angular momentum provided a method for deriving the orbital gyromagnetic factor, namely $g_K = 0.69^{+0.19}_{-0.16}$ for the ground-state band. The odd-$Z$ $^{249}$Md was studied using $\gamma$-ray in-beam spectroscopy. Evidence for octupole correlations resulting from the mixing of the $\Delta l = \Delta j = 3$ $[521]3/2^-$ and $[633]7/2^+$ Nilsson orbitals were found in both $^{249,251}$Md. A surprising similarity of the $^{251}$Md ground-state band transition energies with those of the excited band of $^{255}$Lr has been discussed in terms of identical bands. Skyrme-Hartree-Fock-Bogoliubov calculations were performed to investigate the origin of the similarities between these bands.

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arXiv2018

Production cross section and decay study of $^{243}$Es and $^{249}$Md

In the study of the odd-$Z$, even-$N$ nuclei $^{243}$Es and $^{249}$Md, performed at the University of Jyv\"askyl\"a, the fusion-evaporation reactions $^{197}$Au($^{48}$Ca,2$n$)$^{243}$Es and $^{203}$Tl($^{48}$Ca,2$n$)$^{249}$Md have been used for the first time. Fusion-evaporation residues were selected and detected using the RITU gas-filled separator coupled with the focal-plane spectrometer GREAT. For $^{243}$Es, the recoil decay correlation analysis yielded a half-life of $24 \pm 3$s, and a maximum production cross section of $37 \pm 10$ nb. In the same way, a half-life of $26 \pm 1$ s, an $\alpha$ branching ratio of 75 $\pm$ 5%, and a maximum production cross section of 300 $\pm$ 80 nb were determined for $^{249}$Md. The decay properties of $^{245}$Es, the daughter of $^{249}$Md, were also measured: an $\alpha$ branching ratio of 54 $\pm$ 7% and a half-life of 65 $\pm$ 6 s. Experimental cross sections were compared to the results of calculations performed using the KEWPIE2 statistical fusion-evaporation code.

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arXiv2008

Search for Fingerprints of Tetrahedral Symmetry in $^{156}Gd$

Theoretical predictions suggest the presence of tetrahedral symmetry as an explanation for the vanishing intra-band E2-transitions at the bottom of the odd-spin negative parity band in $^{156}Gd$. The present study reports on experiment performed to address this phenomenon. It allowed to determine the intra-band E2 transitions and branching ratios B(E2)/B(E1) of two of the negative-parity bands in $^{156}Gd$.

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arXiv1993

Superdeformed rotational bands in the Mercury region; A Cranked Skyrme-Hartree-Fock-Bogoliubov study

A study of rotational properties of the ground superdeformed bands in \Hg{0}, \Hg{2}, \Hg{4}, and \Pb{4} is presented. We use the cranked Hartree-Fock-Bogoliubov method with the {\skm} parametrization of the Skyrme force in the particle-hole channel and a seniority interaction in the pairing channel. An approximate particle number projection is performed by means of the Lipkin-Nogami prescription. We analyze the proton and neutron quasiparticle routhians in connection with the present information on about thirty presently observed superdeformed bands in nuclei close neighbours of \Hg{2}.

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arXiv1993

Microscopic approach to collective motion

An overview of a microscopic framework based on the Hartree-Fock description of the mean field is presented which, starting from an effective interaction allows a description of collective motions. A study of the isotope shifts in the Pb region illustrates the importance of the effective interactions and points to their limitations. Such forces should be improved in order to achieve a better description of nuclei properties especially with the coming availability of exotic beam facilities. The coupling of collective quadrupole and octupole degrees of freedom in $^{194}$Pb is analyzed within the Generator Coordinate Method, which represents a step beyond the simple mean-field approximation. As a last example, we present a study of nuclear rotations. First we discuss results on superdeformed rotating bands in $^{192}$Hg, $^{194}$Hg and $^{194}$Pb obtained without including a treatment of pairing correlations. Preliminary calculations are also presented with these correlations included as well as an approximate projection on nucleon number. Email contact: paul@amoco.saclay.cea.fr

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