Search

The ${0}^{+}$ ground state of the $^{10}\mathrm{He}$ nucleus produced in the $^{3}\mathrm{H}(^{8}\mathrm{He},p)^{10}\mathrm{He}$ reaction was found at about $2.1\ifmmode\pm\else\textpm\fi{}0.2\text{ }\text{ }\mathrm{MeV}$ ($\ensuremath{\Gamma}\ensuremath{\sim}2\text{ }\text{ }\mathrm{MeV}$) above the three-body $^{8}\mathrm{He}+n+n$ breakup threshold. Angular correlations observed for $^{10}\mathrm{He}$ decay products show prominent interference patterns allowing us to draw conclusions about the structure of low-energy excited states. We interpret the observed correlations as a coherent superposition of a broad ${1}^{\ensuremath{-}}$ state having a maximum at energy 4--6 MeV and a ${2}^{+}$ state above 6 MeV, setting both on top of the ${0}^{+}$ state ``tail.'' This anomalous level ordering indicates that the breakdown of the $N=8$ shell known in $^{12}\mathrm{Be}$ thus extends also to the $^{10}\mathrm{He}$ system.

An isomeric state in {sup 255}Lr with a half-life of t{sub 1/2} = 1.4(1) ms and E{sub x}>720-keV has been observed for the first time using the GABRIELA setup at the focal plane of the VASSILISSA separator. Based on its K-forbiddeness, the configuration of the state is most probably formed by coupling the valence proton to a two quasiparticle neutron excitation. Possible three quasiparticle configurations are discussed.

An isomeric state in {sup 209}Ra has been observed for the first time, using the GABRIELA setup at the focal plane of VASSILISSA, to decay to the ground state of {sup 209}Ra via a cascade of 238-keV (M2) and 644-keV transitions. The half-life of the isomer has been measured to be 117(5) {mu}s and from systematics is assigned as a neutron i{sub 13/2}{sup -1} excitation.

The spectrum of $^{9}\mathrm{He}$ was studied by means of the $^{2}\mathrm{H}$($^{8}\mathrm{He}$,$p$)$^{9}\mathrm{He}$ reaction at a laboratory energy of 25 MeV/nucleon and small center-of-mass angles. Energy and angular correlations were obtained for the $^{9}\mathrm{He}$ decay products by complete kinematical reconstruction. The lowest resonant state of $^{9}\mathrm{He}$ is found at 2.0 \ifmmode\pm\else\textpm\fi{} 0.2 MeV with a width of $~2$ MeV and is identified as $1/{2}^{\ensuremath{-}}$. The observed angular correlation pattern is uniquely explained by the interference of the $1/{2}^{\ensuremath{-}}$ resonance with a virtual $1/{2}^{+}$ state (a limit $ag\ensuremath{-}20$ fm is obtained for the scattering length), and with a $5/{2}^{+}$ resonance at energy $\ensuremath{\geqslant}4.2$ MeV.

Excited states in {sup 249}Fm were populated via the {alpha} decay of {sup 253}No and the subsequent decay was observed with the GABRIELA detection system installed at the focal plane of the VASSILISSA recoil separator. The energies, spins, and parities of these states could be established through combined {alpha},{gamma}, and conversion-electron spectroscopy. The first members of the ground-state rotational band were identified. Their excitation energies as well as the observation of a cross-over E2 transition confirm the assignment of 7/2{sup +}624 for the ground state of {sup 249}Fm. Two excited states were also observed and their decay properties suggest that they correspond to the particle excitation 9/2{sup -}734 and hole excitation 5/2{sup +}622. The analysis suggests that the 279-keV transition de-exciting the 9/2{sup -} state has anomalous E1 conversion coefficients.

An experimental search for 5H using a secondary beam of 6He was performed. The transfer reaction p(6He,2He)5H was studied by detecting two protons emitted from the decay of 2He. A peak consistent with the 5H resonance at 1.7±0.3 MeV above the n + n + t threshold was observed, with a width of 1.9±0.4 MeV. The angular distribution of the p(6He,2He)5H reaction was measured as well as the energy correlation of two protons.