Reevaluation of structures in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>Se</mml:mi><mml:mprescripts/><mml:none/><mml:mn>70</mml:mn></mml:mmultiscripts></mml:math> from combined conversion-electron and <mml:ma

In the selenium isotopes various shape phenomena are present, in particular, the emergence of a dominant oblate deformation in the most neutron-deficient isotopes has been observed. The scenario of shape coexisting oblate and prolate bands has been proposed across the isotopic chain, with the crossing point of such bands being located near <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mmultiscripts><a:mi>Se</a:mi><a:mprescripts/><a:none/><a:mn>70</a:mn></a:mmultiscripts></a:math>, where no coexistence has yet been identified. To determine the presence or absence of any low-lying <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msup><b:mn>0</b:mn><b:mo>+</b:mo></b:msup></b:math> state in <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mmultiscripts><c:mi>Se</c:mi><c:mprescripts/><c:none/><c:mn>70</c:mn></c:mmultiscripts></c:math>, confirm the level structure, and interpret the nuclear deformation with theoretical models. A combined internal-conversion-electron and <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mi>γ</d:mi></d:math>-ray spectroscopy study was undertaken with the SPICE and TIGRESS spectrometers at the TRIUMF-ISAC-II facility. Nuclear models were provided by the generalized triaxial rotor model (GTRM) and the collective generalised Bohr Hamiltonian (GBH). Despite a comprehensive search, no evidence was found for the existence of a <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:msup><e:mn>0</e:mn><e:mo>+</e:mo></e:msup></e:math> state below 2 MeV in <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mmultiscripts><f:mi>Se</f:mi><f:mprescripts/><f:none/><f:mn>70</f:mn></f:mmultiscripts></f:math>. Significant discrepancies to the previously established positive-parity-level scheme were found. GBH calculations using UNEDF1 mass parameters were found to reproduce the revised low-lying level structure well. <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mmultiscripts><g:mi>Se</g:mi><g:mprescripts/><g:none/><g:mn>70</g:mn></g:mmultiscripts></g:math> does not have a well-defined axial shape. The <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:msubsup><h:mn>2</h:mn><h:mn>2</h:mn><h:mo>+</h:mo></h:msubsup></h:math> state at 1601 keV resembles a quasi-<i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mi>γ</i:mi></i:math> excitation rather than a member of a shape coexisting band; the presence of such a band is all but ruled out. Published by the American Physical Society 2024