Reduction of spectroscopic overlap across the Z=8 shell in neutron-rich nuclei

The recent discovery and spectroscopic measurements of <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mmultiscripts><a:mi mathvariant="normal">O</a:mi><a:mprescripts/><a:none/><a:mn>27</a:mn></a:mmultiscripts></a:math> and <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mmultiscripts><c:mi mathvariant="normal">O</c:mi><c:mprescripts/><c:none/><c:mn>28</c:mn></c:mmultiscripts></c:math> suggests the disappearance of the <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mi>N</e:mi><e:mo>=</e:mo><e:mn>20</e:mn></e:mrow></e:math> shell structure in these neutron-rich oxygen isotopes. We measured one- and two-proton removal cross sections from <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mmultiscripts><f:mi mathvariant="normal">F</f:mi><f:mprescripts/><f:none/><f:mn>27</f:mn></f:mmultiscripts></f:math> and <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mmultiscripts><h:mi>Ne</h:mi><h:mprescripts/><h:none/><h:mn>29</h:mn></h:mmultiscripts></h:math>, respectively, extracting spectroscopic factors and comparing them to shell model overlap functions coupled with eikonal reaction model calculations. The invariant mass technique was used to reconstruct the two-body (<i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mrow><i:mmultiscripts><i:mi mathvariant="normal">O</i:mi><i:mprescripts/><i:none/><i:mn>24</i:mn></i:mmultiscripts><i:mo>+</i:mo><i:mi>n</i:mi></i:mrow></i:math>) and three-body (<k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:mrow><k:mmultiscripts><k:mi mathvariant="normal">O</k:mi><k:mprescripts/><k:none/><k:mn>24</k:mn></k:mmultiscripts><k:mo>+</k:mo><k:mn>2</k:mn><k:mi>n</k:mi></k:mrow></k:math>) decay energies from knockout reactions of <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mmultiscripts><m:mi mathvariant="normal">F</m:mi><m:mprescripts/><m:none/><m:mn>27</m:mn></m:mmultiscripts></m:math> (106.2 MeV/u) and <o:math xmlns:o="http://www.w3.org/1998/Math/MathML"><o:mmultiscripts><o:mi>Ne</o:mi><o:mprescripts/><o:none/><o:mn>29</o:mn></o:mmultiscripts></o:math> (112.8 MeV/u) beams impinging on a <p:math xmlns:p="http://www.w3.org/1998/Math/MathML"><p:mmultiscripts><p:mi>Be</p:mi><p:mprescripts/><p:none/><p:mn>9</p:mn></p:mmultiscripts></p:math> target. The one-proton removal from <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"><q:mmultiscripts><q:mi mathvariant="normal">F</q:mi><q:mprescripts/><q:none/><q:mn>27</q:mn></q:mmultiscripts></q:math> strongly populated the ground state of <s:math xmlns:s="http://www.w3.org/1998/Math/MathML"><s:mmultiscripts><s:mi mathvariant="normal">O</s:mi><s:mprescripts/><s:none/><s:mn>26</s:mn></s:mmultiscripts></s:math> and the extracted cross section of <u:math xmlns:u="http://www.w3.org/1998/Math/MathML"><u:mrow><u:mn>3</u:mn><u:mo>.</u:mo><u:msubsup><u:mn>4</u:mn><u:mrow><u:mo>−</u:mo><u:mn>1.5</u:mn></u:mrow><u:mrow><u:mo>+</u:mo><u:mn>0.3</u:mn></u:mrow></u:msubsup></u:mrow></u:math> mb agrees with eikonal model calculations that are normalized by the shell model spectroscopic factors and account for the systematic reduction factor observed for single nucleon removal reactions within the models used. For the two-proton removal reaction from <v:math xmlns:v="http://www.w3.org/1998/Math/MathML"><v:mmultiscripts><v:mi>Ne</v:mi><v:mprescripts/><v:none/><v:mn>29</v:mn></v:mmultiscripts></v:math> an upper limit of 0.08 mb was extracted for populating states in <w:math xmlns:w="http://www.w3.org/1998/Math/MathML"><w:mmultiscripts><w:mi mathvariant="normal">O</w:mi><w:mprescripts/><w:none/><w:mn>27</w:mn></w:mmultiscripts></w:math> decaying though the ground state of <y:math xmlns:y="http://www.w3.org/1998/Math/MathML"><y:mmultiscripts><y:mi mathvariant="normal">O</y:mi><y:mprescripts/><y:none/><y:mn>26</y:mn></y:mmultiscripts></y:math>. The measured upper limit for the population of the ground state of <ab:math xmlns:ab="http://www.w3.org/1998/Math/MathML"><ab:mmultiscripts><ab:mi mathvariant="normal">O</ab:mi><ab:mprescripts/><ab:none/><ab:mn>26</ab:mn></ab:mmultiscripts></ab:math> in the two-proton removal reaction from <cb:math xmlns:cb="http://www.w3.org/1998/Math/MathML"><cb:mmultiscripts><cb:mi>Ne</cb:mi><cb:mprescripts/><cb:none/><cb:mn>29</cb:mn></cb:mmultiscripts></cb:math> indicates a significant difference in the underlying nuclear structure of <db:math xmlns:db="http://www.w3.org/1998/Math/MathML"><db:mmultiscripts><db:mi mathvariant="normal">F</db:mi><db:mprescripts/><db:none/><db:mn>27</db:mn></db:mmultiscripts></db:math> and <fb:math xmlns:fb="http://www.w3.org/1998/Math/MathML"><fb:mmultiscripts><fb:mi>Ne</fb:mi><fb:mprescripts/><fb:none/><fb:mn>29</fb:mn></fb:mmultiscripts></fb:math>. Published by the American Physical Society 2024