Disentangling the influence of excitation energy and compound nucleus angular momentum on fission fragment angular momentum

The origin of the large angular momenta observed for fission fragments is still a question under discussion. To address this, we study isomeric yield ratios (IYRs), i.e., the relative population of two or more long-lived metastable states with different spins, of fission products. We report on IYRs of 17 isotopes produced in the 28-MeV <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi>α</a:mi></a:math>-induced fission of <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mmultiscripts><b:mi>Th</b:mi><b:mprescripts/><b:none/><b:mn>232</b:mn></b:mmultiscripts></b:math> at the IGISOL facility of the University of Jyväskylä. The fissioning nuclei in this reaction are <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mmultiscripts><c:mi mathvariant="normal">U</c:mi><c:none/><c:mo>*</c:mo><c:mprescripts/><c:none/><c:mrow><c:mn>233</c:mn><c:mo>,</c:mo><c:mn>234</c:mn><c:mo>,</c:mo><c:mn>235</c:mn></c:mrow></c:mmultiscripts></c:math>. We compare our data to IYRs from thermal neutron-induced fission of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mmultiscripts><e:mi mathvariant="normal">U</e:mi><e:mprescripts/><e:none/><e:mn>233</e:mn></e:mmultiscripts></e:math> and <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mmultiscripts><g:mi mathvariant="normal">U</g:mi><g:mprescripts/><g:none/><g:mn>235</g:mn></g:mmultiscripts></g:math>, and we observe statistically significant larger IYRs in the <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"><i:mmultiscripts><i:mi>Th</i:mi><i:mprescripts/><i:none/><i:mn>232</i:mn></i:mmultiscripts></i:math>(<j:math xmlns:j="http://www.w3.org/1998/Math/MathML"><j:mi>α</j:mi></j:math>, <k:math xmlns:k="http://www.w3.org/1998/Math/MathML"><k:mrow><k:mi>f</k:mi></k:mrow></k:math>) reaction, where the average compound nucleus (CN) spin is 7.7 <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"><l:mi>ℏ</l:mi></l:math>, than in <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"><m:mmultiscripts><m:mi mathvariant="normal">U</m:mi><m:mprescripts/><m:none/><m:mrow><m:mn>233</m:mn><m:mo>,</m:mo><m:mn>235</m:mn></m:mrow></m:mmultiscripts></m:math>(<o:math xmlns:o="http://www.w3.org/1998/Math/MathML"><o:msub><o:mi>n</o:mi><o:mi>th</o:mi></o:msub></o:math>, <p:math xmlns:p="http://www.w3.org/1998/Math/MathML"><p:mrow><p:mi>f</p:mi></p:mrow></p:math>), with average spins of 2.6 and 3.6 <q:math xmlns:q="http://www.w3.org/1998/Math/MathML"><q:mi>ℏ</q:mi></q:math>, respectively. To assess the influence of the excitation energy, we study literature data of IYRs from photon-induced fission reactions, and find that, within current uncertainties, the IYRs indicate no dependency of the CN excitation energy. We conclude that the different IYRs seem to be due to the different CN spins alone. This would imply that the fission fragment angular momentum only partly comes from the fission process itself and is, in addition, influenced by the angular momentum present in the CN.