Shedding Light on the Origin of Pb204 , the Heaviest s -Process–Only Isotope in the Solar System

Asymptotic giant branch stars are responsible for the production of most of the heavy isotopes beyond Sr observed in the solar system. Among them, isotopes shielded from the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mi>r</a:mi></a:mrow></a:math>-process contribution by their stable isobars are defined as <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mrow><c:mi>s</c:mi></c:mrow></c:math>-only nuclei. For a long time the abundance of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:mmultiscripts><e:mrow><e:mi>Pb</e:mi></e:mrow><e:mprescripts/><e:none/><e:mrow><e:mn>204</e:mn></e:mrow></e:mmultiscripts></e:mrow></e:math>, the heaviest <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mrow><g:mi>s</g:mi></g:mrow></g:math>-only isotope, has been a topic of debate because state-of-the-art stellar models appeared to systematically underestimate its solar abundance. Besides the impact of uncertainties from stellar models and galactic chemical evolution simulations, this discrepancy was further obscured by rather divergent theoretical estimates for the neutron capture cross section of its radioactive precursor in the neutron-capture flow, <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mrow><i:mmultiscripts><i:mrow><i:mi>Tl</i:mi></i:mrow><i:mprescripts/><i:none/><i:mrow><i:mn>204</i:mn></i:mrow></i:mmultiscripts></i:mrow></i:math> (<k:math xmlns:k="http://www.w3.org/1998/Math/MathML" display="inline"><k:mrow><k:msub><k:mrow><k:mi>t</k:mi></k:mrow><k:mrow><k:mn>1</k:mn><k:mo>/</k:mo><k:mn>2</k:mn></k:mrow></k:msub><k:mo>=</k:mo><k:mn>3.78</k:mn><k:mtext> </k:mtext><k:mtext> </k:mtext><k:mi>yr</k:mi></k:mrow></k:math>), and by the lack of experimental data on this reaction. We present the first ever neutron capture measurement on <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline"><m:mrow><m:mmultiscripts><m:mrow><m:mi>Tl</m:mi></m:mrow><m:mprescripts/><m:none/><m:mrow><m:mn>204</m:mn></m:mrow></m:mmultiscripts></m:mrow></m:math>, conducted at the CERN neutron time-of-flight facility n_TOF, employing a sample of only 9 mg of <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mrow><o:mmultiscripts><o:mrow><o:mi>Tl</o:mi></o:mrow><o:mprescripts/><o:none/><o:mrow><o:mn>204</o:mn></o:mrow></o:mmultiscripts></o:mrow></o:math> produced at the Institute Laue Langevin high flux reactor. By complementing our new results with semiempirical calculations we obtained, at the <q:math xmlns:q="http://www.w3.org/1998/Math/MathML" display="inline"><q:mrow><q:mi>s</q:mi></q:mrow></q:math>-process temperatures of <s:math xmlns:s="http://www.w3.org/1998/Math/MathML" display="inline"><s:mi>k</s:mi><s:mi>T</s:mi><s:mo>≈</s:mo><s:mn>8</s:mn><s:mtext> </s:mtext><s:mtext> </s:mtext><s:mi>keV</s:mi></s:math> and <u:math xmlns:u="http://www.w3.org/1998/Math/MathML" display="inline"><u:mi>k</u:mi><u:mi>T</u:mi><u:mo>≈</u:mo><u:mn>30</u:mn><u:mtext> </u:mtext><u:mtext> </u:mtext><u:mi>keV</u:mi></u:math>, Maxwellian-averaged cross sections (MACS) of 580(168) mb and 260(90) mb, respectively. These figures are about 3% lower and 20% higher than the corresponding values widely used in astrophysical calculations, which were based only on theoretical calculations. By using the new <w:math xmlns:w="http://www.w3.org/1998/Math/MathML" display="inline"><w:mrow><w:mmultiscripts><w:mrow><w:mi>Tl</w:mi></w:mrow><w:mprescripts/><w:none/><w:mrow><w:mn>204</w:mn></w:mrow></w:mmultiscripts></w:mrow></w:math> MACS, the uncertainty arising from the <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"><y:mrow><y:mrow><y:mmultiscripts><y:mrow><y:mi>Tl</y:mi></y:mrow><y:mprescripts/><y:none/><y:mrow><y:mn>204</y:mn></y:mrow></y:mmultiscripts></y:mrow><y:mo stretchy="false">(</y:mo><y:mi mathvariant="normal">n</y:mi><y:mo>,</y:mo><y:mi>γ</y:mi><y:mo stretchy="false">)</y:mo></y:mrow></y:math> cross section on the <db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline"><db:mrow><db:mi>s</db:mi></db:mrow></db:math>-process abundance of <fb:math xmlns:fb="http://www.w3.org/1998/Math/MathML" display="inline"><fb:mrow><fb:mmultiscripts><fb:mrow><fb:mi>Pb</fb:mi></fb:mrow><fb:mprescripts/><fb:none/><fb:mrow><fb:mn>204</fb:mn></fb:mrow></fb:mmultiscripts></fb:mrow></fb:math> has been reduced from <hb:math xmlns:hb="http://www.w3.org/1998/Math/MathML" display="inline"><hb:mo>∼</hb:mo><hb:mn>30</hb:mn><hb:mo>%</hb:mo></hb:math> down to <jb:math xmlns:jb="http://www.w3.org/1998/Math/MathML" display="inline"><jb:mrow><jb:mo>+</jb:mo><jb:mn>8</jb:mn><jb:mo>%</jb:mo><jb:mo>/</jb:mo><jb:mo>−</jb:mo><jb:mn>6</jb:mn><jb:mo>%</jb:mo></jb:mrow></jb:math>, and the <lb:math xmlns:lb="http://www.w3.org/1998/Math/MathML" display="inline"><lb:mrow><lb:mi>s</lb:mi></lb:mrow></lb:math>-process calculations are in agreement with the latest solar system abundance of <nb:math xmlns:nb="http://www.w3.org/1998/Math/MathML" display="inline"><nb:mrow><nb:mmultiscripts><nb:mrow><nb:mi>Pb</nb:mi></nb:mrow><nb:mprescripts/><nb:none/><nb:mrow><nb:mn>204</nb:mn></nb:mrow></nb:mmultiscripts></nb:mrow></nb:math> reported by K. Lodders in 2021. Published by the American Physical Society 2024