r-process

<h2 id="history">History</h2>
Following pioneering research into the <a href="/facts/Big_Bang/I5T1vTnv">Big Bang</a> and the formation of <a href="/facts/Helium/yRul93TG">helium</a> in stars, an unknown process responsible for producing heavier elements found on Earth from <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a> and helium was suspected to exist. One early attempt at explanation came from <a href="/facts/Subrahmanyan_Chandrasekhar/LQID778M">Subrahmanyan Chandrasekhar</a> and Louis R. Henrich who postulated that elements were produced at temperatures between 6×109 and 8×109 <a href="/facts/Kelvin/E3trzC4C">K</a>. Their theory accounted for elements up to <a href="/facts/Chlorine/y27UwYBM">chlorine</a>, though there was no explanation for elements of <a href="/facts/Atomic_weight/A3vbuz2n">atomic weight</a> heavier than 40 <a href="/facts/Atomic_mass_unit/IMATU3vf">amu</a> at non-negligible abundances.<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a>
This became the foundation of a study by <a href="/facts/Fred_Hoyle/tXjJ0Lbd">Fred Hoyle</a>, who hypothesized that conditions in the core of collapsing stars would enable nucleosynthesis of the remainder of the elements via rapid capture of densely packed free neutrons. However, there remained unanswered questions about equilibrium in stars that was required to balance beta-decays and precisely account for <a href="/facts/Abundance_of_the_chemical_elements/CE9WehBl">abundances of elements</a> that would be formed in such conditions.<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a>
The need for a physical setting providing rapid <a href="/facts/Neutron_capture/elsFpM12">neutron capture</a>, which was known to almost certainly have a role in element formation, was also seen in a table of abundances of isotopes of heavy elements by <a href="/facts/Hans_Suess/Vuc8f8LR">Hans Suess</a> and <a href="/facts/Harold_Urey/j1bPXfDz">Harold Urey</a> in 1956.<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a> Their abundance table revealed larger than average abundances of natural isotopes containing <a href="/facts/Magic_number_(physics)/v9PwXFzQ">magic numbers</a><a class="footnote-ref" id="fnref:10" href="#fn:10">10</a> of neutrons as well as abundance peaks about 10 amu lighter than <a href="/facts/Stable_nuclei/vC0VVvqN">stable nuclei</a> containing magic numbers of neutrons which were also in abundance, suggesting that radioactive neutron-rich nuclei having the magic neutron numbers but roughly ten fewer protons were formed. These observations also implied that rapid neutron capture occurred faster than <a href="/facts/Beta_decay/vxTBlckp">beta decay</a>, and the resulting abundance peaks were caused by so-called waiting points at magic numbers.<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a><a class="footnote-ref" id="fnref:12" href="#fn:12">12</a> This process, rapid neutron capture by neutron-rich isotopes, became known as the r-process, whereas the s-process was named for its characteristic slow neutron capture. A table apportioning the heavy isotopes phenomenologically between s-process and r-process isotopes was published in 1957 in the <a href="/facts/B2FH_paper/MM1X74bh">B2FH review paper</a>,<a class="footnote-ref" id="fnref:13" href="#fn:13">13</a>  which named the r-process and outlined the physics that guides it.<a class="footnote-ref" id="fnref:14" href="#fn:14">14</a> <a href="/facts/Alastair_G._W._Cameron/PgxFRRgw">Alastair G. W. Cameron</a> also published a smaller study about the r-process in the same year.<a class="footnote-ref" id="fnref:15" href="#fn:15">15</a>
The stationary r-process as described by the B2FH paper was first demonstrated in a time-dependent calculation at <a href="/facts/Caltech/3FPcp7BL">Caltech</a> by Phillip A. Seeger, <a href="/facts/William_A._Fowler/WowQ2AlE">William A. Fowler</a> and <a href="/facts/Donald_D._Clayton/fX1v4KSb">Donald D. Clayton</a>,<a class="footnote-ref" id="fnref:16" href="#fn:16">16</a> who found that no single temporal snapshot matched the solar r-process abundances, but, that when superposed, did achieve a successful characterization of the r-process abundance distribution. Shorter-time distributions emphasize abundances at atomic weights less than A = 140, whereas longer-time distributions emphasized those at atomic weights greater than A = 140.<a class="footnote-ref" id="fnref:17" href="#fn:17">17</a> Subsequent treatments of the r-process reinforced those temporal features. Seeger et al. were also able to construct more quantitative apportionment between s-process and r-process of the abundance table of heavy isotopes, thereby establishing a more reliable abundance curve for the r-process isotopes than B2FH had been able to define. Today, the r-process abundances are determined using their technique of subtracting the more reliable s-process isotopic abundances from the total isotopic abundances and attributing the remainder to r-process nucleosynthesis.<a class="footnote-ref" id="fnref:18" href="#fn:18">18</a> That r-process abundance curve (vs. atomic weight) has provided for many decades the target for theoretical computations of abundances synthesized by the physical r-process.
The creation of free neutrons by electron capture during the rapid collapse to high density of a supernova core along with quick assembly of some neutron-rich seed nuclei makes the r-process a primary nucleosynthesis process, a process that can occur even in a star initially of pure H and He. This in contrast to the B2FH designation which is a secondary process building on preexisting iron. Primary stellar nucleosynthesis begins earlier in the galaxy than does secondary nucleosynthesis. Alternatively the high density of neutrons within neutron stars would be available for rapid assembly into r-process nuclei if a collision were to eject portions of a neutron star, which then rapidly expands freed from confinement. That sequence could also begin earlier in galactic time than would s-process nucleosynthesis; so each scenario fits the earlier growth of r-process abundances in the galaxy. Each of these scenarios is the subject of active theoretical research.
Observational evidence of the early r-process enrichment of interstellar gas and of subsequent newly formed stars, as applied to the abundance evolution of the galaxy of stars, was first laid out by <a href="/facts/James_W._Truran/y8EMBrh1">James W. Truran</a> in 1981.<a class="footnote-ref" id="fnref:19" href="#fn:19">19</a> He and subsequent astronomers showed that the pattern of heavy-element abundances in the earliest metal-poor stars matched that of the shape of the solar r-process curve, as if the s-process component were missing. This was consistent with the hypothesis that the s-process had not yet begun to enrich interstellar gas when these young stars missing the s-process abundances were born from that gas, for it requires about 100 million years of galactic history for the s-process to get started whereas the r-process can begin after two million years. These s-process–poor, r-process–rich stellar compositions must have been born earlier than any s-process, showing that the r-process emerges from quickly evolving massive stars that become supernovae and leave neutron-star remnants that can merge with another neutron star. The primary nature of the early r-process thereby derives from observed abundance spectra in old stars<a class="footnote-ref" id="fnref:20" href="#fn:20">20</a> that had been born early, when the galactic metallicity was still small, but that nonetheless contain their complement of r-process nuclei.

Either interpretation, though generally supported by supernova experts, has yet to achieve a totally satisfactory calculation of r-process abundances because the overall problem is numerically formidable. However, existing results are supportive; in 2017, new data about the r-process was discovered when the <a href="/facts/LIGO/jX6jyJzM">LIGO</a> and <a href="/facts/Virgo_interferometer/wfaxuUqc">Virgo</a> gravitational-wave observatories discovered a merger of two neutron stars ejecting r-process matter.<a class="footnote-ref" id="fnref:21" href="#fn:21">21</a> See Astrophysical sites below.
Noteworthy is that the r-process is responsible for our natural cohort of radioactive elements, such as uranium and thorium, as well as the most neutron-rich isotopes of each heavy element.

<h2 id="nuclear-physics">Nuclear physics</h2>
There are three natural candidate sites for r-process nucleosynthesis where the required conditions are thought to exist: low-mass <a href="/facts/Supernova/Pfo2J23l">supernovae</a>, <a href="/facts/Type_II_supernova/623d9YYT">Type II supernovae</a>, and <a href="/facts/Neutron_star_merger/AMLBIl3G">neutron star mergers</a>.<a class="footnote-ref" id="fnref:22" href="#fn:22">22</a>
Immediately after the severe compression of electrons in a Type II supernova, <a href="/facts/Beta_decay/vxTBlckp">beta-minus decay</a> is blocked. This is because the high electron density fills all available free electron states up to a <a href="/facts/Fermi_energy/S4Q8ShGR">Fermi energy</a> which is greater than the energy of nuclear beta decay. However, nuclear <a href="/facts/Electron_capture/EIccoV1u">capture of those free electrons</a> still occurs, and causes increasing neutronization of matter. This results in an extremely high density of free neutrons which cannot decay, on the order of 1024 neutrons per cm3,<a class="footnote-ref" id="fnref:23" href="#fn:23">23</a> and high <a href="/facts/Temperature/QyGe4gmj">temperatures</a>. As this re-expands and cools, <a href="/facts/Neutron_capture/elsFpM12">neutron capture</a> by still-existing heavy nuclei occurs much faster than <a href="/facts/Beta_decay/vxTBlckp">beta-minus decay</a>. As a consequence, the r-process runs up along the <a href="/facts/Neutron_drip_line/k3PfJUoy">neutron drip line</a> and highly-unstable neutron-rich nuclei are created.
Three processes which affect the climbing of the neutron drip line are a notable decrease in the neutron-capture <a href="/facts/Nuclear_cross_section/AtghNytD">cross section</a> in nuclei with closed <a href="/facts/Nuclear_shell_model/1Qgep495">neutron shells</a>, the inhibiting process of <a href="/facts/Photodisintegration/7TKyEJtc">photodisintegration</a>, and the degree of nuclear stability in the heavy-isotope region. Neutron captures in r-process nucleosynthesis leads to the formation of neutron-rich, <a href="/facts/Nuclear_binding_energy/PU8bREHW">weakly bound</a> nuclei with <a href="/facts/Separation_energy/qF9D5g2p">neutron separation energies</a> as low as 2 MeV.<a class="footnote-ref" id="fnref:24" href="#fn:24">24</a><a class="footnote-ref" id="fnref:25" href="#fn:25">25</a> At this stage, closed neutron shells at N = 50, 82, and 126 are reached, and neutron capture is temporarily paused. These so-called waiting points are characterized by increased binding energy relative to heavier isotopes, leading to low neutron capture cross sections and a buildup of semi-magic nuclei that are more stable toward beta decay.<a class="footnote-ref" id="fnref:26" href="#fn:26">26</a> In addition, nuclei beyond the shell closures are susceptible to quicker beta decay owing to their proximity to the drip line; for these nuclei, beta decay occurs before further neutron capture.<a class="footnote-ref" id="fnref:27" href="#fn:27">27</a> Waiting point nuclei are then allowed to beta decay toward stability before further neutron capture can occur,<a class="footnote-ref" id="fnref:28" href="#fn:28">28</a> resulting in a slowdown or freeze-out of the reaction.<a class="footnote-ref" id="fnref:29" href="#fn:29">29</a>
Decreasing nuclear stability terminates the r-process when its heaviest nuclei become unstable to spontaneous fission, when the total number of nucleons approaches 270. The <a href="/facts/Fission_barrier/Xt5xoTpB">fission barrier</a> may be low enough before 270 such that neutron capture might induce fission instead of continuing up the neutron drip line.<a class="footnote-ref" id="fnref:30" href="#fn:30">30</a> After the neutron flux decreases, these highly unstable <a href="/facts/Radioactive_decay/Ya6FVjt4">radioactive</a> nuclei undergo a rapid succession of beta decays until they reach more stable, neutron-rich nuclei.<a class="footnote-ref" id="fnref:31" href="#fn:31">31</a> While the <a href="/facts/S-process/31HVkqkI">s-process</a> creates an abundance of stable nuclei having closed neutron shells, the r-process, in neutron-rich predecessor nuclei, creates an abundance of radioactive nuclei about 10 <a href="/facts/Atomic_mass_unit/IMATU3vf">amu</a> below the s-process peaks.<a class="footnote-ref" id="fnref:32" href="#fn:32">32</a> These abundance peaks correspond to stable <a href="/facts/Isobar_(nuclide)/25ezJVsq">isobars</a> produced from successive beta decays of waiting point nuclei having N = 50, 82, and 126—which are about 10 protons removed from the <a href="/facts/Line_of_beta_stability/Qc1jTB6H">line of beta stability</a>.<a class="footnote-ref" id="fnref:33" href="#fn:33">33</a>
The r-process also occurs in thermonuclear weapons, and was responsible for the initial discovery of neutron-rich almost stable isotopes of <a href="/facts/Actinide/kHC4lxJV">actinides</a> like <a href="/facts/Plutonium-244/XGtUVghF">plutonium-244</a> and the new elements <a href="/facts/Einsteinium/RePoIVN9">einsteinium</a> and <a href="/facts/Fermium/Us3rnvG5">fermium</a> (atomic numbers 99 and 100) in the 1950s. It has been suggested that multiple nuclear explosions would make it possible to reach the <a href="/facts/Island_of_stability/XuxQzqjL">island of stability</a>, as the affected nuclides (starting with uranium-238 as seed nuclei) would not have time to beta decay all the way to the quickly <a href="/facts/Spontaneous_fission/0QuJoVKI">spontaneously fissioning</a> nuclides at the line of beta stability before absorbing more neutrons in the next explosion, thus providing a chance to reach neutron-rich <a href="/facts/Superheavy_element/9PjYvCc3">superheavy</a> nuclides like <a href="/facts/Copernicium/EhgyBkg0">copernicium</a>-291 and -293 which may have half-lives of centuries or millennia.<a class="footnote-ref" id="fnref:34" href="#fn:34">34</a>

<h2 id="astrophysical-sites">Astrophysical sites</h2>
The most probable candidate site for the r-process has long been suggested to be core-collapse <a href="/facts/Supernova/Pfo2J23l">supernovae</a> (spectral types Ib, Ic and II), which may provide the necessary physical conditions for the r-process. However, the very low abundance of r-process <a href="/facts/Atomic_nucleus/orpIm5Xl">nuclei</a> in the interstellar gas limits the amount each can have ejected. It requires either that only a small fraction of supernovae eject r-process nuclei to the <a href="/facts/Interstellar_medium/mokU0WEz">interstellar medium</a>, or that each supernova ejects only a very small amount of r-process material. The ejected material must be relatively neutron-rich, a condition which has been difficult to achieve in models,<a class="footnote-ref" id="fnref:35" href="#fn:35">35</a> so that astrophysicists remain uneasy about their adequacy for successful r-process yields.
In 2017, new astronomical data about the r-process was discovered in data from the merger of two <a href="/facts/Neutron_star/4enfoxBA">neutron stars</a>. Using the gravitational wave data captured in <a href="/facts/GW170817/q78LSs5w">GW170817</a> to identify the location of the merger, several teams<a class="footnote-ref" id="fnref:36" href="#fn:36">36</a><a class="footnote-ref" id="fnref:37" href="#fn:37">37</a><a class="footnote-ref" id="fnref:38" href="#fn:38">38</a> observed and studied optical data of the merger, finding spectroscopic evidence of r-process material thrown off by the merging neutron stars. The bulk of this material seems to consist of two types: hot blue masses of highly radioactive r-process matter of lower-mass-range heavy nuclei (A < 140 such as <a href="/facts/Strontium/ABaA5qQK">strontium</a>)<a class="footnote-ref" id="fnref:39" href="#fn:39">39</a> and cooler red masses of higher mass-number r-process nuclei (A > 140) rich in <a href="/facts/Actinide/kHC4lxJV">actinides</a> (such as <a href="/facts/Uranium/n2sGWBzU">uranium</a>, <a href="/facts/Thorium/ysdWH470">thorium</a>, and <a href="/facts/Californium/AJVjoVut">californium</a>). When released from the huge internal pressure of the neutron star, these ejecta expand and form seed heavy nuclei that rapidly capture free neutrons, and radiate detected optical light for about a week. Such duration of luminosity would not be possible without heating by internal radioactive decay, which is provided by r-process nuclei near their waiting points. Two distinct mass regions (A < 140 and A > 140) for the r-process yields have been known since the first time dependent calculations of the r-process.<a class="footnote-ref" id="fnref:40" href="#fn:40">40</a> Because of these spectroscopic features it has been argued that such nucleosynthesis in the Milky Way has been primarily ejecta from neutron-star mergers rather than from supernovae.<a class="footnote-ref" id="fnref:41" href="#fn:41">41</a>
These results offer a new possibility for clarifying six decades of uncertainty over the site of origin of r-process nuclei. Confirming relevance to the r-process is that it is radiogenic power from radioactive decay of r-process nuclei that maintains the visibility of these spun off r-process fragments. Otherwise they would dim quickly. Such alternative sites were first seriously proposed in 1974<a class="footnote-ref" id="fnref:42" href="#fn:42">42</a> as decompressing <a href="/facts/Neutron_star/4enfoxBA">neutron star</a> matter. It was proposed such matter is ejected from <a href="/facts/Neutron_stars/4enfoxBA">neutron stars</a> merging with <a href="/facts/Black_holes/1r2lmkcF">black holes</a> in compact binaries. In 1989<a class="footnote-ref" id="fnref:43" href="#fn:43">43</a> (and 1999<a class="footnote-ref" id="fnref:44" href="#fn:44">44</a>) this scenario was extended to binary <a href="/facts/Neutron_star/4enfoxBA">neutron star</a> mergers (a <a href="/facts/Binary_star/TYlDe1bD">binary star system</a> of two neutron stars that collide). After preliminary identification of these sites,<a class="footnote-ref" id="fnref:45" href="#fn:45">45</a> the scenario was confirmed in <a href="/facts/GW170817/q78LSs5w">GW170817</a>. Current astrophysical models suggest that a single neutron star merger event may have generated between 3 and 13 <a href="/facts/Earth_mass/G74DaKFv">Earth masses</a> of gold.<a class="footnote-ref" id="fnref:46" href="#fn:46">46</a>

<h2 id="see-also">See also</h2>
<ul>
<li>Astronomy portal</li><li>Physics portal</li></ul>
<ul><li>HD 222925</li>
<li><a href="/facts/Nucleosynthesis/F9MIeADF">Nucleosynthesis</a></li>
<li><a href="/facts/S-process/31HVkqkI">s-process</a></li>
<li><a href="/facts/P-process/X7k6soYm">p-process</a></li></ul>
<h2 id="notes">Notes</h2>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Burbidge, E. M.; Burbidge, G. R.; Fowler, W. A.; Hoyle, F. (1957). "Synthesis of the Elements in Stars". Reviews of Modern Physics. 29 (4): 547–650. Bibcode:1957RvMP...29..547B. doi:10.1103/RevModPhys.29.547. <a href="https://doi.org/10.1103%2FRevModPhys.29.547" target="_blank">https://doi.org/10.1103%2FRevModPhys.29.547</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn:2">neutrons 1,674,927,471,000,000,000,000,000/cc vs 1 atom/cc interstellar space <a href="/wiki/Interstellar_medium" target="_blank">/wiki/Interstellar_medium</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Thielemann, F.-K.; et al. (2011). "What are the astrophysical sites for the r-process and the production of heavy elements?". Progress in Particle and Nuclear Physics. 66 (2): 346–353. Bibcode:2011PrPNP..66..346T. doi:10.1016/j.ppnp.2011.01.032. S2CID 119412716. <a href="/wiki/Friedrich-Karl_Thielemann" target="_blank">/wiki/Friedrich-Karl_Thielemann</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Kasen, D.; Metzger, B.; Barnes, J.; Quataert, E.; Ramirez-Ruiz, E. (2017). "Origin of the heavy elements in binary neutron-star mergers from a gravitational-wave event". Nature. 551 (7678): 80–84. arXiv:1710.05463. Bibcode:2017Natur.551...80K. doi:10.1038/nature24453. PMID 29094687. <a href="https://doi.org/10.1038%2Fnature24453" target="_blank">https://doi.org/10.1038%2Fnature24453</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Frebel, A.; Beers, T. C. (2018). "The formation of the heaviest elements". Physics Today. 71 (1): 30–37. arXiv:1801.01190. Bibcode:2018PhT....71a..30F. doi:10.1063/pt.3.3815. Nuclear physicists are still working to model the r-process, and astrophysicists need to estimate the frequency of neutron-star mergers to assess whether r-process heavy-element production solely or at least significantly takes place in the merger environment. <a href="https://doi.org/10.1063%2Fpt.3.3815" target="_blank">https://doi.org/10.1063%2Fpt.3.3815</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">Cowan, John J.; Thielemann, Friedrich-Karl Thielemann (2004). "R-Process Nucleosynthesis in Supernovae" (PDF). Physics Today. 57 (10): 47–54. Bibcode:2004PhT....57j..47C. doi:10.1063/1.1825268. <a href="https://www.astro.umd.edu/~hamilton/ASTR630/handouts/R-Process.pdf" target="_blank">https://www.astro.umd.edu/~hamilton/ASTR630/handouts/R-Process.pdf</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">Hoyle, F. (1946). "The Synthesis of the Elements from Hydrogen". Monthly Notices of the Royal Astronomical Society. 106 (5): 343–383. Bibcode:1946MNRAS.106..343H. doi:10.1093/mnras/106.5.343. <a href="https://doi.org/10.1093%2Fmnras%2F106.5.343" target="_blank">https://doi.org/10.1093%2Fmnras%2F106.5.343</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">Hoyle, F. (1946). "The Synthesis of the Elements from Hydrogen". Monthly Notices of the Royal Astronomical Society. 106 (5): 343–383. Bibcode:1946MNRAS.106..343H. doi:10.1093/mnras/106.5.343. <a href="https://doi.org/10.1093%2Fmnras%2F106.5.343" target="_blank">https://doi.org/10.1093%2Fmnras%2F106.5.343</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">Suess, H. E.; Urey, H. C. (1956). "Abundances of the Elements". Reviews of Modern Physics. 28 (1): 53–74. Bibcode:1956RvMP...28...53S. doi:10.1103/RevModPhys.28.53. <a href="/wiki/Reviews_of_Modern_Physics" target="_blank">/wiki/Reviews_of_Modern_Physics</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">Neutron number 50, 82 and 126 <a href="/wiki/Neutron_number" target="_blank">/wiki/Neutron_number</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">Burbidge, E. M.; Burbidge, G. R.; Fowler, W. A.; Hoyle, F. (1957). "Synthesis of the Elements in Stars". Reviews of Modern Physics. 29 (4): 547–650. Bibcode:1957RvMP...29..547B. doi:10.1103/RevModPhys.29.547. <a href="https://doi.org/10.1103%2FRevModPhys.29.547" target="_blank">https://doi.org/10.1103%2FRevModPhys.29.547</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">Abundance peaks for the r- and s-processes are at A = 80, 130, 196 and A = 90, 138, 208, respectively. <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
<li id="fn:13">Burbidge, E. M.; Burbidge, G. R.; Fowler, W. A.; Hoyle, F. (1957). "Synthesis of the Elements in Stars". Reviews of Modern Physics. 29 (4): 547–650. Bibcode:1957RvMP...29..547B. doi:10.1103/RevModPhys.29.547. <a href="https://doi.org/10.1103%2FRevModPhys.29.547" target="_blank">https://doi.org/10.1103%2FRevModPhys.29.547</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></li>
<li id="fn:14">Woosley, Stan; Trimble, Virginia; Thielemann, Friedrich-Karl (2019). "The origin of the elements". Physics Today. 72 (2): 36–37. Bibcode:2019PhT....72b..36W. doi:10.1063/PT.3.4134. S2CID 186549912. <a href="/wiki/Stanford_E._Woosley" target="_blank">/wiki/Stanford_E._Woosley</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></li>
<li id="fn:15">Cameron, A. G. W. (1957). "Nuclear reactions in stars and nucleogenesis". Publications of the Astronomical Society of the Pacific. 69 (408): 201. Bibcode:1957PASP...69..201C. doi:10.1086/127051. <a href="https://doi.org/10.1086%2F127051" target="_blank">https://doi.org/10.1086%2F127051</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></li>
<li id="fn:16">Seeger, P. A.; Fowler, W. A.; Clayton, D. D. (1965). "Nucleosynthesis of heavy elements by neutron capture". Astrophysical Journal Supplement. 11: 121–66. Bibcode:1965ApJS...11..121S. doi:10.1086/190111. <a href="https://doi.org/10.1086%2F190111" target="_blank">https://doi.org/10.1086%2F190111</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></li>
<li id="fn:17">See Seeger, Fowler & Clayton 1965. Figure 16 shows the short-flux calculation and its comparison with natural r-process abundances whereas Figure 18 shows the calculated abundances for long neutron fluxes. - Seeger, P. A.; Fowler, W. A.; Clayton, D. D. (1965). "Nucleosynthesis of heavy elements by neutron capture". Astrophysical Journal Supplement. 11: 121–66. Bibcode:1965ApJS...11..121S. doi:10.1086/190111. <a href="https://doi.org/10.1086%2F190111" target="_blank">https://doi.org/10.1086%2F190111</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></li>
<li id="fn:18">See Table 4 in Seeger, Fowler & Clayton 1965. - Seeger, P. A.; Fowler, W. A.; Clayton, D. D. (1965). "Nucleosynthesis of heavy elements by neutron capture". Astrophysical Journal Supplement. 11: 121–66. Bibcode:1965ApJS...11..121S. doi:10.1086/190111. <a href="https://doi.org/10.1086%2F190111" target="_blank">https://doi.org/10.1086%2F190111</a> <a href="#fnref:18" class="footnote-back-ref">↩</a></li>
<li id="fn:19">Truran, J. W. (1981). "A new interpretation of the heavy-element abundances in metal-deficient stars". Astronomy and Astrophysics. 97 (2): 391–93. Bibcode:1981A&A....97..391T. <a href="/wiki/Astronomy_and_Astrophysics" target="_blank">/wiki/Astronomy_and_Astrophysics</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></li>
<li id="fn:20">Frebel, A.; Beers, T. C. (2018). "The formation of the heaviest elements". Physics Today. 71 (1): 30–37. arXiv:1801.01190. Bibcode:2018PhT....71a..30F. doi:10.1063/pt.3.3815. Nuclear physicists are still working to model the r-process, and astrophysicists need to estimate the frequency of neutron-star mergers to assess whether r-process heavy-element production solely or at least significantly takes place in the merger environment. <a href="https://doi.org/10.1063%2Fpt.3.3815" target="_blank">https://doi.org/10.1063%2Fpt.3.3815</a> <a href="#fnref:20" class="footnote-back-ref">↩</a></li>
<li id="fn:21">Abbott, B. P.; et al. (LIGO Scientific Collaboration and Virgo Collaboration) (2017). "GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral". Physical Review Letters. 119 (16): 161101. arXiv:1710.05832. Bibcode:2017PhRvL.119p1101A. doi:10.1103/PhysRevLett.119.161101. PMID 29099225. <a href="https://doi.org/10.1103%2FPhysRevLett.119.161101" target="_blank">https://doi.org/10.1103%2FPhysRevLett.119.161101</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></li>
<li id="fn:22">Bartlett, A.; Görres, J.; Mathews, G.J.; Otsuki, K.; Wiescher, W. (2006). "Two-neutron capture reactions and the r process" (PDF). Physical Review C. 74 (1): 015082. Bibcode:2006PhRvC..74a5802B. doi:10.1103/PhysRevC.74.015802. Archived from the original (PDF) on 2020-08-06. Retrieved 2019-06-17. <a href="https://web.archive.org/web/20200806174821/https://core.ac.uk/download/pdf/101046.pdf" target="_blank">https://web.archive.org/web/20200806174821/https://core.ac.uk/download/pdf/101046.pdf</a> <a href="#fnref:22" class="footnote-back-ref">↩</a></li>
<li id="fn:23">Burbidge, E. M.; Burbidge, G. R.; Fowler, W. A.; Hoyle, F. (1957). "Synthesis of the Elements in Stars". Reviews of Modern Physics. 29 (4): 547–650. Bibcode:1957RvMP...29..547B. doi:10.1103/RevModPhys.29.547. <a href="https://doi.org/10.1103%2FRevModPhys.29.547" target="_blank">https://doi.org/10.1103%2FRevModPhys.29.547</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></li>
<li id="fn:24">Thoennessen, M. (2004). "Reaching the limits of nuclear stability" (PDF). Reports on Progress in Physics. 67 (7): 1187–1232. Bibcode:2004RPPh...67.1187T. doi:10.1088/0034-4885/67/7/R04. S2CID 250790169. <a href="https://people.nscl.msu.edu/~thoennes/personal/papers/rpp67_1187.pdf" target="_blank">https://people.nscl.msu.edu/~thoennes/personal/papers/rpp67_1187.pdf</a> <a href="#fnref:24" class="footnote-back-ref">↩</a></li>
<li id="fn:25">Burbidge, E. M.; Burbidge, G. R.; Fowler, W. A.; Hoyle, F. (1957). "Synthesis of the Elements in Stars". Reviews of Modern Physics. 29 (4): 547–650. Bibcode:1957RvMP...29..547B. doi:10.1103/RevModPhys.29.547. <a href="https://doi.org/10.1103%2FRevModPhys.29.547" target="_blank">https://doi.org/10.1103%2FRevModPhys.29.547</a> <a href="#fnref:25" class="footnote-back-ref">↩</a></li>
<li id="fn:26">Eichler, M.A. (2016). Nucleosynthesis in explosive environments: neutron star mergers and core-collapse supernovae (PDF) (Doctoral thesis). University of Basel. <a href="https://edoc.unibas.ch/59530/1/Thesis_finalsubmit.pdf" target="_blank">https://edoc.unibas.ch/59530/1/Thesis_finalsubmit.pdf</a> <a href="#fnref:26" class="footnote-back-ref">↩</a></li>
<li id="fn:27">Wang, R.; Chen, L.W. (2015). "Positioning the neutron drip line and the r-process paths in the nuclear landscape". Physical Review C. 92 (3): 031303–1–031303–5. arXiv:1410.2498. Bibcode:2015PhRvC..92c1303W. doi:10.1103/PhysRevC.92.031303. S2CID 59020556. <a href="/wiki/ArXiv_(identifier)" target="_blank">/wiki/ArXiv_(identifier)</a> <a href="#fnref:27" class="footnote-back-ref">↩</a></li>
<li id="fn:28">Burbidge, E. M.; Burbidge, G. R.; Fowler, W. A.; Hoyle, F. (1957). "Synthesis of the Elements in Stars". Reviews of Modern Physics. 29 (4): 547–650. Bibcode:1957RvMP...29..547B. doi:10.1103/RevModPhys.29.547. <a href="https://doi.org/10.1103%2FRevModPhys.29.547" target="_blank">https://doi.org/10.1103%2FRevModPhys.29.547</a> <a href="#fnref:28" class="footnote-back-ref">↩</a></li>
<li id="fn:29">Eichler, M.A. (2016). Nucleosynthesis in explosive environments: neutron star mergers and core-collapse supernovae (PDF) (Doctoral thesis). University of Basel. <a href="https://edoc.unibas.ch/59530/1/Thesis_finalsubmit.pdf" target="_blank">https://edoc.unibas.ch/59530/1/Thesis_finalsubmit.pdf</a> <a href="#fnref:29" class="footnote-back-ref">↩</a></li>
<li id="fn:30">Boleu, R.; Nilsson, S. G.; Sheline, R. K. (1972). "On the termination of the r-process and the synthesis of superheavy elements". Physics Letters B. 40 (5): 517–521. Bibcode:1972PhLB...40..517B. doi:10.1016/0370-2693(72)90470-4. <a href="http://lup.lub.lu.se/record/2858722" target="_blank">http://lup.lub.lu.se/record/2858722</a> <a href="#fnref:30" class="footnote-back-ref">↩</a></li>
<li id="fn:31">Clayton, D. D. (1968), Principles of Stellar Evolution and Nucleosynthesis, Mc-Graw-Hill, pp. 577–91, ISBN 978-0226109534, provides a clear technical introduction to these features. A more technical description can be found in Seeger, Fowler & Clayton 1965. <a href="978-0226109534" target="_blank">978-0226109534</a> <a href="#fnref:31" class="footnote-back-ref">↩</a></li>
<li id="fn:32">Figure 10 of Seeger, Fowler & Clayton 1965 shows this path of captures reaching magic neutron numbers 82 and 126 at smaller values of nuclear charge Z than it does along the stability path. - Seeger, P. A.; Fowler, W. A.; Clayton, D. D. (1965). "Nucleosynthesis of heavy elements by neutron capture". Astrophysical Journal Supplement. 11: 121–66. Bibcode:1965ApJS...11..121S. doi:10.1086/190111. <a href="https://doi.org/10.1086%2F190111" target="_blank">https://doi.org/10.1086%2F190111</a> <a href="#fnref:32" class="footnote-back-ref">↩</a></li>
<li id="fn:33">Surman, R.; Mumpower, M.; Sinclair, R.; Jones, K. L.; Hix, W. R.; McLaughlin, G. C. (2014). "Sensitivity studies for the weak r process: neutron capture rates". AIP Advances. 4 (41008): 041008. Bibcode:2014AIPA....4d1008S. doi:10.1063/1.4867191. <a href="https://doi.org/10.1063%2F1.4867191" target="_blank">https://doi.org/10.1063%2F1.4867191</a> <a href="#fnref:33" class="footnote-back-ref">↩</a></li>
<li id="fn:34">Zagrebaev, V.; Karpov, A.; Greiner, W. (2013). "Future of superheavy element research: Which nuclei could be synthesized within the next few years?". Journal of Physics: Conference Series. 420 (1): 012001. arXiv:1207.5700. Bibcode:2013JPhCS.420a2001Z. doi:10.1088/1742-6596/420/1/012001. <a href="https://doi.org/10.1088%2F1742-6596%2F420%2F1%2F012001" target="_blank">https://doi.org/10.1088%2F1742-6596%2F420%2F1%2F012001</a> <a href="#fnref:34" class="footnote-back-ref">↩</a></li>
<li id="fn:35">Thielemann, F.-K.; et al. (2011). "What are the astrophysical sites for the r-process and the production of heavy elements?". Progress in Particle and Nuclear Physics. 66 (2): 346–353. Bibcode:2011PrPNP..66..346T. doi:10.1016/j.ppnp.2011.01.032. S2CID 119412716. <a href="/wiki/Friedrich-Karl_Thielemann" target="_blank">/wiki/Friedrich-Karl_Thielemann</a> <a href="#fnref:35" class="footnote-back-ref">↩</a></li>
<li id="fn:36">Arcavi, I.; et al. (2017). "Optical emission from a kilonova following a gravitational-wave-detected neutron-star merger". Nature. 551 (7678): 64–66. arXiv:1710.05843. Bibcode:2017Natur.551...64A. doi:10.1038/nature24291. <a href="https://doi.org/10.1038%2Fnature24291" target="_blank">https://doi.org/10.1038%2Fnature24291</a> <a href="#fnref:36" class="footnote-back-ref">↩</a></li>
<li id="fn:37">Pian, E.; et al. (2017). "Spectroscopic identification of r-process nucleosynthesis in a double neutron-star merger". Nature. 551 (7678): 67–70. arXiv:1710.05858. Bibcode:2017Natur.551...67P. doi:10.1038/nature24298. PMID 29094694. <a href="https://doi.org/10.1038%2Fnature24298" target="_blank">https://doi.org/10.1038%2Fnature24298</a> <a href="#fnref:37" class="footnote-back-ref">↩</a></li>
<li id="fn:38">Smartt, S. J.; et al. (2017). "A kilonova as the electromagnetic counterpart to a gravitational-wave source". Nature. 551 (7678): 75–79. arXiv:1710.05841. Bibcode:2017Natur.551...75S. doi:10.1038/nature24303. PMID 29094693. <a href="https://doi.org/10.1038%2Fnature24303" target="_blank">https://doi.org/10.1038%2Fnature24303</a> <a href="#fnref:38" class="footnote-back-ref">↩</a></li>
<li id="fn:39">Watson, Darach; Hansen, Camilla J.; Selsing, Jonatan; Koch, Andreas; Malesani, Daniele B.; Andersen, Anja C.; Fynbo, Johan P. U.; Arcones, Almudena; Bauswein, Andreas; Covino, Stefano; Grado, Aniello (2019). "Identification of strontium in the merger of two neutron stars". Nature. 574 (7779): 497–500. arXiv:1910.10510. Bibcode:2019Natur.574..497W. doi:10.1038/s41586-019-1676-3. ISSN 0028-0836. PMID 31645733. S2CID 204837882. <a href="/wiki/Almudena_Arcones" target="_blank">/wiki/Almudena_Arcones</a> <a href="#fnref:39" class="footnote-back-ref">↩</a></li>
<li id="fn:40">Seeger, P. A.; Fowler, W. A.; Clayton, D. D. (1965). "Nucleosynthesis of heavy elements by neutron capture". Astrophysical Journal Supplement. 11: 121–66. Bibcode:1965ApJS...11..121S. doi:10.1086/190111. <a href="https://doi.org/10.1086%2F190111" target="_blank">https://doi.org/10.1086%2F190111</a> <a href="#fnref:40" class="footnote-back-ref">↩</a></li>
<li id="fn:41">Kasen, D.; Metzger, B.; Barnes, J.; Quataert, E.; Ramirez-Ruiz, E. (2017). "Origin of the heavy elements in binary neutron-star mergers from a gravitational-wave event". Nature. 551 (7678): 80–84. arXiv:1710.05463. Bibcode:2017Natur.551...80K. doi:10.1038/nature24453. PMID 29094687. <a href="https://doi.org/10.1038%2Fnature24453" target="_blank">https://doi.org/10.1038%2Fnature24453</a> <a href="#fnref:41" class="footnote-back-ref">↩</a></li>
<li id="fn:42">Lattimer, J. M.; Schramm, D. N. (1974). "Black Hole–Neutron Star Collisions". Astrophysical Journal Letters. 192 (2): L145–147. Bibcode:1974ApJ...192L.145L. doi:10.1086/181612. <a href="https://doi.org/10.1086%2F181612" target="_blank">https://doi.org/10.1086%2F181612</a> <a href="#fnref:42" class="footnote-back-ref">↩</a></li>
<li id="fn:43">Eichler, D.; Livio, M.; Piran, T.; Schramm, D. N. (1989). "Nucleosynthesis, neutrino bursts and gamma-rays from coalescing neutron stars". Nature. 340 (6229): 126–128. Bibcode:1989Natur.340..126E. doi:10.1038/340126a0. <a href="https://doi.org/10.1038%2F340126a0" target="_blank">https://doi.org/10.1038%2F340126a0</a> <a href="#fnref:43" class="footnote-back-ref">↩</a></li>
<li id="fn:44">Freiburghaus, C.; Rosswog, S.; Thielemann, F.-K (1999). "r-process in Neutron Star Mergers". Astrophysical Journal Letters. 525 (2): L121 – L124. Bibcode:1999ApJ...525L.121F. doi:10.1086/312343. PMID 10525469. <a href="https://doi.org/10.1086%2F312343" target="_blank">https://doi.org/10.1086%2F312343</a> <a href="#fnref:44" class="footnote-back-ref">↩</a></li>
<li id="fn:45">Tanvir, N.; et al. (2013). "A 'kilonova' associated with the short-duration gamma-ray burst GRB 130603B". Nature. 500 (7464): 547–9. arXiv:1306.4971. Bibcode:2013Natur.500..547T. doi:10.1038/nature12505. PMID 23912055. <a href="https://doi.org/10.1038%2Fnature12505" target="_blank">https://doi.org/10.1038%2Fnature12505</a> <a href="#fnref:45" class="footnote-back-ref">↩</a></li>
<li id="fn:46">"Neutron star mergers may create much of the universe's gold". Sid Perkins. Science AAAS. 20 March 2018. Retrieved 24 March 2018. <a href="https://www.science.org/content/article/neutron-star-mergers-may-create-much-universe-s-gold" target="_blank">https://www.science.org/content/article/neutron-star-mergers-may-create-much-universe-s-gold</a> <a href="#fnref:46" class="footnote-back-ref">↩</a></li>
</ol>

r-process open-in-new

r-process