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High-energy processes |
Innuclear astrophysics, therapid neutron-capture process, also known as ther-process, is a set ofnuclear reactions that is responsible forthe creation of approximately half of theatomic nucleiheavier than iron, the "heavy elements", with the other half produced largely by thes-process. Ther-process synthesizes the more neutron-rich of the stable isotopes of even elements, and those separated from thebeta-stable isotopes by those that are not often have very lows-process yields and are consideredr-only nuclei; the heaviest isotopes of most even elements from zinc to mercury fall into this category. Abundance peaks for ther-process occur nearmass numbersA = 82 (elements Se, Br, and Kr),A = 130 (elements Te, I, and Xe) andA = 196 (elements Os, Ir, and Pt). Further, all the elements heavier than bismuth, including natural thorium and uranium (and other actinides) must ultimately originate in anr-process nucleus.
Ther-process entails a succession ofrapidneutron captures (hence the name) by one or more heavyseed nuclei, typically beginning with nuclei in the abundance peak centered on56Fe. The captures must be rapid in the sense that the nuclei must not have time to undergoradioactive decay (typically via β− decay) before anotherneutron arrives to be captured. This sequence can continue up to the limit of stability of the increasingly neutron-rich nuclei (theneutron drip line) to physically retain neutrons as governed by the short range nuclear force. Ther-process therefore must occur in locations where there exists a high density offree neutrons. At some time following the neutron captures, the nucleus beta-decays back to the line of stability (just as withfission products) resulting in a stable isotope of the same mass number A, and normally the most neutron-rich of those.
Early studies theorized that 1024 free neutrons per cm3 would be required, for temperatures of about 1 GK, in order to match the waiting points, at which no more neutrons can be captured, with the mass numbers of the abundance peaks forr-process nuclei.[1] This amounts to almost a gram of free neutrons in every cubic centimeter, an astonishing number requiring extreme locations. Traditionally this suggested the material ejected from the re-expanded core of acore-collapse supernova, as part ofsupernova nucleosynthesis,[2] or decompression ofneutron star matter thrown off by a binaryneutron star merger in akilonova.[3] The relative contribution of each of these sources to the astrophysical abundance ofr-process elements is a matter of ongoing research as of 2018[update].[4]
Anr-process-like series of neutron captures (onuranium-238 normally) occurs to a minor extent inthermonuclear weapon explosions, and can be enhanced by purposeful design. The elementseinsteinium (element 99) andfermium (element 100) innuclear weapon fallout, and in general this neutron capture results in isotopes as heavy as A = 257.
Ther-process contrasts with thes-process, the other predominant mechanism for the production of heavy elements, which is nucleosynthesis by means ofslow captures of neutrons. In general, isotopes involved in thes-process have half-lives long enough to enable their study in laboratory experiments, but this is not typically true for isotopes involved in ther-process.[5] Thes-process primarily occurs within ordinary stars, particularlyAGB stars, where the neutron flux is sufficient to cause neutron captures to recur every 10–100 years, much too slow for ther-process, which requires up to 100 captures per second. Thes-process issecondary, meaning that it requires pre-existing heavy isotopes as seed nuclei to be converted into other heavy nuclei by a slow sequence of captures of free neutrons. Ther-process scenarios create their own seed nuclei, so they might proceed in massive stars that contain no heavy seed nuclei. Taken together, ther- ands-processes account for almost the entireabundance of chemical elements heavier than iron. The historical challenge has been to locate physical settings appropriate to their time scales.
Following pioneering research into theBig Bang and the formation ofhelium in stars, an unknown process responsible for producing heavier elements found on Earth fromhydrogen and helium was suspected to exist. One early attempt at explanation came fromSubrahmanyan Chandrasekhar and Louis R. Henrich who postulated that elements were produced at temperatures between 6 billion and 8 billionK. Their theory accounted for elements up tochlorine, though there was no explanation for elements ofatomic weight heavier than 40amu at non-negligible abundances.[6]This became the foundation of a study byFred Hoyle, 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 forabundances of elements that would be formed in such conditions.[6]
The need for a physical setting providing rapidneutron capture, 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 byHans Suess andHarold Urey in 1956.[7] Their abundance table revealed larger than average abundances of natural isotopes containingmagic numbers[a] of neutrons as well as abundance peaks about 10 amu lighter thanstable nuclei 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 thanbeta decay, and the resulting abundance peaks were caused by so-calledwaiting points at magic numbers.[1][b] This process, rapid neutron capture by neutron-rich isotopes, became known as ther-process, whereas thes-process was named for its characteristic slow neutron capture. A table apportioning the heavy isotopes between thes-process and ther-process was published in 1957 in theB2FH review paper,[1] which named ther-process and outlined the physics that guides it.[8]Alastair G. W. Cameron also published a smaller study about ther-process in the same year.[9]
The stationaryr-process as described by the B2FH paper was first demonstrated in a time-dependent calculation atCaltech by Phillip A. Seeger,William A. Fowler andDonald D. Clayton,[10] who found that no single temporal snapshot matched the solarr-process abundances, but, that when superposed, did achieve a successful characterization of ther-process abundance distribution. Shorter-time distributions emphasize abundances at atomic weights less thanA = 140, whereas longer-time distributions emphasized those at atomic weights greater thanA = 140.[11] Subsequent treatments of ther-process reinforced those temporal features. Seeger et al. were also able to construct more quantitative apportionment betweens-process andr-process of the abundance table of heavy isotopes, thereby establishing a more reliable abundance curve for ther-process isotopes than B2FH had been able to define. Today, ther-process abundances are determined using their technique of subtracting the more reliables-process isotopic abundances from the total isotopic abundances and attributing the remainder tor-process nucleosynthesis.[12] Thatr-process abundance curve (vs. atomic weight) has provided for many decades the target for theoretical computations of abundances synthesized by the physicalr-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 ther-process aprimary 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 asecondary 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 intor-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 woulds-process nucleosynthesis; so each scenario fits the earlier growth ofr-process abundances in the galaxy. Each of these scenarios is the subject of active theoretical research.Observational evidence of the earlyr-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 byJames W. Truran in 1981.[13] 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 solarr-process curve, as if thes-process component were missing. This was consistent with the hypothesis that thes-process had not yet begun to enrich interstellar gas when these young stars missing thes-process abundances were born from that gas, for it requires about 100 million years of galactic history for thes-process to get started whereas ther-process can begin after two million years. Theses-process–poor,r-process–rich stellar compositions must have been born earlier than anys-process, showing that ther-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 earlyr-process thereby derives from observed abundance spectra in old stars[4] that had been born early, when the galactic metallicity was still small, but that nonetheless contain their complement ofr-process nuclei.
Either interpretation, though generally supported by supernova experts, has yet to achieve a totally satisfactory calculation ofr-process abundances because the overall problem is numerically formidable. However, existing results are supportive; in 2017, new data about ther-process was discovered when theLIGO andVirgo gravitational-wave observatories discovered a merger of two neutron stars ejectingr-process matter.[14] SeeAstrophysical sites below.
The only natural candidate sites forr-process nucleosynthesis where the required conditions are thought to exist arecore-collapse supernovae (includingelectron-capture supernovae), and now mergers ofneutron stars.
Immediately after the severe compression of electrons in a Type II supernova,beta-minus decay is blocked. This is because the high electron density fills all available free electron states up to aFermi energy which is greater than the energy of nuclear beta decay. However, nuclearcapture of those free electrons still occurs, and causes increasingneutronization of matter. This results in an extremely high density of free neutrons which cannot decay, on the order of 1024 neutrons per cm3,[1] and hightemperatures. As this re-expands and cools,neutron capture by still-existing heavy nuclei occurs much faster thanbeta-minus decay. As a consequence, ther-process runs up along theneutron drip line 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-capturecross section in nuclei with closedneutron shells, the inhibiting process ofphotodisintegration, and the degree of nuclear stability in the heavy-isotope region. Neutron captures inr-process nucleosynthesis leads to the formation of neutron-rich,weakly bound nuclei withneutron separation energies as low as 2 MeV.[15][1] At this stage, closed neutron shells atN = 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.[16] 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.[17] Waiting point nuclei are then allowed to beta decay toward stability before further neutron capture can occur,[1] resulting in a slowdown orfreeze-out of the reaction.[16]
Decreasing nuclear stability terminates ther-process when its heaviest nuclei become unstable to spontaneous fission, when the total number of nucleons approaches 270. Thefission barrier may be low enough before 270 such that neutron capture might induce fission instead of continuing up the neutron drip line.[18] After the neutron flux decreases, these highly unstableradioactive nuclei undergo a rapid succession of beta decays until they reach more stable, neutron-rich nuclei.[19] While thes-process creates an abundance of stable nuclei having closed neutron shells, ther-process, in neutron-rich predecessor nuclei, creates an abundance of radioactive nuclei about 10amu below thes-process peaks.[20] These abundance peaks correspond to stableisobars produced from successive beta decays of waiting point nuclei havingN = 50, 82, and 126—which are about 10 protons removed from theline of beta stability.[21]
As mentioned, an artificial r-process can be made by nuclear explosions. It has been suggested that multiple explosions would make it possible to reach theisland of stability, as the affected nuclides (starting with uranium-238 as seed nuclei) would not have time to beta decay all the way to the quicklyspontaneously fissioning nuclides at the line of beta stability before absorbing more neutrons in the next explosion, thus providing a chance to reach neutron-richsuperheavy nuclides likecopernicium-291 and -293 which may have half-lives of centuries or millennia.[22]
The most probable candidate site for ther-process has long been suggested to be core-collapsesupernovae (spectral typesIb,Ic andII), which may provide the necessary physical conditions for ther-process. Ejectedr-process material must be relatively neutron-rich, a condition which has been difficult to achieve in models,[2] so that astrophysicists remain uneasy about their adequacy for successfulr-process yields.
In 2017, new astronomical data about ther-process was discovered in data from the merger of twoneutron stars. Using the gravitational wave data captured inGW170817 to identify the location of the merger, several teams[23][24][25] observed and studied optical data of the merger, finding spectroscopic evidence ofr-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 radioactiver-process matter of lower-mass-range heavy nuclei (A < 140 such asstrontium)[26] and cooler red masses of higher mass-numberr-process nuclei (A > 140) rich inactinides (such asuranium,thorium, andcalifornium). 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 byr-process nuclei near their waiting points. Two distinct mass regions (A < 140 andA > 140) for ther-process yields have been known since the first time dependent calculations of ther-process.[10] 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.[3]
These results offer a new possibility for clarifying six decades of uncertainty over the site of origin ofr-process nuclei. Confirming relevance to ther-process is that it is radiogenic power from radioactive decay ofr-process nuclei that maintains the visibility of these spun offr-process fragments. Otherwise they would dim quickly. Such alternative sites were first seriously proposed in 1974[27] as decompressingneutron star matter. It was proposed such matter is ejected fromneutron stars merging withblack holes in compact binaries. In 1989[28] (and 1999[29]) this scenario was extended to binaryneutron star mergers (abinary star system of two neutron stars that collide). After preliminary identification of these sites,[30] the scenario was confirmed byGW170817. Current astrophysical models suggest that a single neutron star merger event may have generated between 3 and 13Earth masses of gold.[31]
Nuclear physicists are still working to model ther-process, and astrophysicists need to estimate the frequency of neutron-star mergers to assess whetherr-process heavy-element production solely or at least significantly takes place in the merger environment.
