第30卷增刊Ⅱ2006年12月
高能物理与核物理
HIGH ENERGY PHYSICS AND NUCLEAR PHYSICS
Vol.30,Supp.Ⅱ
Dec.,2006
Nuclear Structure Aspects in Nuclear Astrophysics
Michael S.Smith1)
(Physics Division,Oak Ridge National Laboratory,Oak Ridge,TN,37831-6354,USA)
Abstract Nuclear structure information plays an extremely important role in studies of the evolution and explosion of stars and the cosmic synthesis of the elements.Properties of nuclear ground ,masses, lifetimes,decay branches)and low-lying resonances(excitation energies,spins,parities,decay widths,spec-troscopic factors),especially on unstable nuclei,can quantitativ
ely and qualitatively change predictions of astrophysical simulations.The location of the particle driplines and shell structure far from stability also strongly influence our astrophysical predictions.A number of examples of the dramatic impact that new nu-clear structure information has on simulations of nova explosions,X-ray bursts,and core collapse supernovae are given.Some of these are results of recent measurements with radioactive18F,82Ge,and84Se beams at ORNL’s Holifield Radioactive Ion Beam Facility.A new suite of software tools to help determine the astro-physical impact of nuclear physics studies will also be presented.
Key words nuclear astrophysics,nucleosynthesis,radioactive beam,reaction rates,supernova
1Introduction
This is an incredibly exciting time for astro-physics.New measurements of neutrinos emit-ted from the core of our sun have shownflavor oscillations[1].A map of the entire galaxy in gamma rays emitted from the decay of26Al shows hotspots in “recent”element synthesis[2].Detailed spectral anal-ysis of material ejected from supernovae explosions such as the one in Cas A show anomalous abundances of44Ti[3]and the presence of iron in the outer ejected layers(showing the star turned itself inside out)[4]. Images and spectroscopy of a shell of material blown offnova explosions show isotopic anomalies and spa-tial density inhomogeneities[5].
A diverse set of nuclear structure information on a wide variety of nuclei serves as essential input for simulations that attempt to explain these,and many other,observations of astrophysical phenomena.In-formation on unstable nuclei is particularly impor-tant to understand the nuclear processes occurring in the extremely high temperature and density envi-ronments characteristic of exploding stars[6].Some of the needed structure information,and the relevant astrophysical phenomena,include:resonance param-eters(novae);positron decays,proton separation en-ergies(X-ray bursts);level densities,alpha-nucleus potentials,decay lifetimes,masses,neutron separa-tion energies(supernovae).Also needed are:opti-cal model parameters,2-particle separation energies, single particle energy levels,decay modes,branch-ing ratios,and beta-delayed particle emission prob-abilities.The availability of beams of some of the nuclei involved in stellar explosions with reasonable purity,intensity,and emittance is now making it pos-sible to begin building an empirical foundation for models of stellar explosions.This experimental work, in combination with theoretical estimates of unmea-sured quantities,will enable nuclear structure science to make tremendous contributions to our understand-ing of how stars explode.Below I will give examples of the significant impact of nuclear structure studies
1)E-mail:msmith@l.gov
214—218
增刊ⅡMichael S.Smith：核天体物理中的核结构问题215
–some made with radioactive beams at Oak Ridge National Laboratory(ORNL)–in our understanding of three types of stellar explosions–nova explosions, X-ray bursts,and supernovae.
2Novae
Novae occur in binary star systems in which a main sequence or giant star expands and transfers material to its white dwarf companion star.The accreted material increases in temperature and den-sity until thermonuclear reactions are triggered on the surface of the compact dwarf star,leading to a runaway explosion which generates up to1045ergs of energy in roughly1000seconds and increases the light output by up to a factor of a million.Nuclear reactions on unstable nuclei up to mass40are be-lieved responsible for the nova outburst[7],but the rates of most of the relevant reactions are unmea-sured.Theoretical estimates are particularly difficult to make because individual nuclear resonances can change reaction rates by factors of10—107,dra-matically changing predications of energy generation and element synthesis in these explosions.Therefore, searching for resonances and measuring their proper-ties(resonance energy,spin,partial and total widths) is absolutely essential to understand novae.Nuclei in the sd-shell are the most important,as the burning rarely involves nuclei with mass greater than40.
As an example,the structure of18Ne was inves-tigated at ORNL to improve our estimate of the 17F(p,γ)18Ne reaction.This reaction,crucial in syn-thesis of15N,17O,18O,and18F in novae,is likely dominated by a3+resonance(known in the mirror nucleus18O)not seen in nine stable beam studies of 18Ne.By measuring the interference of resonant and elastic scattering using a radioactive17F beam pro-duced at ORNL’s Holifield Radioactive Ion Beam Fa-cility(HRIBF)[8],we provided thefirst unambiguous evidence for this important resonance,confirmed its spin and parity,and precisely determined the reso-nance energy and total width(to±2keV)[9].The now measured properties of this level changed the 17F(p,γ)18Ne rate calculations by up to a factor of30 over previous estimates using older nuclear structure information.When utilized as input for a nova nucle-osynthesis simulation,the rate based on the new18Ne level information changed the calculated production of17O in novae by factors of5when averaged over the entire exploding envelope,and by a factor of15,000 in the hottest regions of the envelope[10].
3X-ray bursts
A Type I X-ray burst(XRB)is a violent ther-monuclear runaway explosion[11]which is similar to a nova,except transfer of material is onto surface of a neutron star–the exotic remnant of a super-nova explosion.The resulting bursts of X-rays(10−8 erg·cm−2·s−1)can last for tens of seconds and can re-c
ur hourly or daily,and are driven by reactions[(α,p) and(p,γ)]on proton-rich nuclei with masses up to ap-proximately A=100via the“alpha-p process”and “rp-process”[12,13].Large-scale nuclear burning cal-culations are required to determine the energy gen-eration in XRBs that drives the observed X-ray lu-minosity.Nucleosynthesis calculations can also esti-mate the possible contribution of these explosions to the abundances of the rare,low-mass p-nuclides such as74Se,78Kr,92,94Mo,and96,98Ru that are difficult to synthesis in standard p-process scenarios[14].
The peak temperatures of XRBs can be as high as109K.The proton capture reactions driving the burst proceed through resonances above the proton threshold,which for high mass nuclei(A>40)are at excitation energies where the level densities are quite high.For this reason,estimates of cross sec-tions from a statistical model are used for the over-whelming majority of the hundreds of strong interac-tion rates used in XRB computer simulations.How-ever,this approach is invalid for low level densities or near closed shells or subshells where individual res-onances can significantly contribute to the capture rates[15].Shell models can,however,be used to pre-dict the levels,spectroscopic strengths,and reduced transition probabilities needed to calculate resonant reaction rates.One recent study[16]used the shell model ANTOINE to determine the properties of res-onances in the fp shell.Excitation energies of previ-ously measured states are calculated in this approach
216高能物理与核物理(HEP&NP)第30卷
to typically within1MeV of their known energy,and spectroscopic factors are calculated to within40%of the known values.New rates based on this resonance information were then utilized in an XRB element synthesis simulation[16]and found to change the pre-dictions of synthesized abundances of a number of fp shell nuclei by a factor of∼10compared to simula-tions using the older reaction rates based solely on a statistical model.Measurements of level structure, and improved shell model calculations,are needed for accurate predictions of XRB physics.reaction mass
Furthermore,mass models and particle decay properties are also needed for XRB studies.The nu-clear burning in XRBs proceeds by successive proton capture reactions until halted by photodissociation (γ,p)at the proton dripline.Nuclear masses deter-mine the(p,γ)-(γ,p)detailed balance:the ratio of the rates at a temperature T is proportional to exp(−Q(p,γ)/kT)where Q(p,γ)is the Q-value for the proton capture reaction.Recently,a comparison of XRB luminosity predictions using four different mass models[17]found significant qualitative(shape of ini-tial and subsequent peaks)and quantitative(dura-tion,amplitude)changes.Another XRB study of the highest masses synthesized–the endpoint of XRB nucleosynthesis[18]–illustrated the model sensitivity to alpha decays.Their calculations suggested that the synthesis of elements beyond Sn-Sb-Te is difficult because of photo-induced alpha emission.Howe
ver, this relied on the assumption that Te isotopes are alpha-unbound by∼4MeV.Experimental determina-tion of the Qαvalues and other properties are really needed to determine highest mass nuclides synthe-sized in XRBs.
4Supernovae
Supernova explosions are powered not by nuclear reactions but by the gravitational collapse of the Fe core of a massive star.The collapse to densities greater than nuclear matter in the inner core is fol-lowed by a rebound,with inner core material moving outwards while the outer core materials is falling in. This sets up a shock wave which,with help from neu-trino interactions[19]and convection,propagates out-wards through the dense core and then to the lower density outer layers,completely disrupting the star and leaving behind either a neutron star or,for higher mass stars,a black hole.
In this scenario,there is a high-entropy bubble formed above the newly-born neutron star,and the conditions(temperature,free neutron density,num-ber of heavy nuclei present)are just right to quickly form roughly half of all nuclei heavier than iron via the rapid neutron process(r-process)[20].This se-quence of nuclear reactions involves rapid neutron captures on neutron-rich unstable nuclei.Simula-tio
ns of the r-process require nuclear structure in-formation(masses,lifetimes,level structure,decay properties)on thousands of nuclei out to the neutron drip line.Additionally,nuclear reaction information is needed,especially near the N=50and82closed neutron shells[21]where the abundances peak.Be-cause the relevant nuclei have very short lifetimes, information on their structure are challenging to ob-tain experimentally.
These nuclei are also difficult to model theoreti-cally because they are many mass units away from stability and there is a general lack of experimen-tal information to constrain relevant theoretical mod-els.Nevertheless,the structure information is crucial for understanding the r-process,and nuclear masses are particularly important.During the supernova cooling phase,the sequence of reactions followed in the r-process follows contours of constant neutron separation energy.The wide variety of available mass models–phenomenological,microscopic,semi-microscopic–predict significantly different r-process reaction paths,which give radically different predic-tions of the abundances of nuclei synthesized in the r-process[22].Nuclear masses are also direct input into simplified r-process element abundance estimations that utilize the“waiting point”approximation[23]. Furthermore,masses are required to calculate ther-monuclear energy released during r-process burning, and are direct input in calculations of neutron cap-ture cross sections using a statistical reaction model.
Other structure information,such as beta-delayed
增刊ⅡMichael S.Smith：核天体物理中的核结构问题217
neutron emission(βn),is also important for under-standing r-process burning.The neutron-rich un-stable nuclei that are in the r-process reaction path beta decay back to stability as explosion temperature drops;this decay changes the element,the Z,but not the mass value.Older r-process models(which did not includeβn),however,almost always underpredict the observations of nuclei in the mass range124—126 while overpredicting the abundances at the mass130 peak[22].The inclusion ofβn can solve this prob-lem because this decay branch lowers the mass value during decay.Significantly improved agreement be-tween theory and observations is obtained whenβn is included in the calculation[22].However,more exten-sive measurements of this branch are needed,both near mass130and near the other r-process abun-dance peaks at mass80and195.This branch may potentially also be important for light masses as well. There is one model[24]that suggests that the inclu-sion of reactions on light(Z<10)neutron-rich nuclei could modify predicted r-abundances by up to a factor of10.This study utilizes an old calculation of beta-delayed neutron emission[25]which may need updating at these low mass nuclei.New experimental information onβn would help understand the neces-sity for neutron captures on light-element nuclei in the r-process.
Another exciting area of research is into the pos-sible weakening or disappearance of the traditional nuclear shell structure for unstable nuclei approach-ing the neutron drip line.There are numerous the-oretical models of shell gap evolution away from stability[22,26],and the astrophysical implications of this are profound:r-process abundance predictions can be changed by up to a factor of100-1000[22].The current lack of measurements makes shell gap evolu-tion difficult to study offstability.However,beams of neutron-rich unstable nuclei are now enabling an em-pirical foundation to be built for this work.ORNL’s HRIBF has the capability–unique in world–to uti-lize transfer reactions to investigate the level struc-ture of neutron-rich nuclei in and near the r-process path.For example,thefirst(d,p)measurements on nuclei at the N=50closed shell,82Ge(d,p)83Ge and84Se(d,p)85Se,have been measured at HRIBF[27], with the result that a weaker shell closure in83Ge is measured than previously predicted[28].Many more measurements needed to benchmark theory,however. Currently,(d,p)reactions on nuclei at the N=82 closed shell,132Sn and130Sn,are being measured at HRIBF,and many more studies are planned in the future.
5Online nuclear astrophysics soft-ware tools
To examine the astrophysical impact of the latest nuclear measurements and theoretical calculations,it is essential to process the nuclear information into a format that astrophysical simulations can utilize. T
his work,and more,is now greatly streamlined by the Computational Infrastructure for Nuclear Astrophysics[29].This is a unique suite of computer codes,freely available online , that enables anyone to quickly–with a few mouse clicks–incorporate new nuclear physics results in astrophysical simulations,run the simulations,vi-sualize the results,and compare new predictions to those based on older data.Furthermore,the suite enables users to share large datasets(reaction rate libraries,astrophysical simulations)with each other in an online community.The nucleosynthesis calcu-lations utilized the reaction network code of Hix and Thielemann[30].More features are continually being added to this suite,many on the basis of user recom-mendations.For example,tools to visualize theoret-ical mass models and compare them with each other and with experimental masses were recently added to our suite.
6Summary
To understand the evolution and explosion of stars,and the accompanying synthesis of nuclei,it is necessary to determine the structure of subatomic nuclei.Information needed includes masses,life-times,decays,shell structure,resonance properties, and level densities.This is especially important for
218高能物理与核物理(HEP&NP)第30卷
nuclei away from stability,where nuclear theories are the most uncertain and the measurements are the most difficult to make.As measurements and theoretical calculations in nuclear structure improve, significant quantitative and qualitative changes are sometimes made in our prediction of astrophysical phenomena.The availability of beams of radioactive nuclei are now making it possible to build an em-pirical foundation for studies of exploding stars,and the impact of recent can now be quickly estimated with some online software tools.Much future nuclear structure work,both experimental and theoretical,is however still needed to improve our understanding of the cosmos.
ORNL is managed by UT-Battelle,LLC,for the U.S.Department of Energy under contract DE-AC05-00OR22725.
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核天体物理中的核结构问题
Michael S.Smith1)
(美国橡树岭国立实验室物理部TN37831-6354USA)
摘要核结构信息在星体的演化和爆炸以及宇宙元素生成的研究中起着极重要的作用.原子核基态性质(如质量、寿命、衰变分支)，特别是不稳定核的性质，可以定性地和定量地改变天体物理模拟的预言.远离稳定线粒子滴线的位置和壳结构也会强烈地影响天体物理的预言.举几个例子说明新的核结构信息对于新星、X射线爆和核芯塌缩超新星的戏剧性影响.其中的某些方面来自于橡树岭实验室放射性离子束设备上的18F,82Ge和84Se 放射核束的最新实验测量结果.同时展示新一套软件工具如何帮助确定核物理研究对于天体物理的影响.
关键词核天体物理核合成放射性核束反应率超新星
1)E-mail:msmith@l.gov

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