WLW原子核质量模型在γ过程研究中的应用

本工作利用最近提出的WLW宏观-微观原子核质量模型,在经典r过程框架下很好地描述了太阳系r过程核素丰度分布.与常用的FRDM质量模型相比,新计算更好地再现了A≈135和A≈180质量区的太阳系r过程核素丰度,尤其避免了其他相似计算中A=135附近出现的与观测不符的丰度峰问题.分析表明,这种差异可能暗示WLW更为合理地处理了壳结构和对称能.     

贫金属星的中子俘获元素

利用分量系数公式并结合太阳系元素丰度的最新数据,计算了6?4颗贫金属星的中子俘获元素丰度.结果显示,快中子俘获过程(r过程)是贫金属星中子俘获元素的主要来源,且金属丰度小于–2.5的贫金属星的重中子俘获元素产量可能来自于纯r过程;重中子俘获元素与轻中子俘获元素来自于不同的天体物理场所     

太阳系的元素丰度分布与AGB星慢中子俘获元素

通过对AGB星演化模型的理论计算结果和51颗AGB星的观测丰度进行重新分析,发现任何AGB星与慢中子俘获过程(s过程)主要分量对应的重元素(简称SMH元素)丰度分布都与对应的太阳系s过程主要分量的元素丰度分布相似,这表明,任意AGB星SMH元素丰度分布的迭加结果与对应的太阳系s过程主要分量的元素丰度分布相似,由此得出结论:太阳系s过程主要分量的重元素丰度分布模式是一个典型的模式,可以作为标准用于单星重元素丰度的研究.     

一类生灭过程及其在核天体物理计算中的应用

研究一类生灭过程,利用矩阵分解技巧求出了生灭过程微分方程的解,并将之应用于核天体物理中太阳系慢中子俘获过程重元丰度计算问题研究,计算结果与天文观测值一致.     

中能质子入射的裂变产物质量分布计算

采用量子分子动力学(QMD)、统计衰变模型(SDM)和半经验的多模裂变模型方法计算了能量在200MeV附近的中能质子入射重核引起裂变的裂变产物质量分布,得到了与实验相符合的结果;同时对锕系核素和非锕系重核素分别给出了一组合理的多模裂变模型参数.     

质子滴线附近的β缓发质子衰变

简要回顾了实验小组在过去8年中获得的实验结果, 即采用氦喷嘴快速带传输系统 +“p-γ”符合方法, 在稀土区质子滴线附近首次观测了9种新核素的β缓发质子衰变, 在A=90核区的N=Z线附近获得了5种核素的β缓发质子衰变的新数据. 并把这14种核素的半衰期, 自旋宇称, 形变以及生成反应截面的实验值与流行的核模型理论预言进行了系统地对比讨论. 从中看出:(1)⁸⁵Mo, ⁹²Rh以及“等待点”核⁸⁹Ru和⁹³Pd半衰期的实验值比近期Moller等人的宏观-微观理论预言值[At. Data Nucl Data Tables, 66, 131(1997)]长5—10倍, 因而明显地影响天体rp过程生成的核素丰度;(2)实验指认的质子滴线核¹⁴²Ho和¹²⁸Pm的自旋宇称与流行理论预言不符, 但用Woods-Saxon-Strutinsky方法可以计算得到相符的位能面;(3)实验估计的9种稀土核的生成截面比通用的Alice和HIVAP程序的计算值要小1—2个数量级.     

42MeV/u 12C与115In相互作用的靶余核

使用放射化学方法测定了42MeV/u ¹²C与¹¹⁵In相互作用靶余核的生成截面,得到了质量分布及同位素分布.实验得到的质量分布与使用级联的两体衰变模型GEMINI程序计算的结果很好地相符.根据同位素分布的系统性,对利用中能重离子反应生成新的远离β稳定线的缺中子核素进行了讨论.     

关于极端原子核的相对论自洽角动量投影壳模型计算(英文)

相对论自洽角动量投影壳模型是最近发展出来的一个自洽模型, 它对于不同核区具有稳定的参数, 能够很好地描述已知和未知具有稳定形变的各种原子核的性质。 计算了若干包括稳定核素、 极端核素和超重核素, 并把计算结果与现有的实验数据进行了比较。     

等待点N=82附近核素β~-衰变寿命的研究

利用提出的远离稳定线附近的原子核β-衰变寿命的指数规律理论计算公式,对N=82附近快中子过程中等待点核素的β-衰变寿命进行了理论计算,比较了所获得的计算结果与最新的理论结果和实验结果并加以讨论.研究表明,相对于理论复杂和计算时间长的微观理论计算而言,利用考虑壳效应的远离稳定线的原子核β-衰变寿命指数规律理论计算公式能较快且准确地得出快中子俘获过程(R过程)等待点核素的β-衰变寿命.这能为R过程核素合成网络计算研究提供有效可靠的重要物理输入,并对今后天体中核素的合成研究具有重要意义.     

低能中子的俘获γ射线能谱

给出了计算共振区平均中子俘获r能谱的方案。主要考虑了三种反应机制:统计过程、位阱俘获与复弹性道中的俘获脚。具体计算了3s共振区几个核素⁵¹V,⁵²Cr,⁵⁵Mn的热中子俘获r能谱和r光子多重数。结果表明,包括了复弹性道中的俘获这一过程后,理论与实验的符合得到明显的改善,特别是对能谱的高能端。     

R-process nucleosynthesis and nuclei far from the region of β-stability

The gross theory of β-decay is applied to make improved estimates of the absolute magnitude of β-decay half-lives and related variables in the r-process calculation. The time necessary for synthesis seems to be considerably longer than assumed in some of the recent studies of the dynamic r-process. This means the necessity of suitable re-scaling of the expansion rates in the dynamics. Although the application of the improvements to the dynamics is most interesting, the present paper has limited itself to the classical quasi-static model of Seeger, Fowler and Clayton in performing the abundance calculation. The calculation method of the final abundance curve can allow several nuclear processes to interrupt β-decay cascades before the β-stability region is reached. The sudden freezing of neutron flux and temperature is assumed to precede these processes. In particular, the effect of β-delayed neutron emission after freezing is extensively studied. The β-delayed fission is also discussed. The calculated β-strength functions are compared with the recent experimental data obtained at the ISOLDE and OSIRIS facilities.     

On the delayed neutrons at the final stage of the r-process

The gross theory of beta decay is applied to the r-process calculation. In particular, it is explicitly shown that the delayed-neutron emissions at the final stage of the r-process smooth out the abundance curve significantly.     

The r-process of stellar nucleosynthesis: Astrophysics and nuclear physics achievements and mysteries

The r-process, or the rapid neutron-capture process, of stellar nucleosynthesis is called for to explain the production of the stable (and some long-lived radioactive) neutron-rich nuclides heavier than iron that are observed in stars of various metallicities, as well as in the solar system.

A very large amount of nuclear information is necessary in order to model the r-process. This concerns the static characteristics of a large variety of light to heavy nuclei between the valley of stability and the vicinity of the neutron-drip line, as well as their beta-decay branches or their reactivity. Fission probabilities of very neutron-rich actinides have also to be known in order to determine the most massive nuclei that have a chance to be involved in the r-process. Even the properties of asymmetric nuclear matter may enter the problem. The enormously challenging experimental and theoretical task imposed by all these requirements is reviewed, and the state-of-the-art development in the field is presented.

Nuclear-physics-based and astrophysics-free r-process models of different levels of sophistication have been constructed over the years. We review their merits and their shortcomings. The ultimate goal of r-process studies is clearly to identify realistic sites for the development of the r-process. Here too, the challenge is enormous, and the solution still eludes us. For long, the core collapse supernova of massive stars has been envisioned as the privileged r-process location. We present a brief summary of the one- or multidimensional spherical or non-spherical explosion simulations available to-date. Their predictions are confronted with the requirements imposed to obtain an r-process. The possibility of r-nuclide synthesis during the decompression of the matter of neutron stars following their merging is also discussed.

Given the uncertainties remaining on the astrophysical r-process site and on the involved nuclear physics, any confrontation between predicted r-process yields and observed abundances is clearly risky. A comparison dealing with observed r-nuclide abundances in very metal-poor stars and in the solar system is attempted on grounds of r-process models based on parametrised astrophysics conditions. The virtues of the r-process product actinides for dating old stars or the solar system are also critically reviewed.     

Examination of n-T_9 conditions required by N=50,82,126 waiting points in r-process

We examined the conditions of neutron density(n) and temperature(T_9) required for the N = 50, 82,and 126 isotopes to be waiting points(WP) in the r-process. The nuclear mass based on experimental data presented in the AME2020 database(AME and AME ±Δ) and that predicted using FRDM,WS4, DZ10, and KTUY models were employed in our estimations. We found that the conditions required by the N = 50 WP significantly overlap with those required by the N = 82 ones, except for the WS4 model. In addition, the upper(or lower) bounds of the n-T_9 conditions based on the models are different from each other due to the deviations in the two-neutron separation energies.The standard deviations in the nuclear mass of 108 isotopes in the three N = 50, 82, and 126 groups are about rms = 0.192 and 0.434 Me V for the pairs of KTUY-AME and WS4-KTUY models,respectively. We found that these mass uncertainties result in a large discrepancy in the nn-T_9 conditions, leading to significant differences in the conditions for simultaneously appearing all the three peaks in the r-process abundance. The newly updated FRDM and WS4 calculations can give the overall conditions for the appearance of all the peaks but vice versa for their old versions in a previous study. The change in the final r-process isotopic abundance due to the mass uncertainty is from a few factors to three orders of magnitude. Therefore, accurate nuclear masses of the r-process key nuclei, especially for ~(76) Fe,~(81)Cu,~(127)Rh,~(132)Cd,~(192)Dy, and ~(197)Tm, are highly recommended to be measured in radioactive-ion beam facilities for a better understanding of the r-process evolution.     

Long, cold, early r process? Neutrino-induced nucleosynthesis in He shells revisited

We revisit a ν-driven r-process mechanism in the He shell of a core-collapse supernova, finding that it could succeed in early stars of metallicity Z ? 10?3 Z(⊙), at relatively low temperatures and neutron densities, producing A ~ 130 and 195 abundance peaks over ~10-20 s. The mechanism is sensitive to the ν emission model and to ν oscillations. We discuss the implications of an r process that could alter interpretations of abundance data from metal-poor stars, and point out the need for further calculations that include effects of the supernova shock.     

Challenge on the Astrophysical R-Process Calculation with Nuclear Mass Models

Our understanding of the rapid neutron capture nucleosynthesis process in universe depends on the reliability of nuclear mass predictions. Initiated by the newly developed mass table in the relativistic mean field theory (RMF), we investigate the influence of mass models on the r-process calculations, assuming the same astrophysical conditions. The different model predictions on the so far unreachable nuclei lead to significant deviations in the calculated r-process abundances.     

Spin-dependent nuclear weak processes and nucleosynthesis in stars

Spin dependent nuclear weak processes and nucleosynthesis in stars are investigated based on recent advances in shell model studies of stable and unstable exotic nuclei. Three topics on (1) neutrino-nucleus reactions in supernova explosions and nucleosynthesis of light elements as well as Mn, (2) electron capture reaction rates on Ni and Co isotopes at high densities and temperatures in the core-collapse process, and (3) new β-decay half-lives of N=126 isotones obtained by including both the Gamow-Teller and the first-forbidden transitions, and the effects on the element abundance in the r-process at the third peak region (A∼195), are studied with the use of new shell model Hamiltonians.     

Probabilities of delayed processes for nuclei involved in the r-process

Delayed fission, along with induced and spontaneous fission, is responsible for the suppression of the production of superheavy elements both during the r-process and after its completion. Beta-decay strength functions are required for calculating delayed fission. In the present study, respective strength functions are calculated by relying on the theory of finite Fermi systems and by predominantly employing nuclear masses and fission barriers predicted by a generalized Thomas-Fermi model. The probabilities for delayed fission and for the emission of delayed neutrons are calculated for a number of isotopes. On the basis of calculations performed in order to determine the probabilities for delayed processes, it is shown that some of the delayed-fission probabilities calculated thus far were substantially overestimated. The application of these new results to calculating the r-process may change substantially both the r-process path and the yields of superheavy nuclei.