Towards Neutron Capture on Exotic Nuclei: Demonstrating (d,pγ) as a Surrogate Reaction for (n,γ)

Abstract

The neutron-capture reaction plays a critical role in the synthesis of the elements in stars and is important for societal applications including nuclear power generation and stockpile-stewardship science. However, it is difficult -if not impossible -to directly measure neutron capture cross sections for the exotic, short-lived nuclei that participate in these processes. In this Letter we demonstrate a new technique which can be used to indirectly determine neutron-capture cross sections for exotic systems. This technique makes use of the (d, p) transfer reaction, which has long been used as a tool to study the structure of nuclei. Recent advances in reaction theory, together with data collected using this reaction, enable the determination of neutron-capture cross sections for short-lived nuclei. A benchmark study of the 95 Mo(d, p) reaction is presented, which illustrates the approach and provides guidance for future applications of the method with short-lived isotopes produced at rare isotope accelerators.Essentially all of the heavy elements are synthesized in astrophysical environments by processes that involve neutron capture. The slow neutron-capture process (the s process) occurs predominantly in the low neutron flux in AGB stars, yielding a nucleosynthesis path that typically deviates only one or two neutrons from β-stability. In contrast, the rapid neutron-capture process (the r process) involves exotic neutron-rich nuclei and requires explosive stellar scenarios with high neutron fluences. The r process is responsible for the creation of roughly half of the elements between iron and bismuth and synthesizes heavy nuclei through the rapid production of neutronrich nuclei via neutron capture and subsequent β decay.The recent observation of the gravitational waves associated with a neutron-star merger [1], and the subsequent kilonova understood to be powered by the decay of lanthanides [2,3], demonstrated that neutron-star mergers are an important r -process site, especially for the heaviest elements. However, r -process abundance patterns are sensitive to astrophysical conditions (cf. [4]). In a "cold" r process (which could occur in a neutron star merger or with the highly accelerated neutrino-driven winds following a core-collapse supernova), equilibrium between neutron capture (n, γ) and photo-dissociation (γ, n) rapidly breaks down, so the rate at which neutron capture proceeds will affect the final r -process abun-dance pattern. The timescales of the cold r process are such that competition between neutron capture and β decay occurs during the bulk of the r -process nucleosynthesis. Neutron-capture rates on unstable nuclei affect the final observed abundance patterns even in the traditional "hot" r process (thought to occur in the neutrinodriven winds in a proto-neutron star resulting from a core-collapse supernova) during the eventual freeze-out, when (n, γ) (γ, n) equilibrium no longer occurs. Accordingly, neutron capture is influential in determining the final r -process abundance pattern, especia...

Simultaneous Determination of Neutron-Induced Fission and Radiative Capture Cross Sections from Decay Probabilities Obtained with a Surrogate Reaction

“…In particular, we have used the 240 Puð 4 He; 4 He 0 Þ 240 Pu à reaction as a surrogate for the n þ 239 Pu reaction. We also stress a significant difference with respect to [25] and [26], namely that, instead of measuring the probabilities for some selected γ-ray transitions, we measure the γ-emission probability P s;γ , i.e., the probability that the CN releases its entire E à by emitting a cascade of γ rays. Using P s;γ can lead to a more reliable determination of the HF model parameters since it does not involve modeling the details of the γ-ray cascade, which requires including the branching ratios of many low-lying transitions that are often unknown.…”

“…Using P s;γ can lead to a more reliable determination of the HF model parameters since it does not involve modeling the details of the γ-ray cascade, which requires including the branching ratios of many low-lying transitions that are often unknown. This is, however, much less critical for an eveneven CN, as considered in [26], because there is generally a strong-collecting 2 þ → 0 þ γ-ray transition. Yet, measuring the P s;γ of fissionable nuclei is very challenging as it requires removing the intense background of γ rays emitted by the fission fragments.…”

Research program of nuclear astrophysics based on the HIAF

“…在"冷"r-过程中, 温 度相对比较低, 导致中子俘获(n, γ)反应和光致裂解(γ, n)反应没有处于平衡状态, 因此(n, γ)反应将影响最终 的r-过程核素丰度. 即使在"热"r-过程中, 在最终冻结 期间, (n, γ)和(γ, n)反应也处于不平衡状态, 因此丰中 子核的(n, γ)反应将对r-过程元素丰度产生重要影 响 [25,26] . 这都表明, 丰中子核(n, γ)反应率的实验测量 对于理解r-过程发生的天体场所及重元素丰度起源仍 有重要意义 [27] .…”

“…[37] . 另一种方式是通过β衰变来布居目标复合 核的激发态, 通过测量复合核的中子衰变 [38] 或伽马跃 迁来 [26,40] . 替代反应需 要精确测得复合核的数目, 而靶中的杂质和其他一些 出射道容易干扰复合核数目的测量.…”

“…Ne, 19 Ne(p, γ) 20 Na, 56 Ni(p, γ) 58 Cu, 25 Al(p, γ) 26 Si, 14 21 Na(图11)等反应的研究 [47][48][49] , 为将来在HIAF开展 更丰质子核区的相关反应奠定了基础. [50] , 包括近质子滴线 短寿命核素 63 Ge, 65 As, 67 Se和 71 Kr的质量 [51] 和缺中子 短寿命原子核(尤其是 存在的Ca-Sc循环 [52] .…”