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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >A low initial abundance of (CM)-C-247 in the early solar system and implications for r-process nucleosynthesis
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A low initial abundance of (CM)-C-247 in the early solar system and implications for r-process nucleosynthesis

机译：早期太阳系中（CM）-C-247的低初始丰度及其对r过程核合成的影响

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The short-lived nuclide Cm-247 is produced by reprocess nucleosynthesis. When the presolar nebula formed, 247 Cm became isolated from r-process production and its abundance diminished as a result of radioactive decay. Given its short half-life of only similar to 16 million years, Cm-247 is presently extinct, but its former presence should be detectable as small variations in U-235/U-238 in primitive meteoritic material, provided Cm was chemically fractionated from U at the time these solid objects formed. The magnitude of U isotopic anomalies in meteorites can thus be used to elucidate the timing and character of the last r-process nucleosynthetic event for input into models describing the formation and evolution of the early solar system. Here we report coupled U isotopic determinations and Nd/U proxy measurements for Cm[U in a series of acid-etched leachates and mineral assemblages extracted from meteorites containing primitive phases expected to show strong Cm-U fractionations. Using multiple-collector ICPMS, we are able to determine U-235/U-238 with 2 sigma analytical uncertainties of 1 epsilon (1 epsilon = 1 part in 10,000) on sample sizes consisting of < 3 ng of U-238 and < 20 pg of U-235. A double-spiking procedure using a mixed U-236-U-233 spike was employed to allow instrumental mass fractionation to be reliably corrected intemally and at high precision. Uranium isotopic results for almost 40 different phases show no resolvable deviations in U-235/U-238 from the chondritic value, at the similar to 1-2 epsilon level. These data supplement our previous observations for a suite of bulk meteorite samples [C.H. Stirling, A.N. Halliday, D. Porcelli, In search of live Cm-247 in the early solar system, Geochim. Cosmochim. Acta 69 (2005) 1059-1071] and provide evidence for a solar system initial Cm-247/U-235 of < 8 X 10(-5). Such a low value is difficult to explain without a long time-scale of similar to 2.3 X 10(8) years between the last actinide producing r-process event and the formation of the solar system. As such it is difficult to reconcile with a model of actinide production in the same r-process forming event as Hf-182 with a half-life of 8.9 My [G.J. Wasserburg, M. Busso, R. Gallino, Abundances of actinides and short-lived nonactinides in the interstellar medium: Diverse supernova sources for the r-processes, Astrophys. J. 466 (1996) L109-L113]. The alternative models of Hf-182 production via a neutron-rich fast s-process, occurring, for example, in the helium burning shell in a 25 solar mass star during explosive nucleosynthesis [G.J. Wasserburg, M. Busso, R. Gallino, Abundances of actmides and short-lived nonactinides in the interstellar medium: Diverse supernova sources for the r-processes, Astrophys. J. 466 (1996) L109-L113], or via a distinct r-process event that is separate from actinide production [G.J. Wasserburg, M. Busso, R. Gallino, K.M. Nollet, Short-lived nuclei in the early solar system: Possible AGB sources, Nucl. Phys. A (in press)], may provide a viable explanation. However, further studies are also required to assess the veracity of Cm-U systematics, which are critically dependent on the suitability of using Nd and the light rare earth elements (LREEs) as a chemical proxy for Cm. (c) 2006 Elsevier B.V. All rights reserved.
机译：短寿命核素Cm-247是通过再加工核合成产生的。当太阳前星云形成时，247 Cm从r过程产生中分离出来，由于放射性衰变，其丰度降低了。由于Cm-247的短半衰期仅接近1600万年，目前已灭绝，但只要Cm是从化学成分中分离出来的，原始的陨石中U-235 / U-238的微小变化就应该可以检测到它的存在。这些固态物体形成时的U。因此，陨石中U同位素异常的大小可用于阐明最后r过程核合成事件的时间和特征，以输入到描述早期太阳系形成和演化的模型中。在这里，我们报告了从一系列陨石中提取的Cm [U的U同位素测定和Nd / U替代测量的耦合，Cm [U是从陨石中提取的，这些陨石含有原始相，预期显示出强Cm-U分级。使用多收集器ICPMS，我们能够确定U-235 / U-238的2σ分析不确定度为1 epsilon（1 epsilon = 10,000的1份），样本量包括<3 ng U-238和<20 U-235页。采用混合U-236-U-233尖峰的双尖峰程序可以使仪器的质量分数在内部和高精度下可靠地得到校正。几乎40个不同阶段的铀同位素结果显示，在U-235 / U-238中，软骨素值没有可分辨的偏差，类似于1-2ε的水平。这些数据补充了我们先前对一系列大块陨石样品的观察结果。斯特灵（美国） Halliday，D. Porcelli，寻找早期太阳系Geochim中的Cm-247活体。宇宙猫Acta 69（2005）1059-1071]，并提供了太阳系初始Cm-247 / U-235≤8 X 10（-5）的证据。在最后一次产生act系元素的r-过程事件与太阳系形成之间，没有类似于2.3 X 10（8）年的长时间尺度，很难解释这么低的值。因此，在与半衰期为8.9 My的Hf-182相同的r加工过程中，很难与act系元素的产生模型相吻合。 Wasserburg，M.Busso，R.Gallino，星际介质中ides系元素和短寿命非ides系元素的丰富度：r过程的不同超新星源，天体。 J. 466（1996）L109-L113]。通过富中子快速S-过程生产Hf-182的替代模型，例如，在爆炸性核合成过程中，发生在25太阳质量恒星中的氦燃烧壳中[G.J. Wasserburg，M.Busso，R.Gallino，星际介质中大量的act系元素和短寿命的非act系元素：r过程的超新星源，天体。 J. 466（1996）L109-L113]，或通过与act系元素生成不同的独特r过程事件[G.J.瓦瑟堡，M。布索，R。加利诺，K.M。 Nollet，早期太阳系中的短寿命原子核：可能的AGB来源，Nucl。物理A（印刷中）]，可能提供可行的解释。但是，还需要进一步的研究来评估Cm-U系统的准确性，这主要取决于使用Nd和轻稀土元素（LREEs）作为Cm的化学替代物的适用性。 （c）2006 Elsevier B.V.保留所有权利。

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