Departamento de Física Aplicada II, Universidad de Sevilla. ETSA, Seville, Spain.
Departamento de Física Aplicada I. Universidad de Sevilla. ETSIA, Seville, Spain.
J Environ Radioact. 2019 Nov;208-209:106016. doi: 10.1016/j.jenvrad.2019.106016. Epub 2019 Jul 17.
Theoretical and experimental studies have shown that activity concentrations of fallout radionuclides (such as Cs and excess Pb) decrease with particles size in aqueous suspensions. This paper is aimed at reviewing the theoretical fundamentals for granulometric speciation of radionuclides, and at exploring its practical use in the analytical context of gamma spectrometry for the radiometric dating of recent sediments, with view to: i) improving the detection of Cs (since its use as independent chronostratigraphic mark is challenging in the southern hemisphere because its low fallout rate), ii) supporting refined CIC models and normalization techniques in Pb-based radiogeochronologies. The work uses surface sediments sampled from the Tinto Estuary (SW Spain), affected by mining and phosphate-fertilizer industries, and from the Ankobra Estuary (Ghana), affected by intensive artisanal gold-mining. Granulometric classes have been separated by a sieving column with decreasing mesh sizes and the obtained cumulative percentage of mass mathematically described by a Rosin-Rammler particle-size distribution. The target radionuclides for gamma spectrometry were Pb, Ra andCs, complemented with K, Th and Ra. Results revealed that, far from ideal experiments, under actual environmental conditions the increase in activity concentrations with decreasing particle sizes is too moderate, and in general they are affected by larger counting uncertainties due to the small available amount of mass. Indeed, there was no correlation between grain-size and Cs concentrations (p = 0.25), and similarly for excess Pb (p = 0.53). No effect of the organic matter content was observed in Cs (p = 0.58) and excess Pb (p = 0.85) concentrations. Present results pose some concerns to the general use of granulometric speciation in the context of gamma spectrometry for supporting the radiometric dating of recent sediments. A detailed discussion on the use of normalization methods is also presented.
理论和实验研究表明，在水悬浮液中，沉降核素（如 Cs 和过剩 Pb）的活度浓度随颗粒大小的减小而降低。本文旨在综述放射性核素粒度形态分析的理论基础，并探讨其在伽马谱分析中应用于近期沉积物放射性测年的实际意义，以期：i）提高 Cs 的检测能力（因为在南半球，由于其沉降率低，Cs 作为独立的年代地层标记的使用具有挑战性），ii）支持基于 Pb 的放射性年代计中精细的 CIC 模型和归一化技术。本工作使用取自受采矿和磷肥工业影响的西班牙廷托河口（Tinto Estuary）和受密集手工采金影响的加纳安克布拉河口（Ankobra Estuary）的表层沉积物作为研究对象。采用具有递减网眼尺寸的筛分柱分离粒度级，获得的质量累积百分比用罗辛-拉姆勒（Rosin-Rammler）粒度分布进行数学描述。用于伽马谱分析的目标放射性核素为 Pb、Ra 和 Cs，同时还补充了 K、Th 和 Ra。结果表明，与理想实验相比，在实际环境条件下，随着颗粒尺寸的减小，活度浓度的增加幅度适中，而且由于可获得的质量非常少，通常受到更大的计数不确定性的影响。实际上，粒级与 Cs 浓度之间没有相关性（p=0.25），过剩 Pb 也是如此（p=0.53）。Cs（p=0.58）和过剩 Pb（p=0.85）浓度与有机物含量之间没有关系。目前的结果对伽马谱分析中粒度形态分析在支持近期沉积物放射性测年方面的普遍应用提出了一些关注。还对归一化方法的应用进行了详细讨论。
