In-situ X-ray tomographic imaging under extreme conditions: a proof of concept study

在高压/高温（P/T）条件下进行实验使我们能够研究地球内部深处物质的行为，这是理解地球形成和不断改造过程的关键一步。在这种条件下研究地球物质的新技术为我们了解地球的运作提供了新的视角。在x射线微断层扫描（CT）中，样品在x射线束中旋转，探测器记录下通过样品的辐射透射。与医学CAT扫描一样，数据随后被用于构建实体内部结构的3d模型，尽管CT也可以提供复杂系统中组件分布的定性数据。最重要的是，该技术是非破坏性的，并且可以通过CT多次研究样品以观察它们如何随时间演变。因此，CT非常适合于现场调查，在非环境条件下观察样品的变化。我们将测试开发一种新设备的可行性，以研究高P/T条件下材料的内部结构，同时也研究材料的变形。这项概念验证研究结果来自两项最新进展：(1)巴黎-爱丁堡旋转单元（roPEC）的开发，这是一种设计用于在高P/T下进行变形时对样品进行原位调查的设备；(2)爱丁堡大学地球科学学院开发的最先进的CT仪器，专门用于促进地质上重要材料的原位CT研究。迄今为止，只有一种仪器被开发出来，可以在极端条件下进行详细的CT。这个装置是在美国的APS同步加速器上开发的。同步加速器是一种强烈的辐射源，它产生的高能x射线比实验室的辐射源能更深入地穿透材料，非常适合于现场研究。然而，在同步加速器上获得光束时间是非常有竞争力的；原位高P/T CT研究通常需要数天时间进行一次实验，这就限制了详细的研究，并且原位高P/T CT的全部潜力尚未实现。我们希望开发一种新的设备（旋转断层扫描巴黎-爱丁堡细胞，rotoPEC），其关键优势在于它可以用于使用同步辐射和低强度实验室源的原位CT。该装置将基于roPEC，其中样品在2个碳化物砧之间加压，使用内部炉加热，并通过旋转其中一个砧来变形，但修改为允许整个样品在x射线束中完全旋转，如CT所要求的那样。roPEC是为原位研究设计的，允许x射线束以最小的，不必要的吸收到达样品。它也足够小，可以运输和安装在同步加速器源或其他实验室设备上，包括爱丁堡的CT仪器。然而，在建造rotoPEC之前，我们需要进行可行性研究。具体来说，我们将：(1)测试rotoPEC的潜力和局限性(在极端条件下，我们可以在样品中观察到多少细节……我们可以研究的材料类型有限制吗？除了测试rotoPEC的潜力外，在未来的设计中也需要这些信息；(2)测试一种x射线透明的新型砧，它可以增加CT时可以“看到”的样品体积——透明砧在变形过程中的性能是至关重要的，但尚未经过测试；(3)进一步开发用于roPEC的样品组件，以尽量减少不必要的吸收并提高样品分辨率。在进行这项工作的同时，我们还将研究两个重要的地质系统的熔体结构：橄榄岩中的富铁熔体（在行星形成的早期，变形是否帮助地球形成了一个富铁的核心？）和橄榄岩中的玄武岩熔体（岩浆是如何在洋中脊下运输的？）。

Performing experiments under high pressure/temperature (P/T) conditions allows us to study how materials behave in Earth's deep interior, a key step in understanding processes which formed and constantly reform the Earth. Developing new techniques for studying earth materials under these conditions provides new insight into the workings of our planet. In X-ray microtomography (CT) a sample is rotated in an X-ray beam and transmission of radiation through the sample recorded by a detector. As with medical CAT scanning, data is then used to construct 3d models of the internal structure of solid objects, although CT can also give qualitative data on the distribution of components in complex systems. Most importantly, the technique is non-destructive and samples can be studied by CT many times to observe how they evolve with time. As such, CT is ideally suited to in-situ investigations, where changes in a sample under non-ambient conditions are observed. We will test the viability of developing a novel device to study the internal structure of materials at high P/T conditions, and also whilst they are deforming. This proof-of-concept study results from 2 recent advances: (1) development of the rotational Paris-Edinburgh cell (roPEC), a device designed to allow in-situ investigations of samples held at high P/T whilst they are deforming, and (2) development of a state-of-the-art CT instrument in the School of GeoSciences, University of Edinburgh, which was specially designed to facilitate in-situ CT studies of geologically important materials. To date, only one instrument has been developed to perform detailed CT under extreme conditions. This device was developed at the APS synchrotron in the USA. Synchrotrons are intense radiation sources which produce high energy X-rays capable of penetrating much more deeply into materials than lab-based sources, and are well suited for in-situ investigations. However, obtaining beamtime at synchrotrons is very competitive; in-situ high P/T CT investigations typically take several days for one experiment which prohibits detailed investigations, and the full potential of in-situ high P/T CT has yet to be realised. We hope to develop a new device (rotating tomography Paris-Edinburgh Cell, rotoPEC) which has the key advantage that it can used for in-situ CT using both synchrotron radiation and lower intensity lab sources. This device will be based on the roPEC, in which samples are pressurised between 2 carbide anvils, heated using an internal furnace, and deformed by rotating one of the anvils, but modified to allow full rotation of the entire sample in an X-ray beam, as required in CT. The roPEC was designed for in-situ studies, and allows X-ray beams to reach the sample with minimal, unwanted absorption. It is also small enough to be transported and installed at synchrotron sources or on other lab equipment, including the CT instrument at Edinburgh. However, before constructing a rotoPEC we need to conduct a feasibility study. Specifically we will: (1) test the potential and limitations of a rotoPEC (how much detail can we observe in samples under extreme conditions...are there limitations in the types of material we can study?). As well as testing the potential of a rotoPEC this information is also required in its future design; (2) test a new type of anvil which is X-ray transparent and would increase the volume of sample which could be 'seen' during CT -the performance of transparent anvils during deformation is critical, but remains untested; (3) further develop sample assemblies used in the roPEC to minimise unwanted absorption and increase sample resolution. Whilst conducting this work we will also study the structure of melt in 2 geologically important systems: Fe-rich melt in peridotite (did deformation help Earth to form an Fe-rich core during early stages of planet formation?) and basaltic melt in olivine (how is magma transported beneath mid-oceanic ridges?).