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发表于 2008-8-12 16:07:27
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Thursday, July 31, 2008
Status report on six interstellar candidates
2008年7月31日
六颗星际尘埃候选者状态报告
We have finished the analysis now of six of our interstellar candidates -- one by Scanning Transmission X-ray Microscopy (STXM) at the Advanced Light Source (ALS) synchrotron at Lawrence Berkeley National Laboratory, and five by high-resolution Synchrotron X-ray Fluorescence (SXRF) Microscopy at the European Synchrotron Radiation Facility (ESRF). All candidates have also been analyzed using Fourier-Transform Infrared (FTIR) spectroscopy at the ALS. The STXM work was done in collaboration with Tolek Tyliszczak and Anna Butterworth; the SXRF work was done with Alexandre Simionovici, Laurence Lemelle, Pierre Bleuet, Peter Cloetens, and Romain Basset. The FTIR work was done in collaboration with Hans Bechtel and Sasa Bajt.
我们现在完成了对6个星际尘埃候选者的分析——一个是用伯克利劳伦斯国家实验室先进光源(ALS)同步器的X射线扫描透射显微镜(STXM),另5个是用欧洲同步辐射设备(ESRF)的高解析度同步X射线荧光显微镜(SXRF)。所有候选者都用ALS的红外傅立叶变换光谱仪分析过。
First, the mildly disappointing but unsurprising news -- it does not appear that any of our candidates are likely to be interstellar. The one that we looked at by STXM (4589365V1) was a water-clear alumina inclusion near the surface of the aerogel. Of the five that we analyzed at the ESRF, three (9471219V1, 9267050V1, 404198V1) show high Zn/Fe, one (5637295V1) shows Si, S and Ni but no Fe , and one (3602277V1) shows nothing at all in x-ray fluorescence.
首先是稍微有些失望但并不奇怪的消息——看起来没有一个候选者像是星际尘埃。我们用STXM观察的那个(4589365V1)是一个靠近凝胶表面的透明氧化铝内含物。在ESRF分析的那5个,三个(9471219V1, 9267050V1, 404198V1)表现出高Zn/Fe值,一个(5637295V1)显示出Si、S和Ni但没有Fe,另一个(3602277V1)在X射线荧光反应中什么也没显示出来。
The reason that we conclude that these are unlikely to be interstellar candidates is because Zn is a relatively rare element in the universe, while Fe is one of the most common. A high Zn/Fe ratio is most likely an indication of terrestrial material. Similarly, it is unexpected to find Ni without Fe. Lastly, it is possible that an impact could show no detectable material using these techniques if it is purely made of H, C, N and O ("organic" material), but feature 3602277V1 appears to be crater-like rather than track-like, which is unexpected.
我们之所以得出这些不太可能是星际尘埃候选者的结论是由于Zn在宇宙中是相对稀少的元素,而Fe是常见的一种元素。高的Zn/Fe比值很可能是地球物质的一种暗示。与之相似的是出乎意料的之发现了Ni而没有Fe。最后,有可能一个使用这些技术无法探测到的物质造成了冲击,如果它纯粹是由H、C、N、O组成(“有机”物),不过3602277V1的特征看起来更像弹坑而不是轨迹形式,这有些出乎意料。
While our assessment is that none of these features are likely to be interstellar, these samples will be available to investigators for more detailed analyses in the near future, and we could be surprised.
那么我们估计没有一个是星际尘埃,很快这些样本将提供给研究人员做更多详细分析,说不定会有惊喜。
But there is also good news. We have successfully demonstrated the critical techniques that will be important for this project.
不过也有好消息。我们成功的证明了评价技术对该项目将是多么的重要。
First, identification of candidates through the Stardust@home collaboration:
We know from two lines of evidence that the detection efficiency for tracks with the expected size and morphology of interstellar dust tracks is high -- first, because we have measured duster efficiency using calibration images and found the efficiency to be very high, and second, because many very thin (2 micron diameter) bona-fide off-normal tracks have been identified by Stardust@home dusters --- typically >300 dusters have identified each of these. Because of their weaker contrast, these should be more difficult to identify than high-velocity interstellar tracks.
首先,通过Stardust@home合作的识别:从两条线表明对期望的星际尘埃轨迹大小和形态的探测效率是很高的——一是由于我们使用校准图像并发现效率非常高,二是因为很多非常小(直径2微米)、真实而不寻常的轨迹被搜尘者鉴别出来——代表性的每一个都有大于300位搜尘者鉴别出来。由于其对比度较低,就要比那些高速的星际轨迹要更难于识别。
Second, we can now reliably extract tracks directly from the tray with low (but not zero) risk, although the process is slow and tedious (and fortunately mostly automated), and must be tailored to each extraction since the local topography is often complex. Dave Frank has now completed the move from Berkeley to JSC and is working hard at extractions on the interstellar tray. We now permanently preserve trajectory information right in the picokeystone using a thin artificial track.
其次,现在我们可以以低风险(但不是零)从盘中直接取出轨迹,虽然过程慢且枯燥(幸运的是几乎是自动的),并且由于局部地形总是很复杂每一个提取物都要经过裁剪。Dave Frank 现在完成了伯克利给JSC的影片,正埋头于星际尘埃盘上的提取工作。我们现在用一条细小的人造轨迹永久的保存微楔形块中弹道信息。
Third, we can analyze these candidates on at least four different synchrotron beamlines (ALS/1.4.3 FTIR, ALS/11.0.2 STXM, ESRF/ID13 SXRF, ESRF/ID22 SXRF) and next week we expect to have an analysis at the 2-ID SXRF beamline at the Advanced Photon Source (APS) at Argonne National Laboratory with Steve Sutton and George Flynn on at least two more IS candidates. We can do synchrotron fluorescence mapping with ~100nm resolution, map abundances of many major elements (including Mg and Si) and do X-ray Absorption Near-Edge Spectroscopy (XANES) using x-ray absorption mapping with ~25nm resolution, and search for organic materials with ~0.1% sensitivity in micron-scale particles using FTIR, all essentially non-destructively and in situ in the aerogel picokeystones. We expect to have a fluorescence tomography analysis done on one of the IS candidates by Alexandre Simionovici and colleagues at ESRF in the next month or so.
第三,我们可以用至少四种不同的同步光(ALS/1.4.3 FTIR, ALS/11.0.2 STXM, ESRF/ID13 SXRF, ESRF/ID22 SXRF)来分析这些候选者,下周我们期待用阿冈国家实验室先进光子源的2-ID SXRF光线来分析至少两个IS候选者。我们可以做到分辨率约100纳米的同步荧光图,绘制多种主要元素(包括Mg和Si)图;还能做近缘X射线吸收光谱(XANES),分辨率~25纳米;使用FTIR在微米尺度粒子上以~0.1%的灵敏度寻找有机物质,所有都是本质上非破坏性的对微楔形凝胶块的分析。我们期待下月在ESRF的 Alexandre Simionovici 及其同事对一颗IS候选者做的荧光断层分析。
Finally, an important point is that although none of our first candidates appear to be interstellar, it is quite amazing that we are able to say that at all. The fact that we are able to do detailed characterizations and make sensible assessments of these tiny candidates is very important. A lesson from the cometary side of Stardust is how critical but difficult it is to adequately characterize samples before they are sent out to investigators.
最后,重要的是虽然首批候选者没有一个像是星际尘埃,但我们可以对任何人说它太神奇了。事实上我们可以对这些微小的候选者进行细致的描述和明智的评估是非常重要的。来自Stardust彗星方面的教训是在样本送给研究人员之前如何充分的描述其特性,这很重要但是很困难。
Although this is mildly disappointing, it is not unexpected. We are just starting to look at our candidates, and we knew from the beginning that our signal to noise was low. These are still early days -- these candidates comprise about 6% of our current list of candidates. Statistically we could have expected to find perhaps one real interstellar track among these first candidates, so finding none is not at all surprising. We are not at all discouraged by these early results, but even more determined to press on to identify and analyze the first contemporary interstellar dust particles ever returned to earth for analysis.
虽然有些许失望,但这不意外。我们才刚开始着手这些候选者,我们从一开始就明白我们的信噪比低。现在还在早期阶段——这些只是现有候选者列表的6%。从统计上来讲我们期望在这首批候选者中多半能发现一个真正的星际尘埃轨迹,所以没有发现并不意外。对这一早期结果我们一点儿也不气馁,反而会更坚定的投入到鉴别和分析这些当代首次送回地球用以分析的星际尘埃的工作中。 |
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