A Fast-Cycling Magnet, 3He Cryostat Instrument for Spin- Polarized EXAFS and X-Ray MCD

用于自旋偏振 EXAFS 和 X 射线 MCD 的快速循环磁体 3He 低温恒温器仪器

• 批准号: 9317942
• 负责人: Stephen Cramer
• 金额: $ 12.69万
• 依托单位: University of California-Davis
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1994
• 资助国家: 美国
• 起止时间: 1994-06-15 至 1996-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9317942&HistoricalAwards=false
• 关键词: Fast; Cycling; Magnet; 3He; Cryostat

We propose to develop a new instrument for both x-ray magnetic circular dichroism (XMCD) and hard x-ray spin- polarized EXAFS (SPEXAFS) experiments. In 1992, our group was the first to demonstrate soft x-ray MCD on a dilute, paramagnetic bioinorganic sample, using the Fe center in the proteinPyrococcus furiosus rubredoxin. Temperatures below 4K and fields near 6 Tesla were used to fully magnetize the sample. Although this experiment was a successful proof of principal, better instrumentation is needed to fully exploit the tremendous potential of this technique. In the XMCD experiment, the absorption of left- and right- circularly polarized x-rays by a magnetized sample is compared. The resulting difference spectrum can reveal the orientation of the magnetic moment of the x-rav absorbing element. Comparison with the field direction then reveals whether this species is ferromagnetically or antiferromagnetically coupled with respect to the bulk magnetization. In the related SPEXAFS experiment, changes in the EXAFS with photon helicity are used to distinguish the spin-orientation of neighboring atoms. The combination of the two techniques thus reveals a great deal about the local magnetic structure. The goal for the new instrument is to achieve one to two orders of magnitude greater sensitivity for the XMCD and SPEXAFS of dilute bioinorganic samples. We hope to accomplish this primarily by canceling monochromator and beam instabilities with a rapidly switching magnetic field. We also hope to increase the efficiency of fluorescence photon detection by capturing a larger solid angle with a much faster detector. In consultation with commercial vendors, a design with a 1 Tesla superconducting magnet and a ~0.4 K 3He cryostat has been developed that can accomplish this goal. To conduct these experiments, we will initially employ circularity-polarized x-rays from bend magnet beamlines at the National Synchrotron Light Source (NSLS), the Stanford Synchrotron Radia tion Lab (SSRL), and the Advanced Light Source (ALS) in Berkeley. Following these initial studies we will use a more powerful elliptical wiggler source currently under development at the ALS. Better XMCD and SPEXAFS instrumentation will allow characterization of paramagnetic metal clusters in many bioinorganic systems. Mixed-metal and/or mixed-valence clusters appropriate for study include nitrogenase P- clusters and Fe- Mo cofactor, Ni-Fe centers in hydrogenase and CO dehydrogenase, and the photosystem II Mn cluster. The methods developed for these problems will be generally applicable to nearly all paramagnetic transition metal complexes.

本文提出了一种新的X射线磁圆二色性（XMCD）和硬X射线自旋极化EXAFS（SPEXAFS）实验装置。在1992年，我们的小组是第一个证明软X射线MCD的稀释，顺磁性生物无机样品，使用铁中心的蛋白质Pyrococcus furiosus rubredoxin。使用低于4K的温度和接近6特斯拉的磁场来完全磁化样品。虽然这个实验是一个成功的证明的原则，更好的仪器是需要充分利用这一技术的巨大潜力。 在XMCD实验中，比较了磁化样品对左旋和右旋圆偏振X射线的吸收。所得到的差谱可以揭示x-rav吸收元件的磁矩的取向。与磁场方向的比较，然后揭示是否这个物种是铁磁或反铁磁耦合相对于体磁化。在相关的SPEXAFS实验中，光子螺旋度的EXAFS变化被用来区分相邻原子的自旋取向。因此，这两种技术的结合揭示了大量关于局部磁结构的信息。 新仪器的目标是使稀释生物无机样品的XMCD和SPEXAFS灵敏度提高一到两个数量级。我们希望主要通过快速切换磁场来消除单色性和光束不稳定性来实现这一点。 我们还希望通过用更快的检测器捕获更大的立体角来提高荧光光子检测的效率。在与商业供应商的协商中，已经开发了一种具有1特斯拉超导磁体和~0.4 K 3 He低温恒温器的设计，可以实现这一目标。 为了进行这些实验，我们最初将采用来自国家同步加速器光源（NSLS）、斯坦福大学同步辐射实验室（SSRL）和伯克利高级光源（ALS）的弯曲磁体光束线的圆偏振X射线。在这些初步研究之后，我们将使用ALS目前正在开发的更强大的椭圆摇摆器源。 更好的XMCD和SPEXAFS仪器将允许在许多生物无机系统中的顺磁性金属簇的表征。适合研究的混合金属和/或混合价簇包括固氮酶P-簇和Fe-Mo辅因子，氢化酶和CO脱氢酶中的Ni-Fe中心，以及光系统II Mn簇。针对这些问题开发的方法将普遍适用于几乎所有的顺磁性过渡金属络合物。