An Update on Beam Line 11 Dedicated to Molecular Environmental Studies

- T. Rabedeau
- G. E. Brown

During the past few years, there has been increasing demand at the Stanford Synchrotron Radiation Laboratory by national laboratory and university scientists for beam time on high-flux, insertion-device beam lines to characterize the chemical forms (speciation) of radionuclides and toxic heavy metals in environmental samples. To help meet the needs in this new research area, DOE-BES-Chemical Sciences is funding a new wiggler beam line at SSRL (Beam Line 11) devoted to molecular environmental science.

The construction of this beam line has seen major and welcome progress on several fronts in the last year. First and foremost, the contract for a 26-pole wiggler was signed with Danfysik A/S for delivery in June of 1997. By press time, the magnetic structure design review will have been completed and the majority of the magnetic material ordered. The magnet specification details a 17.5 cm period, hybrid wiggler with a 1.9 T peak field and a field profile designed to maintain a high critical energy within the central 6 mrad of the wiggler fan.

During this summer's shutdown of SPEAR, the foundation for the wiggler was set in stone (well, concrete) in preparation for the installation of the wiggler next summer. As of this writing, the conceptual design of the wiggler vacuum chamber is well along with the final design awaiting a few last details of the wiggler vacuum chamber interface geometry. Detailed design of the remaining front end components will commence this fall with installation scheduled for the summer 1997 shutdown.

The current beam line concept envisions optics very similar to that of the XAS branch line (9-3) of the new structural biology beam line. This design employs a vertically collimating mirror for power filtering and improved energy resolution upstream of an intensively cooled double crystal monochromator. A focusing mirror is located downstream of the monochromator. To facilitate actinide research, the Beam Line 11 optics will be optimized for operations up to 23 keV. Higher energy studies, such as cadmium K edge XAS, will be conducted unfocused. The current schedule calls for commissioning of these optics in 1998.

An essential component of this new environmental sciences beam line is a controlled clean laboratory that will be built around the experimental hutch. This facility is required to permit safe sample handling, temporary storage, and XAFS spectroscopy studies on environmental samples containing radionuclides, particularly transuranics, and highly toxic or carcinogenic species. A separate DOE grant provides the funding to construct this controlled experimental area on Beam Line 11.

Because this will be a permanent installation, with all the necessary equipment for radiation monitoring and safe sample handling and containment, it will also permit increased numbers of radioactive and toxic samples to be studied without the time-consuming and inefficient set-up and dismantling times now required on SSRL Beam Line 4-2, where XAFS studies of samples containing these elements are typically carried out. We are currently exploring various design features for this clean lab with members of the Beam Line 11 Advisory Committee and plan to have a detailed plan developed in the coming months. Another DOE grant application has been funded that will provide a dedicated multi-element Ge array detector with computer controlled-electronics and high throughput for use in the XAFS studies.

When Beam Line 11 and the controlled clean laboratory are completed, a major new synchrotron radiation facility for XAFS studies of radioactive and toxic elements in natural samples, man-made waste forms, and simplified synthetic analog systems will be available to the growing number of environmental science users at SSRL. The results of these studies (speciation of contaminants; spatial distribution of different contaminant species; chemical and biochemical factors affecting transformations among species; and the kinetics of such transformations) will have a major impact on improving the technologies needed to address and solve contamination and waste management problems within the U.S. weapons complex and at numerous sites contaminated as a result of past and present agricultural and industrial activities, mining, and manufacturing.

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December 2, 1996