30 J. Appl. Cryst. (1991). 24, 30-37 High-Resolution Small-Angle X-ray Scattering Camera for Anomalous Scattering BY G. G. LONG National Institute of Standards and Technology, Gaithersburg, MD 20899, USA P. R. JEMIAN AND J. R. WEERTMAN The Technological Institute, Northwestern University, Evanston, IL 60208, USA AND D. R. BLACK, H. E. BURDETTE AND R. SPAL National Institute of Standards and Technology, Gaithersburg, MD 20899, USA (Received 1 May 1990; accepted 24 August 1990) Abstract The design and operation of a new small-angle X-ray scattering instrument, optimized for high throughput at a synchrotron source, high angular and wave- length resolution, large sample cross-sectional area, accurate energy tuning, excellent signal-to-noise ratio and harmonic rejection are presented. The principles of design and implementation are given, as are the details of primary calibration of absolute intensity and experimental desmearing. The instrument has been tested for application to anomalous-scattering measurements near the chromium K edge. Prelim- inary results on samples of a heat-treated steel are presented as a demonstration of the capability of this experiment to separate the microstructure evolution as a function of temperature of a chromium-rich precipitate from the thermal behavior of other preci- pitates in the steel. 1. Introduction The small-angle scattering curve and its transform lead in a direct manner to the determination of many important microstructure parameters such as particle radius of gyration, particle volume, particle shape and total surface area. For this reason, small-angle X-ray scattering (SAXS) has been used since the 1940's in a wide range of applications in metallurgy, polymer science and structural biology. Over the past decade, these applications have expanded to include time-resolved microstructural measurements and dif- ferential contrast anomalous-diffraction measure- ments thanks to the availability of intense collimated white sources of X-rays from storage rings. For the three major classes of SAXS experiments - real time, static and differential contrast - the experi- mental requirements are markedly different. Optimal utilization of the X-ray source depends on the scien- 0021-8898/91/010030-08503.00 tific problem that is being addressed (Koch & Bordas, 1983). In particular, the highest priority may be high X-ray flux at the sample, or angular resolu- tion, or narrow-wavelength bandpass. For this reason, one finds many solutions to the problem of creating an optimized SAXS experiment. In real-time applications, the wavelength resolution, Ah./a, is gen- erally made as large as possible, commensurate with the required angular resolution, so that the instru- ment will deliver the highest flux at the sample. For measurements in metallurgy and ceramic science, high angular resolution is often of major importance, where high resolution refers to a small minimum achievable scattering vector hmin, with h = 4rr sinO/A, 20 is the scattering angle and ,~ is the photon wave- length. To obtain a low value for hmin, either slits or crystals are used. If good wavelength resolution (i.e. A~./~."10 -4) is also available, then anomalous SAXS (or differential contrast SAXS) offers the additional possibility of separating the scattering caused by a particular scattering entity from the total scattering in a complex material. Thus it becomes possible, for example, to follow the influence of various forms of high-temperature service (simple aging, creep, cyclic stresses) on the different preci- pitates in a complicated alloy. The geometry of the new SAXS instrument for anomalous scattering that forms the subject of the present paper is described in §2. This instrument delivers photon fluxes of approximately 10 ~° photons s -I in a 3 × 3 mm area at the sample position, AA/A = 6 x 10-4 and hmin =0"005 nm- ~ over the energy range of incident photons from 5 to 11 keV. The camera was optimized to make best use of the most important optical element, the X-ray source. Measurements over the range 0-005 ___ h ___ 2 nm- require approximately 30-40min. This type of operation is made possible through the use of a PIN photodiode detector and its custom-designed electro- © 1991 International Union of Crystallography