Engineering Diffractometer - Equipment

VULCAN Diffractometer Schematic Click image for a larger view.

At present, this instrument is tentatively named VULCAN, after the Roman god of fire and metalworking. Although the primary use of VULCAN is intended for deformation and residual stress related studies, other uses include spatial mapping of chemistry, microstructure, and texture.

The desired performance for VULCAN as determined by the user community is listed below.

• rapid volumetric (3-dimensional) mapping with a sampling volume of 1 mm³ and a measurement time of minutes
• very high spatial resolution (0.1 mm) in one direction with a measurement time of minutes
• ~20 well defined reflections for in-situ loading studies
• ability to study kinetic behaviors in sub-second times
• simultaneous characterization capabilities, including dilatometry, weight, and microstructure
• ancillary equipment such as a furnace and load frame be an integrated part of the instrument

Together, these requirements call for a "compound" engineering diffractometer with a large degree of flexibility for intensity-resolution optimization. The design philosophy is therefore to deliver a diffractometer with the highest desirable Q-resolution over a large angular range. For experiments that do not require such a high resolution, the incident beam divergence can be relaxed for intensity gain at the sample position. The key to this design is an interchangeable guide-collimator system.

This is a 44-m long instrument, consisting of a 3 m in-monolith guide, a frame definition chopper, a 20 m curved guide followed by a 12 m straight guide, and a 5 m interchangeable guide-collimator system. All guides are m=3 supermirror guides and have a cross-section of 12×50 mm².The interchangeable guide-collimator system consists of five-1 m segments, each containing two channels which is either a m=3 supermirror guide or a straight collimator. Each channel can be translated into the beam position at the push of a button. These channels are used in combinations to produce an incident beam of desirable divergence. A 1 m space is allowed between the sample and the exit of the guide-collimator system. In addition to housing the incident slit unit, this space may also be used to accommodate additional shielding or collimators. The detectors cover 60° - 150° in 2q and ±30° in the vertical plane. They are located on a locus designed to give an approximately equal resolution across the covered 2q range. The radius of the detector locus ranges from 1.5 m at 150° to 5 m at 60°.

VULCAN Diffractometer at SNS Click image for a larger view.

VULCAN can be operated flexibly for experiments having very different intensity-resolution requirements. In the high intensity mode, the m=3 supermirror channel will be employed in all segments of the guide-collimator system, effectively extending the supermirror guide all the way to 1 m from the sample. This is the preferred mode of operation for most stress mapping experiments where the specimens already exhibit significant peak broadening due to deformation, for kinetic studies where time resolution is critical, and for texture studies where a high Q-resolution is unnecessary. Note that even in this mode, the resolution is still quite good, ranging from 0.2% for 2q=150º to 0.65% for 2q=60° at d=1 Å. For comparison, the resolution for ENGIN, currently the only dedicated engineering diffractometer at a pulsed neutron source, is 0.7%. For in-situ loading studies where one needs to examine the response of individual reflections, however, it may be necessary to operate the diffractometer in the high resolution mode. In this mode, the collimator channel will be employed in all segments of the guide-collimator system, which produces a resolution of 0.10-0.22% for all detectors. This resolution is already better than that of the 148° (highest resolution) detector bank on GPPD at IPNS, which is sufficient to resolve ~ 20 peaks for cubic materials, such as Fe and Al, and for low-symmetry materials, such as Al₂O₃. Table I summarizes the performance matrix for these two vastly different operation modes. If the spatial variation is not of interest, as is the case for most in-situ loading experiments, the loss of neutron intensity can be more than compensated with the use of a large sample. For a 60 Hz target, the wavelength bandwidth is Dl~1.3 Å, which can be positioned anywhere in the incident beam spectrum. If necessary, this bandwidth can be doubled by eliminating every other pulse using the frame definition chopper, effectively operating the instrument at 30 Hz.

A 20×20 mm² two-dimensional position sensitive detector will be mounted 4 m downstream of the sample for simultaneous diffraction and small angle measurements. By using a small specimen (e.g., < 5 mm width), a poor-man’s SANS can be realized with little disruption to the base-line design. The Q-range was estimated to be 0.01 Å^-1-0.18 Å^-1. When desirable, a larger Q-range can be obtained by eliminating every other pulse using the frame definition chopper. A separate detector can be mounted at the center of the SANS detector to record transmission data. By analyzing Bragg edges in the transmission data, it is possible to obtain information about the chemistry and lattice strains in materials in the incident beam path. While methods for systematic analysis of the transmission data have not been fully developed, this technique has the potential for making measurements an order of magnitude faster than with the conventional diffraction method.

A great challenge lies in developing the technology that enables diffraction measurements with a 0.1 mm spatial resolution. Radial collimators, currently the method of choice at pulsed neutron sources for defining the sampling volume, become inefficient when used to define a 0.1 mm wide diffracted beam. This is because with today’s technology, the minimum blade thickness is 0.1 mm. As part of the research efforts, two technologies will be pursued in parallel to achieve the 0.1 mm spatial resolution, one based on reducing the blade thickness of radial collimator and the other based on a novel Bragg mirror imaging concept.

Xun-Li Wang is the SNS instrument scientist responsible for the design and construction of the Engineering Diffractometer. To find out more about him, visit his webpage. While the primary use of this instrument is intended for deformation and residual stress related studies, other uses might include spatial mapping of chemistry, microstructure, and texture. Furnace, load frame, and other auxiliary equipment for in-situ and time-resolved measurements will be an integrated part of the instrument. A workshop was held on January 20-21, 2000 in Atlanta to define the required performance matrix.

The following documents contain more detailed information about the expected performance and design of the diffractometer:
Science Case For the Vulcan Diffractometer (PDF 99.1KB)
Summary of the Breakout Session on Basic Mechanical Properties (PDF 200KB)
Conceptual Design Report (PDF 201KB)
IDT Meeting Report (March 7, 2001)
IDT Meeting Report #1 (Nov. 18-19, 2002), Oak Ridge — T. M. Holden
Instrument data sheets (PDF)

The following personnel are currently working on the diffractometer:
rennichgq@sns.gov, Engineer
turnerb@sns.gov, Designer