May 27, 2004

Cullie Sparks: In-Memoriam

Cullie Sparks, a charter member of the NSLS users group and one of the first Chairmen of the users executive committee, died March 19 2004. Dr. Sparks made major contributions to materials science, x-ray physics and synchrotron science that continue to have a worldwide impact. He was a particularly enthusiastic supporter of synchrotron radiation and threw himself wholeheartedly into developing beamline X14 at the NSLS when the NSLS was still on the drawing board.

After earning a metallurgical Ph.D. from the University of Kentucky, Sparks joined the Oak Ridge National Laboratory in 1957. In the mid 60’s, Sparks and his group leader, Bernard Borie, used symmetry to interpret variations in the weak so called diffuse x-ray scattering from crystalline alloys. By quantitatively studying patterns in the diffuse x-ray scattering, they found that local structural fluctuations could be measured with unprecedented sensitivity. This early research explored the tendency for some materials to have a long-ranged average structure but with important nanoscale fluctuations about the average. Later this research led to elegant experiments at the NSLS that measured chemically specific bond distances to less than a tenth of a picometer (<0.001 Å).

Their Borie-Sparks method for analyzing diffuse X-ray and neutron scattering is still used worldwide to interpret diffuse neutron scattering and forms the basis for modern diffuse X-ray techniques. Research based on their pioneering work continues at Department of Energy major facilities including the Advanced Photon and National Synchrotron Light Sources and ORNL's High Flux Isotope Reactor.

Also in the 60’s, Dr. Sparks recognized the potential of artificial graphite crystals for high-performance X-ray and neutron monochromators. He worked with researchers at Union Carbide Corporation, to perfect the manufacture and performance of graphite monochromators and became the world's expert on mosaic graphite optics. By combining the natural tendency of mosaic graphite crystals to focus in the plane-of-scatter with out-of-plane focusing based on curved surfaces, he created powerful doubly-focusing crystal optics. Carbide continues to make a range of graphite monochromators that are used worldwide both for x-rays and neutrons.

Armed with a vastly more powerful way to produce intense x-ray beams, Sparks began a systematic search for inelastic x-ray scattering contributions that might contributed background in his diffuse scattering measurements. His careful research uncovered an unsuspected resonant-inelastic scattering mechanism. Although a respected reviewer from Bell Labs, who specialized in inelastic x-ray scattering, could find no fault in Sparks' 1974 Physical Review Letters paper, the reviewer personally performed definitive synchrotron experiments at SSRL to check Sparks' results -and verified, much to his surprise, that Sparks was correct! Resonant Raman X-ray Scattering or “Sparks Scattering” is still widely used to study the dynamics of X-ray-induced atomic transitions.

In 1976, proton microprobe measurements on monozite inclusions with anomalously large halos in micas from Madagascar, indicated the presence of primordial superheavy elements. This “discovery” reverberated throughout the scientific community, as the presence of primordial superheavy elements suggested that the earth might be only a few thousand years old, a compact atomic weapon might be made of these unusual elements, and even the shape of the nucleus might differ from standard materials.

In a crash program to settle the issue, Sparks designed the first synchrotron-based X-ray fluorescence microprobe and led a team of distinguished scientists that installed and executed the critical test at the Stanford Synchrotron Radiation Laboratory. Sparks and his team convincingly showed that primordial superheavy elements do not exist in these micas. As a result, the U.S. and other governments avoided the enormous resources that might otherwise have been expended to understand something for which there is no evidence. This experiment clearly illustrated the critical need for intense synchrotron radiation sources.

In the summer of 1979, during a sabbatical at BNL, Dr. Sparks began studying how to focus x-rays with bent perfect crystals. He was motivated by the fact that crystals, with roughly a 20 x larger scattering angle than mirrors, can collect much larger divergences and focus them onto a sample. Although Dr. Sparks was greatly challenged by computer programming, he worked with scientists at BNL and ORNL to study ways to utilize the potential of crystal focusing. He discovered that, in a nondispersive geometry, the Bragg angle of each ray reflected from a flat crystal is virtually the same off a crystal bent to focus at magnification=1/3. This discovery opened the possibility of dynamically bent sagittal focusing optics. Despite major technical challenges and a general consensus that the method could not work, sagittal focusing optics were demonstrated paving the way for beamline X14. Sagittal focusing optics are now installed in multimillion dollar facilities around the world providing 20 times greater X-ray intensity than alternative focusing methods.

Not only were the sagittal focusing optics widely adopted by the worldwide synchrotron community, but the efficiency and flexibility of sagittal focusing optics on beamline X14 also led to numerous exciting collaborations. First or early measurements included fluorescence tomography, resonant magnetic scattering, multiple wavelength holography, anomalous powder and diffuse x-ray scattering, nuclear resonant scattering, glancing angle scattering from lipids, scattering from liquid crystals, quasi crystals and other experiments.

In addition to his other contributions at the NSLS, in 1985 Cullie and three associates contributed the after dinner show at the NSLS users meeting. This legendary performance of bluegrass songs with synchrotron themes was topped off with a round of authentic moonshine for those NSLS users and staff brave enough to try.

In short, Sparks was a gifted experimentalist and a good friend of the NSLS. His scientific legacy continues to this day through advanced X-ray optics, new fields of atomic physics, materials and synchrotron science.

ARTICLE BY: Gene Ice, ORNL