Biographical Memoirs Volume 86 (2005) / Chapter Skim
Currently Skimming:
Robert M. Walker
Robert M. Walker
Pages 378-400
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 379... ...
He conjectured that meteorites and lunar rocks contain a record of the ancient radiation history of the solar system in the form of fossil tracks of radiation damage. With his colleagues at General Electric Research Laboratory, he made that dream come true.
|
From page 380... ...
He had to walk a mile, sometimes in blizzard conditions, to catch the bus to elementary school. When Bob was 12 they moved to the Bronx, where he went to Thomas Knowlton Junior High School, which he described as "a perfectly awful ghetto school" and a dangerous place where many students carried weapons.
|
From page 381... ...
During his time at Yale, other graduate students told him about their research on cosmic rays, which were then poorly understood. This early contact may have influenced his thinking about cosmic ray tracks in meteorites a few years later.
|
From page 382... ...
H Silk and Barnes had mounted sheets of mica against a thin layer of uranium, exposed the sandwich to thermal neutrons in a reactor, stripped thin layers from the mica, and examined them in an electron microscope.
|
From page 383... ...
Within a few months they solved the problem of track fading with the discovery that immersing the mica in hydrofluoric acid "fixed" the tracks by removing the cylindrical regions of radiation damage, leaving fine holes a few nanometers in diameter. They took passing note of the possibility of creating molecular sieves made of irradiated and etched mica, but concentrated on the question of whether mica was sensitive enough to record etchable tracks of spallation recoils.
|
From page 384... ...
This discovery established that tracks in mica survive more than 100 million years and led them to the invention of the nowfamous fission track dating method of geochronology. To Bob the most important consequence of the fission track dating method was the proof that cosmic ray tracks would be stable over geologic time periods.
|
From page 385... ...
For example, Fleischer decided not to be inhibited by his colleagues' view of a track as a disordered, chemically reactive region. He discovered that two kinds of noncrystalline solids—glasses and polymers—could record etchable tracks, which forced a revision of their model of track structure and ultimately led him to invent the "ion explosion spike" model of track formation.
|
From page 386... ...
Among the early technological applications were production of molecular sieves; deposition of quasi-onedimensional solids inside etched tracks; neutron dosimetry; radon dosimetry; neutron radiography; mapping of spatial distributions of uranium, thorium, boron, and lithium; alpha autoradiography; and uranium exploration via radon fluxes. During Bob's 1962-1963 sabbatical, he was joined by Michel Maurette, then a graduate student at the University of Paris (Orsay)
|
From page 387... ...
For decades after their discovery of trans-iron cosmic rays, Fleischer, Price, Walker, and Russian researchers at Dubna searched in vain for superheavy elements (Z ~ 110) and magnetic monopoles in meteorite crystals, using heavy ion calibrations together with improved etching techniques and various procedures such as partial annealing to get rid of the background of abundant tracks of iron cosmic rays.
|
From page 388... ...
Two technological spin-offs of the nuclear track technique led to the creation in the 1960s of industries owned in part by General Electric. The first and still the best-known is the Nuclepore filter, available in hole size from 0.015 to 10 Elm, made by irradiating a long sheet of thin polycarbonate film with collimated fission fragments in a nuclear reactor and then etching it in hot sodium hydroxide to the desired hole size on an assembly line.
|
From page 389... ...
New faculty members appointed with Bob's leadership included Ray Arvidson, Ghislaine Crozaz, Larry Haskin, and Frank Podosek in Earth and Planetary Sciences, and Charles Hohenberg in Physics. Bob's efforts to create a permanent structure that would ensure the pursuit of space science at Washington University culminated in the establishment of the McDonnell Center for the Space Sciences in 1974 through an initial large gift by the McDonnell Aerospace Foundation.
|
From page 390... ...
During more than three decades, he saw Bob convert the laboratory from a place that concentrated on nuclear track studies into a place with sophisticated microanalytical instruments, such as the ion microprobe, NanoSIMS, scanning electron microscope, transmission electron microscope, and Fourier transform infrared spectrometer. This equipment, and Bob's vision, led ultimately to the discovery of stardust in meteorites and interplanetary dust particles and to their detailed isotopic and mineralogical characterization.
|
From page 391... ...
of 244Pu. Among the topics they studied were dating of lunar rocks and glassy impact spherules, analysis of gardening and erosion rates of lunar soil using solar flare track densities as a function of depth in soil particles, comparison of the average present-day and ancient spectra of heavy ions emitted in solar flares, suprathermal ions from the sun, 244Pu fission tracks in ancient lunar rocks, cosmic ray exposure history of lunar rocks, and tracks of solar wind ions heavier than iron.
|
From page 392... ...
Bob was the principal investigator of an experiment in which various track detectors (glasses, plastic, mica) as well as metal foils for the capture of implanted noble gas solar wind ions that were taken to the moon with the ApoRo 16 and ApoRo 17 flights were exposed to solar radiation.
|
From page 393... ...
These originally included analytical transmission electron microscopy and Fourier transform infrared spectroscopy but were later extended to the ion probe and to two-step laser-desorption laser-ionization mass spectrometry. His successes contributed to a large extent to the establishment of IDP studies as an important branch of meteoritics.
|
From page 394... ...
The fact that such negotiations usually followed a lunch with a fine French meal and ample wine posed a challenge, but Bob lived up to this challenge splendidly. The instrument arrived at Washington University in the spring of 1982.
|
From page 395... ...
The fourthfloor group leads in studies of pre-solar grains with discoveries of new types of stardust, detailed isotopic characterization of countless grains in the ion probe, and mineralogical studies in the transmission electron microscope. Bob was involved in many aspects of this work.
|
From page 396... ...
In his last scientific effort Bob returned to the application of nuclear tracks. His idea was to measure both the 23SU and 235U content of pre-solar silicon carbide grains— the 23sU by spontaneous fission tracks and the 235U by irradiating the grains with both thermal and fast neutrons.
|
From page 397... ...
. WE ARE INDEErED to Ghislai~le Crozaz for her help with this memoir and to Roland Schmitt for sharing with us his unpublished article "A Discovery and its Uses: The Story of Particle Tracks in Solids." HONORS 1967 1970 1971 1964 Co-winner, American Nuclear Society Award for Distinguished Service 1966 Yale Engineering Association Annual Award for Contributions to Basic and Applied Science Doctor, honoris cause, Union College NASA Exceptional Scientific Achievement Award E
|
From page 398... ...
Electron microscope observation of etched tracks from spallation recoils in mica.
|
From page 399... ...
Woolum. Nuclear track studies of ancient solar radiations and dynamic lunar surface processes.
|
From page 400... ...
Isotopic analysis of small pre-solar SiC grains with the NanoSIMS ion microprobe. Meteont.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.