The halo photographs in Henderson's scientific reports showed much more clearly defined rings than those reported by Joly. Both investigators had used the dark mica, biotite, in their halo searches. Henderson, however, had used thinner slices, and this had given the sharper rings. Henderson's uranium halos were the very ones needed to make my own measurements. Was his collection of thin sections still available? Correspondence with the geology and physics departments at Dalhousie was not encouraging. Henderson had died many years earlier, and most of his halo collection had been lost. It seemed that a trip to Nova Scotia was the quickest way to obtain more information about the intriguing halos.
The trip was an experience in frugal living, and for a week it appeared little would come of it. Then, early the next week, the head of the physics department returned from a brief trip and managed to locate the few remaining thin sections from Henderson's original halo collection. A few days later my funds had almost run out, but my studies of the thin sections had barely begun. The trip was made a success when it was agreed the halo specimens could be loaned to me for further study. In addition, the geology department gave me many fresh specimens of mica from their museum collection. I returned to Atlanta, borrowed a microscope, and set up a makeshift laboratory in my home.
Unfortunately, Henderson's remaining thin sections did not contain the best uranium halos pictured in his reports. Some with better ring definition had to be found, and this activity began to consume much of my time outside of my teaching duties. The mica specimens given to me at Dalhousie became the source material for my own search. Halos with large centers were common in these specimens, but such halos did not exhibit the delicate [p. 24] ring structure produced by those with point-like centers. Perfect uranium halos with clearly defined rings were needed to settle the question of variable-sized halo rings that Joly had reported. I spent long, tedious hours scanning different pieces of mica, but the perfect uranium halos remained elusive.
The end of my second year at Georgia Tech was approaching, and the time had come to decide about my graduate program. My interest in learning the scientific truth about the age of the earth was stronger than ever. And so was my conviction that radioactive halos might be the key to unlock that truth. At the same time, the physics department chairman remained firm that research on radioactive halos was not an acceptable thesis topic for my doctoral degree; so I left Georgia Tech at the end of that academic year and spent the summer of 1964 doing independent research on halos, using my own funds. (Fortunately, my wife was in total agreement with this decision.) Savings and borrowed funds do run out, though, and that fall I became a substitute high school math teacher in the Atlanta area.
The A, B, C, and D Halos
In addition to the uranium (and thorium) halos, Henderson had reported four other types which he designated as simply A, B, C, and D halos. Along with searching for perfect uranium halos, my attention focused on the D halos. Under the microscope this halo type appeared as a uniformly colored disk with a somewhat fuzzy periphery. It was only about half the size of a fully developed uranium halo; yet it much resembled a uranium halo in an early stage of development when only the inner rings are visible. I became curious about Henderson's tentative association of this halo type with an isotope of radium having a half-life of about 1,600 years. (Figures 1.1 and 1.3 show where this isotope, 226Ra, fits into the uranium decay chain.) The micas in which the D halos had been found were thought to be so old that all the original radium should have died away; only the stable end product was thought to remain in the centers. Henderson claimed the radioactivity in the D halo centers halos should be dead, or "extinct." However, no one had shown this was true, and I decided it was worth investigating. Who knew? Perhaps some new information about the age of the earth would present itself in the process.
The small number of radioactive atoms in the halo centers meant a low rate of alpha-particle emission—only a few particles per month were expected from the uranium-halo centers. Autoradiography was the only technique that could show exactly where an alpha particle originated; hence it [p. 25] was the only technique which could determine whether the D halo centers were still radioactive. The autoradiographic experiments required the use of a special photographic emulsion capable of recording the passage of a single alpha particle. The first step was to split the mica specimen so that the D halo centers were either exposed on the surface or else very close to it. (The specimens chosen sometimes contained uranium halos and one or more of the A, B, or C halos as well.) Step two consisted of pouring a thin layer of this special emulsion over the exposed surface. Under these conditions, nearly half of all the alpha particles ejected from the various halo centers would pass up into the alpha-sensitive emulsion; there they would leave very short trails of ionized atoms. These short trails would remain invisible until the emulsion was developed; after development they appeared as short black tracks when viewed under the microscope. The emulsion-covered halo specimens were placed in a freezer to insure that the tiny trails didn't fade away during the several-week or more storage time.
In the early experiments the emulsion often slid over the sample during the development process. This slippage destroyed the exact registration between the emulsion and the halo centers and made it impossible to know which, if any, of the alpha tracks were actually from the halo centers. A change in procedure remedied this difficulty, and soon I had a technique for maintaining registration throughout the experiments.
After the emulsion was developed, I sometimes observed a few short alpha tracks radiating from both the uranium and the D halo centers. I expected the tracks from the uranium halo centers, but the tracks from the D halos were a surprise. Something long held to be a fact was not true: the D halo centers were not extinct after all. (Later experiments have strongly suggested that the D halos are just uranium halos in an early stage of development, not really a complete surprise considering their almost identical appearance.) It had taken a lot of effort to come to this conclusion, but in the world of science it wasn't much of a discovery. And it didn't seem to have anything to do with my main interest in the age of the earth.
Unspectacular though they were, I decided to present the results of these initial investigations at the January 1965 annual meeting of the American Association of Physics Teachers in New York City. My wife encouraged me to take this trip, even though it depleted the last of our financial reserves. Some new acquaintances, Drs. C.L. and A.M. Thrash, learned of this venture and soon after became the primary sponsors of my research for the next year and a half. This was a difficult time for us, and my research would surely have ended without their help.
Earth Science Associates