IT'S ELEMENTAL!
ASTATINE D A L E R. C O R S O N , C O R N E L L U N I V E R S I T Y
I
N I 9 3 8 - 4 O , I SERVED TWO YEARS AS
a postdoctoral fellow in the Radiation Laboratory of the University of California, Berkeley, helping construct a new 60-inch cyclotron that produced 32-MeV a-particles. With construction nearing completion, the laboratory staff met in a special session one evening in the fall of 1939 to discuss experiments designed to exploit the newly available high-energy particles. In the course of the discussion, Emilio Segre pointed out the obvious fact that once one looked at the Periodic Table ofthe Elements, adding an a-particle to a bismuth nucleus of atomic number 83 could produce a nucleus of element 85, which had been until then missing in the periodic table. The next day, Robert Cornog, a graduate student colleague who possessed a small piece of bismuth, and I bombarded the shiny metal for a short time with the 32MeV a-particles. I had a nearby laboratory with instrumentation I had built, anticipating the new research opportunities. At the conclusion of the short bombardment, we carried the bismuth to my laboratory and placed it in front of an ionization chamber connected to a linear amplifier. The oscilloscope screen recording the output of the amplifier was alive with large pulses, characteristic of a-particles. We clearly had something of interest. After some preliminary experiments seeking to sort out the radioactivities we
ASTATINE AT A GLANCE Name: From the Greek astatos,unstable. Atomic mass: (210). History: First produced in 1940 by Dale R. Corson, K. R. MacKenzie, and Emilio Segre at the University of California, Berkeley, by bombarding a bismuth isotope with a particles. Occurrence: The rarest of the 92 naturally occurring elements; less than 30 g exists on Earth at any one time as a natural by-product of uranium and thorium decay. Appearance: Solid nonmetal of unknown color at room temperature. Behavior: Highly radioactive; decays very quickly. Uses: None.
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had produced, it was clear that I was going to need help if I were to pursue the effort with vigor. Cornog was already occupied with the discovery of tritium, working with Luis Alvarez. Kenneth MacKenzie was a graduate student ready for a dissertation project, and I invited him to help me. He proved to be an ingenious and energetic colleague. R I N G I N G I N A 1939 photograph of We identified the many radiations we the 60-inch cyclotron group at the had produced: two groups of a-particles, University of California, Berkeley. 7-rays, X-rays, low-energy electrons, and (From left) Don Cooksey, Corson, some energetic positrons. Curiously, all Ernest 0. Lawrence, Bob Thornton, these radiations possessed the same 7.5John Backus, W. W. Salisbury, (above) hour half-life, except for the positrons. We Alvarez, and Edwin McMillan. later showed that these came from copper contamination in the bismuth. Segre's help was essential in determinFor a reason I do not know, our 75-hour ing the general chemical properties of 85. half-life is now recorded in the literature as Some of the properties are similar to those 72 hours. The precision of our measureof iodine, its lower homolog. It also exment did not permit this large an experimental error. It probably has to do with hibits metallic properties, more like its the volatility of the radioactive product we metallic neighbors Po and Bi. were seeking to identify There was some investigation of element 85's behavior in guinea We applied all the ingenuity pigs. I collaborated with J. G. and imagination we could muster, Hamilton, an M.D., in one exseeking to determine the origins periment. In this study, element ofall the observed radiations. We 85 was injected in the animal, finally arrived at the correct raand a few hours later tissue samdioactive decay scheme after a ples were examined for 85 aclengthy series of experiments detivity The element was concensigned to check one possibility after another. Fortunately, bistrated in the tiny thyroid gland CELEBRATING C&EN'S muth has only a single stable isoin much the same way iodine 80TH tope (209). Through an (a-2n) would be, establishing the physANNIVERSARY process —that is, an a-particle iological similarity to iodine. enters the bismuth nucleus and The literature now records two neutrons leave it— the element 85 nusome 20 different isotopes of 85. Small cleus with atomic mass number 211 is proamounts of some naturally occurring 85 duced, with a half-life of 75 hours. The isotopes have been found, but probably no 211 85 nucleus decays in two ways: 4 0 % of more than a few grams total in the entire the time, it decays by capturing a K-shell Earth's crust. I have appeared in the "Guinelectron to go to 211Po, which then decays ness Book of World Records" for having with a very short half-life, emitting an adiscovered the rarest substance on Earth. particle, to go to stable 207 Pb; 60% of the In a note to Nature in 1947, MacKenzie, time, 21185 decays with emission of an aSegre, and I proposed the name astatine particle followed by a P process to go to (from the Greek word meaning unstable) 207 Pb. We failed to identify the p process. for this element, in keeping with the namIdentification of the nucleus emitting ing ofthe other halogens where the name rethe X-rays following the K-capture process lates to some property of the substance. was vital in nailing down the disintegraDale R. Corson is aprofessor of physics emerition scheme. We employed the critical abtus and president emeritus of Cornell University. sorption edges in tungsten and platinum He is a Public Welfare Medalist of the National absorbers to show that the X-rays are polonium X-rays, as demanded by our decay Academy of Sciences anda Bueche Medalist of the NationalAcademy of Engineering. scheme. HTTP://WWW.CEN-ONLINE.ORG
