Bonner, W. Brubaker, W. Burcham and M. The neutrons producing the reaction appeared to be of thermal, or of near thermal energy.
During the period , the thermal-neutron absorption cross-section for nitrogen was found by Fermi's group in Rome to be quite large, compared to most materials. This property indicated in fact that thermal neutrons in air would be expected to react with nitrogen essentially converting it into carbon by this kind of reaction. In the physical sense, the discovery of carbon had thus been established by these studies, but the establishment of its existence in the chemical sense, which was to reveal the most important one, was delayed by many obstacles.
The table of isotope showed that there was a blank at the position mass 14 in the element carbon, so that it seemed reasonable to try to produce it by irradiating nitrogenous materials with neutrons. Lawrence was assembling a group of physicists attracted by the cyclotron, a much stronger particle source than any then existing. But detection of carbon was not an easy task. The new inch cyclotron was in operation on a full schedule when the discovery of nuclear fission burst on the world in January At that time Lawrence decided that both cyclotrons must be diverted to a full-time effort to determine definitely whether long-lived isotopes of hydrogen, carbon, nitrogen, and oxygen did or did not exist.
Lawrence asked Martin Kamen to organize a complete and systematic campaign in this direction and in early , Kamen and Samuel Rubens - who had previously worked under Libby as a doctoral student - succeeded in obtaining enough radiocarbon by bombarding graphite with a strong deuteron beam from the new Berkeley cyclotron.
They were also able to make a rough estimate of its surprisingly long half-life, remarkably close to the disintegration value now accepted of about 5, years.
In the meantime, the cosmic-ray physicist Serge Alexander Korff of New York University had found in that cosmic rays produce showers of neutrons when they strike atoms in the top of the atmosphere. At this point the question was: What will million-electron-volt neutrons do once liberated in the air?
Oxygen is essentially inert to neutrons, but the atmosphere contains about 78 percent nitrogen, and the abundant isotope nitrogen was known to be quite reactive.
Korff answered to this question pointing out that the main part of the total number of neutrons produced as byproduct of cosmic radiations, results in the formation of carbon in the atmosphere. After the war, from the data of Korff and B.
Hammermesh it was in fact possible to estimate that, on average, one or two atoms of carbon would be produced in this way each second for each square cm of the earth's surface. In Libby and R. Cornog had also shown that fast neutrons on nitrogen make tritium and carbon, in analogy to the above mentioned reaction by which slow neutrons on nitrogen make ordinary hydrogen of mass 1 and carbon On this basis, in June Libby called thus the attention to a possible explanation of the tenfold greater abundance of helium-2 as decay product of hydrogen-3 in atmospheric helium, as compared to helium from gas wells.
Such traces of tritium could be thus used as a tracer for atmospheric water. By measuring tritium concentrations, Libby later developed a method for dating well water and wine, as well as for measuring circulation patterns of water and the mixing of ocean waters.
The Radiocarbon Clock At the same time, Libby suggested that the newly formed carbon has high energy at the moment of its formation, so that it rapidly oxidises to carbon dioxide, which spreads out and distributes itself evenly in the atmosphere.
Active and non-active carbon dioxide are dissolved in a constant ratio in the water of the seas and lakes where they are converted into carbonate and bicarbonate, and they are assimilated by trees and plants during photosynthesis, and finally also by all forms of terrestrial life that consume vegetation contain the same proportion of carbon Since the age of the earth - remarked Libby - is much greater than the life of radiocarbon, a radioactive equilibrium must exist in which the rate of disintegration is equal to the rate of production.
It appeared that in this great span of time there is adequate opportunity for the carbon dioxide to form, for the atmosphere to mix, for the oceans to mix, and for the biosphere to cycle many times.
All this formed a grand system which is continually stirred. Assuming that the intensity of the cosmic radiation has been constant during the last few tens of thousands of years, the rate of formation of carbon would be equal to the rate at which it disappears to reform nitrogen The average lifetime of carbon should be sufficiently long to allow for the formation of a stationary state in the concentration of this isotope not only with reference to the atmosphere, but also to the hydrosphere and biosphere as well.
The continuous labelling of the biosphere and living matter must be thus characterized by an expected number of disintegrations per second per gram of carbon. At that time, tracer methodology, an offspring of nuclear science, was already providing essential support for the ever widening and deepening knowledge of structure and function in biological systems. Now it was on the verge of increasing by large factors the already large total benefit which up to that time had been derived from it.
It was clear from the previous set of assumptions that organic matter, while it is alive, is in equilibrium with the cosmic-radiation; that is, all the radiocarbon atoms which disintegrate in living bodies are replaced by the carbon contained in the food. He died in Los Angeles on September 8, Willard Libby made many contributions in physical chemistry and received several international scientific awards over his professional lifetime.
It was while he was at the University of Chicago that he made the discovery that led to his Nobel Prize for Chemistry in Libby had developed a method for measuring the carbon content present in organic materials in archaeological artifacts and geological deposits: radiocarbon dating.
Today, radiocarbon dating is the standard technique for dating organic materials from archaeological sites around the world. Besides developing radiocarbon dating, Libby had an extensive and varied research career.
He found that by measuring the amount of tritium, a radioactive isotope of hydrogen, in water samples, it was possible to trace the source and circulation of fresh waters. He also identified the presence of radioactive strontium, resulting from atmospheric nuclear testing—of which he was a supporter—in milk. In he moved to the Enrico Fermi Institute of Nuclear Studies, Chicago, and began an extensive study of radiocarbon.
The halflife was accurately measured on the artificially produced isotope. The natural isotope was discovered by comparing the radioactivity of methane from sewage and petroleum. The former, only recently out of the biosphere, had a measurably higher activity. These measurements were made on borrowed equipment by an expensive technique known as isotope enrichment, so Libby decided to devise a simpler method using more sensitive apparatus.
Unfortunately, more sensitive counters picked up "background" radiation, much of it due to penetrating cosmic rays. Attempts by Libby to shield the apparatus in various ways met with limited success. The problem was solved by surrounding the counting equipment containing the sample with counters which switched off the central counter whenever an interfering particle muon arrived. With this refined apparatus Libby, with E. Anderson, made radiocarbon dating a practical possibility.
For this work Libby received the Nobel Prize for chemistry in
0コメント