From Hiroshima to the Iceman

The Development and Applications of Accelerator Mass Spectrometry

Harry E Gove

• Publisher: CRC Press
• Year: 2022
• ISBN:
  9781000673753
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

From Hiroshima to the Iceman: The Development and Applications of Accelerator Mass Spectrometry presents a fascinating account of a breakthrough in science and the insights it has brought that would not have been possible without it. Involved since its invention, Harry Gove recounts the story of the development of accelerator mass spectrometry and its use as an ultrasensitive detection technique in many fields of science and the arts. A key advantage of the technique is that it requires only very small samples of material. The book explores the areas where the technique has increased understanding and provided solutions to problems, including the clean-up and storage of nuclear waste, the effects of the atomic bombing of Hiroshima, biomedical research, the settling of the Americas, and carbon dating of many precious artifacts. Objects dated include the Turin Shroud, the Iceman, the elephant bird egg, and the Dead Sea scrolls.

Chapter 1 Introduction

DOI: 10.1201/9780429332463-1

This book is a semiautobiographical account of the historical development of accelerator mass spectrometry (AMS), the instrumentation involved and some of its more interesting, if not to say recondite, applications. AMS has been used to gain information on when man first arrived in the Americas and to date the discovery of America by the first Europeans, to measure the rate of flow and the age of underground water, to establish the exposure ages of rocks on the rims of meteor craters, and thus reveal the time of the meteorite impact, to measure the blast intensity of neutrons from the Hiroshima and Nagasaki atom bombs, to gain insights on the earth’s environmental history, to determine the age of the Turin Shroud [1] and much more. But what is of paramount significance is that it uses samples that are at least a thousand times smaller than those required for any other method. Usually there is no other method.

As will be discussed later, AMS is a technique for the ultrasensitive detection of long-lived radioactive nuclear isotopes of stable atoms. What is a nuclear isotope? The nucleus of any atomic element is made up of positively charged particles called protons and neutral particles called neutrons (particles of almost equal mass to the proton but with no electric charge) packed closely together and bound by the nuclear force. The number of protons in the nucleus determines the number of negatively charged electrons that can be bound in orbits around the nucleus. It is the number of electrons that determine the chemical properties of the atom. But the nucleus of an atom such as carbon with six protons can have, for example, six, seven or eight neutrons. These atoms have atomic masses of 12, 13 and 14 and they are called the isotopes of carbon. The first two are stable but the third, carbon-14 or radiocarbon, is radioactive.

Without doubt the most interesting applications of AMS, at least to the general public, are in the area of radiocarbon dating, and that involves carbon-14. People have an intense interest in the dates of important past events and the ages of important artefacts. Is this small piece of ancient wood really from the true cross? Are the wooden beams found on Mount Ararat in Turkey really from Noah’s ark? Could the Turin Shroud really be Christ’s burial cloth? Was Columbus the first European to discover America? Measuring the carbon-14 in a sample or radiocarbon dating can provide an answer to all these questions and, in fact, already has.

Often, however, available sample sizes are too small or not enough of an artefact can be sacrificed to get a date by the radiocarbon dating method invented by Willard Libby in the late 1940s [2]. That method, called decay counting, uses grams of material. For example Libby would have needed a piece of cloth from the Turin Shroud the size of a man’s handkerchief to carbon date it. No one was going to allow that much of the shroud to be consumed by fire in the process of converting it to the carbon Libby needed to insert into his counters. Or, as another example, there are violins on the market that have internal yellowing labels certifying them to be from the hand of the master violin craftsman Antonio Stradivari of Cremona, Italy, who died in 1737. Dating a Stradivarius violin by the Libby method might be possible, but so much of the instrument would be consumed that it could no longer be played. AMS requires milligrams of material—the Turin Shroud could be dated using a square centimetre of cloth and a violin from a few slivers of wood removed from a non-critical spot. In short the application of AMS to radiocarbon dating means that virtually anything, no matter how small or valuable, can be dated. As with the Libby method, of course, the object must be of organic origin—a product of trees, plants, animals, etc.

Another increasingly important application of AMS lies in the field of biomedical research and, in the foreseeable future, as a diagnostic tool in medicine. Radiocarbon is the most commonly used tracer in biomedical research. Complex hydrocarbons can be labelled at an appropriate site by replacing the stable carbon atom with carbon-14. The way in which a particular hydrocarbon is metabolized in humans and animals can be deduced by measuring the carbon-14 in blood, urine and tissue and even bone marrow samples, as a function of time. AMS permits the radioactive dose of whatever hydrocarbon is to be studied to be reduced to amounts permitted for use in human subjects, and it also reduces the amount of whatever samples are removed for analysis. The field of AMS biomedicine is still in its formative stages but it may well constitute the medical revolution of the future.

Radiocarbon dating, even by the AMS method, is limited at present to objects of organic origin that died 65 000 years ago or less. This limit may, one day, be extended back to 100 000 years but not much further. The limit is, essentially, set by the half-life (the time it takes for half the atoms to decay) of carbon-14, and that is 5730 years. For carbon-14, 60 000 years is ten half-lives or a decrease in the carbon-14 content of a factor of a thousand. To date further back one needs a radioactive element with a much longer half-life. Like radiocarbon, that element must also be produced by cosmic rays, as will be explained later. The next most important cosmogenic radioisotope that fits the bill is chlorine-36. Its half-life is 301 000 years.

AMS can measure chlorine-36 almost as easily as carbon-14 and in equally small quantities. There are, however, not as many dating possibilities for chlorine-36 as for carbon-14. For example, generally it cannot be used to date anything that was once living and growing. It can, however, be used to date the exposure ages of rocks on the earth’s surface, the terrestrial and extraterrestrial ages of meteorites and the age of ground water.

Although the latter may seem arcane, it is of great importance. For example, consider the storage of nuclear waste—a problem currently vexing mankind and one that will become increasingly taxing if we continue to resort to nuclear reactors as sources of electrical energy. Although many people oppose the idea of nuclear power it is probably an option we should continue to keep open. At least reactors do not emit carbon dioxide, an atmospheric pollutant that contributes to global warming. The most obvious place to store used fuel elements from nuclear reactors (usually called, incorrectly, atomic reactors) is deep underground in mines or some other place not directly penetrated by ground water. In the case of mines there is bound to be some ground water but if one could show it was hundreds of thousands of years old it would mean it was not mixing with surface water on a short time scale. Any nuclear waste leached out by such deep mine ground water would not return to the biosphere before its radioactivity was reduced to tolerable levels. That, at least, is the hope. AMS measurement of the chlorine-36 content of the water in the mine can be used to reveal its age.

Another potential storage site for used nuclear fuel is in volcanic tuff well above the water table such as at Yucca Mountain in Nevada. At that site the ground water level lies almost 600 m below the surface and the general geographical area around Yucca is quite arid. Caverns carved out of the mountain should be good, dry storage sites. As we shall see, measurements of chlorine-36 can check this assumption.

One can travel back even further in time by AMS measurements of a third cosmogenic radionuclide, iodine-129. This element has a half-life of almost 16 million years. It can be found in ocean sediments and oil deposits. It is also produced in nuclear reactor fuel elements. It is one of the more abundant products of nuclear fission and can be released into the environment as a result of a reactor accident. For example, when the terrible reactor mishap occurred at Chernobyl in 1986 (see Chapter 9) substantial amounts of iodine-129 as well as the much more lethal iodine-131 and other deleterious fission products were distributed over vast areas surrounding the site and throughout the world, with tragic consequences. At two AMS and several other laboratories the iodine-129 content of soil samples collected at Chernobyl has been measured. It provides information on how much of the short-lived cancer-producing iodine-131 was delivered to the inhabitants of the surrounding region resulting in tens of thousands of deaths, particularly to children and pregnant women. The radioactive contamination produced by that reactor explosion continues to be distressingly high and will remain so for many, many years.

The first application of AMS to natural materials occurred in the spring of 1977 in the Nuclear Structure Research Laboratory (NSRL) at the University of Rochester in upstate New York. That laboratory, funded by the National Science Foundation, commenced operations in 1966. Until 1988 the author was director of the NSRL. The laboratory housed a tandem Van de Graaff accelerator designed for research in nuclear physics and, until its decommissioning in 1995, it was one of the top such university laboratories in the USA. Until 1977 it had made only a few modest forays into fields other than nuclear physics. Since then, until the accelerator was shut down, a fifth of its running time had been devoted to the field of AMS.

Following the description of the historical development of accelerator mass spectrometry in Chapters 2 and 3, the development of both single-ended and tandem electrostatic accelerators is described in Chapter 4. A description of how such tandem accelerators are applied to AMS and the instrumentation required follows in Chapter 5. The latter is not a highly technical description of the subject. A much more technical account has been written by Tuniz et al [3]. The present book is designed to introduce the technique of AMS to potential users and to the general public.

Chapters 6 to 11 are devoted to some applications of AMS. The applications included by no means constitute a complete set of AMS research areas nor a complete account of a particular application. They are, however, ones of wide appeal. Chapter 12 presents some thoughts on the future of AMS.

Chapter 2 Historical Development of Accelerator Mass Spectrometry—1977

DOI: 10.1201/9780429332463-2

My involvement with AMS began during the annual Washington meeting of the American Physical Society in April 1977. A nuclear physics colleague of mine dating back to my Chalk River days, A E Litherland, was meeting with K H Purser, president of General Ionex Corporation, in his hotel suite. Litherland and I collaborated in many pioneering experiments at Chalk River, when I was head of the Nuclear Physics Branch there, using the Chalk River tandem. He left Chalk River in 1966 to become a professor of physics at the University of Toronto. Purser is also a nuclear physicist. Before founding the General Ionex Corporation, he was a senior physicist in my laboratory, the Nuclear Structure Research Laboratory, at the University of Rochester, where he was responsible for the installation of the tandem accelerator. He used it for several years in his nuclear physics research programme. He left the university in 1973 to found a company that manufactures tandem components and small tandems for various industrial purposes. Purser and Litherland had been discussing the possibility of measuring carbon-14 with a tandem accelerator when I joined them. I learned later that it was an idea they had independently been thinking about for some time. As mentioned in Chapter 1, carbon-14 is the key to carbon dating.

Carbon dating was invented by Willard Libby in the 1940s [1] and earned him the Nobel Prize for chemistry in 1960. Radioactive carbon-14 is produced in the atmosphere by cosmic rays and, along with the stable isotopes of carbon, it combines with oxygen to form carbon dioxide gas. All living organisms ingest carbon dioxide. Carbon dating involves measuring the ratio of cosmogenic radioactive carbon-14 to the stable carbon isotopes in organic material. As long as plants or animals are alive this ratio of their carbon component is about one part in a trillion and represents the equilibrium between the production of carbon-14 in the atmosphere and its decay. At the time of death of the organic matter the ratio begins to decrease. If death occurred 5730 years ago (the half-life of carbon-14) the ratio is only half a part in a trillion. A measurement of the ratio determines the time of death with considerable precision.

For many years it had been the dream of carbon daters to employ a mass spectrometer that would detect radioactive carbon-14 directly, at the exceedingly low concentration at which it occurs in living organisms, rather than waiting for it to decay as Libby had done. Such a method would reduce the amount of material required to make a dating by several orders of magnitude over the decay counting technique. For example, as early as 1969 Oeschger et al recognized this and suggested using mass spectrometric methods for radiocarbon dating [2].

However, all attempts had failed because to a high accuracy carbon-14 weighs the same as the abundant and ubiquitous stable element nitrogen-14. Nuclear physicists familiar with tandem accelerators knew that negative nitrogen ions could not be accelerated through a tandem Van de Graaff. It was necessary to tune the ion source to a negative molecular ion of nitrogen. It was, therefore, assumed that negative nitrogen ions were unstable. If they were the tandem’s use of negative ions would instantly solve that most serious interference problem. Traditional mass spectrometers start with positive ions—neutral atoms with one electron removed—and it is as easy to rip one electron from neutral nitrogen as it is from carbon.

Then why not use negative ions in conventional mass spectrometers? The answer is that there is another serious source of mass 14 interference, namely molecular hydrides of stable carbon atoms, and these do form stable negative ions. Such molecules are destroyed if enough electrons are removed in the terminal stripping process.

After I joined the discussions Purser mentioned that he had a patent pending (it had been filed on 1 March 1976) on a tandem accelerator system for the detection of ozone-eating chlorofluorocarbons in the atmosphere that covered the negative ion source and, most importantly, the principle that molecules accelerated to high velocities would be destroyed in the terminal stripping process [3]. The patent did not, however, specifically address the question of detecting long-lived cosmogenic radioisotopes such as carbon-14.

A diagram of the apparatus proposed by K H Purser in his patent for an ultrasensitive spectrometer for making mass and elemental analyses. Courtesy of K H Purser, Southern Cross Corp., Peabody, MA.

The questions were just how unstable was the negative nitrogen ion and how many electrons needed to be removed from a carbon hydride molecule to cause it to blow apart? The best way to answer such questions was to make the appropriate measurements on a tandem Van de Graaff accelerator. Litherland and Purser asked whether the Rochester tandem accelerator might be available for this purpose and would I be willing to collaborate. Although this would represent a considerable departure from the basic resear...

Table of contents

• Cover Page
• Half-Title Page
• Title Page
• Copyright Page
• Dedication
• Table of Contents
• Preface
• Acknowledgments
• Brief Biographical Sketch of the Author
• 1 Introduction
• 2 Historical Development of Accelerator Mass Spectrometry—1977
• 3 Historical Development of Accelerator Mass Spectrometry—1978–80
• 4 The Development of Tandem Electrostatic Accelerators
• 5 Instrumentation for Accelerator Mass Spectrometry
• 6 The Legacy of the Atomic Bombing of Hiroshima and Nagasaki
• 7 The Initial Peopling of the Americas
• 8 The American Indians, the Vikings and Columbus
• 9 Nuclear Power, Nuclear Weapons and Nuclear Waste
• 10 Carbon Dating the Turin Shroud
• 11 The Iceman, the Dead Sea Scrolls and More
• 12 Some Thoughts on Future Developments of AMS
• References
• Index