Radioanalytical Methods in Interdisciplinary ... - ACS Publications

Chapter 1

One Hundred Years of Development in Radioanalytical Chemistry as a Science to Probe the Limits 1

2

Downloaded by 80.82.77.83 on May 31, 2017 | http://pubs.acs.org Publication Date: November 4, 2003 | doi: 10.1021/bk-2004-0868.ch001

Carola A. Laue and Kenneth L. Nash 1

A n a l y t i c a l and Nuclear Chemistry Division, Lawrence Livermore National Laboratory, 7000 East Avenue, L - 2 3 1 , Livermore, CA 94550 Chemistry Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, I L 60439 2

The development of radioanalytical chemistry as a science to probe the limits of our understanding of matter and the universe is tightly bound to the evolution of nuclear chemistry, nuclear physics and nuclear technology. The advancements that have been achieved in all of those areas as well as in nuclear medicine, a field presently attracting significant attention, would not have been possible without the steady progress that has been made in radioanalytical chemistry. The objective of this chapter is to highlight the important historic milestones that have led to the present state of the art in radioanalytical chemistry.

th

Before the turn to the 20 century, chemistry had advanced from the black art of alchemy to a creditable field of scientific inquiry, largely due to significant growth in understanding achieved during the 19 century. Though ten chemical elements had been "known" and widely used since prehistoric times (carbon, sulfur, iron, copper, zinc, silver, tin, gold, mercury and lead), only three new elements were added during the middle ages (when alchemistry enjoyed its heyday). Nineteen new elements were discovered during the 18 century while th

th

2

© 2004 American Chemical Society Laue and Nash; Radioanalytical Methods in Interdisciplinary Research ACS Symposium Series; American Chemical Society: Washington, DC, 2003.

3 th

thirty were identified in the 19 century. A s the numbers of elements increased, Mendelejev and Meyer were able to simultaneously and independently (in 1869/70) develop the basic concept of chemical periodicity and the basic organization of the periodic table, as we understand it today. The pace of discovery of new elements through the end of the 19 century is illustrated in Figure 1. B y the end of the 19 century, 3 naturally occurring elements (polonium, radium, and actinium) had been identified on the basis of their radioactivity and chemical properties. A s with all previously discovered elements, the position of these elements within the periodic table was guided by the trends of chemical properties specific to their periodic groups. Unlike their stable congeners, these elements would not have been discovered without their radioactive characteristics. The discovery of these three elements represents the birth of radiochemistry, as we know it today. th


th

Figure 1: All elements known at 1900 are shown in today's format of the periodic table, as established by Mendelejev, Meyer and Seaborg. The elements with shaded background were known by 1800, all others were identified in the 19f century, including the first three radioactive elements highlighted in black. h

In truth, a historical overview of radioanalytical chemistry cannot be separated from the development of nuclear chemistry. More than a century ago, nuclear chemistry evolved on the basis of several scientific achievements, coincidences, fortunate circumstances, and the persistence of the researchers creating this new field. The 19 century had been the century of chemistry — of the atom and molecule — and at the turn to the new century, renowned scientists freely predicted that only a few great discoveries were left to be made. Looking th

Laue and Nash; Radioanalytical Methods in Interdisciplinary Research ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


4 back we note that, although chemistry had gained a substantial knowledge base and was increasing in sophistication, basic knowledge of chemistry was largely build on an empirical foundation while die fundamental principles still remained mysterious. A t the same time, classical physics had developed to an advanced state and a common thought in the physics community, likewise, was that there were no new fundamental principles to be uncovered. Like the chemists, physicists failed to recognize that there was no understanding of the fundamental structure of matter, which could be used, for example, to explain the principles behind the periodic law and chemical behavior displayed by the elements. The existence of atoms was not even conjectural. Furthermore, an important aspect of the classical thermodynamic problem of black body radiation had not been explained. A t the end o f the 19 century, it became obvious that physics within its existing rigid framework could not explain those phenomena. Development of the physical and chemical sciences in the 20 century relied on some very basic experiments and discoveries made in the last half o f the 19 century. In 1859, Plucker had experimented with cathode rays, but Lorentz, in 1892, first formulated the theory that electricity is due to charged particles. After an initially slow period, scientific observations and breakthroughs appeared at an accelerated pace. The following events (not intended to be comprehensive) were particularly important for the development of the new field of nuclear science: • In 1895 Roentgen discovered X-rays and Perrin stated that cathode rays are negatively charged particles. • In 1896, Becquerel observed for the first time an unexpected property of natural matter —radioactivity— in an uranium ore. • In 1898, Marie and Pierre Curie separated and identified the first naturally occurring radioactive elements, Po and Ra, from pitchblende. The circumstances that led to the identification of those new elements are particularly noteworthy. The Curie's employed common chemical separation procedures, but tracked the new elements in the separation process using the radioactive decay properties of the elements. Shortly thereafter, Becquerel found that at least some of the observed radiated emissions are electrically charged. Rutherford successfully identified two different types of those radiations and labeled them a and p radiation. Villard determined that y rays are not charged. In 1899, M a x Planck started the revolution in physics by introducing a theory, which postulates that matter only absorbs or emits energy in arbitrary units or 'quanta'. In the same year, Soddy uncovered the half-life property associated with radioactivity. Although Becquerel suggested in 1900 that P particles were electrons, the Goldhabers proved this in 1948 settling a long standing argument between the peers. Rutherford determined in 1903 that a particles have a positive charge. During his work with Soddy, they formulated th

th

th



5 the theory on the transmutation by radiation, radioactive decay, which led to coining of the term atomic energy in 1902. Harriet Brooks, a woman not often mentioned, working under the Curies and later with Rutherford, contributed substantially to the formulation of the transmutation theory of one element into another with her work. As early as 1904, radioactivity and radioanalytical chemistry began making contributions to the advancement of science in general. Rutherford estimated the age of the earth by the radioactive dating technique, which had evolved by then from the U/He to the U/Pb technique. B y 1905, the still standing dogmatic castle of classical physics crumbled lastly and forever when Einstein formulated the Special Theory of Relativity, the Law of Mass-Energy Equivalence, the Brownian Theory of Motion, and the Photon Theory of Light. In the same year, Bragg and Kleeman determined that a particles have discrete energies. Thompson revealed the relation between the scattering of X-ray photons and the number of electrons in an atom in 1906. Two years later, Geiger designed a simple device for detecting radiation, and in collaboration with Royds and Rutherford, they successfully identified a particles as He nuclei. In 1910, atmospheric radiation was recognized by Wulf, and in 1911, Hess attributed the radiation increase with altitude as originating in extraterrestrial space. Also in 1911, Rutherford proposed that atomic mass is concentrated in the nucleus, a direct conclusion from his a scattering experiments. Wilson constructed his cloud chamber in 1912 allowing for the first time the detection of protons and electrons. The year of 1913 produced numerous scientific advancements. Geiger formulated the relationship between atomic number and nuclear charge that year. Soddy coined for the first time the term 'isotopes' ("same place" - meaning that they inherit the same position in the periodic table but differed distinctly in their radioactive properties; chemically they are the same but have a different mass), and Moseley utilized X-rays to confirm the relationship between nuclear charge and atomic number. Although Nagaoka proposed a 'Saturnian' model of the atom in 1904, this postulate did not capture the attention of the scientific community. It was left to Bohr in 1913 to revolutionize the understanding of the atom by proposing a "solar system" as a model of the atom, the quantum theory of atomic orbits, and radioactivity as a property of the nucleus. Rutherford and da Costa revised the nature of y rays as high-energy photons, which had been previously identified as neutral by Villard. B y 1919, the alchemist's dream of transforming matter was achieved. Rutherford's group artificially transmuted nitrogen into oxygen and hydrogen. Rutherford proved, furthermore, the existence of protons in die nucleus. A t the same time Aston, working in Thompson's lab, designed a mass spectrometer that allowed him to identify several isotopes of neon, among others. In 1923 de Broglie demonstrated the wave nature of particles on which basis Schroedinger



6 postulated the particle wave equation three years later. In 1929, Bothe and Kolhorster found that cosmic rays are charged particles. The year 1929 is also one of major developments of scientific equipment that proved crucial to the further advancement of the young field of nuclear science. Lawrence designed the first cyclotron and van de Graaff devised the Van de Graaff generator. In the early 1930's a new phenomenon was observed. Although Rutherford had speculated on the existence o f neutrons as early as 1920, Becker and Bothe first observed an unusual penetrating neutral radiation in 1930. Two years later, Chadwick successfully identified the neutron, which led Heisenberg to slate that the nucleus is composed of neutrons and protons. In 1933, Szilard conceived the idea of using a chain reaction of neutron collisions with atomic nuclei to release energy — the possibility of "atomic energy" as a source of human controlled power thus came into existence as a concept. In 1934, Joliot and Curie-Joliot were able to artificially induce radioactivity. B y bombarding a sheet of aluminium-27 with a particles, they created of a new radioactive isotope, or radioisotope, phosphorus-30. In the same year, indications exist that Fermi and Hidin did observe fission, though neither recognized the nature o f their results at this time. The first man-made element (i.e., not existing naturally), technetium, was produced and identified by Perrier and Segre in 1937. One year later, at the dawn of the Second World War, Hahn and Strassman induce fission by exposing uranium to neutrons. B y chemical separation they are able to prove that barium was formed rather than the anticipated transuranium elements, hence, they concluded that fission of the uranium target had occurred. Meitner, a member of the Hahn team, theoretically explains the fission process together with Frisch. Joliot, Curie-Joliot, and Szilard introduced the theory predicting the possibility of a nuclear chain reaction. Those stunning findings, in the climate of the emerging world war, sparked more than scientific curiosity. Along side with fission, fusion was also recognized in those years; Bethe postulated in 1939 that the fusion of hydrogen nuclei forming deuterium releases energy. He suggested further that the energy release of the sun and stars originates from the energy-releasing fusion of four hydrogen nuclei uniting and forming one helium nucleus. One year later, in 1940, Flerov discovered spontaneous fission, the 'natural' break up of a nucleus into two more stable fragments, in uranium. In 1940 Corson, MacKenzie, and Segre synthesized element 85, astatine. Scientific publications were delayed for several years during the wartime (and postwar) in the interest of security. New elements such as neptunium (the last published in 1940), plutonium and americium were artificially produced and studied in great detail for military proposes. This chapter of nuclear science has been thoroughly described previously, and so will not be discussed in detail here. Nevertheless, the reader should note that, in addition to producing the world's first two nuclear weapons, the Manhattan project added much to the



7 advancement of the field of nuclear science and to the development of radioanalytical techniques. Starting with the identification of neptunium, the first transuranium element, a new era in the nuclear sciences began. Probing constantly the limits by designing new irradiation techniques, new separations methods and increasing sensitivities to detect unknown radioactive species, the surge to discover elements heavier than americium began. B y 1961, the last of the transuranium (5f) elements, lawrencium (named after E . O. Lawrence, inventor of the cyclotron), was discovered. Chemical separation methods were crucial in the discovery of the new elements because the limited energy discrimination of the detection devices used at that time (to identify the energy of the emitted particles/y-rays or fission fragments) and the interfering activities produced at the same reaction would have prevented the detection of the element of interest. The elements from neptunium through einsteinium can be produced in weighable amounts. However, for heavier elements only small numbers of atoms could be created making the identification of new elements by its chemical properties challenging. With the ability to create only small amounts of the new elements, innovative approaches to conducting "one-atom-at-a-time" chemistry were developed to enable the identification of the remaining actinide elements. The trans-actinides that have been discovered since have all relied on these techniques for their creation and characterization. With the development of the first silicon detection devices, the search for "Superheavy" elements was revitalized in the 70's and culminated in the 1999 and 2001 discoveries of elements 114 and 116, respectively. The Superheavy elements, or trans-actinides, can be produced only in an one-atom-at-a-time fashion today and the longest lived of them survives barely one minute. Chemical characterization is made extremely difficult by such short life-times thus discovery of new elements today is based solely on the isotopic decay signature. The elements that are thought to occupy a region of enhanced nuclear stability (formerly called "Island of Stability") are expected to live considerable longer, although theoretical predictions on the expected half-lives differ by orders of magnitude. Reaching the proton/neutron ratios necessary for attaining this nuclear stability is the primary obstacle to validation of the existence of this region of increased nuclear stability. Continued development in detection technologies has proven crucial to the advancement of the nuclear and radioanalytical sciences and for the development of technological applications of the science. Those developments dominated the second half of the century of nuclear sciences. Applications such as Neutron Activation Analysis ( N A A ) would have been impossible without the development of nuclear reactors and semiconductor based gamma ray detection techniques. Detecting contaminants in the environment at below ppt levels would



8 be unthinkable without the advances achieved in chemical separation methods combined with the constantly improving detection techniques such as: • scintillation counting, which was in its entity first introduced by Kallmann in 1947 (photomultiplier were known and the phenomenon of scintillating material had already been observed in 1903 by Pierre Curie, who had taken a ZnS coated glass plate that had been bombarded with alphas into the dark to see the sparks); • gas-proportional counting, the logical successor to the earlier electroscopes, ion chambers, and the Geiger-Mueller counters, although the latter are still very valuable for fast survey measurements to ensure safe laboratory practice; • the high resolution y- and a- detection based on the still advancing semiconductor techniques. Such devices have revolutionized the field with its sensitivity and capability to be very isotope selective. This semi-conductor based instrumentation vitalized not only the identification of new elements, the trans-actinides, but also the identification of most of the more than 2500 isotopes known today. These achievements equal the sudden increase in element discoveries that resulted when Bunsen and Kirchhoff designed a spectroscope in 18S9 that enabled identification of elements by their characteristic spectral lines. 1 1 H 3 Li 11 Na 19 K

Mr

20 Ca

15

16

7 6 C N 13 14 15 M Si P 31 32 33 Ga Ge As

8 O

13

I 4 Be 12

14

5

4 22 Ti

6

7

23 24 E l Cr

25

S

8

9

10

11

2$ Mb Fe

27 Co

28 Ni

Cu

T1L

Radioanalytical Methods in Interdisciplinary ... - ACS Publications

Recommend Documents