Joseph John Thomson 1856-1940

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Joseph John Thomson 1856-1940
In 1884, at age 28, J.J. Thomson became Director of the Cavendish Laboratory at Cambridge University. No one was more surprised than Thomson who had been decried as a "mere boy." Nevertheless, this mere boy turned what he described as a "string and sealing wax laboratory" into the world's preeminent center for experimental nuclear physics. It has been said that Thomson, like Michael Faraday, was greater than his discoveries. However, those discoveries were far from insignificant. Thomson and his student Ernest Rutherford were the first to demonstrate the ionization of air by X rays. So fundamental is this phenomenon that the phrase "ionizing radiation" remains the most concise way to characterize the wide range of electromagnetic and particulate radiation emitted by atoms. Nevertheless, Thomson is best known for his investigations into the nature of "cathode rays," (i.e., electrons). By deflecting these "rays" with an electric field, something that had been done previously with a magnetic field, Thomson provided conclusive proof that they were negatively charged particles. He subsequently determined the mass of these particles to be one two- thousandth that of the hydrogen atom, the smallest object known at that time. Thomson was thus the first to identify the existence of subatomic particles. This opened the door to a new world of which his student, Ernest Rutherford, would later master, as well as provide his own significant contributions to nuclear physics. Later, Thomson demonstrated that the interaction between electrons and matter produced X rays and that X rays interacting with matter produced electrons. Although it would fail the test of time, Thomson can also be credited with the first "modern" model of the atom, the so-called "plum pudding" model. In it, he pictured a sphere of positive charges mixed together with an equal number of electrons (i.e., negative charges). For his theoretical and experimental investigations into the electron and the conduction of electricity by gases, Thomson was awarded the 1906 Nobel Prize in physics. Ironically, Thomson, who had characterized the material properties of electrons, would live to see his son George P. Thomson share the Nobel Prize for experimentally confirming the wavelike property of electrons.

Ernest Rutherford 1871-1937
Ernest Rutherford is considered the father of nuclear physics. Indeed, it could be said that Rutherford invented the very language to describe the theoretical concepts of the atom and the phenomenon of radioactivity. Particles named and characterized by him include the alpha particle, beta particle and proton. Even the neutron, discovered by James Chadwick, owes its name to Rutherford. The exponential equation used to calculate the decay of radioactive substances was first employed for that purpose by Rutherford and he was the first to elucidate the related concepts of the half-life and decay constant. With Frederick Soddy at McGill University, Rutherford showed that elements such as uranium and thorium became different elements (i.e., transmuted) through the process of radioactive decay. At the time, such an incredible idea was not to be mentioned in polite company: it belonged to the realm of alchemy, not science. For this work, Rutherford won the 1908 Nobel Prize in chemistry. In 1909, now at the University of Manchester, Rutherford was bombarding a thin gold foil with alpha particles when he noticed that although almost all of them went through the gold, one in eight thousand would "bounce" (i.e., scatter) back. The amazed Rutherford commented that it was "as if you fired a 15-inch naval shell at a piece of tissue paper and the shell came right back and hit you." From this simple observation, Rutherford concluded that the atom's mass must be concentrated in a small positively-charged nucleus while the electrons inhabit the farthest reaches of the atom. Although this planetary model of the atom has been greatly refined over the years, it remains as valid today as when it was originally formulated by Rutherford. In 1919, Rutherford returned to Cambridge to become director of the Cavendish Laboratory where he had previously done his graduate work under J.J. Thomson. It was here that he made his final major achievement, the artificial alteration of nuclear and atomic structure. By bombarding nitrogen with alpha particles, Rutherford demonstrated the production of a different element, oxygen. "Playing with marbles" is what he called it; the newspapers reported that Rutherford had "split the atom." After his death in 1937, Rutherford's remains were buried in Westminster Abbey near those of Sir Isaac Newton.

Niels Bohr 1885-1962
Ernest Rutherford's model of the atom, developed at the turn of the century, pictured negatively charged electrons moving in circular orbits about a positively charged nucleus. Contradictory to electrodynamic theory the electrons did not emit electromagnetic radiation. Niels Bohr provided the explanation by incorporating Max Planck's quantum theory into Rutherford's atomic model. He envisioned specific discrete energy levels (i.e., shells) for the electrons within which they could move yet not emit radiation. Only if the electrons dropped to a lower energy level, or were raised to a higher level, would they emit or absorb electromagnetic radiation. That the energy of the emitted or absorbed radiation must equal the difference between the original and final energy levels of the electrons explained why atoms only absorb certain wavelengths of radiation. To Albert Einstein, Bohr's achievements were "the highest form of musicality in the sphere of thought." In recognition, Bohr received the Nobel Prize in physics in 1922. Later, Louis de Broglie and Erwin SchrÓdinger described the electron as a standing wave rather than as a particle, which "explained" how Bohr's electrons could move about within a defined energy level without emitting radiation. This led Bohr to his famous principle of complementarity, whereby the electron could be interpreted in two mutually exclusive yet equally valid ways: by either the particle or wave models. Later, Bohr hypothesized how an incoming particle could strike a nucleus and create an excited "compound" nucleus. This idea formed the basis for his "liquid drop" model of the nucleus which would provide Lise Meitner and Otto Frisch with the theoretical basis for their explanation of fission.

George de Hevesy 1885-1966
The discoveries of George de Hevesy have done as much as those of any other individual to influence science in the 20th century. Ironically, it was his inability to accomplish a task assigned by Ernest Rutherford in 1911 that led to his greatest discovery: radiotracers. Hevesy had just joined the research group at the University of Manchester headed up by Rutherford who was investigating the radioactive properties of radium-D (lead-210). Much to Rutherford's annoyance, the lead with which the radium-D was associated interfered with his analyses. Not realizing that radium-D was a radioactive form of lead, Rutherford erroneously thought it could be chemically isolated and told Hevesy "My boy, if you are worth your salt, you try to separate radium-D from all that lead." Out of his failure to complete that impossible task, Hevesy conceived the radiotracer technique by which radioisotopes could be used to investigate the behavior of stable atoms. It is a technique second to none in its analytical power. Hevesy not only performed the first radiotracer studies on plants and animals, using both natural and artificial radionuclides, he also performed the first tracer studies employing stable nuclides by using deuterated water to measure the turnover of hydrogen in the body. In addition to these studies, which earned him the 1943 Nobel Prize in chemistry, Hevesy developed the technique of neutron activation analysis, perhaps the most powerful non-destructive technique for the elemental analysis of solid samples. Despite the importance of the radiotracer technique and neutron activation analysis, Hevesy took the greatest pride in his discovery of the element hafnium. In part, this was because of the magnitude of the effort involved and in part because of the important role hafnium played in the organization of the periodic table.

Victor F. Hess 1883-1964
Today, Victor F. Hess is best known for his discovery of cosmic rays in 1911. Originally, however, there was considerable uncertainty about the exact nature of this type of radiation. It was not until 1936, when further research by Hess and others (e.g., Robert Millikan who coined the term cosmic rays) had confirmed the extraterrestrial origins of the radiation, that Hess was awarded the Nobel Prize in physics. Other cosmic ray studies by Hess involved their biological effects, their seasonal variation and the influence of magnetic disturbances on their intensity. However, for most of his career, Hess studied the medical uses of radium and the nature and diagnosis of radium poisoning. Between the years 1945 and 1965, Hess measured the radium body burdens of thousands of radium workers. Many of these measurements utilized extremely sensitive techniques developed by Hess at Fordham University. As a footnote, it was during WW I that Hess was the first to utilize Geiger counters for the detection of gamma rays.

Arthur Holly Compton 1892-1962
Enrico Fermi believed that good looks and height were inversely proportional to intelligence, but he was willing to allow an exception in the case of the tall and handsome Arthur Compton. Compton demonstrated the magnitude of his formidable intelligence very early in his career. In 1919, shortly after receiving his doctorate in physics from Princeton, Compton spent a year in Cambridge working under Ernest Rutherford investigating the properties of scattered gamma rays. In the early 1920's, at Washington University in St. Louis, he continued this line of research using X rays instead of gamma rays. He discovered that the scattering of the X rays by graphite lowered their energy. Compton hypothesized that the X rays must be behaving like particles (i.e., photons) that transferred their energy to the electrons of the graphite in a "collision." This would not happen if X rays behaved exclusively as waves. For example, the wavelength (i.e., pitch) of sound does not change as it is reflected off a surface. This provided experimental proof that electromagnetic radiation could exhibit the characteristics of particles as well as waves. In acknowledgment of the importance of this work, Compton was awarded the 1927 Nobel Prize in physics. His research then shifted to investigations of cosmic rays. Measurements at thousands of locations around the world showed that the intensity of cosmic rays was affected by the earth's magnetic field. This provided conclusive evidence that cosmic rays must consist of charged particles. At the outbreak of WW II, Compton's reputation was such that he was asked to direct the Metallurgical Laboratory. The "Met Lab," as it was called, was the organization at the University of Chicago that helped guide the nation's scientific efforts devoted to the development of the atomic bomb.

Gioacchino Failla 1891-1961
Gioacchino Failla, one of the greatest pioneers in the fields of biophysics and radiobiology, began his career at New York's Memorial Hospital in 1915. Within a few years of joining the staff, he had established the first research program devoted to improving the medical applications of radiation. One of the initial products of this research was the construction of a radon generator, the first in the United States. In 1921, Failla was the first to suggest that radiation doses be expressed as the amount of radiation energy absorbed and made the first dose estimates in radium therapy, in terms of micro-calories per cc of tissue. With the arrival of an X-ray unit at his laboratory the following year, Failla constructed the first human phantom in the U.S. so that he could determine the effects of filtration and distance on X- ray fields in the body. In 1925, upon returning from a one-year sabbatical with Marie Curie in Paris, Failla published protocols and described equipment permitting radiotherapists to deliver the desired doses to their patients accurately. Not the least of his contributions were his roles in founding the International Commission on Radiation Units and Measurements (ICRU), and the Radiation Research Society. Later in his career, Failla left Memorial Hospital for Columbia University where he made important contributions to our understanding of radiation mutagenesis and the induction of cancer by radiation.

Edith Quimby 1891-1982
Edith Hinkley Quimby began her career in 1919 at the Memorial Hospital in New York City where Gioacchino Failla had established the first research laboratory devoted to the medical uses of radiation. Failla needed an assistant and, as Quimby remembers it, "This job turned up. I took it." Although no standard techniques were available at the time, radium was widely used to treat cancer. Radium-containing needles were applied to tumors in a makeshift fashion, with no certainty that the tumors received the required exposures. Quimby was the first to determine the distribution of the radiation doses in tissue from various arrangements of radium needles. The techniques she described in 1932 for choosing the most effective grouping of radium needles were widely adopted in the United States and served as the forerunner of Parker and Paterson's Manchester system. During the same period, she quantified the different doses from beta and gamma radiation required to produce the same biological effect, such as skin erythema (i.e., reddening of the skin). In doing so, she pioneered the concept of the relative biological effectiveness of radiation (RBE). This important concept is still employed by radiobiologists and served as the basis for the quality factor used to convert an absorbed dose measured in rad (or gray) to a dose equivalent in rem (or sievert). Although radiologists had previously used X-ray film to estimate radiation exposures, Quimby was the first (ca. 1923) to institute a full scale "film badge" program, which consisted of cutting X-ray film into strips, covering them with black paper and distributing them among the laboratory personnel. In the 1940s, Quimby and Failla moved to Columbia University and began working with the newly available artificial radioisotopes being produced by accelerators and reactors. The early clinical trials with radioactive sodium and iodine to diagnose and treat various medical disorders established Quimby as one of the pioneers of nuclear medicine. Quimby finished her career at Columbia University by teaching a new generation about radiation physics and the clinical use of radioisotopes.

Ernest O. Lawrence 1901-1958
During the 1920s, the only available method for probing nuclei was that developed by Ernest Rutherford, which consisted of bombarding the nuclei with alpha particles. A major problem with this technique was overcoming the repulsive forces between the positively charged alpha particle and the target's positively charged nucleus. The relatively low energies possessed by alpha particles compounded the problem. Rutherford's method worked reasonably well with light elements, whose nuclear charges were small, but failed with elements of high atomic numbers. To overcome this problem, a number of machines were developed for accelerating charged particles to higher energies, but the cyclotron of Ernest Orlando Lawrence would prove to be the most important tool in high-energy physics. Lawrence conceived the idea for the cyclotron in 1929 after coming across an article by Rolf Wideroe. The article described an accelerator that employed a pair of linearly arranged cylinders and an alternating electric field. Lawrence's inspiration was to reconfigure Wideroe's cylinders as D-shaped chambers and position them between the poles of a magnet. Within the "dees", ions (e.g., protons) were accelerated in a series of steps over a spiral path. As such, the cyclotron could be small yet capable of generating very high energy ions. Even Lawrence's first machine, only 4.5" in diameter, accelerated protons to 80,000 eV. Later, Lawrence used improved versions of the cyclotron to investigate nuclear processes and to produce a variety of new and medically important isotopes (e.g., the phosphorous-32 used in early attempts to treat leukemia). For this work, Lawrence received the 1939 Nobel Prize in physics. Today, descendants of this first cyclotron continue to play an important role in medicine and have proven to be the physicist's most useful tool for exploring the nature of matter.

Glenn Seaborg 1912-
Glenn Seaborg has made major contributions to science as a discoverer, administrator and educator. During the 1930s, 1940s and 1950s at E.O. Lawrence's lab in Berkeley and at the University of Chicago, Seaborg discovered (or co- discovered) the elements plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium and nobelium, as well as a wide variety of radionuclides including iodine-131, technetium-99m, cobalt-60, cesium-137, and iron-55. Indeed, he helped configure the periodic table as we now know it by placing the actinide series under the lanthanide series. For his discoveries of the transuranic elements and his determination of their chemistry, Seaborg was awarded the 1951 Nobel Prize in chemistry. As an administrator, Seaborg guided the nation's nuclear programs for ten years while Chairman of the Atomic Energy Commission. As a scientist, he has provided consultation to every U.S. President since Franklin D. Roosevelt. As an educator, he has been tireless in his efforts to inform the public about the benefits of nuclear power and the use of radionuclides in medicine, industry and the biological and physical sciences. Recently, the discoverers of element 106 have recommended that it be named Seaborgium, in honor of Seaborg's life- long achievements in radiochemistry.

Wilhelm Conrad Roentgen 1845-1923
On November 8, 1895, at the University of Wurzburg, Wilhelm Roentgen's attention was drawn to a glowing fluorescent screen on a nearby table. Roentgen immediately determined that the fluorescence was caused by invisible rays originating from the partially evacuated glass Hittorf-Crookes tube he was using to study cathode rays (i.e., electrons). Surprisingly, these mysterious rays penetrated the opaque black paper wrapped around the tube. Roentgen had discovered X rays, a momentous event that instantly revolutionized the field of physics and medicine. However, prior to his first formal correspondence to the University Physical-Medical Society, Roentgen spent two months thoroughly investigating the properties of X rays. Silvanus Thompson complained that Roentgen left "little for others to do beyond elaborating his work." For his discovery, Roentgen received the first Nobel Prize in physics in 1901. When later asked what his thoughts were at the moment of his discovery, he replied "I didn't think, I investigated." It was the crowning achievement in a career beset by more than its share of difficulties. As a student in Holland, Roentgen was expelled from the Utrecht Technical School for a prank committed by another student. Even after receiving a doctorate, his lack of a diploma initially prevented him from obtaining a position at the University of Wurzburg. He even was accused of having stolen the discovery of X rays by those who failed to observe them. Nevertheless, Roentgen was a brilliant experimentalist who never sought honors or financial profit for his research. He rejected a title (i.e., von Roentgen) that would have provided entry into the German nobility, and donated the money he received from the Nobel Prize to his University. Roentgen did accept the honorary degree of Doctor of Medicine offered to him by the medical faculty of his own University of Wurzburg. However, he refused to take out any patents in order that the world could freely benefit from his work. At the time of his death, Roentgen was nearly bankrupt from the inflation that followed WW I.

Lauriston S. Taylor 1902-
By the time Lauriston Taylor was 26 years of age, he was Chief of the X-ray group at the National Bureau of Standards and had served as one of three representatives of the United States at the Second International Congress of Radiology in 1928. At the Congress, Taylor and G. Kaye of Great Britain were the driving forces behind the creation of the International Commission on Radiological Protection (ICRP), the world's most influential organization in the field of radiation protection. From 1937 to 1950, Taylor served as Secretary of the ICRP, and from 1934 to 1950, Secretary of almost as influential an organization, the International Commission on Radiation Units and Measurements (ICRU). In 1953 Taylor became Chairman of the ICRU, a position which he held until 1969. A year after the Second Congress of Radiology, Taylor established and became the first president of the National Council on Radiation Protection and Measurements (NCRP), the counterpart in the U.S. to the ICRP and the scientific body from which this nation's regulatory agencies seek guidance regarding radiological protection. At the National Bureau of Standards, Taylor's activities focused on the measurement of X rays. In fact, he is credited with building the world's first portable radiation survey meter (ca. 1929). He also built the Bureau's first free-air ion chamber, the primary device for measuring the intensity of X rays and the first such chamber to employ guard wires. The use of guard wires was a tremendous improvement because it created a more uniform electric field and greatly reduced the size and weight of these chambers. In the mid 1930's, Taylor constructed the first pressurized ion chamber in the United States which, for a time, was the only operating free-air ion chamber anywhere in the world. When technical contributions such as these are considered together with his many other achievements, it becomes apparent that no one has played a greater role in shaping the profession of radiation protection than Lauriston Taylor.

Thomas Alva Edison 1847-1931
Thomas Alva Edison's reputation was well established by the time X rays were discovered in November, 1895. He had already received patents for hundreds of inventions, including those for the motion picture camera and the first practical incandescent light. Upon learning of Roentgen's discovery, Edison set about assembling the necessary equipment to investigate this new phenomenon. Because X-ray tubes were difficult to obtain, Edison manufactured his own, something he was well equipped to do owing to his work with incandescent lights. In fact, some of his original X-ray tubes were little more than modified electric light bulbs. Edison's investigations into X rays were wide ranging but most of his initial research was devoted to improving upon the fluorescent screens used to view X-ray images. (Roentgen had discovered X rays by the fluorescence they created from a screen of barium platinocyanide.) After investigating several thousand materials, Edison concluded that calcium tungstate was far more effective than barium platinocyanide. By March of 1896, Edison had incorporated this material into a device he called the Vitascope, later called a fluoroscope, that consisted of a tapered box with a calcium tungstate screen and a viewing port. Similar devices already had been developed, but Edison's version quickly became the standard tool by which physicians viewed X-ray images. During the course of these investigations, Clarence Dally, one of Edison's most dependable assistants, developed a degenerative skin disorder which progressed into a carcinoma. In 1904, Dally succumbed to his injuries - the first radiation related death in the United States. Immediately, Edison halted all his X-ray research noting "the X rays had affected poisonously my assistant, Mr. Dally..."

William David Coolidge 1873-1975
In 1913, William David Coolidge revolutionized the field of radiology by inventing what is now referred to as the Coolidge X-ray tube. No new scientific principles or discoveries were involved, and to Coolidge's employer, the General Electric Company, the invention simply represented a new product. Nevertheless, this new product became a watershed in the field of medicine. The story of its development began in 1905 when Coolidge joined the General Electric Research Laboratory and was given the task of replacing the fragile carbon filaments in electric light bulbs with tungsten filaments. The available tungsten was difficult to work metallurgically, but Coolidge succeeded and his improved light bulb was brought to market in 1911. General Electric also manufactured X-ray tubes and Coolidge recognized that his tungsten filament together with additional modifications could significantly improve the performance of the tube. Coolidge's improved X-ray tube employed a heated tungsten filament as its source of electrons (i.e., the cathode). Since residual gas molecules in the tube were no longer necessary as the electron source, the Coolidge (or hot cathode) tube could be completely evacuated which permitted higher operating voltages. These higher voltages produced higher energy X rays which were more effective in the treatment of deep-seated tumors. In addition, the intensity of the X rays did not show the tremendous fluctuations characteristic of earlier tubes and the operator had much greater control over the quality (i.e., energy) of the X rays. Coolidge later became Director of the laboratory and eventually Vice-President and Director of Research for General Electric. At the beginning of WW II, he was appointed to a small committee established to evaluate the military importance of research on uranium. This committee's report led to the establishment of the Manhattan District for nuclear weapons development. In 1975, with 83 patents to his credit, William David Coolidge was elected to the National Inventor's Hall of Fame, the only person to receive this honor in his lifetime.

Antoine Henri Becquerel 1852-1908
Henri Becquerel was born into a family of scientists. His grandfather had made important contributions in the field of electrochemistry while his father had investigated the phenomena of fluorescence and phosphorescence. Becquerel not only inherited their interest in science, he also inherited the minerals and compounds studied by his father. And so, upon learning how Wilhelm Roentgen discovered X rays from the fluorescence they produced, Becquerel had a ready source of fluorescent materials with which to pursue his own investigations of these mysterious rays. The material Becquerel chose to work with was a double sulfate of uranium and potassium which he exposed to sunlight and placed on photographic plates wrapped in black paper. When developed, the plates revealed an image of the uranium crystals. Becquerel concluded "that the phosphorescent substance in question emits radiation which penetrates paper opaque to light." Initially he believed that the sun's energy was being absorbed by the uranium which then emitted X rays. Further investigation, on the 26th and 27 of February, was delayed because the skies over Paris were overcast and the uranium-covered plates Becquerel intended to expose to the sun were returned to a drawer. On the first of March, he developed the photographic plates expecting only faint images to appear. To his surprise, the images were clear and strong. This meant that the uranium emitted radiation without an external source of energy such as the sun. Becquerel had discovered radioactivity, the spontaneous emission of radiation by a material. Later, Becquerel demonstrated that the radiation emitted by uranium shared certain characteristics with X rays but, unlike X rays, could be deflected by a magnetic field and therefore must consist of charged particles. For his discovery of radioactivity, Becquerel was awarded the 1903 Nobel Prize for physics.

Pierre Curie 1859-1906

Marie Curie 1867-1934
By the time he met Marie Sklodowska, Pierre Curie had already established an impressive reputation. In 1880, he and his brother Jacques had discovered piezoelectricity whereby physical pressure applied to a crystal resulted in the creation of an electric potential. He also had made important investigations into the phenomenon of magnetism including the identification of a temperature, the curie point, above which a material's magnetic properties disappear. However, shortly after his marriage to Marie in 1895, Pierre subjugated his research to her interests. Together, they began investigating the phenomenon of radioactivity recently discovered in uranium ore. Although the phenomenon was discovered by Henri Becquerel, the term radioactivity was coined by Marie. After chemical extraction of uranium from the ore, Marie noted the residual material to be more "active" than the pure uranium. She concluded that the ore contained, in addition to uranium, new elements that were also radioactive. This led to their discoveries of the elements polonium and radium, but it took four more years of processing tons of ore under oppressive conditions to isolate enough of each element to determine its chemical properties. For their work on radioactivity, the Curies were awarded the 1903 Nobel Prize in physics. Tragically, Pierre was killed three years later in an accident while crossing a street in a rainstorm. Pierre's teaching position at the Sorbonne was given to Marie. Never before had a woman taught there in its 650 year history! Her first lecture began with the very sentence her husband had used to finish his last. In his honor, the 1910 Radiology Congress chose the curie as the basic unit of radioactivity; the quantity of radon in equilibrium with one gram of radium (current definition: 1Ci = 3.7E10 dps). A year later, Marie was awarded the Nobel Prize in chemistry for her discoveries of radium and polonium, thus becoming the first person to receive two Nobel Prizes. For the remainder of her life she tirelessly investigated and promoted the use of radium as a treatment for cancer. Marie Curie died July 4, 1934, overtaken by pernicious anemia no doubt caused by years of overwork and radiation exposure.

Robley Evans 1907-
The defining moment in Robley Evans' career came during his graduate studies at Caltech when his supervisor, Robert Millikan, introduced him to the Los Angeles County Health Officer, Frank Crandall. Crandall was concerned about the hazard to the public from radium-containing patent medicines, many of which were being produced in the Los Angeles area. After graduation, Evans accepted a position at the Massachusetts Institute of Technology where he continued to investigate the subject of radium poisoning. Here, Evans built the first whole body counter to measure radium uptake by the radium dial painters and carried out the first quantitative in-vivo measurements of a radionuclide in the human body. Indeed, the scintillation cameras so common in today's hospitals are direct descendants of his original counter. Evans' studies went well beyond measuring radium in the body: he pioneered investigations into its metabolism, its hazards, and methods for mitigating these hazards. He was primarily responsible for promulgating the first limit on radioactive material in the body, 0.1  Ci of radium-226, a value that served for more than four decades as the benchmark for bone-seeking radionuclides. Not the least of his contributions was the first use (ca. 1930s) of radioiodine to evaluate thyroid function in humans, which is a technique that has stood the test of time and remained, well into the 1980s, one of the most potent diagnostic tools available to physicians. It is no wonder Robley Evans is recognized as one of the founders of the field of Nuclear Medicine.

Hermann Joseph Muller 1890-1967
Hermann Muller, the father of radiation genetics, began his career with T.H. Morgan studying mutations in fruit flies (Drosophila). Muller grew impatient with the mutation rate in Drosophila, and was the first to increase the mutation rate using heat. Still not satisfied, he irradiated the flies with 50 kilovolt X rays (ca. November 1926) that resulted in an even greater incidence of mutations. In doing so, he was the first to demonstrate radiation-induced genetic alterations! Moreover, he did so in a quantitative manner that determined the mutation frequency. Nevertheless, it took nearly two decades for this work to be recognized with the Nobel Prize. The delay was in large part due to his left-wing politics, his controversial views on eugenics and his often unpopular opinions about the hazards of radiation. In 1931, the severe criticism and pressure to which these views exposed Muller caused him to leave the United States. A year later he ended up in Leningrad directing the genetics laboratory at the Institute of Applied Botany. Eventually, Stalin's reign of terror and disagreements with Trofim Lysenko led Muller to leave for Scotland, where he and S.P. Ray-Chaudhuri studied mutation frequency and dose rate dependence. About this time, he began warning about needless exposures to radiation and their associated risks of cancer and hereditable genetic effects. By the late 1940s, the nuclear weapons testing program had begun and Muller was back in the United States, a vocal critic of the Atomic Energy Commission's views on the hazards of worldwide fallout. As a result, the AEC did not choose Muller as an official US delegate at the 1955 United Nations International Conference on the Peaceful Uses of Atomic Energy. Nonetheless, Muller attended and after virtually every presenter referenced his work, he was given an extended standing ovation!

Herbert Parker 1910-1984
Herbert M. Parker began his remarkable career in 1932 by developing, along with James R. Paterson, what ultimately became known as the Manchester System for radium therapy. Their techniques enabled physicians to arrange radium needles or tubes in configurations that would maximize the radiation dose to a tumor while minimizing that to healthy tissue. Other techniques had been developed but the Parker and Paterson system was the most comprehensive and widely used, and is considered a milestone in the field of radiology. In 1938, Parker left England for the Swedish Hospital in Seattle where he conducted research in supervoltage therapy. At the start of WW II, he joined the "Metallurgical Laboratory" at the University of Chicago and became one of the first group of radiation protection specialists to adopt the title "health physicist". Soon afterwards, Parker left Chicago for Oak Ridge where he established the health physics program at what eventually became Oak Ridge National Laboratory. In 1944 he returned to the state of Washington and established the health physics program at the Hanford Engineering Works, a program hat he directed until 1956 when he became overall manager for the Hanford Laboratories. Among his many other accomplishments, he was instrumental in the development of the roentgen equivalent physical ("rep") sometimes called the roentgen equivalent parker, and roentgen equivalent biological ("reb") units, predecessors to the rad and rem. He also established the first maximum permissible concentration for a radionuclide in air: 3.1E-11  Ci/cm3 for plutonium- 239.

James Chadwick 1891-1974
In 1907, while enrolling at the University of Manchester, James Chadwick accidentally found himself in the line for those hoping to major in physics. Chadwick, who had intended to be a mathematician, was too shy to admit he was mistaken and stayed in line. Thus began the career of one of this century's most distinguished physicists. In 1913 he received his master's degree and left for Germany to work with Hans Geiger. There, Chadwick was the first to show that beta particles possess a range of energies up to some maximum value. Trapped in Germany when WW I broke out, Chadwick was imprisoned in a horse stall at a racetrack that served as an internment camp. As soon as the war ended and he gained his freedom, Chadwick returned to England and joined forces with Ernest Rutherford. Intrigued by Rutherford's speculation about a subatomic particle with no charge, Chadwick began a series of experiments to demonstrate the existence of such a particle. Initially, none of the experiments succeeded. Then, in 1930, Walther Bothe and Herbert Becker described an unusual type of gamma ray produced by bombarding the metal beryllium with alpha particles. Chadwick recognized that the properties of this radiation were more consistent with what would be expected from Rutherford's neutral particle. When Fr«d«ric and Ir‰ne Joliot-Curie subsequently claimed that Bothe and Becker's "gamma rays" could eject high energy protons from paraffin, Chadwick knew these were not gamma rays. The subsequent experiments by which Chadwick proved the existence of the neutron earned him the 1935 Nobel Prize in physics. Not only did this singular particle provide physicists with a superlative tool for investigating the atom, it was also used to produce a wide variety of new radioisotopes and permitted the initiation of nuclear chain reactions. Hans Bethe has referred to Chadwick's discovery as the historical beginning of nuclear physics.

Jean Frederic Joliot 1900-1958

Irene Curie 1897-1956
In 1925, Frederic Joliot accepted the position of special assistant to Marie Curie. The next year, he married Marie's daughter, Irene, forming one of the most remarkable scientific partnerships of all time: Frederic served the role of chemist, Irene that of physicist. Unfortunately, the early stage of their careers was defined by failure rather than success. Not only did they fail to discover the neutron, misidentifying it as a gamma ray, they also just missed discovering the positron. Later on, however, it was their observations of these particles that led to their discovery of artificial radioactivity, which is considered to be their greatest triumph. Irene and Frederic had noted that the bombardment of aluminum with alpha particles resulted in the emission of neutrons and positrons. As expected, the neutrons were emitted only as long as the aluminum was being bombarded by alpha particles. What astonished Frederic and Irene was the continued emission of positrons long after the alpha source had been removed from the target. Immediately, Frederic and Irene performed careful analyses which showed that the alpha bombardment had produced a positron-emitting radionuclide of phosphorous from the aluminum. Not only had they produced the first artificial radionuclide, they were the first to experimentally confirm transmutation, the conversion of one element into another element! Up to this point, the only radioactive materials available for medical and scientific research were those that occurred naturally. Now a method was available for creating a wide new variety of radioisotopes. The impact was immense, and for this discovery the Joliot-Curies won the 1935 Nobel Prize for chemistry. Later, during WW II, they helped hamper German efforts to develop an atomic bomb by ensuring that the entire stock of heavy water from the Norsk Hydro Plant was secured and shipped to Britain before France and Norway came under German control. After the war, they made major contributions to the construction of France's first nuclear reactor.

Otto Hahn 1879-1968
Otto Hahn was the chemist whose discovery of nuclear fission ultimately led to the ending of WW II. The story of Hahn's discovery began in 1938 with a report by Irene Joliot-Curie that bombarding uranium with neutrons produced a radionuclide of thorium, which they later speculated was a transuranium element similar to lanthanum. The astounded Hahn told Irene's husband, Frederic, that such a thing was nonsense and that he would perform an experiment to prove as much. In the process of duplicating her work, Hahn and co-worker Fritz Strassmann discovered that, among other things, three isotopes of barium had been produced. This was incredible because the mass of barium is about half that of uranium. No known reaction could explain such a huge change. When they published their results (Jan. 6, 1939) Hahn and Strassmann noted that such a thing was "in opposition to all the phenomena observed up to the present in nuclear physics." Hahn, conscious of the fact that as a chemist he was treading in the domain of physics, did not offer an explanation. Instead, he left it up to Lise Meitner, his longtime collaborator, to whom he had sent a letter (December 19, 1938) describing his findings and asking "Perhaps you can suggest some fantastic explanation," which she explained as nuclear fission. Nevertheless, despite the contributions of Strassmann and Meitner, it was Hahn who was awarded the 1944 Nobel Prize in chemistry for the discovery. Unfortunately, Hahn was not at the awards ceremony to receive his prize. At the time he learned of the award, he was being held by the British who were seeking information from him about the failed German effort to develop an atomic bomb. As the Chairman of the Nobel Committee for Chemistry reported "Professor Hahn . . . has informed us that he is regrettably unable to attend this ceremony."

Lise Meitner 1878-1968
Lise Meitner, forever linked in people's minds with the monumental discovery of nuclear fission, made many significant contributions to science throughout a long and productive career. Upon receiving a doctorate in physics in 1906, Meitner went to the University of Berlin where she began her collaborations with Otto Hahn. The first significant result of this collaboration was an important technique for purifying radioactive material that took advantage of the recoil energy of atoms produced in alpha decay. Later, at the Kaiser Wilhelm Institute in Austria, she was the first to explain how conversion electrons were produced when gamma ray energy was used to eject orbital electrons. She also provided the first description of the origin of auger electrons, i.e., outer-shell orbital electrons ejected from the atom when they absorbed the energy released by other electrons falling to lower energies. When Nazi Germany annexed Austria in 1938, Meitner, a Jew, fled to Sweden. In her absence, Hahn and Fritz Strassmann continued experiments they had begun earlier with Meitner and demonstrated that barium was produced when a uranium nucleus was struck by neutrons. This was absolutely startling because barium is so much smaller than uranium! Hahn wrote to Meitner, "it [uranium] can't really break up into barium . . . try to think of some other possible explanation." While visiting her nephew Otto Frisch for the Christmas holidays in Denmark, she and Frisch proved that a splitting of the uranium atom was energetically feasible. They employed Niels Bohr's model of the nucleus to envision the neutron inducing oscillations in the uranium nucleus. Occasionally the oscillating nucleus would stretch out into the shape of a dumbbell. Sometimes, the repulsive forces between the protons in the two bulbous ends would cause the narrow waist joining them to pinch off and leave two nuclei where before there had been only one nucleus. Meitner and Frisch described the process in a landmark letter to the journal Nature with a term borrowed from biology: fission.

Enrico Fermi 1901-1954
Enrico Fermi's first significant accomplishment in nuclear physics was providing a mathematical means for describing the behavior of certain types of subatomic particles, a process concurrently developed by Paul Dirac which came to be known as Fermi-Dirac statistical mechanics. His next major accomplishment was to successfully explain beta decay by incorporating into the process the production of a new particle which he named the neutrino. Despite the significance of his contributions to theoretical physics, Fermi is best known for his experimental work. When Frederic and Irene Joliot-Curie first produced artificial radionuclides by bombarding aluminum with alpha particles, Fermi recognized that James Chadwick's recently discovered neutron offered a means to create radionuclides from targets of higher atomic number. In the course of doing so, Fermi noticed that greater activity was induced when the neutron bombardment was performed on a wooden table. He deduced that the neutrons became more effective because they slowed down after being scattered by the wood. He also recognized that neutron bombardment of uranium had the potential to produce a new class of elements, refered to as the transuranics. For his discovery of new radioactive elements and his work with slow neutrons, Fermi was awarded the 1938 Nobel Prize in physics. However, unknown to Fermi and the Nobel Prize Committee, the "new elements" Fermi characterized (with one exception) were not new, they were fission products, i.e., radioisotopes of known elements produced by splitting uranium. Shortly after receiving his Nobel Prize, Fermi left Italy to join the faculty of Columbia University in the United States. Here he supervised a series of experiments that culminated in construction of the CP-1 Pile, the first controlled self-sustaining nuclear chain reaction. This momentous event took place in a squash court under the west stands of Stagg Field at the University of Chicago on December 2, 1942. Fermi thus became the first to control nuclear fission, the very process that in 1934 had led him to the false conclusion that he had discovered the transuranic elements!

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