HISTORY

a. Man has always lived in a radiation environment. We are continuously bombarded each day by cosmic rays from space, terrestrial radiation from the crust of the earth, and even radiation from radioactive materials within our own bodies. It has only been during this century, however, that we have come to recognize and characterize these radiations and to artificially produce radioactive materials and manufacture radiation-producing devices for the benefit of mankind.

b. In 1895, x-rays were discovered by Wilhelm Conrad Roentgen, a German physicist, but many scientists before him paved the way for his discovery. Many major discoveries relating to electricity had been made during the three centuries that preceded the discovery of x-rays, but it was the study of electrical discharges under high voltage in vacuum tubes that led to the actual discovery of these rays. Scores of scientists had experimented with electrical discharges through different types of vacuum tubes and, no doubt, many of them had produced x-rays but had not recognized them as a new type of ray.

c. Roentgen himself was experimenting with cathode rays when he observed the presence of this new radiation. He was working with a certain vacuum tube (Crookes-Hittorf) through which a current, under high voltage, was being passed. The tube was entirely enclosed in black paper so as to exclude all the light emanating from it. During the experiment, Roentgen observed a fluorescence of some barium platinocyanide crystals coating a piece of cardboard lying nearby. It had been known for sometime that these crystals would fluoresce in the presence of a vacuum tube activated by high voltage, but it occurred to Roentgen that the fluorescence of the crystals was due to some type of ray that could pass through the black paper around the tube. When he picked up the chemically coated cardboard, his fingers came between it and the tube, and he saw the bones of his hand. He realized that he had discovered the presence of a ray that would penetrate solid matter. By replacing the chemically coated cardboard with a photographic plate, he was able to record an image of the internal structure of his wife's hand. He also noted that the rays could not be reflected or refracted by the usual means and that they were not affected by electrical and magnetic fields as were the cathode rays, which he was studying. Because he did not know the nature of these rays, he called them x-rays. Others have called them roentgen rays.

d. Following the discovery of x-rays, man was not long in learning about both the harmful and useful characteristics of this new energy source. E. H. Grubbe, a manufacturer of Crookes’ tubes in Chicago, Illinois, suffered severe hand injuries by exposing his hands to x-rays. Realizing from his own experience the destructive power of x-rays, Grubbe, on January 29, 1896, treated a patient for carcinoma of the breast with his Crookes tube. Numerous applications of the x-ray for medical purposes followed Grubbe’s procedure and today x-ray machines are among the most valuable therapeutic and diagnostic tools available to the clinician.

e. The year following the discovery of x-rays, Henri Becquerel observed that crystals of a uranium salt emitted rays which were similar to x-rays in that they were invisible, were highly penetrating, and could affect a photographic plate. Becquerel's discovery was followed by the identification of other "radioactive" elements and further investigation showed that there were actually three different kinds of radiation from naturally occurring radioactive substances. These three types of radiation were called alpha (a), beta (b) and gamma (¡) from the first three letters of the Greek alphabet.

f. It has been shown that all three radiations are not emitted simultaneously by all radioactive substances. Some elements emit a-rays, others emit b-rays, while g-rays sometimes accompany one and sometimes the other. To date, numerous radioactive elements have been identified or artificially produced. As with x-rays, radioactive elements are used quite extensively for medical purposes for the diagnosis and treatment of a variety of diseases.