were discovered in 1895 by Wilhelm Conrad Roentgen (1845-1923)
who was a Professor at Wuerzburg University in Germany. Working
with a cathode-ray tube in his laboratory, Roentgen observed a
fluorescent glow of crystals on a table near his tube. The tube
that Roentgen was working with consisted of a glass envelope (bulb)
with positive and negative electrodes encapsulated in it. The
air in the tube was evacuated, and when a high voltage was applied,
the tube produced a fluorescent glow. Roentgen shielded the tube
with heavy black paper, and discovered a green colored fluorescent
light generated by a material located a few feet away from the
He concluded that a new type of ray was being emitted from the
tube. This ray was capable of passing through the heavy paper
covering and exciting the phosphorescent materials in the room.
He found that the new ray could pass through most substances casting
shadows of solid objects. Roentgen also discovered that the ray
could pass through the tissue of humans, but not bones and metal
objects. One of Roentgen's first experiments late in 1895 was
a film of the hand of his wife, Bertha. It is interesting that
the first use of X-rays were for an industrial (not medical) application,
as Roentgen produced a radiograph of a set of weights in a box
to show his colleagues.
discovery was a scientific bombshell, and was received with extraordinary
interest by both scientist and laymen. Scientists everywhere could
duplicate his experiment because the cathode tube was very well
known during this period. Many scientists dropped other lines
of research to pursue the mysterious rays. Newspapers and magazines
of the day provided the public with numerous stories, some true,
others fanciful, about the properties of the newly discovered
Public fancy was caught by this invisible ray with the ability
to pass through solid matter, and, in conjunction with a photographic
plate, provide a picture of bones and interior body parts. Scientific
fancy was captured by the demonstration of a wavelength shorter than
light. This generated new possibilities in physics, and for investigating
the structure of matter. Much enthusiasm was generated about potential
applications of rays as an aid in medicine and surgery. Within
a month after the announcement of the discovery, several medical
radiographs had been made in Europe and the United States, which
were used by surgeons to guide them in their work. In June 1896,
only 6 months after Roentgen announced his discovery, X-rays were
being used by battlefield physicians to locate bullets in wounded
to 1912, X-rays were used little outside the realms of medicine
and dentistry, though some X-ray pictures of metals were produced.
The reason that X-rays were not used in industrial application
before this date was because the X-ray tubes (the source of the
X-rays) broke down under the voltages required to produce rays
of satisfactory penetrating power for industrial purposes. However,
that changed in 1913 when the high vacuum X-ray tubes designed
by Coolidge became available. The high vacuum tubes were an intense
and reliable X-ray source, operating at energies up to 100,000
In 1922, industrial radiography took another step forward with
the advent of the 200,000-volt X-ray tube that allowed radiographs
of thick steel parts to be produced in a reasonable amount of
time. In 1931, General Electric Company developed 1,000,000 volt
X-ray generators, providing an effective tool for industrial radiography.
That same year, the American Society of Mechanical Engineers (ASME)
permitted X-ray approval of fusion welded pressure vessels that
further opened the door to industrial acceptance and use.
A Second Source of Radiation
Shortly after the discovery of X-rays, another form of penetrating
rays was discovered. In 1896, French scientist Henri Becquerel
discovered natural radioactivity. Many scientists of the period
were working with cathode rays, and other scientists were gathering
evidence on the theory that the atom could be subdivided. Some
of the new research showed that certain types of atoms disintegrate
by themselves. It was Henri Becquerel who discovered this phenomenon
while investigating the properties of fluorescent minerals. Becquerel
was researching the principles of fluorescence, wherein certain minerals
glow (fluoresce) when exposed to sunlight. He utilized photographic
plates to record this fluorescence.
One of the minerals Becquerel worked with was a uranium compound.
On a day when it was too cloudy to expose his samples to direct
sunlight, Becquerel stored some of the compound in a drawer with
his photographic plates. Later when he developed these plates,
he discovered that they were fogged (exhibited exposure to light).
Becquerel questioned what would have caused this fogging. He knew
he had wrapped the plates tightly before using them, so the fogging
was not due to stray light. In addition, he noticed that only
the plates that were in the drawer with the uranium compound were
fogged. Becquerel concluded that the uranium compound gave off
a type of radiation that could penetrate heavy paper and expose
photographic film. Becquerel continued to test samples of uranium
compounds and determined that the source of radiation was the
element uranium. Bacquerel's discovery was, unlike that of the
X-rays, virtually unnoticed by laymen and scientists alike. Relatively few scientists were interested in Becquerel's findings.
It was not until the discovery of radium by the Curies two years
later that interest in radioactivity became widespread.
While working in France at the time of Becquerel's discovery,
Polish scientist Marie Curie became very interested in his work.
She suspected that a uranium ore known as pitchblende contained
other radioactive elements. Marie and her husband, French scientist
Pierre Curie, started looking for these other elements. In 1898,
the Curies discovered another radioactive element in pitchblende, and named it 'polonium' in honor of Marie Curie's native homeland.
Later that year, the Curies discovered another radioactive element
which they named radium, or shining element. Both polonium and
radium were more radioactive than uranium. Since these discoveries,
many other radioactive elements have been discovered or produced.
Radium became the initial industrial gamma ray source. The material
allowed castings up to 10 to 12 inches thick to be radiographed. During
World War II, industrial radiography grew tremendously as part
of the Navy's shipbuilding program. In 1946, man-made gamma ray
sources such as cobalt and iridium became available. These new
sources were far stronger than radium and were much less expensive.
The manmade sources rapidly replaced radium, and use of gamma
rays grew quickly in industrial radiography.
The science of radiation protection, or "health physics"
as it is more properly called, grew out of the parallel discoveries
of X-rays and radioactivity in the closing years of the 19th century.
Experimenters, physicians, laymen, and physicists alike set up
X-ray generating apparatuses and proceeded about their labors with
a lack of concern regarding potential dangers. Such a lack of
concern is quite understandable, for there was nothing in previous
experience to suggest that X-rays would in any way be hazardous.
Indeed, the opposite was the case, for who would suspect that
a ray similar to light but unseen, unfelt, or otherwise undetectable
by the senses would be damaging to a person? More likely, or so
it seemed to some, X-rays could be beneficial for the body.
Inevitably, the widespread and unrestrained use of X-rays led
to serious injuries. Often injuries were not attributed to X-ray
exposure, in part because of the slow onset of symptoms, and because
there was simply no reason to suspect X-rays as the cause. Some
early experimenters did tie X-ray exposure and skin burns together.
The first warning of possible adverse effects of X-rays came from
Thomas Edison, William J. Morton, and Nikola Tesla who each reported
eye irritations from experimentation with X-rays and fluorescent
Today, it can be said that radiation ranks among the most thoroughly
investigated causes of disease. Although much still remains to
be learned, more is known about the mechanisms of radiation damage
on the molecular, cellular, and organ system than is known for
most other health stressing agents. Indeed, it is precisely this
vast accumulation of quantitative dose-response data that enables
health physicists to specify radiation levels so that medical,
scientific, and industrial uses of radiation may continue at levels
of risk no greater than, and frequently less than, the levels
of risk associated with any other technology.
X-rays and Gamma rays are electromagnetic radiation of exactly
the same nature as light, but of much shorter wavelength. Wavelength
of visible light is on the order of 6000 angstroms while the wavelength
of x-rays is in the range of one angstrom and that of gamma rays
is 0.0001 angstrom. This very short wavelength is what gives x-rays
and gamma rays their power to penetrate materials that light cannot.
These electromagnetic waves are of a high energy level and can
break chemical bonds in materials they penetrate. If the irradiated
matter is living tissue, the breaking of chemical bonds may result
in altered structure or a change in the function of cells. Early
exposures to radiation resulted in the loss of limbs and even
lives. Men and women researchers collected and documented information
on the interaction of radiation and the human body. This early
information helped science understand how electromagnetic radiation
interacts with living tissue. Unfortunately, much of this information
was collected at great personal expense.