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8 Ionizing Radiation and Life
8.5 X-ray Radiation
Natural Sources of X-rays and Gamma-Rays
X-rays were discovered by Wilhelm Röentgen in 1895 when he developed a
photographic plate that had never been exposed to light, but had been placed near
a high-voltage vacuum tube. A month later, he had made a radiograph of his wife’s
hand on a photographic plate, showing bones and a wedding band. In 1903, Ernest
Rutherford collimated the radiation from radioactive substances by putting the
source in a hole in a lead brick. He then separated the emissions using an electric
field, and later a magnetic field. He was able to divide the radiation into three types:
α, which he later showed to be helium nuclei, β, which he showed to be electrons,
or positrons, and an uncharged radiation which he named γ rays. In 1912, Max von
Laue, William Bragg, and his son Lawrence Bragg, deduced that X-ray radiations
were electromagnetic waves by exhibiting their diffraction effects. Rutherford and
Edward Andrade confirmed, in 1914, that gamma rays were electromagnetic waves.
They showed that radium gamma rays form diffraction patterns in small-angle
scattering from rock salt (NaCl) crystal surfaces.
X-rays and gamma rays are copiously produced by stars and violent events in
the Universe, but our atmosphere largely shields life on the surface of the Earth
from these astronomic sources. Radioactive elements in the Earth do expose life
forms, but the level is tolerable, and perhaps even useful to evolution by producing
mutations.
In the early 1950s, before good radiation standards, children who watched TV
close to the high-voltage tube for energizing the Cathode Ray Tube (CRT) display
screen were exposed to X-rays from electrons scattering within the tubes. In the
late 1940s, a shoe store might have a machine that used X-rays and a fluoroscope
for patrons to see how their shoes fit. Kids, instead, would look to see their foot
bones wiggling. 5 Those same kids might have been wearing a radium-dial watch,
with radium mixed in a phosphor painted on the numbers and hands, to make them
glow. Workers who did the painting employed a fine brush, which they might shape
by licking the tip. These workers had a higher-cancer rate than similar non-radium
workers.
X-ray wavelengths are of atomic sizes, while gamma rays have wavelengths
comparable to nuclear diameters, or less. With the rough rule of thumb that wiggles
of naturally bound charges within regions of length L will likely generate/absorb
electromagnetic waves with wavelength of the order of L, we should expect
electrons in atoms to be good X-ray absorbers, and charges in nuclei to be good
gamma ray absorbers. But given the small size of the nuclei compared to their
separations in ordinary material, gamma rays can penetrate lead several centimeters
thick.
5 I was one of them.
8 Ionizing Radiation and Life
8.5 X-ray Radiation
Natural Sources of X-rays and Gamma-Rays
X-rays were discovered by Wilhelm Röentgen in 1895 when he developed a
photographic plate that had never been exposed to light, but had been placed near
a high-voltage vacuum tube. A month later, he had made a radiograph of his wife’s
hand on a photographic plate, showing bones and a wedding band. In 1903, Ernest
Rutherford collimated the radiation from radioactive substances by putting the
source in a hole in a lead brick. He then separated the emissions using an electric
field, and later a magnetic field. He was able to divide the radiation into three types:
α, which he later showed to be helium nuclei, β, which he showed to be electrons,
or positrons, and an uncharged radiation which he named γ rays. In 1912, Max von
Laue, William Bragg, and his son Lawrence Bragg, deduced that X-ray radiations
were electromagnetic waves by exhibiting their diffraction effects. Rutherford and
Edward Andrade confirmed, in 1914, that gamma rays were electromagnetic waves.
They showed that radium gamma rays form diffraction patterns in small-angle
scattering from rock salt (NaCl) crystal surfaces.
X-rays and gamma rays are copiously produced by stars and violent events in
the Universe, but our atmosphere largely shields life on the surface of the Earth
from these astronomic sources. Radioactive elements in the Earth do expose life
forms, but the level is tolerable, and perhaps even useful to evolution by producing
mutations.
In the early 1950s, before good radiation standards, children who watched TV
close to the high-voltage tube for energizing the Cathode Ray Tube (CRT) display
screen were exposed to X-rays from electrons scattering within the tubes. In the
late 1940s, a shoe store might have a machine that used X-rays and a fluoroscope
for patrons to see how their shoes fit. Kids, instead, would look to see their foot
bones wiggling. 5 Those same kids might have been wearing a radium-dial watch,
with radium mixed in a phosphor painted on the numbers and hands, to make them
glow. Workers who did the painting employed a fine brush, which they might shape
by licking the tip. These workers had a higher-cancer rate than similar non-radium
workers.
X-ray wavelengths are of atomic sizes, while gamma rays have wavelengths
comparable to nuclear diameters, or less. With the rough rule of thumb that wiggles
of naturally bound charges within regions of length L will likely generate/absorb
electromagnetic waves with wavelength of the order of L, we should expect
electrons in atoms to be good X-ray absorbers, and charges in nuclei to be good
gamma ray absorbers. But given the small size of the nuclei compared to their
separations in ordinary material, gamma rays can penetrate lead several centimeters
thick.
5 I was one of them.