8.6 Man-made X-rays
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by semiconductors (giving photon energies). The result is called ‘X-ray emission
spectroscopy’.
With precise measurement of the emitted X-rays which came from transitions
from a higher to a lower atomic level, the technique is called ‘Resonant X-ray
Emission Spectroscopy’ (RXES).
If just the absorption by a material of the X-rays of particular wavelengths is
measured, the technique is called ‘X-ray absorption spectroscopy’.
Auger Emission Spectroscopy When a photon or electron beam excites or ionizes
the K or L shell of a heavy atom, the electron that falls into the created hole, while
falling, may interact with an outer electron, causing it to be ejected from the atom.
This is called the Auger effect, discovered by Lise Meitner. 8 Because the lowerenergy ejected electron has a short mean-free path in a solid material, one usually
detects these electrons as they are ejected from the surface of the material placed in
a vacuum.
8.6.9 Mammography
Mammography uses low-energy X-rays (≈30 kV peak) to view the interior of
the human breast, looking for potentially cancerous masses. The X-rays used in
mammograms deposit up to 5 mGy per exposure, with an average of 4 mGy. Natural
background is 2–3 mGy per year.
8.6.10 CAT Scans
A Computer Assisted Tomographic scan (CAT scan), also called a Computed
Tomography (CT) scan, applied to humans, is a computerized axial tomographic
view of body organs, tissue and bones. The idea is that the absorption and scattering
of X-rays depends on the electron density around atoms and molecules and also on
the presence of K-shell electrons in heavy elements. A selected heavy element is
sometimes administered beforehand to increase the scan contrast in certain organs.
By sending X-rays from one direction and confined to a plane which extends through
the body, the absorption pattern in that plane can be measured by detecting the
intensity of the transmitted X-rays. By rotating the direction of the X-rays (but
keeping their plane), a sequence of such intensity patterns is recorded. From these,
a two-dimensional cross-sectional density pattern can be constructed by a computer
analysis. Combining the density patterns for a sequence of adjacent planes, a three8 Lise Meitner, along with Otto Hahn and Otto Robert Frisch, discovered nuclear fission in 1939.
301
by semiconductors (giving photon energies). The result is called ‘X-ray emission
spectroscopy’.
With precise measurement of the emitted X-rays which came from transitions
from a higher to a lower atomic level, the technique is called ‘Resonant X-ray
Emission Spectroscopy’ (RXES).
If just the absorption by a material of the X-rays of particular wavelengths is
measured, the technique is called ‘X-ray absorption spectroscopy’.
Auger Emission Spectroscopy When a photon or electron beam excites or ionizes
the K or L shell of a heavy atom, the electron that falls into the created hole, while
falling, may interact with an outer electron, causing it to be ejected from the atom.
This is called the Auger effect, discovered by Lise Meitner. 8 Because the lowerenergy ejected electron has a short mean-free path in a solid material, one usually
detects these electrons as they are ejected from the surface of the material placed in
a vacuum.
8.6.9 Mammography
Mammography uses low-energy X-rays (≈30 kV peak) to view the interior of
the human breast, looking for potentially cancerous masses. The X-rays used in
mammograms deposit up to 5 mGy per exposure, with an average of 4 mGy. Natural
background is 2–3 mGy per year.
8.6.10 CAT Scans
A Computer Assisted Tomographic scan (CAT scan), also called a Computed
Tomography (CT) scan, applied to humans, is a computerized axial tomographic
view of body organs, tissue and bones. The idea is that the absorption and scattering
of X-rays depends on the electron density around atoms and molecules and also on
the presence of K-shell electrons in heavy elements. A selected heavy element is
sometimes administered beforehand to increase the scan contrast in certain organs.
By sending X-rays from one direction and confined to a plane which extends through
the body, the absorption pattern in that plane can be measured by detecting the
intensity of the transmitted X-rays. By rotating the direction of the X-rays (but
keeping their plane), a sequence of such intensity patterns is recorded. From these,
a two-dimensional cross-sectional density pattern can be constructed by a computer
analysis. Combining the density patterns for a sequence of adjacent planes, a three8 Lise Meitner, along with Otto Hahn and Otto Robert Frisch, discovered nuclear fission in 1939.