Chapter 1
Introduction and Historical Background
1.1 X-rays and the Electromagnetic Spectrum
This is a book about X-rays—how they are created with synchrotron radiation and
how they are used in X-ray spectroscopy. For many of us, our first encounter with
X-rays is as a child in a dentist’s chair, or with a broken bone in the doctor’s office,
or perhaps at an airport baggage inspection. Compared to other forms of electromagnetic radiation, such as visible light that we see with, infrared radiation that we
feel as warmth on our skin, or microwaves that cook our food, X-rays have always
seemed somewhat mysterious—perhaps vaguely associated with Superman in our
unconscious mind. However, as illustrated in Fig. 1.1, X-rays are just another part of
the continuous spectrum of electromagnetic radiation.
There are many different definitions of the X-ray region. From a chemist’s point
of view, X-ray radiation is involved in excitation of the core electrons of atoms. This
ranges from the 55 eV required to dislodge a 1s electron in lithium to the 116 keV
required to do the same for uranium. On the low-energy side, the extreme ultraviolet
(EUV) community has laid claim to photons from 30 to 250 eV, while on the highenergy side there are γ-rays, often produced during nuclear reactions and typically
involving hundreds of keV or more. Even with a conservative definition of 100 eV to
100 keV, X-rays vary in energy or wavelength by three orders of magnitude,
compared to the ~two-fold variation for all of visible light (Fig. 1.1).
Another fuzzy boundary is between “soft X-rays” and “hard X-rays.” X-rays are
called “soft” when they do not penetrate deeply. For our purposes, soft X-rays are
those with energies below ~2 keV, a range normally served by vacuum beamlines
and grating monochromators, while hard X-rays are above ~2 keV and generally use
crystal monochromators. Some have tried to define a middle region as “tender”
X-rays; this seems more cute than useful.
© Springer Nature Switzerland AG 2020
S. P. Cramer, X-Ray Spectroscopy with Synchrotron Radiation, Biological and Medical
Physics, Biomedical Engineering, https://doi.org/10.1007/978-3-030-28551-7_1
1
Introduction and Historical Background
1.1 X-rays and the Electromagnetic Spectrum
This is a book about X-rays—how they are created with synchrotron radiation and
how they are used in X-ray spectroscopy. For many of us, our first encounter with
X-rays is as a child in a dentist’s chair, or with a broken bone in the doctor’s office,
or perhaps at an airport baggage inspection. Compared to other forms of electromagnetic radiation, such as visible light that we see with, infrared radiation that we
feel as warmth on our skin, or microwaves that cook our food, X-rays have always
seemed somewhat mysterious—perhaps vaguely associated with Superman in our
unconscious mind. However, as illustrated in Fig. 1.1, X-rays are just another part of
the continuous spectrum of electromagnetic radiation.
There are many different definitions of the X-ray region. From a chemist’s point
of view, X-ray radiation is involved in excitation of the core electrons of atoms. This
ranges from the 55 eV required to dislodge a 1s electron in lithium to the 116 keV
required to do the same for uranium. On the low-energy side, the extreme ultraviolet
(EUV) community has laid claim to photons from 30 to 250 eV, while on the highenergy side there are γ-rays, often produced during nuclear reactions and typically
involving hundreds of keV or more. Even with a conservative definition of 100 eV to
100 keV, X-rays vary in energy or wavelength by three orders of magnitude,
compared to the ~two-fold variation for all of visible light (Fig. 1.1).
Another fuzzy boundary is between “soft X-rays” and “hard X-rays.” X-rays are
called “soft” when they do not penetrate deeply. For our purposes, soft X-rays are
those with energies below ~2 keV, a range normally served by vacuum beamlines
and grating monochromators, while hard X-rays are above ~2 keV and generally use
crystal monochromators. Some have tried to define a middle region as “tender”
X-rays; this seems more cute than useful.
© Springer Nature Switzerland AG 2020
S. P. Cramer, X-Ray Spectroscopy with Synchrotron Radiation, Biological and Medical
Physics, Biomedical Engineering, https://doi.org/10.1007/978-3-030-28551-7_1
1