8.2 Characterizing Ionizing Radiation
283
are also formed in a gas put in a strong electric field. The tracks are generated by
a cascade of electrons following the ionization path of the particle, i.e. a spark.
• Charge-discharge quartz-fiber dosimeters: In these devices, a gold-plated thin
quartz fiber is used as one electrode in an electroscope. Charging causes the fiber
to bend away from a fixed electrode electrically connected to the gold on the
fiber. Ionizing radiation discharges the device.
• Neutron detection: Neutrons can be detected using the absorption of neutrons
in light nuclei with the emission of a detectable alpha particle, such as in the
reactions 10 B(n, α) 7 Li and 6 Li(n, α) 3 H . The gas BF 3 in a tube will absorb
slow neutrons. Fast neutrons can be slowed in graphite and then detected.
(Graphite is a good moderator of neutrons because 12 C nuclei can easily scatter
neutrons, but will not easily absorb them.)
• Plastic tract-etch detectors/recorders: An energetic charged particle passing
through a clear plastic can leave a track of defects. Etching the plastic can make
these tracks visible. The tracks through the helmets of astronauts have been used
to gauge radiation exposure. 3
• Cherenkov detectors: A charged particle can move through a material faster than
the speed of light in that material. In doing so, an electromagnetic shock wave is
created, generating light.
Electromagnetic radiation with frequencies above about 10 15 Hz will contain
photons capable of breaking the bonds in many organic molecules. Just as in rain, for
which the chances that two drops to hit your nose at once is quite small, the chance
that an individual molecule absorbs the energy of two photons at once is also small
compared to the chance for a single hit, except for extraordinary intensities of light.
The discrete nature of light absorption means that a molecule is damaged not by the
buildup of the light wave energy, but rather by individual hits of photons. A single
photon with enough energy will suffice.
8.2 Characterizing Ionizing Radiation
Radioactive substances occur naturally, but also are manufactured. Background
radiation, which also includes secondary cosmic rays, affects biological systems
and even plays a role in the rate of biological mutation and therefore evolution.
Man-made ionizing radiation is used in medical diagnostics and therapy, as well as
in power devices and other technologies.
3 See Comstock et al., Cosmic-ray tracks in plastics: The Apollo helmet dosimetry experiment,
Science, Apr 9 1971. They find that 8–60 cells in the cerebellum per million would be killed by
cosmic radiation after spending a year in space near the Earth.
283
are also formed in a gas put in a strong electric field. The tracks are generated by
a cascade of electrons following the ionization path of the particle, i.e. a spark.
• Charge-discharge quartz-fiber dosimeters: In these devices, a gold-plated thin
quartz fiber is used as one electrode in an electroscope. Charging causes the fiber
to bend away from a fixed electrode electrically connected to the gold on the
fiber. Ionizing radiation discharges the device.
• Neutron detection: Neutrons can be detected using the absorption of neutrons
in light nuclei with the emission of a detectable alpha particle, such as in the
reactions 10 B(n, α) 7 Li and 6 Li(n, α) 3 H . The gas BF 3 in a tube will absorb
slow neutrons. Fast neutrons can be slowed in graphite and then detected.
(Graphite is a good moderator of neutrons because 12 C nuclei can easily scatter
neutrons, but will not easily absorb them.)
• Plastic tract-etch detectors/recorders: An energetic charged particle passing
through a clear plastic can leave a track of defects. Etching the plastic can make
these tracks visible. The tracks through the helmets of astronauts have been used
to gauge radiation exposure. 3
• Cherenkov detectors: A charged particle can move through a material faster than
the speed of light in that material. In doing so, an electromagnetic shock wave is
created, generating light.
Electromagnetic radiation with frequencies above about 10 15 Hz will contain
photons capable of breaking the bonds in many organic molecules. Just as in rain, for
which the chances that two drops to hit your nose at once is quite small, the chance
that an individual molecule absorbs the energy of two photons at once is also small
compared to the chance for a single hit, except for extraordinary intensities of light.
The discrete nature of light absorption means that a molecule is damaged not by the
buildup of the light wave energy, but rather by individual hits of photons. A single
photon with enough energy will suffice.
8.2 Characterizing Ionizing Radiation
Radioactive substances occur naturally, but also are manufactured. Background
radiation, which also includes secondary cosmic rays, affects biological systems
and even plays a role in the rate of biological mutation and therefore evolution.
Man-made ionizing radiation is used in medical diagnostics and therapy, as well as
in power devices and other technologies.
3 See Comstock et al., Cosmic-ray tracks in plastics: The Apollo helmet dosimetry experiment,
Science, Apr 9 1971. They find that 8–60 cells in the cerebellum per million would be killed by
cosmic radiation after spending a year in space near the Earth.