312
8 Ionizing Radiation and Life
• Mott scattering (of electrons by nuclei): A relativistic electron can be scattered by
the nuclei of atoms. (The probability is highest when the electron wavelength is
comparable to the nuclear size.) Mott scattering does not include the emission of
a photon in the scattering process. Since nucleons are 1836 times more massive
than electrons, even electrons with kinetic energy up to the order of GeV’s will
not kick the nuclei very hard. Rather, the electrons will be scattered away by the
Coulomb force while hardly moving the nuclei of atoms.
• Bhabha scattering (of positrons from electrons): Energetic positrons, in beams or
from radioactive emissions, can scatter from electrons in a material before they
are captured and annihilated by combining with electrons.
• Bremsstrahlung (of electrons with nuclei): An electron, while going around a
nearby nucleus, necessarily accelerates. In quantum theory, this implies that the
electron has a finite probability of emitting a photon. This kind of emission is
called Bremsstrahlung (‘breaking radiation’), and is one of the ways X-rays are
produced. The process is also a mechanism for energy deposit into a material by
a charged particle beam.
• Particle Pair Creation (by a photon near a nucleus): If a photon passing nearby
to a nucleus has an energy greater than 1.022 MeV, it has a chance of emitting a
pair of particles, the electron and the positron. These particles, in turn, contribute
to the ionization produced by a gamma-ray beam.
• Cherenkov radiation (by charged particles moving through materials): If a
charged particle traveling through a material at speed V moves faster than the
speed of light in that material, v, an electromagnetic field shock wave is created
over a trailing cone with vertex at the particle’s location and with an opening half
angle θ c = arcsin (v/V ). (This is analogous to an airplane creating a sound shock
wave by traveling faster than sound in the air. See Sect. 5.13.) The blue glow from
water surrounding a neutron-rich radioisotope is due to the Cherenkov radiation
from the beta emissions. This blue glow can also be seen from the water pool
used in some nuclear reactors to moderate neutrons.
8.13.3 Linear Energy Transfer
In 1930, Hans Bethe calculated the rate of energy loss of a fully-ionized nuclear
particle beam passing through matter. For momenta of the beam particle in the
range 0.05 < p/(mc) < 500, the energy loss of the beam particle is dominated
by collisions of the ions with electrons that act almost free relative to the incoming
ion. Bethe’s expression 13
13 H. Bethe, Zur Theorie des Durchgangs schneller Korpuskularstrahlen durch Materie, Annalen
der Physik 397, 325–400 (1930).
8 Ionizing Radiation and Life
• Mott scattering (of electrons by nuclei): A relativistic electron can be scattered by
the nuclei of atoms. (The probability is highest when the electron wavelength is
comparable to the nuclear size.) Mott scattering does not include the emission of
a photon in the scattering process. Since nucleons are 1836 times more massive
than electrons, even electrons with kinetic energy up to the order of GeV’s will
not kick the nuclei very hard. Rather, the electrons will be scattered away by the
Coulomb force while hardly moving the nuclei of atoms.
• Bhabha scattering (of positrons from electrons): Energetic positrons, in beams or
from radioactive emissions, can scatter from electrons in a material before they
are captured and annihilated by combining with electrons.
• Bremsstrahlung (of electrons with nuclei): An electron, while going around a
nearby nucleus, necessarily accelerates. In quantum theory, this implies that the
electron has a finite probability of emitting a photon. This kind of emission is
called Bremsstrahlung (‘breaking radiation’), and is one of the ways X-rays are
produced. The process is also a mechanism for energy deposit into a material by
a charged particle beam.
• Particle Pair Creation (by a photon near a nucleus): If a photon passing nearby
to a nucleus has an energy greater than 1.022 MeV, it has a chance of emitting a
pair of particles, the electron and the positron. These particles, in turn, contribute
to the ionization produced by a gamma-ray beam.
• Cherenkov radiation (by charged particles moving through materials): If a
charged particle traveling through a material at speed V moves faster than the
speed of light in that material, v, an electromagnetic field shock wave is created
over a trailing cone with vertex at the particle’s location and with an opening half
angle θ c = arcsin (v/V ). (This is analogous to an airplane creating a sound shock
wave by traveling faster than sound in the air. See Sect. 5.13.) The blue glow from
water surrounding a neutron-rich radioisotope is due to the Cherenkov radiation
from the beta emissions. This blue glow can also be seen from the water pool
used in some nuclear reactors to moderate neutrons.
8.13.3 Linear Energy Transfer
In 1930, Hans Bethe calculated the rate of energy loss of a fully-ionized nuclear
particle beam passing through matter. For momenta of the beam particle in the
range 0.05 < p/(mc) < 500, the energy loss of the beam particle is dominated
by collisions of the ions with electrons that act almost free relative to the incoming
ion. Bethe’s expression 13
13 H. Bethe, Zur Theorie des Durchgangs schneller Korpuskularstrahlen durch Materie, Annalen
der Physik 397, 325–400 (1930).