302
8 Ionizing Radiation and Life
dimensional image can also be generated. This computer-generated 3-D image can
be viewed from arbitrary angles on a two-dimensional display screen.
In a CT scan, differences in electron density less than 1% can be discerned.
However, radiation doses may be 100–1000 times higher than conventional chest
X-rays, and so involves an increased cancer risk. Exposure of a fetus to 10 mGy
increases its risk of cancer by the age of 20 from 0.03 to 0.04%.
8.7 Gamma Rays
Gamma rays are made naturally by radioactive materials and by cosmic rays
hitting the atoms in the upper atmosphere. They are made by humans in particle
accelerators by synchrotron radiation, by the scattering of high-energy nuclear
particles, by nuclear reactors, and by particle-antiparticle annihilation. In the
following, we will focus on the application of gamma rays in biological research
and medicine.
8.7.1 Positron-Emission Studies
Positron-emission studies takes advantage of the fact that two oppositely directed
gamma rays are emitted by positron annihilation, so that the location of the emission
region can be pinpointed. For example, in proton beam therapy against cancer
tumors, a small number of radioactive nuclei can be produced. Their beta decay
can produce positrons, which then annihilate with local electrons, producing two
gamma particles moving in opposite direction (without much interaction with the
surrounding tissue). Detecting these gammas lets one map where the energy deposit
of the protons is concentrated.
Positron-emission tomography will be described in more detail in the Sect. 8.9.
8.7.2 Mössbauer Techniques
As an example of the use of gamma rays in biochemical research, we will describe
how the Mössbauer effect can be employed in the investigation of organic molecular
structure.
Because of their short wavelength (of nuclear dimensions), gamma rays are
not emitted by electron transitions in molecules and atoms. But gamma rays can
be emitted by the acceleration of charges in the nuclei of atoms. It is therefore
remarkable that this kind of emission can be used to study the behavior of valence
electrons in molecular bonds. However, even the valence electrons can briefly visit
the interior of nuclei, those small heavy dots at the center of atoms, a few ten
8 Ionizing Radiation and Life
dimensional image can also be generated. This computer-generated 3-D image can
be viewed from arbitrary angles on a two-dimensional display screen.
In a CT scan, differences in electron density less than 1% can be discerned.
However, radiation doses may be 100–1000 times higher than conventional chest
X-rays, and so involves an increased cancer risk. Exposure of a fetus to 10 mGy
increases its risk of cancer by the age of 20 from 0.03 to 0.04%.
8.7 Gamma Rays
Gamma rays are made naturally by radioactive materials and by cosmic rays
hitting the atoms in the upper atmosphere. They are made by humans in particle
accelerators by synchrotron radiation, by the scattering of high-energy nuclear
particles, by nuclear reactors, and by particle-antiparticle annihilation. In the
following, we will focus on the application of gamma rays in biological research
and medicine.
8.7.1 Positron-Emission Studies
Positron-emission studies takes advantage of the fact that two oppositely directed
gamma rays are emitted by positron annihilation, so that the location of the emission
region can be pinpointed. For example, in proton beam therapy against cancer
tumors, a small number of radioactive nuclei can be produced. Their beta decay
can produce positrons, which then annihilate with local electrons, producing two
gamma particles moving in opposite direction (without much interaction with the
surrounding tissue). Detecting these gammas lets one map where the energy deposit
of the protons is concentrated.
Positron-emission tomography will be described in more detail in the Sect. 8.9.
8.7.2 Mössbauer Techniques
As an example of the use of gamma rays in biochemical research, we will describe
how the Mössbauer effect can be employed in the investigation of organic molecular
structure.
Because of their short wavelength (of nuclear dimensions), gamma rays are
not emitted by electron transitions in molecules and atoms. But gamma rays can
be emitted by the acceleration of charges in the nuclei of atoms. It is therefore
remarkable that this kind of emission can be used to study the behavior of valence
electrons in molecular bonds. However, even the valence electrons can briefly visit
the interior of nuclei, those small heavy dots at the center of atoms, a few ten