Q2- Interaction of ionizing radiation with matter / penetration, scattering and absorption.
The interaction of photons (γ-quantums) with matter involves several distinct processes. The relative
importance and efficiency of each process is strongly dependent upon the energy of the photons and
upon the density and atomic number of the absorbing medium.
X-rays interact with matter in three main ways, through photoabsorption, Compton scattering,
and Rayleigh scattering. The strength of these interactions depend on the energy of the X-rays and the
elemental composition of the material, but not much on chemical properties since the X-ray photon
energy is much higher than chemical binding energies. Photoabsorption or photoelectric absorption is
the dominant interaction mechanism in the soft X-ray regime and for the lower hard X-ray energies. At
higher energies, Compton scattering dominates.
a) Photoelectric absorption
The most probable fate of a photon having energy slightly higher than the binding energy of atomic
electrons is photoelectric absorption. In this process, the photon transfers all of its energy to the
electron and its own existence terminates. The electron will escape its orbit with a kinetic energy equal
to the difference between the photon energy and its own binding energy. Photoelectric absorption is
most important for photons below 0.1 MeV if the absorbing medium is water or biological tissue.
However, in high Z (atomic mass number) materials such as lead, this process is relatively important for
photons up to about 1 MeV. As with ionization produced by any process, secondary radiation are
initiated, in this case, by the photoelectron which may have sufficient energy to produce additional
ionization and excitation of orbital electrons. Also, filling of the electron vacancy left by the
photoelectron results in characteristic X-rays.
The probability of a photoelectric absorption per unit mass is approximately proportional to Z3/E3,
where Z is the atomic number and E is the energy of the incident photon. This rule is not valid close to
inner shell electron binding energies where there are abrupt changes in interaction probability, so
called absorption edges. However, the general trend of high absorption coefficients and thus
short penetration depths for low photon energies and high atomic numbers is very strong. For soft
tissue photoabsorption dominates up to about 26 keV photon energy where Compton scattering takes
over. For higher atomic number substances this limit is higher. The high amount of calcium (Z=20) in
bones together with their high density is what makes them show up so clearly on medical radiographs.
A photoabsorbed photon transfers all its energy to the electron with which it interacts, thus ionizing the
atom to which the electron was bound and producing a photoelectron that is likely to ionize more atoms
in its path. An outer electron will fill the vacant electron position and produce either a characteristic
photon or an Auger electron. These effects can be used for elemental detection through X-ray
spectroscopy or Auger electron spectroscopy.
b) Compton scattering / Compton effect
The Compton effect is usually the predominant type of interaction for medium energy photons (0.3 to 3
MeV). In this process the photon interacts with an atomic electron sufficiently to eject it from orbit, the
photon retains a portion of its original energy and continues moving in a new direction. Thus, the
Compton effect has an absorption component and scattering component. The amount of energy lost by
the photon can be related to the angle at which the scattered photon travels relative to the original
direction of travel. The scattered photon will interact again, but since its energy has decreased, it
becomes more probable that it will enter into a photoelectric or Rayleigh interaction. The free electron
produced by the Compton process may be quite energetic and behave like a beta particle of similar
energy, producing secondary ionization and excitation before coming to rest.
Compton scattering is the predominant interaction between X-rays and soft tissue in medical imaging.
Compton scattering is an inelastic scattering of the X-ray photon by an outer shell electron. Part of the
energy of the photon is transferred to the scattering electron, thereby ionizing the atom and increasing
the wavelength of the X-ray. The scattered photon can go in any direction, but a direction similar to the
original direction is a bit more likely, especially for high-energy X-rays. The transferred energy can be
directly obtained from the scattering angle from the conservation of energy and momentum.
c) Rayleigh scattering
When a photon interacts with atom, it may or may not impart some energy to it. The photon may be
deflected with no energy transfer. This process is called Rayleigh scattering and is most probable for
very low-energy photons.
Rayleigh scattering is the dominant elastic scattering mechanism in the X-ray regime. Inelastic forward
scattering gives rise to the refractive index, which for X-rays is only slightly below 1.
d) Penetration
The penetration depth varies with several orders of magnitude over the X-ray spectrum. This allows the
photon energy to be adjusted for the application so as to give sufficient transmission through the object
and at the same time good contrast in the image.
Ionizing Power and Penetration Power are linked.
The penetrating power of nuclear radiation depends upon the ionizing power of the radiation. The
radiation continues to penetrate matter until it has lost all of its energy. The further it can penetrate into
the substance the more spread out the ionization it causes will be, so the more localized the ionization
the less penetrating power it will possess.
Alpha particles are the least penetrating as they are the most densely ionizing. They are absorbed by 10
cm of air, 0.01 mm lead or a sheet of paper. This means that if a given number of alphas are fired at a
target they will all cause ionization near the surface of the material, resulting in the effects of the
radiation being concentrated in a small volume. The double charge and considerable mass of the alpha
in comparison with the other nuclear radiation forms explains why the impact on matter is so great.
Beta particles can penetrate quite deeply into matter before its energy has been used up.
Their penetrating power is therefore moderate (absorbed by 1m air, 0.1 mm lead or 3mm aluminium
sheet). Betas have only about 1/8000 of the mass of an alpha paticle and only half of the charge.
Therefore its interaction with matter as it passes through is far less severe. Therefore the effects of its
interaction (ionization) are much more spread out.
Gamma Rays (X-rays) have an ionizing power so low that they penetrate very deeply into matter before
most of the energy has been used up. Their penetrating power is therefore very high (about 99.9% is
absorbed by 1 km of air or 10 cm lead). Gamma rays are pure energy - no charge and no mass therefore their interaction with matter is much less than the other two.