Interaction of radiation with matter Flashcards
Which of the following statements concerning the interaction of photons with matter is CORRECT?
A. The probability of the photoelectric effect decreases with the atomic number of the absorber
B. The predominant interaction of 10 keV photons with soft tissue is the Compton process
C. In the Compton process, the energy of the scattered photon is less than that of the incident photon
D. Pair production occurs for photons with energies less than 1.02 MeV
E. There is only partial absorption of the energy of the incident photon in the photoelectric effect
C
In the Compton process, a photon interacts with an atom causing the ejection of an orbital electron. The incident photon, now with reduced energy, continues along a deflected path. The probability of the photoelectric effect increases with the atomic number of the absorber (Answer Choice A).
The predominant interaction of 10 keV photons in soft tissue is the photoelectric effect (Answer Choice B).
Pair production occurs for photons with energies greater than 1.02 MeV and results in the complete conversion of the photon’s energy into the production of a positron and electron (Answer Choice D).
For the photoelectric effect, there is complete absorption of the photon’s energy, resulting in ejection of an electron that possesses kinetic energy equal to the difference between the incident photon’s energy and the electron’s binding energy (Answer Choice E).
Which one of the following is a radiolysis product of water responsible for the molecular damage caused by the indirect action of ionizing radiation?
A. e aq
B. 1 O2
C. OH-
D. OH*
E. O2-
D
65-75% of the damage caused by indirect action is mediated by the hydroxyl radical, OH*
Little biological damage is caused by the hydrated electron (e aq; Answer Choice A).
1 O2 is produced primarily by photosensitizers and, rarely, by ionizing radiation (Answer Choice B).
Neither OH- nor O2- are primary radiolysis products, although O2- can be produced secondarily by reaction of eaq with O2 (Answer Choices C and E).
The approximate minimum photon energy required to cause ionization of a water molecule is:
A. 10-25 eV
B. 100-250 eV
C. 1-2.5 keV
D. 10-25 keV
E. 100-250 keV
A
On average, about 25 eV is required to create an ion pair in water, although the minimum energy needed to eject an electron is only 12.6 eV.
Which of the following X-ray interactions with matter is most important for producing high-contrast diagnostic radiographs?
A. Compton process
B. Pair production
C. Photoelectric effect
D. Nuclear disintegration
E. Coherent scattering
C
The photoelectric effect is the predominant interaction responsible for producing high quality diagnostic radiographs. At relatively low photon energies, the photoelectric effect is the most likely photon interaction and is the desirable type of photon/tissue interaction since there is complete photon absorption with no production of secondary photons. The other possible tissue interactions at the photon energies used in diagnostic radiology are the Compton effect and coherent scattering. For these interactions, a deflected photon traveling in an altered direction is
produced at the site of interaction. If these secondary photons are
permitted to reach the film, there would be a reduction in image sharpness and loss of spatial resolution. Furthermore, with the photoelectric effect, absorption of photons is dependent on the cube of the atomic number of the material. The resultant differential of absorption in tissue allows for the ability to differentiate between bone, soft tissue, and air.
Which of the following pairs of photon energy and predominant atomic interaction at the specified photon energy is correct?
A. 1 keV – pair production
B. 50 keV – triplet production
C. 100 keV – compton process
D. 2 MeV – photoelectric effect
C
The predominant atomic interaction for 100 keV photons is the Compton process. Sources provide different answers on minimum energy for triplet production with some stating 2mC2 (1.02 MeV) and some stating 4mC2 (2.04 MeV) The photoelectric effect is predominant for photon energies in the range of 10 keV.
Which of the following statements is correct? High LET radiations:
A. Include 250 kVp X-rays, 200 MeV protons, and 1.1 MV X-rays
B. Produce much higher yields of OH radicals than do either X-rays or gamma-rays
C. Are components of solar flares but not of cosmic rays
D. Produce less dense ionization tracks than X-rays
E. Produce increased numbers of clustered lesions in DNA than X-rays
E
High linear energy transfer (LET), or densely ionizing, radiations include particles such as 290 MeV carbon ions, GeV cosmic particles and neutrons. 250 kVp X-rays, 200 MeV protons and 1.1 MV X-rays are all low LET, or sparsely ionizing radiations (Answer Choice A).
Although high LET radiations produce more clustered lesions (multiply damaged sites) in DNA than low LET radiations (Answer Choice E), they actually produce lower yields of OH radicals because of the extensive ion and radical recombination within spurs and blobs (Answer Choice B). High LET radiations, such as iron or carbon ions, are components of cosmic rays, while solar flares are composed largely of energetic protons (which are low LET; Answer Choice C).
The lifetime of an OH* radical is approximately:
A. 10-15 second
B. 10-9 second
C. 10-1 second
D. 1 second
E. 1 minute
B
The initial ionization process takes approximately 10-15 second. The primary radicals produced by the ejection of an electron typically have a lifetime of 10-10 second. The resulting hydroxyl radical has a lifetime of approximately 10-9 second. The DNA radicals subsequently produced have a lifetime of approximately 10-5 second.
Regarding pair production and annihilation, which of the following is true?
A. The incident photon is scattered with reduced energy
B. Annihilation photons always have an energy of 0.511 MeV each
C. A pair of orbital electrons are ejected from the atom
D. Two positrons are emitted at 180 degrees
E. It cannot occur if the photon energy is above 1.02 MeV
B
Annihilation photons always have an energy of 0.511 MeV each, which is equal to the rest energy of the positron and electron.
Directly ionizing radiation includes all of the following EXCEPT:
A. Electrons
B. Positrons
C. Alpha particles
D. Neutrons
E. Betas
D
Neutrons are not charged particles and, therefore, cannot ionize atoms directly. They do, however, transfer some of their energy to protons or light nuclei, which then cause ionization. They are, therefore, indirectly ionizing.
Concerning fast neutron interactions with matter, which of the following is FALSE?
A. They do not interact with atomic electrons of biological media
B. They interact primarily with oxygen in water
C. They may cause the ejection of an alpha particle
D. They may activate the target nucleus.
E. They may transfer a large fraction of its energy in the process of elastic scattering.
B
Fast neutrons with kinetic energy between a few and several tens of MeV are slowed down in biological media mainly by elastic collisions with hydrogen nuclei (protons) of the cellular water. A fraction of energy lost by fast neutrons in elastic collision with oxygen nuclei is less than 10% of that which occurs with hydrogen nuclei. Neutron beams used in radiation therapy can be made using a cyclotron by accelerating protons into a beryllium target. The fast neutrons then recoil protons in target tissue from elastic collisions and produce a large density of ionizations along their tracks. Neutrons do not interact with atomic electrons but, instead, interact with atomic nuclei. Alpha particles can be produced by neutron capture reactions with isotopes of both carbon and oxygen, but the probability is strongly dependent on the neutron energy and target material.
Example:
17O + n → 14C + α
Neutron absorption in a target nucleus is called activation. This is a process by which neutron radiation induces radioactivity in materials. It occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus often decays immediately by emitting gamma rays, beta particles, alpha particles, fission products, and/or neutrons (in nuclear fission). Neutron activation is a potential health hazard in therapy with high energy photons because when photons with energy > 10 MeV are utilized, neutrons are generated in linacs via the interaction of photons with nuclei of high atomic number materials within the linac head and the beam collimator systems. These
photoneutrons can have an energy of 0.1 to 2 MeV, are highly penetrating, have a quality factor of 20, and can significantly add to a patient’s offfield dose.
Which of the following results from the recombination of the initial water radiolysis products?
A. Solvated electron
B. Solvated proton
C. Hydrogen ion
D. Water
E. Only A and B
D
The main initial products of resulting from irradiation of pure water are the short-lived free radicals, hydrogen radical (H) (10%), hydroxyl radical (OH) (45%), and the solvated electrons (eaq ) (45%). These react with DNA or with each other. Therefore:
OH + H -> H2O
The remaining recombination reactions of free radicals are:
eaq + eaq +2 H2O -> H2 + 2 OH-
OH + OH -> H2O2
H + H -> H2
These reactions always compete with reactions that lead to direct damage of the biological molecules. The relative efficiency of the recombinations will depend on the separation of the short-lived free radicals after the passage of the charged particle, and therefore depend on LET. At low LET values, the spacing of the ionizations is large. As a result, *OH radicals are widely separated thereby decreasing the probability of recombination to form H2O2. As LET increases, the spacing between ionizations decreases and the probability of production of an *OH from one ionization event as well as an *OH from another ionization event along a single track increases. The yield of hydrogen peroxide increases rapidly with LET of about 20 - 150 keV/μm, the range of LET where direct damage to DNA dominates over indirect damage from the free radicals. Note that LET for photons is in the ~1 keV/µm range, while protons are approximately 10 keV/µm, carbon atoms are 10-100 keV/µm, and alpha particles or heavy charged particles are >100 keV/µm.
When a live human cell is irradiated by gamma-rays, which one of the following events may eventually cause most of the damage to DNA?
A. Absorption of radiation energies by the chemical bonds in the DNA molecules
B. Ionization and excitation on atoms within the DNA structure
C. Ionization and excitation on atoms within the histones that are bound to DNA
D. Ionization and excitation of the water molecules that surround DNA
E. Direct damage to the lipids that may later oxidize DNA
D
The indirect effect mediated by free-radical reactions involving water are most responsible to cause DNA damage upon low LET irradiation.
