Interaction of Photons with Matter Flashcards
List the 7 most important photon interactions with absorbing medium in medical physics
The 7 most important photon interactions with absorbing medium are:
(1) Thomson scattering, (2) Compton effect, (3) Rayleigh scattering, (4) Photoelectric effect, (5) Nuclear pair production, (6) Electronic pair production, and (7) Photonuclear reaction (photodisintegration).
For the seven photon interactions with absorbing medium listed in (a) prepare a table with the following entries: (1) Symbol used for attenuation coefficient, (2) Type of target, (3) Photon fate after interaction, and
(4) Charged particles released or produced in the interaction
(1) Symbols used for designation of attenuation and absorption coefficients: Of the seven important photon interactions with atoms of the absorber, two are designated with separate Greek letters: photoelectric effect with τ (tau) and pair production with κ (kappa) where κNPP stands for nuclear pair production and κTP for electronic (triplet) production. Four effects are designated with Greek letter sigma (σ ) that is
in nuclear physics used to designate general interaction cross sections. In radiation physics σ is used with a subscript to designate four specific types of photon interaction: σTh for Thomson scattering, σC for Compton effect, σR for Rayleigh scattering, and σPN for photodisintegration (photonuclear effect).
(2) Type of target. A closer look at photon interaction with absorber atom reveals that the interaction can be either with an orbital electron or nucleus.
(i) Interaction with an orbital electron can be interaction with an essentially “free electron” (Thomson scattering and Compton effect) or interaction with a tightly bound electron (photoelectric effect) or interaction with the electric field of the electron (triplet production).
(ii) Interaction with the nucleus can be interaction with electric field of the nucleus (nuclear pair production) or actual direct interaction with the nucleus (photodisintegration).
(3) As far as the fate of the photon after interaction is concerned there are two
possibilities: (i) photon disappearance and (ii) photon scattering.
(i) In photon disappearance the photon is completely absorbed (photoelectric effect, nuclear pair production, triplet production, and photodisintegration).
(ii) In photon scattering photon is scattered with no energy loss (Rayleigh scattering) or with concurrent loss of the energy (Compton effect).
(4) Charged particles released or produced in photon interaction with absorber atom.
(i) In Thomson and Rayleigh scattering there is no release or production of charged particles. In photoelectric effect, Compton effect, and triplet production orbital electrons are released from the absorber atom.
(ii) In pair production and triplet production an electron / positron pair is produced in the electric field of the nucleus and electric field of the orbital electron,
respectively.
(iii) In photodisintegration heavy charged particles and neutrons may be released from the nucleus.
Describe the salient features of narrow beam geometry and broad beam geometry and draw a schematic diagram for the two geometries.
Narrow beam geometry technique in attenuation measurements of photon beams implies a narrowly collimated source of mono-energetic photons and a relatively small radiation detector. As shown in Fig. 7.5(A), a slab of absorber material of thickness x is placed between the source and detector. The absorber decreases the detector signal (intensity which is proportional to the number of photons striking the
detector) from I (0) measured without the absorber in place to signal I (x) measured with absorber thickness x in the beam. In contrast to the narrow beam geometry that is used in determination of the various attenuation coefficients for photon beam attenuation, one can also deal with the so-called broad beam geometry in which the detector reading is not only diminished by attenuation of the primary beam in the absorber, but is also increased by the radiation scattered from the absorber into the detector, as shown schematically
in Fig. 7.5(B).
The table below shows a comparison between two attenuation experiments on a radiation source using lead as absorber; experiment (1)was carried out under narrow beam geometry conditions, experiment (2) under broad beam conditions. Plot the data for both experiments on a semi-log plot, identify the source type (mono-energetic or heterogeneous beams), and indicate which beam was measured under narrow beam geometry conditions.
The attenuation data for the two experiments are plotted in Fig. 7.6 on semilog graph paper. Two straight lines arise, one for each experiment. Since the data
fall on straight lines, we conclude that the radiation source used in experiments was a mono-energetic source.
The answer to the question on which of the two straight lines represents the narrow beam data is relatively simple, since we are dealing with a mono-energetic source and neither beam hardening nor beam softening effects are expected to happen
in the lead absorber. The lower straight line that appears to give a less penetrating beam in Fig. 7.6 is attributed to narrow beam geometry and the higher straight line is attributed to broad beam geometry.
The term “Compton scattering” usually refers to inelastic scattering of a photon on loosely bound electrons of an absorber. However, other, more exotic, Compton phenomena are also known, such as for example,
nuclear Compton effect and inverse Compton effect. Briefly describe these two phenomena
Nuclear Compton effect is a process in which an incident photon of energy hν interacts with a nucleus, transfers some energy to nuclear recoil, and is emitted with diminished energy hν at scattering angle θ . This effect may occur but it is much less probable than the standard Compton effect in which a photon is scattered off a “free and stationary” orbital electron (see Prob. 182). Inverse Compton effect is a process in which a low energy photon interacts with a highly relativistic electron and gains and the electron loses energy in contrast to the standard Compton effect where through the photon–“free and stationary electron” interaction the photon loses energy and the recoil electron gains energy
Pair production
describe production of an electron-positron
pair (materialization) out of energy (photon) in either the electric field of the nucleus of an absorber atom (called nuclear pair production) or in the electric
field of an orbital electron of an absorber atom (called triplet production or electronic pair production). The threshold for nuclear pair production is slightly larger than the sum of the rest masses of the electron and positron
