Radiation Interaction
Q&A
Robert Metzger, Ph.D.
RAPHEX General Question 2001
G56: The fractional number of photons
removed from a beam per cm of absorber is
called the _______.
A. Linear attenuation coefficient
B. Mass absorption coefficient
C. Scatter coefficient
D. Mean attenuation length
RAPHEX Answer for 2001
Question G56
A. Linear attenuation coefficient.
The mass energy absorption coefficient is the
linear attenuation coefficient divided by the
density (i.e., attenuation per unit mass rather
than per cm.) The mean attenuation length is
the linear attenuation coefficient X 1.44.
RAPHEX General Question 2003
G50: The mass attenuation coefficients for
most materials (except hydrogen) are similar
when________ interactions predominate.
A. Photoelectric
B. Compton
C. Pair Production
D. Photonuclear disintegration
RAPHEX Answer for 2003
Question G50
B. Compton
The mass attenuation coefficient is similar for
most materials in the Compton region, except
those containing hydrogen. This is because
most materials have approximately one
electron per two nucleons (one proton and
one neutron), while hydrogen has one
electron per nucleon, or twice the number of
electrons per unit mass as most other
elements.
RAPHEX General Question 2003
G52: The process whereby energy is
transferred from a photon beam to electrons
in the medium is called ______.
A. Electron capture
B. Absorption
C. Bremsstrahlung
D. Scatter
RAPHEX Answer for 2003
Question G52
B. Absorption.
In the interaction of radiation with matter,
energy is generally transferred to electrons
and photons. The energy reemitted as
photons is called “scatter”. The energy
transferred to electrons is called “absorbed
dose” and is dissipated locally in collisions
with atoms.
RAPHEX General
Question 2001
G60: A photoelectric interaction occurs between an 8
keV photon and a K shell electron. A 3.6 keV
photoelectron is emitted. The binding energy of the K
shell is ________ keV.
A. 3.6
B. 4.0
C. 4.4
D. 8.0
E. 11.6
RAPHEX Answer for 2001 General
Question G60
C. 4.4
The energy of the photon is totally
absorbed by the electron, which
uses 4.4 keV to overcome its
binding energy, leaving 3.6 keV as
the energy of the emitted
photoelectron.
RAPHEX General
Question 2002
G57: At x-ray energies between 40 and 100
keV, ________ absorbs more energy than
________ per gram.
A. fat, muscle
B. muscle, bone
C. iodine, bone
D. fat, air
E. air, muscle
RAPHEX Answer for 2002 General
Question G57
C. Iodine, bone.
The absorbed dose per unit exposure is
called the f-factor, and depends on both Z
and the photon energy. f factors listed from
lowers to highest are: fat, air, muscle, bone,
and iodine.
RAPHEX General
Question 2003
G63: In diagnostic x-ray systems, filters are
used to “harden” the beam. This process is
mainly due to:
A. Coherent scattering.
B. Photoelectric effect.
C. Compton effect.
D. Pair production.
E. A, B, and C only.
RAPHEX Answer for 2003 General
Question G63
B. Photoelectric Effect
With the exception of the K edges,
photoelectric interaction are more likely at low
energy than at high energy. After passing
through a filter, the total beam intensity is
reduced, but the beam contains a relatively
greater number of high-energy photons than
before filtration.
RAPHEX General
Question 2002
G59: The ratio of the number of Compton
interactions in one gram of calcium to one
gram of carbon is about ________ .
A. 2:1
B. 1:1
C. 1:2
D. It depends on the photon energy
E. In the ratio of their densities
RAPHEX Answer for 2002 General
Question G59
B. 1:1
The number of Compton interactions
depends on the number of electrons. Most
materials have about the same number of
electrons per gram. (An exception to this is
hydrogen, which has twice as many
electrons as it has no neutrons in the
nucleus.)
RAPHEX General
Question 2003
G56: if a technologist were to stand 2 m
away from a patient during fluoroscopy
(outside the primary beam) the dose received
by the technologist would be mainly due to:
A. Compton electrons.
B. Photoelectrons.
C. Compton scattered photons.
D. Characteristic x-rays generated in the
patient.
E. Coherent scatter.
RAPHEX Answer for 2003 General
Question G56
C. Compton scattered photons
Even at low KV, coherent scatter contributes
only a small part of the total scatter. The
characteristic x-rays created by photoelectric
interactions within the patient are of very low
energy (because of the low Z of tissue) and
have extremely small range. Compton and
photoelectrons also have a short range and
are unlikely to leave the patient’s body.
RAPHEX General
Question 2003
G58: The probability, per gram, of a Compton
interaction:
A. Increases as energy increases.
B. Is independent of energy.
C. Is proportional to E2Z2
D. Is proportional to Z3E-3
E. None of the above.
RAPHEX Answer for 2003 General
Question G58
E. None of the above.
Compton interactions, per unit mass, are
approximately independent of Z, and
decrease with increasing energy.
RAPHEX General
Question 2001
G63: Concerning pair production, which of the
following is true?
A. Photons can be backscattered.
B. The electron acquires most energy when it is
scattered forward.
C. Compton electrons can be backscattered.
D. Any dose measured at a diagnostic x-ray
operator’s console would be due to
Compton scattered photons.
RAPHEX Answer for 2001 General
Question G63
B. The electron acquires most energy
when it is scattered.
The threshold energy for pair production is
2 mec2 = 1.02 MeV. The electron and the
positron share the photon energy less 1.02
MeV. When the positron loses its kinetic
energy and annihilates with an electron, two
0.51 MeV photons are emitted in opposite
directions.
RAPHEX General Question 2002
G63: In pair production, which of the following is
true?
A. The incident photon is scattered with reduced
energy
B. Annihilation photons always have an energy of
0.51 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
RAPHEX Answer for 2002
Question G63
B. Annihilation photons always have an
energy of 0.51 each.
Annihilation photons have an energy
equal to the rest energy of the positron
and electron.
RAPHEX General Question 2003
G61: After a 1.5 MeV photon undergoes pair
production, production of ____ will always occur.
A. A pair of 0.51 MeV photons
B. A pair of 0.51 MeV positrons
C. A pair of 0.51 MeV electrons
D. A single 0.51 MeV photon
E. An electrons a positron, each with kinetic energy
0.51 MeV
RAPHEX Answer for 2003
Question G61
A. A pair of 0.51 MeV photons
It takes 2 X 0.51 MeV to create the electronpositron pair. The remaining 1.28 MeV is
divided between the two particles as kinetic
energy. This kinetic energy is deposited
locally and when the positron and an
electron combine and annihilate, two 0.51
MeV phtons are emitted.
RAPHEX General Question 2002
A.
B.
C.
D.
E.
G67-G68: Match the type of interaction mainly responsible for
the effect:
Compton scattered photons
Compton electrons
Pair production
Photoelectric interaction
Photo-nuclear disintegration
G67: Differential absorption in diagnostic radiographs
G68: Loss of contrast in a lateral pelvic radiograph of a large
patient.
RAPHEX Answer for 2002 General
Questions G67-G68
G67: D. Photoelectric interaction. This is due to
the Z3 dependence of the photoelectric
interaction
G68: A. Compton scattered photons. The use
of high ratio grids and limiting the field size
will help to reduce the unwanted Comptonscattered photons, which degrade contrast in
the image
RAPHEX General Question 2002
G69: PET scan images are created by
detecting:
A. Positrons emitted from the region of uptake
B. Annihilation photons
C. Protons emitted from the region of uptake
D. The paths of the positrons traveling
through tissue
RAPHEX Answer for 2002
Question G69
B. Annihilation photons
PET (Positron Emission Tomography)
relies on coincidence counting of the
annihilation photons that are emitted at 180
degrees from the point where a positron
and electron annihilate. The positron
emitter is labeled to a substance which is
taken up by, or metabolized by, the tissue
to be imaged.
RAPHEX General Question 2001
G64-G69: Match the most appropriate
interaction to the description (answers may be
used more than once)
A.
B.
C.
D.
Coherent scatter
Photoelectric
Compton
Pair production
G64: Chiefly responsible for loss of contrast in diagnostic
radiograph
G65: Most probably at photon energies between 100 keV and
2MeV
G66: No energy is transferred or locally absorbed
G67: Probability of interaction, per unit mass, does not dpend on Z
G68: Involves “bound” electrons
G69: Increases with increasing photon energy
RAPHEX Answer for 2001
Questions G64-G69
G64: C. Compton scattered photons decrease the
contrast by increasing the level of background fog
G65: D. Compton is actually most probably for
monoenergetic photon energies between 25 kV and
25MV
G66: A. Coherent scatter (also known as Raleigh or
unmodified scatter) occurs only at very low photon
energy, and is of little concern in radiology
G67: C. The probability of a Compton interaction
depends on the number of electrons per gram, which
is roughly the same for all elements except hydrogen
G68: B.
G69: D.
RAPHEX General Question 2001
G70: Which type of nuclear radiation has the
shortest range in tissue? (assume equal
energy)
A. Gamma rays
B. Betas
C. Neutrinos
D. Alphas
E. Neutrons
RAPHEX Answer for 2001
Question G70
D. Alphas
Alphas have the highest linear energy
transfer (LET) because of the their
larger mass and charge.
RAPHEX General Question 2002
G11: Directly ionizing radiation includes all of
the following EXCEPT:
A. Electrons
B. Positrons
C. Alpha particles
D. Neutrons
E. Betas
RAPHEX Answer to 2002
Question G11
D. Neutrons
Neutrons are not charged particles and,
therefore, cannot ionize atoms directly.
However, they transfer some of their
energy to protons or light nuclei, which
then cause ionization. Thus they are
indirectly ionizing.
RAPHEX General Question 2003
G67: An electron, proton, and an alpha
particle each have 20 MeV kinetic energy.
Which of the following statements is true?
A. The alpha particle travels at almost the
speed of light
B. The alpha particle has the least total
energy
C. The proton has the highest total energy
D. The electron travels almost the speed of
light
E. None of the above
RAPHEX Answer to 2003
Question G67
D. The proton has the highest total energy
The electron’s rest mass is low
compared to its kinetic energy so it must
be traveling at nearly the speed of light.
The alpha particle has the greatest total
energy.
RAPHEX General Question 2003
G68: When protons interact with soft tissue,
all of the following are true EXCEPT:
A. Linear energy transfer (LET) increases
towards the end of the proton track
B. Protons have a finite range
C. Protons undergo exponential attenuation
D. The proton track ends in a “Bragg Peak”.
RAPHEX Answer to 2003
Question G68
C. Protons undergo exponential
attenuation
Protons, like all charged particles, have
a finite range; photons are attenuated
exponentially
RAPHEX General Question 2001
G71: Concerning neutron interactions with
matter, which of the following is FALSE?

The neutron:

A. May remain in the target nucleus
B. Interacts primarily with the oxygen in H20
C. May cause the ejection of an alpha particle
D. May induce radioactivity in the target nucleus
E. May transfer a large traction of its energy in the
process of elastic scattering




RAPHEX General Question 2002
G72: Neutrons have a higher Quality Factor
than electrons because:
A. They transfer energy to protons which
have a high LET
B. They slow down in tissue, and deposit a lot
of energy at the ends of their tracks
C. They have a large mass and charge
D. They are directly ionizing
RAPHEX Answer for 2002
Question G72
A. They transfer energy to protons which
have a high LET.
Neutrons are indirectly ionizing. They
transfer energy to protons, which have
a large mass, and are densely
ionizing, especially at the end of their
track (the “Bragg Peak”)
RAPHEX Diagnostic Question
2002
D2-D5: Select the interaction that
predominates in the following situations:
A. Compton Scattering
B. Photoelectric interaction
C. Pair production
D. None of the above
D2: 30 kVp, adipose tissue
D3: 90 kVp, muscle
D4: 90 kVp, iodine contrast
D5: 90 kVp, LaOBr Intensifying screen
RAPHEX Answer to 2002
Question D2-D5
D2: B. Photoelectric interaction. Photoelectric interaction
predominate in tissue below 30 keV, which is
equivalent to a kVp of about 60-90 kVp.
D3: A. Compton Scattering. Compton scatter
predominates above about 90 kVp (30 keV) in
muscle.
D4: B. Photoelectric interaction. The probability of of
photoelectric inteardtion is proportional to Z-3.
Therefore photoelectric interactions predominate at all
diagnostic energies for iodine.
D5: B. Photoelectric interaction. The K-edge is the
reason rare earth screen have a high absorption
efficiency.
RAPHEX Diagnostic Question
2001
D3: Carbon dioxide can be used as an
angiographic contrast medium because:
A. The K absorption edges of CO2 are significantly higher than
tissue
B. The K absorption edges of CO2 are significantly lower than
tissue
C. The linear attenuation coefficient of CO2 is significantly higher
than tissue
D. The linear attenuation coefficient of CO2 is significantly lower
than tissue
E. Of differences between the mass attenuation coefficients
RAPHEX Answer to 2001
Question D3
D. The linear attenuation coefficient of
CO2 is significantly lower than tissue.
This is because the physical density is
much less. The mass absorption
coefficients (attenuation per gram) and
K edges are very similar.