Physical Principles of Ionizing Radiations

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This radiology continuing education module explores physical principles of ionizing radiations. Basic scientific principles of ionizing radiations used in radiology and nuclear medicine are discussed. Some of the topics include: Bremsstrahlung and characteristic radiations, positron production, annihilation reaction, and more.
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Author: Nicholas Joseph Jr. (RT) R
Credits: 3



This activity may be available in multiple formats or from different sponsors. ARRT does not allow CE activities such as internet courses, home study programs or directed readings to be repeated for CE credit in the same or any subsequent biennium”

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Section I: The electrical and physical properties of the atom.

Section II: The fundamental types of radiations emitted from the nucleus during its disintegration

Section III: Basic properties of electromagnetic radiation

Section IV: Bremsstrahlung and characteristic radiation produced by â-radiation

Section V: Positron production by proton to neutron transition

Section VI: Positron interactions and annihilation reaction

Section VII: Electron capture decreases the proton number and raises the neutron count.

Section VIII: Manmade electromagnetic Radiation

Section IX: Basic Interactions of X-rays with Matter

Summary and References

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Upon completion the reader will be able to:
  • Discuss the three fundamental components of the atom and their relationship to ionizing radiation production.
  • Discuss the arrangement of the fundamental particles of an atom and how they contribute to the stability or instability of an atom's nucleus.
  • Discuss the factors that contribute to the emission of particulate and electromagnetic radiation from the nucleus of a nuclide.
  • List and discuss the types of nuclear transformations that are important to nuclear medicine.
  • Discuss the production of positron emission and the subsequent interactions positrons will undergo.
  • Discuss the production of electromagnetic radiation in x-ray tubes and list some of the commonly used x-ray equipment that produces it.
  • Discuss the production mechanisms of bremsstrahlung and characteristic electromagnetic radiations produced in x-ray tubes.
  • Compare the energy and penetrability of particulate and electromagnetic radiation as functions of their deionization potentials.
  • State the relationship between a photon's wavelength and frequency, energy and frequency, and energy and wavelength.
  • Discuss the production of gamma radiation outside the nucleus of an atom by mechanisms that involve the positron annihilation reaction.
  • Explain the mechanisms of K-shell electron capture, Auger electron production, characteristic radiation production.

Outline Part I: Physical Principles of Ionizing Radiations

  1. The electrical and physical properties of the atom
    1. Arrangement of protons, neutrons, and electrons, and their electrical charges.
    2. Atomic number, atomic mass, and electro-chemical stability.
    3. Radioactive decay as an observation of neutron count being above or below the theoretical line of nuclear stability.

  2. The fundamental types of radiations emitted from the nucleus during its disintegration.
    1. Basic differences between particulate and electromagnetic ionizing radiations.
    2. Conditions that cause the emission of beta particles.
    3. How the n:p ratio affects atomic stability and instability leading to nuclear decay.
    4. Products of neutron to proton transition for non-pure and pure beta emitters.
    5. Isobaric transition, isomeric transition, and gamma radiation production by a non-pure beta emitter.
    6. Ionization potential as a function of distance traveled for various particulate radiations and electromagnetic radiation.

  3. Basic properties of electromagnetic radiation
    1. Waveform, a distinguishing characteristic of electromagnetic radiation.
    2. Intrinsic relationships between wavelength, energy, and frequency of electromagnetic radiation.
    3. The electromagnetic spectrum illustrates the important relationships of energy, frequency, wavelength and the penetrating ability of radiation.

  4. Bremsstrahlung and Characteristic radiation produced by beta radiation
    1. The mechanism of bremsstrahlung production by a high speed beta particle is similar to the high speed rapid deceleration of an electron in an x-ray tube.
    2. Mechanism of characteristic radiation produced when a beta particle ejects an inner shell electron of an atom.
    3. Reducing bremsstrahlung and characteristic radiation production through proper shielding materials as a function of the atomic "Z" number.

  5. Positron production by proton to neutron transition
    1. A basic definition of the positron and its mass description.
    2. The unstable atomic nucleus with a low n:p ratio as a fundamental cause of proton to neutron transition with positron production.
    3. Positron emission is linked to a concomitant production of bremsstrahlung radiation, ionization of the parent atom, and annihilation reaction.
    4. Key observations with reference to positron production and the n:p ratio.

  6. Positron interactions and annihilation reaction
    1. A glimpse at the negative acceleration mechanism that causes bremsstrahlung production by a newly created positron.
    2. The unstable atomic nucleus with a low n:p ratio as a fundamental cause of proton to neutron transition with positron production.
    3. Ionization of the atom may also occur by a mechanism intrinsic to the emission of the positron.
    4. Positron collision with electron causes an annihilation reaction that yields two high energy gamma photons.
    5. The Einsteinium conversion of mass to energy occurs in the annihilation reaction of a positron and an electron.

  7. Electron capture decreases the proton number and raised the neutron count
    1. The underlying intention of electron capture is to raise the neutron ratio.
    2. Characteristic radiation is the principle electromagnetic radiation type produced by electron capture.
    3. The production and effect of Auger electrons by positron emission.

  8. Manmade electromagnetic radiation
    1. X-rays are manmade electromagnetic radiation; gamma rays are electromagnetic radiation that is not manmade
    2. Four things are necessary to produce x-rays: a vacuum tube, a means of producing free electrons, a means of rapid acceleration of the electrons, followed by rapid deceleration of electrons.
    3. X-rays are a heterogeneous mixture of bremsstrahlung and characteristic radiations.
    4. The mechanism of bremsstrahlung radiation production in the x-ray tube.
    5. The importance of bremsstrahlung radiation and the continuous ejection spectrum to diagnostic imaging.
    6. Characteristics of electromagnetic radiation produced in the orbitals of atoms.

  9. Basic Interactions of X-rays and Matter
    1. Five basic interactions of X-rays with matter.
    2. Coherent or unmodified scatter.
    3. Understanding why scatter radiation is of great concern in diagnostic imaging.
    4. The photoelectric effect and its significance to radiographic imaging.
    5. Characteristics and properties of Compton scatter and how it may affect the occupation worker.
    6. Pair production and photodisintegration by x-ray photons.
  10. Conclusion:
    1. Summary points to ponder

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Protection of patients, staff, and visitors from energies used in radiology departments are in wide spread use; these energies include electromagnetic ionizing radiation, particulate ionizing radiation, and magnetic energies. Not only do radiology personnel come in close contact with these energies as part of their daily duties, but also do other health care professionals, patients, and visitors who daily traverse areas near their use. All health care workers not just radiology personnel have an obligation to themselves and the public we serve to understand the hazards, protections, and safety principles governing the use of these energies. Safety in the use of radiation and magnetic energies deserves no less understanding and practice than does biological protections and precautions we practice under standard blood and body fluid guidelines. These six issues titled "Protection and Safety For Energies Used In: Diagnostic Imaging, CT, Nuclear medicine and PET, and MRI" will discuss these commonly used energies and safety issues in a general way that promotes understanding across the health care community. These volumes of study will refresh the radiographer's working knowledge, educate health care practitioners, and provide the layperson the assurance that all health providers continually update their knowledge and practice of these safety principles. This series of materials covers the inherent nature of ionizing radiation, biological effects, radiation safety regulations, patient and personnel protection, and MRI safety issues required to safely operate an ongoing diagnostic imaging-MRI department.

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Section I: The electrical and physical properties of the atom.

Understanding the inherent properties of ionizing radiations necessitates a basic fundamental understanding of the electrical physical properties of an atom. Atoms are composed of a central relatively dense nucleus that contain positive charged particles called protons, and particles which have no net charge and are termed neutrons. Protons and neutrons have about the same mass but differ in that protons have a positive charge (+1), and the neutron is characterized by its absence of charge (0). Surrounding an atom's nucleus are discrete volumes of space wherein negatively charged low mass particles termed electrons are found. Electrons are found dispersed in several relatively large distinct volumes of space surrounding the nucleus, much like the planets that surround the sun. These discrete volumes wherein an electron can be found are termed orbitals. The classical arrangement of an orbital will have several suborbitals representing partitioning of the orbital into subshells. Subshells and orbitals represent energy patterns of an atom as well as volumes in space in which an electron in dynamic motion can be found. Generally speaking, a suborbital will contain two electrons maximally separated by repulsive forces their like negative charge creates. Comparatively, the two electrons spin, their relative motions in addition to their orbital motion is one in a clockwise rotation, the other in an opposite counterclockwise manner. Orbital and axis rotations, the motions of electrons, are important to their magnetic properties that are useful in magnetic resonance imaging (MRI).

Protons and neutrons are densely packed into the core of the atom called its nucleus. The number of protons is known as the atomic number or "Z" number, which is a numerical way of stating the identity of an atom. Observing a periodic table of elements, the number of protons an atom contains specifically names that element. For example, all oxygen atoms are composed of eight protons in their nucleus, so that any atom with an atomic number of eight is an oxygen atom. The mass number or "A" number of an atom refers to the number of protons and neutrons in its nucleus. The number of neutrons can be determined by subtracting the atomic number from the mass number; a simple arithmetical formula used to determine the number of neutrons is N = "A - Z".

A stable atom will have the same number of electrons in its orbitals' surrounding the nucleus as there are protons within the nucleus. The number of neutrons varies, but must increase as the atom becomes more complex to maintain nuclear stability, otherwise the nucleus will decay to a stable form.

Observations about the structural arrangement of an atom give us four principles: 1) atoms are generally electrically neutral when the number of electrons are equal to the number of protons, 2) electrons contribute a negligible amount to the atom's weight, approximately 1/2000 the proton weight, so that the nucleus contain most of the atom's weight, 3) orbitals are organized around the nucleus delimiting discrete energy boundaries for the electrons, 4) hydrogen has no neutron; however, for all other elements the number of neutrons must increase at a greater rate than the number of protons.

For stable atoms with fewer than 20 protons, the number of neutrons equals the number of protons; however to maintain nuclear stability as the number of protons increase above twenty, the neutron count increases greater than one to one n:p. This increased neutron count trend continues in atoms with up to eighty-three protons, for example, Bismuth (Bi) has 83 protons and 126 neutrons. Beyond bismuth there are no stable atoms since their proton count is eighty-four protons or greater and there is insufficient neutrons to balance the atom. Polonium 210 contains 84 protons and 126 neutrons and is radioactive. The maximum number of electrons in an orbital can be calculated using the formula 2(n)2 , where (n) is the number of the orbital. The second orbital of an atom could contain 2(2)2 or eight electrons; the third orbital could contain 2(3)2 or eighteen electrons if the number of protons supports them.

Radioactive decay occurs when the number of neutrons in a given atom is above or below a theoretical line for nuclear stability. Atomic decay is a process that may release electromagnetic and particulate ionizing radiations from the nucleus, and will continue until the n:p ratio is adjusted to that of a stable atom. The periodic table of elements presents the known elemental varieties of stable atoms. A periodic table of radioisotopes must be used to follow the complete decay series for a particular radionuclei. To fully understand the structure of the atom and the arrangement of electrons about the nucleus one must have an appreciation for the specific quantum numbers that describe the location and spin of each electron. Quantum numbers are beyond the scope of this study and will not be discussed.

Certain physical or chemical interactions with an atom may cause it to gain an additional electron(s) making it an electronegative species called an anion; likewise, an electron(s) may be removed from it resulting in a positive atomic species termed a cation. Anion and cation species are produced by processes that are collectively termed ionization. Gamma radiation, x-rays, alpha particles, beta particles, and the like, are all capable of removing an electron(s) from an atom and are therefore classified as ionizing radiations. Ionization is the first step in the mediation of biological damage. An atom is most stable when its positive and negative charges are equal and its neutron to proton ratio mimic a stable atom. Electrons removed from the valence shell are replaced by chemical interactions mainly electrochemical bonding, whereas deeper orbital electrons are replaced by physical interactions that create energy in the form of electromagnetic radiation.

The periodic table of the elements represents stable atoms and their isotopes. As they appear on this table, these atoms do not undergo disintegration. Chart developed by Los Alamos National Laboratory as a public service.

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Section II: The fundamental types of radiations emitted from the nucleus during its disintegration

Atomic decay results in several types of radiations being emitted from the nucleus of the atom. Four types of ionizing radiations are released by a radionuclide to attain stability (decay), these are: 1)alpha (α) particles, 2)beta (β) particles, 3) positrons, and 4) gamma photons. These ionizing radiations can be in the form of particulate radiation and/or electromagnetic radiation. Particulate radiations, in simple terms, are those ionizing radiations that have both atomic mass and charge; electromagnetic radiation has no mass, no charge, possesses waveform, and travels at the speed of light. Both types of radiation can be emitted from a decaying atomic nucleus. A common type of particulate radiation encountered in a radiology department, specifically, the nuclear medicine department, is the beta particle (β-particle). Beta particles originate in the nucleus of an atom when the n:p ratio is relatively high. A high n:p ratio causes the nucleus to become very unstable resulting in the conversion of a neutron to a proton. The newly made proton stays in the nucleus, while the beta particle, antineutrino, and gamma radiation are all expelled from the nucleus. If the nucleon is a pure beta emitter, electromagnetic energy will not be released from the nucleus in the decay transformation. The net result of beta emission is that the atom increases its proton count which is an effective method of reducing the n:p ratio towards nuclear stability. β-emission results in the neutron count being decreased while increasing the proton count, the n:p ratio decreased, and excess energy in the form of gamma radiation being released from the nucleus during the decay process.

Table 1.2 (n:p ratio). As an atom becomes more complex, represented by increasing number of protons, a greater number of neutrons (greater n:p ratio) is needed to maintain nuclear stability. An atomic species lying above the theoretical line for nuclear stability needs more protons or fewer neutrons and tend to convert neutrons to protons to become stable. Atoms lying below the line are neutron poor or proton rich; they require fewer protons to become more stable and will convert protons to neutrons. Following the theoretical line for nuclear stability we see that atoms with a balanced n:p ratio, such as those listed on the periodic table of elements will not undergo nuclear disintegration and are stable.

Beta particles are high velocity, negatively charged electrons emitted from the nucleus of radioisotopes during some modes of atomic disintegration. The released particle may be one of two types: a negatively charged electron, termed a negatron (β-), or a positively charged electron, termed a positron (β+). In this module we will refer to the beta particle as the negative electron emitted from the nucleus of an unstable atom. To avoid misunderstandings when we wish to discuss the beta particle as a positive electron we will call it a positron. One manner in which beta particles are produced and emitted from an atomic nucleus is when a pure beta emitter, such as phosphorus 32, undergoes disintegration. The conversion process involves a neutron being converted to a proton, a beta particle, and an antineutrino. A neutrino is a neutral particle of very small mass (nearly zero when at rest) that is emitted during neutron degradation. An antineutrino has the same mass and charge value as the neutrino, but its spin is opposite. The proton remains in the nucleus, and the antineutrino and beta particle are expelled from the nucleus.

The energy yield from a disintegrating neutron of a pure beta emitter radioisotope is shared at production between an antineutrino and a beta particle. The energy the beta particle may have is not a discrete energy; instead, its energy may vary from about one third to the maximum energy, (called Emax) which is calculated from experimental data for that type of atom; the remainder of the energy is given to the antineutrino. Usually the beta particle is quite generous giving most of the energy to the antineutrino, so that it keeps for itself only about 1/3 of the total energy partitioned.


For pure beta emitters when a neutron undergoes conversion to a proton, a beta particle and antineutrino are produced. Beta emitters that are not pure emit electromagnetic radiation in the form of gamma photons. Pure beta emitters are very useful for therapeutic applications because the beta particle's range in tissue is about 0.5 cm and causes thousands of ionizations in its path. If the nucleon is not a pure beta emitter, it may also partition some of the released nuclear energy to a gamma photon that it also releases from its nucleus. In the case of a pure beta emitter such as 32P, its decay series is to 32S an isobar, with the liberation of a beta particle and an antineutrino from the nucleus.

The result of neutron disintegration as we can see from our example above is that the atomic number (phosphorus) is increased from 15 protons to 16 protons, concomitantly the type of element changes from phosphorus to sulfur. There is no change in the mass number because there are still 32 particles in the nucleus; however, the transition product, a proton, does confer upon the atom a plus one charge. The balanced equation simply states that what is lost as a neutron is gained as a proton, beta particle, and antineutrino. This is the pattern of disintegration for pure beta emitters. Beta emission changes the atomic number giving rise to an atom with totally different chemical properties; in our example phosphorus transitions to sulfur. This process is called isobaric transition. An isobar is defined as atomic nuclei that have the same atomic mass number, but different atomic numbers. Notice that isobars comparatively, are atoms that have a different number of protons and neutrons, but have the same total number of nucleons in their nucleus, they do not necessarily have the same chemical properties.

Generally speaking isobaric transition is one of three steps in the decay process, and is followed by isomeric transition and possibly gamma radiation being emitted from the nucleus. Should an atom still contain excess energy following an isobaric transition, and has not attained a final stable state, it will undergo isomeric transition. Nuclei in this intermediate state are isomers that will expel energy in the form of a gamma ray, and will continue transitions until the atom arrives at a ground state of stability. Isomers are atoms with the same atomic number and atomic mass number. An example of isomers is cesium 137, which decays to barium 137 by emitting 2 beta particles. There are three types of isobaric transitions that are of interest to radiology (1) beta emission, (2) positron emission, and (3) electron capture. If we refer back to table 1.2 and table 1.3 below we see radioisotopes that are beta emitters that are above the theoretical line of stable nuclides, while positron emitters and electron capture occurs in atomic species below that line.

There are clear clinical therapeutic values to using pure beta emitters one of which is that beta particles travel a short distance in tissue (approximately 0.42 cm) causing many ionizations along their paths that can cause DNA damage in tumor cells inactivating them.
In nuclear medicine some beta emitters are useful because they are pure beta emitters and do not give off gamma radiation. These types are good therapeutic agents because beta does not leave the patient's body so only local tissues are exposed. Non pure beta emitters give off gamma radiation and it can be detected outside the body for diagnostic imaging. Positron and electron capture are below the theoretical line and are competing reactions among those radioisotopes that are used for positron scanning.
Table 1.4 illustrates the relative resultant energy to the beta particle from the conversion of a neutron to a proton causing beta emission. The total energy released from the nucleus is known to be 1.7 MeV, which is Emax. This energy is partitioned between the beta particle and the antineutrino during beta emission; and between the beta, antineutrino, and a gamma photon if the nucleon is not a pure beta emitter. Beta particles used in nuclear medicine generally have 25-30% of the maximum energy (0.6 MeV), called Eβ- ; the balance is imparted to the antineutrino and/or to the gamma photon. A unique characteristic of electromagnetic radiation is that it will travel in a vacuum, where other types of radiation energy will not.

The net energy produced by neutron conversion to a proton is partitioned between a beta particle and an antineutrino, with a concomitant ejection of them both from the nucleus. Because neither energy nor mass is created nor destroyed, but can be converted from one form to another (the first law of thermodynamics-conservation of energy and mass), we must account for all energy released in the disintegration pattern of a non pure beta emitter. According to Albert Einstein, whenever mass is lost, it can be accounted for by a transformation to pure energy and vice versa. For non-pure beta emitters, along with isobaric transition a portion of the energy goes to the production of a gamma ray, which is also released from the nucleus along with the beta particle and antineutrino. As we have stated, the beta particle is special to radiological science because of its therapeutic applications. Its properties are not useful in terms of nuclear medicine imaging because these particles lack sufficient energy to escape the patient's body and be detected. A closer look at the physical properties of β-particles explains its interactions with other atoms causing ionization. There are two interactions we will discuss related to beta initiated ionization: 1) the specific ionizations caused by a β-particle per centimeter traveled, 2) the energy losses by a beta particle to achieve a deionization state. Deionization is the process of a charged particle loosing all of its kinetic energy through ionization of adjacent atoms to achieve a rest state.

The ionization potential and distance of travel for any particulate radiation is related to its mass and its charge; the larger the particle's size and charge, the greater is its ionization potential although its distance of travel diminishes. A beta particle has the mass of an electron and carries a minus one negative charge. An alpha particle by comparison contains twice its charge and 7400 times its mass. Gamma radiation has no mass and no charge. What this means is that an alpha particle because of its mass and charge travels the shortest distance and has the least penetrability. An α-particle is easily absorbed from air by a sheet of paper. A beta particle having 3.4 MeV will cause about 100,000 ionizations in tissue before achieving its rest state of a free electron, or deionization. In soft tissue the beta particle can travel 1-3 cm, and in tissue a 1 MeV beta has a range of about 0.42 cm. Gamma photons because they have no mass or charge have unlimited range of distance, and have relatively few ionizations in tissue. Pure beta emitters are great therapeutic agents because a beta particle looses 3.4 eV per ionization event. This energy is harnessed to treat clinical states such as leukemia, ascites, polycythemia, and other diseases. Table 1.2 illustrates the ionization and penetrating potentials of some radiations.

Ionization of adjacent atoms by a beta particle is quite different from alpha or gamma radiation mechanisms. When a high speed beta particle passes through an atom's orbitals, it ionizes by repulsion force between it and the atom's electron(s) ejecting them from their orbital(s) causing ionization of the atom.

Beta particles deionize by ejecting orbital electrons through repulsive forces. In each ionization event the β–particle looses 3.4 eV of energy. When all of its kinetic energy is lost it will become a free electron and be acquired by an ionized atom seeking electrical neutrality.
Table 1.2
Classification of the Major Radiations Used In Radiology
Type of Radiation Symbol "Z" # Charge/Origin
Particulate Radiations  
Alpha Particle α 4 +2/Nucleus
Beta Particle β θ +1/Nucleus
Electromagnetic Radiations
Gamma Rays γ θ 0/Nucleus
X-rays Χ θ 0/Orbitals

Electrons are required to maintain a maximum distance from each other because of their like charge (opposites attract, likes repel) according to what is known as VESPER Theory. This creates a strong repulsion force that ejects an orbital electron when the high speed β-particle collides with it.

Beta radiation may also produce electromagnetic radiation consequential to its interaction with atoms in its path. Ionization by particulate radiation can produce bremsstrahlung and characteristic radiations, but not gamma radiation, in the orbitals of atoms. Electromagnetic radiation we stated earlier differs from particulate radiation in several ways, primarily in that it is composed of small discrete packets of pure energy, termed photons that have no mass or charge, and travels at the speed of light. The energy and wave form of electromagnetic radiation can be represented with an oscilloscope.

The energy of a photon is measured in electron volts (eV) and multiples thereof, keV and MeV. A unique property of energy is that it determines the penetrating ability of a photon. Low energy photons do not penetrate a patient's body and are absorbed in bodily, whereas high energy photons will penetrate the patient and contribute to the radiographic image. Any photon with greater than 20 eV is classified as ionizing radiation since any element on the periodic table can be ionized by photon energies ranging from as little as 5 to 20 eV.

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Section III: Basic properties of electromagnetic radiation

Unlike particulate radiation, electromagnetic radiation energy possesses a waveform that is sinusoidal, having amplitude, frequency, and wavelength. The wavelength (γ) may be described as the distance the radiation travels through space in a single oscillation. Radio waves and television wavelengths are often expressed in meters, but light energy such as gamma and x-rays are expressed in shorter units such as the angstrom or nanometer (nm). Photon energy and wavelength are inversely related by the formula:

Relationship Between Energy and Wavelength

E (keV) = 1.24 / λ(nm)

The frequency of a photon describes its rate of oscillations, which is cycles of its wavelength per second; its units are cycles per second (cps) or Hertz (Hz). The Hertz is equal to one cycle per second. This description is almost exclusively used to describe photons at the lower end of the spectrum, called the RF portion, which includes radio waves, television broadcasting, even microwaves, and MRI are all located at the lower end of this spectrum. Photon frequency is not a particularly useful way of describing gamma or x-rays because of their short wavelength. Wavelength and frequency are inversely related so that as the frequency increases so does the photon's energy and its wavelength decreases inversely. X-rays and gamma rays have such short wavelength and high frequency that to describe them in hertz would be incomprehensible. Photon energy (E) and frequency (f) are directly proportional according Max Planck's quantum theory from which the following formula is given:

Relationship between Energy and Frequency

E = hf

E = energy, h = Plank's constant (6.625 X 10-27), f = frequency

The relationship between energy, frequency, and wavelength is illustrated below by the electromagnetic spectrum. Some energy radiations are strong enough to eject an electron from an atom creating an ion pair.

Electromagnetic Spectrum Chart from: Berkeley Lab. Berkeley, CA. with Permission

Frequency, wavelength, and the corresponding energy associated with the photon are what give it its penetrating ability. A photon has great penetrability because of its wave like properties, which is something that particulate radiation does not possess. Frequency and wavelength cannot be used to describe particulate radiations such as alpha and beta radiations because they do not have a waveform. A particle's energy is due to its kinetic or potential energy it may contain, as well as its charge, and mass.

Frequency is useful in describing magnetic resonance because during MRI, protons emit a signal frequency of 42.58 MHz when placed in a 1 tesla magnetic field. Proton frequency is important to MRI imaging; however, frequency is not an especially a useful term to describe the application of photon energy in diagnostic imaging. A further discussion on the application of frequency in MR imaging is discussed in module 6 of this issue.

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Section IV: Bremsstrahlung and characteristic radiation produced by â-radiation

Bremsstrahlung and characteristic radiation can be produced as a result of a beta particles release from the nucleus of a decaying atom. Let's look at the mechanism of production of bremsstrahlung radiation by a beta particle first. Bremsstrahlung radiation is generated by projectile beta particles released from a decaying nucleus. This type of radiation is also called "breaking radiation" because its origin is from a partial deceleration of the high speed β-particle as it passes close to the highly positive charges in the nucleus of an adjacent atom, causing the particle to undergo a slight change direction. But because the incident beta particle is moving at a high velocity it is not sequestered by the nucleus, instead, it is deflected enough to give off energy. The vectorial change results in a loss of kinetic energy by the β-particle, which is released as a photon of electromagnetic energy, termed bremsstrahlung radiation. Bremsstrahlung radiation produced by a β-particle is a two edged sword, first because it is not especially useful for therapeutic purposes because of its low ionization potential; secondly, bremsstrahlung produced by a beta electron is more harmful to the technologist than the beta particle that produces it, because of the penetrability of an electromagnetic ray. For beta particles the amount of bremsstrahlung increases as the density (high Z number) of the material through which it passes increases.

Bremsstrahlung radiation is made as a beta particle passes close to the nucleus of an atom causing the beta particle to be deflected from its original course. The β-particle looses energy as it is slowed down in a "braking" deceleration that creates a photon of energy.

It is for this reason that shielding is an important consideration when using beta emitters. Plastic or Lucite are effective shielding materials for a beta emitter since these materials have a low Z number and therefore produce less bremsstrahlung than lead shielding.

The other type of electromagnetic radiation the beta particle can produce is characteristic radiation. If a high speed beta particle approaches an electron in an inner orbital, it can cause it to be ejected from the atom by the strong repulsive force between it and the electron because of their same charge. When an inner orbital electron is ejected the atom rearranges its electrons to fill the vacancy by moving outer orbital electrons inward. When an electron is moved to a different orbital it must give up some of its energy during the transition; this energy is released in the form of a characteristic ray. These are generally low energy photons depending on the atom type and orbital from which the photon originates. Characteristic radiation produced by beta particles is not useful to diagnostic imaging or to therapy. The production of characteristic radiation by a high speed beta particle is minimal in comparison to bremsstrahlung produced; bremsstrahlung production is the dominant mechanism responsible for bringing the beta particle to a rest state.

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Section V: Positron production by proton to neutron transition

By definition, a positron is a positive charged electron emitted from the nucleus of an atom during disintegration. A positron is identical to the electron or beta particle with the exception that it carries a positive charge. It is the plus one (+1) charge of the positron along with its mass and speed, and interaction outside the nucleus that gives it its properties. Its relative size is that of an electron which is approximately 1/2000 the size of a proton, yet it has the charge of a proton.

The underlying influence driving the production and emission of positrons is an unstable nucleus suffering a low n:p ratio. The ratio is raised by increasing the number of neutrons while simultaneously diminishing the number of protons in the nucleus during transitions that produces positrons. A positron is a type of particulate radiation emitted from the nucleus of a decaying atom when a proton is converted into a neutron. In this type of disintegration a positive electron called a positron (e+ or β+) is created, along with a neutrino, and excess energy. Positron production is therefore the opposite of beta production. With beta decay a neutron is converted to a proton with the release of a beta particle and an antineutrino; in positron decay a proton is converted into a neutron with a positron and a neutrino ejected from the nucleus.


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Section VI: Positron interactions and annihilation reaction

Three important physical interactions occur once a positron is ejected from the nucleus: Bremsstrahlung radiation is produced, ionization of the atom occurs, and the annihilation reaction. Bremsstrahlung radiation production is by a different mechanism than that produced by a beta particle or by a high speed electron as it is deflected from its course toward an atom's nucleus. With positron production there is repulsion between the positively charged nucleus and the positively charged positron. This repulsion causes a "negative" acceleration away from the nucleus, which is nearly opposite the mechanism of deceleration we discussed earlier. The net result is the production of electromagnetic radiation energy which is lost from the atom.

Two interactions are caused by the positron as it traverses matter, ionization and annihilation. Ionization interactions also occur by intrinsic mechanisms that alter the parent atom's stability becoming a cation. Ionization is a conditional reaction to the positron's positive charge as it is ejected from the nucleus and traverses its own orbitals to leave the atom. Along its ejection track it may draw orbital electron(s) towards it and away from the atom, causing the atom to be ionized. Ionization does cost the positron some of its energy which concomitantly cost it some of its speed. This manner is very different from beta ionization in that the positron's positivity attracts orbital electrons from the atom during its ejection flight, causing self ionization. It differs from beta ionization in that a negative beta particle ionizes atoms by repulsion between itself and orbital electrons during its expulsion. When a positron looses all of its kinetic energy it attains a rest state (0 keV) followed by interaction with an oppositely charged rest state electron, which is a highly favorable condition for their annihilation.

When a positron and an electron collide, they will together undergo an annihilation reaction in which their combined mass is converted to pure electromagnetic energy that is partitioned equally to two gamma photons. Annihilation occurs because the positron in addition to being opposite in charge to the electron, it is composed of what is sometimes called antimatter, which causes a reaction quite different than that of a proton with a plus one charge. Eventually, the accumulation of attractive interactions and decelerations of the positron will bring it to a rest state. A rest state positron will undergo its final interaction, annihilation of itself and matter (a negative electron). The process yields two high energy photons of gamma radiation that are expelled from the collision at 180 degrees to each other.

A positive electron (positron) interacts with a negative electron in a reaction that annihilates them both and produces two high energy gamma photons each having energy of 511 keV.

The two particles will together undergo annihilation of their masses, and a simultaneous conversion to pure energy. The equation is an Einsteinium conversion in which the energy produced is equal to the total mass times the speed of light squared. What is interesting here is that the energy produced from the annihilation reaction is divided equally between two gamma photons whose vector is 180 degrees to each other, and each photon will possess 511 keV of energy. The positron is important to radiology because it is the basis of PET scanning which is diagnostic imaging's newest and most promising technology.

The Annihilation Reaction of a Positron and an Electron

E = mc2

"E" is the energy of the resulting photon, "m" is the combined mass of the electron and the positron, and "c" is the speed of light. This interaction and formula tells us that energy and mass are interchangeable.

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Section VII: Electron capture decreases the proton number and raises the neutron count.

The final reaction important to atomic nuclear decay that we will discuss is electron capture. Electron capture is another option for an atom having too many protons or too few neutrons. Currently there are several theories about why this reaction occurs, but what is known about it is that the k-shell electron, and possibly the L-shell electron also, may pass through the strong influence of its own nucleus as a part of its normal orbital path. If the atom contains too many protons and too few neutrons for nuclear stability under certain conditions it may capture the K-shell or even an L-shell electron. When this happens there is sufficient nuclear instability to cause transformation of one of its protons into a neutron. As with all atoms in which an inner shell electron is lost, the atom will fill the vacancy by shifting electrons from its outer shells to fill inner vacant orbitals. Successive shifting of electrons inward results in the release of photons corresponding to the orbital difference between the initial and final orbital in which the electron comes to reside. Radiation released in this manner is termed characteristic radiation because the energy of the photon produced is characteristic of the orbital from which its electron was shifted. Finally, the vacancy that was created by the loss of the K-shell electron is expressed in the outermost valance shell. Here the atom has its best change of obtaining a free rest state electron and returning to a stable state. What important to us about electron capture is that with this type of transition, radiation is not emitted from the nucleus of the atom, instead, radiation is produced as a result of refilling electron vacancies from successive orbitals creating characteristic radiation. Characteristic radiation is the primary type of radiation produced as a result of electron capture. Positron emission and electron capture are somewhat competing reactions, for example, a nuclide that is a positron emitter some atoms of the same series will undergo positron emission and others electron capture. It is characteristic that for some nuclear medicine studies characteristic radiation generated by electron capture reactions is ideal as an in-vivo study and is perhaps one of the strongest rationales for using these nuclides.

The above diagram represents the conversion of a proton to a neutron as a result of electron capture. A neutron is produced, along with characteristic radiation or an Auger electron, which is not demonstrated.

Some radionuclides produce Auger electrons as well as characteristic radiation. An Auger electron is produced when the energy given off by the electron filling the K-shell is transferred to another electron ejecting it from the atom. The ejected electron is called an Auger electron because of the mechanism of its release. Electron capture is a competitive reaction with positron emission. Nuclides that are positron emitters may also undergo reactions that emit Auger electrons; the production ratio between positrons and Auger electrons is specific to the type of nuclide.

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Section VIII: Manmade electromagnetic Radiation

Thus far we have discussed some of the properties of radioactive atoms and the ionizations they can cause. This section reviews some of the properties of electromagnetic radiation produced by x-ray tubes found in different types of x-ray apparatus, such as fluoroscopy units, CT, angiographic, and general diagnostic equipment. We will look at man-made radiation which is either bremsstrahlung or characteristic radiation; gamma radiation is also electromagnetic energy, however, it is not produced artificially in x-ray tubes. There are two basic classes of ionizing electromagnetic radiation: gamma rays which have their origin within the nucleus of decaying atoms, and x-rays which are man-made radiations produced in x-ray tubes. X-rays are heterogeneous photons composed of two types of radiations named according to how they are produced. X-rays are generated by two different processes: bremsstrahlung interactions which produce continuous photon energies, and characteristic radiation, which produce defined peaks of photon energy.

We have already talked about these types as products of particulate radiation interactions, but we should review their origin from the x-ray tube as well. X-rays and gamma rays have the same physical properties, but differ in their origins and energy. Gamma rays have a slightly shorter wavelength than x-rays, and can be easily detected from outside the body because of their high penetrability; x-rays are manmade radiation produced from electrons that are accelerated and rapidly decelerated inside a vacuum tube. The origins of x-rays are in the energy shells that surround the nucleus, they contain no gamma radiation.

Types of Ionizing Electromagnetic Radiations and Their Origins
Gamma (γ) Photon - any electromagnetic radiation that originates from the nucleus of a radioisotope is termed a gamma ray; however, in the case of the annihilation reaction between a positron and an electron, gamma radiation is produced outside the nucleus.
X-ray Photon - Originates outside the nucleus in the orbitals of atoms.

An understanding of x-ray production is important because unlike particulate radiation, x-rays are produce during a short period of time called the exposure time, whereas gamma radiation is produced continuously in a source as part of the decay process of a nuclide. X-ray production requires a vacuum tube, source of free electrons (filament), a rapid acceleration of the electrons (high kilovoltage), and a rapid deceleration of the electrons in the dense anode material (tungsten anode). Electromagnetic radiations are produced in several types of x-ray equipment: CT scanners, angiography machines, C-arm fluoroscopy units, portable x-ray equipment and the like, but MRI and ultrasound machines do not produce x-rays; instead, there are other concerns when working around these energies. It should be understood that during occupational work no employee should be exposed to bremsstrahlung or characteristic radiation directly because they should not be in the primary beam. But as we shall see, radiation may change direction during the brief exposure of the patient in an interaction called the Compton Effect, which produces Compton scatter. Most of occupational radiation exposure dose is from Compton scatter. Bremsstrahlung and characteristic radiations are the two components of the useful radiation beam that produces radiographic images.

The most important type of ionizing radiation useful for diagnostic imaging is bremsstrahlung radiation. It is produced in the anode of an x-ray tube when electrons from the cathode are accelerated towards the anode under the influence of high kilovoltage. These electrons penetrate the anode where they interact with tungsten atoms that make up the target. The subatomic picture is that these electrons pass close to the nucleus of tungsten atoms, and with each pass strong positive charges in the nucleus exerts a strong pull on the electron changing its course and decreasing its speed. These rapid deflection decelerations of the electron releases energy in the form of a photon, which is termed bremsstrahlung radiation, meaning "breaking radiation". Tungsten atoms are especially good producers of synthetic made radiation because their high proton density attracts projectile electrons better than would a lower proton dense atom. The speed of the incident electron when it interacts with tungsten atoms determines the energy range of the bremsstrahlung photons produced. A high speed projectile electron will continue to undergo many bremsstrahlung interactions each of which will reduce its speed slightly until the electron achieves rest mass state. Because tungsten atoms are tightly packed in the anode the electron will come to rest in a few milliseconds. Bremsstrahlung photons can have energy values up to the maximum kilovoltage of the electrons that produces them. As we can see the mechanism of bremsstrahlung production in the x-ray tube is identical to the mechanism of its production by a beta particle.

Bremsstrahlung radiation is part of what is called the continuous ejection spectrum, because these photons are emitted with energies from near zero up to the maximum kVp selected. For example, if 100 kVp is selected, then in order to produce a photon with 100 keV energy, an electron moving at 100 keV would have to come to a complete rapid deceleration giving off all of its energy, 100 keV, to a newly created photon. Instead, what usually happens is that the electron gives off many photons with energies less than its kinetic energy as it undergoes many bremsstrahlung interactions with atoms until it looses all of its energy and comes to rest. The sum of the photons produced is equal to the kinetic energy of the electron that produced them in keeping with the law of energy conservation.

X-rays are produced when electrons are produced by the filament in a process called thermionic emission; these electrons are accelerated towards the target on the anode under the influence of high kilovoltage. Their rapid deceleration through interactions with the anode produces x-ray photons. This process of producing x-rays is not very energy efficient, 99% of the energy of the electrons is converted to heat and approximately 1% is converted to x-ray photon energy. The target upon which the electrons bombard is made of tungsten-rhenium for mechanical strength, and molybdenum or graphite layered beneath the tungsten to conduct heat away from the anode. Tungsten has a high melting point (3400oC) and can withstand high tube current, additionally; the anode rotates at speeds of 3400-10,000 rpm, a modification that increases the surface area over which electrons bombard.

Bremsstrahlung radiation is especially important to radiographic imaging because it is responsible for the heterogeneity of the useful beam and therefore subject contrast. If photons all had the same energy, such as a homogeneous x-ray beam, then they would all have the same fate of either being completely absorbed or being completely transmitted through the patient depending on their energy value. Complete absorption would mean there would be no radiographic density, and complete transmission would yield a single radiographic density and no contrast. What is desirable is that there is radiographic balance that creates subject contrast which demonstrates all four radiographic densities: bone, muscle, fat, and air. This is achieved only by a heterogeneous beam whose energies are selectively absorbed and transmitted by different tissue types, which is an advantage of bremsstrahlung radiation. Bone absorbs more photons than muscle and is radiopaque, whereas, air absorbs few photons and is radiolucent. Selective photon absorption requires heterogeneous x-ray beam energies to produce diagnostic radiographic images.

Bremsstrahlung radiation is also called breaking radiation because it is formed during the rapid deceleration of a high speed electron that is deflected from its course by the strong positive attractive forces in the nucleus of the atom it traverses. Bremsstrahlung radiation produces many photons with varying energy values up to the maximum energy of the electron that produces it. These heterogeneous energies are selectively absorbed and transmitted by different types of tissues contributing to subject contrast.

Photon absorption occurs by a mechanism known as the photoelectric effect (P.E.). The P.E. effect occurs when a photon strikes an inner shell electron and is completely absorbed. The collision caused the ejection of the electron from the atom. At absorption, the photon disappears transferring all of its energy and speed to the ejected electron which is then called a photoelectron because it has characteristics of the photon and the electron. The photoelectron is important to radiology because it is the mediator of biological damage capable of causing ionization of matter. In a biological system photoelectrons interact with biomolecules including the ionization of water, macromolecules, and ultimately can damage the DNA in a cell inactivating it. The P.E. effect is also responsible for radiographic contrast. Differential absorption of photons by different types of tissues in the subject is what allows us to visualize subject contrast for radiographic interpretation. The photoelectric effect is related to the energy (keV) of the photon, at low kVp the P.E. effect increases while at high kVp the P.E. effect diminishes.

Bremsstrahlung radiation is a type of man-made radiation produced in the anode of the x-ray tube. Electrons are accelerated towards the anode under the influence of high kilovoltage. At the anode they are rapidly decelerated by the dense packed tungsten atoms composing it. Their rapid deceleration is a process of many close deflections as these electrons pass close to the nucleus of tungsten atoms. Strong positive nuclear charges exert an electromagnetic pull on the electrons. However, because the velocity of the electrons outweighs the attractive forces, the electron(s) is not sequestered by the atom, but its direction is changed. The process results in the release of energy in this braking process in the form of photons, termed bremsstrahlung radiation. This process continues until the electron looses sufficient energy to obtain a rest state and be absorbed by an atom needing an electron(s). Bremsstrahlung radiation is approximately 90-98% of the primary beam that is used to make a radiographic image.

Characteristic radiation is produced in the x-ray tube when inner shell electrons are ejected from tungsten atoms by high speed projectile electrons, creating a hole or vacancy in those orbitals. Inner shell electron vacancy is a highly unstable state for an atom so it immediately shifts its electrons inward successively filling these inner shell electron vacancies. When an electron is moved to a different energy orbital it is required to release some of its energy, and it does so in the form of a photon which is termed a characteristic ray. The process of electron shifting continues until the vacancy is expressed on the outer valance shell where there is a high probability for the atom to acquire a rest state electron returning it to a stable state. This is because the atom does not give up any energy in the process of filling a valance shell electron. All atoms regardless of the species will respond in this manner to the loss of an inner shell electron. The energy given off by electrons shifting to an inner orbital is characteristic for each atomic species and specific for the orbital from which the photon originates. The energy of a newly created photon is the difference between the binding energy of the ejected electron, and the binding energy of the electron that replaces it. The binding energy of an electron is defined as the energy required removing an electron from its orbital. This is the reason why only K-shell characteristic photons produced in tungsten atoms within the anode are useful to diagnostic radiographic imaging. The K-shell of tungsten has a binding energy of 69.5 keV so that an electron of less than 70 keV cannot remove the k-shell electron of a tungsten atom. Because the k-shell is usually filled by an electron from the L-shell which has a binding energy of 10.2 keV, a photon from this internal electron shift would have a discrete energy of 59.3 keV and no other. It is important to realize that the energy of a characteristic photon can only be a discrete energy and no other. If an electron or photon does not possess at least 70 keV it cannot eject a k-shell electron from tungsten. Therefore, in the tungsten anode of an x-ray tube all photons of the useful beam produced at less than 70 kVp are bremsstrahlung radiations. At greater than 70 kVp, for example 140 kVp used in many CT studies, most of the useful radiation is bremsstrahlung, and only a fraction of the useful beam that contains photons of 69.5 keV is from characteristic radiation.

Characteristic radiation is a type of ionizing radiation that is produced in the orbitals of atoms when a high speed projectile electron strikes and ejects an inner shell electron ionizing it. Characteristic radiation is not particularly useful in diagnostic imaging because most of the energy emitted by the tungsten atom is to low to penetrate the patient. A discrete energy of 69.5 keV can be generated from the k-shell electron of tungsten and these photons are useful in diagnostic imaging; however, these photons are only 3-5% of the useful beam. Other low energy characteristic radiations generated in the x-ray tube from tungsten atoms are easily absorbed by inherent and added filtration or in air before reaching the patient’s skin.


Characteristic radiation is produced when an electron from the cathode end of the x-ray tube is accelerated towards the anode at high speed, and it strikes an inner shell electron of a tungsten atom. This creates a hole or vacancy in the atom that renders it unstable. The atom responds by shifting electrons inward to fill the vacancy. Whenever an outer shell electron is shifted inward it must give up some of its energy in the form of radiation called a characteristic photon. In this manner discrete energy is imparted to the newly formed radiation which is "characteristic" of the atom type and orbital from which it came, thus the name characteristic radiation.

Characteristic radiation is extremely important to mammography because photons that enhance differential absorption in breast tissue maximizing contrast are in the 17-24 keV range. Photons with discrete energy of 19 keV are generated in mammography units using molybdenum-target filtered with 30 μm of molybdenum, a combination which produces little bremmstrahlung rays. Tungsten has an atomic number of 74 which would produce photons in the mammography imaging range; however, there will be many useless photon energies produced both above and below the range as well. Molybdenum has an atomic number of 42, and rhodium, which is also sometimes used as a target material has an atomic number of 45. When rhodium is used as the target material a 50 μm Rh filter is used.

X-ray tubes are used to produce bremsstrahlung and characteristic radiations in conventional CT scanners, mobile radiography equipment, conventional and digital fluoroscopy, interventional radiographic equipment, PET/CT, C-arm, and the like, all produce heterogeneous electromagnetic radiations.


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Section IX: Basic Interactions of X-rays with Matter

Scatter occurs when photons interacts with valance shell electrons of atoms in matter. Scattering is one of the multiparts of interactions of radiation in matter. Almost all scatter in diagnostic radiology imaging is a result of Compton scatter.

There are five basic interactions of x-rays with matter depending on their energy:

  1. Coherent scattering
  2. Photoelectric effect
  3. Compton scattering
  4. Pair production
  5. Photodisintegration

The importance of these interactions is that sometimes the photon is absorbed, while at other times they are merely scattered. When a photon is absorbed it is completely removed from the x-ray beam and ceases to exist, whereas when it is scattered it is deflected into a random direction and is said to no longer carry useful imaging information. Preventing scattered radiation from reaching the image receptor will reduce film fog and image noise.

Coherent Scatter

One of the characteristics of ionizing electromagnetic radiation is that it travels in a straight line vector away from its source. The only way its energy, wavelength, frequency, or direction will change is through interaction with matter. One of the basic types of interaction of ionizing radiation with matter is one in which it is deflected so that its direction is changed. Scatter radiation can be defined as a primary photon that has undergone a change in direction due to interaction with matter. Coherent scatter is a name given to those interactions in which there is a change in the direction of the photon but the wavelength of the photon does not change. This is why coherent scatter is sometimes called unmodified scatter. There are two types of coherent scattering: Thomson scattering and Rayleigh scattering. Essentially, with Rayleigh interaction a very low energy photon interacts with a tightly bound orbital photon and being unable to eject it, the electron is merely set it into vibration at the frequency of the photon. The vibrating electron cast off the excess energy by emitting an electromagnetic photon identical in wavelength to that of the incident photon, but in a different direction. It can therefore be stated that the initial photon is scattered in a different direction without a change in its energy, wavelength, or frequency. Unmodified scatter is not a concern in diagnostic imaging because it occurs with x-ray energies well below the range used in clinical radiology practice. Unmodified scatter does occur nevertheless in diagnostic imaging; however, the photon energy is low enough to be considered insignificant to imaging information. The point with coherent scatter is that there is only a change in the direction of the photon with no detectable change in the photon’s energy, wavelength, or frequency.

Coherent Scatter Production

image027 image028 image029

In coherent scattering there is no loss of energy by the photon and no ionization of the atom as a result of the interaction.

Photoelectric Effect (P.E.)

The photoelectric effect is the mechanism by which photons are absorbed by matter. An incident photon with slightly more energy than the binding energy of the orbital electron it collides with strikes that electron and ejects it from its orbital. In the interaction the photon cease to exist imparting all of its energy to the orbital electron. Most of the energy is required to overcome the binding energy of the orbital electron and the remainder imparted to the electron upon its ejection. The ejected free electron is called a photoelectron which travels a short distance in tissue and is absorbed (charged particles have little penetrability). The atom’s resultant charge is +1 due to the electron void. Subsequently, electrons from outer orbitals drop inward to fill the void, such as from the L-shell to the k-shell and from the m-shell to the L-shell, etc. As electrons drop into inner shells they give up energy in the form of an x-ray photon called a characteristic ray. The three products of the P.E. effect: 1) a negative ion (photoelectron), 2) characteristic radiation, and 3) a positive ion (atom deficient one electron).

image030 image031 image032

There are three simple rules governing the probability of the photoelectric effect occurring:

  • The incident photon must have sufficient energy to overcome the binding energy of the electron. For example, if a k-shell electron has a binding energy of 70 keV and the incident photon have energy of 68.5 keV, it absolutely cannot eject that electron from its orbital.
  • A photoelectric interaction is most likely to happen when the energy of the incident photon exceeds but is relatively close to the binding energy of the electron it strikes. Using our example of a k-shell electron with a binding energy of 68.5 keV, a photoelectric interaction is more likely to occur when the incident photon is 70 keV than if it were 120 keV. This is because the photoelectric effect is inversely proportional to approximately the third power of energy:
  • The tighter an electron is bound to its atom, the more likely it is to be involved in a P.E. interaction. Atoms with high atomic number bind their electrons tighter than atoms with low Z-number. These high atomic number elements are more likely to under go P.E. reactions. The probability of the P.E. effect is nearly proportional to the third power of the atomic number.

Compton Scatter

Almost all of the scatter radiation that we encounter in diagnostic radiology is a result of Compton scatter. The basic process is that an incident photon with relatively high energy strikes a "free" valance shell electron of an atom, ejecting it from its orbit. Both the incident photon and the ejected electron are deflected in a new direction as part of the photon’s energy is imparted to the electron. An analogy of the reaction is a billiard ball (orbital electron) being hit by a cue ball (incident photon). The products of the interaction are a positive atom and a free electron called a "recoil" electron to distinguish it from a photoelectron.

Compton Scatter occurs when a primary beam photon strikes an outer valance shell electron of an atom ejecting it, with a resulting change in the direction of the incident photon. The amount of energy retained by the photon is determined by its angle of change. The initial energy of the photon is split between it and the ejected electron; however, the photon will retain most of the energy. For example, at 110 kev the photon will retain 91 kev or more provided the angle of change is 90 degrees or less.

Scatter is produced in the patient's body during radiation exposure sequences that generate diagnostic images for interpretation; most of which is absorbed in the patient's tissues. When a high kVp is selected in order to penetrate a part or to achieve low radiographic contrast, scattered photons may have sufficient energy to escape the patient’s body and reach the occupational worker present in the exam room. The amount of scatter generated within the patient's body is a function of the kVp used, field size and the amount of tissue exposed. As the kVp increases the number of x-rays that undergo Compton scattering increases along with a simultaneous decrease in the relative number of photoelectric interactions. Because scatter radiation from the patient is the primary source of radiation exposure to imaging personnel, a significant amount of radiation protection practices involve reducing patient exposure, which is beneficial to both the patient and the health care team.

Let’s consider why scatter radiation is so important to imaging professionals. Notice that a primary gamma photon with 140 keV of energy interacting with an atom in the patient's body in a Compton interaction will have a final energy that is dependent on the angle at which it scatters. Forward scatter will have the greatest energy being only slightly reduced from the initial energy. Backscatter originating from a 140 keV photon will have retained about 90-100 keV of energy, which may become significant taken as cumulative occupational radiation dose. Such a dose to an unprotected bystander is unacceptable.

The point we are stressing is that a scattered photon may retain most of its initial energy even at 90o deflection. In the case of our 140 keV photon it would retain approximately 100-110 keV at 90 degree scattering. For the unprotected worker, this would be like standing in the primary beam.

Scatter radiation is really primary beam photons that can be extended beyond the patient in random directions from its original straight line path. Because it is radiation that has undergone a change in direction other areas of the radiographic room can be potentially exposed. Compton scatter is a particularly important problem for radiation workers and other health care personnel monitoring patients during x-ray procedures. This is because depending on how close to its origin one is, it could be very nearly like being in the primary beam. What one should remember is that it is really a primary beam photon that has undergone a slight change in direction and therefore retains most of its initial energy. So, under no circumstances should personnel be allowed to remain in a position where exposure can occur without wearing protective apparel. In addition, the technologist should not assume that because they are wearing "protective" shielding it is appropriate to be exposed to scatter, when it can be avoided altogether by remaining at a safe distance. The radiographer should remember that lower energy photons tend to scatter back at 180o, and high energy photons tend to scatter forward. These are constant observations of photon energies throughout the diagnostic energy range. To calculate the change in wavelength of a scattered photon, the following formula is used:

Δλ = 0.024(1 - cosθ)
Where Δλ = change in wavelength (Angstrom)
θ = angle of photon deflection

Energy of Compton-Scattered Photons
for various Angles of Deflection
Energy of
Photon (keV)
Energy of Scattered Photons (keV)
  Photon Deflection Angle
30° 60° 90° 180°
25 24.9 24.4 24 23
50 49.6 47.8 46 42
75 74.3 70 66 58
100 98.5 91 84 72
150 146 131 116 95

Scatter radiation is not useful in diagnostic imaging, but it does occur as an unwanted interaction of photons with matter. Scatter is the main type of radiation that occupational workers are exposed to and is undesirable to imaging. Scatter does not contribute positively to the diagnostic image; instead it diminishes it by causing image fog and undesired low contrast. Most scatter radiation is removed prior to striking the image receptor by a precision placed grid, the only way to protect the technologist from scatter is to maintain a safe distance and remain behind protective shielding. The patient is protected from scatter by technique adjustments and limiting the exposure field to the minimum area of coverage that will provide the most diagnostic information.

Pair Production and Photodisintegration

Pair production and photodisintegration have no significance in diagnostic imaging at this time; therefore we will keep their discussion brief. In pair production a high energy photon strikes the nucleus of an atom and is completely absorbed. The photon's energy is converted into matter with the production of an electron and a positron. It should be remembered that the mass of an electron is equivalent to 0.511 meV, therefore, pair production does require minimum photon energy of 1.02 meV in order to occur. This energy range is well outside the diagnostic imaging range of energies.

Photodisintegration is a term referring to the process of a high energy photon colliding with the nucleus of an atom causing a partial ejection of nuclear contents. The ejected portion can be a neutron, proton, alpha particle, or a cluster of particles. Again this does not occur in diagnostic radiology imaging because to overcome nuclear binding energies photons must contain 7-15 MeV.

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Points to Ponder! image040
  • Atoms are stable when the number of proton (+) and electrons (e-) are equal.
  • Nuclear stability depends on the n:p ratio and radioactive decay occurs when the number of neutrons is above or below a theoretical line of nuclear stability.
  • Unstable atomic nuclei which lie above the theoretical line of stability are beta emitters through a process that converts a neutron to a proton.
  • Pure beta emitters are important in radiology because beta particles only travel a short distance in tissue and cause much ionization. Ionization is a therapeutic treatment for many conditions such as: ascites,
  • Non pure beta emitters are especially useful in diagnostic imaging because they emit gamma radiation that can be detected with a scintillation camera.
  • Isobars are atomic nuclei that have the same atomic mass, but different atomic numbers. Their chemical properties are different because they are different atoms. A key point is that isobaric transition is part of a three part transition of beta emitters.
  • Isomers are atoms with the same atomic number and atomic mass number. Isomeric transition follows isobaric transition causing the release of a gamma photon.
  • Positron emission and electron capture are competing reactions within the same nuclear species. These nuclei lie below the theoretical line of nuclear stability and seek to convert a neutron to a proton.
  • The penetration ability of charged particles is low due to the high number of ionizations they participate in. Alpha particles travel less distance than beta particles, and beta particles less than gamma or x-rays.
  • Ionization of adjacent atoms by a beta particle is caused by repulsion force being that the beta particle is a high speed electron that approaches orbital electrons. Each ionization costs the beta 3.4 eV and therefore many ionizations are required to deionize the particle.
  • Photon energy and wavelength are inversely related.
  • Photon energy and frequency are directly proportional.
  • The positron is a high speed positively charged electron. In addition to being the opposite charge of an electron it contains antimatter that causes annihilation of matter.
  • Positrons are important in modern radiology because they are the fundamental particles that fuel PET imaging.
  • The driving force of positron production is an unstable nucleus suffering a low n:p ratio and a proton are converted to a neutron plus a positron.
  • The mechanism of bremsstrahlung radiation produced by a positron is a negative acceleration away from the nucleus, whereas bremsstrahlung produced in an x-ray tube is by rapid deceleration.
  • Positrons cause ionization of atoms by attracting electrons to its high velocity positive charge.
  • When a positron achieves a rest state and collides with an electron the product is two gamma photons each with 0.511 MeV. These photons are detected by PET scanners to image the patient.
  • Electron capture is a process by which the k-shell electron is taken into the nucleus of its atom and through transition processes electrons are shifted inward to fill the vacancy. The net product of electron capture is the production of characteristic radiation which is desirable for some nuclear medicine studies.
  • Gamma radiation is emitted from the nucleus of disintegrating atomic nuclei whereas x-rays are generated from the orbitals of atom.
  • Bremsstrahlung radiation is the most important type of ionizing radiation produced in x-ray tubes; it is greater than 95% of the heterogeneous x-ray beam. Bremsstrahlung radiation is part of the continuous spectrum producing x-ray photons of all energies from near zero up to the maximum kVp selected.
  • Characteristic radiation is not so important in imaging by x-ray tube sources; it is less than 5% of the heterogeneous x-ray beam. Characteristic radiation produces discrete energy photons that are of the energy difference between the initial electron shell and its final energy shell.
  • The photoelectric effect (P.E. effect) describes the mechanism of photon absorption in matter. The P.E. effect is important to radiographic imaging because it along with transmitted radiation is the foundation of radiographic contrast.
  • The photoelectron, a product of the P.E. effect is important to radiology because it is the mediator of biological damage causing indirect DNA effects.
  • Of the five basic interaction of ionizing radiation with matter, only Compton scatter and the P.E. effect are important to radiographic imaging.
  • Scatter radiation is a term for photons that undergo deflection in matter. Scatter is important to radiology because a scattered photon will retain most of its initial energy even a deflection angle of 90 degrees.


  1. *Bushong, S.C., Radiologic Science for Technologist: Physics, Biology, and Protection, 7th ed., St. Louis, Missouri: Mosby, Inc. 2001.

  2. *Early, P.J., Principles and Practice of Nuclear Medicine, 2nd ed., St. Louis, Missouri, Mosby Co. 1995.

  3. *Seeram, E., Computed Tomography, Philadelphia, Pennsylvania, W.B. Saunders Co. **1994.

  4. *Selman, J., The Fundamentals of X-ray and Radium Physics, 6th ed. Springfield, Illinois, Thomas Books, 1980.

  5. *Sprawls, P., Physical Principles of Medical Imaging, Rockville, Maryland, 1987, Aspen Publishers, Inc.

  6. Electromagnetic chart, Berkeley Lab, Berkeley, CA. 94720.

Copyright image Copyright 2006 Nicholas Joseph Jr.

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