2. Personnel Dosimetry – Chapter 3.1
1. Interactions of ionizing radiation
with matter
Contents:
• General aspects
• Compton scattering (Compton effect)
• Rayleigh scattering
• Photoelectric effect
• Pair production
3. Personnel Dosimetry – Chapter 3.1
General aspects
passing an absorbing medium, photons may experience various
interactions with the atoms of the medium:
• Interactions with the nuclei of the absorbing medium (e.g. pair
production)
• Interactions with the orbital electrons of the absorbing medium (e.g.
Thomson scattering, Compton effect)
after interaction of the photon with an atom there are two
possible outcomes:
• Photon disappears
• Photon is scattered
most important effect in personal dosimetry: Compton effect
4. Personnel Dosimetry – Chapter 3.1
Compton scattering (Compton effect)
Interaction of a photon with energy hν with a loosely bound
orbital electron of an absorber:
• Incident photon with energy hν
• Interaction with a stationary and
free electron
• Scattered photon with energy hν‘
• Scattering angle θ
• difference between the incident
photon energy hν and the
scattered photon energy hν‘ is
given as kinetic energy to the
recoil electron (Compton electron)
Incoherent scattering Fig. 1: Schematic diagram of the Compton effect [1]
5. Personnel Dosimetry – Chapter 3.1
Compton scattering (Compton effect)
Law of energy conservation:
Conservation of momentum:
with
6. Personnel Dosimetry – Chapter 3.1
Compton scattering (Compton effect)
Energy of the scattered photon and the Compton electron:
with
• Maximum energy transfer for θ = 180° (Backscattering)
• Increasing energy transfer for increasing energy of the incident photon
7. Personnel Dosimetry – Chapter 3.1
Compton scattering (Compton effect)
Energy of scattered photon:
Fig. 2: Energy of scattered
⅗photon hν‘ as a function of
the energy of incident photon
hν and scattering angle θ [1]
8. Personnel Dosimetry – Chapter 3.1
Compton scattering (Compton effect)
Angular distribution of scattered photons and the Compton electrons:
Photons
Fig. 3: Angular distribution [1]
9. Personnel Dosimetry – Chapter 3.1
Rayleigh scattering
• Photon interaction process in which photons are scattered by
bound atomic electrons
• Elastic (coherent) scattering
• The atom is neither excited nor ionized and after the
interaction the bound electrons revert to their original state
• The atom as a whole absorbs the transferred momentum
• Energy of the scattered photon is the same energy as the
original photon
• The scattering angles are relatively small
10. Personnel Dosimetry – Chapter 3.1
Rayleigh scattering
Differential Rayleigh atomic cross section per unit scattering angle θ:
with F = atomic form factor
• No energy transfer to attenuator
• Due to atomic form factor Z and energy dependence
• In tissue and tissue equivalent materials the relative importance of
Rayleigh scattering in comparison to other photon interactions is
small, as it contributes only a few percent to the total attenuation
coefficient
11. Personnel Dosimetry – Chapter 3.1
Photoelectric effect
• Interaction between a photon with energy hν and a tightly bound
electron of an inner shell (highest probability for k shell) of an
absorber atom
• The electron is ejected with kinetic energy EK = hν – EB where EB is
the binding energy of the electron
• The photon is absorbed completely
• The vacancy in the shell
will be filled with a higher
orbit electron
• Probability approx. ~ Z3/E3
(neglecting absorption edges)
Fig. 6: Schematic diagram of the photoelectric effect
EK = hν – EB
hν
Before interaction After interaction
12. Personnel Dosimetry – Chapter 3.1
Pair production
• Production of an electron-positron pair together with a complete
absorption of the photon
• Energetically possible when photon energy hν exceeds 2mec² = 1.02
MeV
• Energy, charge and momentum must be conserved for the effect to
occur
• Total kinetic energy transferred to charged particles (electron and
positron) in pair production:
• Electron and positron do not receive
equal kinetic energy, but average is:
Fig. 7: Schematic diagram of pair production
e+
hν
Before interaction After interaction
e-
13. Personnel Dosimetry – Chapter 3.1
Mass Attenuation Coefficient for Soft Tissue
MassAttenuationCoefficient(cm2/g)
Energy (keV)
Total
Pair
Production
Rayleigh Compton
Photoelectric
15. Personnel Dosimetry – Chapter 3.1
Overview
Compton scattering
Rayleigh scattering
Photoelectric effect
Pair production
Photon interaction
Scattered photon and electron
Scattered Photon
Electron
Electron and positron
Emitted radiation
16. Personnel Dosimetry – Chapter 3.1
2. Effects used in different types of
dosimeters
Contents:
• Ionization in gases
• Ionization in solid state material
• Luminescence
• Photographical effect
17. Personnel Dosimetry – Chapter 3.1
Ionization in gases
• Radiation passes through air or a specific gas
• Ionization of the molecules in the gas
• Free pairs of electrons and ions are produced
• Gas filled detectors:
• High voltage is placed between two areas of the gas filled
space
• Positive ions are attracted by the negative side of the
detector (the cathode)
• Free electrons move to the positive side (the anode)
• These charges are collected by the anode and the cathode
• A small current is formed in the cables connected to the
detector
18. Personnel Dosimetry – Chapter 3.1
Ionization in gases
• By placing a very sensitive current measuring device between
the wires connecting the cathode and anode, the small current
can be measured and displayed as a signal
• The more radiation enters the chamber, the more current is
displayed by the measuring instrument
Voltage Source
Electric
Current
Measuring
Device
Cathode -
Anode +
+ -
Air or other gas
α,β,γ
Fig. 8: Schematic
diagram of a gas
filled detector
19. Personnel Dosimetry – Chapter 3.1
Ionization in solid state material
• In crystalline solids exposed to ionizing radiation free pairs of
electrons and ions are produced (see also ionization in gases)
• If solids are used as ionization detectors, these free charges
generated by radiation must be derived and collected by
external electric fields
• The more ionizing radiation hits the solid state material, the
more current is displayed by the measuring instrument
• Whether a solid can be used as a charge detector particularly
depends on the electrical conductivity of the solid's material
• due to their special electric qualities, semiconductors are
excellent in ionization charges detectors
20. Personnel Dosimetry – Chapter 3.1
Luminescence
• Effect is used in scintillation detectors, luminescent screens
and storage phosphor screens
• Some crystalline solids store the energy transferred by
radiation exposure in durable electron states.
• Light (photon) emission starts when crystal is heated:
Thermoluminescence
• Light (photon) emission starts when crystal is put into light
(optically stimulated): Optical stimulated luminescence
21. Personnel Dosimetry – Chapter 3.1
Photographical effect
• When a photographic film emulsion is exposed to ionizing
radiation, a latent image is formed in the emulsion
• Emulsion consists of small silver halides suspended in gelatin
• Amount of blackening produced by the metallic silver is
correlated with the amount of radiation the emulsion received
22. Personnel Dosimetry – Chapter 3.1
Overview
Effect Indicator Dosimeter
Ionization in gases Electric charge Ionization chambers,
electrometers
Ionization in solid state
bodies
Electric charge Semiconductors
Luminescence Light emission
(spontaneous or when
heated)
Scintillations detectors,
luminescent screens,
storage phosphor screens,
thermoluminescent
dosimeters, optical
stimulated luminescence
dosimeters
Photographical effect Blackening (optical
density)
X-ray film emulsions, film
badge dosimeters
23. Personnel Dosimetry – Chapter 3.1
3. Measurement quantities
Contents:
• Absorbed dose D
• Kerma K
• Exposure X
• Dose equivalent H
• Dose rate D/t or H/t
• Dose-area product DAP
24. Personnel Dosimetry – Chapter 3.1
Absorbed dose D
• Absorbed dose D is a measure of the amount of energy from
an ionizing radiation deposited in a medium
where ∆ε is the mean energy transferred by the radiation
to a mass ∆m
• The biological effect is related to the dose and depends on the
nature of the radiation
25. Personnel Dosimetry – Chapter 3.1
Absorbed dose D
Units
• Gray (Gy)
• SI unit used to measure absorbed dose is the gray (Gy)
• 1 Gy = 1J/kg = 107J/103g = 104 erg/g = 100 rad
• Gy can be used for any type of radiation
• Gy does not describe the biological effects of the different
radiations
• RAD (Radiation Absorbed Dose)
• Old, but still commonly used, not unit of International
System of units (SI)
• 1 rad = 100 ergs/g
26. Personnel Dosimetry – Chapter 3.1
Kerma K
• Kinetic energy released per unit mass
• This is the initial energy that is transferred from the photons to the
electrons (kinetic energy) in the medium
where ∆Etrans is the average energy that is transferred to the
electrons of a volume of a medium whose mass is ∆mmed
Unit: Gray (Gy)
27. Personnel Dosimetry – Chapter 3.1
Exposure X
• Exposure
where ∆Q is the sum of all electric charges produced by ionizing
radiation in a mass ∆ma
• Unit: Coulomb per kilogram (C/kg)
• Former unit roentgen (R) is defined as charge of 2.58x10−4 C
produced per kg of air
X =
DQ
Dma
28. Personnel Dosimetry – Chapter 3.1
Dose equivalent H
• Radiation dose to tissue
• Takes into account different relative biological effects of the
different types of ionizing radiation
• Describes the health hazard caused by radiation
• The absorbed dose multiplied by a radiation weighting (quality)
factor
where D is the absorbed dose and ωR the radiation weighting
factor (which is 1 for gamma rays and beta particles)
• Unit: Sievert (Sv)
29. Personnel Dosimetry – Chapter 3.1
Radiation weighting factor (ICRP 103)
Type of Radiation ωR
X-rays, gamma rays, beta particles, and electrons 1
Protons 2
Neutrons (energy dependent) 2.5-20
Alpha particles and other multiple-charged particles 20
30. Personnel Dosimetry – Chapter 3.1
Dose rate
• Dose rate is the dose absorbed per unit time
• Amount of radioactive dose received by a person within a
certain period of time
• Definition for all dose quantities
Absorbed dose rate:
Exposure rate:
Kerma rate:
• Unit:
• Gy/s or Gy/min or Gy/h for absorbed dose rate and Kerma
rate
• A/kg = C/(s×kg) for exposure rate
31. Personnel Dosimetry – Chapter 3.1
Dose area product DAP
• Surface integral of air kerma to a radiation field
where Ka is the air kerma and A the area of the radiation
field
• Unit: Gy×m2
32. Personnel Dosimetry – Chapter 3.1
Effective Dose
• Effective dose is the sum of the organ equivalent doses times
the weighting factors according to the radio sensitivity of the
organ
E(Sv) = HTå ×wT
34. Personnel Dosimetry – Chapter 3.1
4. From kerma to Hp quantities
• From dose measures to personal dose
• Personal dose
• HP(10)
• HP(0.07)
35. Personnel Dosimetry – Chapter 3.1
From dose equivalent to personal dose
• Dose equivalent Hp is the output parameter for all operative
dose measurement parameters
• Dose equivalent is the absorbed dose multiplied by a
radiation weighting (quality) factor
• dosimeters are designed to measure the dose equivalent,
which is used to determine the personal dose
36. Personnel Dosimetry – Chapter 3.1
Personal dose: HP(10) and HP(0.07)
• Individual-related measurement of radiation exposure to a
specific person caused by external radiation fields
• Determination of personal dose with individual dosimeters
• HP(10):
• Personal dose for penetrating radiation
• Skin depth of 10 mm
• HP(0.07):
• Personal dose for radiation with low depth of penetration
• Skin depth of 0.07 mm
37. Personnel Dosimetry – Chapter 3.1
Summary
• interaction of ionizing radiation with matter
• Compton scattering and photoelectrical effect are
dominant in the energy range of diagnostic imaging
• dosimeter types used in the measurement of personnel dose
• film badges, TLD and OSL dosimeters
(in rare cases ionization chamber)
• Measurement quantities
• absorbed dose, exposure, kerma, dose equivalent
• Personal dose
• HP(10) and HP(0.07)
38. Personnel Dosimetry – Chapter 3.1
Literature and additional reading
1. E. B. Podgarsak (2005) Radiation oncology physics: a
handbook for teachers and students
www-pub.iaea.org/mtcd/publications/pdf/pub1196_web.pdf
Editor's Notes
In this chapter you will get an introduction into the basics of dosimetry, which you will need to understand the physical background in dosimetry in this course. We will give a brief summary on Interactions of ionizing radiation with matter, on Effects used in different types of dosimeters and on measurement quantities and units used in diagnostic imaging and dosimetry of the personnel. In the end of this chapter we will show how to convert from kerma to Hp quantities.
In the beginning of the chapter we will summarize the relevant Interactions of ionizing radiation with matter.
We will explain some general aspects and look at the following effects:
Compton scattering (Compton effect), Rayleigh scattering, photoelectric effect, and pair production.
Initially, we will look at some general aspects. When photons are passing through matter, the photons may have interactions with this material. There are two different types of interactions that may occur. This is either an interaction with a nucleus of this material (e.g. in pair production) or an interaction with the orbital electrons (e.g. in Compton scattering).
Depending on the type of interaction the photon might be disappeared or scattered. It transfers parts or its full energy on the atom or produces one or more new particles. The probability for the different types of interaction depend strongly on the atomic number of the material and the energy of the photon, more details later.
The most relevant effect in personal dosimetry is the Compton effect.
Compton scattering or Compton effect is the most important type of interaction needed to be understood in personal dosimetry, because the largest amount of radiation to the personnel is due to Compton Scattering in the patient.
In Compton scattering there is an interaction of a photon of energy hν, which is Planck‘s constant times the frequency of the photon, with a loosely bound orbital electron of the matter it passes. The steps in this interaction are:
There is an incident photon with energy hν
There is an interaction with a stationary and free electron
The photon will become a scattered photon with energy hν‘
The scattering angle θ for the photon will have a corresponding direction Φ of the recoil electron.
The difference between the incident photon energy hν and the scattered photon energy hν‘ is given as kinetic energy minus the binding energy of the electron to the recoil electron (Compton electron)
This process is a form on incoherent scattering.
The dependences of energy transfer and scattering angle can be quite easily derived from laws of conservation of energy and momentum. The law of energy conservation states that the energies before the interaction and after the interaction have to be identical. The energy before the interaction here is described using Planck relationship hν and the relativistic energy expression of the electron mass me times the speed of light c squared.
The energy after the interaction is hν’ , with a lower energy compared to the energy of the initial photon, plus the relativistic energy expression of the electron mass me plus the kinetic energy of the recoil electron EK.
In the conservation of momentum we look at the initial momentum of the photon which is its energy hν divided by its speed c. After the interaction its momentum is split into the momentum of the scattered photon hν’ divided by c times the cosine of its scattering angle to account for the momentum in the initial direction of the photon, and the momentum of the recoil electron in this direction which is the momentum corrected for the relativistic mass times the cosine of the angle of its movement relative to the initial direction. The correction for the relativistic mass has to be accounted for, because the speed of the recoil electron can be relativistic.
The second term for the energy conservation accounts for the momentum perpendicular to the initial direction of the photon, which is zero before the interaction. After the interaction as above we look at the momentum of the photon and the recoil electron, but now in the perpendicular direction, therefore the cosine is now substituted by sinus.
The equation system can now be solved for the energies of the scattered photon and the recoil electron using the scattering angle θ. The maximum energy for the recoil electron occurs, when θ is 180° , which is called backscattering. In this case for a photon energy of 102.2 keV (equal 1/5 of the electrons rest mass), it is 5/7 of the initial energy equal to about 73 keV. The electron would pick up the differences of the energies as kinetic energy minus the binding energy.
The second point seen here is, that we have an increasing energy transfer for increasing energies of the incident photon.
In this graph the energy of the scattered photon in dependence of its energy and some scattering angles θ is shown. For a scattering angle of zero degrees (no scattering) there is no energy transfer and for backscatter we have the highest energy transfer. In diagnostic imaging with energies between 20 and 150 keV, the energy transfer is not higher than 37%.
To describe the probability for different scattering angles the Klein-Nishina equation was derived from quantum electrodynamics.
Looking at the angular distribution we see that the probability for scatter has a slight prevalence into forward direction especially for higher photon energies. Scatter in the perpendicular to the photon direction has a slightly lower probability than into the back direction in the diagnostic energy range. For diagnostic imaging the differences for the probabilities of different scatter directions are not larger than about 3.
In a very rough estimate, the probability for the scatter directions in the diagnostic energy range can be assumed to be about the same.
The next interaction process we look at is Rayleigh scattering. In Rayleigh scattering, the incident photon interacts with and excites the total atom, and not individual electrons as in Compton scattering or the photoelectric effect (see below). In the scattering process, the oscillating electric field of a electromagnetic wave acts on the charges within an atom, causing all the electrons in the scattering atom to oscillate in phase. The atom’s electron cloud immediately radiates this energy, emitting a photon of the same energy but in a slightly different direction. In this process there is no ionization and no electrons are ejected.
The atom as a whole absorbs the transferred momentum and the energy of the scattered photon is the same as the one of the incident photon. The scattering angles in diagnostic imaging are relatively small.
Now we look at the differential Rayleigh atomic cross section. In this equation, you can see, that this interaction occurs mainly with very low energy x-rays, e.g. the energies used in mammography. In general, the average scattering angle decreases as the x-ray energy increases. Rayleigh scattering has a low probability of occurrence in the diagnostic energy range. In soft tissue, Rayleigh scattering accounts for less than 5% of x-ray interactions above 70 keV and at most only accounts for about 10% of interactions at 30 keV.
In the photoelectric effect there is an interaction between the incident photon and electrons of the inner shells, where the whole energy is transferred to this electron. The electron is ejected with a kinetic energy which is the energy of the incident photon (hν) minus the binding Energy of the electron. The ejected electron is most likely one whose binding energy is closest to, but less than, the incident photon energy. For example, for photons whose energies exceed the K-shell binding energy, photoelectric interactions with K-shell electrons are most probable, which is the case in most cases in diagnostic imaging.
The probability is about Z3/E3 (neglecting absorption edges). In the energy range used in diagnostic imaging the photoelectric effect together with the Compton effect are the most likeliest types of interaction of radiation with matter. In other materials this might be different, e.g. in lead and in this energy range the photoelectric effect has a much higher probability than the Compton effect. Therefore lead is frequently used for shielding.
Pair production is the creation of an electron-positron pair together with a complete absorption of the photon and complete energy transfer. This type of interaction is energetically possible when the energy of the photon (hν) exceeds 2mec² = 1.02 MeV. Therefore this process does not occur in diagnostic imaging, because the threshold energy is well beyond the highest energies used in diagnostic imaging. In this process conservation of energy, momentum and charge can be used to gain more information. The total kinetic energy of both charged particles is the energy of the incident photon minus the rest mass energy of the electron and the positron, which is identical. Electron and positron do not receive equal kinetic energy, but average is half of the total kinetic energy.
In this figure a summary of all types of interactions is shown for soft tissue. The data are presented in a double logarithmic scale (log-log plot) showing the mass attenuation coefficients for the different types of interaction and depending on the energy. The total mass attenuation coefficient is the result of all types of interaction. It can be seen, that the total mass attenuation coefficient strongly decreases with increasing energies. Therefore photons with higher energies are less absorbed than low-energy photons.
In diagnostic imaging the energies of the photon are between 10 keV and150 keV. In that energy range photoelectric and Compton effect dominate the interaction processes. Rayleigh scattering only occurs in the lower energy range with a lower probability. The probability for the photoelectric effect dominates in this energy range but drops rapidly and even gets lower than for the Rayleigh scattering for soft tissue. Compton effect becomes to be the most likeliest effect starting at an energy of about 25 keV. At very high energies beyond the diagnostic range pair production starts to become important and at about 30 MeV it is the most probable interaction process of photons in soft tissue.
Keep in mind, that for personal dosimetry the interesting and relevant energy range in interventional radiology is shifted to lower energies between about 10 keV to about 80 keV.
In this figure is shown, how the probability of photon and Compton effect depends on the energy and the type of tissue, in which the interaction occurs. For soft tissue as shown in the figure before, for low energies the photoelectric effect dominates and at energies above about 25 keV the Compton effect dominates. In bone, which has a higher atomic number, photo effect is dominant to higher energies as shown. In natrium-iodine and lead the photoelectric effect is dominant. Even at energies found in Positron Emission Tomography at 511 keV Compton scattering is less probable then the photoelectric effect. However, the mass attenuation coefficient of lead at 511 keV is significantly lower compared to the one at 100 keV.
In this overview the outcome of the interaction of photons with matter are summarized.
In Compton scattering only a part of the energy of the photon is transferred, after the interaction a scattered photon and an electron with the transferred energy minus the binding energy emerges.
In Rayleigh scattering no energy is transferred, after the interaction a scattered photon occurs.
In the photoelectric effect the whole energy of the photon is transferred, after the interaction an electron with the transferred energy minus the binding energy emerges.
In pair production the energy of the photon is used to generate an electron and a positron.
In the second chapter of the module we will repeat the physical effects, which are used in different types of dosimeters.
We will explain briefly the ionization in gases and in solid state materials and look at luminescence and the photographical effect.
The process of ionization in gases is initiated when radiation passes through air or a specific gas. Caused by the interaction of the radiation with the gas and the thereby transferred energy, molecules in the gas are ionized and free pairs of electrons and ions are produced.
The most important application for the effect of ionization in gases is found in gas filled detectors. In gas filled detectors a high voltage is placed between two areas of the gas filled space. Hereby the positive ions are attracted by the negative side of the detector, which is called cathode. However the negative electrons move to the positive side of the conductor – the anode. These positive and negative charges are collected by the anode and the cathode, whereby a small current is formed in the cables connected to the detector. This current is measured by an electrometer, an electric current measuring device.
In this figure you can see a schematic diagram of a gas filled detector. By placing a very sensitive current measuring device between the wires connecting the cathode and anode, the small current can be measured and displayed as a signal. The more radiation enters the chamber, the more current is displayed by the measuring instrument. The current is measured in Ampere, which is Coloumb/s. Thereby we achieve a measurement of the number of produced charges, which relates to the ion dose, see below.
The next effect is the ionization in solid state materials. On the analogy of the ionization in gases, in crystalline solids exposed to ionizing radiation, free pairs of electrons and ions are produced. If solids are used as ionization detectors, these free charges generated by radiation must be derived and collected by external electric fields. The more ionizing radiation strikes the solid state material, the more current is displayed by the measuring instrument.
Whether a solid can be used as a charge detector particularly depends on the electrical conductivity of the solid's material. Due to their special electric qualities, for example semiconductors are suitable very well as ionization charges detectors. Due to their higher z-value and higher density the probability for interaction of the ionizing radiation with the detector is higher compared to gases used in ionization chambers.
Another important effect in radiation dosimetry is the luminescence. It‘s functionality is the emission of photons in the visible spectral range in solid state materials caused by energy transfer of radiation exposure. This is spontaneous light emission. Luminescence is differenced in two types: The fluorescence and the phosphorescence. In fluorescence the decay time of the light emission is independent to temperature of the solid. However in Phosphorescence the photon emission starts not until stimulation is finished and its intensity is dependent on the temperature of the solid.
The effect of luminescence is used in scintillation detectors, luminescent screens and storage phosphor screens.
Some crystalline solids store the energy transferred by radiation exposure in durable electron states. Through this, light (therefor photon) emission starts when the crystal is heated: This is called Thermoluminescence.
Other types of crystals exhibit fluorescent behaviour, when they are optical stimulated. In all materials, the amount of emitted light is proportional to the absorbed energy.
The last effect we will describe in this chapter is the photographical effect. By exposing a photographic film emulsion to ionizing radiation, a latent image is formed in the emulsion. This effect occurs in small silver halides suspended in the gelatin, constituting the emulsion. In the reaction of the photographical effect, the amount of blackening produced by the metallic silver is correlated with the amount of radiation the emulsion received.The photographical effect is used in many personal dosimeter, the film badges.
In this tabular overview you can see all relevant effects with their specific indicators and the different types of dosimeters they are used in.
In this chapter we will briefly repeat the relevant measurement quantities, which are used in dosimetry. This measurement quantities are: The absorbed dose D, the Kerma K, the Exposure X, the dose equivalent H, the Dose rate and the Dose-area product DAP.
The basic quantity for personal dosimetry is the absorbed dose D. The absorbed dose D is a measure of the amount of energy from an ionizing radiation deposited in a medium. It can be mathematical described through the mean energy ∆ε transferred by the radiation per mass ∆m.
The biological effect, which describes a potential damage of human tissue, is related to the dose. The absorbed dose can be used to describe a risk associated with the radiation. Therefore additional information about the type of radiations and the organs absorbing the dose is needed.
Knowing the average energy needed to create an electron-ion pair in a specific gas, an ionization chamber filled with this gas can be used to perform a direct measurement of the absorbed dose.
To quantify the absorbed dose D the unit of the International System of units Gray is used.
The advantage of the unit Gray is that it can be used for any type of radiation. However Gray does not describe the biological effects of the different radiations.
The former unit of the absorbed dose is „Radiation Absorbed Dose“, shortened RAD. This unit is old-fashioned, but still commonly used in dosimetry. It is no unit of the International System of units (SI). One rad is 100 ergs per gram.
Another important quantity in dosimetry is the Kerma K, which is an acronym for Kinetic energy released per unit mass.
Kerma is the initial energy that is transferred from the photons to the electrons (kinetic energy) in the medium per mass unit.
In analogy to the absorbed dose, the unit of Kerma is also Gray (J/kg).
The exposure X is the sum of all electric charges produced by ionizing radiation per mass. The exposure is described in the unit of charge per mass, this is coulomb per kilogram.
The former unit of exposure is roentgen (R), which is defined as a charge of 2.58 x 10−4 coulomb produced per kilogram mass.
Exposure is a helpful quantity, because it can be directly measured in a gas filled ionization chamber. If air is used, the effective number of air and tissue is approximately the same making the measurement with an air-filled ionization chamber tissue equivalent. Knowing the average energy needed for an ionization process in air (33.97 eV/ ion pair or 33.97 J/C, the absorbed energy can be derived from the exposure.
The next dosimetric quantity we want to look at is the dose equivalent H. The dose equivalent describes the specific radiation dose to a specific tissue. Compared to the absorbed dose, the dose equivalent is taking into account the different relative biological effects of the different types of ionizing radiation. Therefore it describes the specific health hazard caused by a certain radiation.
The dose equivalent is calculated with the absorbed dose multiplied by a specific radiation weighting (respectively quality) factor ωR , which is 1 for gamma rays and beta particles.
The unit of the dose equivalent is Sievert (J/kg).
In this table the radiation weighting factors according to ICRP Report 103 are shown. For X-rays, gamma rays, beta particles, and electrons the weighting factor is 1, for Protons it is 2, for Neutrons the weighting factor is energy dependent and between 2.5 and 20.
For Alpha particles and other multiple-charged particles it is 20.
In diagnostic imaging the factor always is 1.
Now we will look at the concept of the dose rate. The dose rate is defined as the dose absorbed per unit time.
The dose rate is defined for all dose quantities: Absorbed dose rate, exposure rate and Kerma rate.
The unit of the dose rate is Gray per second, respectively Gray per minute or Gray per hour for the absorbed dose rate and Kerma rate. For exposure rate, the unit is Ampere per kilogram.
The last measurement quantity we look at is the dose area product, shortened DAP. The dose area product is calculated through the integral of air kerma to a radiation field within a certain area.
The unit of the dose area product is Gray times square meter. Frequently the DAP meters state the value as cGy per cm2 or Gy x m2. In the DICOM standard the used unit is dGy x cm2
Biological tissues vary in their sensitivity to the effects of ionizing radiation. Therefore tissue weighting factors were established by the ICRP to include this. The sum of the organ equivalent doses times the weighting factors of the organ is the effective dose. This is a tool to describe a risk for a population associated with an exposure. Thereby, a tool is available to compare the risk associated with a partial body irradiation to the risk associated with a whole body irradiation.
However, the effective dose is not meant to describe a risk associated for a specific person, but only for a population.
In this table the tissue weighting factor according to ICRP 103 are shown. For Bone-marrow (red), Colon, Lung, Stomach, Breast, and remainder tissues the weighting factor is 0.12, for the gonads it is 0.08, for bladder, oesophagus , liver and thyroid it is 0.04 and for the bone surface, brain, salivary glands and skin it is 0.01.
Note that most of the radiosensitve organs are in the trunk of the body.
The effective dose for the personnel by occupational exposure is assessed by a measured personal dose equivalent.
The personal dose equivalent named Hp(d) is an operational quantity and defined as the dose equivalent in soft tissue (commonly interpreted as the ‘ICRU sphere’) at an appropriate depth, d, below a specified point on the human body. The unit of personal dose equivalent is joule per kilogram(J/kg) and its special name is Sievert (Sv). The specified point is usually given by the position where the individual’s dosimeter is worn.
The dose equivalent Hp is the output parameter for all operative dose measurement parameters. The dose equivalent is the absorbed dose multiplied by a radiation weighting (quality) factor, which is 1 in diagnostic imaging. The dosimeters are designed to measure the dose equivalent, which is used to determine the personal dose.
The personal dose is described with HP(10) and HP(0.07). This values should describe dose in specific depth of the tissue caused by an external radiation field.
For the assessment of effective dose, Hp(10) with a depth d = 10 mm is chosen, and for the assessment of the dose to the skin and to the hands and feet the personal dose equivalent, Hp(0.07), with a depth d = 0.07 mm, is used. A depth d = 3 mm has been proposed for the rare case of monitoring the dose to the lens of the eye. In practice, however, Hp(3) has rarely been monitored and in diagnostic imaging Hp(0.07) can be used for the same monitoring purpose. Operational quantities are measurable, and instruments for radiation monitoring are calibrated in terms of these quantities. In routine monitoring, the values of these operational quantities are taken as a sufficiently precise assessment of effective dose and skin dose, respectively, in particular, if their values are below the protection limits.
The operational quantities for individual monitoring are Hp(10) and Hp(0.07). The personal dosimeter is worn on a position of the body representative of its exposure, at low doses and under the assumption of a uniform whole-body exposure, the value of Hp(10) provides an effective dose value sufficiently precise for radiological protection purposes.
In Summary: We have discussed the different types of interaction of ionizing radiation with matter and seen, that Compton scattering and photoelectrical effect are dominant in diagnostic imaging. We have shown the different types of dosimeters used in the measurement of the dose of the personnel and looked at the different measurement quantities. In the end, we have presented how the personal dose is measured and described.
