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Radioactivity
Dosimetry and radiation absorption
Authors: Ján Pánik and Daniel Kosnáč version 10/2019
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Content • What is radioactivity.................................................................................................................. 3
• Ionizing radiation distribution................................................................................................... 4
• Electromagnetic spectrum distribution.................................................................................. 5
• Dosimetry and its goals............................................................................................................. 6
• Radioactivity basics.................................................................................................................. 8
• Radiation protection…............................................................................................................. 11
• Radiation dose…....................................................................................................................... 12
• Absorbed dose……................................................................................................................... 14
• Absorbed dose rate………....................................................................................................... 15
• Equivalent dose……................................................................................................................. 16
• Effective dose…........................................................................................................................ 17
• Weighting factors..................................................................................................................... 18
• Summary................................................................................................................................... 19
• Exposure..................................................................................................................................... 20
• Exposure rate………………………………………………………………………………………….. 21
• Biological effects of radiation................................................................................................. 22
• Detectors of ionizing radiation…............................................................................................. 27
• Geiger-Müller tubes………….................................................................................................... 28
• Scintillation detectors............................................................................................................... 29
• Film dosimeters……................................................................................................................... 30
• Thermoluminiscence and OSL dosimeters............................................................................. 31
• Additional literature.................................................................................................................. 32
• Sources....................................................................................................................................... 33
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What is radioactivity • A phenomenon in which the nuclei of a given
radioactive element (parent nuclei) spontaneously transform into another element (daughter nuclei) by emission of particles (a, b, neutrons, protons, fission fragments) or electromagnetic radiation (g-radiation).
• Ionizing radiation is any particle or electromagnetic radiation whose energy is large enough to ionize an atom, i.e. remove an electron from the atomic shell.
• If the electromagnetic radiation (photon) has energy of E > 12.4 eV and the wavelength is l < 100 nm, it is classified as ionizing radiation.
3
Ionizing radiation distribution
4
Distribution of directly (electric
charge) a indirectly (no electric
charge) ionizing radiation.
Electromagnetic spectrum distribution
5
Schematic representation of electromagnetic spectrum with the dependence of wavelength,
frequency and energy.
Dosimetry and its goals
6
• Dosimetry is a part of physics that deals with:
1. Ionizing radiation and its properties.
2. Processes of origin and interaction of ionizing radiation with
matter.
3. Measurement methods and quantities characterizing these
interactions.
IAEA, Information for Patients (chart based on UNSCEAR (United Nations Scientific Committee on the Effects of Atomic Radiation) 2008 Report)
Dosimetry and its goals
7
• Quantities measurement that can characterize biological effects of ionizing radiation.
• Environmental dosimetry – often measured in environment where increased radiation dose is expected – e.g. radon monitoring in soil, nuclear power plant surroundings, workplaces, ...
• Medical dosimetry – mainly radiation dose measurements and calculation received by the patient/doctor.
• Natural and artificial radioactivity is measured (nuclear explosions, accelerators, ...)
Radioactivity basics
8
• Radioactivity is natural, stochastic (random) process –
one cannot predict the decay of a specific atom
• The atomic nucleus consists of protons Z (proton/atomic
number) and neutrons N (neutron number).
Nucleon/mass number A of an atom is given by the sum
of its protons and neutrons: A = Z + N.
• Nuclide labeling: where X is a chemical element
and A, Z and N are mass, atomic and neutron numbers.
Therefore, a nuclide is an atomic nucleus. In terms of the
number of protons Z and neutrons N in the nucleus,
nuclides can be divided into 3 basic groups:
A
Z NX
Radioactivity basics
9
1. Isotopes – nuclides have the same atomic number Z, but different mass number (nucleon number) A
We are talking about isotopes of carbon 12, 13 or 14, or isotopes of uranium 233, 235, 236 and 238.
2. Isobars – nuclides having the same mass number A, but different atomic number Z
3. Isotones – nuclides having the same neutron number N and different atomic number Z
12 13 14 233 235 236 238
6 6 6 92 92 92 92(e.g.: C, C, C or U, U, U, U)
26 26 96 96 96
13 12 40 42 44(e.g.: Al, Mg or Zr, Mo, Ru)
14 15
6 7(e.g.: C, N).
10
0 1 2 3 4 5 6 7 8 9 10 110.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0 1
0.5
0.25
0.125
0.0625
N/N
0
Half-life T1/2
Relative quantity decrease of parent nucleus
Relative quantity increse of daughter nucleus
Radioactivity basics -λ t
0N=N e
Radioactive law. The relative decrease of the parent nuclide due to radioactive decay as a function of the half-life with the formation of the daughter nuclide.
• The role of radiation protection is to reduce the
absorbed dose in the human body to the lowest possible
extent (ALARA - "As Low As Reasonably Achievable").
1. TIME
2. DISTANCE
3. SHIELDING
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Radiation protection
dD
D= t=D tdt
0
2
DD=
r
• Absorbed radiation dose is important regarding
ionizing radiation interaction with human body.
• Radiation dose – is biologically effective amount of
radiation received by the unit of mass (volume) of
an organism.
• Do not confuse it with the dose in Pharmacology!
• The medicine only cares about the radiation
absorbed by the body – if radiation passes through
the body without interaction, it is not included in the
radiation dose.
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Radiation dose
• If interaction occurs during the transition of ionizing
radiation through the human body, then this
radiation transfer energy to the atoms of that body
– body receives radiation dose.
• This dose can be described by 3 similar, but
different quantities:
1. Absorbed dose
2. Equivalent dose
3. Effective dose
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Radiation dose
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Absorbed dose • Absorbed dose D [Gy] – can be calculated as ratio
of mean energy transmitted by the ionizing
radiation to an element of the irradiated substance
and the mass of that element:
• Unit of absorbed dose D is called gray.
• 1 Gy = 1 J/kg = 100 rad
• Absorbed dose D can be calculated for any type
of ionizing radiation.
dε 1 dE
D= = [Gy]dm ρ dV
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Absorbed dose rate • Time increment of absorbed dose D is defined as
absorbed dose rate [Gy/s, W/kg].
• Unit of the absorbed dose rate is gray per second or watt per kilogram (Gy/s = W/kg).
• Absorbed dose does not specify the character of interaction of primary ionizing radiation with matter. If primary particle/radiation is neutral, first, its energy is transferred to the charged particles. Then these secondary charged particles ionize/excite the different place of matter.
dDD= [Gy/s,W/kg]
dt
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Equivalent dose • Biological effect of radiation depends both on the absorbed dose and
the type of radiation.
• In order to protect against ionizing radiation, a system has been
introduced which considers the different relative efficiencies of the
different types of radiation on different tissues:
• Radiation weighting factor WR (quality factor Q), tissue weighting factor
WT.
• The quantity used to evaluate biological effects is called the equivalent
dose [Sv]:
• Equivalent dose is mean absorbed dose in tissue/organ multiplied by
radiation weighting factor WR. DTR is mean absorbed dose in tissue T by
radiation R.
• SI unit is Sievert, 1 Sv = 1 J/kg = 100 rem. Gray and Sievert is the same
unit, but they have to be carefully distinguished.
• The same equivalent dose (different radiation) absorbed by the tissue
has the same biological effect.
T R TRR
H = W D [Sv]
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Effective dose • Effective dose ET [Sv] is the sum of the equivalent
doses HT in all organs/tissues multiplied by the tissue
weighting factor WT:
• The unit is Sievert, since the tissue weighting factor is
only a number which determines different
biological effect in different organs/tissues.
• For the whole-human body exposure:
TT
W =1
T T TT
E = W H [Sv]
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Weighting factors Radiation weighting factors WR
Radiation type WR
Photons (all energy) 1
Electrons, muons (all
energy) 1
Neutrons <10 keV 5
Neutrons <10 keV, 100 keV> 10
Neutrons <100 keV, 2 MeV> 20
Neutrons <2 MeV, 20 MeV> 10
Neutrons >20 MeV 5
Protons >2MeV (except
scattered) 5
a particles, heavy nuclei,
fission fragments 20
ICRP 1990: ICRP Publ. No 60. Recommendetations of ICRP, Vol. 21, No 3, 1991.
Tissue weighting factors WT
Tissue/Organ WT
Large intestine 0,12
Bone marrow 0,12
Lungs 0,12
Breast 0,12
Stomach 0,12
Gonads 0,08
Bladder 0,04
Liver 0,04
Thyroid 0,04
Skin 0,01
Brain 0,01
Bone surface 0,01
Other 0,17
Sum 1
Tissue weighting factors for given organs. Taken from: Harrison a Day, 2008.
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Summary • Absorbed dose D [Gy] is the amount of
radiation energy absorbed in certain mass
of material.
• Equivalent dose HT [Sv] describes the dose
absorbed by a certain tissue/organ
depending on the type of radiation.
• Effective dose ET [Sv] is the sum of all
absorbed doses in dependence of
organ/tissue type and radiation type.
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Exposure • Exposure X [C/kg] is a quantity describing ionizing
effects of photons (rtg., gamma) in air.
• Exposure can be calculated as the total electric
charge of all ions, created in the air released by
photons in the air volume element divided by the
mass dm of that element:
• The unit for exposure is coulomb per kilogram,
C/kg.
• Older unit is Röntgen, 1 R = 2,58.10-4 C/kg.
dQ dQ1
X= = C/kg , m=ρ Vdm ρ dV
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Exposure rate • Exposure explains the interaction of primary
indirectly ionizing radiation (photons) with the
elementary volume of matter – air.
• Similar to the absorbed dose, it is possible to
determine the time increment of the exposure –
i.e. exposure rate:
dXX= C/kg/s, A/kg
dt
• Total biological changes after exposure are consequences of physical, chemical and biological processes.
• Stages of radiation effects in exposed biological material can be divided as follows:
1. Physical (10-17 – 10-13)s: radiation absorption, ionization/excitation of molecules.
2. Physico-chemical (10-14 – 10-10)s: secondary processes – molecule dissociation, free radical creation, charge distribution and recombination.
3. Chemical (10-3 – 1)s: reactions of created ions with the DNA, RNA, enzymes and proteins – the changes in composition and function of these molecules.
4. Biological: respond of the biological system to the newly formed substances. Changes in DNA lead to morphological and functional changes of cells/organs and organism as a whole – mutation, cell or whole organism death.
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Biological effects of radiation
• DNA damage
• Free radical production
• Chromosome damage
23
Damage to DNA by gamma
radiation. [cit. 22.3.2016].
Reprinted from:
http://www.scienceart.co.uk/
Alexander Litvinenko poisoned by
polonium 210 (210Po). He died 22
days after poisoning. [cit. 22.3.2016].
Reprinted from:
http://www.theguardian.com/world
/2016/jan/21/key-findings-who-killed-
alexander-litvinenko-how-and-why
Biological effects of radiation
Biological effects of radiation
• Use:
• Liquidation of the metabolically most active cells - tumors – because of scattering, dose has to be calculated with regard to the surrounding healthy tissue
• This implies that children bear the radiation worse - their cells have higher metabolic activity – they divide more often – core gets more exposure.
• External irradiation, radiopharmaceuticals, imaging.
• Monitoring irradiation of staff working on ionizing radiation devices (CT, Röntgen, PET, ...)
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• 87/2018 Law of 13 of March 2018 (effective from 1 of April 2018) on radiation protection and amending certain laws#.
https://www.slov-lex.sk/static/pdf/2018/87/ZZ_2018_87_20180401.pdf
• Students during specialized training for the profession: E = max. ?* mSv per year
• Persons working with RA sources – max. ?** mSv during 5 consecutive years, but ?*** mSv in one calendar year at most.
*, ** and *** - find yourself
• Note: we receive products of RA decay to the body the natural way - by eating, breathing. E.g.: a particles are almost harmless from the outside (stopped by paper, air) but from within they are the most harmful (see Alexander Litvinenko).
• # - find limits for your native country/homeland
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Biological effects of radiation
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Biological effects of radiation Relative
radiation level
Estimated
effective dose
range for adults
Comparable to time of
natural background
radiation
Examples of tests
Ultrasound, MRI
< 0.1 mSv
0.001 mSv 3 hours Rtg – extremity
0.005 mSv 1 day Rtg – dental
0.1 – 1 mSv 10 days – 4 months
0.1 mSv 10 days Rtg – chest
1 – 10 mSv 4 months – 3 years
2 mSv 8 months CT – head
6 mSv 2 years CT – chest
10 – 30 mSv 3 – 10 years
15 mSv 5 years CT – abd & pelvis
30 mSv 10 years CT – abd & pelvis multiphase
30 – 100 mSv 10 – 33 years
Effective doses comparison of some medical examinations with the time required to obtain a given dose from natural background
radiation.
http://dialitdown.org/radiation-and-ct/
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Detectors of ionizing radiation • Detectors of ionizing radiation (IR) can be divided according to the
principles of detection:
a) Changes in the various properties of certain substances due to the effect of IR (evaluation of photochemical effects of IR – rtg. films, film dosimeters, nuclear emulsions; thermoluminiscence and OSL dosimeters, which use e.g. changes in the color or composition of the substance due to the effects of IR).
b) Electronic detectors, which register directly part of absorbed energy of IR (ionization chambers, proportional counters, G-M detectors/tubes, scintillation and semiconductor detectors).
• Detectors of ionizing radiation can be divided according to the measurement method:
a) Continuous – instantaneous values e.g. the number of quanta, the detector response is proportional to the IR intensity and disappears when the IR source is switched off.
b) Integral – they accumulate a response during the exposure period and can be evaluated even after the ionizing radiation source has been switched off.
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Geiger-Müller tubes
• The detector is insensitive during the time
period of the avalanche discharge, - detector
dead time. For G-M detectors about 100 ms.
• Gas addition – quenching agent (e.g.
methylalcohol vapors), that helps quench the
discharge.
• They are used as contamination detectors and monitoring systems, …
• G-M detector is electronic detector filled with gas (Ne, Ar) of less than atmospheric pressure.
• Electrodes are connected to the high voltage (600 – 1000 V).
• When IR enters the detector, ionization occurs in the gas. This
process is avalanche, it means that 1 primary electron generates up to 1010 secondary electrons, then a strong current pulse is detected.
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Scheme of scintillation detector
operation.
https://en.wikipedia.org/wiki/Scinti
llation_counter#/media/File:Photo
MultiplierTubeAndScintillator.svg
Scintillation detectors • Scintillation detectors take advantage of the property of certain
materials to react with absorbed IR by light flashes – scintillation
detected by the photomultiplier (a dynode system amplifies the
light flash – ~ 105 – 108 electrons is at the end of this system).
• Scintillators can be divided into: Inorganic (especially NaI(Tl); for higher energy detection – Bi4Ge3O12 (BGO), Lu2SiO5 (LSO), LuAlO3
(LAO), ...) and organic (naphthalene, anthracene, stilbene, …).
• High detection efficiency and very short dead time(~ 1 ms) is typical
for scintillation detectors, the amplitude of the output pulse is directly proportional to the energy of the absorbed quantum.
30
Film dosimeters
• Film dosimetry uses photochemical effects of IR. Photographic emulsions consist of AgBr microscopic grains
dispersed in gelatin.
• After passing the IR the film gradually
turns black, film blackening rate is a measure of the integral amount of
radiation during exposure – there is a
linear dependence between the dose
and film blackening.
• Usage in personal dosimetry of workers
with IR, usually placed at a reference
point, regularly developed and
evaluated.
Film dosimeter. It contains windows of
different materials for stopping
different types of radiation – e.g.
aluminium window stops alfa and β-
radiation - the darkening of the film
below that window corresponds to
the number of gamma rays.
31
Thermoluminiscence and OSL dosimeters
• Thermoluminiscence – thermo and OSL – optical stimulated luminiscence are phenomena in which IR causes the excitation
of electrons from the valence to the conductivity band.
Electrons remain at these higher energy levels until they are
excited with additional energy – by heating or lighting.
• Electrons return to lower energy levels while emitting photons;
the radiated energy is proportional to the radiation dose.
• After exposure, the TL dosimeter is heated to 160 - 300 oC, there
is a dependence of the electric signal from the photomultiplier and the temperature – heating curve, the area under the curve
is proportional to the absorbed dose.
• They are used in personal dosimetry, especially LiF(Tl), CaF2,
MgBeO4, ...
• Mainly Al2O3(C) is used for OSL dosimeters, LED illumination is
used for evaluation. The produced luminescence is proportional
to the absorbed radiation dose of IR.
Additional literature
• English sources:
• http://www.arpansa.gov.au/radiationprotection/basics/index.cfm
• http://radiopaedia.org/
• www.atomcentral.com
• http://www.physics.isu.edu/radinf/index.html
• http://www.nuclearconnect.org/know-nuclear
• http://www.radiologyinfo.org/
• http://www.radiationanswers.org/
• http://hps.org/
• Slovak sources:
• http://www.edu.snus.sk/
• http://www.uro.sk/
• http://www.javys.sk/sk/
• http://www.health.gov.sk/Zdroje?/Sources/dokumenty/zahranicne_vztahy/ROV
/ROV_Lekarske_expozicie_17-6-2015.pdf
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Sources • STN ISO 31-9: Veličiny a jednotky. 9. časť Atómová a jadrová
fyzika. Bratislava: Slovenský ústav technickej normalizácie, 1997. 36 s.
• BIERSACK, H.J., FREEMAN, L.M. Clinical Nuclear Medicine. Berlin: Springer Verlag, 2007. 548 p. ISBN 978-3-540-28025-5.
• HALLIDAY, D., RESNICK, R., WALKER, J. Fundamentals of Physics. 9th edition. John Wiley & Sons, 2010. 1136 p. ISBN 978-0-470-46911-8.
• http://radiopaedia.org/ [cit. 22.3.2016].
• Slovenská Nukleárna Spoločnosť. www.snus.sk/ [cit. 22.3.2016].
• www.atomcentral.com [cit. 22.3.2016].
• Zákon o radiačnej ochrane 87/2018 Z.z.
• Nariadenie vlády SR 98/2018 Z. z. o ochrane zdravia a osôb pri lekárskom ožiarení
• HOLÁ, O., HOLÝ, K., Radiačná ochrana, Ionizujúce žiarenie, jeho účinky a ochrana pred ionizujúcim žiarením, Nakladateľstvo STU, Bratislava, 2010, 200 p. ISBN 978-80-227-3240-6.
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Last edit: 23.10.2019