Radioactivity

Dosimetry and radiation absorption

Authors: Ján Pánik and Daniel Kosnáč version 10/2019

1

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

13

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.

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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.

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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