technical basis of occupational dosimetry - general review€¦ · technical basis of occupational...

SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALEN RADIATION AND NUCLEAR SAFETY AUTHORITY

Technical basis of occupational dosimetry-general review

Antti Kosunen, Hannu Järvinen STUK

2017 NACP-RPC COURSE

Occupational dosimetry in interventional radiology, -cardiology and nuclear medicine

27-29 September 2017 Södersjukhuset, Stockholm, Sweden


Occupational dosimetry • Dosimetric quantitities • Basics of personal dosemeters

– Whole body dosemeters – Eye lens dosemeters – Extremity dosemeters

•Principles of calibration of personal dosemeters • Measurement traceability • Principles of evaluation of uncertainties

Antti Kosunen, 2017


Antti Kosunen, 2017

Physical quantities: • to characterize the reference radiation field • basis for protection and operational quantities

Protection quantities • International Commission on Radiological Protection (ICRP) • related to anatomy of body (organ dose) and to biological

sensitivity of tissues and organs • basis for dose limits • non-measurable (complicated phantom set-up)

Operational quantities • The international Commission on Radiation Units and

Measurements (ICRU) • conservative estimation of protection quantities • measurable

Dosimetric quantities


Fluence rate , (flux density) Unit: 1/m2 s

Physical quantities

Antti Kosunen, 2017

The absorbed dose, D, is the quotient of de and dm, where de is the mean energy imparted by ionizing radiation to matter of mass dm

The fluence, Φ, is the quotient of dN and dA, where dN is the number of particles incident on a sphere of cross-sectional area dA Unit: 1/m2

dAdN

=Φ

dtdΦ

=ϕ


Antti Kosunen, 2017


The kerma, K, (Kinetic energy released per unit mass) The kerma is the expectation value of the energy transferred from uncharged particles to charged particles per unit mass at a point of interest, including radiative-loss of energy but excluding energy passed from one charged particle to another

dmdEK tr=

Unit; J kg-1, The special name is gray (Gy).

Physical quantities


Antti Kosunen, 2017


The absorbed dose, D, is the quotient of dε and dm, where dε is the mean energy imparted by ionizing radiation to matter of mass dm

Unit; J kg-1, The special name is gray (Gy).

Physical quantities


7

Equivalent dose in an organ or tissue, HT

• wR radiation weighting factor for a radiation quality R

• DT,R the average absorbed dose, in the volume of a specified organ or tissue T, due to radiation of type R . • The sum is performed over all types of radiations involved.

• The unit of equivalent dose is J kg-1. The special name sievert (Sv).

Protection quantities

Antti Kosunen, 2017


Effective dose, E

Protection quantities

• wT tissue weighting factor

• Weighted sum of tissue equivalent doses

• All organs and tissues sensiive by stochastic effects

• The unit of equivalent dose is J kg-1. The special name sievert (Sv).

1=Σ Tw

Antti Kosunen, 2017


Antti Kosunen, 2017

Protection quantities Effective dose, E


Antti Kosunen, 2017

• Voxel anatomical phantoms representing adult Reference Male and Reference Female

• For whole body irradiations in different geometries the conversion factors are calculated.

• For known exposure geometries effective dose estimates can be made using conv. factors.



Antti Kosunen, 2017


Picture from ICRP 103


Antti Kosunen, 2017

• ”The effective dose serves as the basis for the contractual relationship in the regulatory framework, and it has utility in comparative evaluation of alternative work practices.”

• ”The radiation and tissue weighting factors are invariant with respect to age and sex, and hence the weighted sum, the effective dose, is not applicable to a specific individual.”


ICRP Publication 116


Antti Kosunen, 2017

Protection quantities Picture from ICRP 103


Antti Kosunen, 2017

Operational quantities, NEW work • Work for renewal of operational quantities is in progress • Final Draft of Joint report by ICRP and ICRU is open for comments • Draft not to be referenced


Antti Kosunen, 2017

Operational quantities

In this presentation operational quantities were presented according to current definitions and practise !


Antti Kosunen, 2017


• H, dose equivalent in a point in tissue

• D, the absorbed dose in a point

• Q, physical quality factor based on LET dependence of radiation

• Based on definition by ICRU.

• Unit: Jkg-1. The special name is sievert (Sv).

Dose equivalent, H

QDH =

Presenter

Presentation Notes


Antti Kosunen, 2017

• Q is defined as a function of the unrestricted linear energy transfer, L of charged particles in water:

Operational quantities Dose equivalent, H

• The function is based on considerations of radiobiologicall investigations on cellular and molecular systems and results from animal experimentation.


Antti Kosunen, 2017

ICRU sphere phantom • Aproximates the human body / scattering and attenuation of

radiation • For all types of radiation in area monitoring (ambient and directional dose equivalents)

• Sperical phantom with 30 cm diameter

• Density 1g/cm-3

• Composition (mass %): 76,2 % oxygen

11,1 % carbon 10,1 % hydrogen 2,6% nitrogen



The ambient dose equivalent, H*(d), at the point in a radiation field, is the dose equivalent that would be produced by the corresponding expanded and aligned field, in the ICRU sphere at a depth, d, on the radius opposing the direction of the aligned field

Unit: Jkg-1. The special name is sievert (Sv).

Antti Kosunen, 2017


Expanded and aligned radiation field

Picture from: Calibration of radiation protection monitoring instruments , IAEA Safety reports Series No.16, 2000


The directional dose equivalent, H’(d, Ω), at the point in a radiation field, is the dose equivalent that would be produced by the corresponding expanded radiation field, in the ICRU sphere at a depth, d, on the radius in a specific direction, Ω.

Unit: Jkg-1. The special name is sievert (Sv).

Antti Kosunen, 2017


Expanded radiation field

Picture from: Calibration of radiation protection monitoring instruments , IAEA Safety reports Series No.16, 2000


The personal dose equivalent, Hp(d), at the point in a radiation field, is the dose equivalent in ICRU tissue at depth d below a specified point on a body. Unit: Jkg-1, The special name is sievert (Sv).

Antti Kosunen, 2017

To produce backscatter eq. to body calibrations of dosimeters on phantoms


Definition on ”a body”


Antti Kosunen, 2017

Part of body d (mm) Whole body 10 Skin, hands, wrist, feet 0,07 Lens of the eye 3

Operational quantities: Specified depth d


Antti Kosunen, 2017

Operational quantities - summary

• Hp(0,07) has been used instead of Hp(3) for dose to the lens of eye

• Standards to test and calibrate eye dosimeters for Hp(3) are in progress


Relations of protection and operational quantities

Antti Kosunen, 2017

• Generally and for photon radiation Hp(10) overestimates the effective dose, especially with low < 100 kV range • Depends on irradiation geometry for effective dose

Hp(10)/E

Picture from EC, radiation protection No 160. Technical recommendations for Monitoring Individuals Occupationally E xposed to External Radiation, 2009

Presenter

Presentation Notes

Could be designde without response to backscatter, directly proportional fo Hp. Backscatter from dosemeter itself Radiation protection No 160


Operational quantities (ICRU) : • Ambient dose equivalent, H*(d),

H*(10) • Directional dose equivalent, H’(d,Ω) • Personal dose equivalent Hp(d)

Physical quantities: Absorbed dose D, [Gy] Kerma K, [Gy] Fluence Φ, [n cm-2]

Protection quantities (ICRP): • Mean absorbed dose in an organ or tissue DT , [Gy] • Equivalent dose in an organ or tissue HT , [Sv] • Effective dose E , [Sv]

Phantom models, definitions by ICRP Calculated using weighting factors

Calculated using Q(L) and phantom models (Sphere, slab). Validated by measurements H=QD

Compared by measurements and Calculations in anthropomorfic phantoms

Antti Kosunen, 2017

Family of dosimetric quantities

Actual measurand, device specific

Calibration, type tests


Antti Kosunen, 2017

Passive personal dosemeters

Passive: No power circuitry or inbuild software (to directly indicate the value of quantity) • Thermoluminescence dosimetry, TLD • Optically stimulated luminescence, OSL • Radiophotoluminescence, RPL • Film

• Direct ion storage dosemeters (DIS); passive

as separate readout device is required

Dosemeter photos by Maaret Lehtinen



Thermoluminescence dosimetry TLD Optically stimulated luminescence OSL Both are based on similar basic principle: • Electron-hole pairs produced by ionizing radiation are trapped at specific energy levels at the conduction band in the crystal

⇒ Free charge carrier can be released by heating (TLD) or ⇒ by optical stimulation (OSL) ⇒ Return of free charge to valence band emits extra energy by light

⇒ Emitted light intensity is proportional to ionization ie. to Absorbed dose

Antti Kosunen, 2017

Presenter

Presentation Notes

REFERENSSI


Passive personal dosemeters TLD and OSL

• Crystals used for dosimetry are small in size, storing the dose for

read-out • Separate reader is required: - TLD: heating of crystal typically by hot nitrogen gas flow + readout of

light - OSL. Different stimulation techniques, light sources: lasers, LEDs,

lamps • Personal dosimeter has typically several crystals with diffrent filters - Separation of different dose quantities Hp(10) and Hp(0,07) Similar phosphor materials : TLD: LiF:Mg,Ti, CaF2:Mn, CaSO4:Mn, (Li2B4O2:Mn). OSL: Al2O3:C in routine use for personal dosimetry.

Antti Kosunen, 2017



Some advantages of TLD (depends on material) • Sensitive for low doses of few microGy

• Commersicially available

• Reuseable after annealing procedure, low cost per dosemeter

• Commercial automatic, rapid readers with interfaced software

Antti Kosunen, 2017

TLD Practical materials requires impuries to create lattice defects / optimal traps for electrons - Light emitted for specific temperatures => clow curve - Specific clow peaks for result.



Some advantages of OSL Dosimeters • Result can be read out at room temperature. • No need of correction factors for individual elements. • Dosemeters can be re-analysed several times. • No sensitive for changes in temperature => minimal fading during storage

Antti Kosunen, 2017


Antti Kosunen, 2017


Film • Radiation interaction on film (AgBr) change the

light transmission through film

• Transmisson measured by densitometer and converted to optical density.

• Energy dependence is pronouinced for lower photon energies

• Requires developing/prosessing of film.

• Dosemeters: Filters to separate Hp(10) and Hp(0,07)

Film personal dosemeter. These film dosemeters were used in Finland untill middle of 1990s.

Presenter

Presentation Notes

REFERENSSI



DIS- Direct Ion Storage

• Charge / current produced by ionization is measured by ionization chamber principle

• Modified analog memory element as an ionization chamber

• Also MOSFET detectors

• Separate reader device

• Digital memory for storage of Information

• Results are not zeroed during read-out. Can be read several times Antti Kosunen, 2017

Photo provided by Matti Vuotila/ Mirion-Rados

Presenter

Presentation Notes

REFERENSSI


• Hp(3), eye lens, • Hp(0,07), finger, wrist • Operating principles typically TLD (…OSL, RPL) • Beeta and photons.

Eye lens and extremity dosemeters

Antti Kosunen, 2017

Photo: finger dosemeters on ISO rod calibration phantom. Calibration by beeta standard irradiator


Active personal dosemeters, APDs

• Body dosemeters, mainly Hp(10), some also Hp(0,07)

• Has powered electronic circuitry and associated software for visible or audible indication of integrated dose and/or dose rate

• Preset visual and audible alarms

• Often used as supplementary dosemeters to the passive dosemeter

• Approved APDs for official exposure control exist in some countries.

• Challenges. - Response to pulsed radiation (false counting, if counting for external radiation pulses) - Angular dependence

Antti Kosunen, 2017

EC, radiation protection No 160. technical recommendations

Presenter

Presentation Notes

European Commission, radiation protection No 160. technical recommendations for Monitoring Individuals Occupationally Exposed to External Radiation, Directorate-general for Energy and Transport directorate H- Nuclear Energy Unit H.4-Radiation protection, 2009


Active personal dosemeters, APDs

Challenge with pulsed radiation, high dose rate, Recommendations for interventional radiology /cardiology: • Regular calibration for Hp(10) preferably with X-rays in a calibration

laboratory

• ADP tool to optimize and reduce exposure and wear above the apron

• ADP not recommeded for legal dose record … today more advanced ADPs are available…

Optimization of RAdiation protection for MEDical staff (ORAMED), 2008…

Antti Kosunen, 2017

Presenter

Presentation Notes

European Commission, radiation protection No 160. technical recommendations for Monitoring Individuals Occupationally Exposed to External Radiation, Directorate-general for Energy and Transport directorate H- Nuclear Energy Unit H.4-Radiation protection, 2009


The personal dose equivalent, phantoms

Antti Kosunen, 2017

Body: ISO water slab 300mm*300mm*150mm with PMMA walls Wrist: ISO water pillar with PMMA walls Finger: ISO PMMA rod Eye lens: Head phantom (Behrens, PTB)

Operational quantities -calibration The ambient and directional dose equivalent: Calibration of dose/ doseratemeters in air

Photo on left at STUK. Schematic picture of phantoms from:, IAEA Safety reports Series No.16, 2000


Antti Kosunen, 2017

Operational quantities -calibration

• For calibration of eye lens dosemeters, the fluence - dose to eye lens conversion data exist., Hp(3)

• International standards expected.

• PMMA head phantom for calibration has been introduced

SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALEN RADIATION AND NUCLEAR SAFETY AUTHORITY Antti Kosunen, 2017

Figure from: J. Böhm et. al., ISO recommended reference radiations for the calibration and proficiency testing of dosemeters and dose rate meters used in radiation protection, Radiation Protection Dosimetry Vol. 86, No 2, 1999

Operational quantities -calibration

ISO reference radiation fields

Quantification of radiation fields in terms of air kerma

Conversion to operational quantities: ISO tabulated Conversion factors

SÄTEILYTURVAKESKUS • STRÅLSÄKERHETSCENTRALEN RADIATION AND NUCLEAR SAFETY AUTHORITY Antti Kosunen, 2017

Measurement traceability

• Traceability of measurement: Unbroken chain of measurements/calibrations with their uncertainty evaluations at each step.

• To assure the absolute value of dose • Calibrations of dosemeters either at accredited laboratory or at national laboratory


Antti Kosunen, 2017

Assessment of uncertainties Follow int. guidance:

ISO/IEC Guide 98 Part 3 Guide to the expression of uncertainty in measurement (ISO: Geneva) (1995). ISO/IEC Guide 98 Part 3-1 Guide to the expression of uncertainty in measurement (GUM)-Supplement 1: Numerical methods for the propagation of, distributions. (ISO: Geneva) (2008). BIPM, IEC, IFCC, ISO IUPAC, IUPAP and OIML. Joint Committee for Guides in Metrology Guide to the expression of uncertainty in measurement,(BIPM:Sèvres) (2008). http://www.bipm.org/utils/ EUROPEAN COMMISSION RADIATION PROTECTION NO 160, Technical Recommendations for Monitoring Individuals Occupationally Exposed to External Radiation, 2009

http://www.bipm.org/utils/


Antti Kosunen, 2017

Assessment of uncertainties

True value never precisely known Measurement error not precisely known Measurement uncertainty


Step 1/6. Determination of the model function • Identification of all input quantities

(influence quantities) • Form the measurement model

equation

Step 2/6. Estimation of uncertainties for influence quantities • By standard deviation with

probability density function • Quntification of sub-uncertainties:

- Results from type tests - Resuts from other

tests/experiments

Antti Kosunen, 2017

Assessment of uncertainties, procedure


• Contribution of each source of uncertainty to the standard uncertainty of measurement result

• Reliability of the uncertainty estimate (excellent, good, reasonable, rough)

Antti Kosunen, 2017

Step 3/6. Combination of Type A and Type B uncertainties Step 4/6. Propagation/ summation of uncertainties for model function to obtain combined uncertainty - Sensitivity factors - Degrees of freedom Step 5/6. Determination/estimation of expanded uncertainty using coverage factor (for required confidence probability) Step 6/6. Evaluation reliability of uncertainty estimate - Results from intercomparisons - Results from blind test

Assessment of uncertainties, procedure


Antti Kosunen, 2017

EU 160 report Recommendations Dose ranges based on annual dose limits for public • For Hp(10) for a single field component not below 1 mSv in proportion to the wear period, the combined standard uncertainty < 30% for photon/electron workplace fields and < 50% for neutron fields. • For a measurement of Hp(3) and single field component for a quantity value equal to or greater than 15 mSv in proportion to the wear period, the combined standard uncertainty should < 30%. • For Hp(0.07) for a single field component for a quantity value equal to or greater than 50 mSv in proportion to the wear period, the combined standard uncertainty < 30%. • The combined standard uncertainty for values of assessed annual dose values at or near the dose limit < 20 %, or in a more general probabilistic approach the 95% confidence interval should not exceed 0.67 to 1.5, after all corrections have been made.


Antti Kosunen, 2017

EU 160 report highlights also the importance of factors affecting on accuracy on field: a) Implemented quality system (general laboratory and staff quality, quality management, software, conformity of equipment used, calibration and internal performance tests) b) routine external performance tests of the dosimetry, periodic inter-comparisons between systems providing similar services. c) determination of the dosimetric characteristics of the system by type-testing d) information on the energy and direction characteristics of the radiation field being measured, plus other factors (environmental conditions, dosemeter wear position,etc.).


Antti Kosunen, 2017

ICRP, 2007. The 2007 Recommendations of the International Commission on Radiological Protection. ICRP Publication 103. Ann. ICRP 37 (2-4). ICRP, 2010. Conversion Coefficients for Radiological Protection Quantities for External Radiation Exposures. ICRP Publication 116, Ann. ICRP 40(2-5). J. Böhm et. al., ISO recommended reference radiations for the calibration and proficiency testing of dosemeters and dose rate meters used in radiation protection, Radiation Protection Dosimetry Vol. 86, No 2, 1999 Calibration of radiation protection monitoring instruments, IAEA Safety reports Series No.16, 2000. European Commission, radiation protection No 160. technical recommendations for Monitoring Individuals Occupationally Exposed to External Radiation, Directorate-general for Energy and Transport directorate H- Nuclear Energy Unit H.4-Radiation protection, 2009. R. Behrens, Compilation of conversion coefficients for the dose to the lens of the eye, radiation protection Dosimetry Vol 174, No.3., 348-370, 2016. The international Commission on Radiation Units and Measurements (ICRU), Fundamental quantities and units for ionizing radiation (revised) , ICRU Report No. 85, Journal of the ICRU, Volume 11, No 1, 2011

References


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