Technical basis of occupational dosimetrygeneral

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1 Technical basis of occupational dosimetrygeneral review Antti Kosunen, Hannu Järvinen STUK 2017 NACP-RPC COURSE Occupational dosimetry in interventional radiology, - cardiology and nuclear medicine September 2017 Södersjukhuset, Stockholm, Sweden

2 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

3 Dosimetric quantities 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

4 Physical quantities The fluence, Φ, is the quotient of dn and da, where dn is the The number absorbed of particles dose, D, incident is the quotient on a sphere of de of and cross-sectional dm, where de area is the da mean energy imparted by ionizing radiation to matter of mass dm Φ = dn da Unit: 1/m 2 Fluence rate, (flux density) ϕ = dφ dt Unit: 1/m 2 s

5 Physical quantities The kerma, K, (Kinetic energy released per unit mass) The absorbed dose, D, is the quotient of de and dm, where de is the The mean kerma energy is the imparted expectation by ionizing value radiation of the energy to matter of mass transferred dm 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 K de dm Unit; J kg -1, The special name is gray (Gy). = tr

6 Physical quantities The absorbed dose, D, is the quotient of dε and dm, The where absorbed dε is the dose, mean D, is energy the quotient imparted of de and by ionizing dm, where radiation de is the to matter mean energy of mass imparted dm by ionizing radiation to matter of mass dm Unit; J kg -1, The special name is gray (Gy).

7 Protection quantities Equivalent dose in an organ or tissue, H T w R radiation weighting factor for a radiation quality R D T,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). 7

8 Protection quantities Effective dose, E w T tissue weighting factor ΣwT =1 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).

9 Protection quantities Effective dose, E

10 Protection quantities Effective dose, E 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.

11 Protection quantities Effective dose, E Picture from ICRP 103

12 Protection quantities Effective dose, E 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

13 Protection quantities Picture from ICRP 103

14 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

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

16 Operational quantities Dose equivalent, H H = QD 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).

17 Operational quantities Dose equivalent, H Q is defined as a function of the unrestricted linear energy transfer, L of charged particles in water: The function is based on considerations of radiobiologicall investigations on cellular and molecular systems and results from animal experimentation.

18 Operational quantities 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

19 Operational quantities 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). Picture from: Calibration of radiation protection monitoring instruments, IAEA Safety reports Series No.16, 2000 Expanded and aligned radiation field

20 Operational quantities 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). Picture from: Calibration of radiation protection monitoring instruments, IAEA Safety reports Series No.16, 2000 Expanded radiation field

21 Operational quantities The personal dose equivalent, H p (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). Definition on a body To produce backscatter eq. to body calibrations of dosimeters on phantoms

22 Operational quantities: Specified depth d Part of body d (mm) Whole body 10 Skin, hands, wrist, feet 0,07 Lens of the eye 3

23 Operational quantities - summary Hp(0,07) has been used instead of H p (3) for dose to the lens of eye Standards to test and calibrate eye dosimeters for H p (3) are in progress

24 Relations of protection and operational quantities H p (10)/E Picture from EC, radiation protection No 160. Technical recommendations for Monitoring Individuals Occupationally E xposed to External Radiation, 2009 Generally and for photon radiation H p (10) overestimates the effective dose, especially with low < 100 kv range Depends on irradiation geometry for effective dose

25 Family of dosimetric quantities Physical quantities: Absorbed dose D, [Gy] Kerma K, [Gy] Fluence Φ, [n cm -2 ] Calculated using Q(L) and phantom models (Sphere, slab). Validated by measurements H=QD Phantom models, definitions by ICRP Calculated using weighting factors Protection quantities (ICRP): Mean absorbed dose in an organ or tissue D T, [Gy] Equivalent dose in an organ or tissue H T, [Sv] Effective dose E, [Sv] Operational quantities (ICRU) : Ambient dose equivalent, H*(d), H*(10) Directional dose equivalent, H (d,ω) Personal dose equivalent H p (d) Compared by measurements and Calculations in anthropomorfic phantoms Calibration, type tests Actual measurand, device specific

26 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

27 Passive personal dosemeters 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

28 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, CaF 2 :Mn, CaSO 4 :Mn, (Li 2 B 4 O 2 :Mn). OSL: Al 2 O 3 :C in routine use for personal dosimetry.

29 TLD Passive personal dosemeters 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 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

30 Passive personal dosemeters 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

31 Film Passive personal dosemeters 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.

32 Passive personal dosemeters 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 Photo provided by Matti Vuotila/ Mirion-Rados

33 Eye lens and extremity dosemeters H p (3), eye lens, H p (0,07), finger, wrist Operating principles typically TLD ( OSL, RPL) Beeta and photons. Photo: finger dosemeters on ISO rod calibration phantom. Calibration by beeta standard irradiator

34 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 EC, radiation protection No 160. technical recommendations

35 Active personal dosemeters, APDs Optimization of RAdiation protection for MEDical staff (ORAMED), 2008 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

36 Operational quantities -calibration The ambient and directional dose equivalent: Calibration of dose/ doseratemeters in air The personal dose equivalent, phantoms 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) Photo on left at STUK. Schematic picture of phantoms from:, IAEA Safety reports Series No.16, 2000

37 Operational quantities -calibration For calibration of eye lens dosemeters, the fluence - dose to eye lens conversion data exist., H p (3) International standards expected. PMMA head phantom for calibration has been introduced

38 Operational quantities -calibration ISO reference radiation fields Quantification of radiation fields in terms of air kerma Conversion to operational quantities: ISO tabulated Conversion factors 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

39 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

40 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). EUROPEAN COMMISSION RADIATION PROTECTION NO 160, Technical Recommendations for Monitoring Individuals Occupationally Exposed to External Radiation, 2009

41 Assessment of uncertainties True value never precisely known Measurement error not precisely known Measurement uncertainty

42 Assessment of uncertainties, procedure 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

43 Assessment of uncertainties, procedure 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) Contribution of each source of uncertainty to the standard uncertainty of measurement result Reliability of the uncertainty estimate (excellent, good, reasonable, rough) Step 6/6. Evaluation reliability of uncertainty estimate - Results from intercomparisons - Results from blind test

44 EU 160 report Recommendations Dose ranges based on annual dose limits for public For H p (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 H p (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 H p (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.

45 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.).

46 References ICRP, The 2007 Recommendations of the International Commission on Radiological Protection. ICRP Publication 103. Ann. ICRP 37 (2-4). ICRP, 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, 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, R. Behrens, Compilation of conversion coefficients for the dose to the lens of the eye, radiation protection Dosimetry Vol 174, No.3., , 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

47 Thank you for your attention