Basic definitions. Dosimetry, radiation protection. Nuclear measurement techniques. Interaction of the nuclear radiation with the matter

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1 Dosimetry, radiation protection. Nuclear measurement techniques. properties measurement dosimetry medical applications of the nuclear radiation Basic definitions Nuclear radiation: Produced in the transition of the nucleus α (He 2 ), β (e,e ), γ (em.), n radiation Isotope (same atomic number, different mass number) Radioactive isotope (unstable, decays, emits radiation) Activity (Bq decay/s) Exponential decay law Dr László Smeller Semmelweis University, Dept. Biophysics and Radiation Biology Interaction of the nuclear radiation with the matter nuclear radiation absorption detection interaction energy trasfer α β γ n charged uncharged direct ionisation indirect ionisation Attenuation of nuclear radiations α β charged γray Ionisation energy loss: Runs out of the energy on a finite distance. effective range J 0 J J 0 2 J/4 x D Exponential attenuation law no effective range

2 Detection of the ionizing raditaion scintillation counter gasionization detectors thermoluminescent dosimeter photographic methods (film) semiconductor detectors see practical exercises! Scintillation counter Gasionization detectors Q I t I A R

3 Gasionization detectors GeigerMüller tube I I t Ionization chamber: collects all the ions Measurees the inonizating effect of the radiation see: dosimetry ionization current αparticles electrons Geiger Müller range: avalancheeffect: particle voltage pulse R U t GM tube U t radiation counter advantage: simple construction, disadvantage: no energy selectivity, low efficiency for γray usage: mainly in dosimetry

4 Thermoluminescent dosimeter (TLD) An american astronaut uses the TLD dosimeter Pille produced by KFKI (Photo: NASA ISS002E7814) Personal dosimeters Photochemical detection obsolete

5 Semiconductor detector Semiconductor detectors in the diagnostics Principle: Semiconductor diode connected reverse biased The radiation induces free charges and consequently current A I A electrode semiconductor radiation n p n p electron defectelectron current transistor electrode Semiconductor based dosimeters Biological effect of the ionizing radiation

6 The mechanism of the radiation damage Radiation damage Physical phase: s Ionisation Stochastic Deterministic direct indirect Chemical (biochemical) phase: s: free radical reactions Biological phase: hours: alteration in the tissues daysyears: stomachbowel damage of the haematogenesis somatic damage probability of the radiation damage dose probability of the radiation damage 100% dose dose threshold Stochastic already in case of low dose small number of targets no dose threshold severity is independent of the dose personnel at workplaces using ionisation radiation, patients of Xray and nuclear imaging investigations Deterministic Large dose (>threshold) many targets should be hit only above the threshold severity increases with the increasing dose accidents Radiation protection and dosimetry Main tasks in radiation protection: measurement of the dose rate detection of the pollution measurement and control of the personal dose

7 Dose concepts Absorbed dose : ΔE D [Gy] Δ m absorbed dose: unit J/kg Gy ΔE D Δ m The energy absorbed from the radiation by the mass Δm absorbed energy by unit mass of absorbing medium can be used for all types of radiations How to measure: hard to measure directly (unmeasurably small temperature change ΔT 0,006 C/4 Gy) indirect detection methods: ionisation chamber semiconductor detector thermoluminescent detector... Exposure: unit: C/kg X ΔQ Δm the positive charge produced in the of mass Δm total of Q charge total of Q charge How to measure the exposure: Ionisation chamber Q I t I X t X ΔQ Δm A R Olny for γ and xray in!

8 Exposure: X ΔQ Exposure: Δm X ΔQ Δm The location of the detection and of the ionization Conversion to absorbed dose: Characterises the dose in To produce 1 p of ions one needs 34 ev energy in * 34 ev 34 1, J 1, C 34 J 1 C How to convert it to absorbed dose? Exposure was measured in how to convert it to the dose in the tissue? C J Gy kg kg * In case of electrons. For protons and α particles 35 ev Conversion of absorbed dose measured in to absorbed dose expected in tissue A J E At ΔJ μδxj E J At ΔE ΔJ At D ΔE Δm μδxjt ρδx D μ m ΔJ ρaδx μ m Jt At Conversion of absorbed dose measured in to absorbed dose expected in tissue D D μ tissue m, tissue m, tissue D μ tissue D m, μm, D If E photon <0,6 MeV, for soft tissue: μ μ m, tissue tissue 0 μm, μ m, tissue μ m, f X J f 0 34 C 1.1

9 Dose concepts so far: Physical dose f 0 Technical dose Effectivity of the radiation Sensitivity of the tissue? Biological damage in Radiotherapy (Deterministic effect) typically single type of radiation is used single type of tissue is irradiated Absorbed dose proportional Biological effect Radiation protection (Stochastic effect) typically several types of radiations are absorbed several different tissues are irradiated Absorbed dose weighted summation Biological effect Equivalent dose Absorbed dose weighted summation Biological effect Weighting factors for: effectivity of the radiation tissue sensitivity Equivalent dose: H w T RDT, R [Sv] R The w R radiation weighting factors How many times greater is the effectivity (considering stochastic effects) of the given radiation compared to the xray or γradiation. by definition Weighted sum of the absorbed doses from the different radiations (R) in a given tissue (T). w R : radiation weighting factor E.g.: H skin wα Dskin, α wβ Dskin, β wγdskin, γ

10 Effective dose: Absorbed dose weighted summation Biological effect Weighting factors for: effectivity of the radiation tissue sensitivity The w T weighting factor represents the relative contribution of that organ or tissue to the total damage in case of stochastic effects resulting from uniform irradiation of the whole body Effective dose: E w H T T T [Sv] Weighted sum of the equvivalent doses of the irradiated tissues (T) w T H T gives the contribution of the H T dose to the damage of the whole body. In case of homogenous radiation EH T w T 1 T Summary of dose concepts Radiation protection Physical dose f 0 Technical dose Effectivity of the radiation Sensitivity of the tissue H T E Only for radiation protection For personnel: Justification Rule out the deterministic effect Reduction of the stochastic effect on a rationally acceptable level: ALARA principle Dose limits Patients: Justification Costbenefit principle Measurement and documentation of patient dose values

11 ALARA principle As Low As Reasonably Achievable Cost optimal value radiation damage radiation protection Dose Dose limits ( allowed dose!) For personnel at radiation workplace whole body: 100 msv/5 year and 50 msv/year (c.a.10μsv/working hour)* eye lens: 150 msv/year (will be smaller in the future!) skin: 500 msv/year limb: 500 msv/year *compare.: background dose rate: 0,1 μsv/h Threshold doses for deterministic effects bonemarrow: Reduction of blood production Testis: temprary sterility permanent sterility Eye lens obscurity Cataracta Skin: temporary erythema erythema temporary epilation 0,5 Gy 0,15 Gy 3,56 Gy 0,52 Gy 5 Gy 2 Gy 6 Gy 3 Gy For wholebody irradiation: median lethal dose (LD 50 ) : 4 Gy lethal dose 6 Gy A few characteristic dose values Background raditaion: 2,4 msv/year half of it from Rn. Medical investigations (patient dose) conventional xray image: 0,21 msv CT scan: 28 msv Treatment: Intervention radiology doctor: hand: 100 msv/2 month eye: 30 msv/2 month knee: 20 msv/2 month gonad (under the lead apparel): 0,5 msv/2 month Patient: up to 1 Gy!! Radiotherapy: tipically 4560 Gy (2 Gy fractions.)

12 Dose limits and risks Dose limit allowed dose dose with acceptable level of risk Stochastic effects cannot be avoided even below the dose limit! But! everything is dangerous! Some of the above presented values are taken from: Damjanovich et al.: Medical Biophysics Köteles György: Sugáregészségtan (Medicina) Fehér István, DemeSándor: Sugárvédelem (ELTE Eötvös kiadó) Turák O., Osvay M.: A személyzet dózisa az intervenciós radiológia területén. OSSKI Pellet Sándor, Giczi Ferenc, Gáspárdy Géza, Temesi Alfréda: Az intervenciós radiológia sugáregészségügyi vonatkozásai. Magyar Radiológia 81 (2007) life is dangerous! risk benefit