the external hazard. that (biological) hazard arising from the immersion of a body in a radiation...
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THE EXTERNAL HAZARD Slide 2 That (biological) hazard arising from the immersion of a body in a radiation field External Hazard Source types exhibiting an external hazard Sealed sources Sealed sources Unsealed sources Unsealed sources Electrical equipment generating em radiation Electrical equipment generating em radiation Natural sources Natural sources Slide 3 Current Annual Occupational Limits Skin (incl hands/feet) 500 mSv public 50 mSv Eye 150 mSv public 15 mSv Abdomen of female 13 mSv in any 3 mnth period These are equivalent dose limits Equivalent dose (H T ) = Absorbed dose (D T ) X W R Adequate Shielding Level = 7.5 Svh -1 (unclassified radiation workers) Slide 4 Estimating the External Hazard - Calculation Estimate of biological damage i.e. Absorbed dose Estimate of biological damage i.e. Absorbed dose Type of isotope - // Type of isotope - // Radiation generators e.g. X-ray Radiation generators e.g. X-ray Geometry of source point sources are isotropic Geometry of source point sources are isotropic Activity of source Activity of source Distance from the source Distance from the source Exposure time Exposure time Include natural radiation? e.g aircraft crew Include natural radiation? e.g aircraft crew Slide 5 Alpha emitters Not generally considered to be an external hazard Not generally considered to be an external hazard Penetrate less than 4 cm in air Penetrate less than 4 cm in air Generally considered an Internal Hazard Generally considered an Internal Hazard Bremstrahlung a problem? Bremstrahlung a problem? Slide 6 Dose depends on number of beta particles per unit area Independent of beta energy The range of 14 C (low energy) betas in tissue 1 mm The range of 32 P (high energy) betas in tissue 1 cm The dose rate D in Sv/hr produced by a point source of beta activity M MBq at distance 0.1m is given by D = 1000 M Sv/hr at 0.1m This translates to 1 beta particle cm -2 s -1 1 Sv/hr Beta emitters Slide 7 EXAMPLE: ESTIMATE THE RADIATION DOSE RECEIVED AT THE EYES OF A WORKER USING A SMALL UNSHIELDED 32 P SOURCE OF ACTIVITY 5 MBq FOR A PERIOD OF 15 MINUTES. ASSUME EYE-SOURCE DISTANCE IS 0.3M D = 1000 M Sv/hr at 0.1m DOSE RATE = 5000 Sv/hr at 0.1m DOSE RATE = 5000 Sv/hr at 0.3m (inverse square law) 9 DOSE RECEIVED IN 0.25 HR = 5000 x 0.25 = 139 Sv 9 Slide 8 The radiation dose delivered by a flux of gamma radiation depends upon the number of photons incident on unit area and it also depends upon photon energy, ie the dose increases with photon energy. A point source of gamma radiation of activity M MBq with a total gamma photon energy per disintegration E (Mev) produces a dose rate D = ME Sv/hr at 1.0m (>0.1MeV) 7 To find the dose rate at other distances we apply the inverse square law Gamma emitters e.g. Cr 51, Co 60 Slide 9 EXAMPLE Find the gamma dose rate at a distance of 0.5m from a 60 Co source of activity 50 MBq. Each disintegration of 60 Co results in the emission of two gamma ray photons of energy 1.17 and 1.33 MeV respectively It follows that E = 1.17 + 1.33 = 2.5 MeV Dose rate at 1m = 50 x 2.5 = 17.9 Sv/hr 7 Dose rate at 0.5m = 17.9 x 4 = 71 Sv/hr Slide 10 X-ray generators Slide 11 X-ray tube X-rays produced by the bremstrahlung effect X-rays produced by the bremstrahlung effect Dose rate is dependent on: Dose rate is dependent on: Target material (atomic number) Target material (atomic number) Applied tube voltage (kV) Applied tube voltage (kV) Tube current (mA) Tube current (mA) distance from the source (mm) distance from the source (mm) Filtration? Filtration? Rule of thumb formula: (with 1 mm Be filter) D = 670 ZVImGy/s d2d2d2d2 Slide 12 Example Calculate dose rate at 50 mm from an X-ray tube Using a copper target and tube voltage 50 kV at 10 mA Z = 29 for copper D = 670 x 29 x 50 x 10 / 2500 D = 4 Gy/s i.e. finger dose limit of 500 mSv reached in 125 ms Slide 13 In context: Dose rates @ 1 m for various (unshielded) sources 37 MBq (1 mCi) C 14 0 mSv/h 37 MBq P 32 0.5 mSv/h 400 GBq Cs 137 34 mSv/h X-ray source @ 75 kV and 10 mA 3.5 Sv/h (2 mm Al filter) Slide 14 ESTIMATION OF DOSE RATE BY MONITORING SIGN ON SIDE OF MONITOR GIVES RESPONSE TO 10 Sv hr -1 THUS BY USING MINI MONITOR CPS CAN BE APPROXIMATELY CONVERTED TO DOSE RATE ie, IF 20 CPS = 10 Sv hr -1 15 CPS = 7.5 Sv hr -1 THE ADEQUATE SHIELDING LEVEL Slide 15 Minimising the External Hazard Slide 16 ALARP PRINCIPAL USE LEAST ACTIVITY Slide 17 LEAST ACTIVITY Use the least activity required to get good results Incorporation Counting efficiency / error / noise level etc Standard deviation varies as 1/n i.e. doubling activity only improves statistical error by 2 95% confidence level (2 level) ~ 1000 counts above b/g Slide 18 ALARP PRINCIPAL USE LEAST ACTIVITY USE LEAST TIME Slide 19 LEAST TIME Conduct a dummy experiment to pinpoint deficiencies in the technique before using radioactivity All apparatus required should be ready prior to starting experiment Remember dose = dose rate x time Slide 20 Example: A classified radiation worker is permitted to receive up to 20 mSv per year ~ 400 Sv per week. How many hours of each week can he spend in an area having an average dose rate of 100 Sv/hr ? Dose = Dose Rate x Time 400 Sv = 100 Sv x T T = 4h Slide 21 ALARP PRINCIPAL USE LEAST ACTIVITY USE LEAST TIME USE DISTANCE PROTECTION Slide 22 DISTANCE PROTECTION If we double the distance from a point source of radiation, the number of particles/unit area in a given time is reduced by a factor of 4. This is an example of the inverse square law ie if the dose rate produced by a small radioactive source is 1Sv/hr at 1 m then the dose rate at hand distance (0.1 m) = 100 Sv/hr finger distance (0.01m) = 10 4 Sv/hr = 10 mSv/hr Slide 23 Demonstration Slide 24 ALARP PRINCIPAL USE LEAST ACTIVITY USE LEAST TIME USE DISTANCE PROTECTION USE SHIELDING Slide 25 SHIELDING It is always best and most economical to shield the source not the personnel. Placing a shield round the immediate vicinity of the source requires a small amount of shielding material and also obviates the possibility of irradiation of hands etc Use the correct shielding material for the isotope concerned Slide 26 For alpha emitters thin sheet of paper or plastic For alpha emitters thin sheet of paper or plastic For beta emitters plastic / perspex thickness dependent on energy For beta emitters plastic / perspex thickness dependent on energy For gamma emitters lead shielding or leaded glass For gamma emitters lead shielding or leaded glass For high activity sources bremsstrahlung may be an issue For high activity sources bremsstrahlung may be an issue Slide 27 Demonstration Slide 28 Remember the adage: Time!Distance!Shielding!