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Chapter 5 External Dose Calculations H-117 – Introductory ...

Chapter 5. External Dose Calculations H-117 Introductory Health Physics Slide 1. Objectives Understand how radiation is affected by distance from a point source Using the inverse square law, calculate dose rates Understand U d t d howh the th specific ifi gamma ray constant t t is i used Explain how each photon-emitting radionuclide has a unique gamma constant associated with it H-117 Introductory Health Physics Slide 2. Objectives Use the gamma constant correctly in an exposure calculation Calculate exposure and dose rates from gamma point sources H-117 Introductory Health Physics Slide 3. Time, Distance and Shielding Although exposure to ionizing radiation carries a risk, it is impossible to completely avoid exposure. Radiation has always been present in the environment and in our bodies. We can, however, avoid undue exposure through the following protection principles: . H-117 Introductory Health Physics Slide 4.

Review ¾List the three methods of reducing your exposure/dose: ¾Intensity decreases _____ with the square of the distance from the source due only to the change in _____. H-117 – Introductory Health Physics Slide 31 ¾Using the inverse square law, calculate the dose rate at 4 feet away from a point source if the dose rate is originally 1000 R/hr at 2 feet.

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Transcription of Chapter 5 External Dose Calculations H-117 – Introductory ...

1 Chapter 5. External Dose Calculations H-117 Introductory Health Physics Slide 1. Objectives Understand how radiation is affected by distance from a point source Using the inverse square law, calculate dose rates Understand U d t d howh the th specific ifi gamma ray constant t t is i used Explain how each photon-emitting radionuclide has a unique gamma constant associated with it H-117 Introductory Health Physics Slide 2. Objectives Use the gamma constant correctly in an exposure calculation Calculate exposure and dose rates from gamma point sources H-117 Introductory Health Physics Slide 3. Time, Distance and Shielding Although exposure to ionizing radiation carries a risk, it is impossible to completely avoid exposure. Radiation has always been present in the environment and in our bodies. We can, however, avoid undue exposure through the following protection principles: . H-117 Introductory Health Physics Slide 4.

2 Effect of Distance on Dose Rate 25 mrem/hr @ 6 ft 100 mrem/hr @ 3 ft H-117 Introductory Health Physics Slide 5. Inverse Square Law Applies to the force of Gravity, Light, Heat, Electric Fields, Sound and Radiation. Intensity of a radiation field decreases as distance is increased due to geometry. H-117 Introductory Health Physics Slide 6. Inverse Square Law Equation General formula is: I1(d1)2 = I2(d2)2. where I is the Intensity y ((or dose rate)) and d is the distance from the source Hence, I2 = I1(d1/d2)2. H-117 Introductory Health Physics Slide 7. Problem The exposure rate from a 100 Ci point source of Co-60 at 2 meters is 32 R/hr. Find the exposure rate at 4 meters H-117 Introductory Health Physics Slide 8. Solution I2 = I1(d1/d2)2. I2 = 32 R/hr x (2/4)2 = 8 R/hr H-117 Introductory Health Physics Slide 9. Specific Gamma-Ray Constant The gamma constant, , allows the calculation of exposure rate: for a point source of a gamma-emitting radionuclide for a given activity at a specified distance from the source Typically measured in R/hr at one meter from a 1 Ci source H-117 Introductory Health Physics Slide 10.

3 Units The units of the gamma constant are typically given as: R R cm2. at 1 cm =. hr mCi hr mCi or R R m2. at 1 m =. hr Ci hr Ci H-117 Introductory Health Physics Slide 11. Sample Gamma Constants R m2. =. hr Ci < - - - 1 >1. 109Pd 99Mo 63Ni 59Fe 60Co 133Xe 131I 137Cs 226Ra 24Na 125I 65Zn 95Zr 54Mn 192Ir For the same activity (Ci) and the same distance (m): Co-60 = 3 x Ir-192, 4 x Cs-137, 6 x I-131. Ir-192 = 2 x I-131. Cs-137 = x I-131. H-117 Introductory Health Physics Slide 12. Dose vs. Dose Rate There are two point source equations in which the inverse square law is used: ' A t D =. d2. ' A. Drate =. d2. H-117 Introductory Health Physics Slide 13. Very Small Distances Approximate gamma dose rate to the hand from a 1 Ci Sealed Source R-cm2 Surface Dose Dose Rate at Dose Rate at Isotope hr-mCi Rate (R/min) 1 cm* (R/min) 3 cm* (R/min). 137Cs 513 28 60Co 13 2075 114 16. 192Ir 813 43 226Ra 1310 72 (* depth in tissue) (NCRP Report No.)

4 40). H-117 Introductory Health Physics Slide 14. Problem What is the exposure/dose received by an individual who spends one minute at 3 m from an unshielded 100 Ci 192Ir source? (Given = R m2 hr-1 Ci-1). H-117 Introductory Health Physics Slide 15. Answer What is the exposure/dose received by an individual who spends one minute at 3 m from an unshielded 100. Ci 192Ir source? = R m2 hr-1 Ci-1. A = 100 Ci d=3m t = 1 min = hr D = At/d2 = ( R m2 hr-1 Ci-1 * 100 Ci * hr)/(3 m)2. = R = 91 mR = 91 mrem H-117 Introductory Health Physics Slide 16. Shielding Equation I = I0e(- x). Where: I0 is the unshielded intensity (or dose rate). I is shielded intensity is the linear attenuation coefficient for the shielding material with units of cm-1. x is the thickness of the shielding material H-117 Introductory Health Physics Slide 17. Shielding Calculations To perform shielding Calculations , the linear attenuation coefficient, , for the shielding material must be determined In most tables you will find the mass attenuation coefficient which is / and has dimensions of cm2/g To go from cm2/g to cm-11.

5 ( / )( ) = . Example: What is the linear attenuation coefficient for 1 MeV photons in water? H-117 Introductory Health Physics Slide 18. Attenuation Coefficients vs. Energy ( cm2/g). H-117 Introductory Health Physics Slide 19. Calculating . The mass attenuation coefficient, / , for 1 MeV. photons for water is cm2/g To get the linear attenuation coefficient, we multiply by y the densityy of the absorber material The density of water is 1 g/cm3. ( / )( ) = ( cm2/g)(1 g/cm3) = cm-1. H-117 Introductory Health Physics Slide 20. Shielding Problem What is the dose rate after shielding a source that emits only 1 MeV photons if the unshielded dose rate is 100 mSv/h and the source is shielded by 1 cm of lead? H-117 Introductory Health Physics Slide 21. Shielding Problem First, you need the linear attenuation coefficient, . The mass attenuation coefficient for 1 MeV photons in lead is cm2/g, The density of lead is g/cm3.

6 = ( / )( ) = ( cm2/g)( g/cm3) = cm-1. H-117 Introductory Health Physics Slide 22. Shielding Problem (cont). Now use the shielding equation to determine the shielded dose rate: I = I0e(- x). I = (100 mSv/h) exp[-( cm-1)(1 cm)]. I = (100 mSv/h)( ). I = 46 mSv/h H-117 Introductory Health Physics Slide 23. Attenuation & Buildup The shielding equation does not fully account for photon interactions within shielding material when you have broad beams or very thick shields. To account for scattered photons and other secondary radiations, we use the buildup factor, B. H-117 Introductory Health Physics Slide 24. Buildup Factor I = I0 B e(-:x). B = [1 + 2 / 1 ] $ 1 1 = unattenuated radiation 2 = scattered radiation The buildup factor is dependent on the type and amount of shielding material and the energy of the photon. Buildup factors have been calculated for many different types of shielding materials, and can be found in tables.

7 H-117 Introductory Health Physics Slide 25. QUESTIONS? END OF External . DOSE Calculations . H-117 Introductory Health Physics Slide 26. Review List the three methods of reducing your exposure/dose: Intensity decreases _____ with the square of the distance from the source due only to the change in _____. Us g the Using t e inverse e se square squa e law, a , calculate ca cu ate tthe e dose rate ate at 4 feet eet a away ay from a point source if the dose rate is originally 1000 R/hr at 2 feet. The specific gamma ray constant, , provides the dose rate, typically in units of _____, for a given activity of a _____ source at a specified _____. H-117 Introductory Health Physics Slide 27. Review Given = R-m2/hr-Ci, calculate the dose resulting from standing 10 meters away from a 10. Ci Co-60 point source for 2 hours. H-117 Introductory Health Physics Slide 28. Review Given an initial dose rate of 10 R/hr from a source of 10 MeV photons, calculate the shielded dose rate after applying a 3 meter shield of water.

8 (Ignore buildup; / = cm2/g, and = 1 g/cm3 for water). H-117 Introductory Health Physics Slide 29. Review The shielding equation does not account for _____. To adjust for this, we use the _____ factor, B, in the equation. H-117 Introductory Health Physics Slide 30. Review List the three methods of reducing your exposure/dose: Intensity decreases _____ with the square of the distance from the source due only to the change in _____. Using the inverse square law, calculate the dose rate at 4 feet away from a point source if the dose rate is originally 1000 R/hr at 2 feet. The specific gamma ray constant, , provides the dose rate, typically in units of _____, for a given activity of a _____ source at a specified _____. H-117 Introductory Health Physics Slide 31. Review Given = R-m2/hr-Ci, calculate the dose resulting from standing 10 meters away from a 10. Ci Co-60 point source for 2 hours.

9 H-117 Introductory Health Physics Slide 32. Review Given an initial dose rate of 10 R/hr from a source of 10 MeV photons, calculate the shielded dose rate after applying a 3 meter shield of water. (Ignore buildup; / = cm2/g, and = 1 g/cm3 for water). H-117 Introductory Health Physics Slide 33. Review The shielding equation does not account for _____. To adjust for this, we use the _____ factor, B, in the equation. H-117 Introductory Health Physics Slide 34.


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