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OBJ 2 Internal and External Exposure Control

OBJ 2 Internal and External Exposure Control. RPT-111. External Exposure. Absorbed Dose Measures energy deposited in some given mass Originally defined as “ rad ” (Roentgen Absorbed Dose). External Exposure. Dose Equivalent (and equivalent dose)

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OBJ 2 Internal and External Exposure Control

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  1. OBJ 2 Internal and External Exposure Control RPT-111
  2. External Exposure Absorbed Dose Measures energy deposited in some given mass Originally defined as “rad” (Roentgen Absorbed Dose)
  3. External Exposure Dose Equivalent (and equivalent dose) Modifying absorbed dose by quality factor produces dose equivalent Dose equivalent (HT) = rad X quality factor
  4. External Exposure
  5. External Exposure Sample problem A technologist receives 25 mradγ and 33 mGyβ-. What was the equivalent dose for the job?
  6. External Exposure Effective Dose Equivalent (and effective dose) Quantity used to assess risk from BOTH uniform whole-body and non-uniform partial body exposures Uses weighting factors, wT, to take into account reduced risk of cancer mortality and genetic effects when only some body organs receive a dose
  7. External Exposure
  8. External Exposure Committed Dose Equivalent Applies to radioactivity deposited internally Given symbol HT,50 Represents total cumulative dose to organ or tissue for a 50-yr period beginning the instant uptake occurs Committed Effective Dose Equivalent Given symbol HE,50 Represents radiation risk from internal radioactivity equivalent to risk from uniform whole body external exposure of same size
  9. External Exposure
  10. External Exposure Exposure Rate Determination To control exposure must know what it is Can estimate γ dose from known activity To determine field intensity (I) in R/hr at 1 ft. from a point source Accurate to + 20% for energies 50 keV and 3 MeV where: I = dose rate in Rem/hr @ 1 ft. C = source activity in Curies (Ci) E = gamma energy in MeV N = % photon yield in decimal form
  11. External Exposure To determine field intensity (I) in R/hr at 1m from a point source Accurate to within + 20% for energies 50 keV and 3 MeV where: I = dose rate in Rem/hr @ 1m C = source activity in Curies (Ci) E = gamma energy in MeV N = % photon yield in decimal form
  12. External Exposure If N not given, assume 100% If > 1 photon energy given, each one taken into consideration
  13. External Exposure Sample problem Calculate the dose rate at 1 ft. for a 30 Ci point source of 213Bi, which emits a 440.4 keV (28%) gamma.
  14. External Exposure Sample problem Calculate the dose rate at 1 m.
  15. External Exposure Sample problem Calculate the dose rate at 1 ft. for a 4.2 Ci point source of 133I, which emits the following gammas: 529.9 keV (86.3%), 510.5 keV (1.8%), 706.6 keV (1.5%), 856.3 keV (1.2%), and 1.3 MeV (2.3%)
  16. External Exposure Time Since Dose = DR X t, minimizing time in radiation field reduces dose Stay time is the maximum time allowed in a radiation field to preclude exceeding an allowable dose. Calculated as follows:
  17. External Exposure Example problem A worker must enter a 950 mrem/hr γ radiation field to perform assigned work. Accumulated dose for the month is 152 mrem. If the ACL is 750 mrem, what is her stay time in the area?
  18. External Exposure Distance Radiation field intensity  decreases as distance from source  Point source—an imaginary point in space from which all radiation is assumed to be emanating Point Source radiation intensity  according to Inverse Square Law As distance from point source changes, dose rate  or  by square of ratio of distances from the source Becomes inaccurate close to source (i.e., about 10 times the diameter of the source)
  19. External Exposure Inverse Square Law Where: I1 = Exposure rate at distance 1 (d1) I2 = Exposure rate at distance 2 (d2) d1 = Distance 1 d2 = Distance 2
  20. External Exposure Example problem A point source of 60Co has a γ exposure rate of 3 rem/hr at 5 ft. What would the exposure rate be at 2 ft?
  21. External Exposure Line Source Line source treated as a series of point sources side by side along length of source Relationship between distance and exposure rate can be written mathematically as: Valid to point 1/2 of longest dimension of the line source (L/2), beyond which the point source formula should be used
  22. External Exposure Example problem A small diameter tank containing radioactive liquids is 15 ft. long. The exposure rate at 1 foot is 13.1 rem/hr. What is the exposure rate at 6 ft?
  23. External Exposure Combining Equations Can substitute one equation for a given variable for that variable in another equation. For example, if I=6CEN, then becomes
  24. External Exposure Example problem A 2,000 gal., 20-ft-long tank contains 1500 gal. of a solution with a 137Cs concentration of 72.5 pCi/l. What should the dose rate be at 3 feet from the tank and at 20 feet from the tank. What do I need to know? Liters/gallon 137Cs γ energy and probability
  25. External Exposure Assumptions Activity is homogenously distributed in the solution Shielding afforded by the solution is negligible Shielding afforded by the tank is negligible Data 1 gal. = 3.8 l 137Cs γ: 661 keV (90%)
  26. External Exposure Example problem A 2,000 gal., 20-ft-long tank contains 1750 gal. of a solution with a 137Cs concentration of 72.5 μCi/l. What should the dose rate be at 20 feet from the tank.
  27. External Exposure A 2,000 gal., 20-ft-long tank contains 1750 gal. of a solution with a 137Cs concentration of 72.5 μCi/l. What should the dose rate be at 20 feet from the tank.
  28. External Exposure A 2,000 gal., 20-ft-long tank contains 1750 gal. of a solution with a 137Cs concentration of 72.5 μCi/l. What should the dose rate be at 20 feet from the tank.
  29. External Exposure Sample problem A 1 l. container reads 206 mrem/hr at a distance of 9 ft. If the primary isotope in the container is 137Cs, how much activity is there?
  30. External Exposure Plane Source Plane or surface sources can be floor, wall, large cylindrical or rectangular tank, or any other geometry where width or diameter is not small compared to length Requires calculus to calculate accurate dose rates
  31. External Exposure Relationship can be described for how exposure rate varies with distance from the source When distance to plane source is small compared to longest dimension, exposure rate falls off a little slower than 1/d (i.e. not as quickly as a line source) As distance from plane source increases, exposure rate drops off at a rate approaching 1/d2
  32. External Exposure  @ < 1/d  @ < 1/d2 10 ft. 10 ft. >10 ft.
  33. Internal Exposure Relationship can be described for how exposure rate varies with distance from the source When distance to plane source is small compared to longest dimension, exposure rate falls off a little slower than 1/d (i.e. not as quickly as a line source) As distance from plane source increases, exposure rate drops off at a rate approaching 1/d2
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