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Radiation Dosimeters

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Radiation Dosimeters

  1. 1. General requirements for dosimeters #Dosimeter is a device that measures directly or indirectly • Exposure • Kerma • Absorbed dose • Equivalent dose • Or other related quantities. #The dosimeter along with its reader is referred to as a dosimetry system.
  2. 2. A useful dosimeter exhibits the following properties: • High accuracy and precision • Linearity of signal with dose over a wide range • Small dose and dose rate dependence • Flat Energy response • Small directional dependence • High spatial resolution • Large dynamic range
  3. 3.  Accuracy specifies the proximity of the mean value of a measurement to the true value.  Precision specifies the degree of reproducibility of a measurement. Note: High precision is equivalent to small standard deviation.
  4. 4. Examples for use of precision and accuracy: High precision High precision Low precision Low precision and and and and High accuracy Low accuracy High accuracy Low accuracy
  5. 5. Note: The accuracy and precision associated with a measurement is often expressed in terms of its uncertainty.
  6. 6. New Concept by the International Organization for Standardization (ISO) "Guide to the expression of uncertainty in measurement“  This new guide serves as a clear procedure for characterizing the quality of a measurement.  It is easily understood and generally accepted.  It defines uncertainty as a quantifiable attribute.
  7. 7.  Standard uncertainty: is the uncertainty of a result expressed as standard deviation.  Type A standard uncertainty is evaluated by statistical analysis of a series of observations.  Type B standard uncertainty is evaluated by means other than statistical analysis. This classification is for convenience of discussion only.  It is not meant to indicate that there is a difference in the nature of  the uncertainty such as random or systematic.
  8. 8.  Combined uncertainties: The determination of the final result is normally based on several components. Linearity:  The dosimeter reading should be linearly proportional to the dosimetric quantity.  Beyond a certain range, usually a non-linearity sets in.  This effect depends on the type of dosimeter.
  9. 9. Two possible cases Case A: • linearity • supralinearity • saturation Case B: • linearity • saturation
  10. 10. Dose rate dependence :  M/D may be called the response of a dosimeter system  When an integrated response is measured, the dosimetric quantity should be independent of the dose rate dD/dt of the quantity.  Other formulation: The response M/D should be constant for different dose rates (dD/dt)1 and (dD/dt) 2. M = 􀀁 (M / D)(dD / dt)dt M = (M / D)􀀁 (dD / dt)dt
  11. 11.  Energy: The response of a dosimetric system is generally a function of the radiation energy.  The term "radiation quality" is often used to express a specific distribution of the energy of radiation.  Therefore, a dependence on energy can also be called a dependence on radiation quality.  Since calibration is done at a specified beam quality, a reading should generally be corrected if the user's beam quality is not identical to the calibration beam quality.
  12. 12. A small radiation monitoring device worn by persons entering environments that may contain radiation . # Desirable characteristics  Should be lightweight, durable, and reliable  Should be inexpensive
  13. 13.  Healthcare or laboratory workers in non- emergency environments that may contain radiation  Examples: radiology, nuclear medicine, and radiation oncology department staff  Workers in emergency environments that may contain radiation  Examples: first responders and first receivers  Workers in industrial environments where radiation is used  Examples: nuclear power plant workers or employees at radiation sterilizing facilities
  14. 14.  Flat badges are usually worn on the torso, at the collar or chest level, but can be worn on the belt, or forearm  Ring shaped badges can be worn on the finger when dose to the finger may exceed dose to the badge worn elsewhere on the body  First responders and first receivers  Wear water-resistant personal dosimeters on the outer layer of personal protective equipment (PPE).  Should be able to easily see and hear a dosimeter alarm while wearing PPE  May wear a personal dosimeter underneath waterproof outerwear
  15. 15.  Film badge  Pocket ionization chambers  Thermo luminescent dosimeters (TLD)  Optically stimulated luminescence (OSL)  Solid State
  16. 16.  Most widely used and most economical  Consists of three parts:  Plastic film holder  Metal filters  Film packet  Can read x, gamma, and beta radiation  Accurate from 10mrem - 500rem  Developed and read by densitometer  A certain density value equals a certain level of radiation  Read with a control badge  Results generally sent as a printout
  17. 17.  Lightweight, durable, portable  Cost efficient  Permanent legal record  Can differentiate between scatter and primary beam  Can discriminate between x, gamma, and beta radiation  Can indicate direction from where radiation came from  Control badge can indicate if exposed in transit  Only records exposure where it’s worn  Not effective if not worn  Can be affected by heat and humidity  Sensitivity is decreased above and below 50 keV  Exposure cannot be determined on day of exposure  Accuracy limited to + or - 20%
  18. 18.  The most sensitive personnel dosimeter  Two types  Self-reading  Non self-reading  Can only be read once  Detects gamma or x-radiation
  19. 19.  Small, compact, easy to use  Reasonably accurate and sensitive  Provides immediate reading  Expensive  Readings can be lost  Must be read each day  No permanent record  Susceptible to false readout if dropped or jarred
  20. 20.  Looks like a film badge  Contains a lithium fluoride crystal  Responds to radiation similarly to skin  Measured by a TLD analyzer  Crystal will luminescence if exposed to radiation, then heated  More accurate than a film badge
  21. 21.  Crystals contained in TLD interact with ionizing radiation as tissue does  Determines dose more accurately  The initial cost is greater than that of a film badge  Can only be read once  Records exposure only where worn
  22. 22. • “Captures” information in an Aluminum Oxide matrix • Releases information by laser stimulation • Can be reread after processing • Durable • Landauer Only
  23. 23.  Provides instantaneous information regarding dose accumulation  Simple to use  Not a “legal” record  Dose range device dependent
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