Radiation Dosimeters

1. Radiation Dosimeters

2. INTRODUCTION 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.

3. INTRODUCTION 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

4. PROPERTIES OF DOSIMETERS 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.

5. PROPERTIES OF DOSIMETERS 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

6. PROPERTIES OF DOSIMETERS Note: The accuracy and precision associated with a measurement is often expressed in terms of its uncertainty .

7. PROPERTIES OF DOSIMETERS 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.

8. PROPERTIES OF DOSIMETERS 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.

9. PROPERTIES OF DOSIMETERS 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.

10. PROPERTIES OF DOSIMETERS Two possible cases Case A: • linearity • supralinearity • saturation Case B: • linearity • saturation

11. PROPERTIES OF DOSIMETERS 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 d D / dt of the quantity. Other formulation: The response M/D should be constant for different dose rates ( d D / dt )1 and ( dD / dt ) 2. M = 􀀁 (M / D)( dD / dt ) dt M = (M / D)􀀁 ( dD / dt ) dt

12. PROPERTIES OF DOSIMETERS 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.

13. Personal Dosimeters 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

14. Who should wear a personal dosimeter? 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

15. Where are personal dosimeters usually worn? 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

16. Types of personnel dosimeters Film badge Pocket ionization chambers Thermo luminescent dosimeters (TLD) Optically stimulated luminescence (OSL) Solid State

17. Film Badge 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

18. Advantages And Disadvantages Of The Film Badge 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%

19. Pocket Dosimeter The most sensitive personnel dosimeter Two types Self-reading Non self-reading Can only be read once Detects gamma or x-radiation

20. Advantages And Disadvantages Of The Pocket Dosimeter 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

21. Thermo luminescent Dosimeters(TLD) 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

22. Advantages And Disadvantages Of The Thermoluminescent Dosimeter 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

23. Optically stimulated luminescence (OSL) “Captures” information in an Aluminum Oxide matrix Releases information by laser stimulation Can be reread after processing Durable Landauer Only

24. Solid State Provides instantaneous information regarding dose accumulation Simple to use Not a “legal” record Dose range device dependent

25. How to wear the Radiation Dosimeter ?

26. Thanks