Dosimetry

Dosimetry

1. DOSIMETRY Dr. NANDHINI, M.D.S.,

2. Determining the quantity of radiation exposure or dose. Exposure – measure of radiation on the basis of its ability to produce ionization in air under standard conditions of temperature and pressure (STP).

3. Absorbed dose ( D T ) Energy absorbed by any type of ionizing radiation per unit of mass of any type of matter.

4. Equivalent dose (H T ) compare the biologic effects of different types of radiation on a tissue or organ . H T = W R x D T Particulate radiations have a high LET & are more damaging to tissue than x- rays – this biologic effectiveness of different types of radiation – W R Photons – 1 Neutrons & high-energy protons – 5 α particles - 20

5. Effective dose (E) Estimate the risk in humans. E = Σ W T x H T Radiosensitivity of different tissues measured by W T. Red bone marrow, breast, colon, lung, & stomach - 0.12 G onads - 0.08 B ladder , esophagus, liver, & thyroid - 0.04 B one surface, brain, salivary glands, & skin - 0.01

6. Radioactivity (A) Decay rate of a radioactive material.

7. NCRP recommends Diagnostic Reference Levels of 1.6 mSv skin dose for IOPA & bitewing . Average weekly exposure for patient/ operator - 0.001 Sv . Maximum of 13 week exposure is 0.05 Sv . Dentist and staffs are allowed to receive 20 mSv / yr - 0.2-1% of allowable exposure How much is acceptable ???

8. The principle : Physical & chemical effects produced by the radiation. Ionization Photographic effect Luminescence Scintillation Radiation monitoring devices

9. Ionization Principle : Ionization in air by radiation .

10. Working:

11. T himble ionization chamber

12. Geiger-Muller counter

13. Advantages: Most accurate method. Gives immediate information. Disadvantages: No permanent record of exposure. No indication of the type of energy of the radiation. Not sensitive to low energy radiation. Fragile & easily damaged.

14. Principle: The ability of radiation to blacken the photographic film . Application: Film badges. Photographic effect

15. Principle: The property of certain materials that emit light when stimulated by - chemical/ electrical/ heat . Application: TLD badges Luminescence

16. Working:

17. Principle: Certain crystals - sodium iodide & cesium iodide, absorb radiation & convert it to light. Scintillation

18. Working :

19. Monitor the individuals who are exposed to radiation during the course of their work. Must for all occupationally exposed individuals. Mandatory to wear personnel dosimeter if the annual dose is greater than 1 mSv . Pocket dosimeter, the film badge or the thermoluminescent dosimeter . Personnel Dosimeter

20. Radiation measurement b y time-integrated dose, i.e. the dose summed over a period of time, usually about 3 months . The dose is consequently stated as an estimate of the effective dose equivalent to the whole body in mSv for the reporting period. Dosimeters used for personnel monitoring have dose measurement limit of 0.1–0.2 mSv (10–20 mrem ).

21. Pocket dosimeter Minometer / Condensor type/ indirect reading type Principle: ‘air wall’ ionization Working: Insertion into a charger - voltage potential. Radiation penetrating the chamber causes the current to leak in proportion to the radiation exposure. Reinsert into the charge reader - voltage drop is calibrated. 2 dosimeters are worn at a time. Lower of the two readings taken as more accurate.

22. Direct Reading Dosimeter Principle: Gold Leaf Electroscope. Working: Charging to 200V - quartz fiber is displaced electro statically. Fiber is viewed through a lens & focused on a scale. Exposure to radiation discharges the fiber, allowing it to return to the original position . Can measure radiation up to 50 C/kg (200 mR ) .

23. Film badge monitoring Has small X-ray films sandwiched between several filters that detects radiation. Inexpensive , simple mode of functioning. Detects radiation at or above 0.1 mSv .

24. Wearing of the badge: Monitored every 2 weeks/ 4 weeks. If accidental high exposure be suspected - should be immediately processed .

25. Advantages: Good for measuring any type and energy of radiation. e.g . X-rays, gamma radiations. Continuous assessment is possible. Provides a permanent record of dose received. Simple , robust & relatively inexpensive.

26. Disadvantages : They are insensitive to radiation below 0.1 mSv . They cannot be worn longer than 4 weeks duration at a stretch due to fogging. Accuracy is only 10 to 50 %. The results are dependent on processing, strength, type of developer used & speed of the film used. No immediate indication of exposure- all information is retrospective .

27. Thermoluminescent dosimetry (TLD) Principle Property of certain materials to emit light when they are stimulated by heat.

28. Working: Few electrons become trapped in higher energy levels Light is collected & measured by photomultiplier tube E nergy is released in the form of light by heating 250 C Amount of light is p r oport i onal to the radiation dose (Sv)

29. Glow curve TLD analyzer

30. Wearing of the badge:

31. Advantages: Can measure exposures to individuals as low as 1.3 C/kg ( 5 mR ). Can withstand certain degree of heat, humidity, & pressure. Crystals are reusable. Instantaneous readings are possible if the department has a TLD analyzer. Read out simple and quick. Can store dose over long period of time .

32. Disadvantages: Not cost effective. Read out is destructive, giving no permanent record. Results cannot be checked or reassessed .

33. QUESTIONS ???