Contenu connexe



  2. - The maximum allowable dose is limited to 20 mSv per year for people working with radiation. This limit is 1 mSv for the normal population. - The effects of irradiation on an organism may change according to the dose, type of contamination, and the features of the radiation source. - It is crucial to perform measurements on the radiation generators used for diagnostic or therapeutic purposes. - The measurement of radiation is called dosimetry, and the equipment used for dosimetric procedures is r n I l/"»rl n rlnc rw n ^etector
  3. - Dosimetry is the monitoring of individuals to accurately determine their radiation dose equivalent. - When radiation interacts with the human body, there are no perceptible sensations and usually no immediate effects. - We could therefore receive an amount of radiation that could injure our tissues severely without realizing it at the time. - To protect ourselves and others, we must use and^elyupon instruments to quantify and qualify niliiliMNmeamiiminl
  4. Radiation Controlled area: An area where entry, activities, and exit are controlled to help ensure radiation protection and prevent the spread of contamination. Radiation Restricted Area:- it includes all "Radiation Areas," "High Radiation Areas," rooms or areas in which there is radioactive materials in such quantities that "Caution: Radioactive Material" signs are required , and certain other areas which may be so defined by the "Health Physicist." DANGE R Radiation controlled area Access restricted RADIATION RESTRICTED A.A AREA A.A 4k 4k
  5. METHODS OF MEASURING ABSORDED DOSE:- A> Calorimetry:- Basic Method Of Determining absorbed dose. - It Is Based On Principle That The Energy Absorbed In A Medium From Radiation Appears Ultimately As Heat Energy, thus Resulting In Increase In Temp of Absorbing Medium. - To measure such change in temp ,thermistor is used (it’s a semiconductor which show change in electrical resistance with small change in temp). - Most of these apparatus are difficult to construct ,and considered impractical for clinical dosimetry.
  6. B> Chemical dosimetry:- Energy absorbed from ionizing radiation may produce chemical changes,& if this change can be determined , it can be used as a measure of absorbed dose. - Ferrous sulphate or fricke’s dosimeter is considered to be most developed system for precision measurement if absorbed dose. - Dosimeter consist of 1mmol/l of fes04 ,1 mmol/l of nacl & 0.4 mol/l h2so4 - When solution is irradiated fe2+ —^ fe3 + determined by ieter solution.
  7. C> Solid state methods:- I l Integrating type dosimeter Electrical conductivity (a) thermoluminescent dosimeter crystals (a) Semiconductor (b) radiophotoluminescent junction detector glass (c) films ^Of this most widely used for measurement of abson se are TLD , diodes and films
  8. THERMOLUMINESCENE DOSIMTETRY - Thermoluminescence:- when a crystal is irradiated ,very minute fraction of absorbed energy is stored in crystal .some of this energy can be recovered later as visible light if material is heated. - Two categories - Fluorescence - emission of light during or immediately after irradiation( time < 10 power -8 sec) - Not a useful reaction for TLD use. - Phosphorescence - emission of light after the irradiation period. Delay can be seconds to months. e phosphorescence to detect radiation.
  9. Theory of thermoluminescent dosimetry:- In crystal lattice .electronic energy level by mutual interaction between atoms give rise to energy bands. - Impurities in crystals create energy traps , providing metastable states for the electrons. - When the material is irradiated , some of e- in valence band(ground state)receive sufficient energy to be raised to the conduction band. - The vacancy thus created is called positive hole. The e- and the hole moves independently ,until they fall fall into a trap(metastable state).
  10. Band gap (forbidden band) BAND THEORY OF SOLIDS
  11. - Emission of light during these transition is called fluorescence. - If e- in the trap requires energy to get out of the trap and fall to the valence band, emission of light is called phosphorescence(delayed fluorescence).this process can be speeded up with moderate amount of heating ,the phenomenon is called as thermoluminescence. Conduction Band Electron Trap TL Photon Valence Band a) Irradiation b) Heating FIG. 8.11. A simplified energy -level diagram to illustrate thermoluminescence process
  12. Glow Curves - Plot of thermoluminescence against temp is called glow curve. - As the temp of TL material exposed to radiation is increased the probability of releasing e- increases. - Area under this curve is directly proportional to the amount of radiation that was absorbed in the chip. -The individual glow peaks are numbered and correspond to different trap depths. 190*C
  13. - The light emitted (TL) first increases, reaches a maximum value and falls again to zero. - Most phospor contain a number of traps at various energy levels in the forbidden band.
  14. - TLD must be calibrated before it can be used for measuring an unknown dose, because response of tld is affected by their previous radiation and thermal history. - The material must be annealed to remove residual effect. - Standard preirradiation annealing procedure for liF is 1 hour of heating at 400c and then 24hour at 80c - Slow heating removes peak 1 and 2 of glow curve, making it more stable. - Tld is available in many forms and sizes, hence can be used for measuring dose in build up region ,around brachytherapy sources, and for personnel dose monitoring
  15. ANNEALING - Annealing is used to determine trap of interest - Low temperature traps fades away with time at room temperature. - Basically just want high temperature traps to remain. - 400 d celsius for 1 hour reset trap structure and eliminates any electron in residual trap. 80 d celsius for 24 hour eliminates the trap ;esult in peak 2 to stabilize glow curve.
  16. TLD READER CONSTRUCTION - Irradiated material is placed in heater cup or planchet - Emitted light is measured by photomultiplier tube converts light into electrical energy. current is then amplified and measure by recorder
  17. - TL dosimeters most commonly used in medical applications are LiF:Mg,Ti, LiF:Mg,Cu,P because of their tissue equivalence. - Other TLDs, used because of their high sensitivity, are CaSO4:Dy, Al2O3:C and CaF2:Mn. - TLDs are available in various forms (e.g., powder, chips, rods, ribbon, etc.). - Their range of measurement spans anywhere from <1 mrem up to as much as 100000 rem.
  18. PROPERTIES OF TL MATERIAL - It should have a single glow curve no interfering glow curve - High TL sensitivity i.e. more light output per unit of dose - Emissive spectra in visible, preferably in the range 400500 nm - Negligible thermal fading (loss of TL signal due to ambient conditions like temperature etc.) - Glow peak preferably around 200 degree celsius - Should be tissue equivalent - Should be cheap, easy to manufacture and simple annealing procedure - Linearity between dose and light output over wide range
  19. - In India CaS04:Dy(l :3) embedded Teflon TLD disk are used for personnel monitc Characteristics of CaSQ4:Dv - Effective Atomic No- 1 5. - Main Peak -200 degree Celsius - Emission Maximum480-570 nm - Fading- less than 5 % per month(at 25 degree Celsius) - Self fading - 0.01 mSv/month - Linearity in Response :linear upto 30 Gy - Colour of emitted light :yellowish -white
  21. Regions of TLD Cassette Front: Cu (1.0mm thick) 3.5 cm x 1.5 cm Dosimeter +AI (1-0 mm thick) disc of dia 15.6 mm. Discl(DI) Back: Cu (1.0mm thick) disc of dia 15.6mm +Al (1.0 mm thick) disc of dia 15.6 mm. Dosimeter : Front Perspex 180mg.cnrr2 Disc2(D2) Back! Same as front Dosimeter : Openl 12mg.cnr2 (identification paper + Disc3(D3) polythene seal) on the card
  22. -A TLD badge based on CaSO4: Dy Teflon discs has been designed and is in regular Personnel Monitoring use since 1975. - The TLD badge has shown satisfactory performance for monitoring beta and gamma doses of radiation workers. - At present about 40,000 radiation workers are covered with TLD monitoring service in our nuclear industry, medical and industrial institutions as well as research institutions. - The complete Personnel Monitoring TLD badge consists of a TLD card and a plastic cassette for holding the TLD card. The badge is affixed to the clothing of a person with the help of a crocodile clip attached to the badge.
  23. -Adults, minors, and declared pregnant women who are likely to exceed of 10% of the below values from external sources of radiation must be monitored. Also, monitors must be worn by anyone who enters a high radiation area. TABLE 1 - SUMMARY OF IDNS DOSE LIMITS ANNUAL OCCUPATIONAL LIMITS ADULTS MINORS Total Effective Dose Equivalent (TEDE) - cr - Total Organ Dose Equivalent (TODE) 5.000 mrem - or - 5(LOGO mrem 500 mrem - or - 5,000 mrem Lena of Eye (LDE) 15r000 mrem 1.500 mrem Skin cf the Whole Body (SDEAVB) 50.000 mrem 5.000 mrem Extremities (SDE.ME) 50.000 mrem 5.000 mrem OTHER APPLICABLE LIMITS Dose to Embryo Fetus (Declared pregnancies) - 500 mreni Members of the Public - 100 mrem year Unrestricted areas - 2 mrem in any one hour
  24. APPLICATION OF TLD - Measurement of output from Co-60 units and accelerators used in medicine and industry. - Area survey of medical (diagnostic and therapeutic) and industrial radiographic installations. - Measurement of stray and leakage radiation around X-ray tubes and source containers. - Monitoring of high levels of contamination from beta sources. - Estimation of activities of various radionuclides used in brachytherapy and nuclear medicine. - To measure dose rates in rectum and bladder of patient undergoing treatment with Cobalt on Cesium implants for carcinoma of uterine cervix. - Personnel Monitoring.
  25. rv/" O- O O- Teflon Discs ^ jj> 1d2 ifsO-- Personnel No. —♦-405C003 - Name —>M. R. Sane Rad.Type--*► X - Rays Dec. 1997- Sealed polythene pouch Card with wrapper Period of use Ni -plated aluminium card (bare) Card to be loaded in cassette and to be returned to PMS after use Metallc filter 11100 mg/cm-*0 Plastic (transparent) (180 mg / cm2) Open window -2 (Paper 12mg /cmz) Fig. 1. Assembly of TLD badge Front view,of TLD badge
  26. POSITIONING OF THE BADGE - One badge should be worn at chest level. - Should be worn below the lead apron if used. - If selectively high doses are expected to hands and head- additional wrist and head badges may be used. • Load the card properly in the TLD cassette, name and personnel NO. should be in the front, visible from outside • Use TLD card of the valid service period. • Handle the TLD badge with care ation free place when not in use DO’s INSTRUCTION FOR USERS OF TLD BADGES iation incident to your RSO
  27. INSTRUCTIONS FOR USERS OF TLD BADGES Don’t - Don’t share your TLD badge with someone else, your badge is your own. - Once loaded don’t open the badge till the end of the service period. - Don’t pierce or open the sealed polythene pouch of the TLD card - Don’t use a damaged or broken cassette of which filters have come out ask for replacement. - Don’t leave the badge in radiation area, in washing machines or near the vicinity of hot plates or ' naces.
  28. ADVANTAGES (AS COMPARED TO FILM DOSIMETER BADGES) INCLUDES: - Able to measure a greater range of doses. - Small in size - point dose measurements possible. - Available in various forms. - Some are reasonably tissue equivalent . - Not expensive.
  29. TLD DISADVANTAGES - Lack of uniformity - batch calibration needed - Storage instability - Fading - Light sensitivity - Spurious TL (cracking, contamination) - No permanent record
  30. GEIGER -MULLER COUNTER - A Geiger-Muller counter, also called a Geiger counter, is a type of particle detector that measures ionizing radiation. - It detects the emission of nuclear radiation alpha particles, beta particles, and gamma rays by the ionization produced in a low-pressure gas in a Geiger-Muller tube. - In wide and prominent use as a hand-held radiation survey instrument, it is perhaps one of the world's best-known radiation instruments.
  31. - A GM counter consists of a tube filled with "Q-gas” (98% helium and 1.3% butane) - As in an ion chamber, the detector records every interaction instead of measuring the average current that occurs after several reactions. In other words, one ionizing event will produce a pulse or a count in the GM tube. - GM counter does not differentiate between types of radiation or their energies. For this reason, most GM counters are calibrated to give counts per minute (CPM). i 4 * - GM counters are usually used to simply detect the presence of radioactive material. - GM counters are used to detect low- energy X and gamma rays.
  33. CaS04:Dy and LiF:Mq, Cu. P thermoluminescent dosimeters for environmental monitoring in ambient areas of a nuclear power plant. Zeng XS1, Zeng JX, Tan GX, Mai WJ. Author information Abstract This paper describes CaSO4:Dy and LiF:Mg, Cu, P which were used for ambient environmental monitoring before the nuclear power plant operation in Guangdong Daya Bay, China, in 1991. Since LiF:Mg, Cu, P was first used as an environmental dosimeter in this laboratory, the intercomparison of both thermoluminescent dosimeters, including laboratory irradiation and environmental exposure in Beijing reference spots, was conducted in cooperation with National Institute of Metrology and Laboratory of Industrial Hygiene, measured values of both thermoluminescent dosimeters were in agreement with the error being less than +or- 2% for the laboratory irradiation. The results of measurement by both thermoluminescent dosimeters were quite in agreement with environmental reference exposure rates measured by a pressurized ionization chamber. The largest error of CaSO4:Dy environmental monitoring results in Daya Bay also showed that the differences of measurement results between two thermoluminescent dosimeters were not significant. The experiment results indicated that Cu, P was a good environmental dosimeter. PMID: 8 PubMed - indexed for MEDLINE]
  34. THANK YOU.