Transcript Film Badges
Personal Monitoring Dosimetry IRAD2731 RADIATION DETECTION AND MEASUREMENT II Agenda Color dosimeters Film TLDs PICs OSLs Track Etch EPD Characteristics Small Light weight Sensitive to only the radiation of interest Insensitive to other influences Cheap Tissue equivalent Linear response Rugged No dosimeter has all these properties Early Colorimetric Method – color change Small metal box containing several clear tubes of liquid The solution consists of chloroform and the dye brome-creosol purple. When the chloroform absorbs radiation energy, hydrochloric acid is produced which changes the dye color from purple to yellow. Sodium hydroxide is added to adjust the sensitivity. High ranges 50R and above Personnel Radiation Monitoring Devices Film Badges: Photographic film used for measurement of ionizing radiation exposure for personnel monitoring purposes. The film badge may contain two or three films of differing sensitivities, and it may also contain a filter that shields part of the film from certain types of radiation. Pocket ion Chamber: A small tubes that has a charged on a piezoelectric crystal, when ionizing radiation interact with the crystal the it is discharge. This discharge moves the needle down the scale indicating total dose. Personnel Radiation Monitoring Devices Thermoluminescence Dosimeter: A small device used to measure ionizing radiation by measuring the amount of visible light emitted from a crystal in the detector. Heat is used to release trapped energy in the form of visible light. Optically Stimulated Luminescence: A small device used to measure ionizing radiation by measuring the amount of visible light emitted from a crystal in the detector. A laser is used to release trapped energy in the form of visible light. Examples of Personnel Monitoring Devices Dosimeters-Film Badges Film Badges - the amount of exposure of the film is proportional to the dose that the badge was exposed to. Oldest from of dosimetery 1903 used for patients 1920s first film badge Film badge holder is designed to be able to obtain photon energy and direction. Where should you wear it? Dosimeters-Film Badges Good permanent record of dose. Time lag between exposure and reading Used to be industry standard, replaced by TLDs and OSLs Still used when pregnant female badge is requested Will over respond at low energies Materials Silver Halide grains are mixed with gelatin matrix Ionizing radiation “sensitizes” the grains Development converts to metallic silver and washes away unused grains Color change (optical density)of film is compared to calibrated dose color change Color change can be influenced by temperature, humidity, light, and processing Can be used either as a cumulative dose or single particle tracks Film Holder Special design Used to flatten response curve of the film out over larger energy rage unfiltered film will over estimate at low energies Can be used to determine energy of incoming photons by the use of different filters Can be made to determine thermal neutron exposure All filters are locked in and provide for separation of different types of radiations. They make possible the elimination of energy dependence of the film when exposed to different radiation energies. Open Window Plastic Filter #1 Plastic Filter #2 Aluminum Filter Lead/Tin Alloy Filter Another filter may be added for thermal neutron determination. Dosimeters-TLD Thermoluminencent Device (TLD) is a material that absorbs radiation and traps the energy in the matrix of the crystal As opposed to scintillators that release the energy right away When heated the crystal releases this energy as light The amount of light released is proportional to the energy absorbed by the TLD Energy absorbed is proportional to dose TLD electrons Dosimeters-TLD Can be used many times over. Process of annealing clears out the electrons and readies the crystal for reuse Needs special machine to read. Time lag between exposure an reading No permanent record after reset. Fading Spontaneous loss of stored energy over time Each material has its own fading rate Can be affected by heat TLD materials CaSO4:Mn- very sensitive due to traps are close to the band gap, but also fades rapidly CaF2:Mn- not as sensitive as CaSO4:Mn, but fades a great deal less, suitable for long term use LiF- most popular, negligible fading or energy dependence, atomic number close to tissue Neutron badges 6Li/7Li- -6Li is sensitive to slow neutrons Can be used to determine dose due to beta/gamma/neutron in mixed field Glow curve Optically Stimulated Luminesence Same as TLDs but the light is released when the crystal is hit by a laser rather than being heated up Has same characteristics as TLDs but OSLs do not fade Can be read several times. Al2O3 is most common material used Not affected by many external things Dosimeters-PIC Pocket Ion Chamber (PIC)- a charged piezoelectric cell discharges as it interacts with radiation and moves a needle across a screen to indicate exposure. Can check on exposure real time Can use over many times several persons can use But permanent record is lost when reset Can be confusing if multiple people use it and no good records are kept Pocket Chamber and Charger Track Etch Same idea as film but emulsion is thicker Simple and inexpensive Used for heavy charged particle monitoring Insensitive to electrons, gamma rays and fast neutrons Used for Rn detection and space applications Can be used to determine direction of radiation Track Etch Radiation interacts with emulsion Leaves physical and ionizing tracks in emulsion Emulsion is processed Chemicals are washed over the emulsion Chemical enlarges the holes made by the particles Computer can count number of holes per unit area Used to be done by eye and a bright light Number of holes is proportional to particle flux Size of holes is proportional to particle energy Electronic Dosimeter (EPD) Combines the ability to have permanent record and notify person immediately of changing dose rates Can set alarms for total dose and dose rate Info can be loaded to computer Can be used by several people with proper record keeping Not all good Mechanical failure Data loss Heavier than TLD Response vs battery life Some EPDs do not detect betas or neutrons Affected by magnetic fields Several external influences affect EPD Issuing PDs Film, TLDs, OSLs are all person specific Should not share or swap badges EPDs and PICs can be used by several people In order to be used as dose records need good record keeping Person it was checked out to and their initials Serial number of unit Date and time of issue and return Initial reading , final, and net reading Can be used as supplemental info(along with other dosimetry) or if used as official dose record need to be calibrated annually Questions?