IRAD2731 RADIATION DETECTION AND MEASUREMENT II Personal Monitoring Dosimetry
Feb 23, 2016
IRAD2731RADIATION DETECTION AND
MEASUREMENT II
Personal MonitoringDosimetry
Agenda
Color dosimetersFilm TLDsPICsOSLsTrack EtchEPD
Characteristics
SmallLight weightSensitive to only the radiation of interestInsensitive to other influencesCheapTissue equivalentLinear responseRuggedNo dosimeter has all these properties
Early
Colorimetric Method – color changeSmall metal box containing several clear
tubes of liquidThe 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 DevicesFilm 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 ofPersonnel 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 readingUsed 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 grainsDevelopment converts to metallic silver and
washes away unused grainsColor change (optical density)of film is
compared to calibrated dose color changeColor change can be influenced by
temperature, humidity, light, and processingCan 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 awayWhen heated the crystal releases this energy
as light The amount of light released is proportional
to the energy absorbed by the TLDEnergy 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 reuseNeeds special machine to read.Time lag between exposure an readingNo 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 usedNot 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 timeCan use over many timesseveral persons can useBut permanent record is lost when resetCan 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 thickerSimple and inexpensiveUsed for heavy charged particle monitoringInsensitive to electrons, gamma rays and fast
neutronsUsed for Rn detection and space applicationsCan be used to determine direction of
radiation
Track Etch
Radiation interacts with emulsionLeaves physical and ionizing tracks in
emulsionEmulsion is processed
Chemicals are washed over the emulsionChemical enlarges the holes made by the
particlesComputer 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 computerCan be used by several people with proper record keeping
Not all good
Mechanical failureData lossHeavier than TLDResponse vs battery lifeSome EPDs do not detect betas or neutronsAffected by magnetic fieldsSeveral 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?