Miscellaneous Detectors

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Miscellaneous Detectors
The Radiology Physics Laboratory

• November 11, 2003
• Prof. Tsi-chian Chao

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Thermoluminescent Dosimeters (TLD)
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Glow Curve
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Readout Cycle

• Pre-heat period
• Without light integration to discriminate against
  unstable low-temperature traps
• Read period
• Spanning the emission of the part of the glow
  curve to be read as a measure of the dose
• Annealing period
• During which the remainder of the stored energy
  is dumped without light integration
• Cool-down period
• After the heater-pan power is turned off

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Trap Stability

• Annealing
• Apply after TLD signals have been read
• To avoid trap configuration change
• TL phosphors give best performance as dosimeters
  if they receive uniform, reproducible, and
  optimal heat treatment before and after use
• Ex. LiF (TLD-100)
• 400 0C for 1 h, quick cooling, then 80 0C for 24 h

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Advantages

• Wide useful dose range
• From a few mrad to ?103 rad linearly
• Dose-rate independence
• 0-1011 rad/s
• Small size
• Chips, rods, powder
• Commercial availability
• Reusability
• Can be reused many times

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Advantages

• Economy
• Reusability reduces cost
• Availability of different types with different
  sensitivities to thermal neutrons
• TLD-700 (7LiF)
• Sensitive to photons only
• TLD-100 (93 7LiF 7 6LiF)
• Sensitive to both neutrons and photons
• TLD-600 (96 6LiF)
• Sensitive to neutrons only

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Advantages

• Readout convenience
• Readout rapidly (lt30 s)
• Automation compatibility
• Automatic reader for mass amount of TLDs
• Accuracy and precision
• Reproducibility of 1-2
• 1-2 accuracy through individual calibration and
  averaging of several dosimeters in a cluster

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Disadvantages

• Lack of uniformity
• Sensitivity varies from batch to batch, even from
  dosimeter to dosimeter of the same batch
• Storage instability
• Sensitivity varies with time
• Fading
• Gradual loss of the latent TLD signal

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Disadvantages

• Light sensitivity
• Sensitive to lightespecially to UV, sunlight, or
  fluorescent light
• Spurious TL
• Scraping, chipping, or surface contamination by
  dirt or humidity can cause spurious TL readings
• Loss of a reading
• No second chance at getting a reading

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Disadvantages

• Memory of radiation and thermal history
• Sensitivity increased or decreased after
  receiving a large dose of radiation
• Reader instability
• Reader constancy is difficult to maintain over
  long time periods

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Photographic Dosimetry

• Exposure
• Radiation hits photographic emulsion and generate
  ion pairs near AgBr grains then converting Ag
  ions to Ag atoms
• Chemical processing
• Developing
• in the chemical process all of the Ag converted
  to Ag atoms, leaving behind an opaque microscopic
  grain of silver
• Stop bath
• Hypo

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Optical Density of Film
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Energy Dependence

• Increased response for energy lt 200 keV due to
  photoelectric effect
• Use of filter to eliminate this over response

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Advantages

• Spatial resolution
• Unrivaled in spatial resolution
• Reading Permanence
• The record is permanent
• Commercial availability
• Geometry
• Thin and flat shape allow simple use
• Can approach B-G cavity dimensions
• Linearity vs. dose
• Dose-rate independence

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Disadvantages

• Wet chemical processing
• Require careful control of wet-chemical
  development process
• Energy dependence of X rays
• Over-response for energy below 300 keV due to
  photoelectric interactions with silver bromide
  grains
• sensitivity to hostile environments
• Double-valued response functions
• Over-saturate of film response cause
  double-valued response
• Blindness to low-energy neutrons

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Chemical Dosimetry

• Chemical Dosimetry
• Basic Principles
• Radiation interacts with water
• produce chemically active primary products (free
  radicals, such as H2 H2O2) in about 10-10 s or
  less
• initially distributed heterogeneously, close to
  the charged-particle tracks
• by 10-6 s, diffuse to become more homogeneous,
  simultaneous with their chemical interactions
  with the solutes present

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Chemical Dosimetry

• Radiation chemical yield (G-value)
• Defined as the number of chemical entities
  produced, destroyed, or changed by the
  expenditure of 100 eV of radiation energy
• in moles/J
• Calculation of absorbed dose
• ?M (mole/liter)
• the change in molar concentration of product X
  due to the irradiation
• ? (g/cm3 or kg/liter) solution density

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Chemical Dosimetry

• Popular example
• Fricke Ferrous Sulfate Dosimeter
• Fe2 ? Fe3 oxidation reaction
• Composition
• 0.001 M FeSO4 or Fe(NH4)2(SO4)2 and 0.8 N H2SO4

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Chemical Dosimetry

• Advantages
• Z, ?en/? ? similar to water
• Liquid dosimeters can be made similar in shape
  and volume to the studied object
• Absolute dosimetry possible
• Different chemical dosimeters can be used to
  cover various dose ranges 10-1010 rad
• Linear dose response vs. dose in useful ranges

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Chemical Dosimetry

• Disadvantages
• Lack of storage stability prevents commercial
  availability, requiring wet chemistry in the
  users lab
• Useful dose ranges too high for personnel
  monitoring or small source measurements
• Some degree of dose-rate and LET dependence
• Dependence on the temperature of the solution
  during irradiation and during the readout
  procedure

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Calorimetric Dosimetry

• Calorimetric Dosimetry
• Direct measurement of the full energy imparted to
  matter by radiation
• Closest of any method for absolute dose
  measurement
• ?T temperature change
• h thermal capacity (cal/g 0C or J/kg 0C)
• ? thermal defect
• The fraction of E that dose not appear as hear,
  due to competing chemical reactions

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Calorimetric Dosimetry

• Advantages
• Absolute dosimetry
• Closest of any method being a direct measurement
  of the energy involved in the absorbed dose
• Almost any material can be employed in the
  sensitive volume
• Dose-rate independent
• No LET dependence
• Relatively stable against radiation damage

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Calorimetric Dosimetry

• Disadvantages
• Temperature rise small, limiting measurement to
  relatively large doses
• Apparatus bulky, difficult to transport and set
  up
• For low dose rates, thermal leakage limits the
  accuracy and precision achievable
• Thermal defect problem