Integrating Dosimeters II

1
Integrating Dosimeters II

• Photographic Dosimetry
• Chemical Dosimetry

2
Photographic Process Photographic Emulsion

• The emulsion consists of microscopic grains of
  silver bromide (AgBr), dispersed in a gelatin
  layer on either one or both sides of a supporting
  film
• Incident charged particles produce ion pairs in
  or near the grains, and their effect is to
  convert Ag ions to Ag atoms
• A few such Ag atoms on a grain (containing
  typically 1010 Ag ions) constitute a latent
  image, which renders the grain developable by a
  chemical process

3
Photographic Emulsion (cont.)

• In that process all of the Ag ions are converted
  to Ag atoms and the bromine is removed, leaving
  behind an opaque microscopic grain of silver
• The presence of this elemental silver may be
  detected optically and quantitatively related to
  the absorbed dose

4
Photographic Process Chemical Processing

• This usually comprises three steps
• Developing. The developer molecules would reduce
  the Ag ions to Ag atoms in all grains
  eventually, whether ionized or not. Those having
  a latent image are reduced much more rapidly,
  however, and the developing process can then be
  terminated. Thorough agitation of developing
  fluid and close temperature constancy are
  important for homogeneous and reproducible
  development.

5
Chemical Processing (cont.)

• Stop Bath. Immersion of the emulsion in a dilute
  acetic acid stop bath terminates development
  quickly. This is necessary for quantitative
  photographic dosimetry, since the optical density
  depends on the developing time as well as
  temperature, agitation, and developer
  characteristics.
• Hypo. Sodium thiosulfate (hypo) solution then
  is used to dissolve out the remaining undeveloped
  grains of AgBr, that is, those that did not
  contain a latent image. The film is finally
  washed in pure water and air-dried.

6
Optical Density of Film

• In x-ray emulsions the radiation effect is
  measured in terms of the light opacity of the
  film, as measured by a densitometer
• Opacity is defined as I0/I, where I0 is the light
  intensity measured in the absence of the film,
  and I the intensity transmitted through the film
  in a direction perpendicular to the plane
• The optical density (OD) is defined as log10
  (I0/I)

7
Optical Density (cont.)

• If a is the average area (cm2/grain) obscured by
  a single developed grain of silver, and n is the
  number of developed grains per cm2 of film, then
• and
• so long as n ltlt N, where N is the number of
  AgBr grains per unit area (cm2) in the unexposed
  film

8
Optical Density (cont.)

• Making the following additional assumptions leads
  to a simple but useful model
• Incoming x-rays give rise to a secondary-electron
  fluence of ? (e/cm2) passing perpendicularly
  through the film
• A single electron hit renders a grain developable
• All grains have the same projected area a, which
  is assumed not to change during development.
  That is, the target area for electron hits is the
  same as the light-stopping area of a silver grain

9
Optical Density (cont.)

• For this case we can write for the fraction of
  grains struck and made developable
• which can be substituted into the equation
  for OD to give

10
Optical Density (cont.)

• From this relation we can see that, for a small
  fluence ? (i.e., where n ltlt N), the OD is
  proportional to ? (and consequently also to the
  dose) in the emulsion
• The OD is also proportional to the emulsion
  thickness, since N ? thickness
• Furthermore, the OD is proportional to the square
  of the grain area, or the fourth power of the
  grain diameter

11
Optical Density (cont.)

• Film-density measurements are sometimes expressed
  in terms of the standard density (SD), defined
  as
• where (OD) is the optical density of the
  exposed film, (OD)f is that of the unexposed
  film, and (OD)m is the maximum optical density
  measured if all the grains are developed, that
  is, if n N

12
Optical Density (cont.)

• Three types of film-density plots vs. dose are
  commonly used, as shown in the following diagram
• Graphs like A and C are most useful for
  dosimetry, since they are linear at low doses for
  the case where the single-hit response dominates,
  as is usually found
• The second plot (B) is called the H and D curve
• It is of greater use in photography or
  radiography, since its slope, called contrast,
  measures the ability of the film to distinguish
  between two nearly equal exposures by OD
  difference

13
Three common types of plots of film dosimeter
response (A) standard density (SD) vs. dose (D)
in tissue or water (B) SD vs. log10 D, and (C)
log10 SD vs. log10 D.
14
Practical Exposure Range forX-Ray Film

• Typical dosimetry film (Kodak Type 2) shows an OD
  increase of about 0.15 for an x-ray exposure of
  100 mR at quantum energies above the
  photoelectric region (gt0.3 MeV)
• This roughly doubles the OD observed in unexposed
  film, depending upon the temperature and humidity
  conditions to which the film has been subjected
  and how long the film has been worn by personnel
  being monitored

15
Exposure Range (cont.)

• For oncology dosimetry applications, the useful
  ranges of other Kodak films are

16
Dose response curve for Kodak EDR2 film
17
X-Ray Energy Dependence

• Photoelectric effect in the AgBr grains causes
  the film to absorb x-ray energy 10-50 times more
  readily for h? lt 0.1 MeV than does tissue or air,
  as shown in the following diagram

18
Relative response per unit of x-ray exposure,
normalized to 60Co ?-rays, for a typical
film-badge dosimeter with and without a
compensating filter
19
Energy Dependence (cont.)

• This overresponse can either be compensated for
  by enclosing the film in a high-Z filter, as
  shown, or by making the film badge into a crude
  spectrometer by using different metal-foil
  filters over different segments of the films
  area
• Measuring the OD in the different film areas (at
  least two), accompanied by suitable calibration
  with x-ray beams of known energy spectra, allows
  the film badge to yield useful spectral
  information about the x-rays in addition to the
  dose reading

20
Nuclear Track Emulsions

• Aside from use of nuclear track emulsions in
  cosmic ray research, their main application is in
  the dosimetry of fast neutrons for personnel
  monitoring
• Fast neutrons deposit energy in an emulsion (or
  in tissue) mainly by elastic scattering
  interactions with hydrogen nuclei (protons)
• The absorbed dose from these (n, p) reactions in
  emulsion is proportional to the number of recoil
  protons produced per gram, and their average
  energy

21
Nuclear Track Emulsions (cont.)

• The protons energy can be determined from
  microscopic measurement of the length of its
  track in the emulsion, and reference to
  range-energy tables
• Such a procedure is absolute inasmuch as
  calibration in a known neutron field is not
  needed
• Neutrons below ? 0.7 MeV do not make recognizable
  proton tracks because they are too short hence
  the nuclear emulsion is blind to lower energy
  neutrons

22
Photographic Advantages

• Spatial Resolution
• Reading Permanence
• Commercial Availability
• Geometry
• Linearity vs. Dose
• Dose-Rate Independence

23
Photographic Disadvantages

• Wet Chemical Processing
• Energy Dependence for X Rays
• Sensitivity to Hostile Environments
• Double-Valued Response Functions
• Blindness to Low-Energy Neutrons

24
Chemical Dosimetry Introduction

• In chemical dosimetry, the dose is determined
  from quantitative chemical change in an
  appropriate medium, which may be liquid, solid,
  or gaseous
• We will consider primarily aqueous liquid
  systems, especially the Fricke dosimeter, which
  is the most common and generally the most
  relevant to the measurement of dose in tissue or
  other biological material

25
Basic Principles

• Since aqueous dosimeters usually consist of
  dilute solutions, one can generally assume that
  radiation interacts with the water, producing
  chemically active primary products in about 10-10
  s or less
• These products including free radicals like H
  and OH which have an unpaired electron, and
  molecular products such as H2 and H2O2 (hydrogen
  peroxide) are distributed heterogeneously,
  close to the charged particle tracks

26
Basic Principles (cont.)

• By 10-6 s after the initial interaction, the
  spatial distribution of these primary products
  tends to homogenize due to diffusion,
  simultaneous with their chemical interactions
  with the solutes present
• The LET dependence (if any) of the dosimeter
  depends on the reaction rates during this
  interval, that is, before the initial spatial
  distribution is obliterated.
• Dense tracks (high LET) usually encourage
  competing reactions or back reactions, thus
  reducing the yield of the desired product to be
  measured

27
Basic Principles (cont.)

• The yield of the measured product is expressed as
  a G-value, or more recently in terms of the
  radiation chemical yield, G(X), w.r.t. the
  product X
• The G-value is the number of chemical entities
  (e.g., molecules) produced, destroyed, or changed
  by the expenditure of 100 eV of radiation energy
• G(X) is expressed in units of moles/J, and can be
  obtained from the corresponding G-value by
  multiplying it by 1.037 ? 10-7

28
Basic Principles (cont.)

• Since G(X) is usually of the order of 10-6 10-7
  moles/J in aqueous chemical dosimeters, a dose of
  10 Gy then requires measurement of 10-5 10-6
  M solutions of the product with acceptable
  accuracy
• This requires sensitive detection methods and
  careful procedures, and rules out the measurement
  of small doses by this means

29
General Procedures Preparation of Vessels

• To minimize errors due to chemical interference
  by impurities on the inner surface of storage or
  irradiation vessels, Vycor (fused silica) is
  preferred
• After thoroughly washing and rinsing in
  triple-distilled water, vessels are heated at 550
  C for 1 h to burn out any remaining organic
  impurities
• Irradiation vessels are then filled with
  dosimeter solution for storage until use, when
  the old solution is discarded and replaced with
  fresh solution

30
Preparation of Vessels (cont.)

• As an alternative to heat cleaning, the cells can
  be filled with triple-distilled water and
  irradiated to 103 104 Gy, then rinsed out with
  dosimeter solution and stored with that solution
• This method can also be used with plastic cells,
  which are preferable to Vycor from the viewpoint
  of matching the atomic number of solution and
  cell material

31
General Procedures Cavity-Theory Considerations

• Since it is impractical to make irradiation
  vessels for aqueous dosimeters small enough to
  behave as B-G cavities, it may be advantageous
  instead to make their diameter large compared
  with the range of secondary charged particles, so
  that wall effects become negligible and CPE or
  TCPE is achieved in the dosimeter solution itself
  for photon or neutron irradiations

32
Cavity-Theory Considerations (cont.)

• Alternative to using large vessels, the use of
  polystyrene (C8H8) or Lucite (C5H8O2) vessels
  provides close enough matching of atomic numbers
  to water so that cavity wall effects are
  minimized
• Burlin theory predicts that if the ratio
  (?en/?)/(dT/ ?dx)c is the same for the wall
  material and the cavity material, cavity size no
  longer affects the dose in the cavity

33
Matching of vessel walls to aqueous dosimeters
for 60Co ?-rays
34
Cavity-Theory Considerations (cont.)

• For electron beams, wall matching to the solution
  in the irradiation vessel is controlled by
  stopping-power and electron-scattering
  considerations
• Again the choice of polystyrene or Lucite for the
  vessel is to be preferred, to minimize
  perturbation of the electrons passing through

35
General Procedures Attenuation in Vessel Walls

• Polystyrene has a density ? 1.04 g/cm3, which is
  so close to that of water that the difference in
  radiation attenuation is negligible when such a
  thin-walled vessel is immersed in a phantom
• For Lucite ? ? 1.18 g/cm3 even in this case a
  1-mm vessel wall immersed in a water phantom
  would only attenuate a photon beam by ? 0.04
  more than the water it displaces

36
Attenuation in Vessel Walls (cont.)

• For SiO2, ? ? 2.2 g/cm3, hence an attenuation
  correction is called for when such a vessel is
  immersed in a water phantom
• For photons, (?en/?)SiO2 (?en/?)H2O may be used
  as an approximate net mass attenuation
  coefficient, assuming the straight-ahead
  approximation to broad-beam attenuation
• For electron beams, SiO2 irradiation vessels
  should be avoided because of scattering
  perturbations

37
General Procedures Reagents and Water Supply

• The highest-purity reagents available should be
  used to minimize unwanted reactions, and
  triple-distilled water stored in heat-cleaned
  fused-silica (Vycor) containers should be used
  for all rinsing and solution mixing

38
General Procedures Calculation of Absorbed Dose

• The average absorbed dose in the dosimeter
  solution is given by
• where ?M (mole/liter) is the change in molar
  concentration of product X due to the
  irradiation, and ? (g/cm3 or kg/liter) is the
  solution density
• This assumes that G(X) (mole/J) applies to the
  production of X throughout the molar range ?M

39
Fricke Ferrous Sulfate Dosimeter

• This is the chemical dosimeter of choice for most
  applications calling for a linear dose range from
  40 to 400 Gy
• Suitable special procedures are available for
  extending this range downward to ? 4 Gy or upward
  to 4 ? 103 Gy
• The following discussion pertains to the normal
  dose range, however, unless otherwise noted

40
Fricke Dosimeter Composition

• The standard Fricke dosimeter solution is
  composed of 0.001 M FeSO4 or Fe(NH4)2(SO4)2 and
  0.8 N H2SO4, prepared from high-purity reagents
  and triple-distilled water
• A 0.1 M or 0.01 M stock solution of ferrous
  sulfate may be added to 0.8 N H2SO4 to complete
  the mixture

41
Composition (cont.)

• Stock solutions of ferrous sulfate (FeSO4)
  gradually oxidize to ferric sulfate Fe2(SO4)3
  over time
• This process can be slowed by dark storage in a
  refrigerator
• Since it simulates the effects of radiation, a
  background control reading from the same batch of
  solution is essential, and fresh solution should
  be prepared just before use for optimal results

42
Composition (cont.)

• Adding 0.001 M NaCl to the above mixture
  desensitizes the system to organic impurities,
  and is therefore beneficial except where very
  high dose rates (e.g., pulsed electron beams) are
  to be measured, in which case the NaCl reduces
  the ferric ion yield, and should be avoided

43
Fricke Dosimeter Measurement of Ferric Ion
(Fe3) Production

• This can be done by chemical titration of the
  irradiated and unirradiated samples to obtain ?M
  of ferric ion
• Absorption spectroscopy is more convenient and
  sensitive, and requires only a small sample ( 1
  cm3)
• Usually an absorption cell of 1-cm pathlength is
  used, at a wavelength of 304 nm in a
  constant-temperature chamber to control the
  effect of the 0.69/C temperature variation of
  the molar extinction coefficient for Fe3, which
  is ?(Fe3) 2187 liter/mole cm at 25 C

44
Measurement of Ferric Ion Production (cont.)

• The ratio of the transmitted light intensity
  through the irradiated sample to that through the
  unirradiated sample is
• where ?(OD) is the corresponding increase in
  optical density, given by

45
Measurement of Ferric Ion Production (cont.)

• Substituting for ?M we have
• where ? 2187 liter/mole cm at 304 nm and
  25C,
• l 1 cm (usually),
• G(Fe3) 1.607 ? 10-6 mole/J for low-LET
  radiations such as 60Co ? rays,
• ? 1.024 kg/liter for standard
  Fricke solution at 25 C

46
Measurement of Ferric Ion Production (cont.)

• Hence
• Thus the normal dose range of the Fricke
  dosimeter (40 400 Gy) corresponds to ?(OD)
  values of ? 0.14 to 1.4 for a 1-cm
  spectrophotometer cell at 304 nm
• The following diagram gives the approximate
  variation of G for Fe3 production as a function
  of photon energy

47
G value for ferric ion production as a function
of photon energy
48
Fricke Dosimeter Irradiation Conditions

• The solution must be air-saturated during
  irradiation for the Fe2 ? Fe3 oxidation
  reaction to proceed with the expected G value
• Stirring the sample or bubbling air through it
  during irradiation may be necessary to avoid
  local oxygen depletion in case of inhomogeneous
  irradiation
• The system is dose-rate-independent at least up
  to 2 ? 106 Gy/s
• G(Fe3) has a temperature coefficient probably
  lying between 0 and 0.1 /C

49
Fricke Dosimeter Extending the Dose Range

• The upper limit of the Fricke system can be
  extended at least from 400 to 4000 Gy by raising
  the ferrous sulfate content from the usual 0.001
  M to 0.05 M, and bubbling oxygen through the
  solution during irradiation
• The lower dose-range limit of the standard Fricke
  system can be reduced to ? 4 Gy simply by
  increasing the spectrophotometric light path to
  10 cm

50
Other Chemical Dosimeters

• A variety of other chemical dosimeters have been
  described
• Most are limited to dose ranges still higher than
  the upper limit of the extended Fricke system (gt
  4 ? 103 Gy)
• One especially versatile dosimeter is the
  radiochromic dye-cyanide system, which is
  commercially available in some forms
• The following diagram gives typical response
  curves for that and some other dye dosimetry
  systems exposed to 60Co ?-rays

51
Typical optical-density response curves of
various dye-type dosimeters exposed to 60Co ?-rays
52
Advantages and Disadvantages of Aqueous Chemical
Dosimeters

• Dilute aqueous solutions have an effective Z and
  ?en/? that are close to those of water, which in
  turn is fairly similar to muscle tissue for
  photon energies over the entire range of
  practical interest. The density of dilute
  aqueous solutions approximates ? 1.00 g/cm3,
  like water. Thus a dosimeter cell immersed in a
  water phantom does not require a
  polarization-effect correction, such as is needed
  for applying cavity theory to gaseous ion
  chambers for high energies (gt 1 MeV)

53
Advantages and Disadvantages (cont.)

• Liquid dosimeters can, if desired, be irradiated
  in a container similar in shape and volume to the
  object being studied. Mixing the dosimeter
  solution irradiated in this manner, before taking
  a sample in which to determine the amount of the
  dosimetric radiation product, gives a measure of
  the average dose throughout the sensitive volume

54
Advantages and Disadvantages (cont.)

• In the unit-density solution it is relatively
  easy to achieve a large-size dosimeter, in the
  Burlin-theory sense. However, it is difficult to
  satisfy the B-G conditions.
• Absolute dosimetry is possible, at least for the
  Fricke dosimeter system
• Different chemical dosimeters can be used to
  cover various dose ranges within the limits 10
  1010 rad
• Linear response vs. dose is found in some
  chemical dosimeters over limited but useful ranges

55
Advantages and Disadvantages (cont.)

• Liquid dosimeters can be used to measure the
  energy fluence of relatively nonpenetrating beams
  (e.g., electron beams), as shown in the following
  diagram. In the example shown, small positive
  corrections would be needed for energy losses due
  to electron backscattering and x-ray production

56
Energy-fluence measurement by a liquid chemical
dosimeter.
If D is the average dose (Gy) in the m kilograms
of dosimeter solution, then the energy spent is
mD and the electron energy fluence ? mD/A
(J/m2) at the collimator of area A (m2)
57
Advantages and Disadvantages (cont.)

• Lack of storage stability prevents commercial
  availability, requiring careful wet chemistry in
  the users laboratory, a pronounced disadvantage.
• Useful dose ranges tend to be too high for
  personnel monitoring or small-source measurement.
• Individual systems usually show some degree of
  dose-rate and LET dependence, as well as
  dependence on the temperature of the solution
  during irradiation and during the readout
  procedure