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 << 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 << 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

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 (>0.3 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:

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? < 0.1 MeV than does tissue or air, as shown in the following diagram

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 film�s 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 proton�s 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

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 25�C, 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

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 (> 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

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 (> 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

57. Advantages and Disadvantages (cont.) Lack of storage stability prevents commercial availability, requiring careful wet chemistry in the user�s 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