Dosimetry Fundamentals II

1
Dosimetry Fundamentals II

• General Characteristics of Dosimeters

2
Absoluteness

• An absolute dosimeter is one that can be
  assembled and used to measure the absorbed dose
  deposited in its own sensitive volume without
  requiring calibration in a known field of
  radiation
• It may, however, need some kind of calibration
  not involving radiation, such as
  electrical-heating calibration of a calorimetric
  dosimeter

3
Absoluteness (cont.)

• Three types of dosimeters are now generally
  regarded as being capable of absoluteness
• Calorimetric dosimeters
• Ionization chambers
• Fricke ferrous sulfate dosimeters

4
Absoluteness (cont.)

• These are not always employed as absolute
  dosimeters, however, because calibration offers
  certain advantages
• A calibration can be stated in terms of some
  quantity of interest other than the absorbed dose
  in the sensitive volume, e.g., tissue dose or
  exposure
• It can also provide traceability to an
  authoritative standardization laboratory such as
  the NRCC

5
Absoluteness (cont.)

• When an absolute dosimeter is used independently,
  it relies on its own accuracy instead of
  referring to a standard dosimeter in common with
  other radiation users
• Thus an error may go undetected in an absolute
  dosimeter unless comparisons with others are
  carried out, or a calibration is obtained at a
  standardization laboratory

6
Absoluteness (cont.)

• The calorimetric dosimeter has the fundamental
  advantage of directly measuring the heat to which
  the absorbed dose degrades, without dependence on
  any coefficient of conversion such as to
  ionization (W) or to chemical yield (G)
• Thus if there is a hierarchy of dosimeter
  absoluteness, the calorimeter ranks at the top
• Note that the absoluteness of a dosimeter is
  independent of its precision or its accuracy

7
Precision and Accuracy

• The concept of the precision or reproducibility
  of dosimeter measurements was discussed earlier
  it has to do with random errors due to
  fluctuations in instrumental characteristics,
  ambient conditions, and so on, and the stochastic
  nature of radiation fields
• Precision can be estimated from the data obtained
  in repeated measurements, and is usually stated
  in terms of the standard deviation
• High precision is associated with a small
  standard deviation

8
Precision and Accuracy (cont.)

• The accuracy of dosimeter measurements expresses
  the proximity of their expectation value to the
  true value of the quantity being measured
• Thus it is impossible to evaluate the accuracy of
  data from the data itself, as is done to assess
  their precision
• Accuracy is a measure of the collective effect of
  the errors in all the parameters that influence
  the measurements
• In experiments that are limited to relative
  measurements, only the precision, not the
  accuracy, is important

9
Precision and Accuracy (cont.)

• Clearly precision and accuracy are separate
  characteristics
• Measurements may be highly precise but
  inaccurate, or vice versa, or may be strong in
  both or neither of these virtues
• If one speaks of a dosimeter as being a
  high-precision instrument, one means that it is
  capable of excellent measurement reproducibility
  if properly employed

10
Precision and Accuracy (cont.)

• Poor technique, a hostile environment (e.g., high
  atmospheric humidity) or faulty peripheral
  equipment (e.g., ion-chamber cables or
  electrometer) may cause poor reproducibility
• Accuracy depends on the type of radiation being
  measured, and dosimeter calibrations are more or
  less specific in that respect
• A dosimeter that is accurately calibrated to
  measure the exposure at one x-ray quality may be
  significantly in error at another

11
Precision and Accuracy (cont.)

• The quantity that a dosimeter is inherently the
  most capable of measuring accurately, and that is
  the least influenced by changing the type or
  quality of the radiation, is the absorbed dose
  deposited in the dosimeters own sensitive volume

12
Dose Range Dose Sensitivity

• To be useful, a dosimeter must have adequate dose
  sensitivity (dr/dDg) throughout the dose range
  to be measured
• A constant dose sensitivity throughout the range
  provides a linear response, that is desirable for
  ease of calibration and interpretation
• However, single-valuedness of the function
  r(Dg), even if nonlinear, may be acceptable,
  though it requires that the calibration be
  carried out at a multiplicity of doses to provide
  a calibration curve

13
Dose Range Background Readings and Lower Range
Limit

• The lower limit of the useful dose range may be
  imposed by the instrumental background or
  zero-dose reading
• The is the value of r r0 observed when Dg 0
  sometimes it is referred to as spurious
  response, since it is not caused by radiation
• Examples of r0 include charge readings due to
  ion-chamber insulator leakage, and
  thermo-luminescence dosimeter readings resulting
  from response of the reader to infrared light
  emission by the dosimeter heater

14
Background Readings (cont.)

• The instrumental background should be subtracted
  from any dosimeter reading
• The usual procedure for determining this
  correction is to make measurements with the same
  dosimeter treated in the same way (including
  duration of the time) except for the absence of
  the applied radiation field
• In this way the quantity one measures is r0 plus
  the radiation background reading rb

15
Background Readings (cont.)

• If a background reading is very reproducible from
  run to run, subtracting it from a dosimeter
  reading may have little effect on the precision
  of the instrument
• In many cases, however, the background reading
  exhibits significant nonreproducibility
• The lower limit of the practical dose range of a
  dosimeter is usually estimated to be the dose
  necessary to double the instrumental background
  reading

16
Background Readings (cont.)

• Evaluation of the precision of the measurements
  from repeated readings of both the radiation and
  the background will of course provide more
  quantitative information
• If ?? is the S.D. of the average of a group of
  radiation readings r, and ??0 is the S.D. of the
  average of the background readings r 0, then the
  S.D. of the net radiation reading r r0 is given
  by
• (Note that these are not percentage S.D.s)

17
Background Readings (cont.)

• If the background reading is negligibly small,
  then the lower dose limit is imposed by the
  capability of the dosimeter readout instrument to
  provide a readable value of r for the dose to be
  measured, Dg
• If r is less than 10 of full scale on analogue
  instruments, or contains fewer than three
  significant figures on digital readouts, the
  precision and accuracy may both become
  unsatisfactory
• A more sensitive scale should then be used

18
Dose Range Upper Limit of the Dose Range

• The upper limit of the useful dose range of a
  dosimeter may be imposed simply by external
  instrumental limitations, such as reading off
  scale on the least sensitive range of an
  electrometer
• Alternatively some kind of inherent limit may be
  imposed by the dosimeter itself

19
Upper Limit (cont.)

• Causes of this type include
• Exhaustion of the supply of atoms, molecules, or
  solid-state entities (traps) being acted upon
  by the radiation to produce the reading
• Competing reactions by radiation products, for
  example in chemical dosimeters
• Radiation damage to the dosimeter (e.g.,
  discoloration of light-emitting dosimeters, or
  damage to electrical insulators)

20
Upper Limit (cont.)

• Usually the upper limit of the dose range is
  manifested by a decrease in the dose sensitivity
  (dr/dDg) to an unacceptable value
• It may be reduced to zero, or to a negative
  value, as in the following diagram, which causes
  the dose-response function to become double-valued

21
Illustrating a double-valued dose-response
function resulting from a decrease in the
dosimeter sensitivity at high doses
22
Upper Limit (cont.)

• In such a case other information is needed to
  decide which dose is represented by a large
  r-value, as shown in the figure
• It is of course possible in principle to make use
  of the negative-slope part of a dose-response
  curve such as that in the figure for dosimetry
  purposes if it is sufficiently reproducible

23
Dose-Rate Range For Integrating Dosimeters

• If a dosimeter is to be used for measuring the
  time-integrated dose (not the dose rate), then it
  is necessary that its reading not depend on the
  rate at which the dose is delivered, at least
  within the range of dose rates to be encountered
• Usually there will not be any low-dose-rate
  limitation except that imposed by the lower dose
  limits already discussed

24
Integrating Dosimeters (cont.)

• One case of a genuine low-dose-rate limitation is
  reciprocity-law failure in photographic film
  dosimeters
• It occurs only with low-LET radiation (e.g., x
  rays or electrons) and is due to the necessity
  for several ionizing events to occur in a single
  grain of silver bromide to make it developable
• Low-LET radiation can only create one ion pair at
  a time in a small volume like a AgBr grain in
  photographic emulsion, and after a time the ions
  can recombine

25
Integrating Dosimeters (cont.)

• Thus the grain repairs itself at low enough dose
  rates, and never produces a latent image, that
  is, reaches a condition of developability
• Consequently it never contributes to the opacity
  of the film, which is the parameter used to
  measure the dose
• Biological damage by low-LET radiation exhibits
  similar time-repair characteristics

26
Integrating Dosimeters (cont.)

• The upper limit of dose-rate independence usually
  occurs when charged-particle tracks are created
  close enough together in space and time to allow
  the ions, electron-hole pairs, or active chemical
  products such as free radicals to interact
  between tracks
• In an ion chamber this is called general or
  volume ionic recombination
• Similar back reactions also occur in solid or
  liquid dosimeters, resulting in a loss of
  contribution to the reading r

27
Dose-Rate Range For Dose-Rate Meters

• It is desirable in dose-rate-measuring dosimeters
  that the reading r be proportional to the dose
  rate dDg/dt, or at least to be a single-valued
  function of it
• Jamming or paralysis of an instrument, causing
  it to read zero or a small response at high dose
  rates, as can occur in Geiger-Müller counters
  when the dead time overlaps and becomes
  continuous, is intolerable, especially in
  personnel monitoring meters

28
Dose-Rate Meters (cont.)

• The upper limit on the usable dose-rate range
  more usually takes the form of some kind of
  saturation of the reading vs. dose rate, due to
  ionic recombination or other results of track
  proximity
• The counting of two or more events as one when
  they occur temporally too close together in
  pulse-counting dosimeters also may cause
  saturation
• Other modes of saturation may also occur in
  various kinds of dosimeters

29
Dose-Rate Meters (cont.)

• In dose-rate measurements the response time
  constant, while not a limitation on the dose-rate
  range, is also an important consideration
• It is defined as the time it takes for the
  reading in a constant field to rise to within 1/e
  of its steady-state value, or to decay to 1/e of
  that value upon removal from the field
• A long time constant will cause a dose-rate meter
  to seek a mean reading value in a repetitively
  pulsed radiation field

30
Stability Before Irradiation

• The characteristics of a dosimeter should be
  stable with time until it is used
• That includes shelf life and time spent in situ
  until irradiated (e.g., worn by personnel if a
  health-physics monitoring dosimeter)
• Effects of temperature, atmospheric oxygen or
  humidity, light, and so on can cause a gradual
  change in the dose sensitivity or the
  instrumental background
• Photographic, chemical, or solid-state dosimeters
  are generally more susceptible to these
  influences than ion chambers or counters

31
Stability After Irradiation

• The latent reading in some types of integrating
  dosimeters (e.g., photographic, chemical,
  solid-state) may be unstable to some extent,
  suffering fading losses during the time
  interval between irradiation and readout
• Again, harsh ambient conditions of elevated
  temperature or humidity, direct sunlight or
  bright fluorescent lighting, and so on, may
  aggravate this effect

32
After Irradiation (cont.)

• If such time-dependent fading losses are
  unavoidable, it is advantageous to make them as
  reproducible as possible through standardization
  of laboratory technique so that a fading
  correction can be applied to the readings
• The following diagram outlines a protocol for
  measuring both the pre- and postirradiation
  instabilities of a group of identical dosimeters

33
Protocol for measuring pre- and postirradiation
instability effects in integrating dosimeters,
where a common dosimeter preparation time tp is
used
34
Energy Dependence Specification

• Generally speaking, the energy dependence of a
  dosimeter is the dependence of its reading r, per
  unit of the quantity it is supposed to measure,
  upon the quantum or kinetic energy of the
  radiation, as illustrated in the following
  diagram
• Pane A shows the reading r obtained from an
  imaginary dosimeter vs. some dosimetric quantity
  J (such as exposure, absorbed dose in water under
  CPE conditions, etc.)

35
Illustration of the general concept of energy
dependence
36
Energy Dependence (cont.)

• Let us suppose that the calibration curves shown
  have been obtained at the three different
  radiation energies (or qualities) E1, E2, and E3,
  as shown
• In this example the dosimeter response is assumed
  to be linear at energy E1, but becomes
  progressively more nonlinear at E2 and E3
• The corresponding plots of r/J vs. J are shown in
  B
• The energy-dependence curves for the two J-values
  J1 and J2 are given in C, and are seen to differ
  in this case for E gt E1

37
Energy Dependence (cont.)

• If only a single curve of r vs. J, for instance
  the E3 curve in A, were obtained at all energies,
  then the dosimeter would be energy-dependent at
  all J-levels
• For each value of J a horizontal line would
  result as in D, producing a family of such
  energy-dependent r/J curves for different
  J-values
• If the single energy-independent calibration
  curve were linear, then a common horizontal line
  would result in D, providing a single r/J value
  that would be applicable to all J-values and all
  radiation energies

38
Energy Dependence ? Dependence of the Dosimeter
Reading, per Unit of X- or ?-ray Exposure, on the
Mean Quantum Energy or Quality of the Beam, r/X
vs. E

• This usage is commonly found in health-physics
  personnel monitoring or any application in which
  exposure is commonly referred to
• 60Co ?-rays are frequently used as the reference
  energy for normalization, producing
  energy-dependence curves looking typically like
  the following figure for dosimeters made of
  materials higher than, equal to, and lower than
  air in atomic number

39
Typical energy-dependence curves in terms of the
response per unit exposure of x- or ?-rays
40
r/X vs. E (cont.)

• The rise in the top curve below about 0.1 MeV is
  caused by the onset of photoelectric effect in
  the sensitive volume of the dosimeter
• The flat maximum usually occurs at about 30-50
  keV, below which the curve may slowly descend due
  to attenuation in the dosimeter, onset of
  photoelectric effect in the reference material
  (air), and LET dependence of the dosimeter

41
r/X vs. E (cont.)

• The shape of the curves can be estimated by
• where the subscript g refers to the material
  in the dosimeters sensitive volume

42
r/X vs. E (cont.)

• This equation is based on the assumptions that
• The dosimeters sensitive volume is in
  charged-particle equilibrium, and the wall medium
  w g
• Attenuation is negligible in the dosimeter, both
  for incident photons and for fluorescence photons
  generated in the dosimeter
• A given absorbed dose to the sensitive volume
  produces the same reading, irrespective of photon
  energy (i.e., the dosimeter is LET-independent)

43
r/X vs. E (cont.)

• These assumptions are all questionable and may
  require suitable corrections for their failure in
  specific cases
• Matching the wall medium to the material in the
  dosimeters sensitive volume can satisfy
  assumption 1
• Substituting (?/?)g for (?tr/?)g in (?en/?)g has
  the effect of assuming the local reabsorption of
  all fluorescence photons generated in the
  sensitive volume, thus providing an upper limit
  for the influence of that effect

44
r/X vs. E (cont.)

• Attenuation of photons entering the dosimeter can
  be simply estimated by the straight-ahead
  approximation
• Failure of assumption 3 is referred to as LET
  dependence of a dosimeter
• The total effect of assumptions 2 and 3 may cause
  a perfectly air-equivalent dosimeter to decrease
  its reading at low photon energies, as indicated
  by the dashed curve

45
Energy Dependence ? Dependence of the Dosimeter
Reading per Unit of Absorbed Dose in Water on the
Photon or Electron-Beam Energy

• This usage is commonly found in radiotherapy
  literature, where absorbed dose always refers
  to water (or muscle tissue) unless otherwise
  specified
• Inasmuch as water and tissue are not identical,
  one should say which is meant, but since the
  differences are small (1) in the megavolt
  region, this choice frequently remains unspecified

46
Absorbed Dose in Water (cont.)

• For x rays the equation by which a homogeneous
  dosimeters energy dependence can be estimated is
• which depends on water as a reference material
  and 60Co ? rays for normalization
• The following figure illustrates this equation
  over the energy range from 1.25 to 50 MeV for LiF
  and bone-equivalent dosimeters

47
X-ray energy dependence estimated for a LiF and a
bone-equivalent dosimeter, in terms of response
per unit absorbed dose in water, normalized to
60Co ? rays
48
Absorbed Dose in Water (cont.)

• Because of the large secondary-electron ranges at
  these energies, this equation is only satisfied
  to the extent that TCPE is present, g wall w,
  and ? is the same in water as in the dosimeter
• Also, considerable x-ray attenuation occurs in
  the thick walls, and the size of the resulting
  dosimeter may be impractical anyway
• In radiotherapy dosimetry these problems are
  usually avoided by doing the measurements in a
  phantom, letting it comprise most of the
  dosimeters wall thickness

49
Absorbed Dose in Water (cont.)

• For electron beams of kinetic energy T (MeV), the
  corresponding equation for estimating energy
  dependence in terms of the dose to water,
  normalized to T 1 MeV, is

50
Absorbed Dose in Water (cont.)

• This approximation assumes that
• CPE exists for ?-rays entering and leaving the
  sensitive volume
• The incident electrons lose only a very small
  fraction of their energy in traversing the
  dosimeter
• Electron scattering is the same in g as in water
• The reading per unit dose to the dosimeters
  sensitive volume remains energy-independent
  (LET-independent)

51
Absorbed Dose in Water (cont.)

• Items 1 and 3 are suspect, while 2 and 4 are
  easily satisfied in the energy region above 1 MeV
• The following figure illustrates this equation
  for an air-cavity chamber, LiF, and
  bone-equivalent dosimeters
• Clearly the lack of polarization effect in the
  gaseous air relative to water causes a large
  energy dependence in that case
• Neither LiF nor a bone-equivalent dosimeter shows
  much dependence
• This illustrates the fact that collision
  stopping-power ratios are insensitive to electron
  energy unless the polarization effect is involved

52
Electron-energy dependence estimated for LiF, a
bone-equivalent dosimeter, and an air-filled ion
chamber, in terms of response per unit absorbed
dose in water, normalized to T 1 MeV
53
Energy Dependence ? Dependence of the Dosimeter
Reading per Unit of Absorbed Dose to the Material
in the Sensitive Volume Itself, on the Radiation
Energy or Beam Quality

• This kind of energy dependence is the most
  fundamental, inasmuch as it reflects the
  dosimeters energy efficiency, i.e., the ability
  of the dosimeter to give the same reading for the
  same amount of absorbed energy in its own
  sensitive volume, regardless of radiation type or
  quality

54
Dose to Sensitive Volume (cont.)

• It is often called LET dependence because it
  usually manifests itself as a change in the
  reading per unit dose as a function of
  charged-particle track density, due to ionic
  recombination or other second-order effects that
  depend on the proximity of radiation products to
  the dosimeter
• For example, ion chambers show such LET
  dependence only at radiation energies low enough
  (?10 keV) so the value of W for the gas is no
  longer constant but begins to rise

55
Energy Dependence Modification

• The energy dependence of a dosimeter can be
  changed to some extent, especially when the
  photoelectric effect is causing an overresponse
• In that case a medium-Z (e.g., tin), high-Z
  (e.g., lead), or composite filter can be
  incorporated into the design of the dosimeter
  capsule
• The thickness t can be chosen to correct the
  overresponse at about 100 keV, using e-?t as a
  guide

56
Energy Dependence Modification (cont.)

• Having determined the thickness needed to correct
  the response at 100 keV, it will be found to have
  been overcorrected at lower energies, the reading
  being essentially zero at 50 keV
• This can be rectified by perforating the filter
  using the unfiltered height of the maximum
  overresponse as a guide
• Experimental testing is of course required to
  verify and finally adjust the design

57
Energy Dependence Modification (cont.)

• This approach to the modification of energy
  dependence adds weight and size, and introduces
  directional dependence to the dosimeter reading,
  influenced by the geometrical design
• A sophisticated example of a design that
  minimizes the directional dependence is shown in
  the following diagram

58
A perforated filter to reduce the photoelectric
overresponse of a dosimeter while retaining
response to photoelectrons below ?80 keV. The
spherical design minimizes directional dependence.
59
Miscellany

• The configuration of a dosimeter sometimes is
  crucial to its use
• It may be necessary to simulate as closely as
  possible the geometry of the test object
• A thin plastic-film dosimeter might best measure
  the dose in a layer of biological cells
• Finally, small size of a dosimeter is of primary
  importance in its application in vivo in patients
  or test animals

60
Miscellany (cont.)

• A dosimeter needs a relevant calibration that is
  appropriate to the radiation type and quality, as
  well as to the quantity to be measured
• A calibration in terms of the dose to the
  dosimeters own sensitive volume is more
  generally applicable than other types of
  calibrations

61
Miscellany (cont.)

• If different types of radiation coexist in the
  field to be measured, attention must be paid to
  the relative sensitivity of the dosimeter to the
  different components
• It may be possible in specific cases to
  discriminate against one type of radiation (e.g.,
  by attenuation) so that another may be measured
  without competition

62
Miscellany (cont.)

• There are many dosimeters in the literature that
  may be useful but have never become commercially
  available
• Given a choice, a commercial system is usually
  easier to apply, so long as it satisfies other
  requirements
• Chemical dosimetry must still be done on the
  basis of local preparation, primarily because of
  shelf-life instability
• Calorimetric dosimetry has also eluded commercial
  manufacture, so far

63
Miscellany (cont.)

• The reusability of a dosimeter has several
  important implications
• Reusable dosimeters such as TLDs can be
  individually calibrated single-use dosimeters
  such as film badges cannot
• The latter can only be batch-calibrated by
  irradiating and measuring representative samples
• The precision of the measurements refers to the
  reproducibility of readings obtained from
  different members of the dosimeter batch after
  they have been given identical irradiations

64
Miscellany (cont.)

• The advantage of reusability of a dosimeter
  depends on how difficult or convenient it is to
  restore it to its original condition
• If it cannot be fully purged of the effects of an
  earlier dose, some of the advantage is lost
• A shift in sensitivity, for example, means that
  dosimeters must be segregated on the basis of
  prior history and recalibrated before reusing
• Economies that may be realized through reuse of
  dosimeters may thus be limited