Integrating Dosimeters I

1
Integrating Dosimeters I

• Thermoluminescence Dosimetry

2
The Thermoluminescence Process Phosphors

• The sensitive volume of a TLD consists of a small
  mass (1-100 mg) of crystalline dielectric
  material containing suitable activators to make
  it perform as a thermoluminescent phosphor
• The activators provide two kinds of centers
• Traps for the electrons and holes, which can
  capture and hold the charge carriers in an
  electrical potential well for usefully long
  periods of time
• Luminescence centers, located at either the
  electron traps or the hope traps, which emit
  light when the electrons and holes are permitted
  to recombine at such a center

3
Phosphors (cont.)

• The following energy-level diagram illustrates
  the TL process
• At left it shows an ionization event elevating an
  electron into the conduction band, where it
  migrates to an electron trap
• The hole left behind migrates to a hole trap
• At the temperature existing during irradiation,
  these traps should be deep enough in terms of
  potential energy to prevent the escape of the
  electron or hole for extended periods of time,
  until deliberate heating releases either or both
  of them

4
Energy-level diagram of the TL process (A)
ionization by radiation, and trapping of
electrons and holes (B) heating to release
electrons, allowing luminescence production.
5
Phosphors (cont.)

• At right in the diagram the effect of such
  heating is shown
• We will assume that the electron is released
  first, that is, that the electron trap in the
  phosphor is shallower than the hole trap
• The electron again enters the conduction band and
  migrates to a hole trap, which may be assumed
  either to act as a luminescence center or to be
  closely coupled to one
• In that case recombination is accompanied by the
  release of a light photon

6
TL Process Randall-Wilkins Theory

• The simple first-order kinetics for the escape of
  such trapped charge carriers at a temperature
  T(K) were first described by Randall and Wilkins
  using the equation
• where p is the probability of escape per unit
  time (s-1), ? is the mean lifetime in the trap, ?
  is called the frequency factor, E is the energy
  depth of the trap (eV), and k is Boltzmans
  constant

7
Randall-Wilkins Theory (cont.)

• It is evident from this equation that, on the
  assumption of constant values for k, E, and ?,
  increasing T causes p to increase and ? to
  decrease
• Thus if the temperature T is scanned upward
  linearly vs. time, starting at room temperature,
  an increase in the rate of escape of trapped
  electrons will occur, reaching a maximum at some
  temperature Tm followed by a decrease as the
  supply of trapped electrons is gradually exhausted

8
Randall-Wilkins Theory (cont.)

• Assuming that the intensity of light emission is
  proportional to the rate of electron escape, a
  corresponding peak in TL brightness will also be
  observed at Tm
• This is called a glow peak, as shown in the
  following diagram
• The presence of more than one trap depth E gives
  rise to plural glow peaks, which may be
  unresolved or only partially resolved from one
  another in the glow curve

9
A TL glow curve vs. temperature that results from
the gradual heating of an irradiated
thermoluminescent phosphor that contains two trap
depths.
10
Randall-Wilkins Theory (cont.)

• The value of Tm is related to the linear heating
  rate q (K/s) by the following relation from R-W
  theory
• which simplifies to the following approximate
  relationship on the assumption of ? 109/s and q
  1 K/s
• hence Tm 216 C for E 1 eV

11
Randall-Wilkins Theory (cont.)

• Tm increases gradually with q, so that Tm 248
  C at q 5 K/s, and 263C at 10 K/s for the same
  values of ? and E
• The light-emission efficiency may be found to
  decrease with increasing temperature by a process
  called thermal quenching
• Thus at higher heating rates some loss of total
  light output may be noticed
• Aside from this effect, varying the heating rate
  leaves total light output constant, preserving
  the area of glow curves in terms of brightness
  vs. time

12
Glow curves vs. time obtained with a CaF2Mn TL
dosimeter at eight linear heating rates. The
dose to the phosphor was adjusted to be inversely
proportional to the heating rate in each case.
13
Glow curves vs. temperature recorded
simultaneously with the previous curves
14
Randall-Wilkins Theory (cont.)

• It can be seen that employing the light sum
  rather than the height of the glow peak as a
  measure of the absorbed dose is less subject to
  errors caused by fluctuations in the heating rate
• However, the peak height may be used if the
  heating rate is very stable, and this may be
  advantageous in measuring small doses for which
  the upper limb of the glow curve rises due to IR
  and spurious effects

15
TL Process Trap Stability

• The usefulness of a given phosphor trap (and its
  associated TL glow peak) for dosimetry
  applications depends on its independence of time
  and ambient conditions
• If the traps are not stable at room temperature
  before irradiation, but migrate through the
  crystal and combine with other traps to form
  different configurations, changes in radiation
  sensitivity and glow-curve shape will be observed

16
Trap Stability (cont.)

• LiF (TLD-100) is such a phosphor, requiring
  special annealing (e.g., 400 C for 1 h, quick
  cooling, then 80 C for 24h) to minimize
  sensitivity drift
• In general TL phosphors give best performance as
  dosimeters if they receive uniform, reproducible,
  and optimal (depending on the phosphor) heat
  treatment before and after use

17
Trap Stability (cont.)

• The inability of traps to hold charge carriers at
  ambient temperature after irradiation is called
  trap leakage, and of course it becomes greater if
  the ambient temperature is increased
• As a rule of thumb, in typical TLD phosphors a
  glow peak at ?200-225 C is ordinarily found to
  have small enough leakage for practical
  room-temperature dosimetry, having a half-life of
  trapped charge carriers measured in months or
  years

18
Trap Stability (cont.)

• A glow peak at ?150 ?C usually has a half-life
  only of the order of a few days, while a 100 ?C
  peak decays in a matter of hours
• Although short-term dosimetry may still be
  possible with rapidly leaking traps, careful
  timing control is required

19
Trap Stability (cont.)

• Higher-temperature traps than 200-225 C are
  usually even more stable, and would be
  advantageous for dosimetry except for the
  existence of two competing effects
• Heat (Infrared) Signal. As the phosphor and its
  heating tray rise in temperature, the
  short-wavelength tail of the blackbody radiation
  begins to extend into the visible region and
  produce a non-dose-related response in the PMT
  used for measuring the TL light output
• Spurious TL Signal. The combined effects of
  adsorbed gases, humidity, dirt, and mechanical
  abrasion of the phosphor surface tend to produce
  a spurious TL emission

20
Trap Stability (cont.)

• Flowing an oxygen-free inert gas such as N2 or Ar
  through the space above the heater pan, thus
  surrounding the phosphor during the TL readout
  process, allows the stored energy due to these
  surface effects to be released without light
  emission
• Thus N2 flow is often used to reduce spurious
  background TL readings, especially when small
  doses are to be measured

21
TL Process Intrinsic Efficiency of TLD Phosphors

• Only a small part of the energy deposited as
  absorbed dose in a TLD phosphor is emitted as
  light when the substance is heated
• The ratio (TL light energy emitted per unit
  mass)/(absorbed dose) is called the intrinsic TL
  efficiency
• This has been measured as 0.039 in LiF
  (TLD-100), 0.44 in CaF2Mn, and 1.2 in CaSO4Mn

22
Intrinsic Efficiency (cont.)

• The energy budget in LiF (TLD-100) has been
  estimated to account for the loss of the missing
  99.96 of the energy deposited by ionizing
  radiation that ultimately goes into heat
  production
• It should not be surprising that TLDs must be
  used under reproducible conditions to obtain
  consistent results, considering that such a small
  fraction of the absorbed dose energy is relied
  upon as a measure of the entire dose

23
TLD Readers

• The instrument used to heat a TLD phosphor, and
  to measure the resulting TL light emitted, is
  simply called a TLD reader
• Its design principle is shown schematically in
  the following diagram
• The TLD phosphor to be measured is placed in the
  heater pan at room temperature, and heated while
  the emitted light is measured with a
  photomultiplier

24
Schematic diagram of a typical TLD reader
25
TLD Readers (cont.)

• Heating of the sample may be done by means of an
  resistively heated pan as shown, or by preheated
  N2 gas, or by an intense light spot from a
  projection lamp or laser, or other suitable means
• Often the heating program may be more complicated
  than simply linear vs. time

26
TLD Readers (cont.)

• One typical scheme is to heat the phosphor
  rapidly through the unstable-trap region,
  ignoring light emission until some preset
  temperature is reached
• Then the phosphor is either heated linearly or
  abruptly raised to a temperature sufficient to
  exhaust the glow peak of dosimetric interest,
  while measuring the emitted-light sum, which is
  displayed as a charge or dose reading
• Finally, the phosphor may be heated further to
  (say) 400 C to release any remaining charge from
  deeper traps, while ignoring any additional light
  emission

27
Typical programmed readout cycle in a modern TLD
reader, consisting of a preheat period without
light integration to discriminate against
unstable low-T traps, a read period spanning
the emission of the part of the glow curve to be
used as a measure of the dose, an anneal period
during which the remainder of the stored energy
is dumped without light integration, and the
cooling-down period after the heater-pan power is
turned off.
28
TLD Readers (cont.)

• Heating-program reproducibility is vital in
  achieving reproducible TL dosimetry
• In addition, one must provide constant light
  sensitivity so that a given TLD light output
  always gives the same reading
• This requires constant PM-tube sensitivity, and a
  clean optical system
• A constant light source with an appropriate
  spectrum may be built into the reader to
  substitute for a TLD as a check on the constancy
  of light sensitivity

29
TLD Phosphors

• TLD phosphors consist of a host crystalline
  material containing one or more activators that
  may be associated with the traps, luminescence
  centers, or both
• Amounts of activators range from a few parts per
  million up to several percent in different
  phosphors
• The host crystal almost entirely determines the
  radiation interactions, since the activators are
  usually present in such small amounts

30
TLD Phosphors (cont.)

• Many different TLD phosphors have been studied
  and reported in the literature
• A few representative ones are listed in the
  following table
• The following curves show their glow curves at a
  heating rate of 40 C/min and their approximate
  light output vs. 60Co ?-ray exposure

31
Characteristics of TL phosphors
32
Glow curves vs. temperature (upper scale) and
time (lower scale) for four TL dosimetry
phosphors. Heating rate 40 C/min. The
amplitudes are arbitrary.
33
Glow-peak-area response vs. 60Co ?-ray exposure
for several TL phosphors
34
TLD Phosphors (cont.)

• All the phosphors show some degree of
  supralinearity of response, this effect being
  most pronounced in lithium borate
• In CaF2Mn the rise is only ?4 in the
  neighborhood of 104 R, which is too small to be
  seen on this figure
• This supralinearity may be due to the increased
  availability of luminescence centers when the
  charged-particle tracks become closer together,
  or to radiation-induced trap formation, or to
  other causes
• At large enough doses all TL phosphors either
  saturate in their output as all available traps
  become filled, or maximize and then decrease due
  to radiation damage of the phosphor

35
TLD Forms

• The most common forms of TLD are
• Bulk granulated, sieved to 75-150- ?m grain size
• Compressed pellets or chips, usually 3.2 mm
  square by 0.9 mm thick
• A Teflon matrix containing 5 or 50 by weight of
  lt40-?m grain-size TLD powder
• A TLD pellet fastened on an ohmic heating element
  in an inert-gas-filled glass bulb
• Single-crystal plates, cleaved from a larger
  grown crystal boule
• Powder enclosed in plastic tubing that can be
  heated

36
Calibration of TLDs Form

• Solid TLD chips or Teflon-TLD discs are the
  preferred forms of the phosphor for most
  applications
• They can be individually identified and
  calibrated, they do not require containment, and
  they are flat, so that they can be oriented
  perpendicular to a monodirectional radiation
  beam, thus presenting a known cross-sectional area

37
Calibration of TLDs Basis for Calibration

• Most TL phosphors have some threshold dose level
  below which the TL light output per unit mass is
  proportional to the absorbed dose to the
  phosphor, provided that
• the LET of the radiation remains low or
  practically constant, and
• the phosphor sensitivity is kept constant by
  using reproducible annealing procedures

38
Basis for Calibration (cont.)

• Assuming TL-reader constancy, and negligible
  attenuation of light in escaping from the
  phosphor during heating, one can then say that
  the same TL reading will result from a given
  average absorbed phosphor dose in a TL dosimeter,
  regardless of the spatial distribution of
  absorbed dose within it, so long as the dose
  throughout remains in the linear range

39
Basis for Calibration (cont.)

• The practical consequence of this is that a 60Co
  ?-ray calibration in terms of average phosphor
  dose in the TL dosimeter can then be used as an
  approximate calibration for all low-LET
  radiations, including x-rays, ?-rays, and
  electron beams of all energies above 10 keV,
  even if they deposit dose nonuniformly in the
  dosimeter

40
Basis for Calibration (cont.)

• Relating the phosphor dose so measured to the
  dose in a similar mass of tissue hypothetically
  substituted for the TLD requires a separate step
  based on cavity theory
• For the simplest (B-G) case of a thin TLD and
  very penetrating electrons, the dose ratio
  Dtiss/DTLD is proportional to the mass collision
  stopping-power ratio, (dT/?dx)c,tiss/(dT/?dx)c,TLD
  evaluated at the mean electron kinetic energy

41
Basis for Calibration (cont.)

• If the incident radiation beam is completely
  stopped by the TLD, then the incident energy
  fluence can be derived
• The 60Co calibration (under TCPE conditions)
  gives the TL reading per unit of average phosphor
  dose
• Multiplying that dose by the mass of the TLD chip
  allows relating the TL reading to a given
  integral dose, or energy spent in the chip

42
Basis of Calibration (cont.)

• If the chip area presented to the beam is A (m2),
  its mass is m (kg), and the 60Co ?-ray
  calibration factor is kCo (DTLD/r)Co
  Gy/(scale division), where r is the TLD
  reading, then the energy fluence of a stopped
  beam is given by

43
Calibration of TLDs 60Co ?-ray Calibration

• For a free-space 60Co ?-ray exposure X (C/kg) at
  the point to be occupied by the center of the TLD
  in its capsule, the average absorbed dose in the
  TLD, in grays, under TCPE conditions is given by
• where a is a correction for broad-beam ?-ray
  attenuation in the capsule wall plus the half
  thickness of the TLD

44
60Co ?-ray Calibration (cont.)

• For a LiF TLD chip in a Teflon capsule 2.8 mm in
  thickness (for TCPE) the average dose calculated
  from this equation is approximately

45
60Co ?-ray Calibration (cont.)

• If the resulting TLD reading is r scale
  divisions, then the calibration factor is kCo
  (DTLD/r)Co, which applies at the dose value used
  in calibration and throughout the linear
  response-vs.-dose range
• For all low-LET radiations, the average absorbed
  dose in the TLD can then be obtained from the
  observed TLD reading r by

46
60Co ?-ray Calibration (cont.)

• For higher-LET radiations than 60Co ?-rays, TLDs
  typically show some variation in efficiency, and
  consequently a reciprocal change in the low-LET
  calibration factor kCo
• The following diagram gives the results of
  measurements for lithium fluoride, lithium
  borate, and beryllium oxide
• Their LET dependence is seen to be quite
  different, with lithium borate coming closest to
  constancy

47
LET response of BeO, Li2B4O7Mn, and LiF. The
curves give values of kCo/kLET as a function of
LET in water, in keV/?m.
48
60Co ?-ray Calibration (cont.)

• The following diagrams show the photon energy
  dependence of lithium fluoride and lithium borate
• Curves A were obtained from
• showing the CPE dose in the phosphors per
  unit of exposure, normalized to 1.25 MeV (60Co)

49
Thermoluminescent response of LiF per roentgen
and per rad for photon energies from 6 to 2800 keV
50
60Co ?-ray Calibration (cont.)

• Curves B show the TL response per unit exposure,
  and curves C the TL response per unit of absorbed
  dose in the phosphors
• Thus curves C represent the LET-dependence of the
  TL efficiency relative to 60Co, or kCo/kLET

51
Thermoluminescent response of Li2B4O7Mn per
roentgen and per rad for photon energies from 6
to 2800 keV
52
TLD Advantages

• Specific characteristics vary from phosphor to
  phosphor, and are available from the manufacturer
• We will describe the most widely used, LiF
  (TLD-100), where specifics are referred to
• Wide useful dose range, from a few millirads to
  103 rad linearly, plus another decade (103-104)
  of supralinear response vs. dose.
• Dose-rate independence, 0-1011 rad/s.

53
TLD Advantages (cont.)

• Small size passive energy storage. Small TLDs
  can be used as dose probes with little
  disturbance of the radiation field in the medium.
  They can be made thin enough to approach B-G
  conditions at high energies, but TCPE is easier
  to achieve because of their condensed state.
• Commercial availability. TLDs and readers are
  available from a number of suppliers.

54
TLD Advantages (cont.)

• Reusability. By employing appropriate annealing
  procedures to release all the prior stored
  energy, and checking for possible alteration in
  radiation sensitivity, TLD phosphors can normally
  be reused many times until they become
  permanently damaged by radiation, heat or
  environment. Thus it is feasible to calibrate
  individual dosimeters.
• Readout convenience. TLD readout is fairly rapid
  (lt 30 s) and requires no wet chemistry.

55
TLD Advantages (cont.)

• Economy. Reusability usually reduces cost per
  reading.
• Availability of different types with different
  sensitivities to thermal neutrons.
• Automation compatibility. For large
  personnel-monitoring operations automatic readers
  are available.
• Accuracy and precision. Reading reproducibility
  of 1-2 can be achieved with care. Comparable
  accuracy may be obtained through individual
  calibration and averaging of several dosimeters
  in a cluster, since their volume is small.

56
TLD Disadvantages

• Lack of uniformity. Different dosimeters made
  from a given batch of phosphors still show a
  distribution of sensitivities, and different
  batches of phosphor generally have different
  average sensitivities. Thus individual dosimeter
  calibration, or at least batch calibration, is
  necessary for acceptable accuracy and precision.

57
TLD Disadvantages (cont.)

• Storage instability. TLD sensitivity can vary
  with time before irradiation in some phosphors,
  as a result, for example, of gradual
  room-temperature migration of trapping centers in
  the crystals. Controlled annealing of the TLDs
  can usually restore them to some reference
  condition again.

58
TLD Disadvantages (cont.)

• Fading. Irradiated dosimeters do not permanently
  retain 100 of their trapped charge carriers.
  This results in a gradual loss of the latent TLD
  signal. This must be corrected for, especially
  in applications (e.g., personnel monitoring) that
  involve long time delays.

59
TLD Disadvantages (cont.)

• Light sensitivity. TLDs all show some
  sensitivity to light especially UV, sunlight,
  or fluorescent light. This can cause accelerated
  fading, or leakage of filled traps. Or it can
  produce ionization and the filling of traps, thus
  giving rise to spurious TL readings.
• Spurious TL. Scraping or chipping of TLD
  crystals or surface contamination by dirt or
  humidity also can cause spurious TL readings.
  However, the presence of an oxygen-free inert gas
  during readout suppresses these signals.

60
TLD Disadvantages (cont.)

• Memory of radiation and thermal history. The
  sensitivity can be either increased or decreased
  after receiving a large dose of radiation and
  undergoing readout. Additional annealing
  procedures are necessary to restore the original
  sensitivity, if possible. It may be more
  economical to throw away the phosphor after a
  single use, especially for large doses.

61
TLD Disadvantages (cont.)

• Reader instability. TLD readings depend on the
  light sensitivity of the reader as well as on the
  heating rate of the phosphor. Thus reader
  constancy is difficult to maintain over long time
  periods.
• Loss of a reading. The measurement of light out
  of a TLD erases the stored information. Unless
  special provision is made (e.g., a spare TLD),
  there is no second chance at getting a reading.
  Reader malfunction can lose a reading.