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CHAPTER 10. Overview of Radiation Detectors and Measurements
Jenghwa Chang, Ph.D. D.A.B.R.
Associate Professor Radiation Medicine, Hofstra Northwell School of Medicine at Hofstra University
Associate Adjunct Professor Physics and Astronomy, Hofstra University
J. Chang, PhD, DABR
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Dosimeter
• Dosimeter: a detector (det), when placed in medium (med) and irradiated by a beam of ionizing radiation of quality 𝑄, can provide
– A reading ഥ𝑀det,𝑄 from the exposure of its radiation
sensitive volume (RSV) to 𝑄, that is
– A measure of the generic quantity 𝒢med,𝑄 𝑃 at position P
in the medium deposited
– “ ഥ ” indicates that it is an averaged value
– ഥ𝑀det−raw,𝑄 is the detector reading before corrected for
the effect of the environment
– Generic quantity ҧ𝒢 can be dose, kerma, exposure…
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= det
wall
med
ҧ𝒢med,𝑄
ҧ𝒢det,𝑄
ഥ𝑀det,𝑄
RSV +
𝑄
Detector Response and Calibration Coefficient
• Let ҧ𝒢det,𝑄 denote the corresponding quantity in the sensitive material
of the detector, where “ ഥ ” indicates this is necessarily a mean value over the RSV.
• Detector response (or sensitivity): 𝑅𝒢,det,𝑄 = ഥ𝑀det,𝑄 ҧ𝒢det,𝑄
• Calibration coefficient: 𝑁𝒢,det,𝑄 = 1
𝑅𝒢,det,𝑄 =
ҧ𝒢det,𝑄 ഥ𝑀det,𝑄
• Example: air-filled ionization chamber
– ഥ𝑀det,𝑄: integrated charge corrected for recombination, temperature, pressure…
– ҧ𝒢det,𝑄 in RSV is the absorbed dose to air, ഥ𝐷air ≡ ഥ𝐷det
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Conversion factor 𝑓
• The final, essential dosimetry step is to relate ҧ𝒢det,𝑄 to the desired
quantity in the medium, 𝒢med,𝑄 𝑃 in the absence of the detector:
𝒢med,𝑄 𝑃 = ҧ𝒢det,𝑄𝑓med, det, 𝑄 𝒢det→𝒢med
• The conversing factor 𝑓 med, det,𝑄 𝒢det→𝒢med
– Depends on
• Medium
• Detector: complete detector including RSV and all material surrounding RSV (e.g., the wall)
• Radiation quality 𝑄
– “𝒢det → 𝒢med”: the conversion is between 𝒢det in RSV and 𝒢med in medium
– Is determined using the cavity theory.
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Absolute Dosimetry
• An absolute dosimetry: the quantity of interest is determined from fundamental principles consistent with the definition of the quantity and realized with a primary measurement standard.
• Example: absorbed dose to air in a primary ionization chamber:
ഥ𝐷air = 𝑞 Τ𝑊air 𝑒
𝑚air =
𝑞 Τ𝑊air 𝑒
𝑣𝜌air
• It may need some calibration not involving radiation (e.g., electrical-heating for a calorimetric dosimeter)
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Absoluteness
• Three types of dosimeters are generally regarded as being capable of absoluteness:
– Calorimetric dosimeters
– Ionization chambers
– Fricke ferrous sulfate dosimeters
• Calorimetric dosimeter has the fundamental advantage of measuring directly the heat to which the absorbed dose degrades, not relying on any conversion coefficient, e.g., to ionization or to chemical yield
• The absoluteness of a dosimeter is independent of its precision or accuracy
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Reference Dosimetry
• Reference dosimetry: absorbed dose (𝐷) is determined at the user’s facility using a reference detector calibrated at a standards laboratory under the same reference conditions. For example:
– Reference conditions: 𝐷 to water (𝑤) in a beam quality 𝑄 for 10×10 cm2 field
– The user’s chamber is calibrated at the standards lab: 𝑁𝐷,𝑤,𝑄 = 𝐷w,𝑄 lab
𝑀w,𝑄 lab
– At user’s facility this reference chamber is used to measure 𝐷 to 𝑤 :
𝐷w,𝑄 user = 𝑀w,𝑄
user𝑁𝐷,𝑤,𝑄
• Calibration provides traceability to a standardization laboratory thus minimizing errors that may go undetected
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Relative Dosimetry
• Relative dosimetry: 𝒢med,𝑄 is determined using relevant ratios and/or
appropriate corrections of measurements:
– Between non-reference to reference conditions. For example:
– At 𝑧max and 100 MU, use reference dos 𝐷w,𝑄 user 𝑓ref for 𝑓ref = 10 × 10 cm
2 to
determine 𝐷w,𝑄 user 𝑓non−ref of the same MU for 𝑓non−ref = 5 × 5 cm
2
– 𝐷w,𝑄 user 𝑓non−ref = 𝐷w,𝑄
user 𝑓ref 𝑀w,𝑄 user 𝑓non−ref
𝑀w,𝑄 user 𝑓ref
– If 𝐷w,𝑄 user 10 × 10 cm2 =100 cGy, 𝑀w,𝑄
user 10 × 10 cm2 = 10 nC, and
– 𝑀w,𝑄 user 5 × 5 cm2 = 9.2 nC ⇒ 𝐷w,𝑄
user 5 × 5 cm2 = 100 9.2
10 = 92 (cGy)
• Assuming the constancy of the calibration coefficient with field size.
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General Characteristics and Desirable Properties of Detectors
• Linearity
• Reproducibility
• Dose range
• Dose rate range
• Stability
• Energy dependence
• Miscellany (configuration, relevant calibration, reusability, etc.)
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Linearity
• ഥ𝑀det,𝑄 should ideally be linearly
proportional to ҧ𝒢det,𝑄 in the RSV:
𝑅𝒢,det,𝑄 = ഥ𝑀det,𝑄 ҧ𝒢det,𝑄
• However, outside a certain dose range or interval, non-linearity may occur.
• The linearity interval and the non- linear behavior outside this interval depend on the physical characteristics of the dosimeter.
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Figure 10. Response characteristics of two dosimeters. At low doses, curve A first exhibits linearity, then supra- linear behavior, and finally saturation. Curve B first exhibits linearity and then saturation at high doses. (From Izewska and Rajan (2005). Reproduced by the permission of IAEA.)
Reproducibility
• Reproducibility (or Precision):
– Spread of measurements due to fluctuations in
– Instrumental characteristics, ambient conditions…, and stochastic nature of radiation fields
– Estimated from repeated measurements
– Stated in terms of the standard deviation (SD)
– High precision indicates small SD
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• A high-precision instrument is capable of excellent measurement reproducibility if properly used
Figure 10,2 Diagram illustrating the concepts of accuracy and reproducibility (or precision).
Accuracy
• Accuracy of measurements
– Proximity of their expectation value to the true value of the quantity being measured
– A measure of the collective effect of the errors that influence the measurements
– Relevant to absolute dosimeter only
– Can not be determined from the data itself
• In experiments that are limited to relative measurements, only the precision is important
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Figure 10,2 Diagram illustrating the concepts of accuracy and reproducibility (or precision).
Dose range: sensitivity
• To be useful, a dosimeter must have adequate dose sensitivity (𝑑𝑀/𝑑ഥ𝐷det) throughout the dose range to be measured
• A constant dose sensitivity throughout the range provides a linear response (𝑑𝑀/𝑑ഥ𝐷det is constant), and is desirable for ease of calibration and interpretation
• Knowing the function 𝑀(ഥ𝐷det), even nonlinear but single- valued, may be acceptable, though it requires that the calibration be carried out at multiple doses to provide a calibration curve
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Dose range: lower limit
• The lower limit of the useful dose range may be imposed by the instrumental background or zero-dose reading 𝑀 = 𝑀0, observed when ഥ𝐷det = 0 (sometimes referred to as “spurious response”)
• Examples of 𝑀0 include charge readings due to ion-chamber insulator leakage, and TLD readings resulting from response of the reader to infrared light emission by the dosimeter heater
• The instrumental background 𝑀0 should be subtracted from any dosimeter reading
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Dose range: lower limit
• 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
• If 𝑠0 ഥ𝑀0 is the SD of the average of the background readings ഥ𝑀0, and 𝑠 ഥ𝑀 is the SD of the average of a group of radiation
readings ഥ𝑀, then the S.D. of the net radiation reading ഥ𝑀 − ഥ𝑀0 is given by
𝑠net = 𝑠 2 + 𝑠0
2
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Dose range: lower limit
• 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 𝑴 for the dose to be measured ഥ𝐷det
• Readable value 𝑀 is typically considered to be 10% of full scale on analogue instruments, or contain more than three significant figures on digital readouts
• The stochastic nature of the radiation field itself will ultimately limit the reproducibility of a low-dose measurement
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Dose range: Upper limit
• The upper limit of the useful dose range of a dosimeter may be imposed simply by external instrumental limitations (reading off scale of an electrometer)
• Alternatively, an inherent limit may be imposed by the dosimeter itself due to:
a) Exhaustion of the supply of atoms, molecules, or solid-state entities (“traps”) being acted upon by the radiation to produce the reading
b) Competing reactions by radiation products, for example in chemical dosimeters
c) Radiation damage to the dosimeter (e.g., discoloration of light-emitting dosimeters, damage to electrical insulators)
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Dose range: Upper limit
• Usually the upper limit of the dose range is manifested by a decrease in the dose sensitivity (𝑑𝑀/ 𝑑ഥ𝐷det) to an unacceptable value
• It may be reduced to zero, or to a negative value, which causes the dose-response function to become double-valued
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Figure 10.3 Illustration of a double-valued dose- response function resulting from a decrease in the dosimeter sensitivity at high doses
Dose-rate range: integrating dosimeters
• If a dosimeter is used for measuring the time-integrated dose (not the dose rate), then its reading should not depend on the dose rate
• Usually the low-dose-rate limitation is imposed by the lower dose limits of the dosimeter
• One case of a genuine low-dose-rate limitation is reciprocity-law failure in photographic film dosimeters
– It occurs only with low-LET radiation (x rays or electrons)
– It is due to the necessity for several ionizing events to occur in a single grain of silver bromide to make it developable
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Dose-rate range: integrating dosimeters
• 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 reactions can occur in solid or liquid dosimeters, resulting in a loss of contribution to the reading 𝑀
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Dose-rate Meters
• The dose-rate-measuring dosimeters should provide reading 𝑀 proportional to the dose rate 𝑑ഥ𝐷det/𝑑𝑡, or at least to be a single- valued function of it
• The upper limit on the usable dose-rate range is usually imposed by saturation such as ionic recombination, etc.
• The counting of two or more events occurring close together as one in pulse-counting dosimeters may close together cause saturation
• The response time constant characterizes the capability of the dosimeter to resolve separate pulses and possibly measure the pulse shape in a pulsed radiation field
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Stability: before irradiation
• Properties 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 as 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
• Protection from deleterious influence are often designed into casing or packaging.
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Stability: after irradiation
• After irradiation the latent reading in some types of integrating dosimeters (e.g., photographic, chemical, solid-state) may be unstable, suffering “fading” losses during the time interval between irradiation and readout
• Harsh ambient conditions may aggravate this effect
• If 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
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Measuring pre- and post-irradiation instability in integrating dosimeters
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• Figure 10.4 Protocol for measuring pre- and post- irradiation instability effects in integrating dosimeters, where a common dosimeter 'preparation' time tp is used.
• Group 1 dosimeters are promptly irradiated at time ti then promptly read out at time tr.
• Groups 2A, 3A, etc are irradiated promptly but stored for various times before being read out at time tr.
• Groups 2B, 3B, etc are stored for various times before being irradiated, then promptly read out.
• The A groups indicate the post-irradiation instability, and the B groups the pre-irradiation instability.
Energy dependence of a dosimeter
• Energy dependence: dependence of dosimeter reading 𝑀, per unit of the dosimetric quantity 𝑆 it is supposed to measure, upon the quantum or kinetic energy 𝐸 of the radiation
– Energy independent: 𝑀/𝑆 not a function of 𝐸
– Energy dependence: 𝑀/𝑆 varies with 𝐸
• Can be estimated by backward calculation of cavity theory:
– Thick wall: 𝑆 ⟹ 𝑊 ⟹ 𝑀
– Thin wall: 𝑆 ⟹ 𝑀
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= det
wall
med
ҧ𝒢med,𝑄
ҧ𝒢det,𝑄
ഥ𝑀det,𝑄
RSV +
𝑄
General concept of energy dependence
• A: The calibration curves obtained at three different energies
• B: 𝑀/𝑆 vs. 𝑆 for these three energies
• C: 𝑀/𝑆 vs. 𝐸 energy-dependence curves for two values 𝑆1 and 𝑆2. They are different for 𝐸 > 𝐸1
• D: Energy-independent dosimeter where 𝑀/𝑆 vs. 𝐸 is the same for all energies
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Figure 10.5 Illustration of the general concept of energy
dependence of a dosimeter that provides a M reading M
as a result of an irradiation that is quantified in terms of
the quantity (representing air kerma, absorbed dose to water, etc.). See discussion in the text.
Energy dependence in health physics
• Detector response per unit air kerma 𝑀/𝐾air, as a function of beam quality 𝑘
• 60Co -rays used as the reference quality for normalization
• Energy-dependence curves for ҧ𝑍det >, =, and < ҧ𝑍air
• Shape of curves can be estimated by:
𝑀/𝐾air 𝑘
𝑀/𝐾air 60𝐶𝑜 ≈
ΤΤ𝜇𝑒𝑛 𝜌 det Τ𝜇𝑒𝑛 𝜌 air 𝑘 ΤΤ𝜇𝑒𝑛 𝜌 det Τ𝜇𝑒𝑛 𝜌 air 60𝐶𝑜
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Figure 10.6 Typical curves showing the detector response
M/Kair relative to that at 60Co rays, as a function of photon energy, for detector materials with atomic number greater or
less than that of air. Estimated from Eq. (10.9) using Τ𝜇𝑒𝑛 𝜌 med values from the electronic Data Tables.
Assumptions energy dependence in health physics
𝑀/𝐾air 𝑘
𝑀/𝐾air 60𝐶𝑜 ≈
ΤΤ𝜇𝑒𝑛 𝜌 det Τ𝜇𝑒𝑛 𝜌 air 𝑘 ΤΤ𝜇𝑒𝑛 𝜌 det Τ𝜇𝑒𝑛 𝜌 air 60𝐶𝑜
= ΤΤ𝜇𝑒𝑛 𝜌 det,𝑘 Τ𝜇𝑒𝑛 𝜌 det,60𝐶𝑜
ΤΤ𝜇𝑒𝑛 𝜌 air,𝑘 Τ𝜇𝑒𝑛 𝜌 air,60𝐶𝑜
• This equation is based on the assumptions:
1. The dosimeter’s sensitive volume is in charged-particle equilibrium, and the wall material equivalent to the detector material
2. Photon attenuation is either negligible in the dosimeter or is very similar in the dosimeter and in the medium it displaces, both for incident photons and fluorescence photons generated in the dosimeter
3. A given absorbed dose to the sensitive volume produces the same reading, irrespective of photon energy (i.e., the dosimeter is LET-independent)
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Energy dependence in radiation therapy: absorbed dose
• Dependence of the dosimeter reading per unit of absorbed dose in water on the photon or electron-beam energy
• “Absorbed dose” always refers to water (or muscle tissue) unless otherwise specified
• In MV region the differences between water and tissue are small (~1%)
• Assumptions:
– PCPE in the detector
– Detector wall is RSV equivalent
– for PCPE is approximately the same in water as in the dosimeter
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Energy dependence in RT: photon absorbed dose
• For x rays, the homogeneous dosimeter’s energy dependence for reference conditions of water and 60Co rays can be estimated as 𝑀/𝐷w 𝑘
𝑀/𝐷w 60𝐶𝑜 ≈
ΤΤ𝜇𝑒𝑛 𝜌 det Τ𝜇𝑒𝑛 𝜌 w 𝑘 ΤΤ𝜇𝑒𝑛 𝜌 det Τ𝜇𝑒𝑛 𝜌 w 60𝐶𝑜
= ΤΤ𝜇𝑒𝑛 𝜌 det,𝑘 Τ𝜇𝑒𝑛 𝜌 det,60𝐶𝑜
ΤΤ𝜇𝑒𝑛 𝜌 w,𝑘 Τ𝜇𝑒𝑛 𝜌 w,60𝐶𝑜
• This equation can be used over the energy range from 1.25 to 50 MeV for LiF and bone-equivalent dosimeters.
• PCPE requires wall thickness that would produce
– Considerable x-ray attenuation, and
– Just be impractical for the size of the resulting dosimeter
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Energy dependence in RT: photon absorbed dose
• 𝑀/𝐷w 𝑘
𝑀/𝐷w 60𝐶𝑜 ≈
ΤΤ𝜇𝑒𝑛 𝜌 det Τ𝜇𝑒𝑛 𝜌 w 𝑘 ΤΤ𝜇𝑒𝑛 𝜌 det Τ𝜇𝑒𝑛 𝜌 w 60𝐶𝑜
• Photon energy dependence for a silicon diode, a LiF TLD, and a diamond detector in terms of response per unit absorbed dose in water, normalized to 60Co rays
• The rise at higher energies results from increase in pair production for higher Z materials.
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Figure 10.7 Photon energy dependence for a silicon diode, a LiF
TLD, and a diamond detector in terms of response per unit
absorbed dose in water, normalized to 60Co rays. Estimated from Eq. (1 0.1 0) using Τ𝜇𝑒𝑛 𝜌 med values from the electronic Data Tables.
Energy dependence in RT: electron absorbed dose
• For mono-energetic electron beams of 𝐸 MeV, equation for estimating energy dependence of the dose to water, normalized to energy 𝐸0, is
𝑀/𝐷w 𝐸
𝑀/𝐷w 𝐸0 ≈
ΤΤ𝑆el 𝜌 det Τ𝑆el 𝜌 w 𝐸 ΤΤ𝑆el 𝜌 det Τ𝑆el 𝜌 w 𝐸0
= ൗΤ𝑆el 𝜌 det,𝐸 Τ𝑆el 𝜌 det,𝐸0 ൗΤ𝑆el 𝜌 w,𝐸 Τ𝑆el 𝜌 w,𝐸0
• This approximation assumes that:
1. CPE exists for knock-on electron (also know as -ray equilibrium)
2. Incident electrons lose a very small fraction of energy in traversing dosimeter
3. Electron scattering is the same in ‘det’ as in water
4. RSV is “LET-independent”
5. Items 1 and 3 are suspect; 2 and 4 are easily satisfied for 𝐸 above 1 MeV
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Energy dependence in RT: electron absorbed dose
• Electron-energy dependence for LiF, a diamond detector, a silicon diode, and an air-filled ion chamber, in terms of 𝑀/𝐷w, normalized to E = 1 MeV.
• Absence of the density effect in Τ𝑆el 𝜌 air relative to water gives rise to a marked energy dependence.
• LiF and a diamond detector show weak dependence since both are close to water ITO density effect and I-value.
• Si diode has similar 𝜌 to LiF and density effect to water, but a very different I-value.
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Figure 10.8 Electron-energy dependence for LiF, a diamond detector,
a silicon diode, and an air-filled ion chamber, in terms of response
per unit absorbed dose in water, normalized to E = 1 MeV. Estimated
from Eq. (10.11 ) using Τ𝑆el 𝜌 det values from the electronic Data Tables.
Intrinsic energy dependence
• Dependence of the dosimeter reading per unit of absorbed dose to the material in the RSO, on the radiation energy or beam quality
• The most fundamental as it reflects the dosimeter’s 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
• It is often called “LET dependence” as 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
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Miscellany
• Spectrum of radiation detectors is huge.
• Configuration of a dosimeter sometimes is crucial to its use; e.g., a small-size detector is of primary importance in its application in vivo in patients or test animals
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Figure 10.9 Approximate range of absorbed dose values
and the respective applications where radiation dosimetry plays a significant role.
• A dosimeter needs a relevant calibration appropriate to the radiation type and quality, and to the quantity to be measured
• Reusability: reusable TLDs can be individually calibrated; single-use dosimeters such as film badges cannot
