Dosimeters & Detectors Flashcards
(35 cards)
What is a radiation dosimeter?
“A radiation dosimeter is a device, instrument or system that measures or evaluates, either directly or indirectly, the quantities exposure, kerma, absorbed dose or equivalent dose, or their time derivatives (rates), or related quantities of ionizing radiation. A dosimeter along with its reader is referred to
as a dosimetry system.”
To function as a radiation dosimeter, the dosimeter must possess at least
one physical property that is a function of the measured dosimetric quantity
and that can be used for radiation dosimetry with proper calibration.
List some desirable characteristics of dosimeters.
(1) Accuracy
(2) Precision
(3) Linearity
(4) Dose / Dose Rate dependence
(5) Energy Response
(6) Spatial Resolution
(7) Directional dependence **
Which dosimeter is commonly recommended for beam calibration in clinical reference dosimetry?
Ionization Chamber
Which dosimeter is commonly recommended for detecting radiation leaks?
Geiger-Muller Tube
Explain what is meant by ‘accuracy’ and ‘precision’ of a dosimetry system.
“The precision of dosimetry measurements specifies the reproducibility of the measurements under similar conditions and can be estimated from the data obtained in repeated measurements. High precision is associated with a small standard deviation of the distribution of the measurement results. The accuracy
of dosimetry measurements is the proximity of their expectation value to the ‘true value’ of the measured quantity. Results of measurements cannot be absolutely accurate and the inaccuracy of a measurement result is characterized as ‘uncertainty’.”
Explain ‘uncertainty’ of a measurement.
” Results of measurements cannot be
absolutely accurate and the inaccuracy of a measurement result is characterized as ‘uncertainty’.
The uncertainty is a parameter that describes the dispersion of the
measured values of a quantity; it is evaluated by statistical methods (type A) or
by other methods (type B), has no known sign and is usually assumed to be
symmetrical.”
Explain ‘error’ of a measurement.
“The error of measurement is the difference between the measured value
of a quantity and the true value of that quantity.”
What is a type A standard uncertainty?
“The standard uncertainty of type A, denoted u_A, is defined as the standard deviation of the mean value
The standard uncertainty of type A is obtained by a statistical analysis of repeated measurements and, in principle, can be reduced by increasing
the number of measurements.”
The standard deviation of the mean value is standard deviation divided by the root of the number of measurements, N.
Explain ‘linearity’ of a dosimeter.
” Ideally, the dosimeter reading M should be linearly proportional to the dosimetric quantity Q. However, beyond a certain dose range a non-linearity
sets in. The linearity range and the non-linearity behaviour depend on the type of dosimeter and its physical characteristics. “
What is a type B standard uncertainty?
“Type B standard uncertainties cannot be estimated by repeated measurements; rather, they are intelligent guesses or scientific judgements of non-statistical uncertainties associated with the measurement. They include influences on the measuring process, application of correction factors or physical data taken from the literature.”
Explain ‘dose rate dependence’ in dosimetry systems.
Integrating systems measure the integrated response of a dosimetry system. For such systems the measured dosimetric quantity should be independent of the rate of that quantity. Ideally, the response of a dosimetry system at two different dose rates should remain constant. In reality, the dose rate may influence the dosimeter readings and appropriate corrections are necessary, for example recombination corrections for ionization chambers in pulsed beams.
Explain ‘energy dependence’ in dosimetry systems.
The response of a dosimetry system M/Q is generally a function of radiation beam quality (energy). Since the dosimetry systems are calibrated at a specified radiation beam quality (or qualities) and used over a much wider energy range, the variation of the response of a dosimetry system with radiation quality (called energy dependence) requires correction.
Ideally, the energy response should be flat (i.e. the system calibration should be independent of energy over a certain range of radiation qualities). In
reality, the energy correction has to be included in the determination of the quantity Q for most measurement situations. Ιn radiotherapy, the quantity of interest is the dose to water (or to tissue). As no dosimeter is water or tissue equivalent for all radiation beam qualities, the energy dependence is an important characteristic of a dosimetry system.
Explain ‘directional dependence’ in dosimetry systems.
The variation in response of a dosimeter with the angle of incidence of radiation is known as the directional, or angular, dependence of the dosimeter.
Dosimeters usually exhibit directional dependence, due to their constructional details, physical size and the energy of the incident radiation. Directional dependence is important in certain applications, for example in in vivo dosimetry while using semiconductor dosimeters.
Therapy dosimeters are generally used in the same geometry as that in which they are calibrated.
Explain ‘spatial resolution’ and ‘physical size’ of a dosimeter.
Since the dose is a point quantity, the dosimeter should allow the determination of the dose from a very small volume (i.e. one needs a ‘point dosimeter’ to characterize the dose at a point). Τhe position of the point where the dose is determined (i.e. its spatial location) should be well defined in a reference
coordinate system.
Thermoluminescent dosimeters (TLDs) come in very small dimensions and their use, to a great extent, approximates a point measurement. Film dosimeters have excellent 2-D and gels 3-D resolution, where the point measurement is limited only by the resolution of the evaluation system. Ionization chamber type dosimeters, however, are of finite size to give the required sensitivity, although the new type of pinpoint microchambers partially overcomes the problem.
What is ‘readout convenience’ and ‘convenience of use’ in dosimeters?
Direct reading dosimeters (e.g. ionization chambers) are generally more convenient than passive dosimeters (i.e. those that are read after due
processing following the exposure, for example TLDs and films).
While some dosimeters are inherently of the integrating type (e.g. TLDs and gels), others
can measure in both integral and differential modes (ionization chambers).
Ionization chambers are reusable, with no or little change in sensitivity within their lifespan. Semiconductor dosimeters are reusable, but with a
gradual loss of sensitivity within their lifespan; however, some dosimeters are not reusable (e.g. films, gels and alanine).
Some dosimeters measure dose distribution in a single exposure (e.g. films and gels) and some dosimeters are quite rugged (i.e. handling will not influence sensitivity, for example ionization chambers), while others are sensitive to handling (e.g. TLDs).
Dosimetry Principles:
Give the radiation weighting factors for the following:
(i) X-rays
(ii) Gamma rays
(iii) Beta particles
(iv) Protons
(v) Neutrons
(vi) Alpha particles
(vii) Fission products & Heavy nuclei
(viii) Muons
(ix) Charged pions
(i) X-rays : 1
(ii) Gamma rays : 1
(iii) Beta particles : 1
(iv) Protons : 2
(v) Neutrons : 5-20 (depends on energy)
(vi) Alpha particles : 20
(vii) Fission products & Heavy nuclei : 20
(viii) Muons: 1
(ix) Charged pions: 2
What is RBE?
Relative Biologic Effectiveness: defined as the ratio of the doses required by two radiations to cause the same level of effect. Thus, the RBE depends on the dose and the biological endpoint
Discriminate between an ‘ideal’ and ‘real’ detector/dosimeter.
Ideal: Responds to one radiation type only Includes radiation quality factor Uniform energy response Gives equivalent dose (H) or equivalent dose rate
Real: Need to discriminate between particles and gamma radiation using probe - shield • Non-uniform energy response • Often gives exposure rate (X / t) only (Milli-Roentgen per hour)
Summarize differences between various types of gas detectors.
Ionization chamber has relatively low
sensitivity, good for high radiation fields, has
energy info.
• Proportional counter as neutron detector
with BF3
as filling gas (slow neutrons
undergo n-alpha reaction). Has energy info.
• GM has large dead time (~100 micro-sec),
saturation in high radiation field, very
sensitive, no energy info.
What is a radiation detector?
A radiation detector is a sensor that upon interaction with radiation
produces a signal that can preferably be processed electronically to give the
requested information.
Explain the three modes of operation of radiation detectors.
In radiology and radiotherapy, radiation detectors are operated in current
mode. The intensities are too high for individual counting of events. In nuclear
medicine, on the contrary, counting mode is primarily used. Observing individual
events has the advantage that energy and arrival time information are obtained,
which would be lost in current mode. In the case of a personal dosimeter, the
detector is used in integrating mode. The dose is, for example, measured monthly.
Furthermore, instead of real time observation, the information is extracted at a
much later time after the actual interaction.
What are two major differences between the three types of gas detectors?
(1) Applied Potential Difference
(2) Sensitivity
Give essential features of an ionization chamber.
- Use of inert gases like air , oxygen, nitrogen, helium, argon, hydrogen, methane etc..
- No saturation
- Low sensitivity due to low p.d. (small events may not be counted)
- Operates in current mode or pulse mode
- Can be used for neutron detection if inside wall of chamber is coated with thin layer of boron or if the chamber is filled with BF3.
- Pressure within chamber can be controlled
- Provides energy information
- May show differences between particles and photons
- measures exposure rates up to 1000 R/min
- Operates in the full ionization region of Pulse Height vs Voltage curve
ADVANTAGES
- Can measure very high dose rates
- Reusable
- Little or no loss of sensitivity in lifespan
- No dead time, no saturation due to no charge amplification
- Preferred for beam calibration
- Neutron detection
- Uniform response to gamma radiation
- Low energy dependence
- Simple to use
- Has some energy info
DISADVANTAGES
- No charge amplification (low pd)
- Relatively low sensitivity
- Relatively slower response time than proportional chambers
- Requires a thin window for the detection of alpha and beta radiation.
- Low density, therefore gamma radiation deposits less energy in the ionization chamber.
- Easily affected by moisture
- Need electrometer external circuit for measuring current (unless it has a capacitor which is the case for direct reading dosimeters)
Give essential features of a GM counter.
- High sensitivity due to high p.d.
- Large dead time for high dose rates- saturation effect
- Measures low exposure rates
- Provides no energy info
- Use of inert gas
- Operates in GM region of Pulse height vs Voltage curve
- Operates in count mode
- Can measure low exposure rates (0.1 mR/hr)
ADVANTAGES
- High sensitivity ( around 100%)
- High Amplification: large output signal
- Portable
- Measures low exposure rates
DISADVANTAGES
- No particle identification
- No energy resolution
- Large dead time
- Saturation in high exposures and so cannot measure high radiation rates accurately due to large dead time.
