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Measurement of Ionizing Radiation
DILSHAD KP | DEPARTMENT OF PHYSICS | UNIVERSITY OF CALICUT | KERALA
Skin Erythema Dose (SED) is the received
quantity of x- or 𝛾- radiation that causes diffuse
redness over an area of skin after irradiation
SED depends on:
- The type of skin
- The quality of radiation
- The extent of skin exposed
- Dose fractionation
- Differences between early and delayed skin reactions
In the early days measurement of absorbed
ionizing radiation were done on the basis of
chemical and biological effects such as:
• Radiation effects on photographic
• Changes in the color of chemical
• Reddening of the human skin
Because the amount of radiation required to produce the erythema
reaction varied from one person to another, it was a crude and
inaccurate way to measure radiation exposure.
In , ICRU adopted the as the unit of measuring 𝑥- and 𝛾- radiation exposure
The SI unit of 𝑒𝑥𝑝𝑜𝑠𝑢𝑟𝑒 is 𝑐𝑜𝑢𝑙𝑜𝑚𝑏 𝑝𝑒𝑟 𝑘𝑖𝑙𝑜𝑔𝑟𝑎𝑚 (𝐶/𝑘𝑔), but the special unit is
Old definition of roentgen:
ICRU defines as the quotient of 𝑑𝑄 by 𝑑𝑚 where 𝑑𝑄 is the absolute
value of the total charge of the ions of one sign produced in air when all the electrons
(negatrons or positrons) liberated by photons in air of mass 𝑑𝑚 are completely stopped
Current definition of roentgen:
1 𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑠𝑡𝑎𝑡𝑖𝑐 𝑢𝑛𝑖𝑡 = 3.333 × 10−10
𝑜𝑓 𝑎𝑖𝑟 𝑎𝑡 𝑆𝑇𝑃 = 1.293 × 10−6
An 𝑥-ray beam passing through air
sets in motion electrons by various
interaction. These high-speed
electrons produce ionization along
Because of the electric field produced
by the voltage applied across the ion-
collecting electrodes, the positive
charges move toward the negative
electrode and the negative charges
move toward the positive electrode.
This constitutes a current. The
collected charge of either sign can be
measured by an electrometer.
200 400 600 800 1000
150 250 350
The charge collected by the electrode in the
ionization chamber will be less than the
charge produced in the gas. This is because
of within the gas.
An ionization chamber is said to be
to the degree that such ionic
recombination is absent.
Ionic recombination can be decrease by
increasing the applied voltage. But, higher
applied voltage will result in
, in which the free electrons
gain enough kinetic energy to produce
secondary ionization in the gas (extra
ionization). It is desired in proportional
chambers, but not in ionization chambers.
Also known as “ ”
Used at the National Bureau of Standards (NBS) to calibrate
cavity ion chambers with low photon energy
It is enclosed in a lead shielding box to exclude scattered
𝑥-rays from elsewhere.
At the front the box is a tungsten-alloy diaphragm that is
aligned with the 𝑥-ray beam central axis.
The cavity chambers are centered at the axial point, 𝑃 to
be compared with the free-air ionization chamber.
Inside the box, there are
three coplanar electrodes
on one side of the beam
and a parallel high-voltage
electrode opposite. The
guard electrodes are
grounded and the
collecting electrode is
connected to a charge
electrodes are all parallel to
the 𝑥-ray beam axis and
equidistant from it.
The guard wires provide a
uniform electric field
between the electrodes.
These wires are
electrically biased in
uniform steps to establish
planes between the
electrodes. The guard
electrodes also assist in
According to the definition of roentgen,
the by the photons in a
(region of ion collection)
The electrons like colored will
remain in the volume 𝑉′
and thus all
their ionization will be collected and
𝑉 is the actual volume of origin of
secondary electrons whose ionization
we wish to measure. These
ionizations are collected from the
by the collector electrode.
The electrons like colored, which
originate within the volume 𝑉′
carry some of their kinetic energy out
and the remaining ionization
will produce there. These ionizations
will not collected by the collector
electrode, but will go to the grounded
guard electrode. This ionization lost
must be replaced by the electrons like
colored, which originate in the
beam outside of volume 𝑉.
Consequently the volume 𝑉′
as a whole in charged particle equilibrium. That is the ionization produced
by all of the electrons originating in the beam within 𝑉 is equal to all of the ionization produced
, and the correct amount of charge is thus measured.
Conditions to exist the in the volume 𝑉′
1. The distance from the boundaries of 𝑉 to each end of the lead box must be greater
than the maximum electron range in air
2. The beam intensity (photon fluence per unit time) must remain constant across the
length of the volume 𝑉
During CPE, we are neglecting the small effects of
scattered photons, bremsstrahlung, and ionic recombination
If ∆𝑄 is the charge collected in Coulombs and 𝜌 is the density of air in 𝑘𝑔 𝑚3
, then the exposure (in
𝐶/𝑘𝑔) at the centre of the specified volume (point P) is:
where, 𝑨 𝑷 is the cross-sectional area of the beam at point P (in 𝑚2
𝑳 is the length of the collecting volume (in 𝑚)
To change 𝑿 𝑷 from 𝐶/𝑘𝑔 to 𝑅 (1 𝑅 = 2.58 × 10−4
Suppose 𝑓1 and 𝑓2 are the distances of the X-ray source to the diaphragm (D) and point P, respectively.
Now, the exposure of the X-ray beam at D and P can be relate by the inverse square law as:
Similarly, by the inverse square law, the area of cross-section at D and P can be relate as:
So, the exposure (in 𝐶/𝑘𝑔) at D is:
To change 𝑿 𝑫 from 𝐶/𝑘𝑔 to 𝑅 (1 𝑅 = 2.58 × 10−4
where, 𝑨 𝑫 is the cross-sectional area of the beam at D (in 𝑚2
If ∆𝑄 is the charge collected in the volume 𝑉′
, the exposure (in 𝐶/𝑘𝑔) at point D
where, is the air attenuation coefficient (in 𝑚−1
is the distance from D to P (in 𝑚)
To change 𝑿 𝑫 from 𝐶/𝑘𝑔 to 𝑅 (1 𝑅 = 2.58 × 10−4
NOTE : Typically 𝜇 ≅ 2 − 3% per meter of air and 𝑒 𝜇𝑥 ≅ 1.02
1. As the photon energy increases, the range of the electrons liberated in air increases rapidly. This
necessitates an increase in the separation of the plates to maintain electronic equilibrium.
2. Large electrode separation creates problems of non-uniform electric field and greater ion
3 𝑀𝑒𝑉 photons produce electron tracks 1.5 𝑚 long. This thickness of air will attenuate
this photon beam by 5.4% and so large corrections are required to correct for the
attenuation by the thickness of air between the diaphragm and the sensitive volume.
Although the plate separation can be reduced by using air at high pressures, the
problems still remain in regard to air attenuation, photon scatter, and reduction in the
efficiency of ion collection.
They consist of a solid envelop surrounding a in which an electric field is
established to collect the ions formed by radiation
1. They can be made very compact, even for high energy use
2. They can measure multidirectional radiation fields
3. Through the application of cavity theory, the absorbed dose can be determined in
any material of which the cavity wall is made
4. They are capable to design for the dose measurements of charged particles and
neutrons as well as photons
5. Gas cavities can be designed to be thin and flat and as a result point dose
measurement is possible as a function of depth
6. Collected charge can be measured in real time by connecting the chamber to an
electrometer (condenser type chambers can be operate without the cable)
The chamber wall is shaped like a “ ”
They are designed with air cavity volume in the range of 0.1 − 3 𝑐𝑐
The high voltage (HV), usually ± 200 − 500 𝑉 is applied to the thimble wall, with the central
electrode (collector) connected to the electrometer input at or near ground potential.
The guard ring electrode that encircle around the insulator is also in ground potential (not shown
The insulator arrangement exemplifies a fully guarded ion chamber. That is
In order to be equivalent to a free-air ionization chamber, the wall of the thimble chamber (thimble
wall) should be . The most commonly used wall materials are made either of
graphite (carbon), Bakelite or a plastic coated on the inside by a conducting layer of graphite or a
conducting mixture of Bakelite and graphite.
The effective atomic number of the thimble wall is generally a little less than that of air. It is closer
to that of carbon (𝑍 = 6)
The graphite coated inner surface of the thimble wall is act as one electrode. The other electrode is
a rod of aluminum (𝑍 = 13) held in the centre of the thimble but electrically insulated from it.
The ion collecting rod in thimble-type chamber should be made of the same material as the wall, if
possible. However, an aluminum rod is sometimes used in an air-equivalent-walled chamber to
boost the photon response below ~ 100 𝑘𝑒𝑉 by the photoelectric effect, thus compensating for the
increasing attenuation of the 𝑥-rays in the wall.
Polystyrene, polyethylene, Nylon, Mylar and Teflon are all excellent electrical insulators for ion
chamber use (Teflon should be avoided where total doses exceeding ~104
𝐺𝑦, to avoid radiation damage).
In dealing with air, as a mixture of molecules, it is convenient to describe the air by an effective atomic
number. Air contains nitrogen (Z = 7), oxygen (Z = 8) and argon (Z = 18).
Effective atomic number (Z) is the atomic number of an element with which photons interact the same way as
with the given composite material.
Since photoelectric effect is highly 𝑍 dependent, effective atomic number is considered only for
photoelectric interactions by the photons of energy range from 30 𝑘𝑒𝑉 𝑡𝑜 80 𝑘𝑒𝑉.
where, 𝑎1, 𝑎2, … 𝑎 𝑛 are the fraction of electrons/gram of each element to the total number of
electrons in the mixture
𝑍1, 𝑍2, … 𝑍 𝑛 are the atomic numbers of each element in the mixture
If the chamber is designed to measure the absorbed dose at a point of interest in a charged particle
• Chamber volume must be small
• Chamber wall must be thin
For practical purposes a wall thickness of ≅ 15 𝑚𝑔/𝑐𝑚2
(the range of a 100 𝑘𝑒𝑉 electron) should
suffice, as most 𝛿-rays resulting from electron-electron collisions have energies less than that.
By compressing the volume of air required for electronic equilibrium, we can reduce its dimensions.
In fact, the air volume required for electronic equilibrium can be substituted by a small air cavity with
surrounding so that the electronic equilibrium is maintained in the air cavity.
The wall thickness of a thimble chamber must be equal to or greater than the maximum range of the
electrons liberated in the thimble wall. Since the density of the solid air-equivalent wall is much
greater than that of free air, the thicknesses required for electronic equilibrium in the thimble chamber
are considerably reduced.
The wall thickness of the thimble for X-ray energy range 100 – 250 𝑘𝑉𝑝 is about
1 𝑚𝑚 , and in the case of 𝐶𝑜60
𝛾 -rays ( 𝑎𝑣𝑒𝑟𝑎𝑔𝑒 𝑒𝑛𝑒𝑟𝑔𝑦 ≈ 1.25 𝑀𝑒𝑉 ), it is
approximately 5 𝑚𝑚.
If the wall is not thick enough, a close-fitting buildup cap is required to provide
electronic equilibrium for the radiation in question. These buildup caps are required
when making measurements in free air.
Conditions for the direct measurement of exposure by a thimble chamber are:
1. The chamber must be air-equivalent
2. Its cavity volume must be accurately known
3. Its wall thickness must be sufficient to provide electronic equilibrium
Under the above conditions, the exposure is given by
where 𝑄 is the ionization charge liberated in the cavity air of density 𝜌 and volume 𝑣; 𝐴 is the fraction
of energy fluence transmitted through the air-equivalent wall of equilibrium thickness.
The factor 𝐴 is slightly less than 1 and is used here to calculate the exposure for the
energy fluence that would exist at the point of measurement in the absence of the
Hence, thimble chambers are against
A free-air ionization chamber for low energy X-rays (up to few hundred
A standard cavity chamber (with nearly air-equivalent walls and accurately
known volume) for higher energies (up to 𝐶𝑜60
It is almost to construct
a thimble chamber that is exactly
It is to determine accurately the directly
1 2 3 4 5 6 7
Adequate to achieve electronic equilibrium
Required wall thickness
for maximum ionization
of the beam in the wall
Too few electrons
are generated in
True exposure in free air
(without chamber) is given by
where, 𝑀 is the electrometer
reading which is connected to
This exposure value is free from the
wall attenuation or the perturbing
influence of the chamber
Special form of thimble-type chamber
Operates while being irradiated
In 1955, Farmer designed a chamber that provided a stable and reliable secondary standard
This chamber connected to a specific electrometer
is known as the Baldwin-Farmer substandard dosimeter.
The original design of the Farmer chamber was later modified by Aird and Farmer to provide
: Graphite or plastic such as PMMA (acrylic), nylon,
A.E. (air-equivalent) plastic and T. E. (tissue-equivalent)
: Inner surface of the thimble wall coated with a
: A thin aluminum rod of 1 𝑚𝑚 diameter. It delivers the
ionization current to the electrometer
: A cylindrical conductor that wraps around the
insulator surrounding the central electrode in the stem
of the chamber. It has two functions:
1. To prevent leakage current from the central
2. To improve the signal to noise ratio
: Cylindrical air cavity with a nominal volume of 0.6 𝑐𝑐.
The cavity radius is ≈ 0.3 𝑐𝑚.
: Polytrichlorofluoroethylene (PTCFE)
Sensitivity is directly proportional to the chamber sensitive volume
The sensitivity of a Cutie Pie survey meter (600 𝑐𝑐) used for
measuring low-level 𝑥 rays is approximately 1,000 times the
sensitivity of a Farmer-type thimble chamber (0.6 𝑐𝑐) used
for calibration of a treatment beam.
It is the ionization
A chamber is known to have stem leakage if it records (the ionization produced
anywhere other than chamber sensitive volume (chamber stem and cable))
It depends on:
• Chamber design
• Beam quality (modality and energy)
• Irradiation conditions (the extent of stem and cable
exposed to radiation)
Fully guarded Farmer-type chambers have almost immeasurable stem
leakage. However, the stem leakage must be checked periodically.
Measurements are made with the chamber oriented in each of the two
positions shown. A number of points in the field are selected for such
measurements and correction factors are obtained as a function of the
stem length exposed in the field relative to the length of the stem
exposed during calibration
It is the voltage difference between the electrodes of an ion chamber
As the bias voltage is increased from
zero, the ionization current increases
at first almost linearly and later more
slowly. The curve finally approaches
a value for the given
The initial increase of ionization
current with voltage is caused by
incomplete ion collection at low bias
voltages (ion recombination). This
recombination can be minimized by
increasing the field strength (𝑉/𝑐𝑚).
If for a given exposure, the
, then the chamber is said to show a polarity effect
Major causes of polarity effect:
High-energy electrons (Compton electrons) ejected by high-energy
photons constitute a current (Compton current) independent of air cavity ionization
It is collected outside the sensitive volume (stem, cable and
inadequately screened collector circuit points)
Magnitude of polarity effect depends on:
• Chamber design
• Beam energy and modality
• Adequacy of bias voltage
• Irradiation conditions
The errors caused by the polarity effect can be minimized but not eliminated by reversing the
chamber polarity and taking the mean value of the collector current
The difference between collector currents measured with positive and negative polarity should be
< 0.5% for any radiation beam quality
It is the ratio of the number of ions collected to the number of ions produced
The chamber bias voltage should be within
the saturation region to minimize
under the conditions such as:
• Chamber design
• At very high ionization intensity
In the case of pulsed beams, significant loss of
charge by recombination may occur even at
bias voltages in the saturation region.
Correction has to be applied for these losses.
(To determine ion recombination correction 𝑃𝑖𝑜𝑛)
Select two bias voltages 𝑉1 𝑎𝑛𝑑 𝑉2 such as 𝑉2 = 𝑉1 2 and take electrometer
reading with them. The ratio of the two readings (𝑄1 𝑄2) is related to 𝑃𝑖𝑜𝑛.
1.02 1.04 1.06 1.08 1.10 1.12 1.14
The calibration laboratories provide chamber
calibration factors for reference environmental
conditions of temperature and pressure
If the ion chamber is open to the atmosphere,
its response is affected by temperature and pressure of the air in the chamber cavity
1. Minimal variation in sensitivity or exposure calibration factor over a wide range of photon energies
2. Suitable volume to allow measurements for the expected range of exposures
(Sensitivity is directly proportional to the chamber sensitive volume)
3. Minimal variation in sensitivity with the direction of incident radiation
4. Minimal stem leakage
5. Calibrated for exposure against a standard instrument for all radiation qualities of interest
6. Minimal ion recombination losses (sufficient voltage, typically 300 𝑉)
Absorbed Dose Determination in External Beam Radiotherapy:
Medium energy X-rays above 80 𝑘𝑉 and an HVL of 2 𝑚𝑚 𝐴𝑙
High energy photon beams
Electron beams with energy above 10 𝑀𝑒𝑉 approximately
Therapeutic proton and heavy ion beams
Volume : 0.1 𝑐𝑐 𝑡𝑜 1 𝑐𝑐 (this size range provides sufficient sensitivity and point dose measurements)
Internal diameter : ≤ 7 𝑚𝑚
Internal length : ≤ 25 𝑚𝑚
• Chamber must be aligned in a way such that
; it should equilibrate rapidly with the ambient temperature and air pressure
All electron energies (below 10 𝑀𝑒𝑉 their use is mandatory)
Low energy 𝑥-rays
Photon beams, only when a calibration in terms of absorbed dose to water is available
Proton and heavy ion beams (specially for beams having narrow SOBP)
At the center of inner surface of entrance window for all beam qualities and depths
• For the requirements concerning scattering perturbation effects and effective point of measurement, the
The ratio of cavity diameter to the cavity height should be 5 or more (cavity height should be ≤ 2 𝑚𝑚)
Diameter of the collecting electrode should be ≤ 20 𝑚𝑚; to reduce the influence of radial non-
uniformities of the beam profile
Width of guard electrode not smaller than 1.5 times the cavity height
Thickness of the front window should be 0.1 𝑔 𝑐𝑚2
or 1 𝑚𝑚 of PMMA; to make measurements at
shallow depths possible