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84
7 Gas-Filled Detector
energy of these radiations. Ionization chambers are primarily used for measuring
high-intensity radiation such as X-ray beams and high activity of radiopharmaceuticals. Ion chamber survey meters, dose calibrators, and pocket dosimeters are the
common ionization chambers used in nuclear medicine.
7.2.1 Ion Chamber Survey Meter
An ion chamber survey meter consists of a metallic box tted with a voltage circuitry operated by batteries to measure the ionization current or ion pairs produced
by interaction of radiations with the gas molecules in the chamber. The readings are
displayed in analog or digital mode on scales over four or ve decades (background
to 50R/h). They can be used to measure α, β, and γ-ray exposure rates; however, for
measurement of γ-ray exposure alone, a retractable beta shield (1000mg/cm2) is
placed beside a thin window (7mg/cm2) of the chamber. The shield is removed
when measuring α and β particle exposures. However, in the case of α- particle
exposure, the pulse mode counting is applied due to large voltage pulses produced,
whereas in the case of β and γ rays, the current mode is employed. Most meters are
operated at ambient atmospheric pressure, whereas in some, high pressure is
employed using argon or similar gas. In the latter, the probability of photon interaction increases and hence the sensitivity. However, the operation of the meter at
atmospheric pressure is affected by variations in temperature and atmospheric pressure at different geographical locations causing a change in gas density, and hence,
readings will differ from the calibration value. Correction circuits are installed in
current ion chambers to correct for the temperature and pressure changes. A typical
ion chamber is shown in Fig.7.3.
According to NRC regulations, the ion chamber must be calibrated annually with
a calibration source,
for calibration of these ion chambers and other survey meters. The sensitivity of
137
Cs (t
=30year) with its 662 keV photon is commonly used
1/2
Fig. 7.3 An ion chamber
survey meter. (Courtesy of
Ludlum Instruments, Inc.
Sweetwater, TX)
7.2 Ionization Chamber
Fig. 7.4 The relative
energy-independent
response of ionization
chamber
85
these meters is quite linear over a broad range of photon energies relative to
137
Cs
response except at low energies (Fig.7.4). These meters can be used for a wide
range of exposure readings except at low energies. The output is integrated and
averaged over a period of time by the use of an RC circuit, and integration can be
made slower or faster by the use of a switch.
Standard ion chambers are used for measurement of moderate to high exposure
rate readings (e.g., 99Mo–
99m
Tc generator, X-ray beam intensity, etc.), whereas pressurized ion chambers can be used for low exposure rate readings. Sometimes scintillation detectors fabricated in the form of a probe are also used to measure the
exposure rates in low to moderate range.
7.2.2 Dose Calibrator
The dose calibrator is an ionization chamber and one of the most essential instruments in nuclear medicine for measuring the activity of radionuclides and radiopharmaceuticals. Since it measures the current produced by activity, it does not have
deadtime effects. It is a cylindrically shaped, sealed chamber with a central well and
is lled with argon and traces of halogen at high pressure (~5–12 atmospheres). Its
operating voltage is about 150 V.A typical dose calibrator is shown in Fig.7.5.
Because radiations of different types and energies produce different amounts of
ionization (hence current), equal activities of different radionuclides generate
86
Fig. 7.5 A typical dose
calibrator. (Courtesy of
Biodex Medical Systems,
Inc, Shirley, NY)
7 Gas-Filled Detector
different quantities of current. For example, the amount of current produced by 1
mCi (37 MBq) of
99m
Tc differs from that produced by 1 mCi (37 MBq) of
131
I.Isotope
selectors provided on the dose calibrator are the feedback resistors to compensate
for the differences in ionization (current) produced by different radionuclides so that
equal activities produce the same reading. In most dose calibrators, isotope selectors
for common radionuclides are push-button type, whereas those for other radionuclides are set by a continuous dial. An activity range selector is a variable resistor
that adjusts the range of activity (μCi, mCi, Ci, or kBq, MBq, GBq) for display.
In the past, the NRC required the calibration of dose calibrators for constancy,
accuracy, and linearity of their operation and geometry of samples and accordingly
prescribed specic recommendations for these tests. However, current NRC regulations (10CFR35) require only to have these calibrations performed according to
nationally recognized standards or the manufacturers’ instructions. In the absence
of specic recommendations, the earlier frequency and other related requirements
of these calibration tests have been given as follows:
1. Constancy (daily)
2. Accuracy (at installation, annually, and after adjustment or repairs)
3. Linearity (at installation, quarterly, and after adjustment or repairs)
4. Geometry (at installation and after adjustment or repairs)
7.2 Ionization Chamber
87
7.2.2.1 Constancy
Daily constancy check is performed by measuring a long-lived radioactivity (e.g.,
137
Cs) in the dose calibrator and observing the variation not to exceed ±10% relative
to the previous day reading. If the variation exceeds ±10%, the unit must be repaired
or replaced.
7.2.2.2 Accuracy
Accuracy of the dose calibrator is determined by measuring the activity of at least
two long-lived radionuclides (e.g.,
137
Cs and 57Co) certied by the National Institute
of Standards and Technology (NIST) in the dose calibrator and comparing the measured activity with the activity reported by the NIST.The measured value should not
differ from the standard value by more than ±10%. If it exceeds ±10%, the unit must
be repaired or replaced.
7.2.2.3 Linearity
(a) Decay Method
The linearity test indicates the dose calibrator’s ability to measure the activity
accurately over a range of values. It is performed by measuring a radioactive source
99m
(e.g.,
Tc), containing the highest activity normally used in the clinical setting, in
the dose calibrator at different time intervals until the source decays down to less
than 30 μCi (1.1 MBq). The measured activities are plotted against time on a semilog paper and the “best t” line is drawn (Fig.7.6). If the deviation of any point from
the line exceeds ±10%, the dose calibrator needs to be replaced, or a correction factor must be applied to the data in the nonlinear region.
Fig. 7.6 The time–
activity curve for decay
method
88
7 Gas-Filled Detector
(b) Shielding Method
The advantage of this method is that it is less time consuming and is easy to
perform. The method utilizes a commercial kit, called Calicheck, that contains
seven concentric tubes or “sleeves.” All sleeves except the innermost one are leadlined with increasing thickness simulating the various times of decay. When an
activity source is measured by using, rst the inner sleeve followed sequentially by
increasingly thick sleeves, the data represent the activities at different decay times.
Calibration factors are calculated by dividing the innermost tube reading by each
outer tube reading. For subsequent linearity tests, identical measurements are made
using the sleeves, and each measurement is multiplied by the corresponding calibration factors. Each corrected sleeve reading should give an identical value, and the
average of all values is calculated. If an individual reading exceeds the average
value by ±10%, then the calibrator needs replacement, or a correction factor needs
to be applied.
It should be noted that before the shielding method can be instituted, the linearity
test must be rst performed by the decay method for a new dose calibrator.
7.2.2.4 Geometry
Variations in sample volumes or in geometric congurations of the container can
affect the accuracy of measurements in a dose calibrator, particularly for low-energy
radiations. Thus, 1 mCi (37 MBq) in 1-ml or 30-ml volume, or 1 mCi (37 MBq) in
1-cc syringe, 10-cc syringe, or 10-cc vial, or containers of different materials (plastic or glass), may give different readings in the dose calibrator. Correction factors
must be determined for these geometric variations and applied to the measured
activities, if the error exceeds ±10%.
7.2.3 Pocket Dosimeter
The pocket dosimeter operates on the principle of a charged electroscope equipped
with a scale inside. It consists of a quartz ber electroscope inside the chamber.
Initially, the dosimeter is fully charged by means of an external power supply (a
dosimeter charger), and the scale then reads zero. After exposure to radiation, charge
is lost, and the loss of charge is proportional to the amount of radiation exposure,
which is read on the inside scale in mR.This reading can be seen through a viewing
window at the end of the dosimeter. After complete discharge of the dosimeter, it
can be charged and used again. It is primarily used to determine personnel exposure
while working with radiation and has the advantage of giving immediate readings.
These dosimeters are available in full-scale readings of 200 mR, 500 mR, and 1
R.Discharge due to leakage is the major disadvantage of these dosimeters.
7.4 Geiger–Müller Counter
89
7.3 Proportional Counter
Proportional counters are gas counters operated in the proportional region of the
applied voltage as shown in Fig.7.2. Because of the increased voltage beyond the
region of saturation, the electrons released by the initial ionization become energetic enough to cause further ionization of the gas molecules resulting in the amplication of the pulses. These counters are normally hemispherical in shape with
various congurations of the anode and the cathode. Proportional counters do not
use air as ionizing gas, and instead use a specic gas called P10 (a mixture of 90%
argon and 10% methane), which is allowed to ow through the counting chamber.
Because of the large magnitude of the signals, they operate in pulse mode and are
useful in counting α and β particles. Since the specic ionization of α particles is
greater than that of β particles, the pulses from these particles are distinguishable.
To facilitate the entry of these particles, a thin foil of Mylar is used as a window of
the counter. Mylar foil is prone to rupture because of thinness, so caution should be
exercised in handling them and not to increase the gas pressure in the counting
chamber. The proportional counters are not used as survey meters, but mostly used
in various laboratory counting.
7.4 Geiger–Müller Counter
The Geiger–Müller (GM) counter operates in the Geiger region of the voltage, as
shown in Fig.7.2. As already mentioned, in this region, an avalanche of ionizations
occurs as a result of high voltage. Once an ionization is initiated, the avalanche of
ionizations can lead to repetitive discharges unless the process is interrupted by the
quenching technique. An electronic technique of quenching can be applied in which
the voltage applied to the GM tube is temporarily reduced below the Geiger region
until all ion pairs return to their de-excited states. This happens in a few tenths of a
millisecond. The original voltage is then restored for the detection of the next event.
This technique is no longer in use.
The common technique of quenching is to add a small quantity of a quenching
gas to the counting gas. Either organic solvent vapors (e.g., ethyl alcohol, xylene, or
isobutane) or halogen gases (chlorine or bromine) are commonly used as the
quenching gas. These molecules transfer electrons to the “positive” ion cloud and
become ionized themselves. Ionized molecules of the quenching gas migrate to, and
dislodge electrons from the cathode. When these electrons neutralize the ionized
molecules of the quenching gas, energy is released; which causes the dissociation of
the molecules of the gas but with no UV emissions to prolong the avalanche. This
prevents the continuous discharge of the GM counter. Organic molecules are more
effective quenchers but dissociate irreversibly and therefore give a limited lifetime
8
for the GM tube (~10
–1010 pulses). In contrast, dissociated inorganic molecules
recombine to form the original molecules, and therefore halogen-quenched GM
tubes have innite useful lifetime.
90
a
Fig. 7.7 (a) A Geiger–
Miller survey meter and
(b) A pancake probe.
(Courtesy of Ludlum
Instruments, Inc.
Sweetwater, TX)
b
7 Gas-Filled Detector
A GM meter is shown in Fig.7.7. The meter is a box-type unit with all electrical
accessories (Fig.7.7a) and a probe attached to it (Fig.7.7b) to detect radioactivity.
The probes can be either end-window type or side-window type. The window is
made of thin mica (0.01-mm thick), and gases such as argon, methane, helium, and
neon mixed with halogen are commonly used as the counting gas. The gas pressure
in GM probes is normally kept negative (about 0.8 atmosphere) relative to atmospheric pressure. Different shapes of GM probes are available, such as, cylindrical
or pancake types (Fig.7.7b). Some GM probes are provided with a metal cover that
stops all β-particles and very low-energy γ-radiations, so that only high-energy photons are detected. Without the cover, both β-particles and γ-rays are detected. The
GM counter is usually battery operated at a voltage of 500–900 V.Lower voltages
are used for smaller tubes, and some special tubes are operated even at 1300 V.The
meter connected to the GM probe gives readings in mR/hr or counts per minute.
Some counters are equipped with audible alarms or ashing light alarms that are
triggered by radiation above a preset intensity. The latter counter is often used to
monitor the radiation level in work areas and is called an area monitor.
The GM probes operate in pulse modes and register each event as a single count
displaying the data as counts per minute. However, the individual counts can be
summed up by the measuring circuitry and converted to exposure rate (mR/h) for
display. Counts per minutes or mR/h are displayed over several decades (e.g. 1, 10,
100, and 1000).
The exposure rate (mR/h or counts/min) given by the GM tube for X-rays or
γ-rays depends on the energy of the photons, because they primarily interact with
the walls of the tube rather than with the gas volume. The GM tubes are made of
aluminum or steel with atomic number Z higher than that of air. Since, at low energies the photons primarily interact with the metal via the photoelectric process,
3
which is proportional to Z
, the exposure rate measured by the GM counter will be
overestimated. On the other hand, at medium energies, the Compton interaction
7.4 Geiger–Müller Counter
91
predominates, which is independent of Z and will therefore give correct reading of
exposure rate. The GM counters are commonly calibrated for 662-keV photons of
137
Cs. The response of different photon energies relative to
137
Cs measured by these
GM counters is shown in Fig.7.8. It is seen that the response (exposure) for energies
less than 150 keV is overestimated and needs to be corrected. Currently, the correction is made by using a lter that can be snap-tted onto the face of the pancake
probe. There are two types of lters, one for exposure and the other for ambient
dose equivalent, which atten the response to within ±20% relative to 662 keV over
the energy range of 33 keV to 1.2MeV.Alternatively, energy-compensated detectors have been devised in which a thin layer of high Z material such as tin is placed
around the inside of the detector. The increased photoelectric absorption of low
energy photons in tin signicantly attens the response of the detector. A disadvantage of this detector is that the low-energy sensitivity is greatly reduced.
The GM survey meters are more sensitive than ionization chambers by a factor
of about ten. Because voltage pulses generated in GM tubes are independent of the
energy deposited, they cannot discriminate between energies and types of radiations. These counters are almost 100% efcient for counting β-particles but have
only 1–2% efciency for counting γ-and X-rays. The dead time, or resolving time
(Chap. 8), of the GM counters is about 80–500μs. This limits the count rates to
about 15,000–20,000 counts per minute (cpm) for these counters, and at higher
activities they tend to saturate, thus losing counts. The GM counters are normally
used for area survey for contamination with low-level activity. According to the
NRC regulations, these survey meters must be calibrated annually with standard
calibrated sources such as
137
Cs.
Fig. 7.8 The relative
energy-dependent response
of the GM counter
92
7 Gas-Filled Detector
7.5 Questions
1. Describe the principles of gas-lled detectors.
2. What are the differences between an ionization chamber and a Geiger–Müller
counter?
3. What is the function of a push-button isotope selector on a dose calibrator?
4. Can you discriminate between 140-keV γ-rays, 364-keVγ-rays, and 5-MeV
α-particles using a GM counter?
5. What type of instruments would you use for:
(a) Survey of the laboratory?
(b) X-ray beam exposure?
(c) Area survey around X-ray room?
(d) Spill of 50 μCi (1.85 MBq) of
(e) Background radiation?
(f) Radiation survey of a diagnostic X-ray installation?
6. (a) Why are halogen gases added to GM counters?
(b) What is the typical dead time for GM counters?
(c) How often do the GM counters need to be calibrated?
(d) Why cannot the GM counters be used for detecting high-activity samples?
(e) What are the typical detection efciencies of the ionization chamber and
GM meter?
(f) When and why is a specic lter used in GM meters?
7. What are the typical voltages applied to the ionization chambers and GM
counters?
8. Describe the various tests of the dose calibrator and mention the frequency of
each test.
201
Tl ?
Suggested Readings
Cherry SR, Sorensen JA, Phelps ME. Physics in Nuclear Medicine. 4th ed. Philadelphia:
W.B.Saunders; 2012.
Hendee WR, Ritenour R. Medical Imaging Physics. 4th ed. NewYork: Wiley-Liss; 2002.
Knoll, G. Radiation Detection and Measurement. 4th ed. NewYork: Wiley; 2010.
Ouseph PJ. Introduction to Nuclear Radiation Detectors. NewYork: Plenum Press; 1975.
Robinson CV. Geiger–Müller and proportional counters. In: Hine GJ, ed. Instrumentation in
Nuclear Medicine. NewYork: Academic Press, 1967:57–72.
Scintillation andSemiconductor
Detector
8.1 Scintillation Counter
As stated in Chap. 7, the detection efciency of γ- and x-rays in gas detectors is very
low, because these penetrating radiations travel through the low-density gas with
little interaction. To improve detection efciency for these radiations, solid and liquid scintillation detectors with high density are used. These detectors have the
unique property of emitting scintillations or ashes of light after absorbing γ- or
x-radiations. The γ- or x-rays interact with scintillation detectors via photoelectric,
Compton, and/or pair production mechanisms, whereby the detector molecules are
raised to higher energy states through ionization or excitation. These high-energy
states return to ground states by emitting light photons. The time to reach the ground
state is called the scintillation decay time. The light photons produced are converted
to an electrical pulse by means of a photomultiplier (PM) tube (described later). The
pulse is then amplied by a linear amplier, sorted by a pulse-height analyzer
(PHA), and then registered as a count. Different solid or liquid detectors are used for
different types of radiation. For example, sodium iodide detectors containing a trace
of thallium (NaI(Tl)) are used for γ- and x-ray detection, whereas organic detectors
such as anthracene and plastic uors are used for β− particle detection.
In liquid scintillation counting, a β− emitting radioactive sample and an organic
scintillator are dissolved in a solvent. The β− particle interacts with solvent molecules emitting electrons. The latter interact with the organic scintillator, whereby
light photons are produced, which are then directed to two PM tubes coupled in
coincidence. A pulse is generated by the PM tube, which is registered as a count, as
in the solid scintillation counting.
Organic scintillators usually have a lower density and, hence, a lower detection
efciency than inorganic scintillators. The decay time also limits the efciency of a
detector at high count rates. The faster decay time allows high count rate capability.
The decay time for organic scintillators is much shorter than that for inorganic
scintillators. For example, the decay time for NaI(Tl) is 0.25 μs and that for
8
© The Author(s), under exclusive license to Springer Science+Business Media, LLC,
part of Springer Nature 2025
G. B. Saha, Physics and Radiobiology of Nuclear Medicine,
https://doi.org/10.1007/978-1-0716-4816-2_8