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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5255_Библиотеки_им_академика_М_И_Перельмана
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Radiation Detectors
G.S. Pant
Radiation detectors have wide range of applications in science, industry, medicine and
biomedical research. In radiology and radiation medicine, they are used to measure the
radiation dose, dose rate or count rate. Such measurements are required for radiation dosimetry
in addition to their valuable role in diagnosis and treatment of human diseases. Radiation
detectors are also used to measure radioactivity, radioactive dating and background radiation.
When radiation passes through any matter (such as detector material) it produces
ionization. The magnitude of ionization or ion current is directly proportional to the amount
of radiation absorbed. Radiation detectors can measure different types and energies of radiation
with suitable design. The detectors, which have relevance in nuclear medicine, are described
below.
1. Gas filled detector
2. Scintillation detectors (solid and liquid)
3. Semi conductor detector
4. Thermoluminescient detector
5. Neutron detector
Gas Filled Detector
They have a positively charged central electrode in the form of a thin wire and an insulated
negatively charged chamber wall. The chamber is filled with a suitable gas or mixture of
gases. Ion pairs are produced during the passage of radiation through the chamber volume.
On applying adequate voltage, ions are attracted towards the respective electrodes. The
amount of charge collected (current) by the electrodes is directly proportional to the quantity
of radiation absorbed. A typical gas filled detector is shown in figure 1. Three types of gas
filled detectors, which are normally used in nuclear medicine include ion chamber,
proportional counter and Geiger counter. They, however, differ in the type of gas used, gas
pressure and applied voltage between the electrodes.
8 2
Radiation Detectors
Figure 1: Gas filled detector
83
The applied voltage influences the amount of ionization produced in the gas chamber
(with given type of gas and its pressure) as shown in figure 2. The various portions (regions)
of the curve are appropriately utilized for measurement of radiation with corresponding
range of applied voltage.
Ionization Chamber
When radiation passes through the gas chamber, electrons, and positive ions are produced as
a result of ionization and drift towards opposite electrodes to produce ionization current.
With no voltage between the electrodes, the ion pairs, produced by radiation, recombine.
Thus no current is observed in the circuit. When voltage is applied between the electrodes,
the ion pairs are attracted towards the opposite electrodes and current flows in the circuit.
With small applied voltage, only few electrons may reach the electrode and rest will recombine
to form a neutral atom. The region, where possibility of recombination of charges exists due
to insufficient applied voltage, is known as recombination region (Region I in figure 2).
Figure 2: Graph shows increase in current intensity (response) with increase in the applied
voltage in a gas filled detector
On increase in the applied voltage, the detected ion current increases and saturation
occurs at a given voltage beyond which the current does not increase with further increase in
84
Radiation Detectors
voltage (saturation or ionization region) as can be seen in figure 2, region II. In ionization
region the applied voltage between the opposite electrodes is sufficient enough to collect all
the positive and negative ions produced by radiation in the chamber. Ion chamber type
survey meters, radionuclide dose calibrators, pocket dosimeters, primary and secondary
standard dosimeters utilize the ionization region of the curve. However, the design of
ionization chambers varies depending upon the use and type of radiation to be detected. The
most common type utilizes a concentric cylinder as cathode and a central wire as anode.
The gas in an ionization chamber is usually dry air at atmospheric pressure but sometimes
argon or helium is also used in the mixture. To increase detection efficiency gas is filled at
high pressure.
Ionization chamber can measure both dose and dose rate depending upon its design.
The survey meters measure dose rate whereas the pocket dosimeter and secondary standard
dosimeter measure the quantity of radiation during a given a period of time. Ionization
chambers can precisely measure high radiation intensities. They are very frequently used for
measuring the amount of radioactivity in nuclear medicine. Working principle and quality
control of radionuclide dose calibrator and radiation survey meters are described separately
in this chapter. Most of the ionization chambers are sealed to avoid corrections for temperature
and pressure. With the use of thin mica sheet at end window they can detect energetic beta
radiation also
Proportional Counters
With further increase in voltage beyond ionization region, secondary ionizations occur in
the immediate vicinity of the primary events. The secondary ionization depends upon the
intensity of primary ionization, which in turn depends upon the energy of primary radiation
absorbed. This region is termed as proportional region (Region III, figure 2). Proportional
counters utilize this region to detect individual events and to discriminate radiation of
different energies (alpha and beta particles). They operate at atmospheric pressure to allow
introduction of samples directly into the detection volume. The detection chamber is briefly
flushed with one of the inert gases (Argon, Neon, or Krypton) mixed with a small amount of
polyatomic gas (methane or isobutene). The addition of polyatomic gas quenches and absorbs
the de-excitation energy and prevents spurious counts from occurring and maintains the
proportionality characteristics of the counter.
The resolving time of proportional counters is of the order of few micro seconds, thus
allowing high count rate without significant dead time loss. If only detection is required, a
resolving time of less than a microsecond may be achieved but it increases significantly
when energy determination is also required. Their use in nuclear medicine is limited to the
monitoring of effluent from gas chromatographs. They are sensitive to detect and
emitters.
Radiation Detectors
85
Geiger Muller (GM) Counters
With further increase in voltage beyond the proportional region, there is practically no
increase in current regardless of the number of ion pairs initially produced by incident
radiation. The count rate does not appreciably increase with increasing voltage over a wide
range. This region is called plateau region of GM counter (Region IV, figure 2).
The central anode is insulated from cathode in a GM tube (Figure 3), which is filled
with a mixture of argon and quenching gas. The GM counter takes advantage of the enormous
amplification of ionization in the gas under appropriate conditions.
Figure 3: Diagrammatic representation of GM tube
The primary ionizing event produces an ion pair, which are accelerated towards the
respective electrodes. Because of their high velocity, electrons hit other molecules of the gas
and produce secondary ion pairs. The surge of electrons that reaches the anode produces
light or ultra-violet (UV) radiation. Some of them may also produce electrons when absorbed
by gas atoms. Thus there is an avalanche of electrons near the anode as shown in figure 4.
While electrons reach the anode quickly, the positive ions take little longer to reach the
cathode because of their heavy mass, in comparison to electrons, prolonging the dead time
of the counter.
The slow moving positive ions build up a sheath around the anode. When the voltage
gradient falls below a value that is necessary for ion multiplication, the avalanche is
terminated. The positive heavy ions on hitting the cathode may also produce UV radiation,
which may ionize the gas molecules and prolong the avalanche further. The process of this
secondary ionization by positive ions has to be quenched so that counter is ready for detecting
another event.
Three methods are normally used for quenching the secondary electrons at the cathode
namely electronic, organic and halogen quenching.
The first method needs a special electronic circuit that temporarily reduces the applied
86
Figure 4: Electron avalanche in a GM tube
Radiation Detectors
voltage below threshold such that the ion pairs return to their de-excited state. This method
significantly reduces the detector response and is not in common use.
In the second method known as organic quenching, a small amount of organic vapors
(ethyl alcohol, xylene or isobutane) is added to the gas in the tube. The positive ions on their
way to cathode hit the organic molecules and transfer their charge to them. The quenching
gas also absorbs UV radiation released from the cathode when it is hit by positive ion. The
UV radiation dissociates organic molecules and prevents new avalanche. The dissociated
molecules of organic gas do not recombine. So when all the quenching gas is dissociated,
GM counter cannot be used any further. Because of finite lifetime, this method of quenching
is infrequently used.
The third method utilizes halogen gas for quenching. About 0.1 % halogen (usually
chlorine or bromine) is added to the tube. Though the halogen quenching is less effective
than the organic vapors but their dissociated molecules recombine and provide long life to
the GM tube. Halogen quenching is the most frequently used method in GM counters.
The design of GM counters is modified for detecting alpha and beta radiation. The
tubes are either end window or a side window type. End window tubes are made of thin
mica to maximize penetration by alpha and beta radiation. The counter efficiency is very
high (up to 100 %) for alpha or beta but for gamma their efficiency drops to 1 to 2 percent.
The number of events recorded by the counter is a direct measure of radiation intensity.
The pulse size in GM counter is independent of primary ionization hence it is not possible
to discriminate between different radiation types (alpha, beta, or gamma). To achieve high
sensitivity an adequate voltage (above the threshold in the range 900-1200V) has to be
applied between the electrodes.
Radiation Detectors
The last portion of the curve (Figure 2, region V) is known as discharge region. In
this region the detector cannot be used due to continuous discharge. Application and quality
control of GM survey meters is described later in this chapter,
87
Scintillation detectors (Solid state detectors)
Certain crystals emit light for a short duration on interaction with radiation. The process is
called scintillation and is very commonly used for the detection of nuclear emissions. The
desirable properties of good scintillators include high efficiency for conversion of radiation
energy to visible light in proportion to radiation energy absorbed, transparency to the emitted
light and short decay time. The other requirements such as density, state of matter and
versatility in size and shape of scintillating material vary with the intended application of
the detector. Zinc sulphide is often used in the detection of heavy particles such as protons
and alpha particles, plastic scintillators are commonly used for beta particle detection. For
gamma counting, sodium iodide (NaI) crystals containing a trace of thallium (Tl) as an
activator are exclusively used. Thallium forms an intermediate band in the forbidden gap of
sodium iodide molecule that facilitates the emission of light energy of desired wavelength.
The NaI(Tl) scintillators are quite efficient at room temperature but are fragile, hygroscopic
and with relatively long decay time. Improved detection efficiency for high energy photons
(e.g. 511 keV) can be obtained with other solid state detectors such as bismuth germanate
(BGO), lutetium oxyorthosilicate (LSO), germanium oxyorthosilicate (GSO).
Principle of operation
Ionizing radiation causes scintillation because of the excitation or ionization produced in the
detector material. In good scintillators the de-excitation results in light emission, which
occurs, in about 10-8 sec or less. Most of the energy of excitation and ionization produced in
the scintillator is converted into heat with a small percentage that coverts into visible or
ultra violet radiation. This visible light when falls on photocathode emits electrons. The
number of electrons emitted depends upon the amount of light received by the photocathode,
which in turn is proportional to the amount of energy absorbed in the crystal. A photocathode
usually consists of bialkali metals (Sb-Cs) with very low work functions. Modern
photocathodes use alkali metal such antimony-rubidium-cesium (Sb-Rb-Cs) or antimonypotassium-cesium (Sb-K-Cs). They have similar spectral response range as of Sb-Cs
photocathode, but with higher sensitivity that is well matched to the most common scintillator
materials.
At a short distance from the photocathode is a metal plate called dynode coated with a
material having good emission characteristics. Dynodes are made of metals on which coating
of some special substance (such as CsI deposited on tantalum substrates) is done to increase
electron multiplication.
88
Figure 5: Schematic diagram of a scintillation detector
Radiation Detectors
They are maintained at a positive potential relative to photocathode and therefore, attract
the photoelectrons. A high speed photoelectron on striking the dynode surface ejects several
secondary electrons from it. The secondary electrons ejected from the first dynode are
attracted by the second dynode, which is maintained at higher positive potential relative to
first one. The process goes on like this as the dynodes are sequentially maintained at
increasing voltages. The electron multiplication process also goes on repeating at each
dynode till the last one is reached. Typical electron multiplication factors are x3 to x6 per
dynode. The total electron multiplication is very large e.g. 6
10
for a ten stage tube with an
average multiplication factor of 6 at each dynode. Thus, a relatively large pulse of current is
produced when the tube is stimulated by a weak light signal. The current pulse is converted
into voltage pulse at the anode using a resistor or capacitor and passed on to pre-amplifier
(and amplifier if needed) before going through pulse height analyzer (PHA) for energy
discrimination and selection. The signals, which pass through PHA, are qualified for being
recorded. A schematic diagram of scintillation detection system is given in figure 5. Most of
the nuclear medicine equipment such as well counters, thyroid probe, and gamma camera
are based on scintillation detection. They are described separately in this book.
Liquid Scintillation counter
Liquid scintillation (LS) counters are used for counting beta particles. They are particularly
useful for counting low energy beta emitters such as 3H and 14C, which might not be
detected otherwise due to their absorption in the vial itself. Similarly radionuclides that emit
very soft x or gamma rays may also be counted in LS counters. The system uses organic
scintillators for the detection of beta particles. A beta emitting radioactive sample and
organic scintillators are dissolved in a solvent. Electrons are emitted when beta particles
interact with solvent molecules. These electrons interact with organic scintillator and produce
Radiation Detectors
89
light photons. These photons are detected by a pair of PM-tubes coupled in coincidence on
either side of the sample holder as shown in figure 6.
Figure 6: Schematic diagram of a liquid scintillation counter
The signals from the PM tubes are processed and transmitted to the output device for
recording. The average beta energy (E
av
= E
max
/3) from
3
H and 14C is so small that the
thermal noise overlaps the actual signal. Coincidence detection technique is therefore used
which effectively reduces the noise even better than pulse height analysis.
Commonly used liquid scintillators are p-terphenyl, 2,5diphenyl-oxazole(PPO) and
2,5-bis-2 (5-t-butylbenzoxazolyl)-thiophene (BBOT). To shift the wavelength of light emitted
from the scintillator to an optimum value, an additional substance like 4-di- {2-(5phenyloxazolyl)}-benzene (POPOP) is added. The solvents used for scintillator are xylene
and toluene.
Semiconductor Detectors
Free electrons and holes are produced in a semiconductor detector, when exposed to
ionizing radiation. The number of electron-hole pairs is proportional to the energy absorbed.
Under the influence of an electric field (bias voltage), electrons and holes travel to the
respective electrodes resulting in a flow of charge that can be measured.
Usually high purity silicon or germanium is used as semiconductor detector. Silicon and
Germanium have a valency of four. When they are doped with an impurity, such as
phosphorous or arsenic with valency of five, the impure atom after replacing the
90
Radiation Detectors
semiconductor atom will have one excess electron. Semiconductors using these extra electrons
as their primary mode of current conduction are called n type semiconductors. Similarly if
silicon or germanium is doped with impurities such as boron or aluminum with valency of
three, the impurity atom replacing the semiconductor atom will have one electron less. This
material is said to have electron holes. Such electron deficient semiconductors using these
positively charged holes as primary mode of current conduction are called p type
semiconductors. By means of special crystal growth techniques, semiconductors can be
made partly p and partly n type.
The forward-bias and reverse-bias properties of the p-n junction imply that it can be
used as a diode. A p-n junction diode allows electric charges to flow in one direction, but
not in the opposite direction; negative charges (electrons) can easily flow through the junction
from n to p but not from p to n and the reverse is true for holes. When the p-n junction is
forward-biased, electric charge flows freely due to reduced resistance at the junction. When
the p-n junction is reverse-biased, however, the junction barrier (and therefore resistance)
becomes greater and charge flow is minimal.
If a p-n semiconductor is kept between two electrodes, with negative electrode connected
to p type and positive electrode connected to n type, the semiconductor junction is said to be
reverse biased as shown in figure 7.
Figure 7: Reverse biased pn junction
The applied voltage will pull free electrons and holes out of the region where both types
of semiconductors are joined. The central region devoid of any charge is known as depletion
region. When a radiation/charged particle enters the depletion region it interacts with the
semiconductor material producing electron-hole pairs, which are swept away by the applied
electric field. The current produced is proportional to energy lost by the ionizing radiation
in the depletion region.
Radiation Detectors
91
Semiconductor detector requires only about 4 eV to produce an ion pair, whereas in gas
chambers or in scintillation detectors, roughly about 35 to 50 eV energy is required to
produce a single ion pair. Hence, semiconductor detector has a higher resolution performance
than either gas chamber or scintillation detectors. Various types of semiconductor detectors
such as surface barrier detectors, lithium drift detectors are available for measurement of
radiation.
Semiconductor detectors such as Cadmium-Telluride (Cd-Te) have been found useful as
gamma probes during surgery and endoscopy for the detection of focal areas of increased
radioactivity. Cd-Te detectors are made up of Cd and Te metals, which provide high efficiency
for -ray detection because of their high atomic number. Small size detectors are being
made (2mm thick and 2mm diameter) with very high efficiency for 100 keV photons. The
energy resolution is also very good for such detectors (operated at room temperature) for a
wide range of gamma ray energies.
Thermoluminescence dosimeters (TLD)
Many crystalline materials exhibit the phenomenon of thermoluminescence. When such a
crystal is irradiated, part of the absorbed energy is stored in the crystal lattice, which can be
recovered latter, as visible light by thermal means (thermoluminescence). If there is
instantaneous emission of light owing to these transitions, the phenomenon is called
fluorescence. If an electron in the trap acquires energy to raise itself to the conduction band
and then falls to the valence band, then the light emission is called phosphorescence (delayed
fluorescence). The phosphorescence at room temperature may be quite slow but can be
speeded up significantly with a moderate amount of heating. A plot of intensity of visible
light (thermoluminescence) against temperature is called the glow curve.
Figure 8: Schematic energy level diagram showing the thermoluminescence phenomenon, a)
Irradiation lifts the electron from valence band to conduction band which fall from there to the
trap where it remains till exposed to heat b) On heating the TL material the electron is lifted to
the conduction band where from it falls to the valence band emitting a photon of visible light.
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