chapter

7 
Radiation Detectors
When radiations from a radioactive material radiation and secondary ionization from δ
pass through matter, they interact with rays (see Chapter 6, Section A.1). The elec-
atoms and molecules and transfer energy to trons produced by ionization are attracted to
them. The transfer of energy has two effects: the positive electrode and the ionized atoms
ionization and excitation. Ionization occurs to the negative electrode, causing a momen-
when the energy transferred is sufficient to tary flow of a small amount of electrical
cause an orbital electron to be stripped away current.
from its parent atom or molecule, thus creat- Gas-filled detectors include ionization
ing an ion pair (a negatively charged electron chambers, proportional counters, and Geiger-
and a positively charged atom or molecule). Müller (GMâ•›) counters. The use of these detec-
Excitation occurs when electrons are per- tors in nuclear medicine is somewhat limited
turbed from their normal arrangement in an because their stopping power and detection
atom or molecule, thus creating an atom or efficiency for x rays and γ rays are quite low;
molecule in an excited state. Both of these however, they find some use for applications
processes are involved in the detection of in which detection efficiency is not a major
radiation events; however, ionization is the factor and for detection and measurement 
primary event, and hence the term ionizing of nonpenetrating, particle-type radiations.
radiation is used frequently when referring Some of their applications are discussed in
to the emissions from radioactive material. Chapters 12 and 23.
Radiation interactions were discussed in
detail in Chapter 6. In this chapter, we 2.  Ionization Chambers
describe the basic principles of radiation In most ionization chambers, the gas between
detectors used in nuclear medicine. the electrodes is air. The chamber may or may
not be sealed from the atmosphere. Many dif-
ferent designs have been used for the elec-
trodes in an ionization chamber, but usually
A.  GAS-FILLED DETECTORS
they consist of a wire inside of a cylinder or a
pair of concentric cylinders.
1.  Basic Principles For maximum efficiency of operation, the
Most gas-filled detectors belong to a class of voltage between the electrodes must be suf-
detectors called ionization detectors. These ficient to ensure complete collection of ions
detectors respond to radiation by means of and electrons produced by radiation within
ionization-induced electrical currents. The the chamber. If the voltage is too low, some of
basic principles are illustrated in Figure 7-1. the ions and electrons simply recombine with
A volume of gas is contained between two one another without contributing to electrical
electrodes having a voltage difference (and current flow. Figure 7-2 shows the effect of
thus an electric field) between them. The voltage difference between the electrodes on
negative electrode is called the cathode, the the electrical current recorded by an ioniza-
positive electrode the anode. The electrodes tion chamber per ionizing radiation event
are shown as parallel plates, but they may detected. Recombination occurs at low volt-
be a pair of wires, concentric cylinders, and ages (recombination region of the curve). As
so forth. Under normal circumstances, the the voltage increases there is less recombina-
gas is an insulator and no electrical current tion and the response (electrical current)
flows between the electrodes. However, radi- increases. When the voltage becomes suffi-
ation passing through the gas causes ioniza- cient to cause complete collection of all of the
tion, both direct ionization from the incident charges produced, the curve enters a plateau
87
88 Physics in Nuclear Medicine

Voltage source
 

 Anode
Current
Air or measuring
Incident device
e other
ionizing e
e gas
radiation
 e e I





 Cathode

FIGURE 7-1  Basic principles of a gas-filled detector. Electrical charge liberated by ionizing radiation is collected by
positive (anode) and negative (cathode) electrodes.

Saturation
region
Recombination
region
Amplitude of output pulse

Saturation
voltage, Vs

Applied voltage
FIGURE 7-2  Voltage response curve (charge collected vs. voltage applied to the electrodes) for a typical ionization
chamber. In usual operation, applied voltage exceeds saturation voltage Vs to ensure complete collection of liberated
charge.

called the saturation region. The voltage at ionization event in air is approximately
which the saturation region begins is called 34╯eV.* Thus a 1-MeV β particle, for example,
the saturation voltage (Vs). Typically, Vs ≈ causes approximately (106/34) ≈ 3 × 104 ioniza-
50-300╯V, depending on the design of the tions in air and releases a total amount of
chamber. Ionization chambers are operated at electrical charge of only approximately 3 ×
voltages in the saturation region. This ensures 10−15 coulombs.
a maximum response to radiation and also
that the response will be relatively insensi- *â•›The average energy expended in producing a single
tive to instabilities in the voltage applied to ionization event is symbolized by W. This is not the same
the electrodes. as the average energy required to ionize an air molecule,
The amount of electrical charge released in but is the average energy expended per ionization by the
ionizing particle, including both ionization and excitation
an ionization chamber by a single ionizing effects. This is discussed in detail in Chapter 6, Section
radiation event is very small. For example, A.4. Values of W for some detector materials are listed in
the energy expended in producing a single Table 7-1.
 7  •  Radiation Detectors 89

TABLE 7-1â•…
SOME PROPERTIES OF DETECTOR
MATERIALS USED AS IONIZATION
DETECTORS

Ge(Li)
Si(Li) or Ge CdTe* Air
ρ(g/cm )3
2.33 5.32 6.06 0.001297
Z 14 32 48 & 52 ~7.6
†
W(eV) 3.6 2.9 4.43 33.7
CdTe, cadmium telluride; Ge, germanium; Li, lithium; Si,
silicon.
*Cadmium zinc telluride (CZT) is CdTe in which some of
the Te atoms (typically 20%) are replaced by zinc atoms.
CZT has properties similar to CdTe.
†
Average energy expended per electron-hole pair created
or per ionization.

FIGURE 7-3  A battery-powered radiation survey meter.

An ionization chamber is contained in the base of the
Because of the small amount of electrical unit, with the entrance window on the bottom face of the
charge or current involved, ionization cham- device (not shown). The meter indicates radiation level.
bers generally are not used to record or count The rotary switch is used to select different scale factors.
(Courtesy Ludlum Measurements, Inc., Sweetwater, TX.)
individual radiation events. Instead, the total
amount of current passing through the
chamber caused by a beam of radiation is
measured. Alternatively, the electrical charge
released in the chamber by the radiation are discussed in Chapter 23. A typical survey
beam may be collected and measured. meter can measure exposure rates down to
Small amounts of electrical current are approximately 1╯mR/hr or air kerma rates
measured using sensitive current-measuring down to approximately 10╯µGy/hr.
devices called electrometers. Two devices con- Dose calibrators are used to assay activity
sisting of ionization chambers and electrom- levels in syringes, vials, and so forth contain-
eters in nuclear medicine are survey meters ing materials that are to be administered to
and dose calibrators. A typical ionization patients. Unlike other types of ionization
chamber survey meter is shown in Figure 7-3. chambers discussed in this section, dose cali-
The survey meter is battery operated and por- brators employ sealed and pressurized cham-
table. The ionization chamber consists of an bers filled with argon gas. This eliminates the
outer cylindrical electrode (metal or graphite- effect of changing barometric pressure on
coated plastic) with a wire electrode running output readings. Dose calibrators typically
down its center. There is often a pro�tective are calibrated to read directly in units of
cap on the end of the chamber for most mea- activity (becquerels or curies), with switches
surements; however, it is removed for mea- to set the display for different radionuclides.
surement of nonpenetrating radiations such Dose calibrators are discussed in detail in
as α particles, β particles, and low-energy Chapter 12, Section D.1.
(10 keV) photons. A device that records total charge collected
Survey meters are used to monitor radiation over time is the pocket dosimeter. The basic
levels for radiation protection purposes (see principles are illustrated in Figure 7-4. The
Chapter 23, Section E). Ionization current is ionization chamber electrodes are a central
displayed on a front-panel meter. Many older charging electrode and the outside case of the
units are calibrated to read traditional units dosimeter. They are insulated electrically
of exposure rate in roentgens per hour (R/hr) from one another and form an electrical capac-
or mR/hr. Newer units are calibrated to read itor. The capacitor is first charged to a refer-
Systeme International units of air kerma in ence voltage V by connecting the charging rod
grays per hour (Gy/hr), mGy/hr, and so forth, to a separate charging unit. If the capacitance
or have a switch-selectable option for choosing between the charging electrode and the case
between the two systems of units. The defini- is C, the charge stored on the capacitor is Q =
tions and relationships between these units V × C. When the chamber is exposed to
90 Physics in Nuclear Medicine

Outside
case

C
 Charging
electrode

 Insulator

FIGURE 7-4  Schematic representation of a pocket dosimeter.

radiation, electrical charge ΔQ is collected by dosimeters are suitable for measuring radia-
the electrodes, discharging the capacitor. The tion exposures down to approximately 10╯ mR
voltage change across the capacitor is mea- (air kerma of 0.1╯ mGy) to an accuracy of
sured and is related to the amount of electrical approximately 20%.
charge collected by the ionization chamber A basic problem with ionization chambers
electrodes (ΔQ = ΔV × C). is that they are quite inefficient as detectors
Pocket dosimeters are used in nuclear for x rays and γ rays. Only a very small per-
medicine to monitor radiation levels for radi- centage (<â•›1%) of x rays or γ rays passing
ation protection purposes. A typical system is through the chamber actually interact with
shown in Figure 7-5. The ionization chamber and cause ionization of air molecules. Indeed,
is contained in a small metal or plastic cyl- most of the electrical charge released in an
inder (~1.5╯ cm diameter × 10╯ cm long) that ionization chamber by photon radiations
can be clipped to a shirt pocket or collar. comes from secondary electrons knocked loose
Electrodes recessed into one end of the from the walls of the chamber by the incident
chamber are used to connect the dosimeter radiations rather than by direct ionization of
to a separate charger unit to charge up the air molecules. The relatively low detection
capacitor to the reference voltage. Voltage on efficiency of ionization chambers is not a
the capacitor causes a fine wire within the serious limitation in the applications described
chamber to be deflected. The position of the earlier; however, it precludes their use for
wire changes as the voltage on the capacitor most other applications in nuclear medicine,
changes. The wire is observed through a such as imaging.
viewing window at one end of the chamber. Two additional problems with ionization
Its position is read against a scale that has chambers should be noted. The first is that for
been calibrated in terms of the total radia- x rays and γ rays, their response changes with
tion recorded by the chamber, usually in photon energy because photon absorption in
units of air kerma (gray) or exposure (roent- the gas volume and in the chamber walls (i.e.,
gens) (see Chapter 23, Section E). Pocket detection efficiency) and relative penetration

FIGURE 7-5  Pocket dosimeter with charging system. (Courtesy Ludlum Measurements Inc., Sweetwater, Tx.)
 7  •  Radiation Detectors 91

1.2

End-cap off

1.0

Exposure rate (indicated/actual)

0.8

0.6

End-cap on
0.4

0.2

0.0
10 100 1000
Photon energy (keV)
FIGURE 7-6  Energy response curve for a typical ionization chamber survey meter with and without a removable
protective end cap.

of photons through the chamber walls are increased to a sufficiently high value, the
both energy-dependent processes. Figure 7-6 electrons liberated by radiation gain such
shows a typical energy-response curve for a high velocities and energies when accelerated
survey meter. A second problem is that in toward the positive electrode that they cause
unsealed chambers the density of the air in additional ionization in collisions with other
the chamber, and hence its absorption effi- atoms in the gas. These electrons in turn can
ciency, changes with atmospheric pressure (ρ cause further ionization and so on. This
∝ Pâ•›) and temperature (ρ ∝ 1/T ). Most cham- cascade process is called the Townsend ava-
bers are calibrated to read accurately at sea- lanche or the gas amplification of charge. The
level pressure (Pref = 1.013╯N/m2 = 760╯mm╯Hg) factor by which ionization is increased is
and average room temperature (Tref = 22°C = called the gas amplification factor. This factor
295K). For other temperatures T and pres- increases rapidly with applied voltage, as
sures P the chamber reading must be cor- shown in Figure 7-7. The gas amplification
rected (multiplied) by a temperature-pressure factor may be as high as 106, depending on
correction factor the chamber design and the applied voltage.
Detectors that operate in the ascending
CTP = ( Pref × T ) /( P × Tref ) (7-1)
portion of the curve shown in Figure 7-7 are
Temperature must be expressed on the Kelvin called proportional counters. In this region,
scale in this equation (K = °C + 273). The the ionization caused by an incident radiation
correction is significant in some cases, for event is multiplied (amplified) by the gas
example, at higher elevations (P ≈ 0.85╯N/m2 amplification factor. The total amount of
≈ 640╯mm╯Hg at 1600-meter elevation). Note charge produced is equal to the number of
that temperature-pressure corrections are not ionizations caused by the primary radiation
required with sealed chambers, such as in event (at 34╯eV/ionization in air) multiplied
most dose calibrators. A defective seal on such by the amplification factor. Thus the total
an instrument obviously could lead to errone- charge produced is proportional to the total
ous readings. amount of energy deposited in the detector by
the detected radiation event.
3.  Proportional Counters Actually, proportional counters are not
In an ionization chamber, the voltage between simply ionization chambers operated at high
the electrodes is sufficient only to collect voltages but are specially constructed cham-
those charges liberated by direct action of the bers designed to optimize the gas amplifica-
ionizing radiations. However, if the voltage is tion effect, both in terms of the amount of