Radiation Measurement
The technique for detecting the intensity and
characteristics of ionizing radiation, such as alpha, beta, and gamma rays or
neutrons, for the purpose of measurement.
Radiation interactions in matter
Interactions of heavy charged particles
Interactions of fast electrons
Interactions of gamma rays and X rays
Photoelectric absorption
Compton scattering
Pair production
Role of energy and atomic number
Interactions of neutrons
Slow neutrons
Fast neutrons
Applications of radiation interactions in detectors
The term ionizing radiation refers to those subatomic particles and
photons whose energy is sufficient to cause ionization in the matter with which
they interact. The ionization process consists of removing an electron from an
initially neutral atom or molecule. For many materials, the minimum energy
required for this process is about 10 electron volts (eV), and this can be taken
as the lower limit of the range of ionizing radiation energies. The more common
types of ionizing radiation are characterized by particle or quantum energies
measured in thousands or millions of electron volts (keV or MeV, respectively).
At the upper end of the energy scale, the present discussion will be limited to
those radiations with quantum energies less than about 20 MeV. This energy range
covers the common types of ionizing radiation encountered in radioactive decay,
fission and fusion systems and the medical and industrial applications of
radioisotopes. It excludes the regime of high-energy particle physics in which
quantum energies can reach billions or trillions of electron volts. In this
field of research, measurements tend to employ much more massive and specialized
detectors than those in common use for the lower-energy radiations.
For the purposes of this discussion, it is convenient to divide the
various types of ionizing radiation into two major categories: those that carry
an electric charge and those that do not. In the first group are the radiations
that are normally viewed as individual subatomic charged particles. Such
radiation appears, for example, as the alpha particles that are spontaneously
emitted in the decay of certain unstable heavy nuclei. These alpha particles
consist of two protons and two neutrons and carry a positive electrical charge
of two units. Another example is the beta-minus radiation also emitted in the
decay of some radioactive nuclei. In this case, each nuclear decay produces a
fast electron that carries a negative charge of one unit. In contrast, there are
other types of ionizing radiation that carry no electrical charge. Common
examples are gamma rays, which can be represented as high-frequency
electromagnetic photons, and neutrons, which are classically pictured as
subatomic particles carrying no electrical charge. In the discussions below, the
term quantum will generally be used to represent a single particle or photon,
regardless of its type.
Only charged radiations interact continuously with matter, and they are
therefore the only types of radiation that are directly detectable in the
devices described here. In contrast, uncharged quanta must first undergo a major
interaction that transforms all or part of their energy into secondary charged
radiations. Properties of the original uncharged radiations can then be inferred
by studying the charged particles that are produced. These major interactions
occur only rarely, so it is not unusual for an uncharged radiation to travel
distances of many centimeters through solid materials before such an interaction
occurs. Instruments that are designed for the efficient detection of these
uncharged quanta therefore tend to have relatively large thicknesses to increase
the probability of observing the results of such an interaction within the
detector volume.
The term heavy charged particle refers to those energetic particles whose
mass is one atomic mass unit or greater. This category includes alpha particles,
together with protons, deuterons, fission fragments, and other energetic heavy
particles often produced in accelerators. These particles carry at least one
electronic charge, and they interact with matter primarily through the Coulomb
force that exists between the positive charge on the particle and the negative
charge on electrons that are part of the absorber material. In this case, the
force is an attractive one between the two opposite charges. As a charged
particle passes near an electron in the absorber, it transfers a small fraction
of its momentum to the electron. As a result, the charged particle slows down
slightly, and the electron (which originally was nearly at rest) picks up some
of its kinetic energy. At any given time, the charged particle is simultaneously
interacting with many electrons in the absorber material, and the net result of
all the Coulomb forces acts like a viscous drag on the particle. From the
instant it enters the absorber, the particle slows down continuously until it is
brought to a stop. Because the charged particle is thousands of times more
massive than the electrons with which it is interacting, it is deflected
relatively little from a straight-line path as it comes to rest. The time that
elapses before the particle is stopped ranges from a few picoseconds (1 10-12
second) in solids or liquids to a few nanoseconds (1 10-9 second) in gases.
These times are short enough that the stopping time can be considered to be
instantaneous for many purposes, and this approximation is assumed in the
following sections that describe the response of radiation detectors.
Several characteristics of the particle-deceleration process are important
in understanding the behavior of radiation detectors. First, the average
distance traveled by the particle before it stops is called its mean range. For
a given material, the mean range increases with increasing initial kinetic
energy of the charged particle. Typical values for charged particles with
initial energies of a few MeV are tens or hundreds of micrometers in solids or
liquids and a few centimeters in gases at ordinary temperature and pressure. A
second property is the specific energy loss at a given point along the particle
track (path). This quantity measures the differential energy deposited per unit path length
(dE/dx) in the material; it is also a function of the particle energy. In
general, as the particle slows down and loses energy, the dE/dx value tends to
increase. Thus, the density with which energy is being deposited in the absorber
along the particle's track tends to increase as it slows down. The average dE/dx
value for charged particles is relatively large because of their short range,
and they are often referred to as high dE/dx radiations.
Energetic electrons (such as beta-minus particles), since they carry an
electric charge, also interact with electrons in the absorber material through
the Coulomb force. In this case, the force is a repulsive rather than an
attractive one, but the net results are similar to those observed for heavy
charged particles. The fast electron experiences the cumulative effect of many
simultaneous Coulomb forces, and undergoes a continuous deceleration until it is
stopped. As compared with a heavy charged particle, the distance traveled by the
fast electron is many times greater for an equivalent initial energy. For
example, a beta particle with an initial energy of 1 MeV travels one or two millimeters
in typical solids and several meters in gases at standard conditions. Also,
since a fast electron has a much smaller mass than a heavy charged particle, it
is much more easily deflected along its path. A typical fast-electron track
deviates considerably from a straight line, and deflections through large angles
are not uncommon. Because a fast electron will travel perhaps 100 times as far
in a given material as a heavy charged particle with the same initial energy,
its energy is much less densely deposited along its track. For this reason, fast
electrons are often referred to as low dE/dx radiations.
There is one other significant difference in the energy loss of fast
electrons as compared with that of heavy charged particles. While undergoing
large-angle deflections, fast electrons can radiate part of their energy in the
form of electromagnetic radiation known as bremsstrahlung, or braking radiation.
This form of radiation normally falls within the X-ray region of the spectrum.
The fraction of the fast-electron energy lost in the form of bremsstrahlung is
less than 1 percent for low-energy electrons in light materials but becomes a
much larger fraction for high-energy electrons in materials with high atomic
numbers.
Ionizing radiation also can take the form of electromagnetic rays. When
emitted by excited atoms, they are given the name X rays and have quantum
energies typically measured from 1 to 100 keV. When emitted by excited nuclei,
they are called gamma rays, and characteristic energies can be as high as
several MeV. In both cases, the radiation takes the form of photons of
electromagnetic energy. Since the photon is uncharged, it does not interact
through the Coulomb force and therefore can pass through large distances in
matter without significant interaction. The average distance traveled between
interactions is called the mean free path and in solid materials ranges from a
few millimeters for low-energy X rays through tens of centimeters for
high-energy gamma rays. When an interaction does occur, however, it is
catastrophic in the sense that a single interaction can profoundly affect the
energy and direction of the photon or can make it disappear entirely. In such an
interaction, all or part of the photon energy is transferred to one or more
electrons in the absorber material. Because the secondary electrons thus
produced are energetic and charged, they interact in much the same way as
described earlier for primary fast electrons. The fact that an original X ray or
gamma ray was present is indicated by the appearance of secondary electrons.
Information on the energy carried by the incident photons can be inferred by
measuring the energy of these electrons. The three major types of such
interactions are discussed below.
In this process, the incident X-ray or gamma-ray photon interacts with an
atom of the absorbing material, and the photon completely disappears; its energy
is transferred to one of the orbital electrons of the atom. Because this energy
in general far exceeds the binding energy of the electron in the host atom, the
electron is ejected at high velocity. The kinetic energy of this secondary
electron is equal to the incoming energy of the photon minus the binding energy
of the electron in the original atomic shell. The process leaves the atom with a
vacancy in one of the normally filled electron shells, which is then refilled
after a short period of time by a nearby free electron. This filling process
again liberates the binding energy in the form of a characteristic X-ray photon,
which then typically interacts with electrons from less tightly bound shells in
nearby atoms, producing additional fast electrons. The overall effect is
therefore the complete conversion of the photon energy into the energy carried
by fast electrons. Since the fast electrons are now detectable through their
Coulomb interactions, they can serve as the basis to indicate the presence of
the original gamma-ray or X-ray photon, and a measurement of their energy is
tantamount to measuring the energy of the incoming photon. Because the
photoelectric process results in complete conversion of the photon energy to
electron energy, it is in some sense an ideal conversion step. The task of
measuring the gamma-ray energy is then reduced to simply measuring the
equivalent energy deposited by the fast electrons. Unfortunately, two other
types of gamma-ray interactions also take place that complicate this
interpretation step.
An incoming gamma-ray photon can interact with a single free electron in
the absorber through the process of Compton scattering. In this process, the
photon abruptly changes direction and transfers a portion of its original energy
to the electron from which it scattered, producing an energetic recoil electron.
The fraction of the photon energy that is transferred depends on the scattering
angle. When the incoming photon is deflected only slightly, little energy is
transferred to the electron. Maximum energy transfer occurs when the incoming
photon is backscattered from the electron and its original direction is
reversed. Since in general all angles of scattering will occur, the recoil
electrons are produced with a continuum of energies ranging from near zero to a
maximum represented by the backscattering extreme. This maximum energy can be
predicted from the conservation of momentum and energy in the photon-electron
interaction and is about 0.25 MeV below the incoming photon energy for
high-energy gamma rays. After the interaction, the scattered photon has an
energy that has decreased by an amount equal to the energy transferred to the
recoil electron. It may subsequently interact again at some other location or
simply escape from the detector.
A third gamma-ray interaction process is possible when the incoming photon
energy is above 1.02 MeV. In the field of a nucleus of the absorber material,
the photon may disappear and be replaced by the formation of an
electron-positron pair. The minimum energy required to create this pair of
particles is their combined rest-mass energy of 1.02 MeV. Therefore, pair
production cannot occur for incoming photon energies below this threshold. When
the photon energy exceeds this value, the excess energy appears as initial
kinetic energy shared by the positron and electron that are formed. The positron
is a positively charged particle with the mass of a normal negative electron. It
slows down and deposits its energy over an average distance that is nearly the
same as that for a negative electron of equivalent energy. Therefore both
particles transfer their kinetic energy over a distance of no more than a few millimeters
in typical solids. The magnitude of the deposited energy is given by the
original photon energy minus 1.02 MeV. When the positron member of the pair
reaches the end of its track, it combines with a normal negative electron from
the absorber in a process known as annihilation. In this step both particles
disappear and are replaced by two annihilation photons, each with an energy of
0.511 MeV. Annihilation photons are similar to gamma rays in their ability to
penetrate large distances of matter without interacting. They may undergo
Compton or photoelectric interactions elsewhere or may escape from detectors of
small size.
The probability for each of these three interaction mechanisms to occur
varies with the gamma-ray energy and the atomic number of the absorber.
Photoelectric absorption predominates at low energies and is greatly enhanced in
materials with high atomic number. For this reason, elements of high atomic
number are mostly chosen for detectors used in gamma-ray energy measurements.
Compton scattering is the most common interaction for moderate energies (from a
few hundred keV to several MeV). Pair production predominates for higher
energies and is also enhanced in materials with high atomic number. In larger
detectors, there is a tendency for an incident photon to cause multiple
interactions, as, for example, several sequential Compton scatterings or pair
production followed by the interaction of an annihilation photon. Since little
time separates these events, the deposited energies add together to determine
the overall size of the output pulse.
Neutrons represent a major category of radiation that consists of
uncharged particles. Owing to the absence of the Coulomb force, neutrons may
penetrate many centimeters through solid materials before they interact in any
manner. When they do interact, it is primarily with the nuclei of atoms of the
absorbing material. The types of interaction that are important in the detection
of neutrons are again catastrophic since the neutrons may either disappear or
undergo a major change in their energy and direction.
In the case of gamma rays, such major interactions produce fast electrons.
In contrast, the important neutron interactions result in the formation of
energetic heavy charged particles. The task of detecting the uncharged neutron
is thus transformed into one of measuring the directly observable results of the
energy deposited in the detector by the secondary charged particles. Because the
types of interaction that are useful in neutron detection are different for
neutrons of different energies, it is convenient to subdivide the discussion
into slow-neutron and fast-neutron interaction mechanisms.
These are conventionally defined as neutrons whose kinetic energy is below
about 1 eV. Slow neutrons frequently undergo elastic scattering interactions
with nuclei and may in the process transfer a fraction of their energy to the
interacting nucleus. Because the kinetic energy of a neutron is so low, however,
the resulting recoil nucleus does not have enough energy to be classified as an
ionizing particle. Instead, the important interactions for the detection of slow
neutrons involve nuclear reactions in which a neutron is absorbed by the nucleus
and charged particles are formed. All the reactions of interest in slow neutron
detectors are exoenergetic, meaning that an amount of energy (called the
Q-value) is released in the reaction. The charged particles are produced with a
large amount of kinetic energy supplied by the nuclear reaction. Therefore, the
products of these reactions are ionizing particles, and they interact in much
the same way as previously described for direct radiations consisting of heavy
charged particles. Some specific examples of nuclear reactions of interest in
slow-neutron detection are given below in the section Active detectors: Neutron
detectors.
Neutrons whose kinetic energy is above about 1 keV are generally
classified as fast neutrons. The neutron-induced reactions commonly employed for
detecting slow neutrons have a low probability of occurrence once the neutron
energy is high. Detectors that are based on these reactions may be quite
efficient for slow neutrons, but they are inefficient for detecting fast
neutrons.
Instead, fast neutron detectors are most commonly based on the elastic
scattering of neutrons from nuclei. They exploit the fact that a significant
fraction of a neutron's kinetic energy can be transferred to the nucleus that it
strikes, producing an energetic recoil nucleus. This recoil nucleus behaves in
much the same way as any other heavy charged particle as it slows down and loses
its energy in the absorber. The amount of energy transferred varies from nearly
zero for a grazing angle scattering to a maximum for the case of a head-on
collision. Hydrogen is a common choice for the target nucleus, and the resulting
recoil protons (or recoiling hydrogen nuclei) serve as the basis for many types
of fast-neutron detectors. Hydrogen provides a unique advantage in this
application since a fast neutron can transfer up to its full energy in a single
scattering interaction with a hydrogen nucleus. For all other elements, the
heavier nucleus limits the maximum energy transfer in a single scattering to
only a fraction of the neutron energy. In any elastic-scattering interaction,
the energy that is not transferred to the recoil nucleus is retained by the
scattered neutron which, depending on the dimensions of the detector, may
interact again or simply escape from the detector volume.
A number of physical or chemical effects caused by the deposition of
energy along the track of a charged particle are listed in the first column of
the table. Each of these effects can serve as the basis of instruments designed
to detect radiation, and examples of specific devices based on each effect are
given in the second column.
One category of radiation-measurement devices indicates the presence of
ionizing radiation only after the exposure has occurred. A physical or chemical
change is induced by the radiation that is later measured through some type of
processing. These so-called passive detectors are widely applied in the routine
monitoring of occupational exposures to ionizing radiation. In contrast, in
active detectors a signal is produced in real time to indicate the presence of
radiation. This distinction is indicated for the examples in the table. The
normal mode of operation of each detector type is also noted. These include
pulse mode, current mode, and integrating mode as defined below (see Active
detectors: Modes of operation). An indication is also given as to whether the
detector is normally capable of responding to a single particle or quantum of
radiation or whether the cumulative effect of many quanta is needed for a
measurable output.
In the descriptions that follow, emphasis is placed on the behaviour of
devices for the measurement of those forms of ionizing radiation consisting of
heavy charged particles, fast electrons, X rays, and gamma rays. Techniques and
devices of primary interest for the measurement of neutrons are discussed
separately in a later section because they differ substantially in operation or
composition or both. The detection methods that are included also are limited to
those that are relatively sensitive to low levels of radiation. There are a
number of other physical effects resulting from exposure to intense radiation
that can also serve as the basis for measurements, many of which are important
in the field of radiation dosimetry (the measurement of radiation doses). They
include chemical changes in ionic solutions, changes in the color or other
optical properties of transparent materials, and calorimetric measurement of the
heat deposited by intense fluxes of radiation. |
