Despite of knowing little about radiation, most of the people
usually associate the term 'radiation' with certain negative
feeling or with something dangerous. For scientists, radiation
usually refers to transmission of matter or energy, and such
transmission does not require any media. According to different
classification schemes, for example the energy, the nature
or the interaction with the target matter, there are a number
of categories of radiation. By its name, ionization radiation
is the radiation that can introduce ionization (i.e. kicking
out electron from the atomic core) on the implanted materials.
 ray
and X-ray are indeed electromagnetic radiation and are also
ionizing. Despite their dangerous nature, with proper handling,
they are extensively employed in numerous applications, for
example, imaging and radioactive tracing in medical use. In
this article, we are going to briefly discuss the detection
of gamma ray and X-ray.
A complete /X
ray detector system usually consists of a detector, a signal
processing unit and a data acquisition system. In the detector,
the energy of the /X
ray is converted into electrical signal. This electrical signal
is then input to the signal processing unit, which consists
of at least one signal amplifier. More modules may be required
depending on application purposes. The signal amplifier enlarges
and modifies the signal from the detector so that the output
signal can be processed by the acquisition unit. In this article,
we will focus on two of the commonly used /X
ray detectors, namely high purity germanium (HPGe) detector
and sodium iodide (NaI) detector.
Before we discuss the working principle of the HPGe detector,
let us introduce some semiconductor physics of germanium.
Like silicon or diamond, germanium is a group IV element and
thus it has four outermost shell electrons. The lattice structure
of germanium is tetrahedral, which is identical to those of
silicon and diamond, and the electronic bonding between each
of the atom core is covalent bond. Therefore, if there is
no excitation (i.e. energy) given to the lattice system, such
kind of structure would have no free electron and is thus
non-conducting. However, if energy equal to or larger than
the covalent bonding energy is given to the lattice, the covalent
bond will be broken and the corresponding electrons will become
free conducting. This process is sometimes called the valance
band to conduction band excitation. The excitation can be
of the form of thermal energy, light illumination or from
the energy of the /X
ray.
Consider now a X-ray or a gamma ray enters a HPGe crystal,
the energy from the /X
ray will create a number of free conducting electrons. A high
voltage is usually applied across this detector crystal so
that the generated free electrons will be drifted by the electric
field towards the electrode and then be collected at the electrode.
These electrons will then flow through an electronic circuit
and an electrical signal will be outputted at the final stage.
As the number of free electron generated within the detector
crystal depends on the energy of the /X
ray, the amplitude of the output signal reveals the energy
of the /X
ray. In order to eliminate other excitation by thermal energy
or background light that would generate free electron, the
detector crystal is operating in darkness (i.e. in a light
sealed environment) and at a low temperature (77K) using liquid
nitrogen.
NaI detector is another kind of /X
ray detector which is much cheaper but has a much lower energy
resolving power. Moreover, NaI detector can be operated at
room temperature and this can also reduce the running cost
of consuming liquid nitrogen for operating the HPGe detector.
NaI detector consists of two parts, namely the NaI scintillator
(Sc) and the photomultiplier tube (PMT). The NaI converts
the incoming /X
ray into visible or UV light. The PMT is a device that is
usually used to detect visible or ultraviolet UV light. As
the /X
ray enters the Sc, visible or UV light will be emitted and
then be introduced into the cathode of the PMT through an
optical window. Electron will be emitted from the surface
of PMT cathode by the excitation of the visible/UV light (photoelectric
effect). This electron will then be accelerated by the applied
high voltage, gain kinetic energy, and knock on the dynode
of the PMT. With the energy given by the energetic implanting
electron, more than one slow electron will be released from
the dynode surface. Electrons from the dynode will then be
accelerated by the high voltage to the next dynode. This process
will be repeated from dynodes to dynodes and finally large
number of electrons will be generated by this multiplication
process. These electrons finally arrive at the anode, flow
through a circuit and an electrical signal will be output.
(Figure1 A /X
ray detector system)
(Figure2 Lattice
structure of diamond, Si and Germanium.)
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