2.7 Classification of X-ray Detectors
65
will be ionized once again. The closer are the electrons to the anode, the stronger the
electric fields and the greater the probability of ionization. The secondary ionizations
will also produce more secondary electrons and positive electrons. The process of
ionizing proliferation is called electron avalanche effect. The amplification of the
electron avalanche effect is equal to the number of the charges collected in the anode,
and then an electric voltage pulse signal is produced. Meanwhile, the amplitude of
the pulse signal is proportional to the energy loss of the incoming photons. Generally,
collimators may be added in front of the detector window to reject X-ray background
photons.
The gas proportional counters are mainly applied to detecting X-rays with an
energy range from 1 to 50 keV. Using this type of detector, the photon arrival-time
accuracy may usually get to microsecond level, limited by the positive ion and anode–
cathode spacing. However, since the gas easily loses or leaks in practical application,
the detector’s performance declines gradually, its lifetime shortens and its reliability
is low relatively.
2.7.1.2 MCP Detectors
The MCP is a thin plate made from highly resistive material of typical thickness 2 mm
with a regular array of micro-channels leading from one face to the opposite, densely
distributed over the whole surface. It can be used to detect single-particle (electron,
ion and neutron) and low-intensity impinging radiation (ultraviolet radiation and
X-rays). The micro-channels are tiny tubes or slots, typically less than 25 µm in
diameter, and spaced apart from each other by approximately 15 µm. The microchannels are parallel to each other and often enter the plate at a small angle with its
normal direction, about 8 degrees.
Generally, materials to excite photoelectrons are coated on the surface of the
MCP and electrodes also installed on the upper and lower surfaces. There is a highvoltage electric field of about 2 kV between the upper and lower surface electrodes.
By applying the strong electric field across the MCP, each individual micro-channel
becomes a continuous-dynode electron multiplier. When an incident X-ray photon
impacts on the surface of the MCP to produce one or two photoelectrons and enters
into one of near micro-channels through a small orifice, the photoelectron is guaranteed to hit the wall of the channel due to the channel being at an angle to the
plate. The impact starts a cascade of electrons that propagates through the channels,
and thus a single impinging is turned into a cloud of electrons to amplify the original signal by several orders of magnitude. And finally, the electrons eject out on
the opposite side of the plate where they are collected on an anode. Some anodes
are designed to allow spatially resolved ion collection, producing an image of the
photons incident on the plate. Although in many cases the collecting anode functions
as the detecting element, the MCP itself can also be used as a detector. Obviously,
the mechanism of the MCP detector is a photoelectric multiplier effect. A schematic
diagram of the MCP detector is shown in Fig. 2.9. After the cascade of electrons,
the micro-channel usually takes time to recover before it can detect another signal.
65
will be ionized once again. The closer are the electrons to the anode, the stronger the
electric fields and the greater the probability of ionization. The secondary ionizations
will also produce more secondary electrons and positive electrons. The process of
ionizing proliferation is called electron avalanche effect. The amplification of the
electron avalanche effect is equal to the number of the charges collected in the anode,
and then an electric voltage pulse signal is produced. Meanwhile, the amplitude of
the pulse signal is proportional to the energy loss of the incoming photons. Generally,
collimators may be added in front of the detector window to reject X-ray background
photons.
The gas proportional counters are mainly applied to detecting X-rays with an
energy range from 1 to 50 keV. Using this type of detector, the photon arrival-time
accuracy may usually get to microsecond level, limited by the positive ion and anode–
cathode spacing. However, since the gas easily loses or leaks in practical application,
the detector’s performance declines gradually, its lifetime shortens and its reliability
is low relatively.
2.7.1.2 MCP Detectors
The MCP is a thin plate made from highly resistive material of typical thickness 2 mm
with a regular array of micro-channels leading from one face to the opposite, densely
distributed over the whole surface. It can be used to detect single-particle (electron,
ion and neutron) and low-intensity impinging radiation (ultraviolet radiation and
X-rays). The micro-channels are tiny tubes or slots, typically less than 25 µm in
diameter, and spaced apart from each other by approximately 15 µm. The microchannels are parallel to each other and often enter the plate at a small angle with its
normal direction, about 8 degrees.
Generally, materials to excite photoelectrons are coated on the surface of the
MCP and electrodes also installed on the upper and lower surfaces. There is a highvoltage electric field of about 2 kV between the upper and lower surface electrodes.
By applying the strong electric field across the MCP, each individual micro-channel
becomes a continuous-dynode electron multiplier. When an incident X-ray photon
impacts on the surface of the MCP to produce one or two photoelectrons and enters
into one of near micro-channels through a small orifice, the photoelectron is guaranteed to hit the wall of the channel due to the channel being at an angle to the
plate. The impact starts a cascade of electrons that propagates through the channels,
and thus a single impinging is turned into a cloud of electrons to amplify the original signal by several orders of magnitude. And finally, the electrons eject out on
the opposite side of the plate where they are collected on an anode. Some anodes
are designed to allow spatially resolved ion collection, producing an image of the
photons incident on the plate. Although in many cases the collecting anode functions
as the detecting element, the MCP itself can also be used as a detector. Obviously,
the mechanism of the MCP detector is a photoelectric multiplier effect. A schematic
diagram of the MCP detector is shown in Fig. 2.9. After the cascade of electrons,
the micro-channel usually takes time to recover before it can detect another signal.