68
2 Observations of Radio and X-ray Pulsars
and can also be measured. As the amount of energy required to create an electron–
hole pair is known and independent of the energy of the incident photons, measuring
the number of electron–hole pairs allows the intensity of the incident photons to be
determined.
The SDD is fabricated from high-purity silicon with a large-area contact on the
entrance side facing the incoming X-rays, and consists of a large volume of n-type
silicon bulk depleted from both sides: a homogeneous, shallow p–n junction on
the side where the incoming X-ray photons enter into the detector and a structure
of circular p+ drift rings on the opposite side. By applying a negative voltage on
the homogenous back side (X-rays incoming window) and an increasingly negative
voltage on the drift rings, a potential field distribution is created inside the detector
such that the electrons generated by the absorption of ionizing radiation drift toward
a small-sized collecting anode located at the center of the device.
For the germanium detectors, as the germanium as main detecting material has
very high electrical resistivity, the thickness of their depleted layers can generally get
to several centimeters. Therefore, the detectors are also called high-purity germanium
detectors. However, the high-purity germanium detectors require a low temperature
to work normally, and to reduce the background noise of the detectors.
Obviously, the semiconductor detectors can directly detect X-ray photons by
collecting the electron–hole pairs which result from ionizing radiation. An average
energy to grow an electron–hole pair is 3.5 eV for the silicon, and 2.94 eV for
the germanium. The detecting energy range is usually 2–100 keV for the semiconductor detectors, and their detection efficiencies and energy resolutions are higher.
The photon arrival-time accuracy using the semiconductor detectors can often get to
microsecond level.
2.7.1.4 CCD Detectors
The CCD is an integrated circuit containing an array of linked or coupled capacitors,
and a device for the movement of electrical charge, usually from within the device
to an area where the charge can be manipulated, such as conversion into a digital
value. In principle, CCD detectors are also the semiconductor ones, and thus sometimes also called CCD semiconductor detectors. An insulating layer of SiO 2 is
made on the substrate of doping p-type semiconductor. Grooves with about 0.1 µm
deep are etched on the insulating layer, and thus a lot of metal oxide semiconductor
capacitors are composed of a set of electrodes and the substrate. Each three electrodes are grouped into three-phase structure. As a corresponding positive voltage
is put on each phase, the potential wells are produced by inducting depletion layer
inside the substrate below the electrodes. When the incident X-ray photons pass
through the depletion layer and enter into the substrate, electron–hole pairs will be
produced. The electrons are absorbed to the electrodes under the electric fields and
the inversion layer of n-type is formed inside the depletion region. And then, the electrons are stored in corresponding potential wells while the holes reversely enter into
the ground to disappear. During whole exposure of the CCD, these photo-induced
2 Observations of Radio and X-ray Pulsars
and can also be measured. As the amount of energy required to create an electron–
hole pair is known and independent of the energy of the incident photons, measuring
the number of electron–hole pairs allows the intensity of the incident photons to be
determined.
The SDD is fabricated from high-purity silicon with a large-area contact on the
entrance side facing the incoming X-rays, and consists of a large volume of n-type
silicon bulk depleted from both sides: a homogeneous, shallow p–n junction on
the side where the incoming X-ray photons enter into the detector and a structure
of circular p+ drift rings on the opposite side. By applying a negative voltage on
the homogenous back side (X-rays incoming window) and an increasingly negative
voltage on the drift rings, a potential field distribution is created inside the detector
such that the electrons generated by the absorption of ionizing radiation drift toward
a small-sized collecting anode located at the center of the device.
For the germanium detectors, as the germanium as main detecting material has
very high electrical resistivity, the thickness of their depleted layers can generally get
to several centimeters. Therefore, the detectors are also called high-purity germanium
detectors. However, the high-purity germanium detectors require a low temperature
to work normally, and to reduce the background noise of the detectors.
Obviously, the semiconductor detectors can directly detect X-ray photons by
collecting the electron–hole pairs which result from ionizing radiation. An average
energy to grow an electron–hole pair is 3.5 eV for the silicon, and 2.94 eV for
the germanium. The detecting energy range is usually 2–100 keV for the semiconductor detectors, and their detection efficiencies and energy resolutions are higher.
The photon arrival-time accuracy using the semiconductor detectors can often get to
microsecond level.
2.7.1.4 CCD Detectors
The CCD is an integrated circuit containing an array of linked or coupled capacitors,
and a device for the movement of electrical charge, usually from within the device
to an area where the charge can be manipulated, such as conversion into a digital
value. In principle, CCD detectors are also the semiconductor ones, and thus sometimes also called CCD semiconductor detectors. An insulating layer of SiO 2 is
made on the substrate of doping p-type semiconductor. Grooves with about 0.1 µm
deep are etched on the insulating layer, and thus a lot of metal oxide semiconductor
capacitors are composed of a set of electrodes and the substrate. Each three electrodes are grouped into three-phase structure. As a corresponding positive voltage
is put on each phase, the potential wells are produced by inducting depletion layer
inside the substrate below the electrodes. When the incident X-ray photons pass
through the depletion layer and enter into the substrate, electron–hole pairs will be
produced. The electrons are absorbed to the electrodes under the electric fields and
the inversion layer of n-type is formed inside the depletion region. And then, the electrons are stored in corresponding potential wells while the holes reversely enter into
the ground to disappear. During whole exposure of the CCD, these photo-induced