The quantum efficiency or intrinsic efficiency ε of an X-ray detector is the ratio of
detected photons to the actual number incident on a device. This efficiency depends
critically on the materials used in the active region for detection, the physical length
of that active region, the nature of any physical or effective windows on the device,
and finally the energy of the X-rays being detected. For example, an air-filled 20 cm
ion chamber could have a 99% quantum efficiency around 3 keV but less than 1%
efficiency at 30 keV.
The energy resolution R is the breadth of the measured energies vs. the actual
energy: R ¼ FWHM/E 0 , where FWHM is the full width at half maximum for a peak
in the intensity vs. energy curve and E 0 is the mean value of the same peak. The
energy resolution can also be defined in absolute energy terms simply using the
FWHM. The dead time is the amount of time required to process one photon. If
another photon arrives during the dead time, it will not be correctly counted and
processed. As explained later in this chapter, the detector response during the dead
time depends on whether it is a paralyzable or non-paralyzable device. The mathematics are illustrated Appendix F.
The dark noise is the amount of output readout in the absence of any incoming
signal. With integrating detectors, for example, any electrical readout system will
have voltage fluctuations known as thermal noise or “Johnson noise”, V 2 ¼ 4 B kTR L
from the load resistor R L . If the detector is a photodiode, there is also a dark current
which is the reverse saturation current from random carrier generation in the diode.
In pulse-counting systems, the dark noise is the number of counts observed in the
absence of ionizing X-rays. The source of the dark noise might be purely electrical,
but it might also include events produced by cosmic rays. The dark noise of a
detector system sets a lower limit on the strength of an X-ray signal that can
reasonably be measured in an experiment.
The dynamic range is the ratio between the largest and smallest signal intensities
that a detector system can successfully process. The lower limit is set by the dark
noise, while the upper limit is set by the detector properties and signal processing
electronics.
We first discuss devices most commonly used as integrating detectors—which
provide a signal related to the number of X-rays over a certain time interval.
Examples include film, ion chambers, photodiodes, and charge-coupled devices
(CCDs). This is followed by treatment of devices most commonly used as photoncounting detectors—which record the arrival of individual X-ray photons. In general, photon-counting is used for the best S/N ratio until the count rate exceeds the
maximum linear response, at which point integrating devices such as ion chambers
are employed.
5.3 Film and Image Plates
Photographic plates or films detect X-rays in the same manner in which they detect
visible light. The energy from the X-ray causes the photoreduction of the silver in the
silver halide, and the latent image is then chemically enhanced. Since the efficiency
5.3 Film and Image Plates
109
detected photons to the actual number incident on a device. This efficiency depends
critically on the materials used in the active region for detection, the physical length
of that active region, the nature of any physical or effective windows on the device,
and finally the energy of the X-rays being detected. For example, an air-filled 20 cm
ion chamber could have a 99% quantum efficiency around 3 keV but less than 1%
efficiency at 30 keV.
The energy resolution R is the breadth of the measured energies vs. the actual
energy: R ¼ FWHM/E 0 , where FWHM is the full width at half maximum for a peak
in the intensity vs. energy curve and E 0 is the mean value of the same peak. The
energy resolution can also be defined in absolute energy terms simply using the
FWHM. The dead time is the amount of time required to process one photon. If
another photon arrives during the dead time, it will not be correctly counted and
processed. As explained later in this chapter, the detector response during the dead
time depends on whether it is a paralyzable or non-paralyzable device. The mathematics are illustrated Appendix F.
The dark noise is the amount of output readout in the absence of any incoming
signal. With integrating detectors, for example, any electrical readout system will
have voltage fluctuations known as thermal noise or “Johnson noise”, V 2 ¼ 4 B kTR L
from the load resistor R L . If the detector is a photodiode, there is also a dark current
which is the reverse saturation current from random carrier generation in the diode.
In pulse-counting systems, the dark noise is the number of counts observed in the
absence of ionizing X-rays. The source of the dark noise might be purely electrical,
but it might also include events produced by cosmic rays. The dark noise of a
detector system sets a lower limit on the strength of an X-ray signal that can
reasonably be measured in an experiment.
The dynamic range is the ratio between the largest and smallest signal intensities
that a detector system can successfully process. The lower limit is set by the dark
noise, while the upper limit is set by the detector properties and signal processing
electronics.
We first discuss devices most commonly used as integrating detectors—which
provide a signal related to the number of X-rays over a certain time interval.
Examples include film, ion chambers, photodiodes, and charge-coupled devices
(CCDs). This is followed by treatment of devices most commonly used as photoncounting detectors—which record the arrival of individual X-ray photons. In general, photon-counting is used for the best S/N ratio until the count rate exceeds the
maximum linear response, at which point integrating devices such as ion chambers
are employed.
5.3 Film and Image Plates
Photographic plates or films detect X-rays in the same manner in which they detect
visible light. The energy from the X-ray causes the photoreduction of the silver in the
silver halide, and the latent image is then chemically enhanced. Since the efficiency
5.3 Film and Image Plates
109