capacitance—make the contact smaller. The silicon drift diode or SDD accomplishes
this using a field gradient applied by ring electrodes on its back surface. The field
gradient directs the electron flow to a small region at the center of the ring, where an
integral or closely located FET amplifies the initial signal (Fig. 5.9). The lower
capacitance allows drift diodes to run at maximum count rates almost 10x those of
conventional HPGe or Si detectors.
5.10.4 Diode Array Detectors
The count rate limits of individual detectors can also be overcome by creating arrays
of devices. In an early incarnation of this idea, Canberra built an instrument with
13 Ge detectors mounted on a common cryostat (Fig. 5.9) [181]. This was followed
by devices designed for operation in a vacuum [183] (for soft X-ray work) and up to
36 elements. Since the outer devices might be at less favorable angles with respect to
the sample, some vendors have made arrays with more focused individual elements.
The dead space between devices can be minimized by employing “monolithic”
detectors arrays. Devices with 100 detectors are commercially available, and a
384-element detector has been deployed at NSLS-II (Fig. 5.9) [182]. A beautiful
synthesis of SDD and array concepts has been planned—a 384-element array of
SDD detectors [184]!
5.10.5 pnCCD Detectors
An even higher level of pixelation is achieved by so-called “pnCCD” detectors
[185]. One design, originally built for astronomy (of course), involved a 256 Â 256
field of 75 μ Â 75 μ pixels. Parallel readout of each 256-pixel channel is achieved in
5 ms, allowing a frame rate of 200 images per second. Thanks to the very low
electronic noise, ~123 eV (FWHM) resolution is observed at Mn Kα and 47 eV at C
Kα, close to the Fano noise limit. A typical application is as a position-sensitive
detector with energy resolution for single-shot experiments at the LCLS [186].
5.11 Avalanche Photodiodes (APDs)
In order to achieve a faster time response, especially in nuclear spectroscopy
experiments, many scientists have turned to the avalanche photodiode or “APD”
detectors [187, 188]. The APD behaves somewhat like a solid-state version of a gas
proportional counter. It is a diode constructed so that when a large reverse bias is
applied, there is a volume of semiconductor with high enough electric field
(!10
5 V cm
À1 ) to enable migrating electrons to create secondary ionization events
120
5 X-ray Detectors and Electronics
this using a field gradient applied by ring electrodes on its back surface. The field
gradient directs the electron flow to a small region at the center of the ring, where an
integral or closely located FET amplifies the initial signal (Fig. 5.9). The lower
capacitance allows drift diodes to run at maximum count rates almost 10x those of
conventional HPGe or Si detectors.
5.10.4 Diode Array Detectors
The count rate limits of individual detectors can also be overcome by creating arrays
of devices. In an early incarnation of this idea, Canberra built an instrument with
13 Ge detectors mounted on a common cryostat (Fig. 5.9) [181]. This was followed
by devices designed for operation in a vacuum [183] (for soft X-ray work) and up to
36 elements. Since the outer devices might be at less favorable angles with respect to
the sample, some vendors have made arrays with more focused individual elements.
The dead space between devices can be minimized by employing “monolithic”
detectors arrays. Devices with 100 detectors are commercially available, and a
384-element detector has been deployed at NSLS-II (Fig. 5.9) [182]. A beautiful
synthesis of SDD and array concepts has been planned—a 384-element array of
SDD detectors [184]!
5.10.5 pnCCD Detectors
An even higher level of pixelation is achieved by so-called “pnCCD” detectors
[185]. One design, originally built for astronomy (of course), involved a 256 Â 256
field of 75 μ Â 75 μ pixels. Parallel readout of each 256-pixel channel is achieved in
5 ms, allowing a frame rate of 200 images per second. Thanks to the very low
electronic noise, ~123 eV (FWHM) resolution is observed at Mn Kα and 47 eV at C
Kα, close to the Fano noise limit. A typical application is as a position-sensitive
detector with energy resolution for single-shot experiments at the LCLS [186].
5.11 Avalanche Photodiodes (APDs)
In order to achieve a faster time response, especially in nuclear spectroscopy
experiments, many scientists have turned to the avalanche photodiode or “APD”
detectors [187, 188]. The APD behaves somewhat like a solid-state version of a gas
proportional counter. It is a diode constructed so that when a large reverse bias is
applied, there is a volume of semiconductor with high enough electric field
(!10
5 V cm
À1 ) to enable migrating electrons to create secondary ionization events
120
5 X-ray Detectors and Electronics