Silicon Pixel Devices

A variety of technologies have been studied double-sided silicon strip detectors, silicon pixel devices, gaseous microstrip detectors, and micro- and radial time projection chambers. 

It is important that good spatial resolution be combined with low multiple Coulomb scattering, especially in the beampipe and first layer of a silicon tracking device. In addition, the position and mounting of devices at small polar angles must be designed to perform the measurement as close as possible to the beampipe, in order to minimize the effects of scattering in the beampipe. 

Pixel detectors have qualities such as superb spatial resolution and improved tolerance to backgrounds from synchrotron radiation or scattered particles. That makes them an attractive choice, although they are at an earlier stage of development. We have included a pixel design as an alternative technology. 

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Among the various material systems that make up smart pixels, liquid crystals are particularly interesting because of their transparency, broadband electro-optical response, and other unique material properties. Liquid crystal on silicon (LCOS) devices, liquid crystal spatial light modulators (LCSLMS), incorporating neural net functions enabled by the backplane electronics and inter- and intrapixel electronic cormections, such as optoelectronic neuron array, LC/silicon retina, and winner take-all circuit, have been developed previously. In these devices, the photosensitivity needed for sensing the input comes from the photoconductive layer processed into... 

The detector setup consists of four 256 x 256 pixel amorphous silicon technology sensor flat panels with 0.75 x 0.75 mm pixel size, having an active area of 192 x 192 mm [5j. These sensors are radiation sensitive up to 25 MeV and therefor well suited for detecting the LINAC radiation. The four devices are mounted onto a steel Irame each having the distance of one active area size from the other. With two vertical and two horizontal movements of the frame it is possible to scan a total area of about 0.8 x 0.8 m with 1024 x 1024 pixel during four independent measurements. 

At any temperature above 0°C, there will be spontaneous generation of elec-tron/hole pairs that is unrelated to incident light intensity. In addition, defects in the silicon may be sources of electrons and contribute dark electrons. Both of these processes are exponentially dependent on temperature, and CCDs must be actively cooled to reduce dark current to acceptable levels. The dark current is usually expressed as e pixel s (electrons per pixel per second) and depends both on the specific device and the temperature. A few examples are listed in Table 8.6. As an approximate rule of thumb, ihe dark current doubles for each 5°C increase in temperature. 

The linear photodiode array (LPDA) is a transducer developed to enable simultaneous measurement of light intensity at many wavelengths. The diode array consists of a number of semiconductors embedded in a single crystal in a one-dimensional linear array. A common procedure is to use a single crystal of doped silicon that is an n-type semiconductor. A small excess of a group 3A element, such as arsenic, is embedded into the surface at regular intervals. This creates local p-type semiconductors. The semiconductor device ideally has a cross-section such as that shown in Fig. 5.24. The surface contains a linear series or array of pn junctions, each of which is a photodiode. The individual diodes are called elements, channels, or pixels. 

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