Siemens MAMMOMAT NovationDR, Installation Instructions Manual, Page: 28 / 95

Theory of Operation
February 5, 2004
2-11
Figure 2-7.
Cross-Sectional View of the “Mushroom” Charge-Collection Electrode
The layer above the detector element array is the actual X-ray conversion material.
To achieve sufficient X-ray absorption, a thickness of 500
µ
m of a-Se is used. This
material has been widely used in xeroradiography, so its electrical properties and
vacuum deposition processes are well understood.
The next layer is an insulating material with a high dielectric strength. Thickness
uniformity, good adhesion to selenium, optical transparency, and a low defect count
to avoid high field breakdowns are some key requirements for this dielectric
blocking layer.
A thin layer of metal is subsequently deposited on the dielectric material, serving
as an electrode to which several kV can be applied to separate the electron-hole
pairs. An additional material is deposited on top of the electrode material to
environmentally isolate the detector.
Electronic Readout
The readout of the charges starts immediately at the end of the X-ray exposure.
While all FETs (which are operated here as simple on–off switches) are not
conducting during the X-ray exposure, a positive gate pulse applied to the first gate
line simultaneously turns on all FETs connected to the first gate line. This causes
the image charge to flow to a bank of charge amplifier modules connected to the
data lines. The net X-ray signal is multiplexed and shifted out to a 14-bit
analog-to-digital converter (ADC) and subsequently stored in computer memory.
The readout of the charges continues row by row until the digital value of the
charge in each storage capacitor has been transferred to memory. At the end of the
readout cycle, a charge erase cycle is initiated to refresh the detector panel for the
next exposure.
The current commercial 139-
µ
m pitch DirectRay Detector has approximately
8 million pixels in the detector element matrix and a readout time of 4 seconds.
Once in computer memory, the image data can be processed and displayed on a
video monitor or routed into a network system.