A new photon-counting area detector, based on parallel-plate gas amplification with a resistive anode and remote readout electrode is described. The detector is sealed and has sensitive area of 14x14 cm2. The detector is unique in its ability to achieve high gain at high counting rates. A local counting rate >10e5 counts/mm2-sec has been achieved at a gain of 105 in a radiation-hard, non-polymerizing gas mixture. The global readout rate is limited by the delay line and electronics to <106 counts/sec but more sophisticated readout schemes should allow this rate to be increased by more than an order of magnitude. The operating characteristics of the detector are described and preliminary x-ray diffraction data are presented.
Phosphor-coupled CCDs are established as one of the most successful technologies for x-ray diffraction. This application demands that the CCD simultaneously achieve both the highest possible sensitivity and high readout speeds. Recently, wafer-scale, back illuminated devices have become available which offer significantly higher quantum efficiency than conventional devices (the Fairchild Imaging CCD 486 BI). However, since back thinning significantly changes the electrical properties of the CCD the high speed operation of wafer-scale, back-illuminated devices is not well understood. Here we describe the operating characteristics (including noise, linearity, full well capacity and CTE) of the back-illuminated CCD 486 at readout speeds up to 4 MHz.
A new high efficiency, low-bandgap phosphor, ZnSe:Cu,Ce,Cl is described which exhibits a significantly higher quantum gain than conventional x-ray phosphors and more closely matches the spectral sensitivity of silicon sensors. For many imaging applications this phosphor thus promises significantly superior performance compared to conventional phosphors.
A novel sealed gaseous PPAC detector is described which is significantly less prone to discharges and can consequently achieve high gas gains at high counting rates. The detector has demonstrated stable gains greater than 104 at counting rates in excess of 107 counts/mm2-sec.
We have prototyped and characterized a very large format X-ray detector for macromolecular crystallography. The X-ray field strength is converted to visible light in a phosphor film. Light from the phosphor is focused onto a CCD imager by a lens specially designed for this detector, that has a very high numerical aperture. The CCD is very large (61 mm, 4,096 × 4,096 pixels), and employs a very low-noise on-chip preamplifier.
Lens coupling between phosphor film and CCD avoids many of the optical imperfections of fiber optic coupling, but it remains a challenge to make a lens system with optical transfer efficiency matching or exceeding that of fiber optical systems. We have met this challenge by enhancing system gain in our detector through implementation of modern lens technologies and imaginative CCD design. At this point the system gain equals that of conventional CCD-based X-ray crystallography detectors, which couple the CCD to the phosphor through a fiber optic taper. Although many of our technical developments could also be used in fiber optic detectors, the overriding virtues of the lens-couple detector are simplicity, optical perfection, and cost.