J. Pyyhtiä

Both charge integrating and single photon identifying X- and gamma ray imaging devices constructed of CdTe pixel detectors bump bonded to CMOS ASICs have been developed, tested and utilized in a variety of applications. The charge integrating devices apply either frame mode or time delayed integration (TDI) signal readout schemes on the CMOS pixels depending on the requirements of the application. Frame mode readout is applied in real time and tomographic imaging as well as in low speed scanning, and TDI readout in high speed (up to 50 cm/s) industrial on-line scan imaging. The small pixel size of 100 μm of the charge integrating sensors combined with the high absorption efficiency of CdTe, optimized CMOS readout circuitry and real time calibration result in exquisite performance (high DQE) throughout a wide X-ray energy window (10 – 300 keV) and superb image quality very close to the theoretical ideal. Additionally, these sensor systems exhibit good stability over time and negligible afterglow. System level descriptions of various customized X-ray imagers are presented including up to 30 cm long scanning multiple line TDI cameras and small field of view real time area cameras which can be custom made to desired shapes of active area. Example images acquired in several different applications are shown. The unique photon identifying device (PID) comprises eight CdTe-CMOS hybrids each having 2048 pixels with a pitch of 350 μm. Every pixel accommodates a charge sensitive preamp followed by peak/hold, comparator and user selectable ADC/counter circuitry. The PID hybrids can be operated in several functional modes providing photon counting and energy dispersive imaging and timing measurements. 8 bit pixel amplifier offset and gain tuning yields a full field energy resolution close to the single pixel resolution which is measured to be 7 keV FWHM for the Am-241 main peak (60 keV). The PID system description is presented together with energy selective X- and gamma ray images and energy spectra of Am-241 and Co-57 sources. The PID is to be utilized in gamma imaging, X-ray back scatter imaging, energy selective photon counting X-ray imaging, X-ray diffraction experiments etc.

We present an update on a novel direct digital X-ray imaging device and system. The system comprises a mosaic of hybrid solid state semiconductor devices removably mount onto a master plane covering an imaging surface of any desirable shape and size. Each imaging device comprises a pixel semiconductor detector flip-chip joined to a CMOS ASIC. Monolithic CdZnTe and Si pixel detectors with dimensions 12.2 X 4.2 mm<SUP>2</SUP> and 18.9 X 9.6 mm<SUP>2</SUP> have been implemented with a pixel pitch of 35 micrometer. Each circuit on the ASIC, corresponding to a detector pixel, is capable of accumulating thousands of X-rays in the diagnostic energy spectrum with high efficiency (CdZnTe) and user accumulation times ranging from just a few ms to a few s. Individual, removable tiles are combined in a mosaic providing continuous large area imaging with no inactive regions. This tiling approach allows for cost efficient replacement of defective tiles. The packaging delivers a compact, lightweight, portable cassette whose thickness is around 2.0 cm. The basic hybrid detector design and tiling scheme are generic and may be used in mammography, conventional radiography and fluoroscopy. A special tiling scheme has been designed for use in intraoral imaging. We present our measured Modulation Transfer Function (MTF) and Detective Quantum Efficiency (DQE). Images taken with hard objects, phantoms and soft tissue further demonstrate system functionality and provide a comparison with radiographic film and CR plates. The first application of the new technology is intended for the field of dental imaging, mammographic biopsy and other small area medical applications (approximately 10 - 30 cm<SUP>2</SUP> imaging area) as well as Small Area Non Destructive Testing.