Francis Glasser

Applications of CdTe detectors in x-ray imaging and metrology

Operating as a photoconductor, the sensitivity and the impulse response of semi-insulating materials greatly depend on the excitation duration compared to electron and hole lifetimes. The characteristic of ohmic contact for these compounds is briefly discussed. Before developing picosecond measurements with integrated autocorrelation system, this paper explains high energy industrial tomographic application with large CdTe detectors (25 X 15 X 0.9 mm3) where spatial resolution, contrast, and wide dynamic are the main criteria. The excitation is typically microsecond(s) range. X-ray flash radiography with 10 ns burst is in an intermediate time domain where excitation is similar to electron life-time in cadmium telluride. In a laser fusion experiment the excitation is in the range of 50 ps and we develop for such high band devices photoconductive structures able to study very short x-ray emission. Thin polycrystalline MOCVD CdTe films with picosecond response are an alternative material suitable to perform optical correlation measurements of single shot pulses with a very large bandwidth (approximately 50 GHz).

Preliminary characterizations of a new hybrid CdTe structure for x-ray dental imaging

A new 2D imaging system structure is being studied in CEA-LETI for medical and industrial applications, beginning with dental applications. It consists of a bulk CdTe:Cl detection medium connected to a 2D electronic read-out circuit using the indium bumps techniques developed for infrared imaging. The feasibility of such a structure was tested first with 64 X 64 pixels, 100 micrometers pitch. The 900 micrometers thick CdTe sample suits well for x rays up to 100 keV. High absorption efficiency and high spatial resolution can be reached together by using this new x-ray detector. Due to the fact that the electric field channelizes the created charges, unlike structures using scintillators, this new structure requires no compromise in defining the thickness of the x-ray detector medium. Characterization was performed with 70 kV x rays from a standard dental x ray source. The performances (linearity, signal-to-noise ratio, spatial resolution) and the images obtained with the first prototypes confirm the advantages of such detectors. Based on these results, a 20 X 30 mm2 imager for dental applications is now being developed by SOFRADIR and CEA- LETI.

Preliminary characterization of a new hybrid structure with CdTe: x-ray imaging capabilities

This paper presents a new structure of a 2-D imaging system devoted to radiology. The detection upper medium, made with bulk cadmium telluride, is connected to the electronic 2D readout circuit through indium bumps. The 60 X 60 micrometers 2 electrodes, 100 micrometers pitch, are made on the CdTe:Cl detector with standard lithography and ion etching techniques. The silicon circuit is made of n X n independent integrated amplifiers with serial multiplexing readout. The feasibility of such arrangement is made with 64 X 64 pixels. The thickness of 900 micrometers is well suited for 100 keV x rays. Characterization is performed with 10 ms x-ray pulses. Due to electric field the charges are well channelled and high spatial resolution is available in addition with a very high absorption efficiency. The direct absorption of x ray in the readout circuit is negligible. It does not affect either the signal to noise ratio, or the lifetime of the silicon low level analogue ASIC. The presentation includes linearity, sensitivity, noise FTM, and dynamic image discussion.

Recent developments in a CdTe-based x-ray detector for digital radiography

The performance of a new CdTe based x-ray detector devoted to digital radiography are presented. The detectors consist of a 6 cm2 CdTe 2D-array connected to CMOS readout circuit by indium bumps. The final image has 400 X 600 pixels with a 50 micron pitch. This solid-state detector presents the advantages of direct conversion, i.e. high stopping power with high spatial resolution and a significantly higher signal than commercially available scintillator/photodetector systems. The experimental results show excellent linearity, spatial resolution and detective quantum efficiency. The MTF was measured by the angled-slit method: 20 to 30 percent at 10 1p/mm depending on the incident x-ray energy. The measured DQE is about 0.8 at 40 KeV and 100 (mu) Gray dose. Our simulation shows that these experimental results do not reach the theoretical limit. Further improvements are in progress. The first industrial application will be dental radiography due to the small size and the excellent performances. We also tested the detector with x-rays form 20 KeV to 1.25 MeV. Of course the CdTe thickness should then be adapted to the incident x-ray energy.