Marc Cuzin

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.

Power switching with a new material: CdTe:Cl

CdTe material, with its high molecular weight (240), is widely used for radiation detection. The bandgap energy of CdTe is 1.45 eV. When doped with chlorine, which compensates the acceptor level introduced by cadmium vacancies, CdTe is intrinsic and presents a very high resistivity (> 108 (Omega) cm). The contacts were made by electroless metal deposition and further annealing; they were characterized with DC and pulsed voltages. For the first time, this paper presents power switching experiments with CdTe material. We investigated different types of crystals and contact geometries with gap size varying from less than one mm to a few millimeters. The switches were activated by a YAG laser 10 ns FWHM pulses (1.06 micrometers ) or with 160 kV X-rays 30 ns FWHM pulses. The time constant of recovery was found to be more than 10 ns. In some cases, for high voltages corresponding to fields higher than a few kV/cm, large recovery times of more than 100 ns were measured. This apparently long carrier lifetime, combined with the high resistivity, make CdTe an alternative material to Si and GaAs for some switching applications.

Single-pulse analysis with a 35-GHz optical sampler

A compact optical sampler has been developed for the imaging of single-shot optical or electrical pulses. The analyzed duration is 400 ps, and the temporal sampling rate is 4 ps (250 GSa/s). The spectral density of the input signal is restored in the resulting data until 35 GHz. The device consists of a 50 Ohms propagation microstrip structure, lined by periodically arranged ultra-fast photoconductive switches. When illuminated, these switches short the propagation line to storing capacitors. To operate the device, the input signal is injected onto the line; when the signal to analyze is displayed along the line, facing the optical gates, these gates are switched on, and the storing capacitors collect an amount of charges related to the voltage present on the line. When an optical or X-ray pulse is on analysis, the electrical signal injected into the propagation line is delivered by a CdTe very fast detector. This device has been operated with a femtosecond laser. The analysis of a single pulse through the optical sampler results in an amplitude versus time curve, exhibiting temporal characteristics very close to those measured with a classical sampling head (repetitive signal) and much shorter than those measured with single shot existing analyzer.

Optoelectronic sampler: modeling and characterization

In this paper, an optoelectronic sampler is described. The device is made with microstrip propagation lines and ultra- rapid photoconductive switches which are integrated in the same substrate. The sampler is used to measure the output electrical signal provided by an ultra-fast detector. The detector electrical pulse lasts from 20 ps to 100 ps and is sampled every 4 ps. The total record length is 400 ps.

Realization and characterization of an electrical single-pulse ultrafast optosampler

We describe a wide band optoelectronic device for short electrical single pulses sampling. The devices are made with microstrip propagation lines on a sapphire substrate and ultra-rapid photoconductive switches are integrated on the same substrate (synchronously to the electrical pulse, a femtosecond laser pulse is used to switch the photoconductive elements). The switches are made of thin semiconductor films deposited by molecular beam epitaxy at low temperature. The propagation on the stripline structure has been simulated and characterized. A 16 channels prototype with 18 ps sampling step has been realized and tested. The analysis of one sub- 50 ps electrical transient has been demonstrated. This type of device should easily allow to reach sampling steps down to 5 ps.

Electromagnetic study of an electronic sampler for picosecond pulse measurements

This paper deals with an optoelectronic sampler whose expected time resolution is 5 ps (corresponding to a bandwidth up to 50 GHz). The sampler device is made up of metallic stripline combined with fast photoconductor thin film. This device is studied along two interacting directions: 1) sampler description and operation 2) electromagnetic study In particular, the electromagnetic study is discussed here. The purpose of this study is to understand the electromagnetic behaviour of the optoelectronic sampler.