Christer Fröjdh

First experimental tests with a CdTe photon counting pixel detector hybridized with a Medipix2 readout chip

We present preliminary tests of hybrid pixel detectors consisting of the Medipix2 readout chip bump-bonded to a 1-mm thick CdTe pixel detector. This room temperature imaging system for single photon counting has been developed within the Medipix2 European Collaboration for various imaging applications with X-rays and gamma rays, including dental radiography, mammography, synchrotron radiation, nuclear medicine, radiation monitoring in nuclear facilities. The Medipix2+CdTe hybrid detector features 256x256 square pixels, a pitch of 55 /spl mu/m, a sensitive area of 14x14 mm/sup 2/. Here we analyzed the quality of the detector and bump-bonding and the response to nuclear radiation of the first CdTe hybrids. The CdTe pixel detectors, with Pt contacts, showed an ohmic response when negatively biased to less than 100 V (electrons collection mode). Tests were also performed in holes collection mode, where a non-resistive behaviour was observed above +15 V. We performed a series of imaging tests with gamma radioactive sources and with an X-ray tube. In flood illumination, we observed for all detectors the presence of numerous, stable small-scale structures in the form of small circles, with the central pixels showing a reduced counting efficiency with respect to the periphery (in electrons counting regime).

Directional radiation detector

Many applications likehomeland security, radiation protection, control of fissile material proliferation and other require not only detection of radioactive materials, but also their localization. We are presenting a directional detector based on an array of semiconductor detectors capable to determine direction where the radioactive source is placed. Semiconductor single pad detectors are arranged into rows and separated by a shielding material. Selection of the detectors and shielding material depends on the type and energy of the radiation desired to monitor (i.e. X-rays, gammas or neutrons). Level of the signal, i.e. count rate, in each detector depends on the angle of the incoming radiation. Analysis of the count rate in each detector allows calculating angular position of the source. A series of simulations and evaluating measurements of the directional radiation detection principle is presented.

An X-ray imaging pixel detector based on a scintillating guides screen

We have developed and preliminary tested a new digital X-ray imaging sensor based on a scintillating guide screen. The scintillating guides are used to channel the emitted visible light to the pixel detector. This enables us to avoid the well-known tradeoff between detection efficiency and spatial resolution which is encountered when a non-patterned scintillating layer is used on top of a CCD. A prototype has been fabricated using microtechnologies. The scintillator is CsI:Tl and the low-index cladding material used to channel the light is silicon dioxide. The performance of this prototype has been compared to that of a thick CsI single crystal. The results concerning the spatial resolution are quite promising and demonstrate a proof-of-principle. However, the performance in terms of signal to noise ratio and sensitivity have to be improved. These problems are currently addressed.

Improvement of an X-ray imaging detector based on a scintillating guides screen

An X-ray imaging detector has been developed for dental applications. The principle of this detector is based on application of a silicon charge coupled device covered by a scintillating wave-guide screen. Previous studies of such a detector showed promising results concerning the spatial resolution but low performance in terms of signal to noise ratio (SNR) and sensitivity. Recent results confirm the wave-guiding properties of the matrix and show improvement of the detector in terms of response uniformity, sensitivity and SNR. The present study is focussed on the fabrication of the scintillating screen where the principal idea is to fill a matrix of Si pores with a CsI scintillator. The photoluminescence technique was used to prove the wave-guiding property of the matrix and to inspect the filling uniformity of the pores. The final detector was characterized by X-ray evaluation in terms of spatial resolution, light output and SNR. A sensor with a spatial resolution of 9 LP/mm and a SNR over 50 has been achieved using a standard dental X-ray source and doses in the order of those used at the moment by dentists (around 25 mR).

An X-ray imaging pixel detector based on scintillator filled pores in a silicon matrix

Abstract An X-ray imaging pixel detector has been fabricated and preliminary testing has demonstrated a proof-of-principle. The detector is based on a conventional charge coupled detector (CCD)-imaging detector where a scintillator-filled silicon pore matrix provides enhanced X-ray sensitivity without sacrificing lateral resolution. The scintillator blocks inside the pores provide light guiding of the emitted visible photons to increase the number of photons detected by the CCD element. Preliminary X-ray measurements demonstrate good lateral resolution although non-uniform filling of the pore matrix results in a pronounced fixed noise pattern. The detector has primary applications in dental imaging but would also be of importance in other imaging techniques where the X-ray absorption length exceeds lateral pixel size.

X-ray imaging using a 320 x 240 hybrid GaAs pixel detector

The authors present room temperature measurements on 200 {micro}m thick GaAs pixel detectors, which were hybridized to silicon readout circuits. The whole detector array contains 320 x 240 square shaped pixel with a pitch of 38 {micro}m and is based on semi-insulating liquid-encapsulated Czochralski (LEC) GaAs material. After fabricating and dicing, the detector chips were indium bump flip chip bonded to CMOS readout circuits based on charge integration and finally evaluated. This readout chip was originally designed for the readout of flip chip bonded infrared detectors, but appears to be suitable for X-ray applications as well. A bias voltage between 50 V and 100 V was sufficient to operate the detector at room temperature. The detector array did respond to x-ray radiation by an increase in current due to production of electron hole pairs by the ionization processes. Images of various objects and slit patterns were acquired by using a standard X-ray source for dental imaging. The new X-ray hybrid detector was analyzed with respect to its imaging properties. Due to the high absorption coefficient for X-rays in GaAs and the small pixel size, the sensor shows a high modulation transfer function up to the Nyquist frequency.

Simulation of the X-ray response of scintillator coated silicon CCDs

Silicon CCDs are used for X-ray imaging in fields as dentistry and materials testing. Due to the low X-ray absorption in silicon the CCD is generally coated with a scintillating layer. In order to evaluate the response for different combinations of scintillator material, scintillator geometry and detector we have developed a simulation program. The program calculates the signal to noise ratio and spatial resolution based on both the signal from the scintillating layer and the signal from direct detection of X-rays in the detector. Results obtained with this program indicate that the signal to noise ratio in the system is optimized by using a scintillator with high X-ray absorption and high light output while minimizing the signal from direct detection in the CCD. The spatial resolution can be increased by defining pixels in the scintillator.

Performance of scintillating waveguides for CCD-based X-ray detectors

Scintillating films are usually used to improve the sensitivity of CCD-based X-ray imaging detectors. For an optimal spatial resolution and detection efficiency, a tradeoff has to be made on the film thickness. However, these scintillating layers can also be structured to provide a pixellated screen. In this paper, the study of CsI(Tl)-filled pore arrays is reported. The pores are first etched in silicon, then oxidized and finally filled with CsI(Tl) to form scintillating waveguides. The dependence of the detector sensitivity on pore depth, varied from 40 to 400 /spl mu/m here, follows rather well theoretical predictions. Most of the detectors produced in this work have a detective quantum efficiency of the incoming X-ray photons of about 25%. However, one detector shows that higher efficiency can be achieved approaching almost the theoretical limit set by Poisson statistics of the incoming X-rays. Thus, we conclude that it is possible to fabricate scintillating waveguides with almost ideal performance. Imaging capabilities of the detectors are demonstrated.

Metallized and oxidized silicon macropore arrays filled with a scintillator for CCD-based X-ray imaging detectors

Silicon charge-coupled devices (CCDs) covered with a scintillating film are now available on the market for use in digital medical imaging. However, these devices could still be improved in terms of sensitivity and especially spatial resolution by coating the CCD with an array of scintillating waveguides. In this paper, such waveguides were fabricated by first etching pores in silicon, then performing metallization or oxidation of the pore walls and finally filling the pores with CsI(Tl). The resulting structures were observed using scanning electron microscopy and tested under X-ray exposure. Theoretical efficiencies of macropore arrays filled with CsI(Tl) were also calculated, indicating that the optimal pore depth for metallized macropore arrays is about 80 /spl mu/m while it is around 350 /spl mu/m for oxidized ones. This result, together with the roughness of the metal coating, explains why lower SNR values were measured with the metallized macropores. Indeed, the macropore arrays had depths in the range of 210-390 /spl mu/m, which is favorable to oxidized structures.

Full band Monte Carlo simulation of electron transport in 6H-SiC

A study of electron transport in 6H-SiC is presented using a full band Monte Carlo simulation model. The Monte Carlo model uses four conduction bands obtained from a full potential band structure calculation based on the local density approximation to the density functional theory. Electron–phonon coupling constants are deduced by fitting the Monte Carlo simulation results to available experimental data for the mobility as a function of temperature. The saturation velocity perpendicular to the c axis is found to be near 2.0×107 cm/s, which is in good agreement with the experimental data available. In the c-axis direction the saturation velocity is much lower (4.5×106 cm/s). There are no direct experimental results available for the saturation velocity in the c-axis direction. A comparison between two-dimensional simulations of a 6H-SiC permeable base transistor, using transport parameters obtained from the Monte Carlo simulations, and experimental I–V characteristics confirms the low value. The physical m...