A. Castoldi

Performance of the UA6 large-area silicon drift chamber prototype

Abstract This report presents results on the performance of a large-area silicon drift detector (∼ 4 × 4 cm 2 ), which has been designed for use as a high-resolution tracking device in the experiment UA6 at the CERN p- p collider. We give here the basic characteristics of the design, and report the first experimental results. The influence, on the detectors performance, of the adopted design criteria and of the quality of the semiconductors has been experimentally determined and is discussed. Results of the first drift-time calibration using an on-board device for charge injection are also given.

A new silicon drift detector with reduced lateral diffusion

Abstract We present a method to reduce the effect of diffusion in the direction transverse to the drift in silicon drift detectors. This is achieved by creating regions of deep p-implant parallel to the drift direction that act as rigid guidelines during the drift of the charge cloud generated by a radiation interaction. The influence of the deep implanted acceptors on the lateral confining field is discussed. A prototype has been designed, fabricated and tested. First experimental results are reported which demonstrate the achieved reduction of the lateral width of the electron cloud with respect to free broadening.

A quantitative x-ray detection system for gold nanoparticle tumour biomarkers

X-ray fluorescence techniques have proven beneficial for identifying and quantifying trace elements in biological tissues. A novel approach is being developed that employs x-ray fluorescence with an aim to locate heavy nanoparticles, such as gold, which are embedded into tissues. Such nanoparticles can be functionalized to act as markers for tumour characteristics to map the disease state, with the future aim of imaging them to inform cancer therapy regimes. The uptake of functionalized nanoparticles by cancer cells will also enable detection of small clusters of infiltrating cancer cells which are currently missed by commonly used imaging modalities. The novel system, consisting of an energy-resolving silicon drift detector with high spectral resolution, shows potential in both quantification of and sensitivity to nanoparticle concentrations typically found in tumours. A series of synchrotron measurements are presented; a linear relationship between fluorescence intensity and gold nanoparticle (GNP) concentration was found down to 0.005 mgAu ml(-1), the detection limit of the system. Successful use of a bench-top source, suitable for possible future clinical use, is also demonstrated, and found not to degrade the detection limit or accuracy of the GNP concentration measurement. The achieved system sensitivity suggests possible future clinical usefulness in measuring tumour uptake in vivo, particularly in shallow tumour sites and small animals, in ex vivo tissue and in 3D in vitro research samples.

Spectroscopic-grade X-ray imaging up to 100 kHz frame rate with controlled-drift detectors

Controlled-drift detectors are fully depleted silicon detectors for X-ray imaging that combine good position resolution with very fast frame readout. The basic feature of the controlled-drift detector is the transport of the charge packets stored in each pixel column to the output electrode by means of a uniform drift field. The drift time of the charge packet identifies the pixel of incidence. Images of an X-ray source obtained with the controlled-drift detector up to 100-kHz frame rate are presented and discussed. The achievable energy resolution as a function of the operating temperature and frame rate is analyzed.

Room-temperature 2D X-ray imaging with the controlled-drift detector

The controlled-drift detector is a single-photon counting X-ray imaging silicon detector that features excellent energy and time resolution. Its distinctive feature is the readout of the signal charge packets stored in each pixel column by means of an electrostatic field in few /spl mu/s. The drift time of the charge packet identifies the pixel of incidence. Several one-dimensional flat images of single pixel columns have been acquired to investigate the achievable position resolution and the improvement of the energy resolution at high frame rates (up to 125 kHz). At 62.5 kHz, the room-temperature energy resolution at the Mn K/spl alpha/ line is better than 300 eV full-width at half-maximum. The first two-dimensional X-ray images of different masks carried out at frame frequencies in the range 10-100 kHz, both with radioactive and synchrotron light sources, will be presented.

Three-dimensional analytical solution of the Laplace equation suitable for semiconductor detector design

In this paper we present a method to obtain the three-dimensional (3-D) solution of the Laplace equation in closed form suitable for semiconductor detector design, The method applies to planar geometry cases (microstrip detectors, pixel detectors, drift detectors, etc.) with possible mixed boundary conditions on the surfaces of the wafer. The solution inside the detector volume is expressed by a series of elementary functions, The method requires lower CPU time and memory than methods based on 3-D discretization, in particular for the study of 3-D carrier trajectories inside the volume of semiconductor detectors and for inter-electrode capacitance calculations.

New electrode geometry and potential distribution for soft X-ray drift detectors

Abstract A new electrode geometry for silicon drift chambers is proposed, minimizing the surface of the detector covered with oxide, still maintaining a reasonably high drifting field with no limitation in the det ector size. This Letter describes the design criteria and discusses the peculiar trajectory of signal electrons within the device. The proposed detector is particularly suited for fine spectroscopy measurements of visible light of soft X-rays, where radiation is totally absorbed within a few microns from the interface.

A method for doping fluctuations measurement in high resistivity silicon

This paper presents a method for the measurement of doping fluctuations in high resistivity silicon wafers. Spatial fluctuations of doping are derived from the knowledge of the electrostatic potential in a completely depleted semiconductor bulk. The potential variations are indirectly measured through the analysis of the trajectories of majority carriers drifting within the depleted semiconductor material. Regions of semiconductors up to the full wafer can be investigated. An example of mapping along parallel lines of a floating zone 2 K Ω cm silicon wafer over an area of 0.4×0.8 cm2 is presented. The relative sensitivity of the method is better than 1%.

Electron injection in semiconductor drift chambers

Abstract The paper reports the first successful results of a simple new MOS device for injecting electrons at given sites in silicon drift chambers. The device allows on-line calibration of the time of flight of electrons within the detector, making the reconstruction of their position independent of the temperature-dependent mobility of electrons. Several of these auxiliary devices can be implemented in silicon drift chambers without additional steps in the fabrication process.

Beam test of a large area silicon drift detector

Abstract The results from the tests of the first large area (4 × 4 cm 2 ) planar silicon drift detector prototype in a pion beam are reported. The measured position resolution in the drift direction is σ = 40± 10 μ m.