Andrea Castoldi

Conception and design criteria of a novel silicon device for the measurement of position and energy of X-rays

In this paper we present the working principle and the design criteria of the Controlled-Drift Detector, a novel device suitable for the simultaneous measurement of energy and two-dimensional position of X-rays. In the Controlled-Drift Detector the pixel structure typical of a charge-coupled device and the fast readout typical of a silicon drift detector are joined. When the radiation is to be detected, suitable potential barriers are generated to prevent the drift of the signal electrons that are confined within a matrix of integration wells. The potential barrier which prevents the drift is selectively removable to allow the fast transport of the confined signal electrons to the readout electrodes by means of a static drift field. In principle such a device can achieve excellent energy resolution and readout times of some tens of microseconds. Three different confining mechanisms that allow control of the drift of the stored electrons are studied, and their advantages and drawbacks are discussed. First prototypes of the Controlled-Drift Detector are currently in production.

Multiple delay line shaping: a new class of weighting functions suitable for digital signal processing

Abstract We introduce a whole class of weighting functions, that can be implemented with an analog prefilter, an analog-to-digital converter and a FIR or IIR digital filter. These weighting functions feature a finite duration and can have a flat top within less than 1%. General rules are given about the design of such general class of filters. The shape of the obtained weighting function is strictly related to the chosen prefilter, and its duration may be effectively shrinked by the digital processing. However the number of samples required in order to keep a finite duration of the weighting function is shown to increase with the complexity of the prefilter. The developed class of weighting functions can be alternatively implemented with analog delay lines or switched-capacitor filters.

A 55 em2 cylindrical silicon drift detector

Abstract AZTEC, a large area cylindrical silicon drift detector was designed, produced and tested. AZTEC will be the building block of the NA45 and WA98 micro vertex detectors at CERN. Two AZTEC detectors are placed down stream from the target to measure trajectories of charged particles produced in the forward direction. The active area of AZTEC is practically the full usable surface of a 100 mm diameter wafer. The electrons drift radially from the center towards the outside. The sensing anodes are located at a radius of 42 mm. The center of the wafer is cut out and forms a passage for the non interacting beam. With a minimal radius for this hole the active region of the drift detector starts at an inner radius of 3.1 mm. Any larger radius can be selected if necessary. With this geometry and a typical operating voltage the maximum drift time is less than 4μs. Due to constrains in the mask layout the readout region and field electrodes are designed along the 360 sides of a symmetric polygon. All structures on one surface of the wafer are rotated by 0.5° with respect to the other surface. In the middle plane of the detector, where the electrons are mostly transported, the effective geometry is close to a smoothed polygon with 720 sides, cancelling practically all effects of the non-perfect cylindrical symmetry. The radial position of fast charged particles is measured by the electron drift time within the detector. The drift velocity can be monitored by 48 injection points at three different radii. The azimuthal angle is measured by the 360 readout anodes. Each anode is subdivided into five segments, which are interlaced with the neighbouring anodes. By this methode the azimuthal resolution is improved and corresponds to a 720 channel read out.

On the implementation of multiple delay line filters with digital signal processing techniques

By using an analog prefilter and a sampled data processor, it is possible to easily implement an entire class of weighting functions. Their main feature is a finite time duration; an additional interesting property is the small number of samples needed to process a pulse. Flat tops within better than 1% accuracy can also be achieved. Possible implementations of these filters are discussed with emphasis on causes of inaccuracy. In particular, these filters come out to be time-variant. Synchronization issues are therefore developed and thoroughly discussed. In a mixed analog-digital implementation, special regard should be given to the A/D converter: the requirements on its characteristics are derived. In particular, a simple way to evaluate its resolution (number of bits) is derived, and the interrelationships between the achievable accuracy, ballistic deficit reduction, and the characteristics of the analog prefilter are discussed. Strategies aimed at achieving and/or at avoiding the synchronization of the input pulses with the digital acquisition are proposed. >

Electron confinement in drift detectors by means of "channel-stop" implants: characterization at high signal charges

Electron confinement in the direction transverse to the drift can be implemented in silicon drift detectors by means of deep p-implants. The reduced broadening of the electron cloud due to the deep p-implants has been tested as a function of the signal amplitude up to 200000 electrons. The maximum number of electrons for which full confinement is achieved has been measured. The dependence of this threshold charge on the potential barrier generated by the deep p-implants, the size of the confinement, and the detector operating conditions are reported.

A NEW HIGH RESOLUTION X-RAY IMAGING DETECTOR WITH FAST READ-OUT

Abstract A new silicon detector design suitable for X-ray imaging is presented. The new detector features energy and position resolution comparable with a fully depleted pn-charge coupled device and read-out times of few tens of microseconds.

A New Class Of Weighting Functions Readily Available With Sampled Data Processors

By using an analog prefilter and a sampled data processor, it is possible to easily implement an entire class of weighting functions. Their main feature is a finite time duration; an additional interesting property is the small number of samples needed to process a pulse. Flat tops witlun better than 1% accuracy can also be achieved. Possible implementations of these filters are discussed with emphasis on causes of inaccuracy. In particular, these filters come out to be time-variant. Synchronization issues are therefore developed and thoroughly discussed. In a mixed analogdigtal implementation, particular regard should be gven to the A/D converter: the requirements on its characteristics are derived. On the contrary, once a pulse is sampled and digitized, the realization of a delay line is straightforward, since it can be obtained with memory elements arranged, e.g., in a shift regrster. The availability of analog to Qgital converters of ever increasing speed and quality makes it also possible to conceive Delay Line shapers obtained with Qgital techniques. In fact, one processor based on switched capacitor filters has been reported, suitable for the LHC experiment at CERN [2]. In this paper, we examine the conditions that the digital filter should fulfill in order to obtain a signal shaping that retains a finite duration of the impulse response once a particular analog prefilter is chosen. The well known Delay Line, Double Delay Line shapers as well as those shapers described in [2] are obtained as particular cases. Some effects due to the non idealities of the filters are also dealt with, together with a discussion of synchronization issues.

SILICON DEVICES FOR LOW-ENERGY X-RAY DETECTION : COMPARISON BETWEEN THE CHARGE-COUPLED DEVICE AND THE SEMICONDUCTOR DRIFT CHAMBER

Abstract The present work summarises the performance of two silicon devices, a fully depleted charge-coupled device and a semiconductor drift chamber, as X-ray fluorescence detectors at low excitation energy (2–10 keV). The definition of the requirements of the detection system in typical X-ray Absorption Fine Structure (XAFS) experiments is discussed. The main topic of the work is the analysis of the charge (energy) resolution and maximum interaction rate when using state-of-the-art external front-end electronics and when at least the first transistor of the preamplifier is integrated on the detector wafer. A comparison study between the charge-coupled device and the semiconductor drift chamber, both in the case of a “classical” and a multi-linear architecture, has been conducted. The experimental results obtained with both devices demonstrate the possibility of excellent performance in XAFS experiments carried out in synchrotron radiation facilities.

High charge effects in silicon drift detectors with lateral confinement of electrons

A new drift detector prototype which provides suppression of the lateral diffusion of electrons has been tested as a function of the signal charge up to high charge levels, when electrostatic repulsion is not negligible. The lateral diffusion of the electron cloud has been measured for injected charges up to 2/spl middot/10/sup 5/ electrons. The maximum number of electrons for which the suppression of the lateral spread is effective is obtained.