High Drift Fields Research Articles

Design and Experimental Characterization of Multilinear Silicon Drift Detectors for 2D Position-Sensing Operating at High Drift Fields

We describe the design and the experimental characterization of multilinear silicon drift detectors suitable for two-dimensional (2D) position-sensing applications with very fast readout (i.e., operating at high drift fields). The specific design issues taken into account in order to operate at high drift fields (e.g., able to achieve drift speeds of the order of 1 cm/μs) will be discussed. The designed detectors feature the possibility to independently bias the field-plates covering the interstrip oxide in order to investigate the impact of the field-plate voltage on the transport properties and on the surface generated leakage current. In order to test the effectiveness of the proposed designs, a fast (100 ps) infrared pulsed laser, generating the desired signal charge, has been focused to a small spot (10 μm FWHM) on the front side of the detector and displaced along the drift and lateral directions to study the detector response. The experimental characterization, carried out close to room temperature, includes the assessment of the maximum drift field, of the achievable spatial resolution, and of the lateral charge confinement.

Applications of blow-up theory to thyristor turn-on

An analysis of a symmetric double-gated thyristor is performed. By further imposing that the carrier transit times depend on the drift field, we arrive at a nonlinear partial differential equation. It describes the transverse behavior and admits blow-up-type solutions leading to current filamentation, i.e., a possible explanation for hot spots. The shortening of the bases during turn-on, associated with high drift fields expanding from the central junction toward the cathode junction is also predicted. These results could provide a simple interpretation for experimental data in GTOs published by other authors.

Micro-CAT with redundant electrodes (CATER)

High-rate X-ray or neutron counting introduces the problem of hit multiplicity when 2D position reconstruction is demanded. Implementation of a third readout electrode having a different angle than the anode or cathode allows to eliminate multiplicity problems. We present experimental results of a new type of gas-filled micro-patterned radiation detector, called “Compteur à Trous à Electrodes Redondantes (CATER)”, that disposes of such an extra readout channel in the form of a ring-shaped electrode that is positioned between the anode and the cathode. The ionic signal is shared between the ring–electrode and the cathode strip in a way that can be controlled by their potential difference. We observe a strong signal dependence on the drift field, which can be understood by the reduced transparency for the primary charge at high drift fields.

Suppressing drift chamber diffusion without magnetic field

The spatial resolution in drift chamber detectors for ionizing radiation is limited by diffusion of the primary electrons. A strong magnetic field along the drift direction is often applied (Fancher et al., Nucl. Instr. and Meth. A 161 (1979) 383) because it suppresses the transverse diffusion, improving the resolution but at considerable increase in cost and complexity. Here we show that transverse track diffusion can be strongly suppressed without any magnetic field. This is achieved by using a gas additive which reversibly captures primary ionization electrons, forming negative ions. The ions drift with thermal energies even at very high drift fields and low pressures ( E/P=28.5 V/ cm torr), and the diffusion decreases with increasing drift field. Upon arrival at the avalanche region of the chamber the negative ions are efficiently stripped and ordinary avalanche gain is obtained. Using this technique, r.m.s. transverse diffusion less than 200 μm has been achieved over a 15 cm drift path at 40 torr with zero magnetic field. The method can provide high spatial resolution in detectors with long drift distances and zero magnetic field. Negative ion drift chambers would be particularly useful at low pressures and in situations such as space-based or underground experiments where detector size scaleability is important and cost, space, or power constraints preclude the use of a magnetic field.

A two-stage, high gain micro-strip detector

A two stage position-sensitive gas proportional counter has been constructed by tightly coupling a Gas Electron Multiplier (GEM) with a Micro-Groove Detector (MGD). The GEM was used as the first amplifying stage and was optimised to transmit close to 100% of the primary charge even at very high drift fields (10 kV/cm). Very narrow GEM-MGD separations (0–600 μm) were used so that the active volume of the detector is still very thin (3–3.6 mm) and the required drift field could be maintained using an acceptable drift voltage (around 4000 V). Very high combined gains (up to 3×10 5) were obtained with this system. The detector was found to be spark-free in the presence of HIPs (alpha particles) up to gains in excess of 20 000.

The micro-groove detector

We introduce the Micro-Groove Detector (MGD), a new type of two-dimensional position-sensitive gas proportional counter produced using advanced Printed Circuit Board (PCB) technology. The MGD is based on a thin kapton foil, clad with gold-plated copper on both sides. An array of micro-strips at a typical pitch of 200 μm is defined on the top metal layer. Using as a protection mask the metal left after the patterning, charge amplifying micro-grooves are etched into the kapton layer. These end on a second micro-strip pattern defined on the bottom metal plane. The two arrays of micro-strips can have an arbitrary relative orientation and so can be used for read-out to obtain 2-D positional information. First results from our systematic assessment of this device are reported: gas gain >15 000 , rate capability above 10 6 mm −2 s −1, energy resolution 22% at 5.4 keV, no significant charging or aging effects up to 5 mC/cm and full primary charge collection efficiency even at high drift fields.

Optimization of microstrip gas chamber design and operating conditions

We discuss recent experimental and theoretical work aimed at optimizing the structure and operating conditions of microstrip gas chambers. In a systematic set of measurements we have found gas mixtures allowing high stable gains; the best results were obtained using argon-dimethylether in equal percentages (50-50), and mixtures of dimethylether and carbon dioxide. Detectors exhibit lower noise and better energy resolution when using high drift fields, i.e. above 5 kV/cm; in these conditions, stable gains in excess of 10 4 could be achieved. We also discuss a model to characterize the breakdown depending on the combined field strength at the edges of the anode and cathode strips; using a program that computes fields and gains and applying the discharge criterion, we have estimated the optimum geometry of the detectors. The results indicate that higher gains before discharges can be obtained for wide cathodes: as an example, for 10 μm anodes we predict an increase by a factor of 4 of the maximum gain when increasing the width of the cathode strips from 80 μm to 160 μm.

Theory of hot electrons on the surface of 4He

We report here the theoretical analysis of the hot electon mobility on the liquid He surface. We consider the proper form of the electron-ripplon interaction as well as the electron-gas interaction and treat the inter-subband and the intra-subband collisions on the same footing in order to analyze mobility at low temperatures and at high drift fields. Dramatic non-linear behaviors are predicted to occur at low temperature where the electon-ripplon interaction is dominant.

Mobility of electrons on the surface of liquid helium

A time-of-flight mobility measurement ($1.17<T<2.7$ K) is made on low-density electron pulses ($n<{10}^{4}$ ${\mathrm{cm}}^{\ensuremath{-}1}$) moving above the surface of liquid helium with the electrons constrained against the liquid surface by a net dc electric field. Below 1.45 K the mobility starts to level off in the manner predicted by Cole although at the lowest temperatures the experimental value is slightly higher than Cole's value. This is direct evidence that the electron-ripplon interaction is important at these temperatures. The mobility at high drift fields is also reported.