Giovanni Tassielli

Single-hit resolution measurement with MEG II drift chamber prototypes

Drift chambers operated with helium-based gas mixtures represent a common solution for tracking charged particles keeping the material budget in the sensitive volume to a minimum. The drawback of this solution is the worsening of the spatial resolution due to primary ionisation fluctuations, which is a limiting factor for high granularity drift chambers like the MEG II tracker. We report on the measurements performed on three different prototypes of the MEG II drift chamber aimed at determining the achievable single-hit resolution. The prototypes were operated with helium/isobutane gas mixtures and exposed to cosmic rays, electron beams and radioactive sources. Direct measurements of the single hit resolution performed with an external tracker returned a value of 110

The CluTim algorithm: an improvement on the impact parameter estimates

\mu

A full acquisition and elaboration system for Drift Chambers

m, consistent with the values obtained with indirect measurements performed with the other prototypes.

The Use of FPGA in Drift Chambers for High Energy Physics Experiments

A Drift Chamber (DC) is a detector used in high energy physics experiments for determining charged particles trajectories. It consists of a gas volume and of an array of thin anode wires at high voltages generating high electric fields. Charged particles passing through the gas ionize it creating electron/ion pairs along their path, which, accelerated by the electric fields, produce signal pulses on the sense wires. The signal pulses from all the wires are then collected and the particle trajectory is tracked assuming that the distances of closest approach (the impact parameter) between the particle trajectory and the wires coincide with the distance between the closest ion cluster and the corresponding nearest wire. The widespread use of helium based gas mixtures, aimed at minimizing the multiple scattering contribution to the momentum measurement for low momentum particles, produces, as a consequence, a low ion ionization clusters density (12cluster/cm in a 90/10helium/isobutanemixture), thus introducing a sensible bias in the impact parameter assumption, particularly for short impact parameters and small cell drift chambers. Recently, it has been proposed an alternative track reconstruction (Cluster Counting/Timing) technique, which consists in measuring the arrival times on the wires of each individual ionization cluster and combining these times to get a bias free estimate of the impact parameter. Typical time separations between consecutive ionization acts, in a He-based gas mixture, range from a few ns, at small impact parameters, to a few tens of ns, at large impact parameters. Therefore, in order to efficiently applying the cluster timing technique, it is necessary to have read-out interfaces capable of processing high speed signals, in which one can easily isolate pulses due to different ionization clusters. The wire signals generated by the drift chamber, before being processed, are converted from analog to digital with the use of flash-ADCs. Requirements on drift chamber performance impose the conversions at sampling frequencies of at least 1 GS/s with at least 8-bit resolution. These constraints, together with the maximum drift times, usually of the order of 1 microsecond, and with the large number of acquisition channels, typically of the order of tens of thousand, mandate some sizeable data reduction, which, however, must preserve all the relevant information. Identifying both the amplitude and the arrival time of each peak associated to each individual ionization cluster is the minimum requirement on the data transfer for storage. More specifically, a fast readout algorithm (CluTim) for identifying, in the digitized drift chamber signals, the individual ionization pulse peaks and recording their time and amplitude has been implemented and tested on a Virtex 6 core FPGA board. The CluTim algorithm, that we have been developed, is able to process the data in real-time and in particular it:  identifies, in the digitized signal, the peaks corresponding to the different ionization cluster;  stores each peak amplitude and timing in an internal memory;  sends the data stored to an external device when specific trigger signals occur. This algorithm has been implemented tested on different xilinx fpga getting a data reduction factors of more than one order of magnitude respect a the older algorithms.

Ultra light drift chambers for precision physics

We propose a fast acquisition and elaboration system in order to process signals coming from Drift Chambers. The system is made of an analog Front End, designed in our laboratory for signal acquisition and a Demo Board by Texas Instruments for data elaboration. The Front End electronics is a multistage amplifier board based on high performance commercial devices. Texas Instruments board includes an Analog to Digital Converter and a FPGA (Field Programmable Gate Array) in order to implement algorithm for Cluster Counting and Timing purposes.

Cluster counting/timing techniques for drift chambers

In this chapter, we describe the design of a field programmable gate array (FPGA) board capable of acquiring the information coming from a fast digitization of the signals gen‐ erated in a drift chambers. The digitized signals are analyzed using an ad hoc real‐time algorithm implemented in the FPGA in order to reduce the data throughput coming from the particle detector.

A readout electronics for drift chambers signals processing

We present a novel low mass drift chamber concept, developed in order to fulfill the requirements imposed by the search for extremely rare processes, which require high resolutions (order of 50-200 keV/c) for particle momenta in a range (50-300 MeV/c) totally dominated by the multiple scattering contribution. The innovative concept in the chamber design is the separation of the gas containment and the mechanical support structure. We describe a geometry optimization procedure and a new semi-automatic wiring strategy with a feed-through-less wire anchoring system.

The construction technique of the high granularity and high transparency drift chamber of MEG II

We describe the advantages of the cluster counting techniques over the traditional ways of integrating the ionization charge for the purpose of particle identification. We also discuss the improvement in the determination of the impact parameter resolution in a drift cell using cluster timing techniques instead of considering only the arrival time of the first drifting electron. Lastly, we illustrate a possible way to define a fast trigger/filter for high granularity drift chambers.

Application of the Cluster Counting/Timing techniques to improve the performances of high transparency Drift Chamber for modern HEP experiments

A low noise, high speed board designed for drift chamber signals processing has been developed. The Front End electronics is a multistage amplifier based on high performance commercial devices. In addition, a fast readout algorithm for Cluster Counting and Timing purposes has been implemented on a Xilinx-Virtex 4 core FPGA. The algorithm analyzes and stores data coming from a Helium based drift cell and represents the outcome of balancing between efficiency and high speed performance.