A. Miccoli

The CluTim algorithm: an improvement on the impact parameter estimates

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 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.