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A Drift-velocity Monitor for FOPIs Drift-chambers

N. Zernezki1, A. Schüttauf1, K. D.Hildenbrand1, Y. J.Kim1, M. Kiš1, P. Koczon1, Y. Leifels1, X. Lopez1, M. Marquard1, X. Zhang1, and the FOPI Collaboration 1GSI, Darmstadt, Germany We use a modified version of the Goofi-drift-monitor, developed at GSI and MPI-Munich [1][2][3] to test a new electronic chain and readout concept, which can be implement into the existing FOPI slow control system [5]. The design goal for the drift-velocity monitor is to measure the drift-velocity (vd) with a relative error below Δvd vd ≤1% at an absolute value vd ∼ 4 cm/μs which is the operation value in FOPIs drift chambers. The realization of the drift-monitor is rather simple: we use a field-cage of 25 cm length and 3 proportional counters to determine the drift-velocity by a time measurement knowing the distance between the counters. Two proportional-counters (Top/Bottom) are placed within the field-cage and serve as start counters for the time measurement; the third counter (Pickup) is placed at the end of the cage and delivers the stop signals. The start signals are delivered by two Am − α-sources (4 kBq). These α-particles ionize, on their path through the field-cage, the detector gas and are finally detected in the Top and Bottom unit. The third counter (Pickup) collects the electrons which are created by the α-particles in the gas. The time difference between Top-to-Pickup and Bottom-to-Pickup is used for determining the drift-velocity.