The HADES spectrometer including all major detector units, read-out, data-acquisition and trigger electronics has been tested in the commissioning beam time in November 2000. It consists of ring imaging Cherenkov counters, multiwire drift chambers (MDC) of planes I to III, time of flight scintillation hodoscopes (TOF) and pre-shower modules [1]. The first large-area drift chamber MDCIII for the third tracking plane produced at FZ Rossendorf was installed in sector four of the HADES spectrometer (Fig. 1).

Fig. 1 MDCIII mounted on the support frame of the HADES spectrometer. Under normal working condition the drift chambers are hidden inside the spectrometer core.

A 1.5 AGeV carbon beam with an intensity of 5x10⁶ per spill hits a carbon target. The spill length amounted to 5 seconds. Most of the reaction products crossing the spectrometer were protons and pions.
The chamber volume of 150 l has been filled with a gas mixture of 40% iso-butane and 60% helium at a flow rate of 10 l/h. The leakage rate was 0.5 l/h

Fig. 2 The analog signals monitored with a scope. The signals shown from top to bottom were measured on MDC planes I to III.

The applied high voltage amounts to -2 kV for both the cathode and field wires. A selected anode wire is connected to an analog output to check the amplified signals as shown in Fig. 2. The shape of the signal shows a risetime of 30 ns up to the saturation level of the preamplifier.
The MDCIII is equipped with dedicated preamplifier/shaper/discriminator boards inserted into motherboards which itself contain the digitizers (TDC). These daughter- and motherboard components are mounted onto the detector frame under accurate consideration of the grounding [3].
The data acquisition was triggered by the TOF multiplicity ³ 4. A typical TDC spectrum is shown in Fig.  3. The drift velocity was estimated to be 33 mm/ns based on a drift time distribution of 180 ns (FWHM) and the lateral half cell size of 6 mm. There is no visible effect of the strong magnetic field (0.2 T), created by superconducting solenoids in front of the MDCIII, onto the drift time spectrum.

Fig. 3 Drift time distribution of leading electrons in one anode layer of MDCIII (0.5 ns/channel). The time measurement started by a common signal, created by the TOF trigger, and stopped by an individual anode signal.

Fig. 4 Two prongs of an event align between the target and the points of intersection in all three MDC's of sector four (bottom) of the HADES spectrometer. Each prong is characterised by the point of intersection of all fired signal wires in the two dimensional projection plane. The MDC plane numbers have been labeled by colors.

A stereo optical positioning system [2] monitors with a resolution of 1 mm the three dimensional displacement between the MDC's before and behind the magnet. The mean observed displacement during all runs was less than 10 mm.
Fig. 4 shows as a highlight of the commissioning, a plot of the reconstruction of the linear flight paths of charged particles through three MDC planes without magnetic field. In each drift chamber the hit trajectory is defined by at least five anode layers giving a coincident signal. The hit positions have been defined by the dimensions of the drift cells only. The drift time information is not used for this plot.

¹ANL, Argonne, USA
²JINR, Dubna, Russia

References

[1] http://www-hades.gsi.de
[2] H. Alvarez-Pol, privat information
[3] K. Heidel et al., this Report
 IKH 06/19/01 © F. Dohrmann