Tests of Thin-wall Drift Tubes Developed for Panda Trackers

The PANDA experiment will be carried out at the international FAIR facility in the GSI laboratory (Darmstadt, Germany). According to the Technical Design Report, the Forward Tracker (six stations FT1– FT6) [1] and the Central Straw Tube Tracker (STT) [2] will be composed from thin-wall drift tubes (called straw-tubes below) in large quantities, 13500 and 4600 tubes, respectively. The straw-tubes used here were produced by winding and gluing two Mylar aluminized films of 12 μm thickness (the wall thickness including the glue was ~ 27 μm). The tube wall was used as a cathode (Rtube = 5 mm), and the coaxial goldplated W-Re wire was used as an anode of the counter (Rwire = 10 μm). The Mylar material was preferred to the Kapton one because of its better mechanical properties – a higher Young’s modulus and tensile strength. The drift tubes are filled with two-component gas mixture Ar(90 %) + CO2(10 %) at the 1 bar overpressure, i.e. at the 2 bar absolute gas pressure. The overpressure stretches the tube and prevents the wire sagging. The detector plane consisted of two mono-layers of drift tubes shifted by the radius Rtube, see Fig. 1a. The radiation length of a single tube X is characterized by X / X0 = 0.05 %, where X0 is the media radiation length. As a calculation shows, the maximum sag of a 20 μm wire inside of a 1.5 m long horizontal straw tube due to its weight is smaller than 35 μm at the wire tension of 50 g. In this report, we summarize various laboratory measurements made on straw tubes of 0.75 m and 1.5 m lengths (Fig. 1a), as well as on two module prototypes. The experimental set-up is shown in Fig. 1b. The modules were installed horizontally (as in the STT) and also vertically (as in the FT). An intense 1.3 GBq Cs γ-source (660 keV) was used for gas gain measurements, because a very low ionization current (pA) had to be detected. A low intensity Fe X-ray source producing a point-like ionization in only one straw tube within the module was used for gas gain uniformity tests and to investigate the counting characteristics and cross-talks. A collimated Sr β-source (emitting electrons of ≤ 2.3 MeV) with the intensity 11 MBq was used for efficiency measurements and for estimation of the spatial resolution of a single straw tube within the straw-module prototype. The gas gain measurements were performed in order to specify the operational voltage at the 2 bar absolute gas pressure and to study the gas gain variations versus voltage, pressure, temperature and wire diameter. The ionization current corresponding to the unity gas gain was measured using the intense 1.3 GBq Cs γ-source. In order to eliminate offsets in measurements of very small currents, we used batteries (as a floating voltage source) at both positive and negative voltages applied to the anode wire with respect to the cathode. The auto-ranging Keithley-485 pico-ammeter with the sensitivity of 0.1 pA was connected between the cathode and the ground. The ionization current I0 = 1.68 ± 0.17 pA was measured at ± 50 V and I0 = 1.73 ± 0.12 pA at ± 100 V. The resulting value of I0 = 1.70 ± 0.21 pA was obtained by averaging the partial results. The gas gain at any voltage V was calculated as the ratio I / I0, where I was the current corresponding to the voltage V. Now the voltage was incremented in the range 1000–1900 V and the corresponding gas gain changed from 10 to 10, see Fig. 2a. The gas gain versus voltage at several gas overpressures (above the atmospheric pressure) at fixed temperature is presented in Fig. 2b. The results of measurements were fitted by the Diethorn formula with two parameters, Emin and ∆V [3]: