General Description

-- PeterWintz - 04 Nov 2013

The central Straw Tube Tracker (STT) in the PANDA target spectrometer features a high-resolution spatial and momentum reconstruction of charged particle trajectories in a broad momentum range from about 100 MeV/c to 8 GeV/c. In addition, a particle identification of protons, kaons, and pions in the momentum region below 1 GeV/c has to be done exclusively by the STT, with a measurement of the particles' specific energy-loss. The STT design is optimised for a minimal material budget by the technique of self-supporting straw layer modules. The detector is essential for the online track reconstruction and track association to events in the PANDA specific environment of about 150 tracks from 40 individual events within 2 µs.

The detector consists of 4636 straw tubes, each with a length of 1400 mm and 10 mm inner diameter. The wall thickness of the aluminised mylar film tube is 27 µm. The tubes are pressurized to 1 bar overpressure (2 bar absolute pressure) and glued together to close-packed, self-supporting multi-layers which sustain the wire tension and tube stretching equivalent to 230 kg and 3.2 tons (sum for 4636 tubes). The straws are arranged in two semi-barrels around the beam pipe with a few cm gap to leave space for the vertical target pipe.

STT Dimensions
Inner radius	15.0 cm
Outer radius	41.8 cm
Total length	171 cm
Semi-barrel gap (vertical)	5.0 cm
Material budget (radial)	1.23 % (X/X0)

Main dimensions of STT system

Cross-section view with axial oriented straws marekd in green and the skewed straw double-layers in blue (+2.9°) and red (-2.9°).

Specifications
Number of straws	4636
Straw length	1400 mm
Straw diameter (inner)	10 mm
Straw pitch	10.14 mm
Straw film tube	Mylar (aluminised)
Straw wall thickness	27 µm
Sense wire	W/Re
Wire diameter	20 µm
Number of axial layers	15-19
Number of stereo layers	8
Stereo angle	+2.9, -2.9 deg
Gas mixture	Ar/CO2(10%)
Gas pressure	2 bar (absolute)
Maximum drift time (2T-field)	~ 200 ns
Spatial resolution (sigma)	150 µm (xy), 2-3 mm (z)
Energy resolution (dE/dx)	< 10 %
Maximum rate (innermost layer)	< 1 MHz/straw, < 7-14 kHz/cm
Mean charge load	0.2 C / cm

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