-- PaolaGianotti - 20 Sep 2008

PANDA Straw Tube Tracker (STT)

General Description

* General view of Straw Tube Tracker:

A straw tube is basically a single channel drift tube, i.e. a gas filled tube with a wire along its axis. A high voltage of some kV is applied between the wire and the tube so that an electric field is present in the gas filled area. When a charged particle transverses the straw tube, ionization takes place; the electrons drift toward the wire, while the ions drift toward the wall. Once the electrons are near enough to the wire (~50 μm), an avalanche multiplication takes place with an amplification of 10⁴ - 10⁵ of the primary charge signal which allows the readout of the electric signal. The arrival time of the signal defines the drift radius, and the charge collected is proportional to the particle energy lost by ionization. The PANDA Straw Tube Tracker consists of an ensemble of 150 cm long drift tubes, arranged in an cylindrical volume around the beam axis. The basic module is a planar double-layer of tubes in order to resolve the left-right ambiguity of the track position with respect to the wire.

STT internal radius	15cm
STT externalrnal radius	42cm
Number of double-layers	12
Skew angle of double-layer 5	+3o
Skew angle of double-layer 6	-3o
Tube wall thickness	30μm
Tube internal diam.	10mm
Axial tubes length	150cm
Wire diameter	20μm
Tube wall material	Al-mylar
Wire material	Au-plated W/Re
Gas mixture	Ar(90) - CO2(10)
Single tube transparency	3.7x10-4 X/X0
ρ/φ plane resolution	150μm
z resolution	1mm

Table 1: Straw Tube Tracker main parameters.

The first and the last four double-layers are parallel to the beam axis, while the intermidiate ones are arranged with a skew angle of ± 3^o. This is necessary to reconstruct the particles z coordinate. Among the tracking devices straw tubes show many advantages:

• small signal's cross-talk;
• insensitivity of neighboring straws in case of faults;
• good mechanical stability in case of close-packed multi-layers;
• high tracking efficiency;
• good spatial resolution;
• high rate capability.

Physics Requirements

The requirements for the charged particle tracking of the PANDA detector are derived from physics channels we want to measure. A small set of channels which are regarded as the important for the tracking system of PANDA have been identified and are used as the tracking benchmark channels. The following table is reporting the full list of tracking benchmark channels.

The main task of the STT is the efficient reconstruction of charged particle trajectories with high resolution and almost full solid angle coverage. In addition to the precise determination of the momenta of charged particles, the capability to reconstruct decay vertices of longer-lived neutral particles, e.g. hyperons, is also required. The identification of different particle species by their specific energy loss is an additional task, to which the STT is expected to contribute for momenta below 1 GeV/c.