(for muon software developers)

The muon system is divided into two detectors: the central muon detector and the forward muon detector.

The central muon detector consists of a toroid magnet, large drift chambers of the Wide-Angle-Muon-System (WAMUS), the "Cosmic Cap" and "Cosmic Bottom" scintillation counters, and A-phi scintillation counters. The central muon detector covers the region |eta|<1.

The toroid magnet, WAMUS chambers and Cosmic Cap counters were used in Run I. The Cosmic Bottom and A-phi scintillation counters are added for Run II.

Table 1: Channel count for the central muon detector

In Run I, the toroid magnet was excited for internal fields of up to 1.95 Tesla with 2500 Amps of current. In Run II, the current will be reduced to 1500 Amps. But the reduction in magnetic field will be approximately 6%.

The magnetic fields are in X-Y plane; at the top and bottom parts of toroid, the fields are in X direction and at the sides, the fields are in Y direction. Muons are bent in R-Z plane. The wires in WAMUS drift chambers are strung along the direction of fields. The drift distance measurement is done in Y-Z plane (top and bottom) or in X-Z plane (sides).

The WAMUS consists of three layers of drift chambers, one layer inside (A) and two layers (B and C) outside the central toroid magnet. Approximately 55% of the central region is covered by three layers of PDT's, close to 90% is covered by at least two layers. The drift chambers (PDT's) are large, typically 100 x 220 in.², and made of rectangular extruded aluminum tubes. The PDT's outside the toroid magnet (B and C layers) have three decks of drift cells; the layer inside the toroid (A layer) has four decks with the exception of the bottom PDT's (these have 3 decks). The cells are 10.1 cm across, with typically 24 columns of cells per chamber.

The drift chambers produce the following measurements for each hit: the drift time T to the anode wire, the difference dT in the arrival time of the hit between a hit cell and the neighbor connected to it at the far end, providing the distance along the wire, and the charge deposition on inner and outer vernier pads, providing a more accurate measurement of the distance in the direction along the wire. Two neighboring wires are connected to save the number of electronics channels and each wire is read out only in one end.

The drift velocity is approximately 10 cm per micro second, for a maximum drift time of 500 ns. The drift distance resolution will be approximately 500 microns. The resolution of the dT measurement varies depending on whether the muon passes through the cell close to or far from the readout electronics. If the hit occurs far from the electronics, the resolution is approximately 10 cm. If close, the dispersion of the wire signal which propagates two wire lengths to the other end causes the resolution to degrade to 50 cm. The dT measurement is used to identify the particular vernier pads where the muon passed through. The position measurement using charge division with the vernier pads provides a resolution of 5 mm.

The pad readout will be instrumented for the whole A-Layer in order to provide the precise non-bend view measurement for matching with inner tracker tracks. Only about 10% of the pad readout in the B and C-Layers are instrumented in order to monitor the PDT gains.

The Cosmic Cap scintillation counters cover the top and sides of the WAMUS C-Layer. A time resolution of 2.5 ns was achieved after offline corrections (online, the counters have a time resolution of about 5 ns) in Run I. These counters have three sizes depending on the size of the WAMUS chamber on which they are mounted. There are 12 divisions in phi and 20 divisions in eta with the long counter dimension along phi.

The Cosmic Bottom counters are being built for the bottom B and C-Layers. These counters are located underneath the C-Layer chambers where possible. But due to space limitations, some of the counters are located underneath the B-Layer chambers.

These counters will provide a timestamp for muons which pass through the WAMUS PDT's in order to tag in which crossing the muons occurred, in addition to being a part of the muon trigger.

Table 2: Location, quantity and sizes of Cosmic Cap and Cosmic Bottom counters.

The A-phi counters cover the WAMUS PDT's mounted between the calorimeter and toroid magnet (A-Layer). The phi segmentation is ~ 4.5 deg, appropriate for the expected multiple scattering for high-p_T muons. The Z segmentation is 33 1/4 in., with a length appropriate to the necessary time resolution and matching well to the size of the WAMUS PDT's.

There are three counter sizes accommodating approximately a constant 4.5 deg angular coverage for each counter. The large counters are in the corners. The arrangement in the phi direction on the sides of the A-Layer is three wide, three medium, eight narrow, three medium, and three wide counters. For the bottom, the arrangement is three wide, one medium, two narrow, one medium, and three wide counters. For the top it is three wide, one medium, twelve small, one medium, and three wide counters. The counters overlap in phi to reduce inefficiency due to muons which escape through the cracks. The average overlap between counters is about 3%.

Table 3: Quantity and sizes of A-phi counters.

The forward muon detector, also called as Forward Angle Muon System (FAMUS), consists of a toroid magnet, three layers of mini-drift tubes (MDT's) and three layers of scintillation counters (Pixel counters) on both sides (north and south) of the D0 detector. The pixel counters aid in the triggering on events with muons in the forward region. The forward muon detector covers 1.0<|eta|<2.0.

The mini-drift tubes are arranged in three layers (A, B and C) that consist of three (B and C-layers) or four (A-Layer) planes. The planes are made up of tubes and every tube has eight 1x1 cm² cells.

The layers are divided into 8 octants. Each octant contains tubes of different lengths and is an independent assembly unit. The A-layer has a cut in the two bottom octants to accommodate the calorimeter support. The bottom octants of layers B and C are shorter than others because their size is restricted by the collision hall floor. Layers A and B are mounted directly on the EF toroid. The tubes are oriented along the magnetic field lines.

An individual tube has eight cells, each with a 9.4x9.4 mm² internal cross section with a 50 micron anode wire in the center. The accuracy of wire position within a tube is 160 microns. The maximum drift time is about 60 ns. The accuracy of track coordinate measurement will be ~ 0.7mm.

Table 4: MDT system

There are three layers of scintillation counters on each side of the D0 detector. Counters are arranged in R-phi geometry to match the central fiber detector (CFT) trigger segmentation. The muon trigger will combine track candidates from the CFT with information in the muon system. The effective eta range of this trigger is defined by where CFT trigger ends, i.e. at |eta|~1.7. The muon trigger in the 1.7<|eta|<2.0 region will be provided by the scintillation counters and the mini-drift tubes.

The phi segmentation is 4.5 deg and matches CFT trigger sectors. The eta segmentation of the first nine rows of counters is 0.12 and for the last three it is 0.07. Each layer of counters is divided into octants with 96 counters per octant. The size of the largest C-layer is 12x10m².

Some detector drawings and pictures:

• Cut-away view of the Upgrade Muon system
• PDT Cells
• A photograph of the MDT octants installed in the D0 detector.
• Some pictures of the pixel layers being assembled - picture1, picture2

For more information, click here. 

Last updated: Jan. 17, 2001 by Pushpa Bhat

First Version: T. Yasuda