MDT Tube


A charged particle traversing the gas-filled drift tube ionizes the gas. The electrons drift toward the central wire (held at a positive potential, 3080 V)). The gas pressure is 3 atmospheres (Argon/Carbon Dioxide).

An avalanche develops where the electric field is high, very close to the wire. The wire diameter is 50 microns.

The signal induced on the wire is mainly due to the positive ions moving away from the wire. The leading edge of the signal is related to the minimum distance of the particle track from the wire.

The drift time is recorded by a Time-to-Digital Converter (TDC) and the size of the electrical pulse induced on the wire. is recorded by an Analog-to-Digital Converter (ADC). Particles crossing the tube close to the wire (short drift time) contribute a large ADC value (which is spread in time). The ADC values are not linear with charge; the values saturate.

Drift Time


A typical drift time distribution (TDC values) for a parallel beam of incident charged particles.

The drift velocity is about 21 microns per ns. The maximum drift time in the 15 mm radius tube is about 730 ns.


The relationship between the drift distance in the MDT tube and the measured drift time.



MDT chambers are constructed with two multilayers of MDT tubes. Each multilayer consists of either three or four layers of drift tubes.

Most MDT chambers (Middle and Outer layers) have three-layer multilayers.

A few chambers in special locations have fewer multilayers and layers and in some cases hold smaller diameter tubes

The MDT system has a total of more than 1000 chambers and a little more than 350,000 tubes.

Chamber names

Cross-sectional view in the r-z projection. Barrel MDT chambers are green; Endcap MDT chambers are blue.

Eta region numbers are indicated on the chambers. There are 16 phi sectors in both barrel and endcap.

Barrel Muon chambers. View is perpendicular to the beam axis.

MDT chambers are blue (small sectors; even sector numbers) and orange (large sectors, odd sector numbers).

Examples of the MDT chamber naming convention:

    EML2A07.   Endcap Middle station, Large chamber. Side A, Eta region 2, phi sector 7

    BOS5C16   Barrel Outer station, Small chamber, Side C, Eta region 5, phi sector 16


MDT BIL barrel chambers with two four-layer multilayers..

The drift time circles in each tube are shown as well as the reconstructed track..

A track segment is a part of a track with at least three hits in a chamber. Segments are then linked into tracks allowing for curvature. The dark circles are in the tubes first chosen to construct a segment.


Chamber Readout


Twenty four MDT tubes are connected to a Mezzanine board. The Mezzanine board holds a TDC chip and also Amplifier/Shaper/Discriminator (ASD) chips which provide analog-to-digital conversion. Chambers contain up to 18 Mezzanine boards.

Up to 18 Mezzanine boards are connected to Chamber Service Module (CSM). Up to 6 CSM's are connected to an MROD (MDT Readout Driver). The MROD's are VME modules in crates that are located in USA15. There are 16 crates, four each for Barrel and Endcap sides: BA, BC, EA, EC.

Run 2 Muon data quality monitoring Twiki.


Relevant Links

MDT Shifter Histograms


ADC Spectra

        ON Segment

Description of variable: ADC values for MDT hits that are required or not required to be on a segment

Recall that ADC values are not linear with collected charge.

Expected features: The main noise peak should be below 50. A second peak may be below 80. MuonSegment applies noise cut of 80.

TDC Spectra

        ON Segment

Description of variable: TDC value for MDT hits that are required or not required to be on a segment. An ADC cut is applied.

Expected features: The shape of the on-segment spectrum should be similar to what is shown, with a fast rise at t0 and a fast drop at tmax.

Expect a t0 around 80 ns and a tmax around 810 ns. The drift time is approximately 730 ns.

Total Hits


        NO ADC Cut

Description of variable: Number of MDT hits per event without and with ADC cut.

Expected features: The mean will depend on beam conditions.

Regional (BA, BC, EA, EC)

ADC and TDC distribution histograms are included per region.

Number of Hits

Description of variable: Total number of MDT hits for each multilayer. Hits have an ADC cut of 80.

Expected features: Look for chambers that are significantly higher or lower than their neighbors.

Drift Time

Drift Time by sector. The expected drift time is approximately 730 ns.

Occupancy Vs LB

Description of variable: Chamber occupancy shown as a function of Lumi Block (LB) as the run progresses.

Each horizontal bin is 3 Lumi Blocks. A Lumi Block duration is 1 minute.

The occupancy is organized by region and separated by layer. Outer and Extra are put in the same histogram.

Expected features: There will be a gradual decrease in occupancy during a run as the luminosity decreases.

Look for abrupt changes in occupancy during a run.


Description of variable: Efficiency is defined as the number of hits obtained on a segment divided by the number of tubes expected to be traversed by the segment.

Expected features: Efficiencies should generally be in the ~0.6-1.0 range across the region.

Low efficiency rates in one of the multilayers; note this in whiteboard entry.

Single white areas may be indicative of dropped chambers, but widespread blocks can simply mean low statistics (no need to note this in most cases):

Ex pert Contact List

* For questions about the MDT offline monitoring code, contact Heather Russell:

Major updates:
-- JuliaLynneGonski - 2015-07-27

Responsible: JuliaLynneGonski
Last reviewed by: Never reviewed

Topic attachments
I Attachment History Action Size Date Who Comment
JPEGjpg MDTtube.jpg r1 manage 46.0 K 2015-09-22 - 07:31 JosephRothberg MDT Tube
JPEGjpg MEZZ.jpg r1 manage 78.9 K 2015-09-28 - 08:44 JosephRothberg MEZZ
JPEGjpg diehldrift.jpg r1 manage 49.3 K 2015-09-22 - 13:18 JosephRothberg Drift
JPEGjpg mdtlayers.jpg r1 manage 105.8 K 2015-09-22 - 08:36 JosephRothberg Multilayers
Unknown file formatpptx mezz.pptx r1 manage 63.8 K 2015-09-28 - 08:37 JosephRothberg CSM
JPEGjpg segments3.jpg r1 manage 61.8 K 2015-09-30 - 13:18 JosephRothberg segments
JPEGjpg timedist.jpg r1 manage 33.4 K 2015-09-21 - 18:04 JosephRothberg Time Distribution
JPEGjpg timedist2.jpg r1 manage 60.9 K 2015-09-21 - 18:15 JosephRothberg Time Disribution
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