Introduction Central Tracking System Calorimeter Muon System
Exhibit numbers for the Muon System are the following: 1, 2, 3, 4, 5, 6, 7, 8.
The outermost layers of the detector detect muons. Muons are unstable particles but they live long enough to leave the detector. High energy muons are a good sign of interesting collisions. Unlike most common particles they don't get absorbed in the calorimeter so by putting particle detectors outside it, muons can be identified. The muon system is very large because it has to surround all of the rest of the detector, and it is the first thing that you see when looking at D0.
The individual elements making up the muon system, the muon chambers and scintillation counters, come in several shapes and sizes, depending on where they are located in the detector. Some of them are placed just outside the calorimeters. Most of them, however, are located at the outermost edges of the detector. The muon detectors consist of two parts: layers of tracking detectors and layers of scintillation trigger counters.
Muon tracking detectors
The muon drift chambers are located on the very
outer edges of the detector; each chamber consists of a single wire suspended across a gas-filled aluminum cylinder. The muon enters the chamber and ionizes the gas; different voltages are applied to the wire and to the aluminum container, causing the wire to pull and accelerate freed electrons towards it. The wire transmits the electrical signal to the readout system.
drift chambers give scientists an accurate measurement of a muon's position;
but the time it takes for a muon to ionize the gas, and for the freed electrons
to travel to the wire, makes it difficult to assess the passage of a muon through the chamber quickly enough to trigger on the event.
Central Muon System (1)
- measure the position in 3 points with a magnetic field in between; the magnetic field will bend the muon direction. By reconstructing the bending of the trajectory we measure the momentum.
Proportional Drift Tube (2)
The muon ionizes the gas inside the chamber and the position is determined by measuring the time it takes the muon to drift to the wire.
The PDTs outside of the toroidal magnet have three decks of drift cells; the A layer has four decks with the exception of the bottom A-layer PDTs which have three decks. The cells are 10.1 cm across; typical chambers are 24 cells wide and contain 72 or 96 cells. Along with an anode wire at the center of each cell, vernier cathode pads are located above and below the wires to provide information on the hit position along the wire. The wires are ganged together in pairs within a deck and then read out by electronics located at one end of each chamber.
Mini Drift Tube - Forward Muon System (3)
Mini drift tubes were chosen for their short electron drift time (below 132 ns), good coordinate resolution (less than 1 mm), radiation hardness, high segmentation, and low occupancy.
The MDTs are arranged in three layers (A, B, and C, with A closest to the interaction region inside the toroidal magnet and C furthest away), each of which is divided into eight octants. A layer consists of three (layers B and C) or four (layer A) planes of tubes mounted along magnetic field lines (the field shape in the forward toroids is more “square” than “circular”). The entire MDT system contains 48,640 wires; the maximum tube length is 5830 mm in layer C. Since the flux of particles drops with increasing distance from the beam line, the occupancy of individual tubes is the same within a factor of two over an entire layer.
A MDT is made from commercially available aluminum extrusion combs (0.6 mm thick) with a stainless steel foil cover (0.15 mm thick) and are inserted into PVC sleeves. They are closed by endcaps that provide accurate positioning of the anode wires, wire tension, gas tightness, and electrical and gas connections.
The MDT system uses a CF4-CH4 (90%-10%) gas mixture. It is non-flammable, fast, exhibits no radiation aging, and has a wide operational (high voltage) plateau.
Muon scintillation trigger counters
A scintillator is a type of material that emits light when a charged particle passes through it. This light is then carried by light guides to photosensitive devices that convert the light into an electrical signal, which is then read out by an electronic system.
Scintillators often (depending on their size and shape) don't give scientists a very accurate a measurement of the incoming particle's position, but they work very quickly: the scintillation light is extracted from the detector almost instantly. Working together, the muon chambers and scintillators can allow physicists to trace the muon back to the particle it decayed from.
Scintillation Trigger Counter - Forward Muon System (4)
Each layer of forward muon scintillator counters forms an overlapping
("fish-scaled") set of aluminum-covered plastic plates, which produce a burst of
visible light photons when a muon passes through. This light is then fed into
photon counters and converted to an electric current. Because the detection
medium is light-based, the information is available very soon after the initial
proton-antiproton collision and is hence used to make a decision about whether
or not to save (trigger) the event for later use. Triggering is essential to
select the roughly 100 events per second that can be saved to tape, out of the
several million proton-antiproton collisions in this time.
Scintillation Trigger Counter - Central Muon System (6)
Part of the Trigger system: a system designed to quickly register particles from a collision event and analyze their data on a simplified level, and to discard the events that are less likely
to contain the rare processes that will teach us new things about the universe.
Scintillation tile counter without enclosure (8)
The Scintillation Counter is a device which measures the ionizing radiation. It consists of either a crystal or a piece or sheet of plastic, the scintillator, which fluoresces when it is traversed by ionizing radiation. This fluorescence light is then sensed by a photo detector, for example a photomultiplier tube (PMT). This PMT is attached to an electronic circuitry which counts and measures the signals received from the PMT.
In the photo sensor field, photomultiplier tubes (or PMT) are known to have particularly high sensitivity and high-speed response time.
Photomultiplier tube (7)
Cylindrical devices that turn a small light signal (a few photons) into an electric signal and amplify it into a signal that can be picked up by the readout electronics.
Readout electronic (5) counts and measures the radiation by amplifying the signals received from the PMT