The NOMAD Detector and its design
From SydneyHEPWiki
The "NOMAD" acronym stands for "Neutrino Oscillation Magnetic Detector".
The NOMAD collaboration is searching for oscillations from the mu-neutrinos produced by the West Area Neutrino Facility (WANF) at CERN, to tau-neutrinos. If mu-neutrino to tau-neutrino oscillations occur, a small fraction of the resulting tau-neutrinos will undergo charged-current interactions inside our detector, forming tau leptons. These rapidly disintegrate into lighter particles. The guiding purpose behind the design of NOMAD is the need to distinguish these "decay products" from other particles.
A description of the NOMAD detector can be found in the following reference: The NOMAD Collaboration, J.Altegoer et al., Nuclear Instruments and Methods A 404 (1998) 96-128 (also CERN-PPE/97-059 Preprint).
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The Magnet
The various subdetectors are built in and around the magnet from the UA1 experiment, which operated at the CERN Super Proton Synchrotron in the 1980's. This magnet was reinstalled in the hall that housed the Big European Bubble Chamber (BEBC) at a distance of 835 m from the target used to produce the neutrinos. A photograph of the NOMAD detector inside the BEBC Hall can be seen here.
A more detailed diagram showing most of the subdetectors is included below. The 0.4 Tesla magnetic field runs horizontally across the detector volume, "out of the page" towards the viewer. Charged particles traversing the detector volume from right to left follow curved paths in the field, curving up (+ve) or down (-ve), aiding both particle identification and momentum measurement.
The magnet is shown in red and the iron flux return in pale blue. From right to left, the flux return is formed by the two front "I"-shaped pieces, eight pairs of "C's", and then six further "I's" at the rear. The "I's" also provide mechanical support for the metal cage or "cradle" which holds the various subdetectors inside the magnet. Both the "I's" and "C's" are formed from iron plates interspersed with air gaps: in NOMAD, the gaps in the front and back "I's" are filled with scintillation detectors to form the Front and Hadronic Calorimeters respectively (see below).
The neutrino beam produced by WANF is shown approaching NOMAD from the right.
NOMAD Subdetectors
Moving from right to left on the picture, the components of the detector are:
- Veto (olive)
- Front Calorimeter (light blue), installed in the two front "I's"
- Drift Chambers (purple)
- Trigger Planes (at front and back of the TRD)
- TRD - Transition Radiation Detector (light olive)
- Preshower (black, left of the TRD)
- Electromagnetic Calorimeter (blue)
- Hadronic Calorimeter (light blue, installed in the back "I's")
- Muon Chambers (green)
The Veto
The NOMAD "veto" is a set of 59 scintillation detectors , made from 2.1 and 3.0 metre slabs of the plastic NE110, which emits small flashes of light when traversed by charged particles. The light signal is amplified at both ends of the slab by photomultiplier tubes, the double-ended readout allowing precise timing (~ 0.3 ns) of the throughgoing particle. Each of the scintillator slabs is wrapped in black plastic to exclude environmental light - pinprick and smaller "light-leaks" can noticeably reduce the performance of the veto.
As we are interested in neutrino interactions in the main detector volume, and not in charged particles entering the detector (from cosmic rays, or the leftover muons accompanying the neutrino beam), the event-reconstruction software sets aside events caused by particles passing through the veto.
The veto has been built and is operated by the Australian group, using scintillators and photomultipliers on loan from Padova University.
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Front Calorimeter
The front calorimeter (FCAL) of NOMAD was instrumented by the Dortmund group at the beginning of 1995. It is a sampling calorimeter consisting of scintillators inserted into slots inside the iron plates that make up the front support (called the "I") of the detector. As a neutrino interacts in the iron, it creates a shower of particles that create flashes of light in the scintillation detectors. The total amount of light recorded by these detectors is a measure of the energy deposited by the given event.
The total mass of the front calorimeter is nearly 18 tons. This large mass inside the intense CERN neutrino beam, followed by the accurate NOMAD spectrometer provides an ideal medium to study neutrino interactions with high statistics (about 10 times the interaction rate inside the NOMAD target).
One of the topics that are under study is the production of opposite sign muon pairs ("dimuons") from neutrino interactions. This signal is a signature for the production of a charm quark (when a neutrino strikes either a down or a strange quark inside the nucleon) and its subsequent decay. The interest of these studies is to measure the mass of the charm quark with unprecedented accuracy, exploiting the fact that one needs a minimum threshold energy to produce charm quarks and this is observed by the energy dependence of the number of charm events that can be produced. It will also be able to measure the number of strange quarks in the "sea" of the nucleon (each proton and neutron not only have three "valence" quarks that determine the charge of the nucleon but also quark-antiquark pairs that make up the so-called sea).
Another topic of interest is the search for exotic neutral heavy objects (like heavy neutrinos) that would interact in the FCAL and would subsequently decay inside the NOMAD volume. If such an object were found, it would provide evidence for a new type of particle not seen before, or, in the case of a negative result, provide the most stringent limits for the existence of such particles.
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Drift Chambers
The front of the magnet volume is occupied by 44 drift chambers ; another 5 chambers are interspersed with the transition radiation detectors (see below). Related to the well-known Geiger-Muller counter, drift chambers are widely used in high-energy physics experiments and perform the crucial role of tracking charged particles in the detector.
The NOMAD design
NOMAD drift chambers have an area of 3 metres by 3 metres, are ~ 91 mm thick, and are stacked with the large surface parallel to the front of the detector, i.e. like plates in a dishwasher (see the large NOMAD drawing; drift chambers are shown in purple). Within a chamber there are three 8mm-thick gas volumes, each containing a plane of anode wires. An elementary particle travelling through NOMAD (from right to left on the diagram) will thus pass through many wire planes, one after the other. If the particle is charged, it will leave an ionized track in each gas volume, and the electrons thus produced will drift towards the nearest anode wire in that plane, producing a signal on the wire. Using these signals we can trace the path of the particle through the detector.
Although the wires in each plane are 64 mm apart, we can measure track positions to fractions of a millimetre by recording the time taken by electrons to drift from the particle track to the wire. The speed of drift is proportional to the electric field in the chamber, so by setting up a uniform field, we obtain a simple relationship between the drift time and the distance of the track from the wire. The voltages of cathode wires (placed half-way between the anode wires) and metal-foil cathode strips (lining the walls of each gas volume) are carefully chosen to give a near-uniform electric field.
This only gives position information in one direction, however. In NOMAD the anode wires run along the x-direction, parallel to the magnetic field, so the drift-time yields a high-resolution measurement (~250 microns) of the y-position of each track, i.e. its perpendicular distance from the wire. To obtain information in the x-direction the wires in the front and back planes of each chamber are offset by -5 and +5 degrees from x. This gives a kind of "stereo" view of the particle track in each chamber, allowing an x-position measurement which is relatively coarse (resolution ~1500 microns = 1.5 millimetres) but adequate for our purposes. The three closely-placed wire planes also provide some redundancy, so that the chamber as a whole can reliably produce data on particle tracks which pass through it. The momentum of a charged particle is estimated from the radius of curvature of its path in the magnetic field. As the field runs along the x-direction, particle tracks bend in the y-z plane (corresponding to a vertical, lengthways slice through the detector), so the momentum estimate is obtained from a series of (high-resolution) y-position measurements, giving high accuracy (of order 5%).
Breaking new ground ...
An innovative feature of the drift chambers in NOMAD is the use of Aramid fibre honeycomb to provide the chamber walls. These walls provide the "target" material with which the incoming neutrinos interact. For precise tracking of particles it is desirable to have readings as close as possible to (1) the primary interaction vertex where the incoming neutrino collides with an atomic nucleus, and (2) any "kinks" in charged particle tracks. These are chiefly caused by bremsstrahlung (literally, "braking radiation"), the emission of a gamma ray as the particle passes through the electric field of an atomic nucleus; the chance of this occuring goes up dramatically with atomic number , or Z (as Z squared). The low atomic number of the fibre material (mostly carbon atoms, Z=6) and the absence of any metal framework, mean that the chance of a kink occuring between measurements is never greater than 1%; the greatest distance to such an event is just over 2 centimetres.
... and striking a few problems
Full data-taking in NOMAD was delayed by the formation of gas bubbles at the surface of the drift chamber walls, between the kevlar coating of the walls and the mylar sheets which carry the cathode strips. This problem was solved by changing the glue used to bind the mylar sheets to the kevlar, and both newly-built and repaired chambers were installed throughout early 1995.
The drift chambers are the responsibility of the Department of Astrophysics, Particle Physics, Nuclear Physics and Instrumentation at Saclay, France, and their collaborators at the University of Paris VI and VII.
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Trigger Planes
The NOMAD "trigger" detector consists of two separate planes, each of 32 plastic scintillation detectors. Unlike the veto scintillators, these detectors must operate inside the the large (0.4 Tesla) magnetic field of NOMAD, so the scintillation light is read out using special "proximity mesh" photomultiplier tubes than can operate effectively under these conditions.
Each of the trigger planes covers a 280 cm by 286 cm area, about the size of a NOMAD drift chamber. One plane is placed immediately behind the drift chamber target, while the other follows the transition radiation detector. If there is a signal in each plane at the same time, one can assume that at least one charged particle is traversing the NOMAD detector, causing small flashes of scintillation light in the two trigger planes: such a particle might be a cosmic ray (part of the radiation striking the earth from space, or the by-product of such radiation), or a muon accompanying the neutrino beam, or part of a neutrino interaction in NOMAD itself.
We are most interested in this last case, and we "trigger" the readout of all the NOMAD subdetectors - to get a detailed look at what is going on - every time both trigger planes fire, and the veto does NOT fire. This rules out most muons accompanying the neutrino beam, leaving the neutrino interactions and the cosmic rays. Since the neutrino facility produces neutrinos in 6 millisecond bursts, twice every 14.4 second cycle, most cosmic rays trigger the detector at the wrong time and can be excluded. Finally, analysis of the data from all the subdetectors reveals whether the "event" (as we call it) was a cosmic ray, a muon or other particle "leaking" through the veto, a neutrino interaction in the heavy iron flux return of the magnet (which are very common, and sometimes "leak" into the detector), or a neutrino interaction in NOMAD.
The trigger and its associated electronics were built by the group from Dortmund University in Germany.
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Transition Radiation Detector
The Transition Radiation Detector (TRD) was designed to separate electrons from pions. In the quest for muon neutrino to tau neutrino oscillations, the tau particle from the tau neutrino interaction is identified by its decay modes. The tau can decay to one electron and two neutrinos, and since the number of background electrons in the target (from electron neutrino interactions) is small, this is the most sensitive channel in which to search for oscillations.
The TRD is able to identify electrons by measuring the amount of transition radiation produced in a series of thin polypropylene foils (9 modules with 315 foils each module). Transition radiation is produced when a relativistic particle traverses from a medium with one refractive index to another. This disturbs the electric field of the particle, and produces a number of photons (causing the particle to lose a very small amount of energy). A small fraction of these photons are emitted at X-ray energies, and can be measured with proportional gas tubes (like long thin Geiger counters that measure energy). We improve the chance of an X-ray being emitted by forcing the particles to undergo hundreds of changes of medium (from polypropylene to air and back again) between each measurement.
For a given particle momentum, the chance of emitting a transition radiation X-ray is inversely proportional to the mass of the particle. Since pions are 270 times heavier than electrons, this produces a dramatic difference between the two types of particle: pions only deposit energy in the gas tubes by ionization, while electrons deposit energy by ionization and transition radiation X-rays. As can be seen in the figure, the average signal observed for electrons is larger than that for pions or muons. By making nine such measurements (one for each module) and combining their information, we can reject 99.9% of the pions (a rejection factor of 1000) in a sample of particles while keeping 90% of the electrons.
The TRD was built by the group from LAPP Annecy (France); the radiators of the detector were built at CERN.
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Preshower
The preshower serves two purposes in NOMAD: it provides an independent means of separating electrons from other particles, and it provides improved granularity to the coarse electromagnetic calorimeter. The preshower consists of a 9 mm thick slab of lead (with a 4% content of antimony) followed by two planes of proportional tubes, each tube 1 cm wide, that can give both vertical and horizontal coordinates. When an electron or a gamma-ray photon interacts with the lead converter, it initiates an electromagnetic shower, leaving large signals in each of the planes of proportional tubes. Since muons and hadrons do not produce electromagnetic showers, these exhibit smaller signals than electrons and photons. This provides an independent means of differentiating electrons from hadrons.
The electromagnetic showers created by photons, electrons and positrons are "contained" and measured inside the electromagnetic calorimeter (see the next section). By "starting off" most of the showers in the preshower detector, we can pin-point them to within a centimetre or so - a tenth of the width or height of the blocks of the calorimeter. This allows us to reconstruct the paths of photons more precisely than with the calorimeter alone, and to separate the overlapping "blobs" of energy deposited when many particles and photons strike the calorimeter close to each other.
The preshower was built by a group from the University of Lausanne, Switzerland.
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Electromagnetic Calorimeter
The electromagnetic calorimeter (ECAL) consists of 875 Cerenkov counters - blocks of lead-glass, 79 mm by 112 mm in cross-section and 494 mm deep, arranged in a matrix of 35 rows and 25 columns; six of them are shown in the following figure. Cerenkov counters measure the energy of photons, electrons and positrons that strike them, by causing an electromagnetic shower: a photon, passing through the electric field of an atomic nucleus, "converts" into a electron-positron pair; each electron or positron, passing close to a nucleus, "brakes" or loses energy in the form of a gamma-ray photon (a process we call by its German name, bremsstrahlung, literally "braking radiation"). This photon then undergoes a conversion to an electron-positron pair, and the process continues until the energy of the photons, electrons and positrons drops below a certain (low) level.
Cerenkov light
Cerenkov light is emitted when a particle travels faster than the speed of light inside a medium (this is possible in glass because the speed of light in glass is about 0.67 of the speed of light in vacuum). This creates a shock wave, similar to the sonic boom when a plane crosses the sound barrier, that is visible as a bluish cone of light inside the glass. Special two-stage photomultipliers, tetrodes, which can operate inside the NOMAD magnetic field, collect the light and convert it to electrical signals which are then amplified.
Telling one thing from another
As well as providing energy measurement for photons, electrons and positrons, the electromagnetic calorimeter (together with the preshower) can distinguish between electrons and other particles. While an electron or positron will create an electromagnetic shower, "dumping" all its energy in the lead-glass, a muon will pass straight through the lead-glass leaving a single long track, generating a Cerenkov signal equivalent to a 560 MeV gamma ray (on average) - regardless of the energy of the muon. Some pions will do the same, although most will undergo a nuclear interaction somewhere in the lead-glass, producing a handful of particles which then escape - the deposited energy is still less than the energy of the pion.
Since we can already estimate the energy of a particle from its measured momentum in the drift chambers, this allows us to pick out particles which are not electrons. We can reject 99% of pions (a "rejection factor" of 100) while keeping 90% of the electrons in a given sample of particles. This rejection of the hadronic "background" to studies of electrons is quite independant of the rejection already provided by the TRD; this allows us to search for the electronic decay of tau particles above the background of hundreds of thousands of events full of pions, protons and other particles.
The electromagnetic calorimeter was built by all the Italian groups in NOMAD and INR Moscow.
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Hadronic Calorimeter
The hadronic calorimeter was added to the NOMAD experiment at the beginning of 1995. The main aim of this calorimeter is to detect neutral hadrons that would not be measured by the electromagnetic calorimeter, and to perform a measurement of the energy of charged hadrons, complementary to that provided by the momentum measurement from the drift chambers.
Like an electromagnetic calorimeter (ECAL), an hadronic calorimeter (HCAL) estimates the energy of a particle by measuring a shower of particles and photons produced when the particle interacts with a heavy medium. Unlike an ECAL, an HCAL relies on the hadronic cascade produced when a particle strikes an atomic nucleus: protons and neutrons, more pions, and gamma-ray photons are released, which then strike other nuclei and produce yet more particles, and so on, resulting in an extended shower of particles. By measuring the many particles in the shower, one can obtain an estimate of the energy of the original particle. Strongly interacting particles such as pions and protons produce hadronic cascades, as do neutrons and other electrically neutral particles which would otherwise pass through NOMAD undetected.
The hadronic calorimeter, like the front calorimeter, is a sampling calorimeter, in which the energy measurement is inferred from the energy loss of shower particles inside the iron slabs that separate scintillators. All the signals from all the scintillators along the depth of the calorimeter are read-out by one photomultiplier tube at each side (there are 18 horizontal scintillator modules covering different heights). The HCAL is 3.1 interaction lengths deep - an interaction length is the average distance a particle will travel before interacting with a nucleus, about 17 cm in iron - while the ECAL and the coil of the magnet combined are 2.1 interaction lengths deep. This means that measurements from the ECAL and the HCAL must be combined in order to estimate the energy of a particle. The result is a rather coarse measurement (with a resolution of about 57% for particles of 5 GeV, and about 36% for particles of 20 GeV) but it is sufficient to carry out the oscillation search.
The hadronic calorimeter was built by Harvard University.
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Muon Chambers
The muon chambers used in NOMAD were previously used by the UA1 experiment. The muon chambers are outside the NOMAD magnet and provide tracking for particles that can make it through the iron behind the magnet. In the majority of cases these particles will be muons. The chambers are divided into five large area modules (3.75 m X 5.55 m), with the first three modules making up a first "station" and the last two modules making up a second station which is separated from the first by an iron wall 80 cm thick. This second station provides confidence in the identification of a particle as a muon and improves the tracking resolution. Typical resolution for a hit in a given station is about 400 micrometres.
The average track reconstruction efficiency is of the order of 92% for muons with momenta above 6 GeV/c (that can efficiently pass through both stations). The muon chambers were originally constructed at Aachen University for the UA1 experiment, but are now maintained by the CERN group working on NOMAD.