UA1 experiment

The UA1 detector was a particle detector at the Super Proton Synchrotron at CERN.


Looking for a quick and inexpensive way to produce the hitherto undiscovered W and Z bosons, a proposal was in 1977 at CERN submitted that just a year before, went into operation in the Super Proton Synchrotron (SPS ) into a proton- antiproton rebuild in order to have enough energy to produce the new particles available collider. For the detection of the particles two detectors were planned, UA1 and UA2. In 1978, the plans for the reconstruction of the PLC and the construction of the UA1 detector were decided. The development was led by Carlo Rubbia.

This converted PLC and the new detectors went in July 1981 in operation.

In 1983, the observation of the W and Z bosons was published for the following year the Nobel Prize in Physics Carlo Rubbia and Simon van der Meer were awarded.

In 1987, the luminosity of the SPS was taken by about a factor of 10 the " Antiproton Collector" in operation to increase.

With the UA1 detector you participated in the search for the hitherto undiscovered top quark. The covered energy range corresponding to a quark mass of 60 GeV / c.

The operation ran until 1989. According to the operating end of the UA1 detector at the SPS, the magnet in the years to 1999 was in the NOMAD experiment at CERN Neutrinooszillations continue to be used in 1991. In 2005 it was decided to donate the now stored in free magnet for the T2K neutrino oscillation experiment to study the J- PARC at Tokai. The magnet has since been built in Japan again.


The UA1 detector had dimensions of approximately 6x6x10 m weighed about 2000 tons and consisted of several concentrically arranged around the center collision systems.

Micro Vertex Detector

From the year 1985, the UA1 detector through a drift chamber possessed within the central detector, the Micro Vertex Detector ( MVD). With the MVD particle tracks could be reconstructed with an accuracy of up to 65 microns.

MVD had an outer diameter of 18 cm, centrally led through the jet tube of beryllium with a 5 cm diameter and 1 mm wall thickness. The chamber had a length of 8 m and was filled with a mixture of 53% argon and 47 % ethane at 3 bar. Wires ran in parallel to the jet pipe was connected to the said longitudinal spatial resolution in each case, a faster comparator for evaluating the difference signal between the two ends of the signal wires 256.

Later, the metal tubes of the detector to reduce the scatter radiation have been replaced by copies of carbon fiber.

Central drift chamber

The central drift chamber served the reconstruction of particle and allowed a spatial resolution of the trajectories of 100-300 microns. The function of the drift chamber oriented strongly to the hitherto usual bubble chambers.

The central chamber was 6 m long, had 2.2 m outer diameter and was filled at atmospheric pressure with 60 % ethane and 40% argon. The Outer shell had a thickness of 5 cm and was built in plastic sandwich construction. By train the wires it came to a deformation of the shell around 8 cm. The correct tension of each wire was set as the voices of a piano by the wire mechanically vibrated and was tuned to the right frequency.

6000 signal recording wires were parallel to the magnetic field, arranged in layers and evenly distributed in 25 m3 chamber volume. The spacing of the wire planes directed according to the drift velocity of the ions and of the frequency to be associated with the SPS while bunches collision. The repetition time of the PLC was 3.8 microseconds, the maximum drift time of 3.6 microseconds and spacing of the wire layers 18 cm.

The chamber can be seen since 1999 at the CERN Microcosm museum.


Calorimeters are used to determine particle energies, plus the particles to be measured must be absorbed in the detector.

At the start of operations in 1981, the UA1 detector was equipped with an inner and an outer hadronic calorimeter electronic. The calorimeters were composed of scintillators and lead absorbers and connected via fiber optics above the magnetic core arranged photomultipliers.

Since 1984 significant radiation damage were already apparent in the scintillators, a replacement was planned by radiation-resistant calorimeter, especially as more Luminositätssteigerungen were planned. In the years 1987 to 1989, the detector without the electromagnetic calorimeter was operated, the conversion to the new calorimeter lasted until 1989. The new calorimeter worked on the principle of an ionization chamber and consisted of layers of 3.3 mm thick, with tetramethyl- pentane filled cells, used alternately with layers of 2 mm depleted uranium, an outer Kalorimeterschicht 5 mm thick uranium absorber plates.

With the calorimeters almost the entire space has been sealed to the center around the collision, only an opening angle of 0.2 ° at the nozzle was unobserved.


UA1 in the detector, a magnetic field of 0.7 T is generated perpendicular to the beam direction, in a volume of 80 m3. The winding of the magnet was made of aluminum. In the iron core later large-area muon detectors were installed.

Muon detector

Muons can penetrate the calorimeter and by a further layer detectors recorded. The entire magnet was surrounded by multi-layered, crossed for two-dimensional localization muon drift chambers.

From late 1984 until August 1985, the muon detection system has been extended by additional drift chambers with a total area of ​​800 m2 and 50 000 channels. The new chambers are thereby incorporated in the iron core of the magnet. The walls of said iron core are made of three mutually riveted iron blocks of 20 cm in thickness, the drift chambers are between these blocks, and in addition, attached to the inside of the magnet. With the new chambers including a pursuit of the curved trajectory of muons in the iron core was possible. The resolution of the system was between 300 microns and 1.2 mm. The chambers were operated with a fill gas consisting of 75 % isobutane and 25% argon at 5 mbar pressure.