Big Bang Science Big Bang ScienceResearchers working at British universities have contributed to avariety of detectors for the LEP experiments.
At Imperial College, London, a team has built the inner trackingchamber for ALEPH. The detector fits around the beam pipe inwhich the annihilations occur. It reveals the first few points on thetracks of charged particles that spray out into the remainder ofthe apparatus and allows the physicists working with ALEPH toreconstruct these tracks more precisely. The detector haspioneered the use of a new timing technique to locate thepositions of tracks along the length of the detector
The next layer of ALEPH is a large tracking detector. Thisconsists of a gasfilled chamber, with wires and metal pads at theends to pick up the tiny amounts of electrical charge left in thewake of an energetic charged particle. This detector was built atCERN, but a team from Glasgow University worked on a systemto calibrate it. The technique is to use a laser beam to releasecharge in the chamber thereby simulating the passage of charged particles. In addition, a team at Royal Holloway andBedford New College, London, has worked with RAL on thetrigger electronics, which use signals from the tracks in decidingwhether to record an annihilation.
The UK's other major contribution to ALEPH was to build theend caps that close up the barrel of the electromagneticcalorimeter These are layers of lead and particle detectors, which wereconstructed in the workshops at Glasgow University and atRAL. Royal Holloway and Bedford New College and Lancaster andSheffield Universities also provided valuable assistance in buildingthese parts.
Some of the most innovative work for the LEP detectors lies inthe DELPHI experiment, for which RAL designed and built thelargest superconducting solenoid in the world. RAL has alsocontributed to the smallest part of DELPHI - a detector made ofsilicon that reveals precisely the first points on the tracks ofparticles as they emerge from the beam pipe. One of DELPHI'sstrongest points lies in a comprehensive system for identifyingparticles based on detectors called ring imaging Cherenkov counters.To accommodate these, the tracking chamber issmaller than usual but this does not mean that DELPHI loses outin precision in tracking charged particles. A layer of additionaltracking detectors lies outside the Cherenkov counters to providemore points on the tracks. These chambers which have beenbuilt at Liverpool University in collaboration with local industry,can locate tracks to a precision of the width of a human hair.
Oxford University has also contributed toDELPHI in providing the muon detectors thatform the outer layer of the apparatus. identifying muons and measuring theirtracks precisely is an important feature of allthe experiments. Muons are emitted when particles containing heavier quarks, such ascharm and bottom, change into lighterparticles through the agency of the weakforce. The muons are a 'signature' of theoriginal charm and bottom particles.
In OPAL British universities are again involvedin the initial measurement of tracksemerging from the beam pipe. A team fromQueen Mary College, London, CambridgeUniversity and RAL has provided electronics toprocess the signals from the innermost detector which was built in Canada. Thisteam has also worked on the 'trigger' thatuses information from the tracking detectors within OPAL's magnet.In additionthe universities developed a system to monitor the detectors in the end portions ofthe electromagnetic calorimeter which were built at RAL. They consist of large, shaped blocks of polished lead-glass in whichelectrons, positrons and photons producesprays of light. The light is detected and converted to electric signals by devicesdesigned especially to work in the highmagnetic field in the region of the end caps.
The UK has also supplied the outermostlayer of OPAL - the muon detectors. Thecylindrical barrel of detectors that surroundsOPAL was built at Manchester University. The physicists atManchester developed a new way of finding the positions wheretracks cross wires in the detector. This enables them to locate thetracks to an accuracy of 1 millimetre, although the wiresthemselves are 10 metres long.
The ends of the muon barrel are closed by detectors of aslightly different construction. These end-caps are theresponsibility of Birmingham University and were built at RAL.
All the LEP experiments aim to detect as many of the particlesproduced in an annihilation as possible. One difficulty, howeverlies with those particles that emerge close to the path of theoriginal electron and positron beams In the extreme case, theseparticles disappear along the beam pipe, but those produced atslightly larger angles emerge from the pipe at some distancefrom the annihilation point. To observe these particles, eachexperiment contains 'forward' detectors.
In the OPAL experiment, these detectors are partly theresponsibility of a team from Birkbeck and Univerrsity Colleges,London, and Brunel University. Components to track theseforward-going particles have been built in the workshops atUniversity College and at Brunel. The team from UniversityCollege intends to apply the same techniques in future to X-raydetectors for medical use.