Difference between revisions of "CLAS12 R1 Stringing Manual"

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==Purpose==
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A copy of the manual can be found [[media:CLAS12_Region_1_Drift_Chamber_Wire_Stringing_Manual.pdf|here]].
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This manual is written for the detector wire stringing and instrumentation crews who may not be familiar with the technical aspects of the detectors they will be building.  It is intended to provide a basic project orientation for the personnel hired as stringers and fabricators for this part of the project.  This is a training manual as well as a practices and procedures manual for detector fabrication, stringing, testing, and instrumentation.  Requirements and procedures are listed and explained and safety issues as they pertain to the tasks outlined.
 
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==Introduction==
 
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The CLAS12 is a multiple component detector system designed to track and identify subatomic particles resulting from an electron beam or photon beam interaction with a target.  The drift chambers track the path of these particles as they pass through the detector.  The 18 drift chambers will be configured in 3 regions of 6 sectors each to track these particles.  Each of the 18 detectors has the same basic geometry.  Each of the 6 sectors in each region are identical.  Region 3 (R3) is approximately 50% larger than region 2 (R2) and region 2 is approximately 50% larger than region 1 (R1).  When complete, they will be arranged into a 360&deg array of approximately 45&deg acceptance.
 
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Each region of drift chambers will be strung by a different group in a different location.  The R1 detectors will be strung at Idaho State University (ISU), the R2 detectors will be strung at Old Dominion University (ODU), and the R3 detectors at JLAB.  Each location will have it's own team to string and instrument the detectors they are responsible for.  The same basic stringing procedures and requirements will apply to all 3 regions of detectors.
 
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The scientific basis for detector operation is unrelated to the structure or type of mechanical framework used to position and support the wires.  A drift chamber (DC) operates because of the gas, wires, high voltage, and electronics.  DC detectors have been in use for experiment or experimental station at a particle accelerator or collider.  DC design and operational characteristics have been well studied and documented.  DC operation can be explained using elementary physics. 
 
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As a charged particle or photon passes through a gas-filled chamber containing field and sense wires biased at negative and positive high voltages, it ionizes the gas due to the strong electric field. The resulting electrons drift toward the sense wire further ionizing the gas creating more electrons in an avalanche effect until they reach the sense wire where they induce a current pulse.  This current pulse is amplified by the on-board electronics and then to the data acquisition system.  The position of the original ionizing track can be determined from the time it takes the ionization electrons to travel to the sense wire as both the electron drift velocity and signal processing times are known.  The drift velocity is determined from the voltages, size, and shape of the drift cells.  The data acquisition system then processes the data to reconstruct the particle tracks. 
 
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CLAS12 central detector consists of 3 regions of drift chambers and a superconducting toroidal magnet.  The magnetic field causes the particle track to bend according to it's momentum.  The particle then leaves the magnetic field and enters the R3 detector to track the resulting path.  This information used in conjunction with the data from the other parts of the spectrometer can then be used to identify the particles.
 
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==The Detectors==
 
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The detector is made up of 2 end plates which position the wires and support the wire tensions.  A back plate, nose plate, and frame support position the end plates relative to one another.  The gas volume is contained by aluminized mylar windows, 0.001" thick in R1 and R2, but for R3, the upstream window will also be a 1 mil aluminized mylar window, but the downstream seal will be a composite shell for structural strength.  Each of the 18 detectors will have 1440 sense wires and 3600 field and guard wires.  The total force on the end plates due to wire tension is 1000 lbs.
 
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Each of the detectors is assembled in a precise manner and to align the end plates relative to one another.  This will position the wires within the design tolerance. Wire position must be precise in order to track the particles within design resolution.  Because of the high voltages required to operate this type of detector, materials and cleanliness standards must be strictly adhered to.  High voltage breakdown can be caused by an inviable fingerprint on a surface.  High voltage can cause chemical breakdown and chemical reactions in contaminant materials and vapors.  Requirements for an operational detector are a pure and exact gas mixture, perfectly clan and smooth wires, the proper combination of wire diameters, proper voltages, infinite ground planes, and an operational data acquisition system.
 
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The total lifetime of a detector is determined by 4 major factors:
 
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#Total radiation exposure
 
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#Gas characteristics
 
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#Detector design
 
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#Fabrication Techniques
 
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The total radiation that a detector can be exposed to represents the maximum possible life span a detector can have under perfect circumstances.  This is the bottom line in detector longevity.  This is determined by wire damage due to current.  Nothing lasts forever and even gold plated wires will wear out.
 
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Perfect circumstances include a perfect gas mixture which has no contaminants.  All contaminants cause detector degradation.  Some contaminants can rapidly and permanently degrade a detector.  The planned gas mixture of Argon and CO2 is a clean mixture of pure gases.  Most of the contaminants in these gases are not harmful.  So, in our case the greatest change for contamination is during the fabrication and stringing process.
 
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Solvents and lubricants used in the manufacturing process of many detector components must be removed and the components cleaned to vacuum standards.  This is not a viable option for some components which may require alternative manufacturing and fabrication techniques to produce clean components free from contaminants.  The use of solvents, glues, epoxies, tape, lubricants, and improper materials will shorten detector life.  Using materials that out gas, using the wrong glues or epoxies, using excessive amounts of glues or epoxies will create an ongoing long term source of contamination.
 
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Detectors are strung in a clean room with all persons wearing appropriate clean room clothing.  Gloves are worn and are changed frequently to prevent any contamination from human touch.  Hair nets are worn will all hair tucked inside to prevent hair contamination.  Masks are worn in the case you must work in close
 
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proximity to the wires to prevent breathing on the wires or interior surface of the detector.  A single human hair can permanently short out the wires it touches and coat the wires around it creating excessive current.  A fingerprint may cause a high voltage breakdown with permanent carbon tracks requiring that area of the
 
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detector to be turned off. Proper quality control is a critical factor for detector lifetime.
 

Latest revision as of 15:43, 23 September 2011

A copy of the manual can be found here.

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