CEBAF On-line Data Acquisition

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The Continuous Electron Beam Accelerator Facility (CEBAF) is an electron beam particle accelerator constructed using funding from the US Department of Energy and constructed in the city of Newport News Virginia. The accelerator was designed to study the structure of the atomic nucleus, specifically the interaction of the quarks that make up protons and neutrons when those particles are part of a nucleus. With that aim in mind an accelerator design was chosen which consists of two LINACs (Linear Accelerators) that accelerate an electron beam and are connected by arcs of bending magnets at each end so that the electrons are directed around a racetrack shaped loop with acceleration on the straight stretches rather than the classical ring shaped accelerator shape found at CERN or Fermilab. Each time around the loop the beam passes through each of the two LINAC accelerators but through a different set of bending magnets. The electrons make up to five passes through the LINAC accelerators. Effectively CEBAF is a linear accelerator like SLAC at Stanford that has been folded up to a tenth of it's normal length. The design of CEBAF allows the electron beam to be continuous rather than the pulsed beam typical of ring shaped accelerators. (There is some beam structure but the pulses are very much shorter and closer together) The electron beam is directed onto targets in each of three underground chambers (referred to as experiment halls).

When a nucleus in the target is hit by an electron from the beam an interaction, or event, occurs and particles are scattered into the hall. Each hall contains an array of particle detectors that track the physical properties of the particles produced by the event. The detectors generate electrical pulses that are converted into digital values by analog to digital converters (ADCs), time to digital converters (TDCs) and pulse counters (scalers). This digital data must be gathered and stored so that the physicist can later analyze the data and reconstruct the physics that occurred. The system of electronics and computers that perform this task is called a data acquisition system. Since CEBAF has three complementary experiments running simultaneously it was decided that the three data acquisition systems should be as similar as possible so that physicists moving from one experiment to another would find a familiar environment. To that end a group of specialist physicists was hired to form a data acquisition development group to develop a common system for all three halls. CODA, the CEBAF Online Data Acquisition system was the result.

[edit] What is CODA

CODA is a set of software tools and recommended hardware that allow a data acquisition system for Nuclear Physics experiments to be implemented. In nuclear and particle physics experiments the particle tracks are digitized by the data acquisition system but the detectors are capable of generating a large number of possible measurements, or data channels. The ADC, TDC and other digital electronics typically take the form of large circuit boards with connectors at the front edge that provide input and output for digital signals and a connector at the back that plugs into a backplane. A group of boards are plugged into a chassis, or crate, that provides physical support, power and cooling for the boards and backplane. This arrangement allows electronics capable of digitizing many hundreds of channels to be compressed into a single chassis.

In the CODA system each chassis contains a board that is an intelligent controller for the rest of the chassis. This board, called a Readout Controller (ROC). The ROC configures each of the digitizing boards at the start of data taking, reads the data from the digitizers and formats the data into a format that is suitable for later analysis.