Introduction

Introduction

Purpose and Scope

This thesis describes a system capable of digitally recording the charge content from the analog outputs of the Region 1 tracking system detectors used by the Qweak experiment at Jefferson Laboratories. High-energy, charged particle position information is made available by using a process known as Gas Electron Multiplication (GEM) whereby the original particle generates an avalanche of charged particles large enough to be detected by large-scale electronics. The final stage of this GEM detector contains analog voltage signals on individual copper strips. The analog signals on these individual strips are processed by a custom-made integrated circuit (IC) containing preamplifiers and pulse-shaping networks. A digital hit-or-miss comparison is based on a programmable threshold voltage after having passed through these preamplifiers and pulse-shaping networks. These fast-acting integrated circuits (ICs) developed by CERN each possess the ability to process 128 channels simultaneously. These cards will hereafter be referred to as VFAT cards.[1][2][3] The VFATs’ discriminators used to identify charge levels are also programmable via the VFATs’ registers. All of the programmable registers of the VFAT cards are accessed using its I2C interface.

For the purpose of querying multiple VFAT cards simultaneously, it was necessary to use the V1495 module built by CAEN which operates on a VME backplane.[4] The V1495 module contains two Field-Programmable Gate Arrays (FPGA), the first FPGA is programmable by the user, and it used as the main interface to the external I/Os on the faceplate of the V1495, the second FPGA controls the interfacing of the User FPGA and the VME backplane. Both FPGAs are members of the Cyclone device family.[5] The complete operation and underpinnings of the V1495 by itself are worthy of a separate thesis; the Cyclone Device Handbook, Volume 1, contains 385 pages. Of this, an understanding of perhaps 10 percent is sufficient to comprehend, operator, and effectively alter the properties of the Cyclone FPGA for the Region 1 detector.

Where necessary this thesis will reference and attempt to explain other portions from various manuals; all of these pertinent manuals are included on a CD with the original copy of this thesis. For the user firmware and the ROC (Read Out Controller) portions of the detector, UML diagrams have been included to simplify the process of familiarizing the reader with this detector and its respective algorithms for data acquisition from the VFATs. Unfortunately, due to the finite nature of this thesis there are many assumptions that are made about the level of technical proficiency of the reader; hopefully, these are minimal, but certainly they are unavoidable.

The main portion of this thesis begins where the analog voltage signals exit the GEM detectors and ends where the hand off of data occurs at the Linux computer level. Many of the topics for which discussion is necessary to understand the full scope of this project will be briefly touched upon in this introduction.

Qweak Experiment at Jefferson Laboratories

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