Hybrid Survey Method for the CMS Forward Pixel Detector

Abstract

The Compact Muon Solenoid (CMS) experiment is one of the principal particle physics science-gathering facilities at the Large Hadron Collider (LHC), located at the European Organization for Nuclear Research (CERN), near Geneva, Switzerland. CMS includes the forward pixel detector, a system of hybrid silicon sensors used for precision measurements of the tracks made by the debris of proton-proton collisions. The sensor substrates contain fiducials that can only be measured using an optical coordinate measuring machine (CMM). Several silicon sensors are mounted on a “blade,” which, in turn, is assembled on a carrier known as a half-disk. Two half-disks are mounted in each of four 2.2-meter-long half-cylinders to be installed in the CMS apparatus. Because of the high density of very delicate electronic circuits within each half-cylinder, it was deemed too problematic to use a touch-probe or optical CMM probe to measure the location of the half-disks in each half-cylinder to establish precision alignment of the system. To overcome this problem, two additional features were introduced so that photogrammetry could be applied as a “minimum contact” part of the measurement process. First, a series of 8 mm white ceramic balls were mounted on each half-cylinder installation fixture so that they could be measured using a large-format mechanical CMM and a metric camera (INCA3 from Geodetic Systems Inc.). Second, a 1 mm retrotarget was developed that was suitable for observation by the opt