The world’s latest (and greatest) pixel detector

Steve Nahn

Steve Nahn

This article, and the work it represents, is dedicated to our dear friend and colleague Gino Bolla.

There’s a new pixel detector in the world, and it’s one of our own.

Members of the U.S. CMS FPIX team discuss the installation of discs into the half-cylinder shell.

Members of the U.S. CMS FPIX team discuss the installation of discs into the half-cylinder shell.

The Forward Pixel detector, part of the CMS Phase 1 upgrade, is taking shape out at SiDet. The new device lies at the heart of the CMS detector, providing micron-scale resolution on charged particle trajectories, which in turn provides precision position measurements of the proton-proton collisions. This detector is composed of 672 modules, each of which has 16 readout chips. Each chip reads out the signal from charge deposited on one of an array of 52×80 pixels.

That means nearly 45 million pixels to sample at 100 kHz, the nominal trigger rate of CMS. The modules are mounted onto inner and outer custom carbon fiber “half-disks,” and three inner-outer pairs are mounted in the custom carbon fiber support structure, called a “half-cylinder.” The half-cylinder also brings the innovative dual-phase carbon dioxide cooling fluid and power to the half-disks and carries the data away via optical fibers.

In addition to the physical detector, there are also the external components, such as the power supply and interlock system and the Data Acquisition system. To say it is a complex apparatus is a rather gross understatement. Though it is based on the original detector, this version supplies an extra layer of tracking with less mass and is more capable of handling the increased instantaneous luminosity expected in LHC Run 2 and Run 3, which will bring us up to 2023.

This close-up shows three disks in one half cylinder, a quarter of the full FPIX detector.

This close-up shows three disks in one half cylinder, a quarter of the full FPIX detector.

Over the last three years, the U.S. CMS FPIX team, including contributions from 21 institutions, has designed, prototyped, extensively tested, and fabricated the detector components. They are in the last stages of assembly and on time for installation during the upcoming winter technical stop of the LHC. The entire enterprise exploited the unique strengths of this dispersed team. For example, the sensors were procured by the University of Kansas; the data-concentrator ASIC was designed by Rutgers University; the modules were assembled at Purdue University and the University of Nebraska and tested by x-ray exposure at Kansas State University and the University of Illinois-Chicago.

Kansas State, Vanderbilt University, the University of Mississippi and Cornell University are developing data acquisition and control systems, while Fermilab has focused on mechanical structure, microelectronics and integration. And right now it is all coming together at SiDet.

The FPIX team stands by their handiwork.

The FPIX team stands by their handiwork.

Over the past six months, modules have been arriving at SiDet, where a small army of students, postdocs, faculty (and even a project manager, when they let him) have been testing the modules at 17 degrees and at -20 degrees Celsius. In parallel a team of talented technicians and engineers has been assembling the mechanical structure. It hasn’t been without hiccups, notably the initially low yield of modules due to splinters (from cutting up wafers) damaging readout chips and the discovery of a whole new (to us, anyway) potential failure mode. This mode, known as “hot cracking,” is when impurities in the small stainless steel tubing spread out around stress lines during the molten cooling phase of laser welding, forming cracks only a handful of microns wide but several hundred long, which initially withstand pressure testing but may fail after extended thermal cycling.

However, due to the dedication and talents of the teams involved, these problems have been overcome. We are well on the way to sending this new detector to CERN. If you have time, you should head out to SiDet to see it before it goes and gets buried deep in the heart of CMS, but you’ll have to hurry! The first of four half-cylinders complete with modules has been tested and is at CERN already, with the second one following this week, and the third and fourth within the next few weeks. Look for one of the people in the photo above, who will be happy to show you the fruits of their labor.

Check out time-lapse footage of the assembly and integration of the first of the four half-cylinders of the FPIX upgrade detector (courtesy of CMS/Harry W.K. Cheung).

Steve Nahn is the U.S. CMS detector upgrade project manager.