The recent R&D 100 Award given to the Fermilab and the Nevada National Security Site’s Remote Sensing Laboratory (RSL) is an example of how the depth of experience and infrastructure at the lab can enable an efficient approach to innovative projects. Andrew Green of RSL, who had earned his Ph.D. working on the DZero experiment at the Tevatron, approached me about collaborating on a demonstration of a silicon tracking system that could be used for muon tomography, providing a more compact and maintainable system than the current approach, which uses drift tubes.
Muon tomography takes advantage of muons’ ability to traverse solid objects to view what’s inside them, much like an X-ray machine. Our group was in a period between large projects when resources were available, and the laboratory agreed to work with RSL. We formed a collaboration where Fermilab provided the mechanical and electrical design and assembly work, and the Remote Sensing Laboratory provided simulation, overall design, analysis software and funding.
Fermilab has enormous experience in building silicon detectors, starting with fixed-target experiments that demonstrated the technology in the 1980s, through the CDF and DZero vertex detectors, followed by the CMS outer tracker and pixel detectors. Because of this, we were able to quickly put together a design that made efficient use of both people and materials.
We chose sensors we had “on the shelf” that were rejected by CMS because of production flaws but were adequate for a technology demonstration. These sensors set the scale and characteristics of the planes to be produced. Greg Selberg built some early design prototypes. Bill Cooper, a physicist who led the DZero silicon detector mechanical design, came out of retirement to work part-time to design the planes and associated support structure. A crucial part of the design was the carbon fiber support structure with embedded copper that acts both as a support and a ground plane following a technique pioneered in DZero layer 0. The carbon fiber structure and other support parts were fabricated by Dave Butler, Otto Alvarez and his group in the PPD mechanical shop, based on their long experience of carbon fiber fabrication.
At that time Paul Rubinov’s group in PPD Electrical Engineering was supporting CMS high-granularity calorimeter (HGCAL) test beam prototypes. They had developed hardware and software to read out HGCAL modules in the test beam with what’s called a SKIROC chip. We chose to use that chip for the muon tomography project to take advantage of the design expertise and software experience in Paul’s group. Paul developed the conceptual design of the system. Cristian Gingu adapted and refined his SKIROC readout firmware for the muon application. Mike Utes designed the readout boards, debugged the overall system, and managed the day-to-day production and testing work. Johnny Green managed parts procurement. Other boards were adapted from parts designed by the University of Minnesota for the HGCAL testing.
The actual assembly was done at SiDet. Bert Gonzalez, who has been a part of the CDF, DZero, CMS and many other construction projects, provided the precision assembly of the planes. He was able to achieve a precision of 10 microns without the use of specialized jigs or fixtures. Michelle Jonas and Tammy Hawke provided the precision wire bonding of the large area sensors to each other and to the readout board. Vale Glasser and Rich Prokop, summer students from the Community College Internship program, worked during the last two summers to test and analyze data from the assembled devices.
As you can see, this work was a real team effort. It is a team with levels of experience and capabilities that lead the world. That such people were available is the result of more than 30 years of detector design, assembly and testing at the laboratory. We are fortunate to have people with this level of experience, knowledge and breadth of skill available at Fermilab.
Ron Lipton is a Fermilab scientist and the Fermilab lead for the muon tomography project.