Research Interests Johnson's work is in the areas of experimental particle physics and, more recently, applications of particle-physics technology to high-energy astrophysics and medical physics. In the area of high-energy astrophysics, Johnson has worked since 1994 on a NASA/D.O.E. project named the Fermi Gamma-ray Space Telescope (formerly named GLAST), an orbiting gamma-ray telescope based on the silicon-strip technology in which SCIPP specializes. The Fermi Large Area Telescope (LAT) instrument is a pair-conversion telescope that delivers up to two-orders-of-magnitude improvement in sensitivity to astrophysical sources of high-energy gamma rays, compared with the highly successful EGRET experiment on the Compton Gamma Ray Observatory, which flew in the 1990s. The LAT gamma-ray converter and charged-particle tracker includes almost 885,000 silicon-strip channels operating on only 160 Watts of electrical power. Johnson concentrated on development of the low-power, low-noise readout electronics needed for this application. A CMOS VLSI chip set was designed in the SCIPP lab, together with the necessary supporting electronics. Prototypes were extensively tested in a full-scale tracker module assembled in the SCIPP lab and operated in December 1999 and January 2000 in the SLAC test beam and in 2001 on a high-altitude balloon flight. Other SCIPP faculty who contributed to the Fermi-LAT design and fabrication are Bill Atwood (the originator of the LAT conceptual design, on which he began detailed Monte-Carlo simulations in 1992), Hartmut Sadrozinski, most notable for leading the effort to design and procure the silicon-strip detectors, and Terry Schalk, who contributed to the management of the flight software development. The
Fermi Gamma-ray Space Telescope is providing a vast amount of data on gamma ray
sources in our galaxy, such as pulsars, as well as on extra-galactic sources
such as active galactic nuclei and the even more mysterious gamma-ray bursts.
It also enables us to search for signals from massive particles
hypothesized to constitute the dark matter of the universe. The Fermi mission includes both the Large-Area Telescope (LAT)
and a smaller Gamma-Ray Burst
Monitor. The silicon-strip based instrument design was selected by
NASA for the LAT instrument in March of 2000. Johnson was the manager for
the design and fabrication of the silicon-strip
tracker subsystem of the LAT, which also includes a CsI crystal calorimeter
subsystem and a plastic-scintillator veto shield subsystem. The Critical Design Review for the
Tracker subsystem was held in March of 2003, after which the collaboration ( Since the launch of Fermi, Johnson's group has concentrated on several areas of data analysis, including ongoing work to improve the background rejection and purity of the gamma-ray sample and a longer term project to make major improvements to the instrument simulation package and the offline event reconstruction algorithms. The principal science topics of the group are the diffuse production of gamma rays in the Milky Way and nearby galaxies and the associated searches for annihilation of dark matter into gamma rays, and searches for radio quiet gamma ray pulsars. In the latter area, the group has discovered more than two dozen previously unknown pulsars by searching for periodicity in the gamma-ray signal, the first time that has been accomplished by any gamma-ray telescope. See the publications page for details. A much smaller NASA supported project is AESOP-Lite, a balloon payload containing a magnetic spectrometer and a Cerenkov counter for studying low-energy electrons and positrons from cosmic rays as they enter the atmosphere in the polar regions. The PI of the project is John Clem at the Bartol Institute of the University of Delaware. Johnson and his student, Sarah Mechbal, built the silicon-strip tracking modules that together with a permanent magnet, a Cherenkov detector, and a scintillator hodoscope make up the spectrometer. The first flight of the payload launched from Esrange, Sweden on May 16, 2018 and landed on Ellesmere Island, Canada on May 21. Here is a video of the launch. Here is an inflight photo, over the coas of Norway at about 135,000 feet. See also the Stratocat article about the flight. See the collaboration web page for more details. The mission is also described on this web page. We have published scientific results from the flight in the Astrophysics Journal: Sarah Mechbal et al., Measurement of Low-Energy Cosmic-Ray Electron and Positron Spectra at 1 au with the AESOP-Lite Spectrometer, ApJ 903 (2020) 21, arXiv:2009.03437. Johnson is also collaborating with Hartmut Sadrozinski and the Loma-Linda University Medical Center on development of a proton CT scanner (proton computed tomography) to be used in treatment planning for proton radiation therapy. This technique, which is made possible by recent advances in particle detection and in computation, promises to reduce the errors in treatment planning for proton therapy, a powerful cancer treatment that is rapidly gaining in use as new facilities are developed worldwide. Johnson provided silicon-strip detectors and integrated circuits from the Fermi-LAT tracker project, together with expertise in their use, toward the fabrication of a prototype scanner. The scanner has been operational since 20155 and continues to be used to investigate by how much this imaging mode can improve planning for proton radiation therapy. Many of the experiments have been carried out at the Chicago Proton Center, a medical facility of Northwestern Medicine. It was also operated on two occassions in a helium nuclei beam in Heidelberg Germany and the dkfz HIT facility. Fabrication of the scanner was supported by a grant from the National Institute of Health. In 2017 Johnson began collaborating on the Heavy Photon Search experiment, which completed its first main run at Jefferson Lab in the summer of 2019. The tracking system differed from that used in previous engineering runs by the addition of an additional layer of silicon-strip detectors located closer to the target and beam, for improved vertex resolution. Johnson has developed a Kalman-Filter based tracking code to improve pattern recognition in the silicon-strip tracking detector system, and that code will be used to process the data from the 2019 run. The experiment is looking for hints of a 'dark sector' that might explain the nature of dark matter by way of searching for a massive particle that couples to ordinary matter much in the way that photons do, but far more weakly.
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