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 (USA, Italy, Japan) engaged in manufacturing and assembly of the flight hardware.  In addition to the management responsibilities, Johnson’s UCSC group was responsible for delivering the Tracker readout electronics, including 648 multi-chip modules, each with 1536 amplifier channels, and the flexible-circuit readout cables.  The first tracker tower module was completed in Italy in January of 2005, and in October of 2005 the last of the 18 Tracker tower modules (including 2 spares) was completed and tested.  Here you can find a view of the 16 flight Tracker modules mounted in the LAT at SLAC, along with 16 calorimeter modules (not visible).  The remainder of the instrument was completed in the winter of 2006, and in September of 2006 the LAT successfully completed its environmental testing at the Naval Research Laboratory and was ready to be integrated with the spacecraft (photo of the completed, tested instrument).  The instrument was integrated with the spacecraft at General Dynamics in Arizona (photo of the LAT and GBM NaI detectors on the spacecraft).  The integrated observatory completed thermal-vacuum testing at the Naval Research Laboratory in early 2008 and was then transported to Florida (photo of completed observatory, photo of observatory on rocket).  The spacecraft was successfully launched on a ULA Delta-II rocket at 12:05 EDT on June 11, 2008 (photo1, photo2, photo3,video).  After more than five years in orbit, all of the detector systems are still working as designed, and many results have been published since the early autumn of 2008.

      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.

    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 that is now operational.  Currently he is leading the development of a new scanner that is capable of tracking and measuring a million protons per second.  This work is supported by a grant from the National Institute of Health.

     Prior to his work on Fermi, Johnson contributed to the design and construction of an experiment (BaBar) at the B-Factory accelerator of the Stanford Linear Accelerator Center (SLAC). The B-Factory, which began running in 1999, has made detailed studies of CP violation in decays of B hadrons. Johnson was responsible for the design, prototyping, and testing of part of the fast, radiation-hard readout electronics of a silicon-strip vertex detector that is at the heart of this new detector. He worked in a collaboration of physicists and engineers from UCSC, LBNL, and INFN institutes in Pavia and Pisa to design a 128-channel CMOS VLSI chip to perform the amplification, discrimination, buffering, and data acquisition functions necessary for readout of the 150,000 channel detector. The SCIPP group was also responsible for the data transmission electronics of the BaBar vertex detector and for much of the software development needed for analysis of the data.  This experiment operated into 2008 and has published many measurements, including parameters of CP violation in the B system.

     Before coming to UCSC Johnson spent five years working at the CERN laboratory in Geneva , Switzerland on the ALEPH experiment at the Large Electron-Positron collider (LEP). After coming to UCSC he continued working on that experiment in collaboration with Prof. Alan Litke. The ALEPH experiment collected data from 1989 to 1995 at the energy of the Z resonance and later collected data above the threshold for production of W pairs. Members of the SCIPP group made important contributions to the installation, debugging, and operation of the Aleph silicon-strip vertex detector. They also contributed to the exploitation of that state-of-the-art device for physics analysis, both for studies of the production and decays of B hadrons at the Z resonance and for searches for new phenomena, such as the Higgs boson and supersymmetry, in the LEP-II high-energy operation.