Richard C. Lamb

Richard C. Lamb, professor emeritus at Iowa State University, was born in Lexington, Kentucky, on 8 September 1933. Known affectionally as Dick, he began his career as a nuclear physicist and subsequently transitioned to high-energy physics and finally to high-energy astrophysics. He was a founding member of the Whipple and VERITAS collaborations in TeV $\mathrm{gamma}$-ray astronomy. Following a period of illness, Dick passed away in Lexington on 10 February 2018.

Dick received a BS from MIT in 1955. After two years in the US Army, he continued in physics at the University of Kentucky, obtaining his MS (1960) and PhD (1963). His doctoral thesis, under the supervision of Marcus T. McEllistrem, was an experimental study of inelastic scattering of neutrons from iron, aluminum, and manganese. Dick was a research associate (1963) and then an assistant scientist (1964–67) at Argonne Laboratory. He was appointed associate professor of physics at Iowa State University in 1967 and full professor in 1972. During those years he transitioned into high-energy physics, leading or collaborating on research conducted at the Zero Gradient Synchrotron. Specifically, there was a series of counter experiments that used detectors consisting of scintillating plastic observed with photomultiplier tubes, which fed signals to fast logic circuits. The first of these was a study of the elastic scattering of pions and protons from deuterium. The second was a search for meson resonances produced in reactions of pions with protons; detection of a final state neutron and measurement of its time-of-flight allowed the calculation of the mass of the other products of the reaction. Another of these missing-mass experiments used the large aperture magnet and wire proportional chambers of the Argonne Effective Mass Spectrometer to observe the charged decay products of the neutral meson state produced in the reaction of negative pions with protons. This latter work resulted in one of the first high resolution measurements of the mass and decay width of the f0(980) meson. The meson, because of its quantum numbers, cannot be explained by the standard model and consequently has attracted considerable theoretical interest to the present day. Much of the same apparatus was also used, in a collaboration with Dick Edelstein’s group at Carnegie Mellon University, to observe the charge-exchange scattering of neutral K long mesons with protons, producing a K⁺ and a neutron.

In 1974, Dick began to study gamma-ray astronomy, and during the academic year 1975–76 he spent his faculty leave from ISU at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Here he led an investigation aimed at detection of cosmic gamma rays with energy greater than 35 MeV based on observations made with the SAS-2 (Small Astronomy Satellite) telescope. During the 1970s there was considerable interest in searching for x-ray and $\gamma$-ray emission from x-ray binary objects (XRB), with Cygnus X-3 classified as a particularly interesting source, based primarily on synchronized 4.8 h binary orbital modulation of IR and x-ray emission. The SAS-2 observations of this source yielded strong evidence for emission of gamma rays based on observation of the 4.8 h periodicity synchronous with x-ray emission. The flux of (4.4±1.1) x 10^–6 photons cm^–2 s^–1 indicated that Cygnus X-3 was the most luminous gamma-ray point source identified at energies above 35 MeV, up to that time. Other XRB objects were monitored, but no statistically significant signals were detected. However, useful upper flux limits were established.

The successful detection of low-energy gamma rays from Cygnus X-3 led Dick to contemplate experiments to search for gamma rays of much higher energies. Plans were initiated based on the application of ground-based optical techniques employing large reflectors which were originally developed during the late 1970s for solar energy research purposes. Cherenkov light emitted as charged particles within extensive air showers (EAS) that pass through Earth’s atmosphere was discovered by J.K. Galbraith and J.V. Jelley in 1952. It was realized that very energetic particles (≃10¹² to 10¹³ eV) of cosmic origin were responsible for initiating the EASs, suggesting that discrete cosmic-ray point sources might be detectable through construction of cheap and simple ground-based telescopes. A small number of first-generation detector systems developed and operated from 1960 until 1976 failed to establish the discipline of TeV gamma-ray astronomy. Those detectors were simply incapable of selectively rejecting the overwhelming background of isotropic proton initiated showers that dominated possible gamma-ray events from targeted sources. Typical flux sensitivity of first generation detectors being no better than 12.5 x 10^–12 photons cm^–2 s^–1 was insufficient for detection of TeV sources.

In association with collaborators from the Jet Propulsion Laboratory (JPL) and the University of California, Riverside, Lamb spent much of 1980 planning a Cherenkov detector employing twin 11 m diameter solar research concentrators located at Edwards Air Force, California. The scientific objective was to make coincidence high count-rate observations of Cygnus X-3 , at a mean threshold energy of 500 GeV (300 GeV for observations close to the zenith). Observations were carried out through 1981–82. Tentative evidence for detection of 500 GeV photons in phase coincidence with maximum x-ray emission seen by other experiments was estimated at 8% of the 500 GeV cosmic ray background, between phases 0.5 and 0.7 of the Cygnus X-3 4.8 h periodicity. While encouraging, unfortunately the JPL site was not ideal for these measurements, since it was not an astronomical observatory and the optical pollution from many sources of background light proved problematic. Those considerations led to termination of the program in 1982.

However, 1982 was to be a seminal year in the history of TeV gamma-ray astronomy. From about 1977 onward, Monte Carlo simulations of TeV EASs suggested it should be possible to distinguish between electromagnetic- and hadronic-initiated showers based on intrinsic differences of Cherenkov light pools on the ground. Since the Cherenkov light from EASs is extremely directional, a large-aperture narrow field-of-view optical reflector, used in combination with a very fast high-resolution focal plane camera (an array of photomultiplier tubes) seemed promising. Recorded individual images could be analyzed offline, following some form of shower image parameterization. Those concepts formed the basis for development of the Imaging Atmospheric Cherenkov Telescope (IACT) technique. In 1978, Trevor Weekes (Smithsonian Astrophysical Observatory), together with David Fegan and Neil Porter (both University College Dublin), with little financial support, initiated design and construction of the first IACT. Progress was slow, and by 1982 Weekes realized that the small team of researchers needed expansion, preferably with US-based collaborators. Both Dick Lamb and Vic Stenger (University of Hawaii) then became members of what soon became known as the Whipple collaboration.

Establishing the IACT technique was not straightforward. The first Cherenkov light camera placed at the focal plane of the Whipple Observatory’s 10 m reflector had a relatively small number of photomultiplier tubes salvaged from other projects. This prototype camera experienced difficulties with noise (after-pulsing, non-uniformity, night-sky fluctuations). The camera’s sensitivity was simply not adequate for detection of point sources of cosmic gamma rays. Many unforeseen difficulties which were not obvious at the outset needed resolution, causing the project to evolve slowly. It was difficult to maintain an observational program on target sources that might (or might not) be emitting TeV gamma rays in the presence of an overwhelming background of hadron-initiated showers. However, by the mid 1980s, Lamb was instrumental in facilitating an upgrade of the front-end electronics and data-acquisition system, based on his expertise in high-energy physics. Simultaneously, Michael Hillas (University of Leeds) became a member of the collaboration, and his powerful suite of shower simulations indicated that there was indeed sufficient inherent information in the individual images to facilitate separation of showers initiated by gamma rays from those showers initiated by the hadronic background. The data analysis was done offline, using a procedure (Hillas parameterization) based upon the new shower simulations.

Initially, the results emerging from this young field were erratic. There were numerous reports of both gamma-ray-like and hadronic radiation from various targeted cosmic point sources. Most of these were statistically marginal detections of objects that often exhibited sporadic behavior. Cygnus X-3 manifested itself as a particularly enigmatic object for which there were several claimed marginal detections—even some based on detection of muons underground! In those early years (most of the 1980s) the Whipple collaboration didn’t know whether: (1) there were any objects with detectable gamma-ray emission at TeV energies, (2) there were a few objects with sporadic emission just at the sensitivity limit of the Whipple detector, (3) the detector had problems and issues that were not understood, or (4) the IACT method just failed to work. However, in 1989, after more than a decade of incremental technique development, the Crab Nebula was finally detected with a statistical significance of 9 sigma, the first galactic TeV source of high-energy gamma rays. Three years later, the first extragalactic TeV source, Markarian 421, was also discovered by the collaboration. The IACT technique was proven and vindicated, soon to be emulated by many groups worldwide as the basis of a second generation of TeV Cherenkov detectors. By 2018, in excess of 200 TeV sources had been catalogued by the TeV community now operating much more sensitive third-generation arrays of Cherenkov detectors. A frequently overlooked aspect of the work of Lamb and his colleagues was that XRB sources were debunked as possible TeV gamma-ray emitters during the late 1980s, based on the powerful background rejection capabilities of the IACT technique. In essence, none of the data from XRB sources monitored by Whipple ever produced evidence of TeV gamma rays once the Hillas parameterization methodology had become established. To date it has been a peerless method of extracting gamma-ray showers from the overwhelming cosmic-ray hadronic background.

Dick Lamb joined Whipple at a time when meagre funding was a matter of grave concern, and he quickly helped remedy that situation. Dick’s excellent expertise in high-energy experimental physics (experiment design, particle detectors, nanosecond electronics, data collection and analysis by computer, honest and effective communication with funding agencies, etc.) was to serve him and his colleagues in high energy astrophysics extremely well. With his gregarious and fun-loving personality, he brought an infectious enthusiasm to the Whipple collaboration and was a very important and positive influence during the early years of operation, when much had to be accomplished by a small team struggling with many technical and logistical issues. He was tremendously helpful to, and supportive of, the younger postgraduate students as they battled many unforeseen operational problems throughout the 1980s. In addition, his was the guiding hand responsible for TeV gamma-ray astronomy prospering at Iowa State University, as the ISU group expanded and made many important contributions to Whipple and the field of TeV gamma-ray astronomy during its emergence as a viable branch of astroparticle physics.

Dick will surely be remembered by his many colleagues, students, and friends as a talented, productive, successful scientist, a caring and enthusiastic teacher, and a warm and decent person blessed with seemingly inexhaustible energy. May we all be so fortunate.