Drasko Jovanovic is a high-energy experimental physicist at Fermi National Accelerator Laboratory. He has been involved in meson and neutrino experiments using accelerators at Brookhaven National Laboratory, University of Chicago, Daresbury Laboratory in England, Argonne National Laboratory, Cornell University and at Fermilab. Currently, he is a member of the CDF collaboration.
Jovanovic was born in 1930 and holds a B.S. degree in physics from the University of Belgrade, Yugoslavia, and M.Sc. and Ph.D. degrees from University of Chicago. Before coming to Fermilab in 1972 Jovanovic worked at the University of Chicago, the University of California in San Diego and Argonne National Laboratory. In 1968, he participated in the initial design of Fermilab. He has over 180 publications in open literature and has given numerous seminars and lectures on high-energy physics.
In addition to his experimental work, Jovanovic is teaching physics at Northwestern University. He is in charge of the Saturday Morning Physics Program at Fermilab and is very active in precollege and college education sponsored by Fermilab and Friends of Fermilab. Recently, he served as the elected chairman of the ASP Forum on Education.
Introductory Comments: DetectorsDetectors are extensions of our senses. In vision, one shines the light on the object to be observed and the retina in the eye detects photons of visible light that have bounced off it. A simple extension of this principle is what is involved in the particle detector. One bombards the object, target, with the projectiles, protons, pions, gamma rays, etc. and "observes" the deflected incident protons not with the eye, but rather with special devices designed for this purpose.
Charged particles are easiest to detect because they continously interact by ionizing the medium through which they are passing. The trail of ions that a charged particle leaves behind, in its passage through matter, can be made visible either by condensing droplets of liquid, grains of photographic emulsion or developing bubbles of liquid. These are direct tracking detectors, called cloud chambers, nuclear emulsions and the bubble chambers. Neutral particles like neutrons, gamma rays and neutrinos can also be detected but only after they interact with the medium, usually in one "catastrophic" event. One infers the trajectory of these particles by knowing the origin and the point of interaction.
In the last two to three decades these visual detectors have been supplanted by the modern electronic detectors. One of these is called a drift chamber. Here, the wake of ions that the charged particle leaves behind in passing through the chamber, drifts toward a positively charged thin wire. Each wire --and there may be thousands in such a detector --has an electronic "stop watch" attached to it. By measuring the length of time it takes this small cloud of ions to arrive and hit the wire, one deduces how far they have drifted. This information is translated into a coordinate in space by a fast computer. By receiving hundreds of such "stop watch" numbers, computers reconstruct a trajectory of the particle detected.
Another set of detectors very important in high-energy physics are called calorimeters. Here one employs the fact that the high-energy particle dissipates all of its energy in a meter or so of steel or some other high-density material. In a series of violent interactions, a whole avalanche of new particles builds up in a path of the incident projectile, and finally dissipates in the subsequent layers. One therefore samples the number of particles in the avalanche, called shower, at many intervals perpendicular to the particle path. From this information one infers the total energy of the incoming particle.
A combination of drift chambers, with some 40,000 wires and hundreds of calorimeters, is what constitutes most modern particle detectors employed to study results of beam-beam collisions at the highest man-made energy on earth: 1800 GeV. A good collision rate is 10,000 collisions per second. The electronic detectors take the computer-digested snapshots of each and every individual collision and form a three-dimensional representation of the event.