Perspectives on the Development of Track Physics

LET ME thank all of you for being here, Tony Starace, for having concewed this meeting, and Bob Wood and Matesh Varma, for over 20 years of support without which there would have been no track physics. There are many others to thank: the students, postdoctorates, and senior visitors who actually did all the work, and the many mvesugators around the world who made measurements that proved to be essenttal to developing and testing the notions of particle tracks. This enterprise began when I undertook to rewrite an introductory physics text by Henry Semat to adapt it to a calculus-based course. That became Physzcs, by Henry Semat and Robert Katz, pubhshed m 1958. In that writing I became persuaded of the beauty of the magnetic monopole as a pedagogic device in the teaching of elecncity and magnetism. Later I looked into special relatwity to check the validity of my notions That resulted m a Momentum book sponsored by the Commission of College Physics which was titled An Introduction to the Spectal Theory of Relattvzty. There I showed how easy it was to make relativistic transformations of the electric and magnetic fields, if only one admitted the use of poles I may have been the first to write an explicit expression for the Lorentz force on a pole. That equation said that a moving pole would describe a helical path m a uniform electric field. This became one possible basis for the ldentlficauon of the pole In the summer of 1958 I taught at the Umversity of Illinois, and as luck would have it, Bob Hdl--who had been my thesis adwsor--was there. He was working with nuclear emulsions and became interested in my story about poles. Together we tried a hasty expenment exposing some nuclear emulsion (subject to an elecnc field) to the Betatron, to look for a particle that moved in a circular path The experiment was a failure, but then Bob Hdl offered the nouon that I should look mto the track itself. That suggestion matured into track physics. Somettme before, Dlrac had shown quantummechanically that the existence of a quanuzed pole would justify a quantized electric charge. His required pole strength was large compared to the electron's charge. Consequently It would be heavily tomzing, since a moving magneuc pole generates an electric field just as a moving charge generates a magnetic field. Cosmic ray mvesugators had recently sent emulsions aloft m balloons and had observed very heavy tracks which they identified as being due to heavy nuclei. I wondered whether their identification was possibly m error, whether somewhere in these heavy tracks a pole was hiding undetected, because no one knew its s~gnature I thought tt would be an easy task to define the s~gnature of a pole. I would look m the literature, find a theory of the tracks of heavy ions m emulsion, make a few changes and 1o, the theory of the track of a pole 1 found no such theory of heavy ton tracks. And so I set about making one. At that time the physics department at Kansas State had no optical microscope. Through Bob Hill, I obtamed some cosmic-ray-exposed emulsion, and ultimately the department bought a microscope It was a long time before the model of a heavy ton track emerged The essential ingredient was Jim Butts, who was my graduate student He projected the ~mage of tracks, via a mirror placed above a m~croscope eyepiece, onto pieces of paper, and traced around them to get data on the variation of track width with range. There were a couple of preliminary models around that had been apphed to the stopping ends of tracks of hghter ions, neither one of which fitted these data One model took as ~ts crttenon the electron flux, the other the energy flux, both from delta rays Our change was to calculate the radial dtstnbution of dose, and to see whether the track width could be associated with a fixed dose level. It worked. I remember my delight, speaking with Jim and Ed Kobetich, who was then an undergraduate, m which I allowed that we had struck gold. Then, I didn't know how right I was. I had been teaching nuclear physics, and knew of roentgens, rads, and rems, and knew that I didn't understand those rems. I didn't understand RBE.* I didn't understand quahty factors. When we got our