These bubble chambers took pictures on film at the rate of about one per second. Many millions were produced. These had to be scanned, and the events of interest measured and reconstructed. At first we used the simple, manual techniques for scanning and measuring inherited from our cloud chamber predecessors: simple projection tables, protractors for angles, templates for the measurement of the track curvatures and manual computers.
However, just at that time commercial electronic computers were beginning to appear. We learned to construct digital measuring devices that would automatically punch the track co-ordinates onto cards, and to write increasingly sophisticated programs that utilized the rapidly evolving power of computers to reconstruct interesting physical quantities. This was an essential element in the power that the technique developed. It was one of the early challenges to the evolving computer industry, and the bubble chamber community was able to contribute to the advancement of this technology.
At Nevis, in 1956, within a few months of the first chamber, we had our first chamber with a magnetic field. It was a propane chamber, 12 inches (30 cm) in diameter. The volume had increased eight-fold and the magnetic field was 1.3 T. One of the technical innovations was the introduction of a third camera, so that the field of view was photographed from three angles rather than two. This was essential to the automatic measurement and reconstruction of tracks parallel to the plane of two of the cameras.
In the first exposure we were able to discover the S° hyperon (figure 4) and measure its mass.(14) Together with the previously known positively and negatively charged Ss, the three formed an isotopic triplet, the first experimental evidence supporting the flavour SU(3) symmetry, later dramatically confirmed by the W-.
The same magnet as well as optics also served our first hydrogen chamber, with dimensions similar to the propane one. This began operation at the Cosmotron in 1957. The expansion of the liquid was accomplished with the help of stainless steel bellows, with the associated risk of rupture after many cycles of operation. This would have been an interesting accident involving substantial quantities of hydrogen. Nevertheless, I don't remember ever, at the time, trying to understand the likely consequences.
Soon after, Ralph Shutt at Brookhaven demonstrated that equally effective, but safer, expansion could be achieved with a piston sealed with teflon piston rings, and this was the method generally adopted afterwards. The 12 inch hydrogen chamber was used at the Cosmotron to continue the study of strange particle production, their decays and other properties. One of the first results (15) was the demonstration of parity non-conservation in L decay, now with about 10 times as many statistics as with the premature experiment of 1956 (figure 5).
This combined the results obtained in the 12 inch propane and hydrogen chambers with those obtained in a somewhat smaller propane chamber by my mentor and inventor of the bubble chamber, Don Glaser. Similar results were also obtained by the Berkeley pioneers in the hydrogen bubble chamber development at the Bevatron.(16)
This experiment was followed by a determination of the spins of the L and S hyperons. (17) It was natural to assume these to be 1/2, the same as those of the proton and neutron, in line with SU(3) symmetry, but this was not known experimentally.
One of the main interests of the bubble chamber community in the early 1960s was the discovery and study of meson and baryon resonances, and the determination of their properties and relationships to each other.
Resonances are exited states of hadrons that decay rapidly through the strong interaction, and
therefore have poorly defined total energy or mass. The resonance "widths", or energy spread, are typically of the
order of 100 MeV, corresponding to lifetimes of the order of 10-23 s.
The first hyperon resonance was seen in Berkeley, which then had a 15 inch (38 cm) hydrogen chamber in operation at the Bevatron. Berkeley had also pioneered in the electrostatic separation of particle species, making use of the difference in velocity of particles previously selected to have the same momentum.
Using a negative kaon beam to produce Lpp, the Berkeley group found a resonance in the Lp system with mass 1.38 GeV and width 37 MeV. (19) The data favoured the assignment of spin 3/2. Dozens of these resonances were found in rapid succession. This knowledge contributed to our understanding of the strong interaction, which crystallized in 1973 in the form of Quantum Chromodynamics.
At Nevis, in the meantime, we constructed two more chambers, again one using propane and one hydrogen. They were 30 inches (75 cm) in diameter, substantially larger than their predecessors, but when they came into use, late in 1961, there were already larger chambers in operation. Some 12 million pictures were taken in the 30 inch hydrogen chamber.
In tune with the times, we did some work on the production and decay properties of resonances. In one set of measurements, antiprotons of a separated beam were brought to rest in the hydrogen chamber. The antiprotons combine with protons and annihilate, with many different possible final states, of different numbers of pions and kaons, and one could try to exploit these to gain some insights. One rather particular use we made of this exposure was the first determination of the widths of the w and f resonances. These were among the more interesting meson resonances that had been observed, and distinguished by the fact that their widths, or equivalently their inverse lifetimes, were too small to be measurable in the usual experiment.
In the case of the w we were able to select a few proton-antiproton annihilations giving wK
Searching for resonances at random was not my style, and I never looked for, nor found, a new one. I preferred to focus on something specific. In one experiment, negative kaons were stopped in the chamber in order to study the relatively rare leptonic decays of S hyperons. In the same kaon exposure we could measure the relative parity of the S° and L hyperons.
Of these 30 inch chamber experiments, this was probably the one we valued most highly. This was also the thesis of a doctoral student who, in the meantime, had become my wife, after the first one had decided to throw in the towel in 1961.
End of an era
This was pretty much the end of my bubble chamber adventure. Since my first steps in physics in 1947, particle physics had advanced and changed very much. Cosmic rays had been entirely replaced by accelerators, experiments took more time, and were carried out by larger groups, more often 10 people than one or two. Quite a bit had been learned. Four elementary particles had swollen to dozens, the weak interaction had witnessed a wave of clarification in the wake of the discovery of parity violation, and particle detectors had advanced, with the advent of the scintillation counter and the bubble chamber.
I took great pleasure not only in contributing to our understanding of the physics, but also in the design and mechanical construction of the detectors: counters, liquid hydrogen targets, bubble chambers, even the electronics, where I did not shine particularly.