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Bubble Chambers I Have Known and Loved
A personal memoir
Ian Hughes
University of Glasgow
The occasion when I first heard about bubble chambers is one of those scenes which is
engraved on my memory so that even today, almost 50 years later, it is sharp to recall. In
summer of 1954 I was a graduate student in Glasgow working on 1GeV p – p interactions
using nuclear emulsions. That summer a big international Nuclear and Particle physics
conference was held in Glasgow and one of those attending from the USA showed some of us
a few photographs from Glaser’s new chamber. The potential of this new technique was
immediately obvious and I resolved to seek an opportunity to gain experience of bubble
chambers to be applied to the exciting new particle world just starting to be revealed by the
new accelerators.
The opportunity came my way three years later when I joined Martin Block’s group at
that time completing construction of the first liquid helium chamber. We commissioned the
chamber and used it at the Bevatron to measure the K-Λ relative parity. We had no
technicians so as one of three RAs I got to know that chamber rather well. The moment when
I first saw the liquid helium condensing like a fine mist and filling up the chamber is another
of those lodged crystal clear forever in my memory.
I returned to a vacancy on the Glasgow staff where I became responsible for the
embryonic bubble chamber group. There was already a small (12”) propane chamber which,
with the aid of a colleague, Bill Morton, and two students, was developed and modified so
that we could use it to do an experiment on near-threshold pion photoproduction using the
Glasgow 300 MeV electron synchrotron. We designed a tube running through the chamber,
which could be filled with pressurized hydrogen or deuterium as a target for the gammas. The
challenge of course was to make the tube walls thin enough for the pions to penetrate without
too much energy loss while still thick enough to stand the pressure. Fortunately we had no
hydrogen or propane explosions.
It quickly became clear at this time that the most interesting physics lay with bubble
chambers in separated particle beams from the new high energy accelerators and for us in
particular, in the CERN 32 cm hydrogen chamber. We were invited to join the 1.47 GeV/c
K- experiment led by H. Filtuth, and Bert Turnbull who had recently joined our group went to
CERN for an extended period to work on the beam and prepare for the run. This chamber
marked a turning point for us in the way bubble chambers operated in that the chamber was
not run primarily by the physicists but by a dedicated technical crew. Partly because it was
relatively small the CERN 32 cm chamber optics was such that it produced the best pictures
of any chamber I have used.
Around this time it was becoming clear that the UK bubble chamber groups needed to
make a major step into second generation measuring facilities and much more powerful
computing facilities. I was one of a group of three who made a tour of such projects in the
USA which resulted in a plan for such installations in several of the UK groups. Glasgow was
the first of these with installation of three SMP (Scanning and Measuring Projectors) and
associated IBM computers. This installation and its upgrades served us well and in due
course measured over one million events.
Having started my bubble chamber career with kaons I was happy to be able to stick
with these richly interesting particles for the rest of it. Our next venture was a study of 3.5
GeV/c K- in the Saclay 81cm chamber at CERN. The physics yield from this experiment
included the first observation of the K*(1400).
Unfortunately, though 3.5 GeV/c is just above the threshold for Ώ- production, the
cross section is too small and we found none in these runs. The 81cm chamber was replaced
at CERN by the British National 1.5m hydrogen chamber and we were able to run in a
separated 6GeV/c beam. In these data we at Glasgow found our first Ώ- but Brookhaven beat
us to the post having already reported four.
The CERN 2m HBC replaced the BNHBC 1.5m and was the sixth and last chamber
from which I got data but the series of runs which the Bologna, Glasgow, Rome, Trieste
(BGRT) collaboration carried out in a comprehensive study of low energy K+ - nucleon
interactions took data first using the 81cm and then the 2 m chambers. This study was
triggered by the observation of bumps in the total K+p and K+d cross sections and the
Glasgow and Italian groups came to the Track Chamber Committee with very similar
proposals. This coincidence led to one of the happiest and most successful collaborations of
my bubble chamber career.
My remaining bubble chamber experiment was also a happy and successful one using
the CERN 2m chamber in collaboration with Birmingham to study 10 GeV/c K+p interactions
using the rf separated beam but by the time this was nearing completion in the early seventies
it was becoming clear that for much of the physics which most interested some of us we
needed a selective trigger not possible in bubble chambers. The Glasgow bubble chamber
group carried on for a number of years particularly on the massive study of K-p at 8.25 GeV/c
in the CERN 2 m chamber led in Glasgow by Peter Negus.
I believe that I was very fortunate to have worked in particle physics at the time when this
wonderful instrument became available to study and unravel the complex set of states which
formed one of the two main pillars of the quark model. Not only was the physics exciting but
the collaborations were very modest in size compared with today and allowed one to get to
know and make friends with physicists from groups all over Europe. In retrospect I can see it
was a kind of golden age.