I recently read an article by Nick and Lizzie that described the “weird” architecture at Fermilab. It mentioned the 15 foot bubble chamber, a detector of yore that has now been turned into the “world’s strangest lawn ornament”. I did my thesis work on an experiment that used this detector; I am probably one of the last students to have worked on a large bubble chamber experiment.
Before the advent of high speed electronics and powerful computers, bubble chambers were the detectors of choice. Many crucial discoveries were made with them, e.g., the discovery of the Omega- particle at BNL, which set the foundation for theories based on quarks, discovery of Neutral currents at CERN, which confirmed the validity of Glashow-Weinberg-Salam model of electro-weak unification, not to mention the plethora of particles found in the late 50’s and 60’s at Berkeley.
“A bubble chamber is a vessel filled with a superheated transparent liquid (most often liquid hydrogen) used to detect electrically charged particles moving through it.” You basically shot a beam of particles at the chamber, which would interact with the protons/neutrons in the target liquid. Just as the beam arrived, you would compress “expand” the liquid with a piston (see Kenneth’s comment below); this would cause the liquid to become superheated, and as charged particles moved through the liquid, they would cause local boiling that would show up as bubbles. We took photographs of these “bubbly” tracks, which were scanned (by humans) on specially designed tables that used precise instruments to measure the trajectory of various particles, thus obtaining their momentum. The data was then fed into computers. From this point on, analysis was similar to that on modern detectors, e.g., ATLAS. There was an “army” of scanners who measured the events.
Here are some pictures taken in bubble chambers. In the left panel of Figure 1, you can see the actual photograph of the “famous” Omega- event, and in the right panel you can see an annotated version (you can see much better if you print out the photograph and look at it edge-wise); you should also read the description in the caption. This experiment used a kaon beam.
Figure 1. The discovery of the Omega-
In Figure 2, you can see an event created by a neutrino beam; you see nothing coming in, and then an interaction occurs and a charmed baryon is created, which decays before it can leave a measurable track; its presence is surmised by looking at the decay products. On the right you can see an annotated version of the event (this was the first sighting of this particle). It is a great way to learn particle physics – you can actually “see” the event with your eyes, rather than depending on fancy electronics and reconstruction software!
Figure 2. Discovery of the charmed baryon
As you can see in Figure 3, the 15 foot chamber was quite the beast; it was probably the biggest bubble chamber ever built. At the widest point, it had a diameter of 15 feet, hence the name. In our experiment, it was filled with about 20 tons of liquid Hydrogen and Neon. To measure the momentum of charged particles, we used a very strong magnetic field, 3 Tesla, to bend their trajectories (in contrast, the earth’s magnetic field ranges between 30-60 micro-Tesla; modern MRI machines have comparably large fields). When the field was on, one had to be careful using tools or wearing belts or shoes with metal in them; if you were within 10-15 feet of it, a small ball-bearing would easily fly out of your palm! Since the chamber contained liquid Hydrogen (H2 is highly flammable), the surrounding area was heavily regulated. All electrically powered equipment had to be in boxes that were continuously flushed with dry nitrogen (to prevent sparking). We had detectors for identifying muons in and around the bubble chamber, so we had to be very careful; these detectors typically had 2000 volts running through them (but very little current). I also remember that when the piston compressed the liquid it made this loud booming noise, which happened whenever the neutrino beam arrived, about three times/minute; you eventually got used to it.
Figure 3: The 15 foot bubble chamber
We exposed the chamber to the neutrino beam generated by the Fermilab Tevatron when it first came online in 1984-5; it was the highest energy neutrino beam at the time. Our experiment was basically surveying a new energy regime. We also wanted to study some anomalous effects that were “floating” around at the time (they were eventually disproved). A bubble chamber is a very good detector to use in a new energy regime; since it records everything, results have very few experimental biases. One downside is that these experiments usually had low statistics.
A novel part of this experiment was that we tried to take holographic pictures in an attempt to improve resolution, i.e., to be able to see very short tracks due to particles containing charm quarks. To accomplish this we would fire a high-powered laser into the chamber to coincide with the arriving beam. Of course, initially all this did was to “boil” the liquid!! We learned as we went along.
I was a graduate student at the time, and my responsibility was to keep the muon detectors and the data acquisition computer up and running; there was another team operating the bubble chamber. Since we were allotted six months of beam time, everything had to work 24/7 for the entire run. I spent almost all days and many nights in the control room. As they say, it was the best of times and the worst of times. I learned a heck of lot of experimental particle physics, but I also almost “cracked” under the pressure!
I am still close to some of my colleagues, ones who were in the trenches with me. I remember one physicist, who used to snack on pickled herrings at 5 AM, and another who organized the greatest poker parties. Once our spokesperson was talking about how it was our Christian duty to help physicists from India. On hearing this, one physicist, who was Jewish, remarked “well, that lets me off the hook”. I run into some of them now and then. Another time, our spokesperson had taken a vacation during the run, and I started a pool to predict where he had gone and when he would come back; he was not amused!
Anyway, it is fun strolling down memory lane. I hope you enjoyed it too.
— Vivek Jain, Indiana University
Tags: Bubble chambers, CERN, Fermilab
