Pre-1963 (I was not involved):
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The two original low-pressure cloud chambers (LPCC) arrived in 1963. Tentative information from my distant recall from conversations only is that they were a standard prewar design from Dublin, and the pair were operated by Jim McCaughan in beautiful Calgary, Canada before they arrived in Sydney. Lucky Jim was doing his density spectrum, but the great skiing on the soft powder snow must have been nearby. Only Jim can tell us if this is correct, and fill in the information.
1963 (my Physics IV):
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Ron Wand was my Physics IV supervisor in 1963 and we set up these two cloud chambers in support of the 64S EAS core experiment in the old hut out the back of the Physics building. They used Argon gas at 1.5 times atmospheric pressure with saturated vapour from a 3:1 mixture of pure alcohol and water in a small glass dish. Triggering came from the Geiger array, and after a delay for ionic diffusion by random walks from the actual particle tracks, the piston attached to a rubber diaphragm was released in an adiabatic expansion to cause the supersaturated vapour droplets to condense (vapour diffusion) on the ions generated by the EAS charged particles. Flash tubes on the sides of the cloud chamber were triggered and stereo photographs were taken with a small purpose-built camera, and a mechanical counter was included in the photograph. After developing, all the photos were delivered to Rosemary and her team for scanning. A front to back high voltage electrostatic clearing field removed the ions between events. Sorry for the lesson, but the information is needed for later developments. One other relevant thing is that magnified positive prints were made of the negatives to use with the 3D stereo viewing setup in the scannery.
1967-1969 (I was not involved with cloud chambers in this period):
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A rather dramatic change occurred in 1967 when the post-doc Hakki Ogelman arrived from Turkey via Cornell. Hakki looked at some of the photos of our cloud chamber tracks and noticed some apparently lightly ionizing tracks. For Mac the quark hunt was on and the group consisted of Ian Cairns, Ray Woolcott and Lawrie Peak. Ian Cairns was the cloud chamber PhD student and he mostly operated the cloud chambers. Two more cloud chambers of the same design were delivered from Jamaica in 1968. The rest is controversial history as it was inferred that 2e/3 quarks were found in EAS cores, due to an estimate of half (i.e. 4/9) the normal track ionization. Let's whimsically call these artefacts "Max Trax" for unique identification. The results were published in 1969 and five lightly ionizing tracks were found. It was a good idea to search for quark tracks in EAS cores, but two later and much better cloud chamber experiments published in 1971 failed to confirm the Max Trax. What were they? More explanation below.
1970-1972 (High-Pressure Cloud Chambers):
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In 1970 Ray Woolcott (emulsions PhD) and Tony Parkinson (SUGAR PhD) were appointed as Project Officers for the new high-pressure cloud chamber (HPCC) project, along with the excellent techo Nick Avramenko plus Jim Cameron, and the PhD student Andrew Pallos. Mac believed that if 2e/3 quarks were observed in the low-pressure cloud chambers (LPCC) then e/3 quarks with 1/9 normal ionization may be observed with 9x the previous pressure. The design of the HPCC pressure vessel to operate at 10 atmospheres pressure was given to Peerless Engineering and supervised by Murray Winn, and Ray took over that aspect of the task in 1970. Five of these pressure vessels were built from stainless steel and installed in the roof room of the Physics building (ten HPCCs were originally intended). The floor beams had to be strengthened to carry the weight. The front glass for viewing and the side glass ports for the flash tubes were of one inch thick Pilkington toughened glass. Most of the rest of the equipment was constructed using the LPCC gear as a prototype. A Geiger tube array was used to provide the triggering signal to the control electronics. The interior of the HPCCs was about 12"x12" and 8" deep.
There were many practical difficulties to be overcome as might be expected for operations at such high pressures. The most difficult problem was getting a fast enough mechanical movement for the (approximate) adiabatic expansion. The piston had to carry the heavy thick rubber diaphragm, and the mechanical stresses led to problems with the welding. There was also a (related) problem in getting a uniform photographic image of the tracks as the stainless steel vessels, as delivered, were not accurately machined. Our excellent workshop used a large lathe to re-machine the vessels to successfully ensure a uniform expansion, and to reduce the mechanical stresses. A practical problem with cloud chamber operations is to have and maintain a "clean" interior with glass and brass piping, and thus the HPCC vessel was made of stainless steel. However, as supplied, the stainless steel was producing contamination in the chamber, and the solution was a multiple coating with Teflon spray. As a matter of interest two clearing field configurations were tried. One was like the LPCCs with a front to back field with wires behind the front glass, and the other was a novel method with a lateral field using conducting glass plates. Photographs of the HPCC and associated equipment, and some sample track photographs showing also the two clearing field arrangements are also posted here.
The first HPCC was pressurised to 4.3 atm in Oct 1970 and normal runs began soon after, while continuing to make improvements. The HPCC was pressurised to 6 atm in Apr 1971, and finally to 8 atm in Aug 1971. This first HPCC ran from Nov 1970 to Feb 1972 producing 3500 EAS core events, and no (credible) lighly ionizing tracks were found. I did most of the experimental work, with excellent technical support, and Ray supervised the scanning of the photographs by the scanners. The next four HPCCs were delivered in late 1971, and most of the components were constructed, but they remained incomplete when experimental work was abandoned in Feb 1972. This occurred because of the theory of the ionization saturation discussed below, which meant the experiment was hardly sensitive to any lightly ionizing tracks. I wrote the full report on the HPCC experiments later in 1972, and Mac used that to terminate the experiment with the ARC funding body.
1970-1973 (concurrent studies Low-Pressure Cloud Chambers):
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Following the Max Trax era, the four LPCCs continued operating under new management by the HPCC group, and no further lightly ionizing tracks were found using the same techniques as before. The main unsolved problem with the LPCCs was the ionization measurements, as there were many more ions than blackened "objects" (often incorrectly called drops or droplets) counted on the photographic negatives or the positive prints. This was pointed out in the literature, and Ray and I discussed the matter, and it was agreed to try and work directly with the negatives, as the object count on the positives was half that on the negatives, which clearly involved a further loss of resolution. Those from an emulsion background are prone to assume that there is a linear relationship between objects counted and ionization. Another remark is that it is quite seductive to view the interior of the cloud chamber tracks with the 3D viewing equipment and think that individual droplets are being observed.
I developed a complete theoretical construction, which involved the simulation and theory of the formation of cloud chamber tracks and their appearance on photographic film. The theory accounted for the experimental object counts due to the chain of physical processes, including the diffusion of the ions in a track, the diffusion of vapour outside and inside the tracks which determines the sizes of the droplets formed on the ions, the light intensity scattered from the droplets which exposes the photographic film and the diffraction discs appearing on the film which overlap to give the final image. I called this the "Film Resolution Clustering Theory".
Next, experimental evidence was required to verify the theoretical predictions. I had exclusive control of the four LPCCs, and I set up good cloud chamber operating conditions by monitoring the condensation efficency on the negative ions at ca 75%. The clearing field could be left on during ionic diffusion to separate the positive and negative columns. It is well known that the condensation efficiency on the positive ions is 100%, and when the condensation effiency on the negative ions reaches 80% the background is foggy. I made runs with varying ionic diffusion times for very narrow to very wide tracks, and with both undoubled and doubled tracks. Ray supervised and collated the careful scanning of the photographic negatives by Audrey Castleman.
The experimental results were in excellent agreement with the theoretical predictions. In fact, the ratio of the undoubled tracks data to the positive column tracks eliminated many theoretical parameters, and established the confidence in the theory. The result is that there is a saturation effect in the ionization estimates from counting the objects on the photographic film. That is, with increasing ionization, a linear relationship applies until the saturation sets in. This means that, if nothing else is amiss, that the LPCCs for the Max Trax are not sensitive to 2e/3 quark tracks, but actually the Max Trax could be consistent with e/3 quark tracks! Also clearly the HPCC experiment was futile due to the saturation effects, and that's why it was abandoned.
Finally, the question of what are the Max Trax could be settled. My analysis concluded that they are the negative ion columns of older pre-trigger tracks separated by the clearing field, which is normally removed by the trigger signal. This explanation was of course considered by the Max Trax team, but their analysis involves a series of minor and major errors. In particular, the major error was an incorrect measurement of the clearing field in LPCC IV where four of the five Max Trax were found - that's no coincidence! That clearing field was twice what was then assumed, and I found and fixed that error in July 1971. A second major error was an attempt to estimate the track age by a subjective estimate of the track width. However, the saturation effect is also relevant to this process. My analysis established that the pre-trigger track explanation for the Max Trax cannot be ruled out, and hence not even e/3 quark tracks were discovered.
1985 (coda):
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I was browsing in a big city bookshop when I was shocked to notice a slim volume entitled "The Quest For Quarks" by one Brian McCusker, and published by Cambridge University Press as a science book. If anyone has seen it, they will conclude it is a travesty. For the present conversation, I will just point out that it ignores all the work after 1969 on the ionization saturation effects of the flawed experiment, and repeats the misunderstandings concerning the object counts on the photographs.
Here are some photos taken of the high pressure cloud chambers:
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