P.J. Doe and the SNO Collaboration
A unique feature of the Sudbury Neutrino Observatory arises from the use of 1000 tonnes of heavy water which enables it to distinguish between different flavors of neutrinos. In order to satisfy the requirements for low radioactivity and excellent transmission of Cerenkov light, the heavy water will be contained in a 40' diameter acrylic sphere. An acrylic vessel of this size has never been constructed before, furthermore, loan of the heavy water by the Canadian government is contingent upon the integrity of the containment vessel. Personnel from the University of Washington have been leading an ongoing R&D program to ensure that the acrylic containment vessel satisfies all its engineering and physics requirements.
The period 1995 to 1996 was a very exciting time, marked by the transition from R&D and component fabrication to installation in the underground laboratory. By July 1996, all 120 thermoformed acrylic panels from which the vessel is constructed had been fabricated and were on site awaiting installation. During the period June to July the five acrylic cylinders and two flanges which comprise the chimney of the vessel were shipped underground and bonded together to form the 6000lb, eight meter tall chimney. This was then hoisted to the top of the detector cavity while the main spherical shell of the vessel was constructed.
Construction of the shell takes place in three stages. In the first stage, a steel framework was erected on the construction platform on the floor of the underground cavity. The upper hemisphere is assembled "igloo fashion" from five rows of acrylic panels placed on the steel framework. The first row, the "equatorial row", consists of 20 panels. Ten of these panels contain the rope grooves from which the finished vessel will be suspended. Assembly of the equatorial row began in August. Once the panels were bonded into a ring, the 13 panels of the next row were positioned above the equatorial row and bonded to form a ring by making 13 vertical bonds between adjacent panels. This ring was then bonded to the equatorial row below it by making a single horizontal bond. The successful completion in early January of this first horizontal bond at the equator of the vessel was a milestone since at 125' it was the longest acrylic bond so far attempted in the world. Construction then continues towards the "north pole" of the vessel by bonding together the next ring of panels. As of the beginning of March 1996, the upper hemisphere of the vessel is now 80% complete.
The second stage is to bond the chimney to the upper hemisphere while it is still sitting on the construction platform on the floor of the cavity. The construction platform will then be raised from the floor of the cavity until the upper hemisphere is in its final location and the load is transferred to the 10 suspension ropes. This is scheduled to begin on 16 April.
The suspension ropes were originally to be made from kevlar fibers, however; accelerated aging tests (conducted by the Los Alamos National Laboratory) of the fibers in ultra pure water, similar to that which will be used in the experiment, showed that the strength of the fibers was reduced by approximately 44% over the 10 year life of the experiment. This was not acceptable and an expedited research program identified vectran fibers as a suitable replacement. Ropes of vectran have now been made, radioassayed and are on site awaiting installation.
The third and final stage, the assembly of the lower hemisphere of the vessel is begun by sequentially assembling rings of panels and bonding them to the suspended upper hemisphere. The vessel is scheduled for completion by mid September, after which it must undergo a series of acceptance tests, prior to completion of the detector.