Deep Sphere: The unique structural design of the Sudbury Neutrinos Observatory buried within the earth

By Dr. Robert Brewer, P.Eng., AGRA Monenco

Consulting engineers helped to build one of the most unusual laboratories in the world, an underground research station to explore the mysteries of the universe.

A mile beneath the earth's surface in northern Ontario, consulting engineers are helping an international consortium of universities and scientific laboratories to explore the structure and properties of neutrinos, one of the fundamental building blocks of nature.

That exploration is taking place at the Sudbury Neutrino Observatory (SNO), which was opened by famed scientist Dr. Stephen Hawking this spring. AGRA Monenco of Oakville and Canatom NPM (an associated company of AGRA Monenco and SNC-Lavalin of Montreal) are the principal engineering, procurement and construction consultants for the SNO Institute, a Canadian/American/ British scientific collaboration led by Queen's University. AGRA Monenco and Canatom were responsible for the preparation of the detailed engineering of the observatory. They also managed the excavation of the underground facilities by INCO (the mine's owners), handled the procurement of materials and equipment, and performed the project and construction management. With the installation of the laboratory complete, the consulting engineers are now overseeing SNO's commissioning and first operation of the detector equipment, including the initial fill of 1,000 tonnes of heavy water (worth $300 million) which is on loan from Atomic Energy of Canada Ltd.

Studying the elusive neutrino

SNO is a unique physics laboratory established in INCO's deepest operating mine-the Creighton nickel mine in Sudbury. Creighton has been in operation since the turn of the turn of the century and its identified reserves of nickel will last for decades yet.

The Sudbury observatory will allow scientists to study neutrinos from the sun. These tiny particles emitted by the fusion processes of stars are known as the most elusive particles in the universe because they are extremely difficult to observe. Neutrinos are so small they pass through almost all matter, so it is nearly impossible to study them. Indeed, until the construction of SNO there was no way to study all three types of neutrinos-the electron, muon and the tau neutrino-and therefore no way to help unravel the scientific puzzle of whether neutrinos have mass. The answer to this essential question is the key to placing neutrinos within the framework of the basic forces and building blocks of nature.

While other neutrino observatories in the United States, Japan, Russia and Italy have been able to detect one type of neutrino at relatively low event rates, SNO's use of a very large volume of heavy water will allow it to detect all three neutrino types at event rates 50 times those of the best existing detectors.

The characteristics that make the SNO detector unique also created the challenges to AGRA Monenco to find cutting edge solutions.

Construction challenges

The SNO detector is housed in a cavern as large as a 10-storey building. In the cavern, two main structural steel trusses are mounted on four corbels attached to the rock walls. The trusses are designed to support 1,200 tonnes and provide the total support ' for the detector systems that are suspended from them.

. Above: A plan view of the SNO laboratory
Above: A plan view of the SNO laboratory (SNO)

Secondary trusses and a reinforced concrete deck provide a large room, which is the access for the water, data acquisition, and calibration systems serving the detector.

The two main components suspended from the deck are an enormous 12-metre acrylic sphere hung by 20 2.5-cm Vectran ropes surrounded by a large 18-metre stainless steel geodesic structure hung by 15 stainless steel cables.

The acrylic sphere holds 1,000 tonnes of ultra-pure heavy water, and is suspended in a bath of ultra-pure regular water. Around the sphere, mounted on the geodesic structure, are 9,600 very sensitive photomultiplier tubes to measure the energy and interaction of tiny amounts of Cerenkov (UV) light emitted by neutrinos as they interact with the heavy water.

The light-sensing tubes, 20 cm in diameter, were made in several stages in Germany, Japan and Queen's University using special low activity glass. They were mounted at Sudbury in 750 triangular panels and cover the geodesic structure.

The underground laboratory facilities are housed in separate drifts adjacent to the detector, and on the same elevation as the cavity deck. An essential aspect of SNO's experiment is to eliminate any possible interference with the neutrino measurements, which means cleanliness is critical. There is a car wash facility in which all materials and equipment going into the laboratory are cleaned, or where sealed packaging is removed if the object has been pre-cleaned. Personnel enter through a separate path where they remove their mine clothes, shower, and don clean room attire. (There are about 25 full-time operations and maintenance staff and 25 scientific staff, working on site at any one time.) Ventilation air is similarly scrubbed by a series of filters to produce a clean room environment. Water purification facilities (both regular and heavy water) are housed in a separate drift next to the cavity, while electronic data acquisition systems and the laboratory monitoring and alarm systems are housed in the short drift connecting the cavity to the rest of the laboratory.

By providing the mine as a site for the laboratory, INCO gave SNO a norite environment with relatively few faults and low natural radioactivity. Two kilometres of Canadian Shield rock and earth screen out cosmic rays from space, which could interfere with the experiment.

The project was able to use the existing infrastructure of ventilation, water and compressed air services, power distribution and elevator access at only the incremental cost. As a result, the facilities were constructed for a capital cost of $74 million. They will cost $3.5 million per year to operate.

The mine services are distributed to each active level in the mine (every 61 metres) by INCO. On the 2,070metre (6,800-feet) level where SNO is located, the services are extended from the mining area to the laboratory entrance.

Because of the onerous requirements for water quality, SNO ran its own plastic pipeline from the access shaft almost two kilometres to the laboratory entrance. Here the INCO potable water is then treated by filtration, reverse osmosis, vacuum degassing, ultra-violet sterilization and ultra-filtration before being used in the detector.

The detector cavity, 22 metres in diameter and 30 metres high, is the largest cavity at that depth anywhere in the world. The detector's design life (which is 15 years ) plus the value of the heavy water, made it very important that the cavity remain stable for a long time, even in the event of a major rock burst in the adjacent mining area. Both rock bolts and cable bolts, supplemented by steel mesh and concrete, help provide the ground control for the cavity.

INCO used its mining expertise, supplemented by geotechnical engineering specialists from both industry and academia, to detail the required reinforcing of the rock surface. An additional geotechnical challenge was the design of the four wall-mounted corbels near the top of the cavity, which use multiple post-tensioned cables grouted 20 metres into the rock to support the cavity deck and all components of the detector. A significant complication was that both the corbels and the ground control were installed progressively from the top down and therefore had to be designed (and monitored) to survive the blasting accelerations as each cavity bench was excavated.

AGRA Monenco had the next major challenge which was the need to shield the neutrino detector from the surrounding natural radioactivity, and to find a way to minimize the trace radioactivity content of the detector itself. Radioactive decay in the rock produces high-speed neutrons and radon (a radioactive gas). Water shields against the neutrons by thermalizing them and thereby preventing their interference in the detector. The thick polyurethane cavity wall coating acts as a diffusion delay for the radon (which has a half-life of about four days) so that it has decayed to insignificant concentrations by the time it reaches the cavity water.

Building the acrylic sphere

The acrylic sphere for the heavy water is 12 metres diameter and 5.5 cm thick, with an acrylic access chimney 1.5 metres diameter and 7.5 metres high. Since everything for the detector and tire laboratory had to fit into the mine hoist cage, the acrylic vessel had to be bonded together in situ from 130 main structural components up to 2 metres x 3 metres large, spherically thermoformed and numerically machined to 0.5 mm tolerances. The cleaned panels were bonded using a catalyzed acrylic syrup which is kept at freezer temperatures until used.

Above: An artist's sketch of the completed SNO detector .
Above: An artist's sketch of the completed SNO detector (SNO)

To form the sphere, horizontal rings of 11 to 13 panels were first bonded together one ring at a time, and then the nine horizontal rings were bonded to each other, starting with the equator ring and doing the upper hemisphere and chimney first. The vessel was then hung, and each lower ring was added on, one at a time. Each bond required a three-day heat treatment at 175 F, after which the acrylic bond area was carefully sanded and polished to restore the original optical finish.

The acrylic material suppliers cleaned their manufacturing plant, and flushed their process equipment, before starting the SNO production because the detector is extremely sensitive to contamination of the acrylic. It was equally important that the acrylic be free of the normal stabilizing additives since they absorb the UV light produced by the neutrino interactions.

Assembling the detector in the cavity was one of the project's biggest cleanliness challenges. This task required the implementation of an air-conditioned clean room environment (better than class 10,000fewer than 10,000 half micron particles per cubic foot) in the depths of an operational hard rock mine. The cleanliness standards were most stringent in the central part of the detector, and therefore the heavy water containment sphere had the most rigorous requirements. Construction activities such as erecting and removing jigs and scaffolding, and particularly the bonding and finishing of the acrylic sphere, naturally produced significant levels of dust. Portable high efficiency particulate filter (HEPA) fan units were used at the work locations to reduce airborne particle levels, and copious amounts of ultra-pure water were used to do the wet sanding of the acrylic, and to carry debris to the bottom of the cavity where it could be collected and removed. Anything not easily cleaned after installation (such as the panels of photo-multipliers) was covered for protection until all construction was essentially complete.

The use of expensive and heavy water borrowed from Atomic Energy of Canada was critical to the experiment as it enables the scientists to more accurately study the neutrinos. It also meant that an extensive risk control program was required to protect the heavy water from loss or downgrading, and this necessity had an impact on the design of the acrylic vessel and water process equipment. Seismic analysis of the detector was also an important part of the engineering, especially during the filling and operation phase. The seismic design of the detector catered for both the seismicity of the Sudbury area and of the local mining area, and it drew from government and mining records, and data available from other mines around the world in similar rock structures. The postulated largest credible rock burst, located 350 metres from the laboratory, turned out to be more severe than the National Building Code seismic requirements and was therefore used as the design requirement.

Logistics and grand opening

The special demands of managing geographically dispersed design and review teams including engineers, technical specialists, and leading scientists required full use of the management systems and experience developed by AGRA Monenco and Canatom NPM.

Famed physicist Dr. Stephen Hawking, who has made numerous contributions to fundamental physics and cosmology, officially opened SNO earlier this year with a visit and presentation. He is best known for his discovery that black holes can emit radiation. His success at overcoming his personal and scientific challenges provided motivation for the SNO scientists, who are preparing for the detector to illuminate some of the deepest questions of Mother Nature and particle physics.

Reprinted courtesy of Canadian Consulting Engineer