LABORATORY PROFILE The Sudbury Neutrino Observatory - 's eye on the universe

Some 2000 m underground in a working mine, physicists have installed one of the world's most sensitive instruments for observing the universe. Operational Neutrino since 1999, the Sudbury Neutrino Observatory Observatory has ambitious plans for the future James Gillies reports.

It all began 1.8 billion years ago when of 40°C, making for a sticky 1.5 km walk geologists believe that a meteorite struck along the "SNO drift" - the tunnel con­ the Earth, creating what is now the necting the mine shaft to SNO's under­ in Canada. The impact ground laboratory. allowed a rich seam of nickel- to rise through the Earth's crust around Cleanliness is the key the rim of the crater. Today the Sudbury Visiting SNO is an adventure in itself. basin is circled with the world's largest Scientists and miners are indistinguish­ concentration of nickel mines, and, in able in all but their conversation as they one of them, scientists accompany the descend in the lift. Overalls, miners' miners on their morning descent to the lamps and safety harnesses are the 6800ft (2000 m) level. order of the day. Everything to be taken The Sudbury landscape still has an into the lab must be carefully wrapped unearthly quality about it. Early in plastic to protect it from the efforts stripped away trees to provide fuel omnipresent mine dust. Arriving at the for smelting the ore, with the result that in lab, boots are rinsed down, clothes are the 1960s the Sudbury basin resembled removed and everyone takes a shower something close to a moonscape. NASA before changing into a clean set of over­ even sent moonshot astronauts there for alls and entering the lab. training. Today the trees are coming 2000 metres underground - excavation of the Scrupulous attention to cleanliness back, thanks to a large degree to the underground cavity for the Sudbury Neutrino is one of the keys to SNO's success. mines themselves, where underground Observatory detector. Incredibly, the laboratory maintains nurseries provide warm stable condi­ class-100 clean-room conditions in the tions for trees to grow. most sensitive areas and all areas are class-3000 or better. That "All you have to add is light," said Art McDonald, director of the means that everywhere within the laboratory there are fewer than Sudbury Neutrino Observatory (SNO) as we stepped off the lift 3000 particles of 1 (jm or larger per 1 m3 of air. A typical room would 2000 m underground. Here the rock is constantly at a temperature give a count of around 100 000 particles and the SNO drift consid-

24 CERN Courier December 2001 LABORATORY PROFILE erably more. Even more impressive protons and an electron. First is that these clean conditions were results published in 2001, taken maintained throughout the con­ together with Superkamiokande's struction of the experiment. previous measurement (CERN The emphasis on purity does not Courier September p5) via the end with the air. Systems for puri­ elastic scattering of boron-8 neu­ fying the SNO detector's light and trinos from electrons with low sen­ heavy water fill most of the avail­ sitivity to neutrino types other able space. The 33 m deep, 22 m than electron neutrinos, provide diameter chamber that houses the compelling evidence for neutrino detector is lined with several layers oscillation. of plastic material that help to The next step for SNO was to keep the radiation level from measure the total boron-8 neu­ uranium and thorium a full nine trino flux to give a complete mea­ orders of magnitude lower than in surement that is independent of the surrounding rock. the Superkamiokande result.To do this, salt has been added to the Herb Chen's experiment heavy water. Salt increases SNO's SNO began collecting data in sensitivity to the flavour-blind pro­ 1999, but its history goes back cess of neutral current neutrino- much further. In 1984 Herb Chen deuteron interactions, which are of the University of California at identified by the detection of the Irvine first pointed out the advan­ photon emitted when the tages of using heavy water as a deuteron's neutron is captured. detector for solar neutrinos. Two Capture on heavy water results in reactions - one sensitive only to a 6.25 MeV photon, whereas cap­ electron-type neutrinos, the other ture on chlorine releases an sensitive to all neutrino flavours - 8.6 MeV photon that is more eas­ would allow such a detector to ily detected. Moreover, the neutron measure neutrino oscillations capture probability in SNO's heavy directly. The Creighton mine in water is around 25%, whereas in Sudbury - among the deepest in A schematic of the SNO detector. salt it rises to 85%. Radioactivity the world - was quickly identified levels are also low for this phase as an ideal place for Chen's proposed experiment to be built and the of the experiment and data analysis is underway. SNO collaboration held its first meeting in 1984. In a third phase of running, scheduled to begin in the second half There were substantial obstacles to overcome before the experi­ of 2002, the salt will be removed and replaced by helium-3-filled ment could be realized, not least of which was the cost of the heavy proportional counters. These will give the experiment an water. It was clear from the start that industrial partners would have independent sensitivity to the neutral current process and allow to be found. INCO, the company operating the Creighton mine, distortions in the solar boron-8 spectrum to be measured more became a key player, putting its infrastructure at SNO's disposal and accurately than before. blasting out a new cavern for the experiment far away from ongoing mine activity. Another key partner was found in the form of Atomic Supernovae warning Energy of Canada Limited, which provided C$330 million of heavy Solar neutrinos form just one strand of SNO's research programme. water on loan, free of charge. "In a sense we're doing a greater than The experiment's ability to single out electron-neutrino interactions C$600 million project for less than C$100 million in terms of capi­ and its high sensitivity to other neutrino types gives it a powerful tal cost," explained McDonald. tool for investigating supernovae by observing the time development The experiment was approved in 1990. Excavation took three between different neutrino types emerging from the explosion. SNO's years and installation a further five. The detector consists of a 12 m data-acquisition system, normally running at around 10 Hz, is set diameter acrylic sphere containing 1000 tonnes of heavy water sur­ up to buffer several hundred events in a window lasting just a few rounded by light water and viewed by 10 000 photomultipliers. seconds if necessary, and it also alerts the shift crew whenever the Filling the sphere with heavy water, flooding the cavern with light event rate rises significantly. This initiates an analysis procedure, water and calibrating the detector was complete by November designed to identify whether noise or physics is responsible for the 1999, allowing data taking to begin. rise. SNO will be part of the Supernova Early Warning System In its first phase of running - to June 2001 - SNO's analysis con­ (SNEWS) along with the LVD (Gran Sasso), Superkamiokande centrated on the measurement of boron-8 electron neutrinos from (Japan) and Amanda (South Pole) experiments. Signals sent to a the Sun.These are detected at SNO via the charged current process central computer in Japan can be studied for time coincidences and of electron neutrinos interacting with deuteronsto produce a pair of the astronomical community can be alerted in the case of a >

CERN Courier December 2001 25 LABORATORY PROFILE

Neutrino observatory pioneer - in 1984 Herb Chen ofUC Irvine The SNO heavy-water purification plant first pointed out the advantages of using heavy water as a detector for solar neutrinos. He died in 1987, long before the via cosmic-ray interactions in the atmosphere. In contrast with Sudbury Neutrino Observatory became operational. detectors under mountains, SNO has a 45° window for measuring downward-moving neutrinos. A clear distinction between downward supernova. The neutrino burst can precede light by several hours. and upward-moving neutrinos will allow SNO to make a model-inde­ The detector's location 2000 m below a flat surface also makes it pendent measurement of atmospheric neutrinos over a three- to a particularly powerful instrument for observing neutrinos created four-year timescale. SNO has a well defined programme until 2006 and ambitious plans thereafter.The scientists envisage a shift in emphasis towards RF Amplifiers more subtle neutrino physics and possible improvements to the SNO detector. Seasonal variations and correlations with the solar cycle QEI Corporation will design and manufacture to are on the agenda. SNO will also turn its attention to other neutrino your RF Amplifier needs. Our 30 years of experience oscillation processes in the Sun. allows us to work with you to complete your requirements. Canadian scientists are hopeful of extending the laboratory Whether you require solid state or tube, air or water beyond the one experiment that it currently houses. The Canadian cooling, CW or pulse, LF to UHF, we can meet your government has recently launched the Canadian Foundation for needs. To discuss your project, contact QEI at 800-334- Innovation International Programme to generate world-class inter­ 9154 (USA), 1-856-728-2020 (International), or via e-mail national research facilities in Canada, and Sudbury is a strong con­ at [email protected]. tender. Having passed the first round of selection, the laboratory has been invited to submit a detailed proposal by February. Under this C$30 million plan, the Sudbury site would acquire a new exper­ imental hall to house at least two new experiments. Final selection is scheduled for June 2002. Herb Chen didn't live to see his brainchild realized. He died in 1987, but his presence at Sudbury is still very strongly felt. Copies of his 1984 Physical Review Letters paper hang proudly around the laboratory and his portrait graces the entrance. SNO has put 65KW, 200MHz, CW, Sudbury firmly on the physics map, but it hasn't lost sight of its Single Tetrode RF Amp roots. "The SNO team is working very hard to accomplish the full 8KW, 200MHz, CW, Water physics objectives while maintaining Herb's memory as a constant Cooled, Solid State RF Amp inspiration," explained McDonald. •

Further reading MI\ Herb Chen 1984 Phys. Rev. Lett 55 14. SNO Collaboration 2000 Nuclear Instruments and Methods A449 QEI Corporation»One Airport Dr«PO Box 805«Williamstown, NJ 08094 USA»800-334-9154 (USA) 1-856-728-2020 (International) • [email protected] (e-mail) •http://www.qei-broadcast.com 172-207. SNO Collaboration 2001 Phys. Rev. Lett. 87 07301 1-6

26 CERN Courier December 2001