(also see Fraser Duncan's Neutrino 2000 poster for a recent description of the calibration system)
The calibration system, including the manipulator (rope-and-pulley) to move sources within the detector and several sources has been and still is a major effort. Queen's is responsible for the manipulator and part or all of many sources.
The laserball (shown below) is a dielectric filled ball which diffuses light supplied to it via optical fibres from a nitrogen/dye laser. This source used for optical calibration. Optical calibration is very important because SNO detects optical photons that result from neutrino interactions in the detector. Thus we must understand how transparent the water and acrylic in SNO are.
In the next photo, Richard Ford, who did his PhD with the Queen's SNO group, is running some tests of his laser system. Note Richard's clean room clothes. The blue walls you see form the Deck Clean Room. This is the cleanest part of the lab (except the water itself!). It is isolated from the main lab (which is itself clean) and has extra air filters to remove dust. The air quality is measured whenever people are present in the room.
The laser system is in the aluminum box. The inside is shown below. The fundamental nitrogen laser (which produces light at 337 nm) is in a copper box. The arrow points to the exit point of the light. The light travels along to a mirror, off-photo, at which point the light is reflected to one of four dye cells. (The dye cells can be spotted with their white plastic caps.) The light then travels through two filter wheels to select the desired intensity of light and on to fibre optics to the detector.
The Nitrogen-16 source supplies a gamma ray of a known amount of energy. Thus it can give SNO an absolute energy calibration. (Optical calibration gives a relative calibration. The two together allow us to understand the response of the SNO detector to the neutrino signal.) Nitrogen-16 beta decays to an excited state of Oxygen-16 which emits the 6.13 MeV gamma ray that SNO detects.
The Nitrogen-16 source was built at Berkeley National Labs in California. The gas transport system to supply the Nitrogen 16 was built by Andre Hamer, a Queen's graduate student now with the Los Alamos National Lab. The Queen's group has been very active in analyzing the signal from this source. Below, we see Mark Boulay, a Queen's graduate student inspecting the source. The red arrow at the top points to the umbilical, the cable which transports N-16 gas to and from the source and the electrical cables necessary to run the source. The green arrow at the bottom points to the region of the source inside of which the radioactive decays take place. The electron from the beta decay is detected in the source. The gamma follows immediately and escapes the source and goes into the SNO detector. Thus we know (from the electron) exactly when a gamma ray leaves the source and enters the SNO detector.
We can install sources while still running the detector. The source is set up in the region pointed to by an arrow in the photo below. There is a gate valve (circled) which is kept closed. When the source is properly hung above the valve by its umbilical and a rope, the gate valve is opened and motors lower the source into the glove box. This is enough to lower the source straight down into the detector. If we want to move the source off this vertical axis, we need to attach more ropes. The yellow arrow in the photo above points to a set of pulleys on the N-16 source. In the photo below, Mark Boulay is using the gloves to attach the so-called side ropes to the N-16 source. (The laserball has a similar set of pulleys.) With these ropes attached we can move the source throughout an entire plane in the detector.
Several other calibration sources are of interest to double-check the optical-energy calibrations and to better understand backgrounds. One such source built at Queen's is the activated NaI (sodium iodide source). This source produces gamma rays of lower energies to help understand the energy spectrum of radioactive backgrounds within SNO. It has not yet been deployed.
The shifts are fairly long underground: 8 to 12 hours. Here's the lunch/coffee room. It is the only place where food is allowed in the lab. In the foreground is Chuck Hearns, a Queen's technician who has made a huge contribution to the on-site effort.
|URL: sno/queens/calibration.html (Last revised Feb 5, 2002)
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