Seeking neutrinos under the ocean.

Seeking neutrinos under the ocean

Astrophysicists have long believed that a variety of highly energetic objects in the sky emit neutrinos. The actual detection of such neutrinos from supernova 1987 A proved the point and got the science of neutrino astronomy off to an observational start. Now interested scientists hope to deploy the largest detector for astronomical neutrinos yet contemplated. DUMAND, or the Deep Underwater Muon and Neutrino Detector, will use a volume of the ocean off the island of Hawaii as its detection medium. The group that wants to build it has just completed the first stage: verification that a string of instuments deployed underwater can detect astronomical neutrinos and determine the direction from which they come.

The second stage of DUMAND, the actual instrument, will consist of 208 photomultiplier detectors, distributed among nine strings each 330 meters long. Eight strings will be at the corners of an octagon and the ninth in the center. They will be attached to the ocean bottom in water 4.8 kilometers deep, 30 km off Keahole Point, Hawaii. With buoys on their upper ends, the strings will float vertically, "like sea grass," says John Learned of the University of Hawaii at Manoa, who is technical director of the Hawaii DUMAND Center, which manages the project. DUMAND is an international cooperation involving institutions in the United States, Switzerland and Japan.

A proposal for funding the second stage is just about ready to be sent to the Department of Energy, says Learned. The latest cost estimate is $9 million. If Congress appropriates money in the next fiscla year, he says, the second stage could be deployed in three years.

Astrophysicists have planned DUMAND for more than a decade. Learned says the two smaller detectors that recorded neutrinos from supernova 1987 A, the Kamiokande detector at Kamioka, Japan, and the IMB detector at Fairport Harbor, Ohio, were planned at a DUMAND workshop in 1976. These two detectors are large tanks of water with photomultiplier tubes lining their sides.

To detect neutrinos, physicists need a large volume of water. Neutrinos interact with other matter only very weakly. A neutrino can pass through the entire thickness of the earth without hitting anything. However, once in a while a neutrino hits an atomic nucleus and produces a muon particle. The muon is electrically charged (the neutrino is not) and emits the kind of light called Cherenkov radiation as it moves through the water. The photomultipliers record the Cherenkov light, and by whatever tubes are triggered in a given ever the computer program can calculate the direction from which the neutrino came.

To observe neutrinos from more distant and possibly fainter objects than supernova 1987 A, larger detectors are needed. DUMAND's planners want an effective detecting area of 20,000 square meters, compared with IMB's 400. For that a tank is impractical, so they chose the ocean itself.

The stage one exercise was a way of proving it could be done. In it, the scientists dangled a single string of detectors from the U.S. Navy stable research platform Kaimalino. They made measurements during the week between Nov. 3 and Nov. 10, 1987, at depths from 2 to 4.8 km. Analysis of the data, just recently completed, indicates that the string detected muons and had an effective collecting area of 900 square meters.

A serious problem researchers had to face was competing sources of light in the ocean. The ocean contains a certain amount of radioactive potassium, which emits beta rays that produce Cherenkov light of their own. Bioluminescence also contributes a background glow. In the test, neither of these seriously compromised the detection of muons.

However, these background measurements revealed a new kind of deep-ocean bioluminescence of unkown origin. Most previously known bioluminescence is confined to the upper 1 km of depth where sunlight penetrates and most of the biota lives. This new form goes deeper, however, and diminishes in brightness by a factor of two every 600 meters. Soviet groups working in a various places have confirmed its existence.

DUMAND's managers are confident that they can build a detector that will record neutrinos from such things as the centers of active galaxies, quasars and possibly other supernovas. A particularly likely class of candidates is the binary star X-ray sources in our galaxy, such as Cygnus X-3, Hercules X-1 and Vela X-1. For an astronomical object to produce neutrinos, Learned says, something in it must produce a flow of energetic protons. These protons hit other matter and produce the particles called neutral pions. The nuetral pions decay into gamma rays and neutrinos. Astronomers have already detected extremely high-energy gamma rays (energies in the tens of trillions of electron-volts) coming from these binary X-ray sources. To them that is prima facie evidence that the neutrinos are also there.