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DUMAND: Neutrinos from Beneath the Ocean

by John G. Cramer

Alternate View Column AV-59
Keywords: gravitation centrifugal force inversion black hole lightlight orbit

Published in the June-1993 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 11/18/92 and is copyrighted ©1992 by John G. Cramer.
All rights reserved. No part may be reproduced in any form without
the explicit permission of the author.

    Neutrinos are perhaps the most elusive and mysterious of known fundamental particles. They steal energy from nuclear beta decay, from the sun, from nuclear reactors, from distant supernovas. An average neutrino from a beta decay could travel through solid lead a light-year thick without interacting. Neutrinos are prime suspects for "dark matter", the mysterious substance that is responsible for most of the mass in our universe. And, as previously reported in this column [Analog - September-92], there is a growing body of evidence that electron neutrinos may be "tachyons", particles that always travel faster than the velocity of light and that slow down when given more kinetic energy. But that is another story.

    In this AV column we will have a look at the DUMAND project, a new $10 million detector funded by the US Department of Energy for the detection of ultra-high energy neutrinos. DUMAND stands for Deep Underwater Muon And Neutrino Detector. It is now under construction in Hawaii and will come into operation in 1993-94. It is to be placed almost 3 miles deep on a level stretch of Pacific Ocean bottom about 18 miles west of Keahole Point on the Island of Hawaii. Floats anchored to the ocean bottom about 40 meters apart and arranged in an octagon around a central junction point will support nine long vertical strings of sensitive light detectors. DUMAND will be connected to its land-based laboratory by bundles of fiber optics cables. AT&T will lay the cables to shore, and the US Navy's manned deep submersible DSV Sea Cliff will be used to connect, service, and repair the parts of the detector.

    But before getting into the details of construction, let's focus on the primary question: "What is DUMAND for?" Briefly, it's for the creation of the new science of high-energy neutrino astronomy, for looking where no one has looked before, for finding the spots in the sky where the most energetic processes in the universe are taking place.

    Our universe contain a small number of very special "objects" that deliver to our upper atmosphere a "rain" of very high energy particles: protons, heavier nuclei, electrons, photons, and neutrinos. These high energy particles from space are called cosmic rays. The details of the processes that produce cosmic rays are not well understood, but we are coming to realize that these very high energies are ultimately the result of the direct conversion of mass to energy near the event horizons of black holes.

    We would like to map the sky for the sources of cosmic rays, to learn where they and what they are. Of the cosmic ray particles listed above, only the electrically neutral photons and neutrinos are useful for "back-tracking" to their sources. The other particles, protons, nuclei, and electrons, have electrical charges that cause them to be deflected in random ways as they pass through the magnetic fields of the galaxy, the solar system and the earth. This scrambles their incoming direction so that it cannot be related to the direction of the source. To locate the cosmic hot spots, therefore, photons or neutrinos must be used.

    Photons, of course, are the mainstay of conventional astronomy. Photons in the microwave, infrared, visible, ultraviolet, and x-ray regions of the electromagnetic spectrum have been used by astronomers to map the universe. There have also been studies of the sky with gamma rays, photons with energies of about 0.5 MeV or higher. But as the photon energy rises, detection becomes more difficult and direction information more elusive. Gamma rays interact too much with Earth's atmosphere, so all studies of cosmic gamma rays must be done in space or using high altitude balloons. The sizes for gamma rays detectors also depend on the gamma ray energy, with very high energy gamma rays requiring extremely large detectors. These constraints have, up to now, severely limited gamma ray astronomy.

    But if detecting gamma rays from space is difficult, detecting neutrinos is even more difficult because the interaction with matter of neutrinos is about 10-7 times weaker than the interactions of gamma rays. If gamma rays interact too much with the atmosphere, neutrinos interact far too little. Typically neutrino detectors must be placed deep underground, where neutrinos can easily reach the detector but other particles are blocked by the shielding of the earth itself. Neutrinos have only been directly detected in a few experiments, all of which have required large quantities of matter and elaborate detection schemes. For example the underground solar neutrino detector located deep underground in the Homestake gold mine, where 15-18 MeV neutrinos from the sun were first detected, used about 380 tons of per-chloro-ethylene cleaning fluid as its detection medium. [See my columns about neutrino detection in the 05/86 and 09/92 issues of Analog].

    The DUMAND detector is designed to detect cosmic ray mu-neutrinos with energies in excess of 1 TeV (1012 electron-volts of energy) as they pass through sea water. Although a neutrino has zero rest mass (or perhaps nearly zero), a 1 TeV neutrino has a mass due to its energy that is greater than the mass of 1000 hydrogen atoms. When such a mu-neutrino (electric charge=0) has a hard collision with a down quark (charge=-1/3) in a nucleus there is some probability that the neutrino and quark will exchange a W boson, with the result that the electric charge of the neutrino drops by one unit while the charge of the quark increases by one unit. The mu-neutrino thus becomes a muon (a mu lepton with electric charge = -1) and the down quark becomes an up quark (charge=+2/3). The newly created muon keeps essentially all of the energy of its parent neutrino, but it is now electrically charged. It will have a gamma factor (or mass-increase factor) of about 10,000 and a velocity only 6 parts in 109 less than the velocity of light in vacuum. But in sea water visible light travels only about 3/4 of its speed in vacuum. Therefore, the 1 TeV muon, newly made from the cosmic ray neutrino, will be traveling about 33% faster than the speed of visible light in water.

    When an airplane exceeds the speed of sound in air (breaks the sound barrier), it makes a shock wave that is popularly known as a "sonic boom". Similarly, when an electrically charged particle exceeds the speed of light in a transparent medium like water, it makes an electromagnetic shock wave. This shock wave is called Cêrenkov radiation, a wave front of blue light that spreads out in a cone from the track of the superluminal charged particle. The cone of Cêrenkov light has a characteristic direction that can be analyzed to determine the direction of the incoming muon (and hence the incoming mu-neutrino) to a directional accuracy of about 1o.

    In a conventional high energy physics experiment, a Cêrenkov detector might be made from a slab of transparent plastic optically coupled to a photomultiplier tube. In DUMAND the plastic slab is replaced by 2,000,000 tons of sea water optically coupled to 216 hemispherical 15" diameter photomultiplier tubes, each housed in a 16" spherical pressure vessel that can sustain the water pressure of 100 atmospheres present in the ocean depths where the detector is located.

    About half of DUMAND's funding, about $4.8 million, comes from the US Department of Energy. The other half, in the form of the photomultiplier tubes (PMTs) and fast electronics, key components of the detector, will be the contributions of Japanese and European collaborators. About half of the PMTs will be made by the Hamamatsu Corporation of Japan and will be conventional "venetian blind" photomultipliers custom made for DUMAND to achieve the required specifications of sensitivity and timing. The European half of the PMTs will be made by the Phillips Corporation. The Phillips PMT uses an innovative 2 component design, a large image intensifier coupled to a small 2" photomultiplier. Each of the PMT types has some advantages, and a mix of the two types in the detector brings additional benefits.

    A light sensitive detector like DUMAND must be placed in a dark environment, because ambient light is a source of background. Fortunately, essentially no daylight can penetrate the ocean to a depth of 3 miles, where the DUMAND array is located, and the principal light sources will be from bioluminescence and from Cêrenkov light from 40K radioactive decays in the sea water. Neither of these is a problem, as demonstrated in November, 1987, when a small DUMAND prototype string of PMTs and associated hardware was tested in the ocean near Hawaii at depths down to 4 km.

    One significant problem of DUMAND is the position calibration of the detector strings, which hang above the ocean bottom on float-supported cables. Currents in the ocean depths can cause significant movement of the cables that could, if not taken into account, lead to significant errors in interpreting the Cêrenkov light from energetic muons. The DUMAND experimenters solve this problem by surrounding the detector with sonar broadcasters producing chirping pulses of sound that are picked up by microphones placed along the detector strings. This locates each microphone on each string to an accuracy of a few centimeters and eliminates potential errors from the motion of the strings.

    Another source of background in DUMAND comes from muons produced in the upper atmosphere by cosmic ray protons, nuclei, electrons, and gamma rays. Some of these muons have enormous energies and can, with some probability, penetrate the ocean even to a depth of 3 miles. These high energy particles may be interesting in their own right, but they are not predominately produced by neutrinos, the particles of primary interest. Fortunately, there is an excellent way of distinguishing neutrino-generated muons from other cosmic-ray generated muons. If the muons are observed to pass upwards or sideways through the detector, they can only come from neutrinos that have passed through the bulk of the earth as neutral particles before being converted to a muon in a collision with a quark. A muon traveling on the same path would have been absorbed by the earth. The muons from the upper atmosphere, on the other hand, must travel downward through the detector. All upward going muons must be the product of neutrino events.

    What cosmic cataclysms can produce neutrinos with such enormous energies? This is a subject of some speculation in the astrophysics community. Recent calculations have predicted that active galactic nuclei, the power source for quasars and other high energy phenomena, can produce enough primary neutrinos to make thousands of ultra-high energy neutrino events per year in the DUMAND detector. Even using more conservative estimates of the cosmic neutrino flux, the DUMAND collaboration expects about 80 neutrino events per year at energies above 10 TeV and 300 events per year at energies above 100 GeV, which is about the threshold of sensitivity for the detector.

    DUMAND is a delightful scientific adventure, an initiative that will show us a new aspect of nature, a technological foray that combines the forefront techniques of electro- optics, microelectronics, communications, high energy physics, and oceanography. And, from the point of view of the experimenters, the shores of Hawaii will be a wonderful spot from which to explore the mysteries of the universe.


References:

DUMAND:
S. Matsuno, et al., Nuclear Instruments and Methods A276, 359 (1989);
J. Babson, et al., Physical Review D42, 3613 (1990).

Active Galactic Nuclei:
F. W. Stecker, et al., Physical Review Letters 66, 2697 (1991).


SF Novels by John Cramer:  my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .

AV Columns Online: Electronic reprints of about 177 "The Alternate View" columns by John G. Cramer, previously published in Analog , are available online at: http://www.npl.washington.edu/av.


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