R. Jeffrey Wilkes, J. G. Learned, and P. W.Gorham
Department of Physics, University of Washington, Seattle, WN 98101, USA,
Department of Physics and Astronomy, University of Hawaii at Manoa,
Honolulu, HI 96822, USA
The announcement in 1996 by the US DOE of the termination of their support for the DUMAND-II project was a great disappointment to the many people who have worked so hard and so long to realize neutrino astrophysics using an underwater Cherenkov detector. The purpose of this paper is to summarize the many accomplishments of the DUMAND Collaboration in advancing our knowledge of techniques, demonstrating the feasibility of constructing a deep undersea detector, and laying the groundwork for future detector designs. The effort to begin high energy neutrino astronomy goes forward today in Lake Baikal, at the South Pole (AMANDA), and in the Mediterranean (NESTOR and ANTARES). We hope that the documentation herein may be of some aid in their success.
The DUMAND concept in its present form (more or less) has been discussed, and construction projects of various degrees of practicality have been proposed, since the mid- 70s[DUMAND 76][Roberts 92]. DUMAND-I was a ship-suspended single prototype string which was successfully operated in 1987[Babson 90]; the array of bottom-moored strings described in the 1988 proposal [DUMAND 88] is called DUMAND-II. The water Cherenkov technique was further refined in the early 80s by the successful construction and operation of large-scale nucleon-decay detectors (later used as low-energy neutrino observatories) by the IMB[IMB 92] and Kamiokande Collaborations[Kamiokande 94]. Experience gained in these projects helped make it possible for DUMAND to move from concept to reality and receive construction funding from the Department of Energy (DOE) in 1990. The cost and risks involved in deep-ocean engineering operations were still a matter of concern to agency officials.
There were also several false starts in terms of scientific and technological attempts, which are discussed below.
The complete DUMAND-II array would have consisted of 216 Optical Modules (OMs: photomultiplier tubes plus front-end electronics, encased in a standard glass oceanographic pressure sphere) deployed on nine vertical strings, moored in an octagonal pattern with 40m sides and one string in the center (Fig. 1). The instrumented portion of each string begins 100m above the ocean floor to avoid benthic boundary-layer effects. In addition to OMs, the strings include sets of hydrophones and other acoustical/environmental equipment, and calibration modules, in which a constant-output laser light source is used to excite a scintillator ball viewed by the PMTs.
In this paper we are not attempting to state every possibly useful result or conclusion to be drawn from the DUMAND labors, but to give a survey with references to guide interested people to the large amount of documentation available. A personal history of DUMAND was written by Arthur Roberts [Roberts 92], and contains many historical details of the development of the DUMAND Project, which we will not repeat here.
It is generally acknowledged that the early DUMAND Workshops ([DUMAND 1975] and [DUMAND 1976] in particular) served as the first major stimulus for astrophysicsts to seriously consider the role of neutrinos in the universe. As neutrino studies proceeded at accelerator laboratories in the 70's, and as astrophysicist learned more about strange and powerful objects throughout the universe, it slowy was realized that not only would neutrinos permit ``seeing'' into such objects, but that neutrinos might play crucial roles in these engines.
It was also noted that neutrinos provide a unique means to detect whether the source region is composed of matter or anti-matter, via the interactions of neutrinos versus anti-neutrinos (see below). This remote sensing of anti-matter regions of the universe, while not immediately going to be carried out, is the only method for doing so now known.
A historical note may be in order as well. The Baikal group has been working in Lake Baikal in Siberia for about the same time as the DUMAND group in Hawaii. Initially, in the mid-1970's, the groups worked together and indeed the 1979 DUMAND Workshop was held on the shore of Lake Baikal. However, political problems developed after the Soviet invasion of Afghanistan, and the US DUMAND group was told by its government that no funding would be available to collaborate with the Soviet groups. Hence the teams reluctantly went their seperate routes. The Baikal group, which has persisted despite enormous social upheaval, and their colleagues, deserve to share credit with the DUMAND Hawaii Collaboration, if there is such credit to be given, for the initial stimulation of neutrino astronomy.
We will not go into detail on the state of neutrino astrophysics here. The field is well launched, as may be tracked through the proceedings of the biannual International Cosmic Ray Conferences, the International Neutrino conferences, and the International Workshop on Neutrino Telescopes series held in Venice (Eight in February 1999). There have also been many specialized meetings, with two being particularly relevant to this text, [Ohashi 86] and [HENA 93]. It was at the latter meeting that the question of substantial high energy ($PeV$ range) emission from AGNs and like objects was seriously considered by a number of the world's leading astrophysicists. It was concluded that such emission was hard to avoid given the observed luminosities, time variations, and inferred source photon densities.
At first it was not clear whether the initial neutrino telesopes should be designed for higher or lower energies. At the 1976 Workshop, groups split the hypothetical telescope designs into three major categories: low energies (such as solar neutrinos and gravitational stellar collapse neutrinos), atmospheric neutrinos, and high energies (as from various high energy cosmic sources). These categories continue to be relevant today.
The lower energy neutrinos, in the range from a few MeV to about 100 MeV seemed at first to be the most profitable avenue scientifically. Unfortunately, the calculations all reached the same conclusions: detectors with low backgrounds, few MeV levels of sensitivity, and masses of kilotons were needed for solar neutrinos (detected via Cherenkov radiation), and the size requirement increased with distance, despite the peculiar fact that the brightest astronomical objects are the most distant. The nearby brightest objects always win, witness that the sky is much brighter in photons in the daytime than by the light of the Milky Way. At this time we concluded that we would need an unaffordable detector of 100 megatons with 10 MeV sensitivity for sensing supernovae out through the Virgo cluster, which would yield an observable SN rate in neutrinos of nearly one per week. We realized that achievable detectors would likely only to be able to monitor our own galaxy. Moreover, there was little hope of a large excess of invisible galactic GSC events over visible SN, and so the observation rate might be only one per 50 years or so, very discouraging. This estimate, refined in the IMB proposal in 1979, was bourne out by the observation of SN1987A by Kamioka and IMB.
The high energy study group concluded that the most effective route to detection of multi-GeV neutrinos and beyond was through the observation of muons, which may come from interactions far from the detector. The detector effective volume is given by the array effectrive area times the mean muon range (which depends upon the neutrino spectrum). Based upon observed astrophysical object photon lumiosities one could set rough upper limits upon neutrino spectra (which could not be much greater in total neutrino luminosity in any model yet conceived). This led to the uncomfortable conclusion that to surely get into the business of high energy neutrino astronomy one needed a detector of effective volume in the range of a cubic kilometer, an estimate that remains valid 20 years later.
The study of atmospheric neutrinos was percieved as an effective avenue for approaching neutrino astronomy. It was clear that neutrino oscillations could be sought and early calculations of the effects of oscillations on the observation of muon angular distributions were made. Moreover the early workshops saw that nucleon decay seraches could be carried out with such instruments, though at that time there was not much interest in the topic in the theoretical community. This situation changed dramatically in about 1978 when the simple GUT $SU(5)$ predictions of proton decay became clear enough to justify a major experimental search. The race for nucleon decay included three large Cherenkov detectors (IMB, HPW and Kamioka). There were people from all of the major institutions engaged in these projects who were active participants in the seminal 1976 DUMAND Workshop. The designs put forward for these detectors were preceded by the ideas worked out at that workshop for low energy neutrino detectors.
A debate has continued over the years as to the relative worth and efficiency of detection of secondary muons from neutrino interactions, versus the detection of the interaction, via detection of the elementary particle cascade in water. In a large array, such cascades appear as directional point sources of Cherenkov radiation. The threshold for detection, effective volume and directional sensitivity depend upon the optics of the medium, detector density, and source spectrum, so general statements are difficult.
However, we have demonstrated with the DUMAND Prototype String [Bolesta 97] that small arrays can search for cascades in huge volumes of ocean, of the order of $0.1~km^3$ at energies in the $PeV$ range. We used a mere 17 hours of data to set the best limits (for a while) on high energy neutrinos from AGNs employing a handful of DUMAND optical modules. At the same time such an array would have an effective area for muons of hundreds of square meters. The effective voilume for muons would be perhaps megatons, for a source with a spectrum similar to the cosmic rays.
On the other hand, should high energy events be seen which are not consistent with atmospheric origin (basically any energy deposition beyond 100 TeV), a small array will have a hard time to reconstruct a direction. This is because the direction of the cascade is not well constrained by a few samples of the light intensity versus direction. There is also a strong corellation between vertex resolution and energy (misreconstructing a wavefront as more flat puts the vertex further away, and the energy estimate substantially higher). Consequently reconstruction of cascades external to the detection array is not promising. One can make explorations, but long term progress demands huge arrays enfolding the search volume. Also, as we discuss below, this tradeoff between cascade and muon detection depends upon the medium (optical scattering).
When we started in this business it was still interesting to measure kinematics of neutrino interactions, since done thoroughly at acclerator experiments. Hence we considered arrays where we might be able to measure, say, both the outgoing muon and the energy and angle of the target-quark induced cascade. In principle one can detect thus, in the case of muon neutrino charged current intereactions, the neutrino energy and direction, and the fraction of the energy transferred to the quark (the ``y distribution''). This is only possible for charged current interactions, and only when the charged lepton can be distinguished from the pion cascade from the target nucleon. While the measurement of the weak interaction kinematics and the nucleon quark momentum distributions are no longer interesting (in this context), the measurement of the neutrino total energy and direction, and energy transfer, remains worthwhile, particularly for oscillation studies, as discussed below.
For neutrino astronomy purposes, observing the whole interaction increases the angular resolution, potentially allowing one to beat the angular scattering, which is roughly $1.5 \deg / \sqrt{E_\nu/TeV}$. Muons in a $km^3$ array will only be contained up to an energy of $\sim 200~GeV$, so such attempts to improve angular resolution for neutrino astronomy below a $TeV$ are worth, though as usual expensive ({\it eg.} more photomultipliers).
If it is possible to measure the average energy transfer fraction
($
The question of measuring the total neutrino-nucleon crossection
versus energy remains an interesting subject, particularly at
extremely high energies. However, at energies up to about 1 PeV,
measurements at DESY can be used to reliably calculate the
crossection (barring the opening with energy of any new channel
unique to neutrinos). The experimental challenge is simply that one
measures a rate of events versus energy, and that rate is the
product of cross section and flux. When $PeV$ neutrinos are detected,
one may employ the varying attenuation of the earth as it rotates
relative to the source (or ensemable of sources) to extract the
crossection. Here the earth attenuation is the product of cross
section and column density of the earth (see below for discussion of
earth tomography). This measurement will not be soon made, but
remains worth watching, as no terrestrial sources of neutrinos of
more than a few TeV are on the horizon.
We have long dreamed of observing neutrino oscillations, in the
ocean with natural neutrinos. This has been spectacularly carried
out now by the Super-Kamiokande experiment [SuperK 98], employing
the atmospheric neutrinos in the energy range around $1~GeV$ with a
$50~kilton$ tank in a zinc mine. The characteristic oscillation
length for muon neutrinos in the $1~GeV$ range appears to be about
$250~km$, but at the time of this writing is not well constrained,
being ucertain by perhaps as much as an orede of magnitude. Thus
one may wonder if a larger neutrino detector also employing the
reliable and farily well known atmospheric neutrino beam, which has
built-in distance variation from $\sim 10 ~ 10,000~km$ could tighten
up the situation. This would seem to be the case, particularly if
the mass difference turns out to be in the range of $<10^{-3}~eV^2$,
where acclerator based studies lose sensitivity (due to limited
range). However, none of the detectors presently being pursued seem
to be able to fill this need, because of their design being aimed
towards high energies and thus sacrificing the resolution needed for
this task. A possible new detector of $10^6~tons$ might suffice
[Learned 98]. Another possiblity is desribed below.
The actual direct detection of tau neutrinos has proved elusive for
20 years now. Their existence has long been presumed, not just from
symmetry, but from the kinematics of tau decay, and the neutrino
flavor counting via observation of the $Z^o$ decay width. Yet no
tau neutrino interactions have yet been discerned. Part of the
problem is that sources of tau neutrinos are not forthcoming, so
there is no known way to make a tau neutrino beam. Secondly taus
themselves are not easy to pick out as they mostly decay (with
lifetime $2.9 \times 10^{-13}~sec$ into many pions and there is
always missing energy/momentum. Thirdly, the tau cross section
comes up slowly (due to the tau mass of $1.777~GeV$), so few tau
neutrino interactions are expected below about $20~GeV$.
A spectacular observation of taus could come about from seeing few
$PeV$ tau production, flight over substantial distance ($\simeq
100~m$) and subsequent decay [Learned 95]. The second (decay) burst
would be typically twice as bright as the first ($\sim 10^{11}$
photons), and both would be detectable at distances of hundreds of
meters in the ocean.
The source of such high energy neutrinos that seems most promising
at the moment is from AGNs [HENA 92]. However, in all seemingly
likely astrophysical circumstances the most likely neutrino flavor
mix is in the range from mostly muon neutrinos to a ratio of 2/1 of
muon neutrinos to electron neutrinos, with negligible tau neutrino
content at the source. Thus substantial tau nuetrino content would
have to arise from oscillations, which are in equilibrium for any
astrophysical distance and such energy, given the parameters we see
in atmospheric neutrino disappearance [Superk 98]. In the referenced
paper [Learned 95], there is a discussion of a hypothetical scenario
where tau neutrinos are observed as well as the resonant
interactions of $\nu_e + e^- \rightarrow W^-$ at $6.4~PeV$, thus
measuring the electron neutrino fraction of the putative
astrophysical neutrino beam. Interesting conclusions for particle
physics and astrophysics can be drawn from suh masurements. The
detector required would have to be of the order of $>1~km^3$ and
have lattice spacing of detection elements no less than $\simeq 100~m$.
Such measurements at once observe the tau neutrino and measure the
mixing angles (assuming a simple three neutrino mixing scheme).
It may seem peculiar to mention the lowest energy expected for
neutrinos, those left over from the Big Bang with thermal energies
at present. Despite much cogitation, physicist have not come up
with any viable proposed means for direct detection of these relic
neutrinos. The only proposal which might work has been on the table
for some years, from Tom Weiler [Weiler 82, 84, 92], involving the
resonant absorption of UHE neutrinos on target relic neutrinos. One
would observe a dip in the neutrino spectrum coming from distant
sources. The last report on this process [Weiler 92] was not very
encouraging owing to the small neutrino mass limits (and thus higher
neutrino beam energies) and limits from the Fly's Eye experiment.
This is another example of physics directly stimulated by DUMAND.
In the early DUMAND Workshop's it was recognized [Bradner and
Learned, cannot find ref] that one might employ VHE neutrinos to
``neutrino-ray'' the earth. The idea is simply that if one has an
external beam of sufficient energy that the flux is measureably
absorbed in passing through the earth, one can observed the time
variation of the interaction rate to study the variation in column
density. The expected attenuation length for muon neutrinos is one
earth diameter at $2~PeV$ (?check?). Given sources at various
declinations, a single detector can make a density profile assuming
a spherically symmetric earth. Given three detectors, one can do
earth tomography, at least in principle. The main interest in
carrying these observations with neutrinos would be to examine the
earth's core, about which there is little knowledge. [ref to Chaincy
Kuo LBL paper here].
Over the years we have studied the ability of large Cherenkov
instruments for the detection of various kinds of hypothetical
particles, typically those invented to solve the ongoing mytery of
the nature of Dark Matter. The Baikal group has actively worked in
this area, and we simply make reference to their limits on magnetic
monopoles which interact with the Rubakov process (instigating
nucleon decay as it traverses the array), searches for quark nuggets
and the like. The basic signature is one of two types: a particle
which traverses the array at a $\beta \simeq 10^{-3 \div -4}$ and
leaving a glowing trail, and a particle which makes a series of
rapid flashes along a similar track. The velocities are
characteristic of galactic objects. In the first case the object
might be any of a number of hypothetical particles, such as quark
nuggets, which heat the medium locally, radiating thermally along
the path. In the second case, the particle may be inducing
interactions as it passes, or sequentially interacting, but the
rardiation is prompt. The difference experimentally is that in one
case the array sees a light which may last for many times the light
crossing time of the array, while in the other the particle would be
detected as many independent events.
It was recognized that calculations were needed for the atmospheric
neutrinos over a large range of energies, from MeV's to 100 TeV or
so. A great deal of work had been done in this area after the
peculiar observations made by the Utah group in the Park City mine
in the late 1960's. These observations, late shown to be in error,
seemed to indicate evidence for the W boson, and set off a number of
searches and calculations. DUMAND stimulated the revisitation of
the calculations about a decade later, and advanced the work both
for atmospheric neutrino fluxes which could take advantage of the
newly made cosmic ray muon measurements both underground and in
spectrometers at the earth's surface to calibrate neutrino flux
expectations in the few to hundred GeV range. The next round of
atmospheric neutrino flux calcualtions got started in the early
1980's in response to the need for reliable background claculations
for nucleon decay searches.
Similarly, DUMAND pushed the calculations of muon fluxes
underground. Important contributions to this were the calculations
made of the various energy loss processes for muons at high
energies, bremmstrahlung, pair produciton and photonuclear
interactions. These have not been much improved upon over the
years.
Also the DUMAND group stimulated people to make serious
calculations fo the Landau-Pomeranchuk-Migdal effect, that can be
thought of as the result of the high energy particle seeing the
medium compressed until some processes such as Bremstrahlung are
being interfered with by neighboring atoms. Multiple calculations
of this (difficult to compute) phenomena were reported at the
[DUMAND 1979]. It is only in recent years that the effect has
actually been studied at SLAC [ref to LPM paper].
A number of things were recognized in the early DUMAND Workshops
relating to the choice of location, detector and array design, which
we will summarize here. In fact most of these judgements have stood
the test of time and remain reasonable today.
One of the first questions was of course the location. People had
the idea of using the almost limitless and free ocean depths since
early days [ref to Reines, Markov and Greisen, dating from the
60's]. However, that assumption was reopened regularly at the
workshops.
Lakes were heavily investigated early on as possible venues for a
large nuetrino detector. It was eventually realized that the deep
lakes in North America were not adequate, at least for an open
array. However, the deepest lake in the world, Baikal continues to
be the host to such an experiment. In the late 80's the Japanese
explored making an instrument (dubbed LENA) in a lake in Japan, but
the prototype was destroyed in a storm and the project abandoned.
In the US, a project called GRANDE was proposed for installation in
a quarry in Arkanses. This device would have attempted to do
neutrino astronomy with upcoming muons. Meanwhile, being at the
earth's surface it could also use uplooking phototubes to study
Extensive Air Showers, in particular attempting to distinguish those
initiated by gamma rays in the TeV energy range. This was largely
promoted a spinoff group of DUMAND collaborators who were frustrated
by the slow progress and daunting techical challenges in the
ocean. That project failed to get funding, as also did several
similar projects in Europe, Russia and Australia. The AMANDA Project
now being brought to life near Los Alamos, for the study of gamma
rays, is in fact a daughter to the GRANDE Project. There are
multiple ties to the DUMAND effort, in senior researchers, students,
detector and calibration techniques, and software.
The early DUMAND workshops concluded that neutrino astronomy would
require detectors with a great deal of overburden for shielding (see
below), and indeed took about a kilometer of water overhead as a
minimum necessary overburden. The choice of venues was then
restricted to deep mines or water. Mines were eliminated on the
basis of cost and the impracticality of making cavities larger than
sone tens of meters, which would collapse. Deep ice was not
considered in these early workshops, becasue nobody thought that the
ice would be clear enough and the access and infrastructure issues
were too daunting. That was later shown to have been an oversite,
witness the AMANDA project making great progress at the South Pole.
The depth issue has generated continuing debate until the present.
It was the early conclusion of the DUMAND group that work in the
ocean requried not less than 2 km depth to avoid biological
activities, and that deeper was better, with the depth of the
abyssal plane of 5km as initial goal. Lessor depths are clearly
tolerable, but at some cost. Of course near arrays located at teh
earth's surface must be enclosed to keep out ambient light. This
was one of the heavy tolls on the GRANDE style array plans: the
combination of cost of bag, water filtration system, and civil
engineering made them very expensive (but of course much more
accessable).
There is no clear break in depth in terms of background. One gains
slowly with depth (as the logarithm) in being able to employ more
solid angle in which neutrino generated muons dominate those from
the surface. A flat overburden is superior to a mountain, and water
to rock for the ability to know the overburden precisely, and thus
manage backgrounds and use the solid angle most effectively.
For underearth arrays, an equatorial location is desirable since
such a detector locally viewing neutrino arrival directions out to
the horizon sacne the shole sky once per day. A polar location is
of course interesting too, as there one sees one half the sky
continuously, with sources moving around at constant elevations.
With the galactic center as a prime candidate for neutrino emission,
a detector in the Northern hemisphere is favored. One the other
hand, many of the best studied astronomical objects are in the
Horthern sky, favoring a Southern hemisphere location for a neutrino
telesope. In sum we jusdged, and it seem true today that if one had
only one choice for location, a near equatorial site would be optimal.
During early DUMAND Workshops it was soon realized that power supply
demanded cable connection to shore. The power requriements were too
great for batteries, nuclear (SNAP) reactors were not allowable by
treaty, and there was no practical means of power generetion in the
deep ocean. Thus it was found, and has been confirmed repeatedly as
the question has been reopened, that connection to shore for power
is unavoidable. Perhaps this will change with sufficiantly low power
electronics. It is obvious that fiber optics are the means of choice
for data tranmission. This is an area of tremendous evolution of the
technology since early DUMAND Workshops, at which time coaxial cable was
the standard.
The issue of cost, reliability and complexity of long cable runs
caused the DUMAND group to discount continental locations, where the
continental shelf drives one out several hundred miles typically in
order to find great ocean depths.
It is obvious that one would prefer a simple flat bottom in the
ocean, one with sufficient bearing strength so that equipment will
not sink into the bottom ooze. This turns out not to be hard to
satisfy, being met by a large fration of the world's surface (the
deep ocean abyssal plains). However, some deep locations considered
in deep trenches should be avoided, due to steep slopes and the not
infrequent ocean equivalent of an avalanche.
One hardly need mention that the availability of locally friendly
territory, with available supplies, communications, transportation
and kindred facilities is strongly desireable. That the Baikal
group and the AMANDA groups are progressing, points out that these
are desirable characteristics not mandatory.
Optics of the natural waters are clearly an important issue. One
happy surprise of the DUMAND adventure, was finding that the deep
ocean waters were in fact about four times (absorption length
$>40~m$ instead of $\simeq 10~m$) clearer than at first thought.
(The amazingly long absorption lengths in the polar ice were a
similar boon.) The effective scattering length in the ocean remains
long compared to absorption, as had been predicted. (The surprise
at the South Pole was that the scattering, with effective length
around $25~m$, dominates the absorption in that environment). The
relative dominance of scattering or absoprtion has a strong
influsence on array deisgn and ultimately physics. For the ocean
case it means that huge cascades may be observed at long distances,
and with sufficient density of photomultipliers the cascade
direction may be reconstructed.
Calculations of anchoring requirements indicated that a closed array
was impractical at the scale of a $km^3$ array. At the scales of
tens os meter dimensions mines with water tanks are the location of
choice. In the intermediate size scale, a closed array may be
practical in the ocean, but expensive. The attraction, of course,
is the avoidance of the predominantly $K40$ induced Chernkov light
in the ocean, amounting to around $200~quanta/cm^2/sec$ in the open
ocean. Large arrays simply have to pay the nearly factor of two
penalty in photocathode area to live with the $K40$ background.
Many studies of structures for arrays were carried out, and are well
documented in early DUMAND Workshops. Economics of material and
construction costs have always lead back to the simplest option of
fabricating the array of elements consisting of what the
oceanographeers call buoys, simple anchored strings with flotation
at the top. It turns out that the typical deep ocean currents (0-
few $cm/sec$) do not much perturb such arrays, and the lack of small
scale rotary current motions (gyres) prevent tangling. Given the
studies of the DUMAND Hawaii site, it would be practical to place
stroings as close as $10~m$ to each other (placement being the more
difficult issue). The fact of motion with the inevitable tidal
drifts, with time salces of hours in changes, permitted a plan to
employ acoustic monitoring of the array position (see below).
An ongoing debate has been focussed around the question of
reliability and the topology of the array connections. The desire
to avoid vulnerability to single point failure of significant
fractions of the array leads to the idea that one should connect
each module independently to shore. While this is impractical, the
tremendous progress in fiber optics has made the cost of individual
fiber optic links similar to the cost of optical modules. Future
arrays will doubtless have multiple paths to shore, with backup paths.
In any case one sould focus upon keeping the expensive and
vulnerable hardware at the ends of the data connection chain, where
failure will remove a minimum element of the system.
Another debate which has gone on over the years is over the degree
of processing to carry out in the ocean. This has changed with
time. At first it was assumed that triggering and filtering would
have to be done at the site of the array, and that is what is done
in Baikal, at least until the present. With the advent of fast
links the DUMAND II group sought to send all data to shore for
triggering and filtering in the laboratory instead of the ocean.
Now with the decreased cost and power of electronics it is possible
to empoy highly reliable and redundant electronics on site, beating
the data rate down to a very small level. Still it will always be
easire to repair and modify the systems on shore.
It is absolutely necessary to design an array which is as simple as
possible to repair and replace. Particularly in the deep ocean,
where one typically works in difficult circumstances, quite often
not being able to know exactly what is taking place.
This is a least partly why much of oceanographic technology is so
primitive, as oceanographsers have typically launched instruments
and when they did not return, as happened frequently, they knew not
why. There is a long standing saying amongst oceanographers that
one should `say goodbye' to an instrument when it is launched into
the sea, as there is a fair chance one will not see it again. This
has often lead to a strategy of making multiple instruments rather
than identifiyiung and remedying existing hardware problems. A good
deal of effort was spent in the DUMAND project trying to reverse
this trend.
The deep ocean also offers the opportunity to reconfigure an array
if one does not make premanent bottom moorings. (This is in
contrast to the situation in ice where once placed, modules are
irretrievable.) The use of buoyed strings of detectors makes this
reconfiguration possible with fairly simple design allowances.
Another long standing debate has been over the commitment to employ
submarines or remotely operated vehciles (ROVs). While the NESTOR
group has chosen to stick with array designs not requireing ROV's
the DUMAND group chose that route. The reason is simply that if one
is not going to make ocean bottom connections, then the whole array
has to be installed of a piece, a frightening thought at the level
of a $km^3$ array, though perhaps doable with initial arrays (as in
NESTOR). ROVs are ever more available, being driven in development
for the deep ocean at this time by the increasing depth of oil
drilling operations.
Much effort was put into studies of triggering for large arrays. In
teh end though, it seems that simple area triggers, such as the
number of nearest neighbor trigger clusters in some woider array
trigger window, works as well as anything and are simple to
implement. Given the relatively low rate of real local physics
events (muons) as compared with EAS arrays, one is not driven to the
necissty of the local storeage and neighbor polling schemes as are
employed in EAS arrays.
Many computer studies have shown that the array shape is not very
critical to performance, within large bounds. Back of the envelope
calculations tell one of the maximum scale of detector spacing,
being of the order of the optical attenuation length in the medium,
if one wants to detect very muon travelling through a region. A
figure of merit which has been multiply verified is that one DUMAND
optical module ($16~inch$ diameter PMT) gets about $100~m^2$
effective are in an array in the ocean. So, one should not put the
modules much closer than $10~m$ on average. With spacing much
larger than that one may miss traversing muons, but spreading the
detectors out gets one more leverage and better angular resolution.
Another ongoing debate has been over placing modules on the outer
surface or throughout the volume. It has turned out in every study
we have conducted that while a furface disposition makes sense for
muons, filling in the volume costs little, and gets one the ability
to detect and reconstruct cascades.
Thus one is lead to the simplest plan of placing strings of optical
modules with spacings of $10-30~m$ floating up from the ocean
bottom. While simplicity in calculation might tempt one to regular
arrays, the breaking of degeneracies in track finding algorithms
favors irregularities (which are naturally going to occur during
placement at the level of a few meters).
The method of deploying an array depends critically upon whether one
demands preconnnection of the array or not. The DUMNAND-II group
chose to employ ROVs, so the deployment was greatly simplified from
earlier plans. To our view this is the only sensible option for
large arrays. Placement of individual strings may proceed in
various ways (we refer the interested reader to the several DUMAND
deployment workshops). It seems very desireable to have strongs
able to be released individually to float to the surface for
recovery (which may then be accomplished by a relatively small and
inexpensive vessel, without the cost of a full expedition).
One of the unanticipated general difficulties with the DUMAND
Project was that many areas of ocean, particularly deep ocean,
engineering was (and still is) far less developed than one might
wish or expect. There is nothing like the space grade of hardware
and electronic qualification. There are Navy standards, but they
tend to be quite out of date technologically and generally
inapplicable to ocean high technology research. We found that we
had to do engineering on such seemingly bread-and-butter items as
mechanical cable (strength) termination methods (amongst a
depressingly long list of such items). The situation is improving
slowly, as oil exploration moves to greater depths and requries
higher technology and remote operations. But future activities
should not underestimate the difficulties which will be encountered.
We would like to highlight several items we feel should be pursued
in studies for a future $KM^3$ deep ocean array. There are the
obvious issues such as reliability, service, data processing,
improvements in optical sensors (see below), and so on. Near the
top of the `obvious' list of technology studies would go pressure
tolerant connectors, the single largest source of difficulties in
the DUMAND Project experience. In this regard, a requirement for
local infrastructure to the assembly laboratory should be easy
access to pressure and temperature test factilities (simultaneous),
large enough to encompass all major pressure housings.
One area we could not afford to engage in DUMAND was the study of
pressure tolerant electronics. The best way to beat the problem of
leaks in housings and connectors is to employ oil filled housings.
We did this successfully with power transformers, connectors and
some cables (inside hoses). However, the central electronics
housing, at some level almost surely a compenent of any deep ocean
array is a candidate for oil filling. Solid state components are
naturally pressure insentitive, and some US NAvy studies have shown
that much off-the-shelf solid state electronics can be used with no
problem under oil at high pressure, as long as there are no voids in
the cases, and as long as pressure was applied and released slowly
(minutes). Of course some items are going to present challenges,
such as changing capaicatances and this is sure to make some effort
for high speed circuits (clocks in particular). A modest
engineering effort seems worth undertaking here.
Another area worth pursuing is the design of a deep ocean resident
robot. The ability to carry out service whenever it is needed
without having to mount an expedition is very attractive. (Such a
device may also be useful for calibration purposes.) Oceanographic
research ships cost typically \$ 10-20 K/day at present, and
submersibles may double that. Commercial costs would be even
greater. Hence with transit and port time, sailing to and from the
site, expedition preparation costs, and so on, a single operation at
the array with an ROV will cost considerably more than \$ 100 K. It
will not take many trips to amortize the cost of a small ocean
bottom resident robot.
The DUMAND optical sensor has become somewhat of a standard, with
variations. Early on we considered developing new types of
photomultipliers which might be their own pressure housings. This
was dropped as it was learned that such housings tend to spall
internally, which could lead to the shedding to photocathode (but
perhaps this is woirth revisiting with improved technology).
Moreover the Benthos 16 inch housing was a well developed standard
of the industry, and our initial concerns about housing dimensional
creep with time were allayed. All the large Cherenkov detectors
(DUMAND, Baikal, AMANDA, NESTOR and ANTARES) employ large PMTs in
spherical glass pressure housings. Additional cylindrical sections
to the pressure housings (which extend the housing and make for easy
insertion of connectors) were explored by DUMAND but not chosen
because of lack of deep ocean experience (and some discouraging test
made in our pressure tank), but have been employed in AMANDA and
Baikal (under less deamnding conditions). There are tricks in
making the seals for glass pressure housings, the major rule being
to use flat ground surfaces without any lubrication. Pressure
joining between materials with differing temperature and pressure
reponse can lead to disasters (spalling followed by collapse).
During the early days of DUMAND many options for optical sensors
were explored, which we will not review here as they are well
documented in the proceedings. We might pass on one general lesson,
which is that despite substantial efforts towards developing light
collectors via wavelength shifting methods, the resulting devices
were impractical for the ocean. Perhaps one might employ wavelength
shifters to gain light collection area on the order of a factor of
two, as was done with the IMB detector (using a rectangular
wavelength shifting plate in the equatorial plane of the PMT).
However, in the ocean wher one would like gains for the most distant
sources, which have already lost all light below $300~nm$, there is
probably not much to be gained (thought the opposite may be the case
for deep ice).
Various alternative configurations of photomultipliers have been
considered, such as cylindrical devices. While these might have
interesting properties, the development time and expense have so far
proved prohibitive. The use of solid state light sensors has not
been even close to practical, either economically or in terms of
background noise level. Spectral response was a problem in the
past, but recent improvements in astronomical detectors have made
high quantum efficiency available in the UV range. The most
profitable area for exploration would seem to be in the possible use
of liquid photocathodes.
All that said, it continues to seem unlikely that one can beat the
use of traditional photomultipliers. The new hybrid
photomultipliers, which employ a solid state electron detector (with
potentially many pixels) inside the vaccum envelope of an image
intensifier may offer new opportunities, though they are more suited
to image resolution than the simple demand for light collection with
modest time resolution ($ns$) of deep ocean high energy arrays of
the DUMAND style.
Alternative to Cherenkov detection have been explored by DUMAND and
other groups over the years. Radio detection has been pursued by
the Russian group at eh South Pole over the years, and there is new
interest. The DUMAND group made a significant atempt in the late
1970's to design an array employing acoustic detection of neutrinos.
The mechanism is simply that the rapid thermal expansion of water
after the heating caused by a the ionization from particle cascade
leads to an outgoing tiny bipolar pressure wave. The group measured
these pulses in the laboratory and in a proton beam at Brookhaven
[acoustic refs]. The conclusion was that there was no unexpected
new mechanism ({\it eg.} microbubble formation) which might have
been more efficient at energy transfer (which is of the order of
$10^{-9}$ for the thermacoustic mechanism). Calculations led to an
energy threshold in the ocean for neutrinos of about $10^{16}~eV$.
As there are no guaranteed sources of neutrinos (or any other
particle) at those energies in the ocean it was not practical to
propose such an experiment, which might have any signals at all.
The DUMAND stragetgy was therefore to build a Cherenkov detector and
piggy-back an acoustic detector, otherwise employed for surveying,
to make a search for UHE neutrinos in multi $km^3$ volumes. The
NESTOR group is keeping this approach alive.
A location near the Island of Hawaii was selected as a site for
DUMAND for a variety of compelling reasons: exceptional water
clarity [DIR-5-86], proximity of an abyssal plain with appropriate
seabed characteristics [DIR-14-83] to a suitable shore site (30 km
away), presence of active particle physics and astrophysics groups
at the nearby University of Hawaii in Honolulu, and pre-existing
laboratory infrastructure at the shore site, due to an ocean thermal
energy research project. The latter feature even provided a
cost-free conduit for the DUMAND shore cable to pass through the
surf zone, since the thermal energy project involved slant drilling
of tunnels into the ocean. The array site is on the ocean floor at
depth 4800m, 30 km due west from the Kona Coast of the Island of
Hawaii (Fig. 2), connected to the shore laboratory at Keahole Point
by a cable combining electrical and fiber optic elements, terminated
in an underwater junction box (Fig. 3). The shore cable contains 12
fibers (including spares) and a copper layer which supplies 5 kW of
electrical power at 350 VDC, using a seawater return system
[Peterson 92]. The underwater site places no inherent limitation on
possibilities for future expansion of the detector. With all 9
strings in place, DUMAND would have an effective detection area of
20,000 m2.
During early 1992, an array of current meters was deployed near the
center of the DUMAND site for the purpose of obtaining a measurement
of bottom currents over a many-month time baseline. Such studies had
not been previously performed over the longer term, and the
information was deemed to be useful for string deployment, since
bottom currents can dominate the placement errors of the
strings. The array had three vector-averaging current meters
deployed at three positions along a 400 m high mooring, one near the
bottom, one at the top and one near the position which corresponds
to the string controller in the middle portion of the string. The
meters were developed and the data collected and analyzed by
Dr. Pierre Flament of the UH Oceanography Department.
The maximum current speed seen over the 5 month baseline was about
14 cm/sec, and currents were typically about 3 cm/sec or less
(Fig. 4). The predominant components of the currents are tidal, with
a NNE-SSW flow pattern superposed on the normal diurnal tidal
flow. Occasionally this superposed flow changes to an ENE-WSW
direction. The tidal behavior of the currents [Flament 92] is such
that a power spectrum of the time series of the current samples
projected to the N-S direction clearly shows strong components at
the harmonics and subharmonics of the solar day (lunar tides tend to
be suppressed relative to the solar tides in equatorial regions).
Prior to 1992, no bottom cores were successfully recovered from the
DUMAND site, and a number of oceanographers expressed concern that
the site might have characteristics that indicated periodic
turbidity flows. Such flows, which could originate due to the
proximity of the steep Hawaii Island slope, could damage or destroy
the array (they have been known to break undersea phone cables);
thus evidence for recent or frequent turbidity flows could impact
the site selection. Turbidity flows leave sediment layers called
turbidites which are usually quite apparent in a bottom core sample.
The coring operations were performed under the supervision of
Dr. Craig Smith of the UH Oceanography Department using a
multi-coring device that his group had developed. A number of core
sets were recovered from very near the nominal site center, using a
ship of opportunity, the Russian research vessel
Yuhzmohrgeologiya. Shipboard analysis of the core profiles indicated
that, although a turbidite layer was present, the depth of the layer
indicated that the turbidity flow associated with it happened of
order 10,000 years ago. The bottom was also found to be relatively
solid sand below a layer of soft mud about 10 cm deep. [DIR-14-92]
The fact that this mud, which lies on the bottom in near colloidal
suspension, was present and relatively thick was also a good
indication that large transport storms were not frequent in the
area.
The 1992 cruise of the R/V Moana Wave at the DUMAND site permitted a
number of deployment and site-related studies to be performed
[DIR-12-92]. The acoustical locating system planned for the
environmental monitor system was tested, and the acoustical
properties of the waters around the site were measured [DIR-13-92].
DUMAND deployment includes string interconnection operations, which
require a manned deep submergence vehicle (DSV) or robot
remotely-operated vehicle (ROV). Despite the fact that operations
such as those required by DUMAND at depths of 4700 m are technically
feasible and not complicated, such operations are rarely performed,
since most similar operations are done within continental shelf
waters at depths less than 1 km. At DUMAND depths, such vehicles are
working at pressures 80% of their rated limit, compared to 15% in
continental shelf operations. Thus deep operations must be tested
before they can be approached with confidence.
Such a test was performed as part of a program sponsored by the US
Navy Deep Submergence Unit (part of the Submarine Development Group
1 in San Diego, California) in October of 1992. The program
solicited proposals for civilian research in areas close to the
Hawaiian Islands, and the proposals competed for a total of 40 dive
days during the September/October 1993 time frame. DUMAND was
selected among these proposals on the basis of its test of forefront
deep ocean technology and was awarded 4 dive days. These dives took
place over the period 29 October-1 November 1992 at the DUMAND site
near Hawaii Island. The operations consisted of two dives with the
submarine Sea Cliff for the purpose of performing a video survey of
the array site. The Sea Cliff manipulators were not functional
during these dives. In addition, a test junction box was stationed
on the bottom just north of the planned site, and the Advanced
Tethered Vehicle (ATV), a state-of-the-art robotic vehicle, was used
to perform umbilical interconnection tests with actual prototype
electro-optic connectors and electro-optic umbilical cable.
The site survey and tests were entirely successful: the site was
shown to be free of any debris that might complicate the placement
of the strings or junction box, and the mechanical interconnection
operations went smoothly and validated the basic connector design.
It is worth emphasizing that even apparently simple mechanical
operations can suffer unexpected failure under the ambient pressure
of the deep ocean. The ambient pressure at the DUMAND site is nearly
4 tons per square inch, of order the same pressure that structural
engineers must deal with in designing mechanical support systems for
skyscrapers. Although the design of a simple latching connector is
straightforward in principle, in practice it may fail to
mechanically couple due to changes in tolerance or material
deformation due to such ambient pressures. Tests of such systems are
difficult and costly to perform, even in the ocean. Thus this test
of the design is of great importance to the project, since it would
be very difficult to recover from a problem with the connector
discovered during an actual mating operation, and would damage the
credibility of the system. In fact, an example of such problems was
provided by the DSV Sea Cliff during this dive series, since the
state-of-the-art Schilling manipulators on the submarine were found
to fail at depths greater than 10,000 ft, and thus could not be used
for our operations.
In addition to tests of the mechanical integrity of the connector
design, the placement of the test junction box and other activities
conducted at the actual site produced important input into the final
design of the base platforms on which both the junction box and
string ballast would rest. Thus we were able to more accurately
predict the degree to which the junction box might be partially
submerged in the mud, and estimate the drag force that might be
expected from trailing the umbilical cables across the sea floor
during the interconnect operations. The fact that no obstructions
such as rocks or other debris were found at the site also relieved
some of the design constraints for these systems.
In June, 1995, the US Navy's Advanced Tethered Vehicle (ATV)
revisited the DUMAND junction box and successfully performed
connecting operations, proving that tethered remotely operated
vehicles (ROVs, which are cheaper and more readily available than
manned submersibles) are also an option for DUMAND underwater
maintenance activities. The ATV removed connectors which were
believed to have failed, and replaced them with protective dummy
plugs. This eliminateded a short circuit observed on the shore cable
power conductor, but the short reappeared after a few hours. It is
now believed that the remaining power line fault is due to
contaminated contacts on the dummy plugs. However, due to the
extremely limited time window available for ATV operations, it was
not possible to complete diagnosis and repair of the junction box.
DUMAND Optical Modules (OMs) come in two models, the Japanese
OM (JOM) and the European OM (EOM). The JOM was based on the
Hamamatsu 15” hemispherical tube designed to fit within a
standard 16” Benthos Sphere (Fig. 5) [DIR-11-83][DIR-14-90][DIR-
4-91][DUMAND-2-93]. The EOM uses a Phillips PMT
[HDC-14-90][DUMAND-3- 93][DIR-5-93][DIR-4-94][DIR-6-94], employing a
novel design in which photoelectrons emitted from the hemispherical
cathode are focussed onto a small scintillator attached to an
internal structure comprising a conventional 10-stage PM lattice.
Signals from the PMTs are pre-processed within the optical modules,
on a custom PC board surrounding the PMT neck (“ring
electronics”)[DIR-20-90][DIR-21-90][DUMAND Optical 90], providing
standard pulses which encode time of arrival (to 1.25 ns accuracy),
pulse area, and time-over-threshold (TOT), a measure of pulse
duration (Fig. 6). The ring boards also carry DC converters which
boost 48V from the junction box to the HV levels required by the
PMT, as well as housekeeping functions.
A different set of experiments were conducted over the course of two
cruises in 1992, aimed at determining the immunity of the array to
the damage caused by catastrophic implosion of the glass pressure
housings used through much of the detector, both to house optical
modules and for floatation. Such implosions are rare enough that
statistics cannot be reliably estimated for the planned life of the
array. The kinetic energy generated in such an event is of order 1
MJ, equivalent to the detonation of a few kg of TNT. The major
concerns were twofold: first, would such events sever the string
riser cables, thus disabling the string; and second, could the shock
wave generated destroy nearby optical modules or disable the
acoustics?
With the help of Benthos Corporation, which donated pre-weakened
glass pressure housings which were set to implode at depths of order
4.5 km, a total of 9 different experiments were performed. As a
result of these experiments, the design of the riser cables was
substantially improved with the addition of titanium "picture frame"
supports around each optical module (Fig. 5). This addition was
found to be adequate to protect the optical, electrical, and tensile
riser cables from any significant damage during an implosion of a
module adjacent to them, even though the implosions in some cases
actually partially damaged the high-tensile-strength titanium frame
(Fig. 8).
As described, in September 1992, a current meter string was
recovered from the DUMAND site after 6 months of data logging. To
our surprise, a number of its 17" float clusters showed implosion
damage, and the damage had apparently destroyed the bottom current
meter; this observation re-inforced the importance of the previous
implosion studies, and care was taken to determine the cause of the
implosions, which was traced to incorrect handling of the spheres in
question. Another important lesson concerning sympathetic implosions
was also learned: spheres within 1 m of another sphere that has
imploded are likely to suffer induced implosions themselves. Thus we
performed some further experiments to determine the minimum safe
distance for adjacent spheres [DUMAND-8-93] Two meters or more was
found to be safe for floats.
Although Benthos provides electrical feedthroughs for their
spherical glass housings, the stock electrical penetrators were
found to be unreliable and were updated after DUMAND testing
suggested a design change. No suitable optical fiber feedthrough
existed. Working with engineers from the Hawaii DUMAND group, the
Swiss firm Diamond SA developed a suitable feedthrough, allowing
conversion of fast data from electrical to optical signals within
the OMs and thus permitting noise and dispersion free transmission
from the OMs to the SC.
Two major in situ tests of the sea-return electrodes have been
conducted: The first took place in August 1991 aboard the RV Alpha
Helix. During the period August 20-26, a series of 32 different
experiments were conducted to determine the behavior of various
electrode configurations, using depths of up to 4.3 km, a few
hundred meters less than the actual operating depth for the DUMAND
array. The various electrode configurations show some differences in
intrinsic resistance, but the time variation of the resistance
remains in the tens of milliohm range without any systematic trends
that suggest long- term increases[DIR-8-92]. This behavior was later
confirmed over a duration of approximately three months using
electrodes prepared for use with the DUMAND junction box as actual
return electrodes. The experiment was performed in the summer of
1993 in the Mediterranean Sea between two shore-based sites. The
results demonstrated that the long-term resistance of such
electrodes remains low. In fact, for the polarity being used in the
DUMAND array, iron electrodes can be used in the deep ocean if the
array is operated at a reasonably high duty cycle, since the deep
sea electrode is effectively galvanically protected by the current
that it passes into the ocean. However, the advantage of using iron
is only a fraction of an ohm relative to a nobler metal such as
titanium; thus the corrosion resistant material is preferable since
it will not suffer during the downtime of the array.
Data from the 24 OMs on each string are digitized and serialized in
the string controller module by a custom 27 channel (including
spares and housekeeping) monolithic GaAs TDC/buffer/multiplexer chip
which operates with 1.25 nsec timing precision and 2-level internal
buffers. Development of this custom designed ASIC (Application
Specific Integrated Circuit) represented a major achievement by the
Boston University group[DIR-2- 95][DUMAND-6-93]. The data stream is
sent to shore via optical fibers (one per string) at 0.5 GHz. A
separate optical fiber carries environmental and acoustical ranging
information which are used to measure the geometry of the array.
To do useful astrophysics, it is necessary to point reconstructed
muon tracks onto the celestial sphere with an accuracy better than
1o (the median angle between primary ? and secondary ? at 1
TeV). This means that relative OM locations must be known to the
order of a few cm, and the overall geographical orientation of the
array must be known to much better than 1 degree. The Global
Positioning Satellite (GPS) system plus conventional oceanographic
acoustical survey techniques allow us to measure the geographical
coordinates of underwater fiducials (acoustical transponders) to
within a few meters, satisfying the geographical orientation
requirement. We were unable to find a commercial system able to
reliably provide the OM positioning accuracies required, so we
developed our own sonar system[HDC-10- 91],[DUMAND-10-93], which
measures acoustical signal transit times with 10 ?sec precision
using frequency modulated chirps and matched filtering via
DSPs[Berns 93]. Other components of the environmental monitoring
system (Fig.9) measure oceanographic parameters such as water
currents, temperature and salinity (needed to calculate the local
speed of sound)[DIR-13-92][DIR-10-94]. The acoustical positioning
system components provided observations of the underwater noise
environment[DIR-3-94][DIR-11-94].
As part of the junction box environmental module subsystem, the
University of Washington group worked with a leading underwater
video equipment manufacturer, Deepsea Power and Light, to develop a
custom camera module integrating a high resolution camera, remotely
adjustable lens and orientation sensors. These components were
mounted in a compact Ti case with an acrylic hemispherical window
which was designed to compensate for optical distortions. Two
modules were manufactured, and mounted on motorized tilt-pan units
which were controlled from the shore station via the JB and
JBEM. DSP\&L also supplied high-efficiency 250W Th-I lamps, which
were mounted on brackets with the camera modules. These lamps
provide highly monochromatic light at a wavelength near seawater
transmission peak and also near the center of efficiency of the Sony
X77 CCD cameras used.
A number of improvements to the construction of the tensile portion
of the string mooring also resulted from the series of Benthos
sphere implosion tests, including the development of a segmented
riser system consisting of 10 m lengths of kevlar forming a chain
with optical module frames as the joining links (Fig. 7). This
system has proven to be more versatile than the originally planned
continuous length of kevlar since it allows sections to be swapped
in or out with much greater ease and reliability.
The Calibration Module (CM) is used to produce fast, bright UV
pulses for time and pulse height calibration of the array
[DIR-4-86]. It consists of two independent lasers with separate
power supplies. The lasers are operated under the control of a
standard OM electronics board, with some changes made to the
computer operating system to accommodate commands specific to the
CM. In addition, the laser intensity is varied by a mechanical
optical filter assembly which is actuated by a steppeing motor under
control of the same computer. The whole system is placed in a
Benthos sphere of the same type used for the OMs. The lasers then
shine through the glass sphere to strike a small plastic
scintillator ball externally mounted 1 meter away. The scintillator
ball makes the pulse more isotropic such that it can be seen by a
large number of phototubes in the detector array. A small PMT
monitors the scintillator flash intensity within the CM. One CM was
constructed and deployed in December 1993 and was subsequently
recovered with the rest of the DUMAND string 1.
One of the greatest concerns has been the difficulty of managing
large and delicate string arrays during the deployment
operations. Two distinct series of tests were aimed at settling
these issues and these tests eventually proved to be invaluable
learning exercises which paid off in a smooth operation when the
actual deployment of the first string took place.
The first tests of deployment took place in September 1992 aboard
the R/V Moana Wave. A short, mechanically accurate section of an
actually string was deployed and recovered a number of times
successfully. This test validated the new string mechanical design
which incorporated the modifications suggested by the earlier
implosion tests. However, fiber-optical and electrical concerns were
secondary to the mechanical ones.
During the next major deployment test, one year later also aboard
the Moana Wave, the deployment scheme was modified further to avoid
any possibility of damaging fiber-optics. This concern arose
because the original method appeared to produce sharp bends in the
riser cables during the lifting and load transfer operations. A
method which solved this problem and also improved the safety of the
operation was found, and implemented.
The deployment tests were crucially important in that the practice
loads used were comparable to the expected loads during the actual
deployment; thus a number of lessons were learned about the support
and transfer of these loads which would have otherwise been missed,
and could have been very costly during the final operations. We were
able to practice lifting and guiding the junction box over the side,
as well as performing the crucial load transfers that would be
required later to manage it. All of the practice acquired in this
shakedown cruise proved invaluable later in the actual deployment.
The shakedown of the surface navigation was also very revealing,
since it was found that the radar ranging system was ineffective at
that distance from shore. This led to a decision to rely on a
redundant DGPS system only for the surface navigation.
In December of 1993, the DUMAND scientific team and the crew of the
University of Washington oceanographic ship R/V Thomas G.Thompson
successfully deployed the first major components of DUMAND,
including the junction box, environmental monitoring equipment, and
the shore cable, with one complete string attached to the junction
box [Grieder 95].
The string was lowered in an inverted position with a sacrificial
anchor and 400 m of line ahead of it. Once the anchor was set on the
bottom, the string was then lowered slowly while the ship moved
ahead to bring the string into an arched position, with its own
buoyancy providing the tension to form the arch (Fig. 10). The
junction box was then placed to complete the arch. Once the string
was in this position, it could be released near the apex of the
arch. The release mechanism was electrically actuated from the
string controller. Unfortunately, the electrical release failed to
operate when actuated. However, there was also a backup magnesium
corrodible link which released the string a few days after
deployment.
The junction box was placed within 90 m of the planned location,
landed upright, and settled into the bottom surface a few inches,
exactly as planned. Cable laying equipment was leased and mounted on
the ship. The procedures for the lowering and cable laying
operations had been worked out in detail by an ocean engineering
contractor[MOE 92].
The deployed string was used to record backgrounds and muon events.
Unfortunately, an undetected flaw in one of over 100 electrical
penetrators (connectors) used for the electronics pressure vessels
produced a small water leak. Seawater eventually shorted out the
string controller electronics, disabling further observations after
about 10 hours of operation.
Of great importance for continuing efforts is the fact that DUMAND
performed a complete end-to-end system test under actual conditions
in situ. This test, including the cable, junction box, and string,
could not be done at ambient pressure and temperature any other way.
The DUMAND shore cable was designed by G. Wilkins of the University
of Hawaii, and comprised 12 single-mode optical fibers in a
stainless-steel tube, surrounded by a copper sheath capable of
transmitting 5 kW of electrical power[DIR-15-90][DIR-16-90]. The
overall resistance of the 32 km cable length laid was about 30
ohms. The cable was covered overall with two jackets of stainless
steel armor with opposite helicity, to make the cable as torque-free
as possible. The cable was manufactured in 8 km lenths which were
spliced together during the laying operation. Despite the splices,
the net attenuation of light along all fibers (except one which
failed for unknown reasons during laying operations) was
unmeasurably close to the diffraction limit [Rosen 94].
Following junction box and string deployment on 14 December 1993,
the R/V Thomas G.Thompson successfully laid the cable from the
DUMAND site to the shore station at the Natural Energy Laboratory of
Hawaii (NELH) at Keahole Point. The cable path very closely adhered
to the surveyed, planned route (Fig. 11). The cable end was brought
ashore by a small boat, and a diver inserted it into the prepared
conduit leading to the shore station. The conduit had been provided
without cost to DUMAND by a separate project, a DOE-supported
investigation of slant-drilling techniques for ocean thermal energy
conversion (OTEC) systems. NELH operates demonstration programs in
a variety of OTEC and related fields.
The data system had been designed to cope with the background rate
from radioactivity in the water (primarily from natural
potassium-40) and bioluminescence and still generate minimal
deadtime for recording neutrino events. The dark counting rate for
a single OM was found to be on the order of 60 kHz, primarily due to
trace K-40 in the huge volume of seawater each tube views. Noise due
to bioluminescence is episodic and expected to be unimportant after
the array has been stationary on the ocean bottom for some time,
since the light-emitting microscopic creatures are stimulated by
motion. K-40 and bioluminescence contribute mainly 1 photoelectron
hits distributed randomly in time over the entire
array. Bioluminescence caused spikes in the singles rate which reach
100 kHz for periods on the order of seconds, but their frequency of
occurrence is relatively low (Fig. 12).
After about 14 hr of operation, the string controller stopped
operating, due (as determined after recovery) to a leak in one
connector. Seawater in the string controller effectively short-
circuited the power line to the junction box, preventing operation
of JB equipment such as the EM and video cameras. The short was
removed when the string was subsequently released and recovered (the
remotely activated release cuts the string umbilical to the junction
box), but reappeared a short time later, evidently due to a similar,
slower leak in the JBEM.
In January, 1994, the disabled string deployed the previous month
was remotely released by an acoustical signal, recovered at sea, and
returned to Honolulu for diagnosis and repair. A local vessel, the
120' salvage ship Noho-loa, was chartered for the recovery. The
recovery was accomplished between 28-30 January 1994, about 44 days
after the string was deployed. After establishing position at the
site, the acoustic release was actuated, and the release operation
was confirmed by simultaneous observations of the fiber continuity
made using an optical time- domain reflectometer at the shore end of
the cable at Keahole point, since the release caused a break in the
fiber about 200 m from the end at the string controller. Release
operations were performed before dawn, so advantage could be taken
of the self-actuating light beacon on the string for location, with
daylight for recovery operations coming shortly thereafter. The
string rose to the surface faster than expected, and the VHF beacon
was heard within 1 hr after the release operation. The surface
strobe light was located about 3/4 hr after that and the ship and a
small boat from shore approached the string cautiously. After the
surface configuration of the string was noted, the recovery began,
and proceeded smoothly. The recovery was completed in about 6 hours,
with no damage to any of the instruments.
This rapid and economical recovery of a damaged string provided a
significant test of the serviceability of the DUMAND array for long
term deployment. The fact that such a complex instrument could be
recovered without further damage with minimal resources gives us
confidence that our original plan for possible replacement of one
string in the final array every two years is both viable and cost
effective.
In mid 1996, DOE determined that further support for DUMAND-II
should be terminated. At that time, the following DUMAND assets were
in place at the site or at UH:
The DUMAND Project represented an extraordinarty scientific
adventure for a number of people over a period of about twenty
years. The lure of attempting to carry out high technology in the
deep ocean using the technology of high energy physics, with the
tantalizing prospect of openning an entirely new window on the
universe, has drawn the energy of many talented people over the
years. While the DUMAND project has ceased to exist in original
organizational form, the scientific goals remain in view and being
actively sought by more than five groups around the world.
We who have participated in the DUMAND Project have had a marvelous
experience and we hope the project has left a legacy of scientific
and technical accomplishments which can be a base for the succeeding
experiments to realize high energy neutrino astronomy in the near future.
This paper was prepared with crucial assistance from many members of
the DUMAND Collaboration. Portions of the text were adapted from
various proposals and reports prepared over the years, and their
original authorship is now impossible to determine. All errors are
our responsibility.
We want to dedicate this final DUMAND summary paper to the
unflagging dedication of Howard Blood, Fred Reines, and Arthur
Roberts. Others desrving mention as crucial contributors in the
early stages are Howard Davis, Peter Kotzer, M. A. Markov, Saburo
Miyake, and George Zatsepin.
In addition to cited works, the following partial lists of DUMAND
publications and internal notes will demonstrate the breadth of
effort by the Collaboration. Only those notes cited in the text are
listed here; complete lists can be obtained from the DUMAND World
Wide Web page at Hawaii.
Internal notes were generally not restricted, but not intended for
use outside the collaboration, and thus often contain DUMAND jargon
without explanation. External notes were usually copies of
conference papers or reports intended for wider distribution. Note
that the numbering system for the external note archive was changed
in 1992 from $HDC-serial-year$ to $DUMAND-serial-year$. Victor
Stenger (U. Hawaii) was primarily responsible for the heroic job of
maintaining these archives.
Further information on DUMAND is available via the DUMAND Home Page
on the World Wide Web. The URL address is:
{\it http://web.phys.washington.edu/dumand}
Neutrino Oscillations
Detecting Tau Neutrinos
Detection of Relic Big Bang Neutrinos
Earth Tomography with Neutrinos
New Particle Searches
Atmospheric Neutrinos
Cosmic Ray Muons
EXPERIMENTAL DESIGN
Location Choice
Ocean, Lakes, Ice and Mines
Depth
Lattitude
Proximity to Shore
Bottom Character, Flatness
Infrastructure
Clarity: Absorption and Scattering
Ocean Array Design Considerations
Some Design Principles
Many design principles for large neutrino detectors to be installed
in the deep ocean were realized in workshops and over the years. We
summarize some of those below, as they do not appear elsewhere.Open Array for $km^3$
Bottom Tethered Strings
Reliability and Connectivity
Send the Data to Shore for Processing
Repair, Replace and Reconfigure
Use ROVs
Triggering
Array Shape and Spacing
Deployment Options
Critical Items for Future Development
Optical Sensor Studies
Acoustic Detection
DUMAND TECHNOLOGICAL ACCOMPLISHMENTS
Hawaii Site characterization
ROV operations
Optical Modules
Implosion tests
Optical feedthrough
Sea return system tests
ASIC digitizer chip (27 channel/ 1.25 nsec TDC)
Chirped sonar system
Video camera system
String mechanical design
Calibration Modules
Deployment and end-to-end test
Cable performance
Shore cable lay
String Operation
String Recovery
Final Status
Of course, the most valuable residual asset of DUMAND-II is the
experience of the physicists and skilled technical personnel who
constructed and deployed the first string in 1993.
Conclusion
Acknowledgements
Bibliography
DUMAND Workshop Proceedings
Vol. 1: Array Studies, 351 pp., edited by A. Koberts.
Vol. 2: UHE Interactions and Neutrino Astronomy, 213 pp., edited by
A. Roberts.
Vol. 3: Oceanographic and Ocean Engineering Studies, 213 pp., edited
by G. Wilkins.
External Reports
Internal Reports
Other References
``A search for very
high-energy neutrinos from active galactic nuclei.'' in press,
Astrophysical Journal (1998).
, {\bf 3}, 267 (1995).
, Phys. Rev. Lett., {\bf nnn},
nnn (1998).
Material Available on the Web
FIGURES
Figure 1: DUMAND-II Array
Figure 2: DUMAND-II site
Figure 3: DUMAND DAQ block diagram.
Figure 4: Water currents at site.
Figure 5: DUMAND Optical Module (JOM) in its hardhat and Ti frame.
Figure 6: JOM electronics block diagram.
Figure 7: DUMAND String 1 components and layout.
Figure 8: Benthos sphere implosion test results.
Figure 9: JB Environmental Monitor block diagram.
Figure 10: DUMAND deployment procedure.
Figure 11: DUMAND cable laying operation accuracy.
Figure 12: 40K background - measured OM singles rate distribution.