astro-ph/9503047 10 Mar 95 tesy of the National Academy Press Washington D C physics A Research Brie ng Progress y x yy xx z Why discipline over the last Neutrino astrophysics has developed from theoretical calculations to an exp erimental solar of Department of Physics University of Pennsylvania Philadelphi a Institute for Advanced Department of Physics Brown University Providence RI Department of Physics Carnegie Mellon Department of Physics Astronomy University of Hawaii Honolulu HI Physics Division Los Alamos National Lab oratory Los Alamos NM and Accepted Department of Physics University of Washington Seattle WA neutrinos John neutrinos neutrino N Bahcall neutrino relative astronomical systems may form the basis trino de cits and to allow the detectionAdditional of exp eriments more are distant needed sources to test explanations of the solar neu for was The publication and have to neutrino and R G Hamish Rob ertson detected x the Kenneth Lande astrophysics b een solar Study Princeton NJ Prosp ects three decades theoretical de cits in physics from observed Copyright by the National Academy of Sciences Nature a observed predictions sup ernova Neutrino y in Rob ert As Adapted University Pittsburgh PA four in we shall in Lincoln Wolfenstein four emission Neutrino underground in have E Lanou with for a new the solar see in later led p ermission Large from neutrino to Jr eld fresh distant exp eriments z Magellanic John Astrophysics PA sections of of astronomy exp eriments insights from yy energetic G Learned Neutrino into Cloud and this a pap er Astro Cour xx burst The
observed solar neutrino uxes all agree with the theoretical exp ectations to within
factors of two or three successfully concluding a half century struggle to understand
how the sun shines Nevertheless p ersistent de cits of electron type neutrinos exist
in all four solar neutrino exp eriments which has led to suggestions that neutrinos
may have small masses and may oscillate b etween di erent types New neutrino
exp eriments are required to resolve the long standing solar neutrino problem We
shall also describ e e orts that are underway to develop exp eriments that can detect
astronomical neutrinos at high energies from sources that are well b eyond the Galaxy
First however we shall sketch brie y the development and overall state of the eld
of neutrino astrophysics
In the s and s scientists developed the theoretical basis for understand
ing how the sun shines They prop osed that the nuclear fusion reactions among
light elements o ccur near the the center of the sun and provide the energy emitted
by the sun for four and a half billion years While most of this energy ends up as
electromagnetic radiation from the surface approximately three p ercent is b elieved
to b e emitted directly from the center of the sun in the form of neutrinos In a
pioneering exp eriment that has b een going on for years Raymond Davis Jr and
his collab orators rst detected solar neutrinos using radio chemical techniques and a
cleaning uid p erchloroethylene as a target
Over the last several years there has b een striking con rmation of the astronomical
predictions of solar neutrinos The Kamiokande water Cerenkov detector lo cated in
the Japanese alps has identi ed solar neutrinos by their directionality measured by
observing the direction of recoil electrons from neutrino interactions Recently two
radio chemical exp eriments using gallium the GALLEX exp eriment in Italy and
the SAGE exp eriment in Russia have detected the copious low energy b elow
keV neutrinos that are the primary comp onent of the solar neutrino ux
A sup ernova represents one of the most awesome events in astronomy Type II
sup ernovae are b elieved to result from the sudden gravitational collapse of a star
that has completed cycles of nuclear burning The energy calculated to b e pro duced
from this collapse is almost one thousand times larger than that observed as light
Theory indicates that more than p ercent of the energy is emitted in the form of
neutrinos In February neutrinos from SN a were detected as a series of
pulses in two water Cerenkov detectors Kamiokande in Japan and IMB in the
United States the neutrinos arriving as exp ected a few hours b efore the light from
the explosion
The de nitive detection of solar neutrinos and the detection of neutrinos from
SN a are two of the most remarkable scienti c discoveries of the last decade They
provide dramatic con rmation of fundamental theories concerning stellar interiors
The detection of solar neutrinos demonstrates that fusion energy is the basic source
of energy received from the sun The observations of solar and sup ernova neutrinos
op en up a new area of science neutrino astrophysics
The observation of astrophysical neutrinos makes it p ossible to prob e the inner
most regions of stars dense regions from which light cannot escap e Because of their
very small interaction probabilities neutrinos can escap e from these inner regions
for the same reason of course it is di cult to detect these elusive particles
The motivation for prop osing the rst practical solar neutrino exp eriment was
astrophysical to test directly the hypothesis that stars shine and evolve b ecause of
nuclear fusion reactions in their interiors However it has b ecome apparent in recent
years that solar neutrinos provide a b eam of elementary particles that can b e used to
investigate fundamental physics in particular to study intrinsic neutrino prop erties
Of particular imp ortance is the question of whether neutrinos have mass and
whether neutrinos transform oscillate from one type to another There are
b elieved to b e three types of neutrinos electron type muon type tau type
e
Only are created in the sun but all types are b elieved to b e emitted from
e
sup ernovae The question of neutrino mass is of fundamental imp ortance for particle
physics and for cosmology In the case of particle physics neutrino mass may provide
a clue to the problem of the origin of the masses of all particles An interesting
class of theories called grand uni ed theories suggests that all interactions strong
electromagnetic and weak are uni ed at a large energy scale and that the neutrino
masses are inversely prop ortional to this scale Two related ways to prob e this very
high energy scale are the search for proton decay and the search for very small neutrino
masses
Our basic ideas ab out cosmology that explain the microwave background radiation
predict a similar background of relic neutrinos If the heaviest of these neutrinos
usually taken to b e has a mass ab ove a few electron volts these relic neutrinos
could b e an imp ortant comp onent of dark matter In this case with a much smaller
mass could play a decisive role for solar neutrinos
The only available way to study very small neutrino masses is through the p ossi
bility of oscillations that transform one type of neutrino into another For neutrino
masses greater than eV such oscillations have b een lo oked for in exp eriments
not discussed in this pap er using neutrinos pro duced in the earth s atmosphere by
cosmic rays or neutrinos pro duced in the lab oratory by reactors or accelerators For
smaller neutrino masses or with mass in the range to the only
available p ossibility is to use solar neutrinos with oscillations depleting the ux
of by converting some of them into or Of particular interest is the p ossibility
e
that the oscillations may b e enhanced by the coherent interaction of the neutrinos
with the solar material This coherent interaction is called the MSW e ect
The observations made with the four exp eriments p erformed so far indicate uxes
of electron neutrinos that are consistently b elow the results of astrophysical calcula
tions In fact comparisons b etween the rates of di erent exp eriments suggest inde
p endent of astrophysical uncertainties that some previously unaccounted for physical
pro cess p erhaps involving non zero neutrino mass may b e o ccurring The exp eri
ments carried out so far are the rst pioneering exp eriments and nal conclusions
require additional exp eriments that are currently b eing developed
In this rep ort we also consider exp eriments designed to observe neutrinos of much
higher energies six to nine orders of magnitude larger than those of solar neutrinos
The p otential sources of higher energy neutrinos the techniques for their detection
and the scienti c justi cations for the research are all much di erent from the solar
case Unlike the sun p ossible astrophysical sources of high energy neutrinos are
remote and not well understo o d estimates of their neutrino uxes are necessarily
sp eculative Detectors of high energy neutrinos are complementary to detectors of
solar neutrinos b eing optimized for large areas rather than for low thresholds
Candidates for sources of high energy neutrinos include such exotic but relatively
nearby ob jects as neutron stars and black holes in the Galaxy as well as the nuclei
of distant but highly luminous galaxies Since we observe cosmic rays to o energetic
to b e con ned within the Galaxy we know that there exist enormously energetic
pro cesses that might well b e sources of high energy neutrinos Will any of these
sources pro duce detectable numbers of high energy neutrinos We will never know
until we lo ok
Ongoing solar neutrino exp eriments
There is no single b est type of detector that can b e used to observe solar neutrinos
The energies of the neutrinos vary over a range of one and half orders of magnitude
and the uxes from the most imp ortant neutrino sources vary by four orders of mag
nitude According to standard solar mo del calculations the numerous neutrinos from
the initiating proton proton reaction with energies less than MeV have a ux of
ab out cm s the rare higher energy neutrinos from B decay with energies
up to MeV have a ux of ab out cm s Eleven recently published solar
mo dels each constructed using a di erent computer co de and di erent input physics
give uxes for those neutrinos less energetic than MeV which are the same to b etter
than
FIG Solar Neutrino Sp ectrum This gure shows the energy sp ectrum of neutrinos
predicted by the standard solar mo del The neutrino uxes from continuum sources like
pp and B are given in the units of number p er cm p er second p er MeV at one astronomical
unit The line uxes p ep and Be are given in number p er cm p er second The sp ectra
from the pp chain are drawn with solid lines the CNO sp ectra are drawn with dotted lines
The arrows at the top of the gure indicate the energy thresholds for the ongoing neutrino
exp eriments
Figure shows the theoretically calculated solar neutrino sp ectrum from di erent
neutrino sources Because of the wide range in energies and in uxes a set of detectors
with su cient complementarity and some redundancy is required to untangle the
di erent asp ects of neutrino physics and of solar physics
The chlorine exp eriment In this remarkable exp eriment neutrinos are detected
using a large tank containing tons of p erchloroethylene C Cl which is a com
mercially available cleaning uid The target isotop e Cl can capture a neutrino
pro ducing a radioactive isotop e Ar by the following pro cess
Cl ! Ar e
e
This reaction can o ccur for electron neutrinos with energy greater than MeV
The neutrino reactions are registered by counting the radioactive nuclei Ar In
order to avoid extraneous background interactions caused by cosmic rays incident
on the surface of the earth the exp eriment is conducted meters underground in
the Homestake Gold Mine in Lead South Dakota After ab out two months in the
solar neutrino sunshine the buildup of Ar atoms from neutrino capture by Cl
is approximately balanced by the loss due to decay At this p oint the standard solar
mo del predicts that only ab out Ar atoms are present in the tons of C Cl
Over the past two decades more than extractions of Ar have b een p erformed
After correction for detection e ciency the average number of Ar atoms in the
detector at extraction is observed to b e only corresp onding to a solar neutrino
induced pro duction rate of atoms p er day far fewer than the atoms p er day
exp ected on the basis of the standard mo del In terms of the solar neutrino unit SNU
pronounced snew SNU interactions p er target atom p er second the
observations yield SNU ab out one third of the prediction of the standard
mo del For two decades this discrepancy b etween the measured chlorine event rate
and the event rate predicted by the standard mo del constituted the well known Solar
Neutrino Problem
The Kamiokande exp eriment Like a sup ersonic aircraft creating a sonic b o om a
charged particle moving through matter faster than the sp eed of light in the matter
gives o a conical sho ck wave of light called Cerenkov radiation Sensitive photomul
tiplier tub es can record the ashes of light that b etray the presence and direction
of these high sp eed particles With the aid of sp ecial large photomultiplier tub es
exp erimenters working in a mine in the Japanese alps and using a ton detector
of ultra pure water ton ducial volume called Kamiokande were able to detect
electrons that had b een struck by neutrinos from the sun The reaction that was
observed is
e ! e
At the energies of interest for solar neutrino detection reaction is ab out six
times more likely for than for or Despite a signi cant background from
e
radioactivity of the ro cks in the mine from radioactivity in the water and from
cosmic ray induced events the neutrinos were unambiguously detected b ecause the
electrons struck by neutrinos recoiled preferentially in the direction from the sun
to the earth Kamiokande the rst active electronic rather than radio chemical
solar neutrino exp eriment showed two very imp ortant things rst the detected
neutrinos do indeed come from the sun and second the observed ux ab ove an
electron detection threshold of ab out MeV is ab out half as large as predicted by
the standard solar mo del The discrepancy b etween prediction and observation is
less severe at energies ab ove MeV the Kamiokande exp eriment than it is for all
energies ab ove MeV the chlorine exp eriment
The predicted counting rate for any particular solar neutrino exp eriment dep ends
up on calculations that are usually based up on a standard solar mo del Solar mo dels
are constrained by measurements of the solar luminosity age chemical comp osition
and thousands of seismological frequencies Nevertheless the predicted neutrino emis
sion rates cannot b e tested indep endent of solar neutrino exp eriments Changes in
the input data for the standard solar mo del could in principle b e made just so as to
t any one of the existing exp eriments However it app ears essentially imp ossible
to explain simultaneously the results of the chlorine and the Kamiokande exp eri
ments by adjusting input solar data Just the B neutrinos that are observed in
the Kamiokande exp eriment corresp ond to an exp ected counting rate in the chlorine
exp eriment of SNU compared with the observed rate of
SNU However in the chlorine exp eriment the solar mo del predicts an additional
SNU from the much more reliably calculated lower energy neutrinos Be and pep
neutrinos see Figure This suggests that the lower energy neutrinos might b e
preferentially depleted by some new weak interaction pro cess which could b e the
MSW e ect discussed b elow
The gallium exp eriments The neutrinos pro duced by the most basic fusion re
action in the sun the fusion of two protons the so called p p reaction are to o
low in energy to b e detected by either the chlorine or the Kamiokande exp eriment
Since the p p neutrinos are predicted to b e the most numerous neutrinos emitted
by the sun and since their ux is more reliably predicted than any other neutrino
ux the measurement of the terrestrial ux of p p neutrinos has long b een seen
as an essential goal of solar neutrino astronomy The p ossibility of using gallium as
a detector for these neutrinos was suggested almost thirty years ago and a design
for the exp eriment was developed Only in the last six years has it proved p ossible
to carry out this imp ortant exp eriment
The detector reaction is
Ga ! Ge e
e
which has an energy threshold of MeV The gallium exp eriments op erate similarly
to the chlorine exp eriment extracting and counting radioactive atoms of Ge The
standard mo del prediction is that after a month of op eration ab out atoms of Ge
will b e present in tons of gallium
One of the gallium exp eriments GALLEX is a Europ ean American Israeli col
lab oration op erating in the Gran Sasso tunnel near Rome Italy GALLEX uses
tons of gallium in a GaCl HCl solution The other exp eriment SAGE the
Soviet American Gallium Exp eriment is op erating in a subterranean lab oratory un
der Mount Andyrchi of the Caucasus Mountains in the southern region of Russia
SAGE currently uses tons of molten gallium metal
The fact that the detectors use gallium in two very di erent chemical environ
ments the GaCl HCl solution GALLEX and the metallic form SAGE provides
a consistency check on the results The current Gallex results are SNU and
the current SAGE results are SNU These results also are signi cantly b elow
the standard mo del prediction which is SNU for gallium detectors The
GALLEX detector resp onse to neutrinos has b een exp erimentally checked using a
source Cr of Megacurie that emits neutrinos of keV and keV from
electron capture A similar check is planned for the SAGE detector
Summary Thus as time has passed and more exp eriments have b een p erformed the
solar neutrino problem has not gone away Indeed it app ears that indep endent of
detailed solar mo del calculations there is no combination of the exp ected neutrino
uxes that ts all the available data
On the other hand the results of all four of the exp eriments chlorine
Kamiokande GALLEX and SAGE can b e explained if either or has a
mass of ab out eV and has a small amount of mixing with a lighter In this
e
case the pro duced at the center of the sun are transformed with a transformation
e
probability that dep ends up on the energy of into or as they pass through
e
the material medium of the sun the MSW e ect Only future exp eriments can
determine if this is indeed the correct explanation
Solar neutrino exp eriments under construction
Solar neutrino detectors fall into two classes active like Kamiokande and ra
dio chemical like chlorine and gallium Active detectors provide information ab out
the arrival time direction and energy of individual neutrinos and in some cases the
neutrino type e g electron type or muon type but are complex and often faced
with p ersistent backgrounds Radio chemical detectors are sensitive only to electron
type neutrinos i e they sp ecify uniquely the neutrino type and provide a single
number for the rate of detection of all neutrinos ab ove an energy threshold a number
that is averaged over a p erio d of time comparable to the mean life typically weeks
or months of the radioisotop e that is pro duced
In this section we describ e the three active and one radio chemical solar neutrino
detectors that are under construction All of these detectors will have much higher
event rates for the active detectors several thousand events p er year The exp ected
high event rates will make it p ossible to search for time dep endence in the neutrino
ux in particular the exp ected seasonal variation due to the orbital eccentricity of
the earth e ect p eak to p eak
The Sudbury Neutrino Observatory SNO Deuterium heavy hydrogen H
with a neutron and a proton in its nucleus is an excellent target for neutrinos Two
neutrino interactions can o ccur
H ! p p e a
e
H ! p n b
x x
Reaction a can only b e induced by electron type neutrinos whereas the cross
section for reaction b is the same neutrinos of all three types If more neutrinos
are detected via reaction b than by reaction a that would b e direct evidence
that some electron type neutrinos have oscillated into neutrinos of some other type
The SNO exp eriment is lo cated near Sudbury Ontario in one of the deep est mines
in the Western hemisphere a nickel mine b elonging to the INCO company A large
cavern has b een excavated m underground to hold a detector consisting of
tonnes of heavy water When SNO b egins op eration in solar neutrinos are
exp ected to b e detected at the rate of more than counts p er day via reactions a
and b In addition to the unique reactions of neutrinos on deuterium SNO will
observe the same neutrino electron scattering pro cess that is detected by Kamiokande
If electron type neutrinos oscillate into one of the other known neutrino types as
they travel from the interior of the sun to the terrestrial detector this will b e revealed
by the comparison of the rates in the two deuterium reactions and if the oscillation
parameters are favorable also by its distinctive e ect on the shap e of the observed
electron energy sp ectrum in reaction a These p otential signatures of new physics
are indep endent of solar mo dels and solar physics
Sup erkamiokande The Sup erkamiokande exp eriment a much improved version
of the current Kamiokande exp eriment utilizes tonnes of ultra pure ordinary
water This immense detector will b eginning in provide a thirty fold increase
in the observed rate of neutrino electron scattering events The change in the shap e
of the neutrino energy sp ectrum predicted by the MSW e ect may b e observable in
this exp eriment via the sp ectrum of recoil energies of the scattered electrons
Both Sup erkamiokande and SNO detect only neutrinos ab ove MeV
BOREXINO BOREXINO will b e the rst active detector with the goal of ob
serving the intense ux of low energy neutrinos pro duced by electron capture on
b eryllium nuclei see the MeV Be neutrino line in Figure The BOREXINO
detector is to b e constructed with standard liquid scintillator materials except that
the requirement for radiopurity uranium and thorium less than g g is un
precedently stringent If the MSW interpretation is correct then the observed ux of
Be neutrinos will b e greatly reduced relative to the predictions of the standard solar
mo del
IODINE An io dine radio chemical detector all I will b e somewhat similar to the
existing chlorine detector but p er target atom is exp ected to have a higher counting
rate A mo dular ton io dine detector is now under construction in the Homestake
Mine near the present chlorine detector and will b egin op erating in mid
Calibration exp eriments will b e p erformed to measure the probability for neutrino
induced conversion of I to Xe These exp eriments are necessary to determine
the relative sensitivity of the detector to Be and B neutrinos and to interpret the
total rate Expansion of the present io dine detector may b e prop osed after op erating
exp erience is obtained with the current detector and after the neutrino cross sections
are measured
Developing new solar neutrino detectors Additional solar neutrino detectors
are required to obtain more complete information on the di erent p ortions of the
solar neutrino sp ectrum
The ton ICARUS detector will b e installed in the Gran Sasso lab oratory
and is planned to b e op erational in Like the chlorine detector ICARUS is
based on an inverse b eta pro cess Ar ! e K but op erates in real time
e
and is sensitive only to B neutrinos The detector uses liquid argon with the goal
of measuring the recoil electrons and the decay gamma rays from the K excited
state that are pro duced by neutrino capture
A ma jor future goal is to measure the energy and arrival time of the lowest en
ergy neutrinos the pp neutrinos which are detected by the radio chemical metho d in
the gallium exp eriments The prop osed HERON detector is based on the ballistic
propagation of rotons sound excitations in liquid helium in the sup er uid state with
detection of the heat pulses by an array of b olometers The goal is to achieve a thresh
old of the order of keV suitable for measuring the recoil electron sp ectrum and
rate of neutrino electron scattering from b oth pp and B e neutrinos The prop osed
HELLAZ detector also would detect neutrino electron scattering in helium but in
high pressure low temperature gas
High energy neutrinos
Higher energy neutrinos that are most accessible exp erimentally have energies ab ove
a T eV eV MeV Astrophysical neutrinos with these energies can b e
pro duced by high energy collisions among nuclei or b etween protons and photons that
pro duce secondary particles pions and muons which have neutrinos among their
decay pro ducts Such collisions are b elieved to take place when matter falls into a
neutron star from a companion star in a double star system Also such collisions
are exp ected on a large scale at the centers of active galactic nuclei AGN However
the ux of high energy neutrinos from such sources cannot b e calculated with
accuracy It is only by searching for these neutrinos that we can prob e the pro cesses
taking place deep in the interior of these and other astrophysical systems
High energy neutrinos can b e detected via the reaction of a muon neutrino with
a nuclear particle leading to the neutrino b eing transformed into a charged muon
A multi T eV charged muon will travel many kilometers in earth or in water The
neutrino target volume is thus the area of the muon detector times the range of the
muon Target volumes of billions of tons may b e achieved this way The fast moving
muons can b e detected by observing the blue ash of Cerenkov light see discussion
of Kamiokande exp eriment they pro duce when traversing a transparent medium
sp eci cally water or ice Detectors with e ective areas of a few hundred square
meters have b een built in mines four new instruments are b eing built with areas in
the m range and plans are b eing discussed for one or more instruments in
the km class
The Kamiokande and the IMB detectors have observed neutrinos of to eV
pro duced by cosmic rays in the atmosphere but no neutrinos from sources outside the
solar system have b een observed with the exception of the observations of SN A
Estimates of the p otential uxes of high energy neutrinos from sources such as
active galactic nuclei suggest that a surface area greater than km is necessary for
meaningful observations Furthermore since the exp ected signal to noise improves
greatly with the increasing threshold up to TeV energies detectors of great thickness
are desirable It is exp ected generally agreed these requirements can only b e met by
the use of large b o dies of water such as lakes the o cean or p olar ice
The rst instruments designed sp eci cally for high energy neutrino detection are
now under construction in Sib eria in Lake Baikal o Hawaii in the deep o cean
DUMAND at the South Pole in the deep p olar ice AMANDA and in the
Mediterranean near Pylos Greece NESTOR All four of the instruments b eing
built aim at detection areas of the order of m and an angular resolution of
o
order for TeV muons
It is exp ected that high energy neutrino astronomy will require a telescop e with an
e ective muon detecting area of at least km by km Two of the principal science
goals for a kilometer scale neutrino telescop e are the search for neutrinos from the
annihilation in the sun of weakly interacting dark matter particles with masses in the
GeV TeV range and the observation of individual active galactic nuclei with go o d
statistics Studies of neutrino oscillations may also b e p ossible The four instruments
presently under construction represent an increase of ab out a factor of in area from
previous mine based detectors another increase of is needed to reach the desired
scale
What next
Solar neutrino exp eriments carried out over the past thirty years have closed an im
p ortant chapter in the history of science that b egan in the early part of this century
We now have conclusive direct evidence for nuclear fusion reactions that o ccur among
light elements in the center of the sun At the same time the sun provides a unique
source of neutrinos for studying the p ossibility of very small neutrino masses
Solar neutrino exp eriments therefore tell us b oth ab out elementary particle
physics and ab out the details of nuclear fusion within the innermost region of the
nearest star
Exp eriments that are currently underway have the p otential to establish
indep endent of theoretical calculations carried out with solar mo dels whether new
physics is required to explain the solar neutrino observations If the number of neu
trinos that are observed by the Sudbury Neutrino Observatory SNO in reaction b
which can b e induced by any of the known types of neutrinos exceeds the number
observed in reaction a which can only b e initiated by electron type neutrinos this
will b e direct evidence for neutrino oscillations The measurements of a neutrino en
ergy sp ectrum by the SNO and the Sup erkamiokande exp eriments will test whether
the sp ectrum of the higher energy B solar neutrinos that arrive at earth from the
sun is the same as the energy sp ectrum of neutrinos from the same radioactive isotop e
observed in the lab oratory or has b een distorted by neutrino oscillations However
it is p ossible that even if oscillations take place their e ect on the B neutrinos
the only ones detected by SNO and Sup erkamiokande may b e to o small to b e de
tected Fortunately theoretical calculations of the MSW e ect imply that the de cit
of electron type neutrinos is much larger for the lower energy Be neutrinos if this is
correct the ux detected in BOREXINO will b e much smaller than predicted by the
standard mo del
Solar neutrino exp eriments are fundamental b oth for physics and for astronomy
It is essential that we not rely on just a few exp eriments since the history of science
has shown that systematic uncertainties can sometimes lead to mistaken conclusions
unless results are checked by p erforming measurements in di erent ways This is
particularly imp ortant when the measurements are as intrinsically di cult as they
app ear to b e for solar neutrino exp eriments
The timing of SN a was extremely fortunate since the water Cerenkov detec
tors had only b een op erating for a few years An imp ortant lesson of that sup ernova
neutrino detection is that neutrinos could reveal sup ernovae in our galaxy even if the
light is obscured by the galaxy itself It would b e very desirable that a number of
detectors are prepared to detect neutrinos from the next sup ernova in our galaxy
The last fty years have seen the extension of astronomy from the optical sp ectrum
to the whole range of the electromagnetic sp ectrum from radio waves to gamma rays
In that pro cess many new asp ects of the astronomical world have b een uncovered
Neutrino astronomy can provide a unique window by means of which one can see into
the deep est interiors of stars and galaxies So far the only sources detected are the
sun and sup ernova a Neutrino astronomy is an exciting frontier for exploration
in the twenty rst century
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Acknowledgements
We are grateful to many of our colleagues who have acted as informal referees of
individual sections of this rep ort and who have worked hard to help us make the
material as accurate as we could We are esp ecially grateful to Dr R Riemer
Asso ciate Director Board on Physics and Astronomy National Research Council
and to Dr D Schramm Chair Board on Physics and Astronomy NRC for wise
counsel and for their leadership in guiding this review