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phenomena. The instruments required for the study of
this previously inaccessible domain are now technically
feasible, and specific missions incorporating the required
capabilities have been proposed. Their implementation
would assure major advances in our understanding of the
solar atmosphere and would enhance the value of solar
physics in the study of magnetohydrodynamics. m e most
important of the new missions proposed for solar astronomy
is the Advanced Solar Observatory, which should be devel-
oped stepwise in a succession of Shuttle payloads that
will ultimately be combined in a long-lived free-flying
facility such as a Scientific Space Platform.
IX. NEUTRINO ASTRONOMY
Every star is a prolific source of low-energy (1- to
15-MeV) neutrinos that are produced as by-products of the
nuclear reactions in the star's core and pass freely
through its outer layers into space. Every stellar
collapse leading to the formation of a neutron star is a
source of a burst of intermediate energy (10- to 50-MeV)
neutrinos. Collisions between high-energy cosmic-ray
nuclei and matter produce high-energy (greater than
50-MeV) neutrinos. And finally, the theory of the big
bang implies that we are now immersed in a sea of relic
neutrinos with a kinetic temperature of 2 K and a density
of 500 cm~3. Thus cosmic neutrinos carry information
about a wide variety of high-energy processes. mis
information, however, is exceedingly difficult to tap for
the very reason that allows neutrinos to escape freely
from sources deep inside of stars--their interaction cross
section is extremely small at all energies. To obtain a
significant rate of detectable interactions a detector
must have a very great number of target nuclei in its
sensitive volume, which means that it
So far, the only positive detection
cosmic sources has been of low-energy neutrinos produced
in the nearest star, the Sun. Efforts are under way, or
under consideration, to detect cosmic neutrinos in the
intermediate- and high-energy regimes. At present there
seems to be no hope of detecting the relic neutrinos
directly even though, according to recent theoretical
speculation, they may be the dominant form of mass-energy
in the Universe and may comprise the so-called missing
mass whose presence is deduced from dynamical analyses of
the motions in galaxies and clusters of galaxies.
must be very heavy.
Of neutrinos from
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Progress and prospects for neutrino astronomy in each
of the three regimes in which direct detection is or may
be possible will be discussed separately.
A. Low-Energy Neutrinos
1. Introduction
The theory of nuclear reactions as they are believed to
occur in standard main-sequence stars has been used to
predict the flux at Earth of electron neutrinos produced
in the Sun by the processes that result in the fusion of
hydrogen into helium. The Sun, being by far the closest
star, is by far the brightest source of stellar neutrinos,
just as it is by far the brightest source of stellar
light. Thus, a crucial test of the theory of stellar
energy is provided by a comparison between the predicted
and the observed fluxes of solar neutrinos.
2. Progress during the 1970's
The history of solar neutrino astronomy is centered on a
remarkable radiochemical experiment that employs a
detector containing 105 gallons of chlorinated hydro-
carbon and operating 1 mile underground in the Homestak
gold mine in South Dakota. Neutrinos with energies of
several MeV interact with 37C1 to produce radioactive
~ 7 _ . _
e
O'er, which undergoes beta decay with a half-life of 34
days. In the experiment, argon gas is extracted from the
detector tank every few days and transferred to a tiny
low-background proportional counter in which the decays
of 37Ar are counted. The measured rate of decay is only
about 1.5 per day. Data accumulated from 1970 through
1979 imply that the total flux of solar neutrinos is 2.2
+ 0.4 SNUS (1 solar neutrino unit = 10 36 events per
37C1 atoms per see). The current standard theoretical
model for the Sun predicts a counting rate of 7.5 SNUS,
with a considerable uncertainty due to uncertainties in
the nuclear cross sections, the optical opacities, and
the conditions in the Sun's core. Nevertheless, to bring
the predicted value below 3 SNUS may require nonstandard
assumptions such as an inhomgeneous composition of the
Sun, repeated deep mixing, a black hole at the center of
the Sun, or perhaps even neutrino decay as implied by
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recent developments in the Grand Unified meory of the
weak, electromagnetic, and strong interactions.
3. Scientific Goals: Present and Future Programs
The 37C1 experiment is sensitive primarily to the high-
energy "fringe" neutrinos from a relatively unimportant
side reaction not to the "pp" neutrinos from the reaction
that is believed to yield most of the solar power. Since
the flux of fringe neutrinos is extremely sensitive to
conditions in the solar core, the 37C1 experiment can
never be a conclusive test of the theory of solar power.
Therefore it is important to carry out an experiment that
tests the predictions for the main-line pp flux. Measure-
ment of the energy spectrum of the solar neutrinos will
then become the next major objective.
m e most promising approach to the measurement of the
pp neutrinos is the so-called ~gallium" experiment, which,
like the 37C1 experiment, is a radiochemical experiment
with 71Ga being the target nucleus and 71Ge being the
separable radioactive product nucleus. A prototype detec
tor module has been developed at the Brookhaven National
Laboratory with 1.4 tons of gallium in an international
collaboration involving the Max Planck Institute in
Germany, the Weizmann Institute in Israel, the Institute
for Advanced Study at Princeton, and the University of
Pennsylvania. m e full-scale experiment will require 50
tons of gallium in 35 detectors like the present proto-
type. We recommend that it be initiated as soon as
possible.
Looking further ahead, several other radiochemical
experiments have been proposed. For example, one uti-
lizing 1l5In measures the pp neutrinos and could provide
significant energy resolution. Experimental investiga-
tions to determine the feasibility of these alternative
approaches should be encouraged.
Certain difficult laboratory measurements in nuclear
physics are of crucial importance to the calculation of
accurate theoretical estimates of solar neutrino fluxes.
This is particularly well demonstrated by the impact on
these estimates of recent results on the 3He + 4He reac-
tion derived from ultra-low-energy experiments. These
difficult measurements should be continued with adequate
support.
-
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4. Research in Other Countries
The 71Ga experiment currently involves an international
collaboration with major contributions from West Germany.
Soviet scientists are currently planning a 37C1 detector
that is four times larger than the Homestake installation,
and they are also contemplating a 71Ga experiment. The
feasibility of the 1l5In experiment is also being studied
in Japan.
B. Intermediate-Energy Neutrinos
1. Introduction
The most easily detectable contributions to the neutrino
flux in the intermediate-energy range from 10 to 50 MeV
are expected to come from gravitational collapses of stars
in our Galaxy, which occur with an estimated frequency of
one per 10 to 30 years. Although many details of collapse
processes are uncertain, there is general agreement on
several theoretical conclusions. One is that in the
formation of a neutron star the bulk of the approximately
1053 ergs of binding energy is radiated away as neu-
trinos. At least 1052 ergs of energy is radiated in
electron neutrinos from the process of neutronization.
The rest is radiated by thermal processes. The time scale
for emission of the neutrinos is dependent on the equation
of state of the collapsing matter because neutrinos coming
from regions with densities greater than 1011 g/cm3 (the
neutrino "photosphere") must diffuse outward by multiple
scattering. Measurement of the n light curve" of the
neutrinos from a collapse may be the only possible way to
obtain direct observational information about the equation
of state of hot, dense collapsing matter.
2. Inventory of Present Resources
Work in the intermediate-energy regime is proceeding
abroad in coincidence with U.S. efforts. Detectors are
currently in operation in the Homestake gold mine, in
Baksan in the Soviet Union, and in the Ukraine. All are
Cerenkov detectors. The Homestake and Baksan detectors
have sensitive masses of the order of 300 tons each and
should be able to detect a collapse event anywhere in our
Galaxy. However, a positive result would probably be
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believable only if both detectors are in operation and
both record the same event in coincidence. Additional
coincidences with signals from smaller detectors would be
possible if a collapse event were closer than the Galactic
center. Experiments aimed at the detection of proton
decays and now under construction will be sensitive to
the higher energy neutrinos from collapse events in our
Galaxy.
In principle these neutrino detectors should provide
an early warning system for any collapse event in our
Galaxy since the neutrino burst will precede the optical
light peak and the x- and/or gamma-ray bursts from the
supernova by about 1 week. However, lack of accurate
directional information will limit the usefulness of the
early warning in preparing for optical and high-energy
photon observations. Unfortunately, the detection of the
relatively frequent supernovae in galaxies of the Virgo
cluster is not within the range of practicality at
present.
3. Scientific Goals and Future Programs
Existing facilities capable of detecting neutrinos from
stellar collapse events in our Galaxy should be adequately
maintained and operated for many years. Meanwhile, theo-
retical investigations that provide links between astro-
physics and particle physics should be encouraged.
Theoretical research is needed on the equation of state
of matter at ultra-high densities, on supernova mechan-
isms, on nucleosynthesis, and on the spectra of neutrinos
produced in various astrophysical processes. Related
work is required on the theory of neutrino cooling in
newly formed neutron stars, an area closely related to
x-ray observations of the stellar remnants of supernovae.
C. High-Energy Neutrinos
1. Introduction
Collisions of high-energy nuclei with matter produce
mesons of which the ultimate decay products are elec-
trons, gamma-ray photons, and neutrinos. Since high-
energy neutrinos are vastly more difficult to detect than
high-energy gamma-ray photons, a source of high-energy
neutrinos that would be detectable by currently proposed
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methods would almost certainly already have been observed
as a source of high-energy gamma rays that is much more
intense than any known gamma-ray source. An exception
would occur if an extremely powerful source of high-energy
particles were shielded from Earth by matter or radiation
close to the source and so thick that the flux of gamma
rays produced in interactions is attenuated to a value
below the level of detectability. It has been suggested
that such a situation may occur in the vicinity of a
pulsar, where magnetically trapped high energy particles
strike the surface of the star, or near the core of an
active galactic nucleus. It should be noted that a dis-
crete source of high-energy gamma rays is not necessarily
a neutrino source because high-energy gamma rays can be
produced without neutrinos by interactions of high-energy
electrons with matter, photons, or magnetic fields. Thus
the measurement of the value or upper limit on the neu-
trino flux from a bright source of high-energy gamma rays
could, in principle, constrain theoretical models of a
gamma-ray source and decide the question as to whether
the mechanism of gamma-ray production is nuclear col-
lisions or electron interactions.
2. Present and Future Programs
All experiments aimed at the detection of high-energy cos-
mic neutrinos have so far detected only events attribut-
able to the background flux of neutrinos produced by
collisions of cosmic rays in the atmosphere. Upper limits
on the diffuse cosmic neutrino flux have been obtained
with detectors located deep underground in gold mines in
India, South Africa, and the United States. These limits
place significant constraints on the amount of deuterium
that was produced after the initial element synthesis in
the big bang. In the near future a large Cerenkov detec-
tor will be completed for the purpose of detecting proton
decays. It will also be sensitive to high-energy neu-
trinos, and a by-product of its operation will be improved
upper limits on the cosmic neutrino flux. None of these
detectors is sensitive enough to detect any cosmic source
above the background of atmospheric neutrinos except a
discrete source or diffuse background of totally
unexpected brightness.
Theoretical and experimental studies by the DUMAND
(Deep Underwater Muon and Neutrino Detection) group have
been carried out to determine the technical feasibility
Representative terms from entire chapter:
neutrino flux
