Nuclear Physics in the 1990's
NUCLEONS, QUARKS, AND GLUONS
The Continuous Electron Beam Accelerator Facility
| The Continuous Electron
Beam Accelerator Facility (CEBAF) scheduled to begin operations
in 1995, will provide us with an electron "microscope"
for studying the nucleus with unprecedented resolving
power. By probing the nucleus at very short distance scales,
physicists hope to study the transition region in which
the conventional model, where a nucleus is made up of
interacting nucleons (protons and neutrons), gives way
to a more fundamental model, where those nucleons are
made up of quarks and gluons.
CEBAF focuses a million billion electrons
per second into a beam the width of a hair. It accelerates
this beam nearly to the speed of light, raising each electron's
energy to four billion electron volts. The electrons bombard
a target, which consists of a thin sheet of material.
When an electron collides with a nucleus,
detectors record the di"RECT"ion of the scattered electron
and the pattern of nuclear fragments produced by the collision.
From the results, physicists hope to determine the number
of heavy "strange" quarks within the nucleus and the distribution
of charge within the neutron.
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 Vertical Test of
Superconducting Cavities
CEBAF
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The electrons in CEBAF's beam accelerate by
"surfing" on electromagnetic waves, which are confined within
hollow metal cavities. CEBAF is pioneering the large-scale application
of superconducting cavities, cooled to within two degrees of absolute
zero. Superconducting cavities can operate continuously with very
low electrical losses.
CEBAF's continuous beam is essential in experiments
where it is necessary to measure the scattered electron and the
nuclear collision fragments simultaneously. Many important measurements
of nucleon and nuclear structure require both high energy and
a continuous beam, and thus can only be done at CEBAF.
The Relativistic Heavy Ion Collider
Under normal conditions, quarks and gluons
are confined within protons and neutrons; it is impossible
to find a "free and unattached" quark or gluon. However,
theory predicts that at very high temperatures and pressures,
quarks and gluons will be free to move independently over
relatively large distances, forming a "quark-gluon plasma."
Creating a quark-gluon plasma is the goal
of another new facility, the Relativistic Heavy Ion Collider
(RHIC), scheduled to open at Brookhaven National Laboratory
in 1999, RHIC will accelerate nuclei in two concentric beams
heading in opposite di"RECT"ions. The beams will collide in
six chambers. Each head-on collision will produce a small
region of enormous energy density where the quark-gluon
plasma may form.
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Streamer Chamber Picture of Two Sulfur Nuclei Colliding
CERN/Univ. of Washington
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These experiments will require new
detectors to record and analyze the collisions. Lawrence
Berkeley Laboratory is developing new silicon detectors
based on a technology which offers lower costs and higher
yields. RHIC's detectors, like CEBAF's cavities, are examples
of new technologies developed for basic research which quickly
find industrial applications.
The quark-gluon plasma may exist in the
core of neutron stars, and it may have existed during the
first millionth of a second after the Big Bang. But on earth,
it represents a new state of matter. RHIC presents the opportunity
to study matter in an unknown regime. Creating a quark-gluon
plasma in the laboratory is one of the outstanding challenges
of modern physics.
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Aerial View of RHIC
Brookhaven National Laboratory |
NEUTRINO PHYSICS
The Solar Neutrino Puzzle
One of the most elusive particles in nature
is the neutrino. Although neutrinos are extremely difficult to
detect, they may be the most common particle in nature, forming
the "invisible matter" that helps bind together galaxies and other
large-scale structures in our universe. They come in three types,
or "flavors" -- electron, muon, and tauon neutrinos. Their properties
-- for example, whether they have a mass and whether a neutrino
of one flavor can transform into one of a different flavor --
hold the key to some of the most important unanswered questions
in nuclear physics, particle physics, and astrophysics.
The sun produces vast quantities of neutrinos
as a byproduct of thermonuclear reactions that occur in the high-temperature
solar core. Approximately ten trillion solar neutrons pass through
each of us every second. Most of them continue all the way through
the earth and beyond, as neutrinos can pass unaffected through
a light year (nine trillion kilometers) of lead. Neutrinos are
our best source of information about how energy is generated in
the solar core; all other forms of solar radiation come to us
from the surface of the sun.
| 
Homestake Mine Experiment
Brookhaven National Laboratory |
Physicists first detected
solar neutrinos in the Homestake Mine Experiment, which
has operated since 1967 in a South Dakota gold mine (neutrino
experiments must stay underground to avoid cosmic rays).
The detector is a tank containing 100,000 galls of perchlorethylene
(dry cleaning fluid). About three times a week, an electron
neutrino interacts with a chlorine atom and changes it into
an argon atom. This experiment detects only 25% of the neutrinos
predicted by the accepted model of solar processes. |
Two recent experiments employ gallium detectors, which can
sample a greater number of neutrinos. GALLEX is a European-Israeli-U.S.
experiment conducted in a tunnel under the mountains east
of Rome. SAGE is a Russian-U.S. effort located in a mine
in the Caucasus Mountains. Both see about two-thirds of
the neutrinos predicted. These results, combined with those
from a fourth experiment carried out in Japan, suggest that
the problem is not in our model of the sun, but rather a
flaw in the "Standard Model" of particle physics. These
measurements support a theory which predicts that solar
electron neutrinos transform into another flavor before
they reach the earth.
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| Canadian, U.S., and British
physicists have designed the next generation of solar
neutrino detector, the Sudbury Neutrino Observatory (SNO).
SNO's heavy-water target will detect all three neutrino
flavors, not just the electron-type neutrino detected
by the chlorine and gallium experiments. (See the photograph
of SNO's photomultiplier support structure; the inset
is a simulation of a neutrino detection event.) SNO is
designed to determine whether the missing solar neutrinos
have transformed into another flavor. If they have, SNO
will help us deduce the neutrino's mass.
Double Beta Decay
The mass of neutrinos has implications
for the rarest event observed in nature: double beta decay,
in which two protons spontaneously transform into neutrons,
thereby changing the nucleus into a different element
two steps removed in the periodic table. During double
beta decay, the nucleus emits two electrons and usually
two neutrinos. But if no neutrinos emerge, the event proves
that neutrinos must have mass of a special kind, a "Majorana
mass." Researchers in several laboratories are trying
to observe double beta decay without neutrino emission.
These experiments are difficult because double beta decay
is so rare; in a one-kilogram sample, it might occur four
times a year.
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SNO's Photomultiplier Support Structure
Lawrence Berkeley Lab
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Dark Matter and the Big Crunch
Spiral Galaxy in Cepheus
National Optical Astronomy Observatories
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The discovery that neutrinos
have mass would not only affect theories of particle physics
but also help us determine the future of the universe.
Will the universe continue to expand forever? Or will
gravitational forces reverse the expansion, causing the
universe to contract and ultimately destroy itself in
the Big Crunch? The answer depends on the amount of mass
in the universe. Estimates of the visible mass account
for only 1% of the mass required for the universe to halt
its expansion. But not all mass is easily visible -- spiral
galaxies rotate in ways that would be impossible if visible
stars were the only sources of gravitational attraction.
To explain the rotational patterns, some form of "dark
matter" would have to account for much of the mass of
the galaxy. One candidate for the dark matter is the neutrino.
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NUCLEAR ASTROPHYSICS
By combining the data from laboratory experiments
with astronomical observations, nuclear physicists have identified
the processes by which our sun and other stars come into being,
produce energy, create the chemical elements, and die.
Big Bang Nucleosynthesis
According to the Big Bang theory, the universe
began 15 billion years ago in a point of extraordinary density
and temperature and has been expanding ever since. Many light
elements, including deuterium, helium, and lithium, were formed
in the first few minutes. Measurements of the corresponding nuclear
reactions in the laboratory have enabled physicists to calculate
precisely those primordial abundances which we observe in very
old stars. These measurements support the Big Bang theory. The
calculations also tell us that the amount of matter existing in
the form of protons and neutrons is less than 20% of that required
to halt the expansion of the universe.
Stellar Nucleosynthesis
Nuclear fusion in the cores of stars
creates many elements of intermediate mass. Our sun converts
6000 million tons of hydrogen to helium every second.
When stars run out of hydrogen fuel, they contract, heat
up, and begin to fuse more massive nuclei. At later stages,
nuclear burning creates still heavier elements such as
silicon and iron. The heaviest elements are created in
hot stellar environments when nuclei capture neutrons.
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The Sun National Optical Astronomy Observatories
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Supernova Explosions
When a large star exhausts its hydrogen fuel,
it fuses successively larger nuclei until it forms an iron core.
Fusion then stops, because fusion of iron and heavier nuclei doesn't
liberate energy; instead, it consumes energy. Deprived of fusion's
energy to counteract gravity, the star collapses. The protons
in the iron nuclei, now under intense pressure, fuse with electrons
to form vast quantities of neutrons and neutrinos. The neutrinos
stream outward, leaving behind a "neutron star," a dense core
composed entirely of neutrons. For the first ten seconds following
the collapse, the energy radiated by the star exceeds the total
energy emitted by everything else in the visible universe. As
the material surrounding the neutron star's core is thrown into
space, the supernova swells to a radius of over 10 billion miles
and shines with the light of billions of suns.
| The supernova explosion
ejects into the interstellar medium the many nuclei synthesized
during the life of the star. This material is later incorporated
into new stars and planets, such as those in our solar
system. Many of the elements that comprise our bodies
are "star stuff" from ancient explosions.
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Crab Nebula in Visible Light (left) and X-Rays (right) Smithsonian
Institute |
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1994 Hubble Photo of 1987a
Space Telescope Science Institute |
The two photographs above
show the Crab Nebula, the remnant of a supernova explosion
recorded in China, Japan, and Korea in the year 1054.
The exploding star was visible in the daylight for three
weeks. The image on the left shows the Crab Nebula at
visible frequencies. In the image on the right, taken
at X-ray frequencies, the bright spot is a pulsar, a source
of radiation that pulses on and off thirty times per second.
In 1987, light from a supernova which
exploded 165,000 years ago reached the earth. Known as
Supernova 1987a (see photo to left), this was the first
explosion of a star which astronomers had photographed
repeatedly before it became a supernova. It was also the
first supernova visible with the naked eye since 1604.
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NUCLEAR STRUCTURE AND RADIOACTIVE BEAMS
Nuclear Structure
Many questions that arise in nuclear physics
and astrophysics require an understanding of the structure of
the nucleus. Nuclei exhibit a remarkable range of phenomena that
challenge both theory and experiments.
Collectivity, Chaos, and Supercomputers
The collective motion of large nuclei
is similar to the rotations and vibrations of drops of water.
"Exotic" nuclei with unusual proton-neutron ratios can sustain
a variety of unusual shapes, such as extremely elongated
footballs or vibrating pears. Researchers have produced
these nuclei with a variety of accelerators and studied
them with sophisticated detectors. Future detectors such
as GAMMASPHERE will allow us to study nuclei at extremes
of rotation, deformation, and excitation. |
Gammasphere (initial phase)
Lawrence Berkeley Laboratory
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Nuclei also exhibit many properties that
are chaotic, impossible to predict except statistically.
The study of chaos and complexity, now a flourishing field
of research and the subject of a popular book, began with
early studies of how nuclei capture an additional neutron.
Experimenters are studying the evolution from chaos to order
as hot nuclei cool. They are also exploiting the statistical
properties of nuclear energy levels to measure the weak
force between nucleons.
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Supercomputers have enabled theorists to describe
the complicated interactions within nuclei which contain three
or four nucleons. With the aid of techniques which randomly sample
the possible interactions, theorists have also made great progress
in understanding the ground-state structure of heavier nuclei.
Radioactive Beams
Radioactive beams are important to nuclear
structure studies and to nuclear astrophysics. Many nuclei that
exist in the hot, dense environments of stars live for very short
times under terrestrial conditions. Thus beams of radioactive
nuclei are an essential tool for experimentalists seeking to study
astronomical phenomena in the laboratory.
Radioactive beams have also opened new windows
on nuclear structure. For example, such studies recently showed
that many unusual properties of the lithium-11 nucleus arise from
a "neutron halo" of remarkable size.
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