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OCR for page 157
7
Interactions with.Other
Areas of Physics and
Technology
The purpose of research in elementary-particle physics is to inves-
tigate the basic nature of matter. energy, space. and time. In the course
of this research elementary-particle physics interacts with other areas
of physics. Subject matter, instruments, and theories from other areas
all have a bearing on elementary-particle research. And elementary-
particle research contributes data, theories, and apparatus to other
parts of physics. In this chapter we briefly describe the interaction
between elementary-particle physics and four other areas: cosmology
and astrophysics, cosmic-ray physics, nuclear physics, and atomic
physics. Those areas are the subjects of separate volumes in this
survey; here we look only at their interaction with particle physics.
Elementary-particle physics interacts with technology in two ways.
First, the technology that is invented and developed for use in particle
physics subsequently finds use in other fields. The foremost example is
the particle accelerator itself, some of whose applications are described
below in the section on Other Applications of Accelerators. The
second way that elementary-particle physics interacts with technology
is that technology from outside particle physics is stimulated and
developed during the design and construction of particle-physics
accelerators and detectors. Prominent examples are superconducting
magnets, described below in the section on Large-Scale Uses of
Superconductivity, and integrated circuits and computers, described in
the section on Support and Stimulation of New Technology.
IS7
OCR for page 158
158 ELEMENTARY-PARTIC'E PHYSICS
COSMOLOGY AND ASTROPHYSICS
Recent years have witnessed a growing symbiotic relationship
between elementary-particle physics, the science of the very small, and
cosmology, the science of the very large. This interdependence has
been fostered by the revolution in our understanding of particle physics
on the one hand and on the other hand by the existence of and
observational support for the big bang model of the origin of the
universe. In that cosmology, the universe evolved from an original
explosion of a dense, hot mixture of matter and energy. In those first
moments of the universe elementary particles were created and de-
stroyed at an enormous rate. Their creation and destruction occurred
through interactions. For example, if two photons collide they can
interact and be destroyed, but as a result of that interaction an electron
and a positron can be created. In big bang cosmology the universe is
expanding as this happens. the matter and energy cooling ok and
becoming less dense. Some particles no longer interact, hence they are
not destroyed, and they remain in our present universe. Such particles
are called relics.
This is the natural interpretation of the observed recession of distant
objects in the universe, and it is supported by the observations of the
3-K microwave background radiation, generally interpreted as a relic
from the recombination of electrons and nuclei to form electrically
neutral matter. Recombination occurred when the universe was thou-
sands of times more compressed than it is today. with a temperature of
several thousands of kelvins. Such an increase of temperature at earlier
times when the universe was denser receives further support from
successful calculations of the astrophysical abundances of light ele-
ments through cosmological nucleosynthesis when the universe was a
billion times smaller than it is today, with a temperature of several
billion kelvins.
The successes mentioned above illustrate the principles enabling big
bang cosmology and particle physics to be interrelated. Our present
understanding of particle physics enables us to extrapolate the Hubble
expansion of the universe backward to earlier times and higher
temperatures and to calculate the abundances of other elementary-
particle relics from the big bang. An example is provided by the question
of how many kinds of neutrinos exist. This is a critical question because
it is one way to find out the number of generations of leptons that
exist. Nucleosynthesis calculations impose the most stringent available
limit on the number of light neutrino species. They are restricted to
OCR for page 159
INTERACTIONS WITH OTHER AREAS 159
three or at most four' which is below the best upper limits currently
available from particle-physics experiments.
As another example, from present theories it can be calculated that
stable neutrino masses must be less than 100 eV, or more than a few
GeV, if these neutrino relics are not to decelerate or reverse the
present expansion of the universe because of their mass. These
constraints on neutrino masses are much more general than those ob-
tained from laboratory experiments, and for the muon and tau neutrinos
they are more stringent.
Another long-standing problem in cosmology has been the existence
of dark matter; that is. the amount of visible matter in the universe
seems to be far less than the amount of mass that is inferred from the
interactions of the visible matter. There seems to be far more matter
out there than we can see; in fact, the amount of matter implied by the
dynamics of the universe on large scales may be even greater than the
limits on the amount of hadronic matter in the universe as obtained
from the big bang synthesis of nuclei.
Neutrinos with small but nonzero mass might constitute this missing
mass, and they could have played a significant role in the formation of
galaxies and other structures in the present universe. More speculative
particle theories provide other relic candidates for these roles, such as
the hypothesized supersymmetric particles and axions. Some limits on
the existence of such particles come from astrophysical considerations
of the energy flow out of the cores of stars at late stages in their
evolution.
In view of these interesting exchanges of information, it is not
surprising that cosmologists and particle physicists have been inspired
to speculate about much earlier epochs of the big bang, when temper-
atures and hence particle energies were much higher than present or
conceivable particle accelerators could provide. Despite our lack of
control of the experimental conditions, the early universe could be a
useful laboratory for testing new particle theories. One of the most
striking examples has been the realization that grand unified theories
that predict baryon decay could also explain the presence of the matter
in the universe today. The interactions and decays of superheavy
particles with masses above 10'° GeV could have assured the present
predominance of matter over antimatter. It would no longer need to be
assumed as an arbitrary initial condition.
Grand unified theories also predict the existence of magnetic mono-
poles potential cosmological relics which, if they exist, could inval-
idate current cosmological theories. Their masses of 10'6 GeV or more
OCR for page 160
.
160 ELEMENTARY-PARTICLE PHYSICS
are so high that only the early universe could have produced them.
Some simple arguments suggest that unacceptably many grand unified
monopoles would have been produced in the conventional big bang
cosmology, a difficulty that was a stimulus for the proposal of so-called
inflationary cosmology. According to this idea, there may have been an
early epoch in the history of the universe during which it expanded
exponentially, driven by the energy released when there was a change
or transition in the state of matter. A large number of particles would
be produced when this transition terminated, and the abundance of
monopoles would then be greatly diluted. Such an inflationary epoch
could also explain many of the greatest cosmological mysteries of the
universe' such as the high degree of homogeneity and isotropy that it
exhibits and its great age. Inflationary cosmology also enables one for
the first time to relate particle physics to the wide range of the large-
scale fluctuations in the present universe. It is a challenge to find a
particle theory that naturally leads to inflation and to this wide range.
This is one of many areas in which the interaction between particle
physics and cosmology will continue to be fruitful in the future.
COSMIC-RAY PHYSICS
Cosmic-ray physics and nuclear physics are the parents of elemen-
tary-particle physics. Many of the fundamental discoveries about
elementary particles were made using cosmic rays. This is because
cosmic rays are a natural source of high-energy particles. Cosmic rays
consist primarily of protons that come from outside the solar system.
When they hit the atmosphere they make other particles. The positron.
the muon, and some of the strange hadrons were discovered and first
studied using cosmic rays.
However, with the development of accelerators, the use of cosmic
rays in elementary-particle physics has gradually decreased. This is
because accelerators provide controllable and much more intense fluxes
of particles. It is only the highest energy cosmic rays that are still useful
for studies in particle physics energies that cannot be attained try
accelerators.
Thus at present the field of high-energy cosmic rays acts as a bridge
between high-energy particle physics and experimental astrophysics.
At and above the highest energies reached by hadron-hadron colliders
the energy spectrum, composition, and possible source directions ~ f
primary cosmic rays are known to varying degrees. At the same time.
the nature of strong interactions at energies above those provided try
colliders must be deduced from extrapolations based on known accel-
OCR for page 161
INTERACTIONS WIT[l OTHER AREAS 161
erator data and from the largely indirect cosmic-ray data. As the
interpretation of these cosmic-ray-data in terms of particle-physics
phenomenology depends on knowledge of the identity of the initiating
cosmic ray (e.g., proton. carbon. or iron nucleus), our knowledge and
understanding of both areas are interrelated, and progress is made in an
iterative manner as we move to higher energies.
We still do not know the source or acceleration mechanism of
high-energy, primary cosmic rays. At the highest observed energies
(about 10~° eV), it appears that cosmic rays would be too energetic to
be trapped in the known magnetic field of our galaxy or to survive
energy loss by photoproduction on the relic blackbody radiation in
propagation over intergalatic distances. They might come to us from
our own local supercluster of galaxies, or they might come from the
core of our own galaxy, bent to the Earth by the (unknown) magnetic
fields in a galactic halo.
Correspondingly' the only source of information concerning the
nature of particle interactions above the highest accelerator energies
comes from cosmic rays. Hints of strange, unanticipated phenomena at
these energies permeate the cosmic-ray literature. In the past, some
cosmic-ray hints, such as evidence for free quarks and monopoles,
have not stood up under closer scrutiny. But others, such as the
increase of the strong interaction cross section with energy, were later
confirmed at particle accelerators.
The problem of studying cosmic rays at energies above 10'4 eV
(greater than those at the CERN proton-antiproton collider) is dis-
cussed in Chapter 6. Above 10'6 eV, the integrated primary cosmic-ray
flux is only one per square meter per year.
Other areas addressed by cosmic-ray experiments that overlap
astrophysics, particle physics, and nuclear physics include the search
for antimatter in cosmic rays and the study of nucleus-nucleus inter-
actions at very high energy. It is quite certain that our local galaxy is
composed of ordinary matter, but if antinuclei as heavy as iron are
found in primary cosmic rays, even at a level of 10-7, this would be
evidence for entire distant galaxies composed of antimatter. Currently
there are no data with which to answer this question. Finally, the study
of cosmic-ray neutrinos with proton-decay detectors may portend a
new field of neutrino astronomy.
NUCLEAR PHYSICS
High-energy physics traditionally is closely linked to an area broadly
termed nuclear physics. Elementary-particle physics grew out of
OCR for page 162
162 ELEMENTARY-PARTIC~E PHYSICS
nuclear physics; nuclei can be used as probes of elementary particles
and vice versa. But the two disciplines are different because they deal
with matter at different levels of elementarily.
Accelerators are tools that are common to high-energy and nuclear
.
physics. Low- and medium-energy accelerators used by the nuclear-
physics community include meson factories, which produce the most
intense beams of protons, pions, and muons, and reactors, which pro-
duce the highest fluxes of neutrinos. Some special questions in particle
physics must be explored with beams of these kinds.
The nucleus itself is a unique high-density laboratory in which
interactions between quarks may be probed, by electron bombardment
or by nucleus-nucleus collisions. Conversely, processes such as the
production of strange protonlike and neutronlike particles—processes
usually associated with particle physics may be used instead to study
nuclear properties, as when the produced strange particle is a nuclear
constituent.
An outstanding puzzle is the existence of multiple generations of
quarks and leptons. In the case of quarks it is known that the up-down,
charm-strange, and bottom-top generations mix, but there is no clear
evidence for mixing between members of the electron, muon, and tan
lepton families. Searches for muon-electron mixing in muon decay
have been carried to exquisite precision with free muons at the
LAMPF accelerator at Los Alamos and with muons in the fields of
nuclei at TRIUMF (Vancouver) and SIN (Zurich). Mixing of lepton
generations in combination with differences in neutrino mass would
produce an oscillatory behavior in the composition of neutrino beams.
Highly restrictive limits on neutrino oscillations have been set at
LAMPF and at reactors in the United States, in France. and in
Switzerland, as well as at high-energy physics facilities. Discovery of
lepton-generation mixing would be a major achievement, sharpening
and expanding our theoretical understanding. Experiments at high
sensitivities will continue with this aim.
Successful unification of the strong and electroweak interactions
depends on identification of the underlying symmetry group. An out-
standing question is the parity symmetry of the electroweak interac-
tion, which at present appears to be fully left-handed. Under the
assumption that the neutrino that would participate in any right-handed
weak interaction is light enough to be produced in muon decay. the
weak-force-carrying particle WR that would mediate that interaction is
required by recent muon-decay data to be at least five times more
OCR for page 163
INTERA CTIONS WI TH OTHER AREAS 163
massive than its left-handed counterpart We. If further assumptions
including electron nonconservation. are made, much more stringent
bounds on the WR mass are set by the limits on neutrinoless double
beta decay, e.g., of germanium or selenium isotopes, by direct or
geochemical observation. Under the same assumptions, these obser-
vations require the electron-neutrino mass to be less than approxi-
mately 10 eV. More direct measurement of the electron-neutrino mass
is possible through extremely precise study of the endpoint spectrum in
tritium beta decay. At present there is unconfirmed evidence from one
experiment of finite electron-neutrino mass, in a range that would
suggest that neutrinos could account for the dark mass of the universe.
The concept of the quark composition of nucleons has had a major
impact on nuclear-physics theory and has led to new ideas in the
description of nucleons. For nearly a decade, one direction of research
at the Lawrence Berkeley Laboratory Bevalac has been based on the
idea that medium-energy heavy-ion central collisions would produce an
instantaneous temperature and nuclear density so high that hadronic
matter would evolve (deconfine) into an as yet unobserved new state of
matter, the quark-gluon plasma. Present estimates indicate that the
plasma can be made in two different environments. The first is found at
lower energies (a few GeV/nucleon usable energy) in which heavy
nuclei are still able to stop in each other, building up high-energy
densities in a baryon-rich zone. At much higher energies (above 2~30
GeV/nucleon usable energy), the two colliding nuclei are transparent to
each other, leaving a hot baryon-free plasma in the central region after
the collision process has taken place. In this central region large
densities can be found, sufficient for Reconfinement to occur. These
large usable energies await the construction of a new accelerator,
called by the nuclear science community the Relativistic Nuclear
Collider (RNC).
Recently, evidence has indicated that nuclei do not behave simply as
a collection of nucleons when high-energy muon, electron, or neutrino
scattering occurs. CERN and SLAC experiments find a difference in
the form of the quark distribution between deuterium and iron nuclei.
This observation will be pursued in the nuclear-physics community
through the construction of the SURA 4-GeV electron linear ac-
celerator/stretcher.
These are some examples of the interactions between nuclear
physics and elementary-particle physics. Such interactions will con-
tinue, not only in the questions that are being studied but also in the
accelerators and detectors that are used.
OCR for page 164
164 ELEMENTARY-PARTICLE PHYSICS
ATOMIC PHYSICS
e
Effects from new particles are most readily observed at the appro-
priate energy required to produce the particle. Historically, however,
the effects seen in new energy ranges have often been correctly
foreshadowed by extrapolation of small deviations from theory ob-
served at lower energies. The extreme precision with which measure-
ments are possible by atomic-physics techniques makes conceivable,
even today, the exploration of energy ranges beyond those currently
obtained. As an example, atomic experiments were recently used to
test the electroweak theory predictions for the synthesis of the weak
and electromagnetic theories. These experiments were based on slight
differences in the absorption of left- and right-circularly polarized light.
Only the extreme precision of laser spectroscopy techniques made
these experiments possible.
A second example is the study of two-particle systems that are
simpler than the hydrogen atom. The hydrogen atom has at its core an
extended, complex object—a proton. In contrast, the positronium
system, composed of a positron and electron combined as a short-lived
atom, consists of two simple' pointlike particles that exhibit effects that
are masked in atomic hydrogen. Muonium~ composed of a muon and an
electron, is another such simple system that can be formed by stopping
muons, produced in an accelerator, in noble gases.
Investigation of the spectra of atomic systems with exotic constitu-
ents can also be used to probe particle structure. Examples are
provided by atoms composed of muons and pions and of electrons and
pions. Deviation from the results expected from pointlike particles
provides insights into the structure and interaction of the pion with
leptons. Another use of spectroscopy has been the insertion of muons,
pions, and kaons into the innermost electron orbits of nuclei to
provide, via x-ray spectroscopy, a measure of the electric-field struc-
ture in the neighborhood of the nucleus and also to provide high-
precision mass measurements of kaons and pions.
A further example is provided by the precise measurements of the
magnetic properties of the electron and the muon. The quantum theory
of electromagnetism predicts that an electron will act as a small bar
magnet and also predicts the strength of that magnet. Atomic-physics
experiments have measured that strength accurately and have thus
made one of the most careful tests of that theory.
One ingenious small-scale experiment has reported positive evidence
for fractional electric charge on small pellets of niobium. An interpre-
tation of these results might be that free quarks on these spheres were
OCR for page 165
/NTERA CT/ONS W/ TH O THER A REA S 1 65
responsible for the observations. However, the results have not been
confirmed elsewhere, and the consensus in the community is to
postpone accepting these results as evidence for free quarks pending
strong confirmation.
These classes of experiments investigate properties of matter that
are of interest to both atomic and particle physics and are thus an
important meeting point for two apparently unrelated areas of physics.
CONDENSED-MATTER THEoRETICAL PHYSICS
There has been and continues to be ~ fruitful and vigorous dialog
between theoretical condensed-matter physics and theoretical particle
physics. Here we sketch some of the topics in which concepts and
techniques of elementary-particle theory enrich condensed-matter
physics and also some of the topics where ideas of condensed-matter
physics illuminate particle physics.
In the late 1950s, the techniques of quantum field theory used in
particle physics started to be employed in condensed-matter physics
with outstanding results. Early on. field-theory techniques were used
to solve the problem of the energy of an electron gas that forms the
starting point for the general discussion of crystals. These techniques
were found to be of great importance in dealing with superconductivity
and superfluidity. The nature of second-order phase transitions was
revolutionized by the use of renormalization group techniques that
were developed by both particle and condensed-matter physicists. In
turn, the general concepts of phase transitions developed by con-
densed-matter physicists have been of much use to particle physics in
two areas. On one hand, a strongly first-order phase transition has been
invoked to produce an inflationary epoch in the early universe that may
solve several outstanding cosmological puzzles. On the other hand,
there is much recent interest in the possibility of a phase transition in
dense, energetic nuclear matter in which the quarks and gluons become
Reconfined and behave as a quark-gluon plasma. This may happen deep
in a neutron star or, perhaps, in high-energy heavy-ion collisions.
It is hoped that the new theory of quantum chromodynamics (QCD)
will explain the mass and structure of the hadrons. The method used is
to replace the space-time continuum by a discrete lattice of points, and
the techniques of calculation employed are quite similar to those first
developed in the condensed-matter context. The QCD calculations are
of a large scale. The methods are often checked by first applying them
to the simpler models of condensed-matter phenomena. Such checks
have often proved to shed light on these models. The QCD calculations
OCR for page 166
1 66 ELEMENTA R Y-PA R TI CLE PH YSI CS
are so extensive that powerful special-purpose computers are being
developed to handle them. This has led to a similar development of
special-purpose computers to deal with lsing model calculations in
condensed-matter physics.
The concept of order parameters. first introduced in condensed-
matter physics' now plays an important role in quantum field theory.
We give some examples. These order parameters are akin to the Higgs
fields of elementary-particle physics. Liquid helium can flow in a circle
about a vortex. Such a flow is analogous to a topological knot. and the
vortex line is a kind of defect. Similarly. the Higgs field can arrange
itself in a pattern like that of the extended spines of a hedgehog. This
is again a type of topological knot. The corresponding defect is a
magnetic monopole. Such magnetic monopoles are exceedingly heavy.
They may have been produced in the early universe, but they have not
yet been detected. Polyacetylene is a long molecule with an alternating
bond structure. The bonds can have a jump just as a canal can have an
extra lump of water. Such objects are called solitons. A similar
situation may occur in the quantum field theory of elementary particles
with hadrons being described at least approximately. as solitons. The
total electronic charge about a soliton in polyacetylene is most pecu-
liar it is half the charge of a free electron. If magnetic monopoles do
exists they would also behave to some extent as solitons~ and they
could induce a fractional electronic charge.
OTHER APPLICATIONS OF ACCELERATORS
In this section we give some examples of how accelerators have been
extended in their applications to other kinds of research and other
kinds of technology.
Synchrotron Radiation
The foremost example of the application of accelerators is the use of
circular electron accelerators to produce synchrotron radiation. As
shown in Figure 7.1, when an electron moves in a circle it emits
electromagnetic radiation in a direction tangent to that circle. That
electromagnetic radiation covers a broad range of frequencies, extend-
ing from the visible to the ultraviolet to the x-ray region of the
spectrum. In addition to the broad frequency spectrum. the intensity of
the emitted radiation is much higher than can be obtained by other
means. For example' the intensity of the x rays within any given
frequency range is much greater than can be obtained from a conven-
OCR for page 167
INTERACT/O^'S WITH OTHER AREAS 167
/
l
Electron
Moving
Clockwise
~ ~yncnro~ron \
\\ Rodiation ~:
.
Circulor Poth
_ of
E lectron
-
FIGURE 7.1 The most important use of electron storage rings outside of elementary-
particle physics is the production of synchrotron radiation. As ~ high-energy electron
moves in a circular orbit it emits an intense beam of x rays called synchrotron radiation.
Synchrotron radiation is used for research in many scientific and technical fields: for
example. solid-state physics. material science. chemistry. and biology.
tional x-ray tube. This wide frequency spectrum of intense radiation
has found many applications in applied physics. material science,
electrical engineering. metallurgy, biology. biochemistry. biophysics.
and chemistry.
Originally synchrotron radiation was obtained only as a parasitic
by-product from circular electron accelerators and from electron-
positron colliders. However. as the importance of research based on
synchrotron radiation has increased. special dedicated accelerators to
produce synchrotron radiation have been built. A list of present-day
synchrotron radiation sources now in operation or being constructed is
given in Table 7.1. There are more than 20 such facilities.
A simple example of the use of synchrotron radiation has to do with
the process called photoionization. in which light is used to eject an
electron from an atom or molecule or from a solid. The frequency of
the light that ejects the electron tells the researcher something about
the structure of the atom, molecule, or solid. Photoionization has been
known about since the turn of the century, but to do efficient advanced
research at present requires intense sources of light with the frequency
involved being known precisely. This is exactly what can be done with
synchrotron radiation.
Synchrotron radiation facilities have now begun to develop special
kinds of accelerator technology to enhance their capabilities. Synchro-
tron radiation is produced when an electron goes in a circle, because
OCR for page 168
168 E! EMENTAR Y-PART/CLE PH YSICS
TABLE 7. 1 Storage Ring Synchrotron Radiation Sources
Electron Energy
Location Ring (lab) (GeV) Notes
.
China
Beijing BEPC (lHEP) 2.~-2.8 Parasitic"
Hefei HESYRL (USTC) 0.8 Dedicated
England
Daresbury SRS 2.0 Dedicated
France
Orsay ACO (LURE) 0.54 Dedicated
DCI ( LU RE) I .8 Partly dedicated
SuperACO (LURE) 0.X Dedicated"
Germany
Hamburg DORIS ( DESY ) 5. ~ Partly dedicated
West Berlin BESSY 0.8 Dedicated
Italy
Frascati ADONE 1.5 Partly dedicated
Japan
Tsukuba Photon Factory (KKK) 2.5 Dedicated
Accumulator (KKK) 6 8 Partly dedicated"
Tokvo TRISTAN (KKK) 30 Parasitic"
Ol;~~aki SOR (ISSP) 0.4 Dedicated
Tsukuba U VSOR ( I MS; 0 6 Dedicated
Sweden
Lund Max 0.55 Dedicated
United States
Gaithersberg. MD SURF (NBS) 0.28 Dedicated
Ithaca. NY CESR (CHESS) 5.~ - Parasitic
Stanford. CA SPEAR (SSRL) 4.() Partly dedicated
Stoughton. Wl Tantalus (SRC) 0.94 Dedicated
Aladdin (SRC) 1.0 Dedicated
Upton. NY NSLS I (BNL) 0.75 Dedicated
NSLS 11 (BNL) 2.s Dedicated
Soviet Union
Karkhov N-100 (KPI) 0.10 Dedicated
Moscow Kurchatov 0.45 Dedicated
Novosibirsk VEPP-2M (lNP) 0 7 Partly dedicated
VEPP-3 (INP) 2.' Partly dedicated
VEPP-4 (INP) 5-7 Parasitic
" Under construction as of April 1985.
any deviation of the path of a particle from a straight line means that
the particle is being accelerated. ln fact. any means by which an
electron can be made to move off a straight line and thus be accelerated
will also produce synchrotron radiation. Therefore modern synchro-
tron radiation accelerators have devices called wigglers or undulators
introduced in the path of the electrons. These devices shake the
electron as it moves through them, causing strong acceleration and the
emission of intense synchrotron radiation in particularly desirable
frequency ranges.
OCR for page 169
INTERACTIONS WITH OTHER AREAS 169
Accelerators in Medicine
The electron accelerator is now one of the major tools used in the
radiation-therapy treatment of cancerous tissue. The usual way to use
such an accelerator is to allow a high-energy beam of electrons to strike
a target and then to form the resulting x rays from that target into a
narrow, well-defined, and intense beam. The radiation therapist then
directs that x-ray beam as carefully as possible onto the tumor that is
to be treated. Most treatments involve.x rays in the a- to 6-MeV range,
but x-ray energies as high as 30 or 40 MeV are sometimes used. Most
of the electron accelerators used for standard radiation therapy are
commercially produced linear accelerators. Some circular electron
accelerators, based on the betatron principle' are also used.
Although the standard method of radiation therapy using accelera-
tors continues to be the use of x rays' during the last decade there has
been a good deal of research on the use of other kinds of particles to
destroy cancer. For example, accelerators have been used to produce
beams of charged pions, which are then used to treat the tumor. Work
has also been done using neutrons and high-energy heavy ions pro-
duced in an accelerator.
An interesting new use of accelerators in medicine involves the
production of short-lived radioactive materials that produce positrons
when they decay. inside the patient these positrons annihilate, and the
resulting photons can then be used in tomography. Because these
materials have short lifetimes they cannot be stored but must be
produced soon before they are used, and cyclotron accelerators are
now being used in hospitals to produce such materials.
High-Intensity Neutron Sources
The scattering of neutrons in mattter has become an important tool
in materials science, solid-state physics, polymer chemistry, molecular
biology, and other areas of applied and pure science. In the past,
nuclear reactors have been the source of the neutron beams used in the
scattering experiment. Reactors are still the major source, but spalla-
tion neutron sources that use technically advanced proton accelerators
are coming into increasing use because they can provide more-intense
and higher-energy neutron beams. In a spallation source, a proton
beam from a rapid cycling synchrotron bombards a uranium or other
heavy-element target, providing a neutron beam.
OCR for page 170
170 ELEMENTAR Y-PARTICLE PHYSICS
Accelerators and Plasma Physics
.
There is an increasing interaction between accelerator physics and
technology and plasma physics and technology. This interaction takes
several different forms. One example is the use of heavy-ion acceler-
ators to produce inertial fusion. Another example is the use of accelera-
tors to inject charged particles into a plasma to add energy to the plasma
as a step toward producing fusion. These ideas are described in detail in
the companion volume on plasma and fluid physics.
LARGE-SCALE USES OF SUPERCONDUCTIVITY
The study o.f superconducting effects and the use of superconducting
phenomena play an important part in many areas of physics. Briefly,
superconductivity is the absence of electrical resistance that some
metals exhibit when cooled to a temperature near absolute zero. That
means that an electric current can circulate through the metal without
any power loss and therefore could literally circulate forever.
While superconductivity has been used on a laboratory scale for a
long time, there have been few large-scale uses of this phenomenon
until recently. The most striking example is the recent construction of
the Fermilab Tevatron accelerator, which uses about 1000 super-
conducting magnets. The liquid-helium refrigeration system used to
cool those magnets is the largest in the world. In Chapter 5 we
discussed the significance of this accomplishment for future construc-
tion of very-high-energy proton-proton or proton-antiproton colliders.
This accomplishment will also help to lead the way to other large-scale
applications of superconductivity.
Large-scale applications of superconductivity require large facilities
for cooling and refrigerating with liquid helium, control systems for
maintaining the temperature of superconducting devices, and emer-
gency systems for absorbing the sudden power surges that occur if the
material suddenly loses its superconducting properties because it
warms up. This kind of technology only becomes practical when there
has been a sufficient amount of development and engineering work and
sufficient experience with big superconducting systems. This is exactly
what has been accomplished with the Fermilab superconducting accel-
erator.
The construction and operation of the 1000-GeV superconducting
proton accelerator at Fermilab is the first large-scale use of su-
perconductivity in the world. The technology developed for this
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I N TERA CTl ONS WI TH O THER A REA S 1 7 1
accelerator and the experience gained in using it will be useful for other
proposed large-scale uses of superconductivity. Some possible appli-
cations are listed below:
· Rotating electrical machinery with superconducting windings;
· Superconducting high-power electrical transmission lines;
· Superconducting current-limiting devices for electrical switch-
gear;
· Superconducting magnet energy storage to smooth peak loads;
· Superconducting coils for separation of materials via their mag-
netic properties;
· Superconducting magnet systems for fusion reactors:
· Superconducting magnet systems for magnetohydrodynamic
power generators;
· Electrodynamic levitation systems for trains using supercon-
.
ductiv~ty.
SUPPORT AND STIMULATION OF NEW TECHNOLOGY
As described in Chapter 6, experiments in elementary-particle
physics depend a great deal on the use of integrated circuits. micro-
processors, and large high-speed computers. Since the particle physi-
cist uses these devices in an experimental situation, it is often possible
to use devices that are not yet fully commercially developed. The
researcher will often buy devices or computers that are in the proto-
type stage in order to have the advantage of using the newest
technology. This supports the development of new technology in
integrated circuits and in computers.
In addition, there is another valuable effect. The research physicist
in elementary-particle physics is often well acquainted with the prin-
ciples, both physics and engineering, of the new device. Therefore the
researcher can often provide information back to the manufacturer
about how the prototype device behaves and how it might be im-
proved. Thus elementary-particle physics, through providing for early
use of new electronic devices, supports the development of new
technology and new devices in electronics and computers.
Superconducting magnet technology is another example. These
magnets as used in the Tevatron and, as proposed for use in the
Superconducting Super Collider (SSC), use large amounts of
superconducting wire. This has stimulated the superconducting metals
industry to develop better and cheaper ways for refining and fabrica-
tion.
Representative terms from entire chapter:
synchrotron radiation
